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The Woman Who Broke Gravity | Claudia de Rham
August 20, 2024
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To be honest, I was certain I had made a mistake. I remembered going through it over and over again. It simply doesn't make sense. What if everything we thought we knew about gravity was wrong? What if the symmetries of Einstein's beautiful general relativity are broken at the fundamental level? We've been told that gravity is just the curvature of spacetime and not a force,
But are we misguided? In this episode, we plunge into the world of massive gravity with Professor Claudia de Rham, a theoretical physicist at Imperial College London and author of The Beauty of Falling. Professor de Rham has upended decades of scientific consensus with her recent radical theory suggesting that gravity itself may have mass, an idea that was long thought impossible.
What does this mean for our understanding of the universe? Professor, please tell me what's something about gravity that most physicists hold as an assumption that you think is deeply mistaken? Okay, that's a good one. And I'm going to have most physicists against me now. Okay, maybe there's two things which are probably related.
The first one is the assumption that general activity and understanding of gravity as we have it at the moment is based on Einstein's principles which are the pillars of modern science, the pillars of general activity related to the equivalence principle and I think
and related to symmetries and nothing actually those we don't need to have those pillars i think you can divide the laws of gravity you can divide general activity not based on.
additional assumptions and principles per se but rather on the requirement that it is a self-consistent theory that it is a stable theory particularly when you embedded in a quantum field theory network and you can actually derive general relativity from the ground up
And therefore you have the Einstein's principles as consequences of stability and self-consistency as opposed to use them as original assumptions. So that's probably one of them. Another thing which perhaps most of my direct colleagues would completely agree with as a whole, the general audience may think they disagree, although I think they would agree of all is
The notion that gravity as described by general activity is not a force you may have heard that and people say that they emphasize a lot the difference between our representation and understanding of gravity with other forces of nature and i think actually that is sometimes misleading because for many aspect.
For many aspects, general relativity is actually a force like the other forces of nature and we can describe general relativity as a force. Actually the detection of gravitational waves is the proof that deep down gravity is a force. Gravitational waves is a representation through the stretching and squeezing of space-time of the force that is hidden within gravity.
Okay, well, firstly, what are those symmetries you think that we can do away with when deriving general relativity? And then also, what is the definition of force? That's a good one. Definition of force is a very hard one. Let me start with what I think is simpler, although maybe the words themselves are not as simple. What is what are the symmetries that are that general relativity, according to Einstein is rooted within
It's a beautiful symmetry, which is the technical term is perhaps coordinate transformation invariance. It is nonlinear deformophism. That's the technical term. But actually that symmetry in its sense is very simple to understand.
It is the realization of the laws of physics should be equivalent wherever we are and have a we describe a phenomenon you should be equivalent independently of the observer so whoever you are even though some elements are relative to one another when it comes to measuring a physical quantity we have to be able to describe it. The way i sent for is it is in any frame of reference so independently of how you decide to represent.
Nature around you have you decide to slice space and time that shouldn't matter that is your own packages but then you can't leave when you come with something which is physically observable we should all agree on that and that is beautiful it is a beautiful symmetry it is a philosophical framework if you want in some sense.
But to my mind, this is actually so beautiful. It's something that is derived from the self consistency of a theory as opposed to setting it up as the basis as the foundations upon which we're going to build general relativity. So that was the first question about symmetry. The second question about what is a force? What do I mean by a force?
i make it very deep to a point where i may not you know terminology and vocabulary is not it's not my strength so what what do we mean by that by a force i think for for most of us for me including um we think of force as as a contact force in itself
When when i push against something that's a notion of pressure in itself that's not really a force per se it's more related to the electromagnetic bonds etc but that's not exactly what we mean by force particularly not when we're dealing with the gravitational force so if you feel if you feel you sitting down for instance if you feel your chair
The bottom of your chair, this is not the gravitational force acting on you. This is the pressure and the contact interaction that is present between the cells in your body and the atoms on the chair. And actually that's related. That's very interesting. I'm going to attend and that's related to Pauli's exclusion principle, which prevents two states versus two fermions, two electrons to occupy the same place at the same time.
And so if we have already some of the states in the chair that are occupying a particular place at a particular time, I cannot just go through it. And so I'm feeling the effect from the pressure of this poverty exclusion principle, which prevents me from having two states at the same place at the same time.
That is not what I mean by a force at the more fundamental level, which I more represent as something related to electromagnetism. For instance, I consider electromagnetism to be a force and it is something in that can act at a distance. So it's not something of a contact. It's something that can act at a distance. And we may be familiar with that with a magnetic force. For instance, if you take a magnet,
You can attract some iron with a magnet at a distance. That is happening through an electromagnetic force.
And deep down, what happens is that there's a field there, there's electromagnetic field that carries the force for us. And that can be represented fundamentally in a particle, in a field theory level. For us, we understand the electromagnetic force has been carried by a messenger, which we call the electromagnetic waves, which we call deep down the photon. The photon is a messenger for the electromagnetic force.
And gravity can be represented as a force. In exactly the same way so you may have heard of gravity according to us that the region activity is not a false rather it is the representation of the curvature of space time and that is a beautiful is an extraordinary beautiful and accurate description of gravity for all sorts of phenomena.
But it doesn't mean that also deep down it isn't represented as a fundamental force, just like electromagnetism. And so if we feel the gravitational attraction from the earth, it is also mediated by a gravitational field, which is carrying a force.
Fundamentally the component of this gravitational field is a particle which we call the graviton and there's a direct analogy there between electromagnetism being a fundamental force carried by photons and gravity which is also a fundamental force that's mediated by gravitational field and carried by a fundamental particle which we call the graviton.
Okay, so for the people listening to go back, the general in general relativity comes from the theory being generally covariant, which is the same as being diffeomorphism invariant. So can you have general relativity, if you remove that any coordinate system is an okay coordinate system? Isn't the whole point of the language of bundles to describe
constructs without reference to coordinates. So then do you remove the underpinnings of differential geometry to GR? So you don't want to do that directly, first of all, because your relativity is a beautiful framework that works extremely well in all sorts of settings. So as soon as you start removing some of the beauty of it directly, then you end up with elements which won't match observations.
And as you said, it is a beautiful symmetry. It is also something that we like to have in a theory of in a description of the world where we would like to make abstraction as much as possible to anything which relies on us making a choice of any sort. And so in the world of bundles or in different representation of general activity, we can extract ourselves from expressing where we are in a frame of reference.
Hi there are some situations where we can think of a description of gravity which is very well described by general activity.
Up to a given level but comes a point where the symmetry of this representation needs to have more to it in reality what happens is that you never break things completely you never break everything down but you start seeing a new structure emerging when you deep dive into it before to understand how to do that you first need to embrace all of the beauty of general activity need to understand how it works.
and only in some special limit, can you allow yourself to go beyond that and understand a generalized framework that has a symmetry in some limit, but then more generally it behaves slightly differently. Now going back to a force, are you calling a force anything that has a force carrying particle? And so that's why you said gravity is a force because there's a graviton associated with it.
So at the fundamental level everything is quantized so it doesn't not be it's not because something has a particle associated with it that you will necessarily be a force but fundamentally all what does she say is all fundamental forces well to my mind necessarily have a particle as a messenger associated with it because everything is quantum and this is all
Even more true for gravity because gravity connects to everything and so if we know the gravity connects with a real for instance electromagnetism for instance electrons and other fundamental particles which are quantum then gravity has to be quantum as well.
You can't just couple something which is fundamentally classical that satisfies fundamental classical properties with something which satisfies quantum probabilities. Actually, what you need to do is having a grander framework where everything is quantum and in some limits some sector may behave classical, but it's only in some limit.
So what do you see as the difficulty in reconciling gravity or general relativity with quantum theory? So we can do that and we do that actually on an almost everyday basis in my work but also in connecting with some people doing observations.
When we dealing with gravity in a not extreme environment so when we thinking about how gravity behaves on earth in the solar system even in most of the galaxy and in most of the universe actually we can reconcile gravity with the quantum laws of nature and with the other quantum theories the quantum theories of the forces and there's no real issue associated with that.
The distinction with gravity and where the problem arises is when we're trying to describe it when the curvature scale is very intense so in a very extreme environment for gravity and there what happens is if we took seriously the quantum laws of probabilities and we applied to general activity in those extreme environments we would end up with some laws of probabilities that stop making sense.
And so that doesn't mean that probably doesn't make sense. What it means is that we need a better framework to understanding and how to reconcile late gravity with those laws of quantum probabilities in those extreme environment. So we don't have access to all of the information, all of the description of how gravity behaves. So if you imagine that you are working with some laws of probabilities and at the end of the day, the outcome is not what you predicted.
It must mean that something else must go on. Now for gravity, it's not like we've been in an extreme region in the universe, for instance, at the center of a black hole, or for instance, at the very beginning of the Big Bang, and tested gravity and the laws of probability there to be able to say that they don't work. That's not what we have done. Already, according to Einstein's theory of general relativity, when I use the standard laws of quantum probabilities associated with it,
I end up with outcomes which simply don't make sense. For instance, you can imagine that typically when you add probabilities, I have a probability for something to happen. I have a probability for something else to happen. When I add things up, I cannot end up with an outcome that has more than a hundred percent probability to happen. But when we take general relativity and we're trying to apply the laws of quantum probability at the very center of the black hole,
All five cents at the very beginning of the universe very close to the big bang i see him to be able to end up with outcomes which would have more than a hundred percent probability to happen sometimes have a negative probability to happen or a complex probability to happen.
And that simply tells me that i am missing something i am not summing up all my probabilities given the all the configuration that i am allowing myself properly something is missing and i need to understand gravity better i need to understand how to go beyond the description of gravity using term relativity to being able to better appreciate how to reconcile my laws of quantum probability
What's my description of gravity in those extreme environments. I see it doesn't have to be that something's missing or could it be that you over counted in the case where it exceeds one hundred percent. So even if you were that i have accounted i need to understand why i have accounted because the possible outcomes to my mind not to my mind per se but when when we do the standard estimations.
Are possible outcomes a possibilities which otherwise could have been realized so if you want to think of an analogy i can take two particles which is something which is done for instance in particle exclamators i can take two particles and i smash them together and then i have a probability of a given outcome.
The given outcome can be two other particles that are scattered with a different angle or some of those particles may have transformed themselves in something different and I can think of all the possible outcomes.
And for general activity, of course, I'm not gonna smash gravitons together. I don't have access to gravity, but I can perform those things, thought experiments, or I can think of doing them for all the particles and understand what would be the impact of having gravity in it as well. I can do all of this and I can sum of all of the outcomes, the possibilities, and they seem to be realizable and they seem to be making sense in themselves.
So if now I'm of accounting in some of those extreme environments, I will need to understand what happens, what happens to those possible outcomes that are no longer a possibility at the center of a black hole. Irrespectively of what precisely happened, whether we are counting and accounting, not counting it right, or giving too much weight in some possible processes,
When i say something is missing is not necessarily something tangible is missing but something in understanding of what is happening is missing.
So I want to get to this massive theory that you and your colleagues came up with in 2010 and literally massive. It's also outlined in your book, The Beauty of Falling, which will be on screen now and people can click it in the description. The subtitle is a life in pursuit of gravity. So firstly, why don't you tell us what is that book about and bring us through the journey that led you to 2010 and that discovery?
Okay, yes, so I can discuss about massive gravity afterwards. The journey itself is very much a journey towards the scientific exploration, the ups and downs of doing research, particularly doing research in theoretical physics, where the connection with the real world is still something that we need to develop and is ups and downs all the time.
so it is about this journey as a researcher and also associated with the journey of myself in in going through different steps in my life but also to me as a scientist right it is that and also in parallel the journey that we have us as human i would say.
In operation appreciation of the laws of nature and particularly appreciation of gravity and how they spin ups and downs in understanding of how gravity is being described and actually we are possibly hitting another possible. Down in the description of gravity failure or falling down in gravity.
I'm related to what i was describing that we know something is missing in our description of gravity there's a point of failure which in itself is part of how we do research we understand that we have a description of nature we have a description of some phenomenon around us which is quite fundamental but is not fully. Basic we need to go deeper in those laws of nature and is a description of nature around ourselves and so.
We can think of the points of failure for instance when general activity breaks down and we need to have a better description of gravity as one of the failures of general activity but actually is an opportunity to look for new underlying framework to better understand nature around us. So the book in itself is framed within that premises of how we doing research and how we going along
trying to understand things but everyday almost as a there's an up and down and embracing this level of knowing that at every stage there will be a point where how we do research and how we connect with the world will have some elements of failure and this is something to be proud of actually and this is something to very much embrace and keep exploring because it's through these points of failures that are we going to make discoveries and being able to
Get access to new layers of understanding new layers in understanding of how nature works around itself. So it's a description of this journey but also associated with my own one and through the understanding of a theory of gravity which is very similar to Einstein theory of general relativity other than it is massive right and and that says be massive.
Doesn't mean that it's huge. It's related to the fact that it does sound like that. It's a massive thing. Actually, it's quite the opposite. It makes gravity smaller in some sense, but I'll explain what I mean by less far reaching. Exactly. Less, less far reaching. So the idea behind massive gravity is related to trying to tackle some of all the problems we have with gravity.
I'm not those are related to not to the reconciliation of gravity with the quantum world which is what happens when we're looking at extreme regions of the universe where the curvature of the land scales the coverage is very high the land scales are very small like the center of black hole at the beginning of universe rather exploring what happens on the other side of the spectrum.
Where we looking at very very small curvature scales so not on the smallest possible distance scales but actually quite the opposite on the largest possible distance scales what we mean by that is that we exploring the behavior of nature around itself on cosmological scales on the largest possible observable scales in the universe so the size of the observable universe itself.
Which is spanning of thousands of trillions of kilometers apart as is the size of the observable universe and so the idea is to try to understand how we can reconcile the behavior of the universe as we see it cosmologically with expectations from gravity and expectations from the other fundamental phenomenon in nature.
Particular the realm of particle physics which postulates the existence of particle describing all the fundamental constituents of matter and the fundamental constituents of the forces. Now we know from particle physics that for instance the Higgs let me go on a tangent a little bit the Higgs phenomenon is a phenomenon where the vacuum is not empty is filled with the Higgs vacuum
sort of a Higgs bath is a Higgs bath where you have interactions between the Higgs and all of the other massive particles. And it is these interactions between the Higgs vacuum, this Higgs bath that changes the dynamics of some of the massive particles and in fact give a mass to some of those particles. So we understand the Higgs phenomenon as the phenomenon that gives a mass to other particles of nature.
So this is just an illustration to tell you that empty space is not at all this boring thing when nothing happens most of the universe from our eyes from our point of view is filled with emptiness we have huge cosmic voids which are millions and millions of kilometers wide they're huge most of the universe is actually empty in that sense is filled with cosmic voids where galaxies struck where galaxies and
Clusters of galaxies and filaments of dark matter are that's just filaments that taking in itself just a small fraction of the universe and most of the universe is actually made out of this cosmic voice and they seem empty to from our perspective but actually they are filled with at the very least they should be filled with this Higgs path because we know that it's thanks to this Higgs path that other fundamental particles carry a mass.
But now this Higgs math, sorry, now this Higgs bath, this Higgs bath. The math of the bath. That's right. Exactly. Now this Higgs bath or the vacuum should also carry energy from possibly all the other particles. And that's according to Einstein's theory of general relativity itself, because of the equivalence principle, because gravity is so universal,
If he has an effect on other particles should also connect with gravity and so we should we should expect this Higgs bath and this energy in the vacuum to gravitate to have an effect on gravity to curve the structure of space time. Yes, that is not something controversial. This is something that has been developed has been understood already since the thirties.
Since the understanding of i sent the original activity and then from the beginning of quantum mechanics the quantum realm of mechanics by pauli and all the other fathers of quantum mechanics in the beginning of the last century it was already understood that we should expect emptiness to be filled with something that seems paradoxal but emptiness should be filled by something.
And that's something should gravitate and if it gravitates then we should expect it to have an effect on the evolution of the universe and in fact it was already understood already in the nineteen thirties that the effect of Francis the energy of the electrons in the vacuum should lead to an accelerated expansion of the universe which would be going so fast that the space between us the earth.
and the moon should be stretching at a speed that exceeds the speed of light. And therefore, if we put those two things together, we shouldn't be able to see the moon. Just a moment. So if in the vacuum, there are these, are you referring to the virtual particles? Yes. Yeah. So if each of those virtual particles, and they're an infinite amount, if you go all the way down to zero, not the plank length and stop there, but there's an infinite, okay, whatever, there's a large amount, even if you put a cutoff,
And each of those has some energy associated with it. That's right. And energy is associated with gravity. So you say they should gravitate, you mean that they should exert a gravitational force. But why would that force be repulsive and not contracted?
Okay so that's very interesting of why the effect of this phenomenon is something which doesn't seem to be the same as an apple falling on the earth which I will classify we typically classify as something attractive as opposed to the acceleration of the universe which we seem to be looking as a repulsion phenomenon and according if and indeed if you look at if you look at this phenomenon according to Newton gravity
You would think that this pushing away this accelerated expansion of the universe should be related to a repulsion or should be related to having an effective negative mass in there. That's how you would describe it according to Newton gravity.
But we are in a state of activity where things don't just happen in one dimension of time or one dimension of space what happens is an entertainment between space and time unified together and so you have things happening in space and you have things happening in time now when you have an effect of energy which is localized that you can think of it as something that happened in space to some extent.
It gives you what you would think a localized must hear the earth and ask being attracted by the earth this is an attraction but this is something which in the realm of general activity is only in one special dimension which is more related to time than to space is a bit of technicalities there.
Now in general activity, if something happens in space is also happening time and vice versa. And for this vacuum energy is not just localized here at one point in space. It's everywhere all the time throughout the universe. And so it acts on for all four of our dimensions and the way it acts in the space and the time dimension opposite. And since we have more dimensional space than we have of time,
Actually what happens along the space dimensions wins over what happens along the space dimension looks like it has the opposite sign as what happens in the time direction and that's why it looks like you have a repulsion but it's all attractive is just what attraction looks like in general activity may have different ways to manifest itself attraction is something that
According to space and time, it looks slightly different in how you represent itself. Maybe another way to see it is that rather than just being some energy localized in space, it is actually also some pressure which is localized in space and time. And this pressure has a negative sign. So it manifests itself as something which looks repulsive in all direction. But actually that's just a manifestation of a negative pressure.
So then in 2D gravity, would there be zero effect because there's one time and one space. So the way acceleration of the universe would work would look slightly different. That is correct. Yeah. But there would still be a positive effect, like a repulsive effect. So you will still have a stretching of space. You will still have the stretching of the space direction in itself. Yes.
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Okay, so now that we're on some opinions of yours that are controversial, what's your controversial take on the expansion of the universe? In other words, tell me the truth behind dark energy, professor. Okay, so dark energy is, I think that's not controversial to some extent. Dark energy is a placeholder for a lack of knowledge of what leads to the accelerated expansion of the universe.
So we don't exactly we don't say necessarily we all agree on what leads to the accelerated expansion of the universe so let's put us in a common ground and let's call that. The source for the acceleration of the universe dark energy now there's different perspective on what dark energy. Could and could not be but there's a very natural candidate for dark energy can be which is this vacuum energy because the vacuum energy by itself.
lead to an accelerated expansion of the universe. So that is, I would say, a very natural candidate for the acceleration of the universe. However, it is controversial because the rate, predicted rate of acceleration of the universe, if dark energy was indeed the vacuum energy, would be way too large, much faster than what we observe today.
And that I already alluded to when I was saying that if you just take the vacuum energy from electrons that we know exist, you should lead to such a fast stretching of space.
Such a fast acceleration of the universe that we wouldn't be able to see the moon that of course is not what is happening so we do have an accelerated expansion universe but by a rate which is way slower than what we would have expected if I counted for all of this vacuum energy from particle physics so.
Face with this dilemma of why doesn't the vacuum energy leads to a much higher rate of expansion of the universe accelerated rate of expansion universe instead what one can do.
Is a well maybe for a reason or another that i haven't yet found this vacuum energy actually doesn't gravitate or maybe it's not that all together or maybe i don't understand it so let me ignore it let me put it aside for a second or for a hundred years and instead let me say there's another source for the accelerated expansion of the universe which we call dark energy.
The reality something we should understand is that if you want a natural source for the accelerated expansion of universe which actually leads to the accelerated rate that we observe this issue that anything natural we would expect.
What's the lead to a much higher rate xx accelerated expansion is still there so you have candidates for dark energy. Various models i can come up with a hundred different names of models of dark energy that explain the accelerated expansion of the universe.
In every single one of them you need to what we call fine tune some parameters so you need to really stretch some screws to a huge level of accuracy in a way which is unstable so if you look at the smallest quantum corrections to that it would be much larger.
As compared to what you the level you tune it out so it's unstable on the quantum corrections all the models of dark energy that we have so far. Ninety nine percent of the models of dark energy that we have so far unstable against quantum correction in one way or another so we will need to resolve this what we call fine tuning issues in those models as well but in some cases they're much better hidden so it's harder to find where the where the tracks of the matter lies.
That's why we're trying to come in, that's why we're trying to say rather than postulating the existence of a new explanation for the accelerated expansion in the universe, dark energy, which typically also come with tuning in itself and it's not stable until quantum corrections. Instead, let's just go back to this original idea that the vacuum energy
is a natural candidate for the accelerated expansion of universe and for that we need to understand why this huge level of right doesn't lead to as high a level of acceleration as what i would have expected.
And there's a lot of models out there that try to address the vacuum energy itself. Try to understand what it is in the quantum field framework. I haven't quite understood. So that the expected vacuum energy is not as large is actually much smaller. So there are some models that do try to do that. There's no successful model so far, but they try to do that still.
Another alternative is to say, okay, well, let me accept actually that this is the way it is from the particle physics side, because after all we have a very high level of control of what happens in the particle physics side. We understand particle physics very well. We actually have access to it. We understand how quantum corrections work extremely well. We look at quantum correction of a quantum correction of a quantum corrections. And this is very much under control in the particle physics side. So let me take that.
As it is and instead it is in making the connection between this and gravity and understanding how the vacuum energy affects gravity that I will try to tweak things. It's interesting. Okay. And so this is where I cannot have just general activity because general activity in all this beauty, it has this level of universality that everything and everyone
Is affected and affects gravity in a universal way so i can't just decide that someone is gonna be affecting always gonna be affected by gravity in a different way i'm not allowed to do that in general activity it is. One of the pillars if to some extent of general activity.
So if i want to have a phenomenon if i want to have vacuum energy which doesn't affect gravity in quite the same way as other sources as for instance a galaxy as localized matter then i need to twig.
Some of these pillars after relativity, every source likely. Okay, you have to tweak it, but do you have to fine tune it so that you're replacing one problem with an equally intractable problem? That's an excellent question. Do I need to? Exactly. So how much is that tweaking fine tune in itself? That's an excellent question.
And you have to tune it you definitely have to tune it the difference in the level of tuning that you need to make is that it is stable and the quantum corrections so you need to have a very small number in the game what ever you do. How ever you try to resolve this paradox you need to bring in a very small number in the game in this case the very small number is the hot by how much you modifying term relativity.
And you do so with a very very small with a very very thin brush but when you have to relativity in itself it doesn't spontaneously. It isn't spontaneously affected by quantum corrections so all of the symmetries often relativity they are protected by quantum corrections.
If you have a symmetry which is present and you do look at how quantum corrections affect them, they will preserve those symmetry typically, unless you have anomalies, but that's for a different question. So these coordinate invariants, for instance, this or this symmetry, which we call covariance or diffimorphism invariance, or this notion that every observer should be equivalent irrespectively of the form of reference. This is not a,
Principle that gets modified by quantum corrections so if you start with that then you're not going to expect it to be modified by quantum corrections but that means that if you depart from this symmetry every so slightly the amount by which you're going to be destabilized by quantum corrections is going to be proportional to how far away you are from it in the first place so if you only
Displace it every so slightly, you're never gonna move very far away from it. So in that sense, you have a very, very small number in the game, which is a small modification of gravity that you bring in. But this small modification doesn't start going completely ballistic. It doesn't start going completely crazy in the sense that it doesn't start growing over time, if you want.
So it is in enough framework of quantum mechanics and naturalness. It is a tuning, but a technically natural tuning. So it's not a fine tuning in the sense that you don't need to keep tuning it at every level in your quantum corrections. But you understand that this is the case. I can say those things and you may or may not believe me, but you understand that this is the case. You need to have actually a very rigorous framework in which you can explore those ideas.
Add any new which you can look at the quantum corrections and see how they affect the small modification of gravity and this is precisely this idea behind massive gravity it is an idea of framework where gravity has a very very small mass the gravity to be precise the particle carrier of the gravitational force of gravity.
Unlike being massless, as would be the case in general activity, it acquires a very small mass. It's extremely small. It's the smallest possible mass that you can ever envision. It would be of the order of 10 to the minus 32, 33 electron volt. By comparison, the neutrino mass, which is the lightest massive particle that we know of for sure, that has a mass of 10 to the minus three electron volts.
Millielectron volt enter minus three electron what roughly so it's roughly thirty orders of magnitude below that it is extremely extremely small. And so this very small departure of general activity you can now look at how the mass of the grab it on good get corrected by quantum corrections.
You know that in general activity the Graviton is massless so the mass is zero and so and you know that you don't start asking yourself is it true that the Graviton remains massless in general activity when you include quantum corrections because the masslessness of the Graviton.
is also related to the symmetries after relativity is related to this equivalence principle is related to this universality is related to covariance all of that is part of a big package in general activity right and so you can't just shake it a little bit around you can't it's simply it's it's a package to stick together now we started to unfold it a little bit and to allow for the graviton to be every so slightly massive
And the amount by which all of the other implication will start losing up is present, but it's the same amount by which you had started shaking it in the first place. And so if the mass of the great one is extremely small, you're going to have a small correction to the equivalence principle. You're going to have a small correction to all of the phenomenon that we discussed about, which is proportional to this graviton mass.
And so it also means that if you have the vacuum, if you have a source, which is present here like the sun or the galaxy or even a cluster of galaxy, these are distance scales, which are still very small.
As compared to the quantum wavelengths of the Graviton. So the quantum wavelengths of is related to the inverse of the mass of the particle, if you want. So if you have a massive particles, one of the aspect of it, which we are after, is the fact that the force associated with it will have a finite range. So you want to have an infinite reach, like in general activity, it would actually just reach a finite distance.
And that's related to the quantum wavelength of the particle. And so any structure that we use to in the universe, for instance, the solar system, a galaxy, even a local cluster of galaxy, will be within the quantum wavelength of the graviton and therefore relatively small distance as compared to the scale at which gravity gets modified. And therefore on those smaller distances,
Gravity looks very similar as in general relativity and we don't see a very big departure from general relativity.
It's only when you start looking at effect which are much larger distances on the scale of the observable universe today that you start seeing a departure of how those effect lead to a curvature of space time and this is precisely where the vacuum energy is coming in so we have a community on the whole.
Of the universe since the beginning of time of billions of years of billions of light years across in distance and over those huge distances this is where we start seeing a weakening of gravity and therefore the effect of this vacuum energy on gravity on cosmonauty and on the evolution of the universe is much weaker as compared to what we would have expected in term relativity.
Interesting. So does this then give an alternate explanation for dark matter? That's a good question. So if you want to understand dark matter, you can and people have indeed tried to understand whether you can use a similar framework to understand dark matter. It is naturally it is difficult to do both at the same time, because the scale involved actually quite different.
We have a very good understanding of the presence of dark matter or something that looks like that matter already on galactic scales. That is very, very much present there. And so the scale at which you need to see this effect to consider it as an alternative to dark matter would be on much smaller distances as compared than what you would need to have it for, for dark energy. So in principle, you can, you can,
Do this in different layers and you can have a modification of one scale and then another modification of another scale you could do that in principle but the reality is these are relatively separate phenomenon so you might as well just consider first what happens for dark matter and another separate phenomenon what happens for dark energy.
Now people have tried to do that for dark matter indeed and their models were similarly to considering what would happen for massive gravity. What if gravity had a mass? What they're considering is model of what we call multi gravity. So rather than having just gravity as we know it, there's many different layers of gravity. There's many different notion of gravity and
You can have some of these alternative gravities which act as dark matter for observations. So this is not something I have worked on. These are the models that are taking on some of the aspects of massive gravity, bringing them on top of gravity itself, on top of general relativity as a new source for dark matter and for other phenomenon in cosmology or in particle physics even.
I see. So for people who are listening and thinking, OK, a massive graviton, the graviton was thought to be massless. Why didn't physicists think about a massive graviton earlier? They did. And there were two problems. One was the VDVZ discontinuity, if I'm not mistaken.
which we can talk about. And then another was that there are some ghosts. There are two types of ghosts, generally speaking. Yeah, exactly. Exactly. Exactly. They have pop off, which are the wanted ghosts are the benign ghosts. And then there's Polly Villiers, if I'm pronouncing that correctly. Yes, that's right. Exactly. Exactly. Okay, so you know, you got you got everything right. Okay, so people have indeed considered the idea that gravity could have a finite reach, which is the essence behind
Massive gravity. In fact, I should say Newton himself. He thought about this idea that according to his law of Newton interest, yeah, Newton law square law of gravity as he as he had it. This is a phenomenon and it has an infinite range. So gravity gets diluted that like the square law. So it gets diluted like the like the distance. And this is in itself very geometrical.
But himself he was thinking about what the gravity could have a finite reach at the end of the day and trying to understand how to make sense of that from a newton perspective. Other other scientists throughout since then like laplace also considered considered that now.
If you wanted to do it just a little bit of newton load that that wouldn't be too challenging the challenge is to do it at the level of a flu fully fledged non-linear non-linear theory of gravity as in anstine theory of relativity with everything that we know about your relativity and then further embedded into a quantum field theory framework as we know has to be the case and so since we know much more since then on how
Gravity works we need to make sure i'm thinking about a massive gravity that it is still satisfies all of the other qualities of gravity as we know them and in fact pauli himself in the nineteen thirties. Fiat and pauli they first started looking at the theory of gravity where the gravity could have a mass.
But one of the issues was pointed out by what you mentioned this vd vz discontinuities and that was in the seventies so vd vz stands for about man zakharov and i'm sorry.
Yeah, sorry. EDVZ stands for Van Dam, Veltman and Zakharov. In the same year, in 1970, they realized that if you take just a theory of massive gravity and then you compare it with a theory of general relativity, you seem to be getting some effects which are different for both cases, even in the limit where the mass is extremely small.
So you may you may have the impression that you can look at let me call it the force of gravity between the earth and the moon and what you would obtain in general activity is the result that we know of and what you would obtain in a massive gravity is a different result which is different even when the mass is as small as you want is different by order one no matter what and the reason for that is quite simple to understand actually.
And it's going back to this idea of, of what carries the force, what happens in there. Okay. And the idea that you have a messenger for gravity, which is related to gravitational waves, the real force of gravity is carried by gravitational by gravitational waves, by by gravitons through gravitational waves. And that part is uncontroversial. Like, that's not just you saying it.
No that part is in conversion and and and controversial so this is this is this is fine i'll tell you when i stop being controversial so the controversy is whether you can have a massive gravity that's why the controversy is but but in terms of what the high gravity behaves and what the issues were at the time
All the way up to to a few years ago with massive gravity that's also uncontroversial so i can go through them and what does issue so if you think of gravitational waves that we have detected we have detected gravitational waves coming from very far away events and the way they work is as the gravitational waves propagate through space and time they actually affect the notion of distance that affects space
How long the line of the line of propagation so they are what we call transfers polarizations so i should do like this sure goes like this you have a squeeze in the stretching like this and the opposite direction is what we call a quadruple and they go along the line transfers to the line of propagation that's uncontroversial now if you think of a theory of massive gravity what happens there is that rather than being a massless particle you have a massive particle
And so if you think of the idea that light travels at the speed of light in the vacuum, light travels at the speed of light in the vacuum because it is a massless particle, because it's carried by a massless particle. You and me, no offense, but we are massive objects in the sense that we're very broad. We are massive objects and we don't, I don't typically travel anywhere close to the speed of light because I'm quite massive.
And so if you have a massive object, you don't no longer travel at the speed of light anymore. You actually can't. You can try to go very close to it, but you can never actually go quite at the speed of light in the first place. But that means also what we can do, though, is control a speed. I can decide to be at rest and I can decide to speed up or slow down. I can do those things. Being a massive particle is actually quite a positive thing.
And so the same thing would be true for massive gravity and gravitational waves could also speed up every so slightly or slow down every so slightly. And so that means that in addition to just having polarization which are transfers to the line of of propagation you could also play with the longitude on our direction a little bit like sound waves if you're thinking of how you.
Not for you and me right now but but how you hear each other it is a sound wave and is a compression of the air pressure and more pressure and less pressure and the along the line of propagation of the wave or just if you drop a stone and on a pond you'll see some waves traveling along the surface of the pond and those are what we call longitudinal waves because they go along the line of propagation of the wave.
And so massive gravity have this additional freedom as well in how gravitational waves can evolve and this additional freedom missing like okay it's great you can do that as well it also mean that in terms of the force of gravity because it carries additional channels.
which gravity can be mediated you would expect gravity in massive gravity on short distances to actually be stronger and that's counterintuitive we came up with a theory or not we but overall when people think of massive gravity in the sense that the particle has a mass one wants to do that because it weakens the behavior of gravity at large distances but what seemed other time price to pay for that would be to have additional
channels of communications for gravity, so an additional way to transmit the force of gravity, which through longitudinal polarizations, which would also mean that gravity on shorter distances would seem at the time to be stronger. And so this is what V, D, V and C discovered in 1970, that because of this additional channel, which seems to be present, even when the gravitational mass is as small as you want,
That means that there's a discontinuity between what happens in term relativity where the mass is exactly zero as compared to what happens in the massless limit of massive gravity. Yes. So that was the original issue. Controversies set up by VD and VDV and Z in 1970. But then two years later in 1972, Weinstein
I understood what happened i can't even stand came along and realize that actually the remit within which this understanding was done this calculations were done and how you think of a notion of force in terms of being mediated by this different channels different polarizations.
Does make sense in some limits, but when you really try want to understand what happens in a very small masses, actually, you can't just neglect a whole sorts of other things that should otherwise be present. So actually, while it is true that those additional polarization are present when the mass is finite, right? Actually, when the mass is becoming very, very small, it becomes extremely hard to excite them. They actually themselves, they freeze in some sense.
But you understand this freezing mechanism, what we now call actually the van stein mechanism. It's a screening mechanism. You need to understand how the self interactions of gravity allow for specific polarizations, which otherwise would not be there in general activity to freeze themselves.
so it's almost if i'm taking too much what is meant by this freezing it's yes so it's almost as if i'm putting too much luggage on myself that i'm no longer able it completely inhibits my emotion and my ability to communicate so it is a it is it is a little bit as if um so we
When we're thinking of the force being mediated by gravity at the level of this different polarization we have a very simple picture in mind but when the mass of the garage becomes very very small this additional polarization interact with itself so it plays it's almost playing the role of a honey.
In which it prevents its own dynamics it's no longer free to move at wish and it does that by itself through its own interaction so it is it is what we call a phenomenon of strong coupling the self interactions of the gravity on a particular sector become so important that they were sent to something which is very different as compared to what one would have expected in the first place.
So i understood that a phenomenon like that had to be the case but you understand how this is implemented in practice how it happens in practice you need to have a fully non linear theory of gravity where all of the nonlinearities all of the interactions of the gravity on coma long just like would happen in general activity now you can think of
You can think of what happens for general activity when the interactions become very strong. When the curvature becomes too strong, you can imagine of having a black hole. You can imagine that if you have general activity and you have a regime where actually not necessarily the curvature, but the nonlinearities of gravity become important. That's where you actually quite far away from Newton and gravity. And this is precisely what happened at the onset of a black hole horizon.
A black hole is precisely where things will be very different as compared to what you would have expected in Newton gravity because you no longer in the week gravity regime you starting having important interactions for gravity. This is the whole realm of black holes testing some new aspects of of general activity in a regime that would otherwise be the case in the solar system for instance where even though we do
Understand the subtle difference between Newton and gravity and general relativity in the solar system. They're still very subtle differences. They're not all the one difference. But when you get close to a black hole, actually the difference between Newton and gravity and general relativity, they are very big. They're very noticeable. Now for massive gravity, in addition to what would seem to be this distance associated with the size of the horizon of a black hole,
You have an additional distance scale related to where the non-linearity is now for the additional polarizations become important. So you have two distance scales. You have the non-linearities are important for the additional modes of the graviton. And then you have a very much smaller distance, which your standard Schwarzschild radius, if you want in, uh, Einstein theory of relativity. Okay. I don't know how familiar people are with the Schwarzschild radius and the idea of horizon.
So what we need to understand is how to make this transition transition between what happens a very, very far, very big distances where there we understand gravity should be weaker and it is in a linear regime. But as you go to look in a theory of massive gravity and you start looking at some other distances, you need to start kicking in the nonlinearities for some sector of gravity.
Which will then suppress the effect of the additional polarization and the departure of massive gravity as compared to general activity within that radius. We call that the branched on radius. Right. And when, and when the, when the gravity mass becomes very, very small, this function radius becomes larger and larger. And as the gravity mass becomes zero.
This function radius becomes infinite and so the whole universe is within its own function radius which means that it looks identical to g.r. interest so we need to understand the non-linearities to make that happen so it exactly smoothed it out didn't just temper the discontinuity that's right it's smoothed it out and and so now we have exact realizations.
Massive gravity where we can see precisely this transition where we have the we can think of the forces of force between the earth and the moon in a theory of massive gravity and we know precisely what happens when the mass of the gravity is smaller and smaller and smaller and we cover precisely the same result as in general relativity when the mass is exactly zero.
So we understand that and therefore in a massive gravity if the mass is sufficiently small as what we would want anyways what we know is that the prediction for gravity in massive gravity would be extremely similar to what they are in general activity the departure would be extremely small it doesn't mean that we may not never see them in the solar system.
Because actually we have very very precise tests of gravity in the solar system but they are very very suppressed.
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Okay, so let's see if I can do a summary so far. So the VDVZ says that if you have a massive gravity theory, sorry, not VDVZ, but generally, you should have your theory. As you take some parameter and you deform it down to zero, agree with a theory where it has zero if you're trying to recover that theory. So for people who are familiar with quantum mechanics, there's h bar and the correspondence principle.
So what that means is as you set H bar to zero, if H bar is supposedly supposed to measure the quantumness and you set H bar to zero, then you should recover classical mechanics. There's a way you can do that with a theorem, although there's some hand waviness, I believe. But it doesn't matter. The point is there's no discontinuity there. And then you think, OK, well, if I have a massive theory of gravity, if I have a massive graviton, it should be straightforward to just put that mass down to zero and recover GR.
But it turns out you don't. And that's quite odd. But then the reason why it's odd is because this this van stein guy realized that we weren't taking into account the nonlinearities and the other mode of polarization. Exactly. So you got exactly that exactly right. It is a there's a lot of subtleties going on. But this is this is exactly the gist of the story so far.
So we still in nineteen seventy two and then the story is not over because still in nineteen seventy two. The same year as what i came along and resolved this video is discontinuity and said hang on a second you can't just be the calculations that were done so far what they were much more similar to what happens in newton and gravity as what happens in.
In a fully nonlinear theory of gravity, like general activity. And he said, you need to account for all of those subtleties. In reality, already in general activity, we need to account for many subtleties that arise for nonlinear interactions. We need to account for them because that's, that's precisely how we understand black holes. This nonlinear effect of gravity are important there. So, so it is the case that general activity is also something that has very important interactions.
Manifest itself in specific frameworks and so for massive gravity it also has to be the case that it's interactions have to be accounted for and we understand how accounting for this non-trivial interactions help us understanding how to take the small mass limit of gravity smoothly to zebra.
I should say it's not a doc is not something we're putting by hand at the end of the story to fudge things so that they work with observation or so that they satisfy a principle that we had in impose on ourselves in advance it is something that comes in naturally you're thinking of a theory of gravity naturally has to be something which carries non-trivial interactions and when you account for these non-trivial interactions.
Naturally you see this function mechanism emerging and a smooth limit to general activity when the mass is small so it's not something a doc is something fully fledged in the model itself understood but but then what in the same year what happened in nineteen seventy two.
These are sunny days are and ball were so two physicists realize that when you account for this non-trivial interactions in massive gravity. This seems to always come in hand in hand with what we call a ghost and it is a ghost which seems to be present in the physical sector. Please explain what ghost particles are.
So I ghost, I ghost should not exist. People who are not physicists, they just think that this is a different channel now. I think the new Ghostbusters movie came out. That's basically what you did in 2010. That's right. And I have a Ghostbusters t-shirt. So I'm not going to go too crazy, but it sounds, it sounds
The word sounds slightly science fiction but it's all really based in the in the rim of a. Very very scientific and i didn't come up with a time ghost myself it's something that was known since a hundred years as well since the nineteen thirties so yeah i'm also hundred years already so i ghost is.
Are the most basic level a particle with negative energy what we mean by negative energy there is negative kinetic energy which means that if you have a ghost and you make it run around at a given speed it's energy will decrease so it will release energy to the system and the faster it goes the more energy it releases to the system.
And the more it does that the more other particles which are normal particle can absorb that energy and that means that the universe as we see it would be completely unstable because you would have this possibility to trade up positive and negative energy so we all of us would be allowed to be as excited as we want and take over the energy
That the ghost is releasing and by making the ghost go as fast as they as they want so this seems like a very unstable process it is different it is different from having a particle. Who's you may have heard having a particle which has a negative potential negative mass square like some of the phase of the expo zone may have had that in the history that is.
Just a transition phase when that happens is that action is just a transition phase where for a while princess you go up the hill with your bike and when you are the top of the of the hill you seem like you can very quickly speed down the hill but that's just a transition phase up to the point where you find again yourself at the bottom of the hill.
And then there'll be no way to go from there are you just when you're the top of the hill you're not on the most stable position but then you always can go and decay to something which is stable and that's around the stable point the point is that is bounded from below.
it's exactly that yeah it's bounded from below because some people will say the problem is negative energy but would it be more accurate to say the problem is that it's not bounded from below you can have negative energy that's bounded exactly that's exactly you're absolutely right so the problem is that it's not bounded from below so it's you're absolutely right so a tachyon is a particle with negative energy but it will still be bounded from below um it's in that case for the tachyon it has negative mass squared
And it will still be bounded from below once you find a right vacuum was for a ghost it's unbounded from below and you know it's unbounded for below because you can decrease its energy. By going faster and faster and there's no limit to how close to the speed of light you wanted to go so it can go as close to the speed of light as you want and that would lead it to as a negative energy as you want it to have so that's the real issue with the ghost you.
You're exactly right the real issue is that it's unbounded from below and so there's no sense in which we can start our life around a stable vacuum where the particles don't go all crazy and where we can build a model from the ground up from that stable vacuum because there's nothing to start from there's no grand basis there's no grand zero this the energy is unbounded from below in the case of the existence of a ghost of a ghost particle.
That's for any type of ghost. So yeah, so this is, this is impressive for any type of ghost. So if, if the ghost is really there in your theory as seem to be the case, um, as observed by, uh, Dazer and Bulwar, Bulwar and Dazer in 1972 for, for massive gravity, that it is an issue. And then that's it.
This is a bit different from the idea of ghosts which are not really ghost which are present in your theory for the fadda fadda of ghosts. They are a mathematical trick they are trick in when you try to quantize some. Some fields which have some nice symmetry embedding into them sometimes it's easier to.
Use an extended framework where you have the impression that they have additional ways to excite themselves which is not the case and then to cancel them directly one by one with fadif pop of ghost so the fadif pop of ghost don't really exist and the additional modes that of the.
Are you trying to contact i'm not really there but you promote them to being there because it makes it easier to look at all this formulation so it is a mathematical trick in some sense but you do so that is an exact constellation between two pairs two pairs etc. I'm afraid of ghost and what would otherwise have been a mode that you are officially introduced in your theory.
So it's not something physical it's a it's a mathematical way to shortcut some of the framework that you try to establish when you try to quantize it. So the fire pop of ghost event though the word ghost is present in this case it's a good ghost in the sunset specifically engineer to patch.
I know the mode that you artificially introduced so you're everything is under control the terminology seems to be the same but in this case is is doing surgery in a way that you know precisely what you include and like with like in such a way that everything is is fine but for massive gravity we never no one ever came along and say oh again i introduce a ghost so that you can patch something up no he just comes up by itself
And that is really the issue because it is there it is physically there and he had as soon as it is present it leads to energy be unbounded from below. It's very existence means that you can reduce the whole of the energy of the universe to as much as what i did once by simply going as fast as it wants and that's that's the real issue with that.
Now I'd like you to take us through to 2010 when you and your colleagues circumvented that ghost and what that experience was like. Yeah. Yeah. So, so you busted that ghost. It was exactly like in the movies, exactly like that. Um, so yeah, so that was in 1972. Uh, ball winders are short that in a theory of massive gravity or in a theory of gravity that has a finite range.
It seems to be always the case that they come hand in hand with the existence of a ghost whether you want it or not it is present and many different people we explore that analysis but why does are we explore this in different languages actually you can think of it at the level of particles at level of energy various different type of levels and then in the middle of the eighties etc people had given up on the idea of massive gravity because
It seemed to be always the case that you can't make it work. To have a smooth limit to general activity where the graviton is massless, you need to implement these non-linearities, but these non-linearities always seem to lead to an excitation of a ghost, which seemed to be impossible. So that was up to mid-80s, but in 1998,
There was the confirmation from different groups that from supernova observations that the expansion rate of the universe was not slowing down as well as was expected rather it was going fast and faster so 1998 was when it was confirmation that the universe is accelerating.
And so from there on, there was this whole emergence of new ideas in understanding whether it is the vacuum energy, whether it is something like the cosmological constant that also plays the role of the vacuum energy, whether it's dark energy and all this tuning and self tuning issues. And so along with all of this, we came the idea that gravity could possibly be modified. It could have a finite range.
So as to tackle what we call this cosmonautical constant problem or the vacuum energy problem related to the accelerated rate of expansion of the universe and this is not where i came in i started thinking about those ideas a few years later.
I was doing my undergrads at the time i started my phd in two thousand and two and there it was already well established people had tried again to understand whether you can have a fear of massive gravity which would have no ghost and you seem to be always coming up with the same impossibility to understand how to make the algebra.
Of the interactions work out in a way that there was no ghost and actually throughout the notice i think you call this the notice from two thousand yeah okay got it is that the notice yeah there's arrows there's been a whatever that period was called lady gaga period that's all i remember okay that's a good name that's a good name um so during that period there's been quite a few very systematic papers being published
Going precisely through the different proves in different languages in different with different logics and showing how for every single one of these way to thinking about it that we always end up with the same issue that you can't have a theory of massive gravity with this nonlinear it is without also coming up with a ghost.
And so it concluded in a set of what we call no go theorems and no go theorem is exactly what the name indicates it tells you that it is a rigorous math mathematical theorem which the answer is no there's a no go you can't do that and what was this theorems name.
The no-go theorem or the no-go theorems in this case. Yes, there was no-go theorems for massive gravity. So there were, you can look it up, no-go theorem for massive gravity or for finite range gravity where you go through an analysis. They wouldn't have had a better name than that in themselves. Okay. So it's not like the Coleman-Mendula theorem. No, no, no, no, no, no, it wouldn't be like that. Yeah.
So this is this was the situation where i was a researcher i in fact at the pyramid institute and not my master and then in geneva in late two thousand nine two thousand ten. And instead of thinking of theory of massive gravity because of the time i was convinced that all of this made sense.
We were trying to start to look for a model of gravity which is based on having extra dimensions extra dimensions were very big at the time that could have some of the properties of massive gravity without this issue with the ghost and we did come up with a model a model based on extra dimension.
In retrospect it wasn't fully finite so it still has some issues at some level but when we looked at how it was consistent with the current framework in simply four dimensions it seemed to be leading to a theory from a four-dimensional perspective that would have looked like a theory of massive gravity with a finite range
And yet by going through this extra dimension we seem to be able to implement something which implement a framework which was free of the ghost so when we were going through the proof for why they should be a ghost we were going through every stage. And somehow everything would agree up to a given level but then at some point we saw that the outcome for our theory was.
To be absent of the ghost was in the no go serum the ghost should have been there. And even though the model we we were working on was a model that was based in a structure in the way that we engineered it was based on extra dimensions and really what i did is that.
The type of bricks are the type of legos that we used to build it were within the logic of the extra dimension as opposed to the logic of four dimensions so we were using a language was which was more appropriate for the symmetries in five dimensions as compared to what we would have used had we been in four dimensions from the outset.
Have a event that we did that it doesn't matter we can still think of a theory based on extra dimension and ask yourself the question of what would a four dimensional observer see in that model what would be the four dimensional characteristics of the theory of gravity from a purely four dimensional perspective.
And on the one hand it was leading to a theory that would look like a massive gravity that would have a finite range gravity where the would be four-dimensional gravity would be massive. And on the other hand we couldn't see any sign of that ghost at least not to the level where it was indicated in all of these no-go theorems. And this is really where pushed us to understand where was the discrepancy between the model that we were having
And all of those no-go theorems. And to be honest, I was certain I had made a mistake. I really, I remember going through it over and over again. I said, it simply doesn't make sense. There must be, the ghost must be there. And where is it? Where I must have hidden it somewhere. It's very easy to do that. It's very easy to convince yourself that things are fine. But then the problem is hidden much deeper. So I spent a year trying to understand
Where this pathologies in my model should be hidden because surely is be should be there somehow but i can't quite see it at the first site and understanding whether i made a mistake or whether things were actually more subtle but the pathology would manifest itself in a given way until i realize. Going back to all of the proofs that actually they had been some shortcut being used.
I'm in some cases there had been some implicit assumption being used in some case there had been. Too many shortcuts being used in such a way that the answer the result wasn't as general and i wasn't applicable to all possible situation as compared to what people thought and the example we had found was precisely almost by miracle one that fit precisely in the box of things that could work out that could make the ghost disappear.
You evaded those assumptions. Yeah, sorry, what would be an example of one of those implicit assumptions? And by the way, when you say that you mean to say that the paper itself didn't make clear or explicit the assumption, it was just embedded in the ethos of their argument, but it was subtle. That's right. That's right. So it's a little bit how what do you mean by a ghost in some sense?
When you try to understand what a ghost is, it's very clear what you mean by that when you can identify different particles and when you can identify the energy, the kinetic energy of every single particles. And that is something we all know how to do very well in the simplest scenario when we're in flat space time and when things are relatively simple.
This is not what we're interested in. We were interested in having a theory of gravity where you want to think of it in potentially quite different geometries where you're not in Minkowski flat space-time where you understand precisely what is your notion of energy, what is your notion of pressure. We want to understand it in curved space-time where things become much more murky on precisely how to separate out your notion of energy with the other notions.
And that's where you can have some mixing between what you think is energy what you think is pressure is adjust. Examples and the different modes actually the different modes of the gravitational waves can start be interacting with one another in a very subtle way so that the way you identify what you would have traditionally called as the standard gravitational polarizations along the transfers mode.
Not in america space time starts getting mixed up with what you would have called being the logical directions and what you would have called the ghost so we needed to formulate a new framework which would allow you to. Separate out this different characteristics if you want if you want imagine you're in a pond is a clear beautiful day and you see you drop you drop a stone and you see this beautiful waves going on the surface.
We can all agree those are the waves on the flat surface of the pond and it's beautiful. Now imagine this you're not on a pond you're in the middle of the ocean is the the huge storm of the century with waves which are bigger than your boat. Who is to say why start the small fluctuation from the little
Stone that you dropped and where are the underlying huge fluctuations huge waves which are bigger than anything else that anyone is very hard it's very hard on first sight to distinguish one for another and yet you need to do that to separate out what you mean by the different polarizations of the gravity tone in that situation. And how they interact with one another and what type of energy they carry out you need to be able to separate all of those things so there was some.
Implicit level of assumption of how you separate those things out there a little bit technical in how to do that but but it's a little bit as if you imagine for your for your ocean you say okay i'm gonna say this is the zero uh depth of the ocean and i'm gonna just calibrate them in the way i would have sort of doing it if i were on a pond
Sometimes that's not the right way to do it you really need to reach change your perspective change completely the way you gonna characterize all of those things so that you end up with something where you can separate out the different modes of the gravity. So when people were identifying the existence of a ghost for the gravity on actually what they were identifying was one of the normal modes of the gravity on.
Because it's difficult to really understand how what type of energy carries. They were just thinking that this was corresponding to the ghost when in reality it was just one of the healthy modes of the Graviton. Okay, so you spent a year checking this over and over. Yeah, it's like that in research. Yes. I mean, I spent I spent a lot of time a lot of a lot of nights going through through all of this. And even even when we understood
That this could be a possibility it's not enough that you understand it's a possibility that you need to understand better how these things work you you still need to understand what is the best way to frame all of these things and to have an explicit realization so that you see whether it can be fully fledgling in a full in a full theory so the model we had in the model of extra dimension that was a model where we could have a first insight of how you could work out in principle
In a limited context but it wasn't the full story because it was still breaking down at some point so but that was enough because we understood how we had the seed of the idea of how things could work out in principle.
And then what you have to do is engineer a model which fits precisely in that box satisfies precisely what was falling between the cracks. Feel those cracking precisely the right way so that you can end up with a massive gravity that evades all of these no-go theorems. So to the young theorist who's watching this sounds inspirational because they may have some theory and their advisor may push against it because it violates some
No go theorem or it produces an anomaly. What would be your advice then to them? So I mean, one of the things is that, of course, that's, that's the beauty of falling, right? Sometimes things don't work. And then it is in the beauty of understanding why they don't work, why they don't work precisely, that you may discover something, something new, so something beautiful, even if that thing is not useful for precisely what you wanted, even in a fear of massive gravity may not be
For the discussion of the universe as we wanted it still has some structure in itself it still has some level of inner beauty which i think is worth in in and of itself but so absolutely sometimes and i have happened to my career multiple times where there's a consensus of what is accepted what are the theorems.
And you should in reality take nothing for granted have in most of the case it is true that this is a deep reasons for why things have been fledged in a particular way so it's not like i just woke up in the morning and i thought okay i just gonna go against the norm and come up with all sorts of different ideas that don't fit the box i think you always need to go back to what has been proven what has been understood so far
And and really understand this to that depth it's by understanding the work that has been done so far to its depth and then you can also understand how what you want to do can come along and how i can complement how i can even contradict what has been presented. What's right trying to understand whether you can have a theory of gravity which is goes beyond general activity i think i'm in a very good position to tell you how.
How incredible general activity is how you may think that it's set up like that because of some assumptions of the pillars of Einstein and some particular assumption that were preset in advance but actually for me it's much more self-consistent in itself and i can see that all of the beauty that is present in general activity is completely self-consistent from the outset it's extremely hard to just challenge a tiny little thing
You need to understand all of this beautiful structure first before you understand how you can start dismantling maybe a small little piece without everything falling apart. Yes. Okay. So it's not throwing away the history of physics. It's deeply understanding it in order to snake your way through a narrow path. That's right. So this is an extremely bold new theory of gravity that in some ways shouldn't exist.
Does it solve any other problems like the information paradox or tell you what happens in the inside of a black hole near the singularity? So by design, the theory of massive gravity is a theory of gravity that generalizes or goes beyond Einstein's theory of general relativity on very large distances or very small curvature regions of the universe, which is precisely the opposite end of the spectrum as compared to what we're interested in
In black holes for instance the horizon of black holes are in black holes themselves so in it's very construction the theory of massive gravity i would say from the outside shouldn't have anything to say about those aspect of.
I am completely agnostic and I think it's good to have a spectrum of different point of views because I think it's good to understand how to connect between those
So far i have no particular preference necessarily on how to which one is right or wrong i think it is useful to understand them all it is useful actually to very much understand them all and very much understand what are the common points and what are the differences so that then we can try much more to distinguish them at different levels let me let me just say that for the same gravity.
Even though i don't have a preference to say what is the ultimate what we call high energy completion of it because it has some some of these additional modes some of those additional polarizations and a radius associated with it which we call the bench and radius which is much bigger as compared to the horizon i would be black hole.
And so in this they are massive gravity even though from the outside that's not the reason it was engineered. It comes in as well with some features where you start needing to understand some phenomenon about let me call them strong coupling or quantum nature of gravity of some of this polarization of gravity are ready at the scale of the balance and radius and not at the scale of the horizon so some of the.
Mysteries of gravity that are present in general activity they are very much present in massive gravity and actually they manifest themselves in some of the modes of the graviton before they would have manifested themselves for general activity now that this.
On one side is going to be very very complicated it is it is very very complicated but on the other hand you can also see it as a play field to explore some of these ideas that you would want to explore for quantum general activity from quantum gravity in a framework of massive gravity where this comes in for just one of the modes so you can just explore the effect for one of these modes and maybe understand some of the phenomenology of quantum gravity in a simplified model.
Now, there's something called the Higuchi bound, which imposes a limit on the mass of a spin two particle in decider space. Yes. So are you within that bound? Are you above it? Are you below it? Yes. Yes. So the Higuchi bound is absolutely right if you are in decider space. So if you're in an accelerated expanding universe, let's say we close to decider space. Now in the universe, you it tells you that the mass of the graviton
Has to be either zero or it has to be larger than twice the herbal parameter or actually the square root of the herbal parameter. It's interesting. It's like a Yang-Mills mass gap. There must be a mass gap. There must be a mass gap. Exactly. Square root two herbal parameter.
Otherwise, you end up with a ghost, which is not the ball, what does it goes to this time? It's called the Gucci ghost. It's not an additional mode, which is a new ghost mode. It's actually one of the mode of the gravity tone, which we discussed before, which itself starts becoming has a negative mass. So this is okay. If you want your theory to take off even further, you should have called it ghost gravity. That's such a catchy name.
Many books can be written about that.
Going back to your question of whether this is okay, it's fine. It's certainly fine today because the herbal parameter today is actually of the same order of the mass of the graviton we would like it to have today. We want the mass of the graviton to be slightly larger than the herbal parameter today, so we would have expected the graviton to be within the realm of mass which satisfies the Iguchi bound.
I have a if you want to understand how this occurred throughout the history universe so if you are the very beginning of the universe you were close to another the city region in universe when are the hubble parameter was way higher and so you would have expected to have the mass of the gravity on the time to be much
Do you need to satisfy any good she found which is much stronger or the gravity mass would have needed to be much higher to satisfy the good she found at the time what what we think of the moment is actually there is some sort of redressing mechanism and that comes in naturally with sad examples.
Where we see that naturally occurring from the environment. So when we are in the very early regions of the universe, the effective mass or what you call the effective mass of the Graviton, you identify as the mass of the Graviton is actually redressed by its environment. So it leads to effects on the Graviton mass, which means that the Graviton mass is actually carried by the environment and carried by the herbal parameter other time. So it satisfies the Gucci bound other time.
And as you have an evolution of the universe, the mass of the graviton also effectively evolves with the curvature and with the evolution of the universe so that it becomes very small today, but it could have been much larger in the earlier parts of the universe. And that in itself is also consistent with with observations. Interesting. So are you suggesting that just like the Higgs so-called gives mass to the fermions,
That there's something that couples like a Yukawa coupling for the Graviton that gives mass to the Graviton. So I'm certainly not suggesting a Higgs mechanism for Graviton. That mechanism is not so much a mechanism where you're in the same way as the Higgs mechanism where it's in the interplay between the Higgs bath and the
Is it is something slightly different where.
Because of the existence of an environment being present, it does carry along with the graviton. So the graviton itself carries along the environment in which it is living. But you don't need an extra particle like the Higgs to lead to this environment in some sense. It is something that you can have at the completely classical level.
There's an article by the economist I'll put on screen called the dominant model of the universe is creaking. And it's about the DESI results from the past couple months or so. Can you go over either the DESI results or any new data that validates that you're on the right track? So
There's been in the past almost ten years or so while all observations completely agree that the accelerate and the universe is accelerated within a given right there's been a deepening in the.
The level of which different type of observation leach slightly different rate for the hubble parameter which we call the hubble tension if you want the rate of accelerated expansion of the universe and this seems to be some slight discrepancy between the rate you seem to be observing depending on the on the type of observation whether you dealing with observation which are.
I'm late on in the age of the universe as compared to early on in the age of the universe and depending on the scale of the surface of the nation so far since between supernovae or what you would have from other observation of form from observation of the cosmic microwave background.
So the result which i'm more recent and we're still expecting much more from the results they see the extremely extremely interesting. I personally think it's a little early to reach too much into the result i think they will have a full.
Many years of observations where they can consolidate summer some of the result but taking that face value today they seem to suggest that one in principle having. Accelerated expansion of the universe and evolution of the universe which is consistent with it being driven simply by pure cosmetical constant.
It seems to be slightly favored to have a dynamical dark energy, so the equation of state parameter that changes every so slightly over time. And what seems to be also very interesting, if correct, is that the equation of state parameter for the would-be dark energy is not within a regime we would have anticipated should be based on typical scalar field models a priori.
And so all of this missing to suggest that there's more much more to the picture of dark energy and to the accelerated expansion of the universe as compared to what the most vanilla pure constant cosmo to call a constant model we seem to suggest i mean it is possible that there's some systematic effects between different type of observations.
Which are playing a role into that. I am, I'm not at all in within this data. I can't say anything about that. But if it is all entirely correct, it seems to suggest that there is some dynamics within the evolution of the accelerated expansion of the universe. And so something else than simply a pure cosmological constant or a pure vacuum energy with the right order of magnitude should be at play to explain those observations.
To me that's really is fascinating because it's it's a sign pose for potentials science of new physics and whether it is done i make all dark energy or modification of gravity or anything in between is is is very much something we have to better understand but it tells us that there's something which is beyond the most simple the simplest possible model.
And so massive gravity where you could have or other models of modified gravity, where you can have a behavior of gravity, which is every so slightly different throughout the ages of the universe is something which could in principle help with understanding some of those questions. Does massive gravity have any implications for anti-gravity? For intergravity, as you, as you think of it, in the sense of having two masses,
The reality is massive gravity is so anchored within the framework of general relativity with very small departure that you're not going to end up with a result which is so radically different as compared to general relativity. It's not going to make something flip sign in itself.
This very notion that you could have anti-gravity or that things could start really fundamentally becoming repulsive as opposed to attractive as in two masses being repulsive that as much in massive gravity than in general activity leads to some instability is also related to negative mass if you want a negative energy which is which is unstable so
A lot of what we do in Massive Gravity is still following very much the same rules as in general relativity. So those type of things would not be directly applicable for Massive Gravity. And does the Witten-Weinberg no-go theorem about the massless spin-2 particle, does it apply to yours or do you see it as evading it? Okay, so what the Weinberg
Sorry, let me go through all that. I knew it.
So some people saw that as suggesting that the graviton shouldn't exist in four dimensions. One of the ways around it is to exceed or shorten your dimensions or to say that the graviton is a composite particle. But it sounds like massive gravity is an evasion of this theorem. Yeah. Yeah. Yeah. Yeah. Yeah. So the Weinberg-Witten theorem tells you that you
A massless graviton in four dimensions cannot be a composite particles. So that's okay for the graviton in general relativity, which is a massless particle, but it would be a fundamental particle. Now, if it is a massive particle, then that evades the theorem altogether. So in principle, it could be a composite particle. So professor, what are you working on now? What are you most looking forward to?
Yes so what i do at the moment is some of these aspects are quite different as compared to massive gravity or they started with massive gravity to some extent i'm trying very much to understand how to make connection with the series.
that we use on a daily basis to describe the world around us, still as a theorist, as a quantum field theorist. So we have the framework of effective field theories. For instance, the standard model of particle physics is an effective description for all of the standard model and all of the constituents of matter and the other forces of nature, aside from gravity. Or general relativity is an effective description of gravity, which works extremely well, we believe,
I'm on low energy scales but we know because of the issues with related to embedded general activity in a quantum world at high energy.
That at some point we need to have a better description of gravity. And so that can be string theory. It can be look on gravity. It can be causal sets. It can be all sorts of different alternatives. It is possible that it is another UV high energy ultraviolet high energy completion of gravity, which we haven't yet come across, which we haven't yet envisioned. So there's all sorts of different possibilities are high energy.
And myself i don't want to be too specific on which particular completion i want to commit into dimension before i like to remain agnostic on the type of completion that i will allow for myself being a string theory etc but i still want physics to make sense ultimately.
So for instance i don't need to know precisely what the laws of physics are at infinitely high energy but it still is meaningful for me to ask that whatever they are the satisfy what we call unitarity so they satisfy some laws of quantum probability so that i think sums up to one that's what i mean by unitarity to some extent.
It's a bit more than that but to some extent I can think of it like that. I can also ask for instance for causality we understand which that is probably not controversial although how you formulate this is probably more controversial but at the bare level the notion of causality is that I would like
The consequence of an effect to happen after the effect not before so if i were to to kick this table i want to be hurting after i kick it and not and not feel it before i kick it this is my notion of of causality.
We can we can stay that in more formal more rigorous terms in saying that i want to have no support of my propagator outside my light cone and i can state it like that because if i also think of fundamentally laurence invariance and then i'm different boosted up servers should be.
Equivalent with respect to one another and so if i have something which seems to be outside my light cone then find a server which is boosted respect to me that may seem to be perceived as something that goes backwards in time. So that there are some relatively easy to formulate or relatively general statement about physics. Which are not too controversial in themselves.
And I still want them to be satisfied in physics in general. So even within a realm of physics for which I don't have direct access, neither theoretically nor observationally, I have no access to it. And yet I want to make sure that physics satisfies those notions because they make sense. Conceptually, they make sense. If things were starting to become a causal, I will need to rethink about everything from the ground up. If things were
Not satisfied unity then i would need to rethink completely about the laws of quantum mechanics so it's not many of them there's a few set of properties of physics i want to make sure they be satisfied at very high energy but those them in themselves whether they are realized in the way that string theory realize them or look on the gravity realizes them or other type of uv completion realizes them
It doesn't matter in which one they still have consequences for the laws of physics in the way that I observe them at the moment and in the way that I can actually probe with my observations not mine but with observation at our disposal or experiments in a particle colliders for instance so there are some features some imprints of high-energy physics based on those assumption
Which should be present on the low energy framework that i'm using to describe the world around me and we used to those actually we used to knowing that the notion of causality at very high energy has for a fact that no one can travel faster than the speed of light at low energy the speed of light in the vacuum.
The notion of causality is something which is actually embedded at very high energy, at infinite energy, because it's related to what we call the front velocity, the infinite frequency limit of the phase velocity.
The front velocity is something I think we all hear but maybe we don't all remember. We all hear for instance that the phase velocity can be superluminal so long as the group velocity can be subluminal because we are thinking of the group velocity as generally carrying the information with the phase velocity something more artificial.
That is that is actually incorrect they are experiments for which the group velocity is superluminal and it doesn't actually violate causality because the very notion of causality is actually set up in the infinite frequency limit.
Off the face velocity so if you're thinking of national causality i want almost to have a discontinuity i want to send you a signal so it cannot be the case that i was sending you a signal since the beginning of time there needs to be a time where i'm not sending your signal.
And then it starts kicking in or sending you a signal. This, this is what has to happen. And so there's a discontinuity there. This is me with no signal sending to you and now I'm sending you a signal. And so it is in this discontinuity that most of the notion of causality starts kicking in, but because it's discontinuous is something if I were to do a Fourier transform, I don't know. Or if you want to think of, of the end, the frequency associated to that, that's something that leaves that infinite frequency.
infinite energy so i don't know how familiar your audience is with with those terminology but it is actually something that doesn't i can't realize this exactly within the really what i have i have.
i have contact right you can imagine a real discontinuity is something that would require so much precision that i can't do it exactly it's the same reason that in heisenberg's uncertainty you can't reduce the position down to delta function down to just one point exactly is exactly the same thing so
Is something which is not within my realm to achieve it is something that would require me an infinite energy to being able to achieve so really to prove the notion of causality to very prove the notion of what happens if i make the transition between not sending your signal and starting.
t zero i start sending your signal then i need to be living at infinite frequency and infinite energy so this is something within the realm of the uv completion of everything in the grand theory of everything that's where the notion of causality really resides but it doesn't mean that it's completely disconnected to how we experience it in our everyday life and we still know that this very notion of causality as embedded at very high energy
Has consequences for everyday life and we know that as a consequence it means we cannot be traveling faster than the speed of light in the vacuum that has a consequence unless there's some.
Different features that emerge from a small violation of unitarity in some particular fluids that we can engineer locally or other things like that. This is this is how this is being played out in all of this engineer system where they managed to achieve having a good velocity, which is faster than light in the in the vacuum. So that's just one example, but there's actually an infinite number of consequences that can be explored.
That we can use to better understand how physics get implemented at high energy so it's almost a two-way street where a inputting some assumption of high energy it can guide us on how to think about the physics in the way we describe it at low energy and you can guide us where to look for signals
What's up what's up what's up what's up what's up what's up
Searches for new physics at low energy that's one way to think about it another way to think about it is in exploring how high energy physics imprints itself at low energy in exploring high energy looks like we may also be able to get a better understanding of how high energy physics.
Looks like and whether some of the assumption that we think we should impose on ourselves at high energy whether they are justified or not maybe we observing low energy physics in such a way that some of those assumption at high energy should be violated and so maybe that could guide us to understanding where we should be in a real of high energy completion is it more to a string theory is it more towards something else.
One of the beauty
way of trying to make connection with the high high energy world which is not specific to string theory is very much in addressing this notion that you may think you may have heard that string theory is not a theory because he doesn't have a specific observable and actually that that may not be true there may be some ways that you can falsify string theory because it comes in with some assumptions which have consequences
For physics that we can observe that we can test within our realm today and so we could come up with a result of an experimental observation that would falsify string theory the very underlying assumptions of string theory. Can you talk briefly once more about how is it that if you were to exceed the speed of light it wouldn't break causality because in the traditional model that you learn
In university, you have the light cone and as soon as you tilt past that, then you can transform your vector in any which way and you would violate causality. It's, it's complicated. So let me see if I can. So, so when we go through the standard explanation of, um, if you have a wave, let's imagine you have a wave, um, which is your signal. And depending on, on,
The velocity of the wave if the group velocity of the wave exceeds the speed of light the traditional picture isn't to say that this is signaling that something is outside your light cone and then you can go into a frame of reference when observer is boosted with respect to you and something which is outside your light cone for them looks like it's traveling backwards in time according to their frame of reference.
So this is how it seems typically to suggest that traveling faster than the speed of light in the sense that if you have a group velocity for a signal away with a particular frequency going faster than the speed of light, this seems to suggest a violation of causality. But this argument is what is mainly true. The reality is this is an idealized scenario where you imagine a signal being emitted by a wave.
Can i wave which is being there since the beginning of time there's no beginning or end to the wave because if they were beginning or end to the wave then you wouldn't correspond to a wave with just a particular frequency you will need to in how this wave dies out or what you need to include the frequency associated with the physics of this dying in and dying out so when we really think much more about the notion of causality.
I want to think of what it means to send you a signal and go from the transition between me not telling you anything. It's embargoed to me starting to tell you something and so I need to switch on my signal.
Which means i can't just be communicating this information with a single frequency wave which has been there since the beginning of time it won't just be one frequency wave and just switching on my signal will lead to a spectrum which also include infinite frequencies so i pretty sharp wavelength in the signal that i'm trying to send you and so it is very much in this very very sharp.
Very very small wavelength part of the signal that information about the high energy physics is encoded and it causality is therefore very much encoded at high energy is not something that i can simply diagnose at low energy but that's an example where high energy physics actually has an imprint in how we think about physics at low energy and how it still tells us that
We shouldn't be able to travel faster than the speed of light in the vacuum.
And I recommend it. It's an honor to be able to speak with you. Thank you. You're great at explaining concepts extremely simply. And you have an effervescence about you that I appreciate and the audience can relate to, I'm sure. Thank you, Kurt. Thank you. That was very, very, that was great. Thanks. Really, really fun. Yeah, feels a lot were very, very technical. I hope your audience likes it. Yeah.
Thank you. Thanks for the questions. They were really great. So you did a PhD in Toronto? No, I did a bachelor's in University of Toronto. Wow, but you know everything. It's incredible. Wow. I do my homework.
Also, thank you to our partner, The Economist.
Firstly, thank you for watching, thank you for listening. There's now a website, curtjymungle.org, and that has a mailing list. The reason being that large platforms like YouTube, like Patreon, they can disable you for whatever reason, whenever they like.
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"text": " What does this mean for our understanding of the universe? Professor, please tell me what's something about gravity that most physicists hold as an assumption that you think is deeply mistaken? Okay, that's a good one. And I'm going to have most physicists against me now. Okay, maybe there's two things which are probably related."
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"text": " The first one is the assumption that general activity and understanding of gravity as we have it at the moment is based on Einstein's principles which are the pillars of modern science, the pillars of general activity related to the equivalence principle and I think"
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"start_time": 230.213,
"text": " And therefore you have the Einstein's principles as consequences of stability and self-consistency as opposed to use them as original assumptions. So that's probably one of them. Another thing which perhaps most of my direct colleagues would completely agree with as a whole, the general audience may think they disagree, although I think they would agree of all is"
},
{
"end_time": 278.831,
"index": 11,
"start_time": 257.824,
"text": " The notion that gravity as described by general activity is not a force you may have heard that and people say that they emphasize a lot the difference between our representation and understanding of gravity with other forces of nature and i think actually that is sometimes misleading because for many aspect."
},
{
"end_time": 303.916,
"index": 12,
"start_time": 279.445,
"text": " For many aspects, general relativity is actually a force like the other forces of nature and we can describe general relativity as a force. Actually the detection of gravitational waves is the proof that deep down gravity is a force. Gravitational waves is a representation through the stretching and squeezing of space-time of the force that is hidden within gravity."
},
{
"end_time": 330.879,
"index": 13,
"start_time": 304.872,
"text": " Okay, well, firstly, what are those symmetries you think that we can do away with when deriving general relativity? And then also, what is the definition of force? That's a good one. Definition of force is a very hard one. Let me start with what I think is simpler, although maybe the words themselves are not as simple. What is what are the symmetries that are that general relativity, according to Einstein is rooted within"
},
{
"end_time": 348.729,
"index": 14,
"start_time": 331.613,
"text": " It's a beautiful symmetry, which is the technical term is perhaps coordinate transformation invariance. It is nonlinear deformophism. That's the technical term. But actually that symmetry in its sense is very simple to understand."
},
{
"end_time": 377.773,
"index": 15,
"start_time": 348.729,
"text": " It is the realization of the laws of physics should be equivalent wherever we are and have a we describe a phenomenon you should be equivalent independently of the observer so whoever you are even though some elements are relative to one another when it comes to measuring a physical quantity we have to be able to describe it. The way i sent for is it is in any frame of reference so independently of how you decide to represent."
},
{
"end_time": 399.326,
"index": 16,
"start_time": 378.2,
"text": " Nature around you have you decide to slice space and time that shouldn't matter that is your own packages but then you can't leave when you come with something which is physically observable we should all agree on that and that is beautiful it is a beautiful symmetry it is a philosophical framework if you want in some sense."
},
{
"end_time": 421.459,
"index": 17,
"start_time": 399.684,
"text": " But to my mind, this is actually so beautiful. It's something that is derived from the self consistency of a theory as opposed to setting it up as the basis as the foundations upon which we're going to build general relativity. So that was the first question about symmetry. The second question about what is a force? What do I mean by a force?"
},
{
"end_time": 438.797,
"index": 18,
"start_time": 421.817,
"text": " i make it very deep to a point where i may not you know terminology and vocabulary is not it's not my strength so what what do we mean by that by a force i think for for most of us for me including um we think of force as as a contact force in itself"
},
{
"end_time": 463.319,
"index": 19,
"start_time": 438.797,
"text": " When when i push against something that's a notion of pressure in itself that's not really a force per se it's more related to the electromagnetic bonds etc but that's not exactly what we mean by force particularly not when we're dealing with the gravitational force so if you feel if you feel you sitting down for instance if you feel your chair"
},
{
"end_time": 491.152,
"index": 20,
"start_time": 463.319,
"text": " The bottom of your chair, this is not the gravitational force acting on you. This is the pressure and the contact interaction that is present between the cells in your body and the atoms on the chair. And actually that's related. That's very interesting. I'm going to attend and that's related to Pauli's exclusion principle, which prevents two states versus two fermions, two electrons to occupy the same place at the same time."
},
{
"end_time": 510.776,
"index": 21,
"start_time": 491.493,
"text": " And so if we have already some of the states in the chair that are occupying a particular place at a particular time, I cannot just go through it. And so I'm feeling the effect from the pressure of this poverty exclusion principle, which prevents me from having two states at the same place at the same time."
},
{
"end_time": 538.268,
"index": 22,
"start_time": 511.425,
"text": " That is not what I mean by a force at the more fundamental level, which I more represent as something related to electromagnetism. For instance, I consider electromagnetism to be a force and it is something in that can act at a distance. So it's not something of a contact. It's something that can act at a distance. And we may be familiar with that with a magnetic force. For instance, if you take a magnet,"
},
{
"end_time": 558.541,
"index": 23,
"start_time": 539.428,
"text": " You can attract some iron with a magnet at a distance. That is happening through an electromagnetic force."
},
{
"end_time": 588.422,
"index": 24,
"start_time": 558.865,
"text": " And deep down, what happens is that there's a field there, there's electromagnetic field that carries the force for us. And that can be represented fundamentally in a particle, in a field theory level. For us, we understand the electromagnetic force has been carried by a messenger, which we call the electromagnetic waves, which we call deep down the photon. The photon is a messenger for the electromagnetic force."
},
{
"end_time": 614.855,
"index": 25,
"start_time": 589.377,
"text": " And gravity can be represented as a force. In exactly the same way so you may have heard of gravity according to us that the region activity is not a false rather it is the representation of the curvature of space time and that is a beautiful is an extraordinary beautiful and accurate description of gravity for all sorts of phenomena."
},
{
"end_time": 636.493,
"index": 26,
"start_time": 615.333,
"text": " But it doesn't mean that also deep down it isn't represented as a fundamental force, just like electromagnetism. And so if we feel the gravitational attraction from the earth, it is also mediated by a gravitational field, which is carrying a force."
},
{
"end_time": 662.534,
"index": 27,
"start_time": 637.261,
"text": " Fundamentally the component of this gravitational field is a particle which we call the graviton and there's a direct analogy there between electromagnetism being a fundamental force carried by photons and gravity which is also a fundamental force that's mediated by gravitational field and carried by a fundamental particle which we call the graviton."
},
{
"end_time": 687.449,
"index": 28,
"start_time": 663.609,
"text": " Okay, so for the people listening to go back, the general in general relativity comes from the theory being generally covariant, which is the same as being diffeomorphism invariant. So can you have general relativity, if you remove that any coordinate system is an okay coordinate system? Isn't the whole point of the language of bundles to describe"
},
{
"end_time": 715.299,
"index": 29,
"start_time": 687.722,
"text": " constructs without reference to coordinates. So then do you remove the underpinnings of differential geometry to GR? So you don't want to do that directly, first of all, because your relativity is a beautiful framework that works extremely well in all sorts of settings. So as soon as you start removing some of the beauty of it directly, then you end up with elements which won't match observations."
},
{
"end_time": 743.353,
"index": 30,
"start_time": 715.725,
"text": " And as you said, it is a beautiful symmetry. It is also something that we like to have in a theory of in a description of the world where we would like to make abstraction as much as possible to anything which relies on us making a choice of any sort. And so in the world of bundles or in different representation of general activity, we can extract ourselves from expressing where we are in a frame of reference."
},
{
"end_time": 753.66,
"index": 31,
"start_time": 743.951,
"text": " Hi there are some situations where we can think of a description of gravity which is very well described by general activity."
},
{
"end_time": 784.036,
"index": 32,
"start_time": 754.224,
"text": " Up to a given level but comes a point where the symmetry of this representation needs to have more to it in reality what happens is that you never break things completely you never break everything down but you start seeing a new structure emerging when you deep dive into it before to understand how to do that you first need to embrace all of the beauty of general activity need to understand how it works."
},
{
"end_time": 809.138,
"index": 33,
"start_time": 784.377,
"text": " and only in some special limit, can you allow yourself to go beyond that and understand a generalized framework that has a symmetry in some limit, but then more generally it behaves slightly differently. Now going back to a force, are you calling a force anything that has a force carrying particle? And so that's why you said gravity is a force because there's a graviton associated with it."
},
{
"end_time": 838.473,
"index": 34,
"start_time": 809.599,
"text": " So at the fundamental level everything is quantized so it doesn't not be it's not because something has a particle associated with it that you will necessarily be a force but fundamentally all what does she say is all fundamental forces well to my mind necessarily have a particle as a messenger associated with it because everything is quantum and this is all"
},
{
"end_time": 857.944,
"index": 35,
"start_time": 838.729,
"text": " Even more true for gravity because gravity connects to everything and so if we know the gravity connects with a real for instance electromagnetism for instance electrons and other fundamental particles which are quantum then gravity has to be quantum as well."
},
{
"end_time": 881.254,
"index": 36,
"start_time": 858.234,
"text": " You can't just couple something which is fundamentally classical that satisfies fundamental classical properties with something which satisfies quantum probabilities. Actually, what you need to do is having a grander framework where everything is quantum and in some limits some sector may behave classical, but it's only in some limit."
},
{
"end_time": 911.8,
"index": 37,
"start_time": 881.869,
"text": " So what do you see as the difficulty in reconciling gravity or general relativity with quantum theory? So we can do that and we do that actually on an almost everyday basis in my work but also in connecting with some people doing observations."
},
{
"end_time": 939.48,
"index": 38,
"start_time": 912.483,
"text": " When we dealing with gravity in a not extreme environment so when we thinking about how gravity behaves on earth in the solar system even in most of the galaxy and in most of the universe actually we can reconcile gravity with the quantum laws of nature and with the other quantum theories the quantum theories of the forces and there's no real issue associated with that."
},
{
"end_time": 969.701,
"index": 39,
"start_time": 940.06,
"text": " The distinction with gravity and where the problem arises is when we're trying to describe it when the curvature scale is very intense so in a very extreme environment for gravity and there what happens is if we took seriously the quantum laws of probabilities and we applied to general activity in those extreme environments we would end up with some laws of probabilities that stop making sense."
},
{
"end_time": 997.978,
"index": 40,
"start_time": 970.162,
"text": " And so that doesn't mean that probably doesn't make sense. What it means is that we need a better framework to understanding and how to reconcile late gravity with those laws of quantum probabilities in those extreme environment. So we don't have access to all of the information, all of the description of how gravity behaves. So if you imagine that you are working with some laws of probabilities and at the end of the day, the outcome is not what you predicted."
},
{
"end_time": 1027.91,
"index": 41,
"start_time": 998.353,
"text": " It must mean that something else must go on. Now for gravity, it's not like we've been in an extreme region in the universe, for instance, at the center of a black hole, or for instance, at the very beginning of the Big Bang, and tested gravity and the laws of probability there to be able to say that they don't work. That's not what we have done. Already, according to Einstein's theory of general relativity, when I use the standard laws of quantum probabilities associated with it,"
},
{
"end_time": 1054.821,
"index": 42,
"start_time": 1028.251,
"text": " I end up with outcomes which simply don't make sense. For instance, you can imagine that typically when you add probabilities, I have a probability for something to happen. I have a probability for something else to happen. When I add things up, I cannot end up with an outcome that has more than a hundred percent probability to happen. But when we take general relativity and we're trying to apply the laws of quantum probability at the very center of the black hole,"
},
{
"end_time": 1072.875,
"index": 43,
"start_time": 1055.503,
"text": " All five cents at the very beginning of the universe very close to the big bang i see him to be able to end up with outcomes which would have more than a hundred percent probability to happen sometimes have a negative probability to happen or a complex probability to happen."
},
{
"end_time": 1099.991,
"index": 44,
"start_time": 1073.046,
"text": " And that simply tells me that i am missing something i am not summing up all my probabilities given the all the configuration that i am allowing myself properly something is missing and i need to understand gravity better i need to understand how to go beyond the description of gravity using term relativity to being able to better appreciate how to reconcile my laws of quantum probability"
},
{
"end_time": 1125.725,
"index": 45,
"start_time": 1099.991,
"text": " What's my description of gravity in those extreme environments. I see it doesn't have to be that something's missing or could it be that you over counted in the case where it exceeds one hundred percent. So even if you were that i have accounted i need to understand why i have accounted because the possible outcomes to my mind not to my mind per se but when when we do the standard estimations."
},
{
"end_time": 1148.609,
"index": 46,
"start_time": 1126.186,
"text": " Are possible outcomes a possibilities which otherwise could have been realized so if you want to think of an analogy i can take two particles which is something which is done for instance in particle exclamators i can take two particles and i smash them together and then i have a probability of a given outcome."
},
{
"end_time": 1162.227,
"index": 47,
"start_time": 1149.155,
"text": " The given outcome can be two other particles that are scattered with a different angle or some of those particles may have transformed themselves in something different and I can think of all the possible outcomes."
},
{
"end_time": 1190.35,
"index": 48,
"start_time": 1162.705,
"text": " And for general activity, of course, I'm not gonna smash gravitons together. I don't have access to gravity, but I can perform those things, thought experiments, or I can think of doing them for all the particles and understand what would be the impact of having gravity in it as well. I can do all of this and I can sum of all of the outcomes, the possibilities, and they seem to be realizable and they seem to be making sense in themselves."
},
{
"end_time": 1214.428,
"index": 49,
"start_time": 1190.725,
"text": " So if now I'm of accounting in some of those extreme environments, I will need to understand what happens, what happens to those possible outcomes that are no longer a possibility at the center of a black hole. Irrespectively of what precisely happened, whether we are counting and accounting, not counting it right, or giving too much weight in some possible processes,"
},
{
"end_time": 1224.684,
"index": 50,
"start_time": 1214.872,
"text": " When i say something is missing is not necessarily something tangible is missing but something in understanding of what is happening is missing."
},
{
"end_time": 1249.787,
"index": 51,
"start_time": 1225.623,
"text": " So I want to get to this massive theory that you and your colleagues came up with in 2010 and literally massive. It's also outlined in your book, The Beauty of Falling, which will be on screen now and people can click it in the description. The subtitle is a life in pursuit of gravity. So firstly, why don't you tell us what is that book about and bring us through the journey that led you to 2010 and that discovery?"
},
{
"end_time": 1275.589,
"index": 52,
"start_time": 1250.811,
"text": " Okay, yes, so I can discuss about massive gravity afterwards. The journey itself is very much a journey towards the scientific exploration, the ups and downs of doing research, particularly doing research in theoretical physics, where the connection with the real world is still something that we need to develop and is ups and downs all the time."
},
{
"end_time": 1295.265,
"index": 53,
"start_time": 1276.135,
"text": " so it is about this journey as a researcher and also associated with the journey of myself in in going through different steps in my life but also to me as a scientist right it is that and also in parallel the journey that we have us as human i would say."
},
{
"end_time": 1318.558,
"index": 54,
"start_time": 1295.265,
"text": " In operation appreciation of the laws of nature and particularly appreciation of gravity and how they spin ups and downs in understanding of how gravity is being described and actually we are possibly hitting another possible. Down in the description of gravity failure or falling down in gravity."
},
{
"end_time": 1347.995,
"index": 55,
"start_time": 1318.848,
"text": " I'm related to what i was describing that we know something is missing in our description of gravity there's a point of failure which in itself is part of how we do research we understand that we have a description of nature we have a description of some phenomenon around us which is quite fundamental but is not fully. Basic we need to go deeper in those laws of nature and is a description of nature around ourselves and so."
},
{
"end_time": 1376.288,
"index": 56,
"start_time": 1348.422,
"text": " We can think of the points of failure for instance when general activity breaks down and we need to have a better description of gravity as one of the failures of general activity but actually is an opportunity to look for new underlying framework to better understand nature around us. So the book in itself is framed within that premises of how we doing research and how we going along"
},
{
"end_time": 1404.36,
"index": 57,
"start_time": 1376.766,
"text": " trying to understand things but everyday almost as a there's an up and down and embracing this level of knowing that at every stage there will be a point where how we do research and how we connect with the world will have some elements of failure and this is something to be proud of actually and this is something to very much embrace and keep exploring because it's through these points of failures that are we going to make discoveries and being able to"
},
{
"end_time": 1428.353,
"index": 58,
"start_time": 1404.462,
"text": " Get access to new layers of understanding new layers in understanding of how nature works around itself. So it's a description of this journey but also associated with my own one and through the understanding of a theory of gravity which is very similar to Einstein theory of general relativity other than it is massive right and and that says be massive."
},
{
"end_time": 1457.619,
"index": 59,
"start_time": 1428.643,
"text": " Doesn't mean that it's huge. It's related to the fact that it does sound like that. It's a massive thing. Actually, it's quite the opposite. It makes gravity smaller in some sense, but I'll explain what I mean by less far reaching. Exactly. Less, less far reaching. So the idea behind massive gravity is related to trying to tackle some of all the problems we have with gravity."
},
{
"end_time": 1481.664,
"index": 60,
"start_time": 1458.148,
"text": " I'm not those are related to not to the reconciliation of gravity with the quantum world which is what happens when we're looking at extreme regions of the universe where the curvature of the land scales the coverage is very high the land scales are very small like the center of black hole at the beginning of universe rather exploring what happens on the other side of the spectrum."
},
{
"end_time": 1507.022,
"index": 61,
"start_time": 1482.176,
"text": " Where we looking at very very small curvature scales so not on the smallest possible distance scales but actually quite the opposite on the largest possible distance scales what we mean by that is that we exploring the behavior of nature around itself on cosmological scales on the largest possible observable scales in the universe so the size of the observable universe itself."
},
{
"end_time": 1531.493,
"index": 62,
"start_time": 1507.346,
"text": " Which is spanning of thousands of trillions of kilometers apart as is the size of the observable universe and so the idea is to try to understand how we can reconcile the behavior of the universe as we see it cosmologically with expectations from gravity and expectations from the other fundamental phenomenon in nature."
},
{
"end_time": 1561.254,
"index": 63,
"start_time": 1532.022,
"text": " Particular the realm of particle physics which postulates the existence of particle describing all the fundamental constituents of matter and the fundamental constituents of the forces. Now we know from particle physics that for instance the Higgs let me go on a tangent a little bit the Higgs phenomenon is a phenomenon where the vacuum is not empty is filled with the Higgs vacuum"
},
{
"end_time": 1590.845,
"index": 64,
"start_time": 1561.527,
"text": " sort of a Higgs bath is a Higgs bath where you have interactions between the Higgs and all of the other massive particles. And it is these interactions between the Higgs vacuum, this Higgs bath that changes the dynamics of some of the massive particles and in fact give a mass to some of those particles. So we understand the Higgs phenomenon as the phenomenon that gives a mass to other particles of nature."
},
{
"end_time": 1619.974,
"index": 65,
"start_time": 1591.647,
"text": " So this is just an illustration to tell you that empty space is not at all this boring thing when nothing happens most of the universe from our eyes from our point of view is filled with emptiness we have huge cosmic voids which are millions and millions of kilometers wide they're huge most of the universe is actually empty in that sense is filled with cosmic voids where galaxies struck where galaxies and"
},
{
"end_time": 1648.626,
"index": 66,
"start_time": 1620.247,
"text": " Clusters of galaxies and filaments of dark matter are that's just filaments that taking in itself just a small fraction of the universe and most of the universe is actually made out of this cosmic voice and they seem empty to from our perspective but actually they are filled with at the very least they should be filled with this Higgs path because we know that it's thanks to this Higgs path that other fundamental particles carry a mass."
},
{
"end_time": 1675.384,
"index": 67,
"start_time": 1649.667,
"text": " But now this Higgs math, sorry, now this Higgs bath, this Higgs bath. The math of the bath. That's right. Exactly. Now this Higgs bath or the vacuum should also carry energy from possibly all the other particles. And that's according to Einstein's theory of general relativity itself, because of the equivalence principle, because gravity is so universal,"
},
{
"end_time": 1700.162,
"index": 68,
"start_time": 1675.845,
"text": " If he has an effect on other particles should also connect with gravity and so we should we should expect this Higgs bath and this energy in the vacuum to gravitate to have an effect on gravity to curve the structure of space time. Yes, that is not something controversial. This is something that has been developed has been understood already since the thirties."
},
{
"end_time": 1727.193,
"index": 69,
"start_time": 1700.435,
"text": " Since the understanding of i sent the original activity and then from the beginning of quantum mechanics the quantum realm of mechanics by pauli and all the other fathers of quantum mechanics in the beginning of the last century it was already understood that we should expect emptiness to be filled with something that seems paradoxal but emptiness should be filled by something."
},
{
"end_time": 1756.425,
"index": 70,
"start_time": 1727.637,
"text": " And that's something should gravitate and if it gravitates then we should expect it to have an effect on the evolution of the universe and in fact it was already understood already in the nineteen thirties that the effect of Francis the energy of the electrons in the vacuum should lead to an accelerated expansion of the universe which would be going so fast that the space between us the earth."
},
{
"end_time": 1785.725,
"index": 71,
"start_time": 1756.886,
"text": " and the moon should be stretching at a speed that exceeds the speed of light. And therefore, if we put those two things together, we shouldn't be able to see the moon. Just a moment. So if in the vacuum, there are these, are you referring to the virtual particles? Yes. Yeah. So if each of those virtual particles, and they're an infinite amount, if you go all the way down to zero, not the plank length and stop there, but there's an infinite, okay, whatever, there's a large amount, even if you put a cutoff,"
},
{
"end_time": 1797.978,
"index": 72,
"start_time": 1786.032,
"text": " And each of those has some energy associated with it. That's right. And energy is associated with gravity. So you say they should gravitate, you mean that they should exert a gravitational force. But why would that force be repulsive and not contracted?"
},
{
"end_time": 1828.712,
"index": 73,
"start_time": 1798.899,
"text": " Okay so that's very interesting of why the effect of this phenomenon is something which doesn't seem to be the same as an apple falling on the earth which I will classify we typically classify as something attractive as opposed to the acceleration of the universe which we seem to be looking as a repulsion phenomenon and according if and indeed if you look at if you look at this phenomenon according to Newton gravity"
},
{
"end_time": 1848.251,
"index": 74,
"start_time": 1829.241,
"text": " You would think that this pushing away this accelerated expansion of the universe should be related to a repulsion or should be related to having an effective negative mass in there. That's how you would describe it according to Newton gravity."
},
{
"end_time": 1875.879,
"index": 75,
"start_time": 1848.797,
"text": " But we are in a state of activity where things don't just happen in one dimension of time or one dimension of space what happens is an entertainment between space and time unified together and so you have things happening in space and you have things happening in time now when you have an effect of energy which is localized that you can think of it as something that happened in space to some extent."
},
{
"end_time": 1896.681,
"index": 76,
"start_time": 1876.305,
"text": " It gives you what you would think a localized must hear the earth and ask being attracted by the earth this is an attraction but this is something which in the realm of general activity is only in one special dimension which is more related to time than to space is a bit of technicalities there."
},
{
"end_time": 1924.855,
"index": 77,
"start_time": 1897.108,
"text": " Now in general activity, if something happens in space is also happening time and vice versa. And for this vacuum energy is not just localized here at one point in space. It's everywhere all the time throughout the universe. And so it acts on for all four of our dimensions and the way it acts in the space and the time dimension opposite. And since we have more dimensional space than we have of time,"
},
{
"end_time": 1951.954,
"index": 78,
"start_time": 1925.265,
"text": " Actually what happens along the space dimensions wins over what happens along the space dimension looks like it has the opposite sign as what happens in the time direction and that's why it looks like you have a repulsion but it's all attractive is just what attraction looks like in general activity may have different ways to manifest itself attraction is something that"
},
{
"end_time": 1979.906,
"index": 79,
"start_time": 1952.142,
"text": " According to space and time, it looks slightly different in how you represent itself. Maybe another way to see it is that rather than just being some energy localized in space, it is actually also some pressure which is localized in space and time. And this pressure has a negative sign. So it manifests itself as something which looks repulsive in all direction. But actually that's just a manifestation of a negative pressure."
},
{
"end_time": 2006.817,
"index": 80,
"start_time": 1980.811,
"text": " So then in 2D gravity, would there be zero effect because there's one time and one space. So the way acceleration of the universe would work would look slightly different. That is correct. Yeah. But there would still be a positive effect, like a repulsive effect. So you will still have a stretching of space. You will still have the stretching of the space direction in itself. Yes."
},
{
"end_time": 2036.783,
"index": 81,
"start_time": 2007.875,
"text": " Close your eyes, exhale, feel your body relax, and let go of whatever you're carrying today. Well, I'm letting go of the worry that I wouldn't get my new contacts in time for this class. I got them delivered free from 1-800-CONTACTS. Oh my gosh, they're so fast. And breathe. Oh, sorry. I almost couldn't breathe when I saw the discount they gave me on my first order. Oh, sorry. Namaste. Visit 1-800-CONTACTS.COM today to save on your first order."
},
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"index": 82,
"start_time": 2037.978,
"text": " Okay, so now that we're on some opinions of yours that are controversial, what's your controversial take on the expansion of the universe? In other words, tell me the truth behind dark energy, professor. Okay, so dark energy is, I think that's not controversial to some extent. Dark energy is a placeholder for a lack of knowledge of what leads to the accelerated expansion of the universe."
},
{
"end_time": 2097.21,
"index": 83,
"start_time": 2068.029,
"text": " So we don't exactly we don't say necessarily we all agree on what leads to the accelerated expansion of the universe so let's put us in a common ground and let's call that. The source for the acceleration of the universe dark energy now there's different perspective on what dark energy. Could and could not be but there's a very natural candidate for dark energy can be which is this vacuum energy because the vacuum energy by itself."
},
{
"end_time": 2123.012,
"index": 84,
"start_time": 2097.5,
"text": " lead to an accelerated expansion of the universe. So that is, I would say, a very natural candidate for the acceleration of the universe. However, it is controversial because the rate, predicted rate of acceleration of the universe, if dark energy was indeed the vacuum energy, would be way too large, much faster than what we observe today."
},
{
"end_time": 2134.94,
"index": 85,
"start_time": 2123.387,
"text": " And that I already alluded to when I was saying that if you just take the vacuum energy from electrons that we know exist, you should lead to such a fast stretching of space."
},
{
"end_time": 2156.254,
"index": 86,
"start_time": 2135.213,
"text": " Such a fast acceleration of the universe that we wouldn't be able to see the moon that of course is not what is happening so we do have an accelerated expansion universe but by a rate which is way slower than what we would have expected if I counted for all of this vacuum energy from particle physics so."
},
{
"end_time": 2169.855,
"index": 87,
"start_time": 2157.073,
"text": " Face with this dilemma of why doesn't the vacuum energy leads to a much higher rate of expansion of the universe accelerated rate of expansion universe instead what one can do."
},
{
"end_time": 2196.988,
"index": 88,
"start_time": 2170.196,
"text": " Is a well maybe for a reason or another that i haven't yet found this vacuum energy actually doesn't gravitate or maybe it's not that all together or maybe i don't understand it so let me ignore it let me put it aside for a second or for a hundred years and instead let me say there's another source for the accelerated expansion of the universe which we call dark energy."
},
{
"end_time": 2213.49,
"index": 89,
"start_time": 2197.671,
"text": " The reality something we should understand is that if you want a natural source for the accelerated expansion of universe which actually leads to the accelerated rate that we observe this issue that anything natural we would expect."
},
{
"end_time": 2229.872,
"index": 90,
"start_time": 2213.729,
"text": " What's the lead to a much higher rate xx accelerated expansion is still there so you have candidates for dark energy. Various models i can come up with a hundred different names of models of dark energy that explain the accelerated expansion of the universe."
},
{
"end_time": 2249.684,
"index": 91,
"start_time": 2230.213,
"text": " In every single one of them you need to what we call fine tune some parameters so you need to really stretch some screws to a huge level of accuracy in a way which is unstable so if you look at the smallest quantum corrections to that it would be much larger."
},
{
"end_time": 2280.418,
"index": 92,
"start_time": 2250.572,
"text": " As compared to what you the level you tune it out so it's unstable on the quantum corrections all the models of dark energy that we have so far. Ninety nine percent of the models of dark energy that we have so far unstable against quantum correction in one way or another so we will need to resolve this what we call fine tuning issues in those models as well but in some cases they're much better hidden so it's harder to find where the where the tracks of the matter lies."
},
{
"end_time": 2307.671,
"index": 93,
"start_time": 2280.879,
"text": " That's why we're trying to come in, that's why we're trying to say rather than postulating the existence of a new explanation for the accelerated expansion in the universe, dark energy, which typically also come with tuning in itself and it's not stable until quantum corrections. Instead, let's just go back to this original idea that the vacuum energy"
},
{
"end_time": 2325.998,
"index": 94,
"start_time": 2308.08,
"text": " is a natural candidate for the accelerated expansion of universe and for that we need to understand why this huge level of right doesn't lead to as high a level of acceleration as what i would have expected."
},
{
"end_time": 2350.435,
"index": 95,
"start_time": 2326.613,
"text": " And there's a lot of models out there that try to address the vacuum energy itself. Try to understand what it is in the quantum field framework. I haven't quite understood. So that the expected vacuum energy is not as large is actually much smaller. So there are some models that do try to do that. There's no successful model so far, but they try to do that still."
},
{
"end_time": 2381.015,
"index": 96,
"start_time": 2351.186,
"text": " Another alternative is to say, okay, well, let me accept actually that this is the way it is from the particle physics side, because after all we have a very high level of control of what happens in the particle physics side. We understand particle physics very well. We actually have access to it. We understand how quantum corrections work extremely well. We look at quantum correction of a quantum correction of a quantum corrections. And this is very much under control in the particle physics side. So let me take that."
},
{
"end_time": 2408.524,
"index": 97,
"start_time": 2381.254,
"text": " As it is and instead it is in making the connection between this and gravity and understanding how the vacuum energy affects gravity that I will try to tweak things. It's interesting. Okay. And so this is where I cannot have just general activity because general activity in all this beauty, it has this level of universality that everything and everyone"
},
{
"end_time": 2427.329,
"index": 98,
"start_time": 2409.019,
"text": " Is affected and affects gravity in a universal way so i can't just decide that someone is gonna be affecting always gonna be affected by gravity in a different way i'm not allowed to do that in general activity it is. One of the pillars if to some extent of general activity."
},
{
"end_time": 2442.858,
"index": 99,
"start_time": 2427.705,
"text": " So if i want to have a phenomenon if i want to have vacuum energy which doesn't affect gravity in quite the same way as other sources as for instance a galaxy as localized matter then i need to twig."
},
{
"end_time": 2462.227,
"index": 100,
"start_time": 2443.285,
"text": " Some of these pillars after relativity, every source likely. Okay, you have to tweak it, but do you have to fine tune it so that you're replacing one problem with an equally intractable problem? That's an excellent question. Do I need to? Exactly. So how much is that tweaking fine tune in itself? That's an excellent question."
},
{
"end_time": 2490.196,
"index": 101,
"start_time": 2462.688,
"text": " And you have to tune it you definitely have to tune it the difference in the level of tuning that you need to make is that it is stable and the quantum corrections so you need to have a very small number in the game what ever you do. How ever you try to resolve this paradox you need to bring in a very small number in the game in this case the very small number is the hot by how much you modifying term relativity."
},
{
"end_time": 2509.957,
"index": 102,
"start_time": 2490.708,
"text": " And you do so with a very very small with a very very thin brush but when you have to relativity in itself it doesn't spontaneously. It isn't spontaneously affected by quantum corrections so all of the symmetries often relativity they are protected by quantum corrections."
},
{
"end_time": 2538.029,
"index": 103,
"start_time": 2510.555,
"text": " If you have a symmetry which is present and you do look at how quantum corrections affect them, they will preserve those symmetry typically, unless you have anomalies, but that's for a different question. So these coordinate invariants, for instance, this or this symmetry, which we call covariance or diffimorphism invariance, or this notion that every observer should be equivalent irrespectively of the form of reference. This is not a,"
},
{
"end_time": 2564.275,
"index": 104,
"start_time": 2538.439,
"text": " Principle that gets modified by quantum corrections so if you start with that then you're not going to expect it to be modified by quantum corrections but that means that if you depart from this symmetry every so slightly the amount by which you're going to be destabilized by quantum corrections is going to be proportional to how far away you are from it in the first place so if you only"
},
{
"end_time": 2590.811,
"index": 105,
"start_time": 2564.889,
"text": " Displace it every so slightly, you're never gonna move very far away from it. So in that sense, you have a very, very small number in the game, which is a small modification of gravity that you bring in. But this small modification doesn't start going completely ballistic. It doesn't start going completely crazy in the sense that it doesn't start growing over time, if you want."
},
{
"end_time": 2621.169,
"index": 106,
"start_time": 2591.783,
"text": " So it is in enough framework of quantum mechanics and naturalness. It is a tuning, but a technically natural tuning. So it's not a fine tuning in the sense that you don't need to keep tuning it at every level in your quantum corrections. But you understand that this is the case. I can say those things and you may or may not believe me, but you understand that this is the case. You need to have actually a very rigorous framework in which you can explore those ideas."
},
{
"end_time": 2644.155,
"index": 107,
"start_time": 2621.408,
"text": " Add any new which you can look at the quantum corrections and see how they affect the small modification of gravity and this is precisely this idea behind massive gravity it is an idea of framework where gravity has a very very small mass the gravity to be precise the particle carrier of the gravitational force of gravity."
},
{
"end_time": 2669.292,
"index": 108,
"start_time": 2644.684,
"text": " Unlike being massless, as would be the case in general activity, it acquires a very small mass. It's extremely small. It's the smallest possible mass that you can ever envision. It would be of the order of 10 to the minus 32, 33 electron volt. By comparison, the neutrino mass, which is the lightest massive particle that we know of for sure, that has a mass of 10 to the minus three electron volts."
},
{
"end_time": 2690.896,
"index": 109,
"start_time": 2670.06,
"text": " Millielectron volt enter minus three electron what roughly so it's roughly thirty orders of magnitude below that it is extremely extremely small. And so this very small departure of general activity you can now look at how the mass of the grab it on good get corrected by quantum corrections."
},
{
"end_time": 2709.292,
"index": 110,
"start_time": 2691.254,
"text": " You know that in general activity the Graviton is massless so the mass is zero and so and you know that you don't start asking yourself is it true that the Graviton remains massless in general activity when you include quantum corrections because the masslessness of the Graviton."
},
{
"end_time": 2737.5,
"index": 111,
"start_time": 2709.548,
"text": " is also related to the symmetries after relativity is related to this equivalence principle is related to this universality is related to covariance all of that is part of a big package in general activity right and so you can't just shake it a little bit around you can't it's simply it's it's a package to stick together now we started to unfold it a little bit and to allow for the graviton to be every so slightly massive"
},
{
"end_time": 2762.619,
"index": 112,
"start_time": 2738.097,
"text": " And the amount by which all of the other implication will start losing up is present, but it's the same amount by which you had started shaking it in the first place. And so if the mass of the great one is extremely small, you're going to have a small correction to the equivalence principle. You're going to have a small correction to all of the phenomenon that we discussed about, which is proportional to this graviton mass."
},
{
"end_time": 2775.282,
"index": 113,
"start_time": 2763.097,
"text": " And so it also means that if you have the vacuum, if you have a source, which is present here like the sun or the galaxy or even a cluster of galaxy, these are distance scales, which are still very small."
},
{
"end_time": 2803.285,
"index": 114,
"start_time": 2775.691,
"text": " As compared to the quantum wavelengths of the Graviton. So the quantum wavelengths of is related to the inverse of the mass of the particle, if you want. So if you have a massive particles, one of the aspect of it, which we are after, is the fact that the force associated with it will have a finite range. So you want to have an infinite reach, like in general activity, it would actually just reach a finite distance."
},
{
"end_time": 2833.319,
"index": 115,
"start_time": 2803.746,
"text": " And that's related to the quantum wavelength of the particle. And so any structure that we use to in the universe, for instance, the solar system, a galaxy, even a local cluster of galaxy, will be within the quantum wavelength of the graviton and therefore relatively small distance as compared to the scale at which gravity gets modified. And therefore on those smaller distances,"
},
{
"end_time": 2842.005,
"index": 116,
"start_time": 2833.609,
"text": " Gravity looks very similar as in general relativity and we don't see a very big departure from general relativity."
},
{
"end_time": 2863.865,
"index": 117,
"start_time": 2842.517,
"text": " It's only when you start looking at effect which are much larger distances on the scale of the observable universe today that you start seeing a departure of how those effect lead to a curvature of space time and this is precisely where the vacuum energy is coming in so we have a community on the whole."
},
{
"end_time": 2892.841,
"index": 118,
"start_time": 2864.224,
"text": " Of the universe since the beginning of time of billions of years of billions of light years across in distance and over those huge distances this is where we start seeing a weakening of gravity and therefore the effect of this vacuum energy on gravity on cosmonauty and on the evolution of the universe is much weaker as compared to what we would have expected in term relativity."
},
{
"end_time": 2918.848,
"index": 119,
"start_time": 2893.456,
"text": " Interesting. So does this then give an alternate explanation for dark matter? That's a good question. So if you want to understand dark matter, you can and people have indeed tried to understand whether you can use a similar framework to understand dark matter. It is naturally it is difficult to do both at the same time, because the scale involved actually quite different."
},
{
"end_time": 2945.35,
"index": 120,
"start_time": 2919.121,
"text": " We have a very good understanding of the presence of dark matter or something that looks like that matter already on galactic scales. That is very, very much present there. And so the scale at which you need to see this effect to consider it as an alternative to dark matter would be on much smaller distances as compared than what you would need to have it for, for dark energy. So in principle, you can, you can,"
},
{
"end_time": 2965.06,
"index": 121,
"start_time": 2945.93,
"text": " Do this in different layers and you can have a modification of one scale and then another modification of another scale you could do that in principle but the reality is these are relatively separate phenomenon so you might as well just consider first what happens for dark matter and another separate phenomenon what happens for dark energy."
},
{
"end_time": 2992.09,
"index": 122,
"start_time": 2965.708,
"text": " Now people have tried to do that for dark matter indeed and their models were similarly to considering what would happen for massive gravity. What if gravity had a mass? What they're considering is model of what we call multi gravity. So rather than having just gravity as we know it, there's many different layers of gravity. There's many different notion of gravity and"
},
{
"end_time": 3021.203,
"index": 123,
"start_time": 2992.551,
"text": " You can have some of these alternative gravities which act as dark matter for observations. So this is not something I have worked on. These are the models that are taking on some of the aspects of massive gravity, bringing them on top of gravity itself, on top of general relativity as a new source for dark matter and for other phenomenon in cosmology or in particle physics even."
},
{
"end_time": 3041.084,
"index": 124,
"start_time": 3022.108,
"text": " I see. So for people who are listening and thinking, OK, a massive graviton, the graviton was thought to be massless. Why didn't physicists think about a massive graviton earlier? They did. And there were two problems. One was the VDVZ discontinuity, if I'm not mistaken."
},
{
"end_time": 3070.145,
"index": 125,
"start_time": 3041.834,
"text": " which we can talk about. And then another was that there are some ghosts. There are two types of ghosts, generally speaking. Yeah, exactly. Exactly. Exactly. They have pop off, which are the wanted ghosts are the benign ghosts. And then there's Polly Villiers, if I'm pronouncing that correctly. Yes, that's right. Exactly. Exactly. Okay, so you know, you got you got everything right. Okay, so people have indeed considered the idea that gravity could have a finite reach, which is the essence behind"
},
{
"end_time": 3097.944,
"index": 126,
"start_time": 3070.503,
"text": " Massive gravity. In fact, I should say Newton himself. He thought about this idea that according to his law of Newton interest, yeah, Newton law square law of gravity as he as he had it. This is a phenomenon and it has an infinite range. So gravity gets diluted that like the square law. So it gets diluted like the like the distance. And this is in itself very geometrical."
},
{
"end_time": 3116.749,
"index": 127,
"start_time": 3098.404,
"text": " But himself he was thinking about what the gravity could have a finite reach at the end of the day and trying to understand how to make sense of that from a newton perspective. Other other scientists throughout since then like laplace also considered considered that now."
},
{
"end_time": 3146.323,
"index": 128,
"start_time": 3117.039,
"text": " If you wanted to do it just a little bit of newton load that that wouldn't be too challenging the challenge is to do it at the level of a flu fully fledged non-linear non-linear theory of gravity as in anstine theory of relativity with everything that we know about your relativity and then further embedded into a quantum field theory framework as we know has to be the case and so since we know much more since then on how"
},
{
"end_time": 3168.712,
"index": 129,
"start_time": 3146.732,
"text": " Gravity works we need to make sure i'm thinking about a massive gravity that it is still satisfies all of the other qualities of gravity as we know them and in fact pauli himself in the nineteen thirties. Fiat and pauli they first started looking at the theory of gravity where the gravity could have a mass."
},
{
"end_time": 3185.23,
"index": 130,
"start_time": 3169.155,
"text": " But one of the issues was pointed out by what you mentioned this vd vz discontinuities and that was in the seventies so vd vz stands for about man zakharov and i'm sorry."
},
{
"end_time": 3214.974,
"index": 131,
"start_time": 3185.606,
"text": " Yeah, sorry. EDVZ stands for Van Dam, Veltman and Zakharov. In the same year, in 1970, they realized that if you take just a theory of massive gravity and then you compare it with a theory of general relativity, you seem to be getting some effects which are different for both cases, even in the limit where the mass is extremely small."
},
{
"end_time": 3242.739,
"index": 132,
"start_time": 3215.401,
"text": " So you may you may have the impression that you can look at let me call it the force of gravity between the earth and the moon and what you would obtain in general activity is the result that we know of and what you would obtain in a massive gravity is a different result which is different even when the mass is as small as you want is different by order one no matter what and the reason for that is quite simple to understand actually."
},
{
"end_time": 3265.367,
"index": 133,
"start_time": 3243.404,
"text": " And it's going back to this idea of, of what carries the force, what happens in there. Okay. And the idea that you have a messenger for gravity, which is related to gravitational waves, the real force of gravity is carried by gravitational by gravitational waves, by by gravitons through gravitational waves. And that part is uncontroversial. Like, that's not just you saying it."
},
{
"end_time": 3288.507,
"index": 134,
"start_time": 3265.674,
"text": " No that part is in conversion and and and controversial so this is this is this is fine i'll tell you when i stop being controversial so the controversy is whether you can have a massive gravity that's why the controversy is but but in terms of what the high gravity behaves and what the issues were at the time"
},
{
"end_time": 3315.862,
"index": 135,
"start_time": 3288.848,
"text": " All the way up to to a few years ago with massive gravity that's also uncontroversial so i can go through them and what does issue so if you think of gravitational waves that we have detected we have detected gravitational waves coming from very far away events and the way they work is as the gravitational waves propagate through space and time they actually affect the notion of distance that affects space"
},
{
"end_time": 3345.077,
"index": 136,
"start_time": 3316.169,
"text": " How long the line of the line of propagation so they are what we call transfers polarizations so i should do like this sure goes like this you have a squeeze in the stretching like this and the opposite direction is what we call a quadruple and they go along the line transfers to the line of propagation that's uncontroversial now if you think of a theory of massive gravity what happens there is that rather than being a massless particle you have a massive particle"
},
{
"end_time": 3373.916,
"index": 137,
"start_time": 3345.316,
"text": " And so if you think of the idea that light travels at the speed of light in the vacuum, light travels at the speed of light in the vacuum because it is a massless particle, because it's carried by a massless particle. You and me, no offense, but we are massive objects in the sense that we're very broad. We are massive objects and we don't, I don't typically travel anywhere close to the speed of light because I'm quite massive."
},
{
"end_time": 3399.957,
"index": 138,
"start_time": 3374.343,
"text": " And so if you have a massive object, you don't no longer travel at the speed of light anymore. You actually can't. You can try to go very close to it, but you can never actually go quite at the speed of light in the first place. But that means also what we can do, though, is control a speed. I can decide to be at rest and I can decide to speed up or slow down. I can do those things. Being a massive particle is actually quite a positive thing."
},
{
"end_time": 3422.978,
"index": 139,
"start_time": 3399.957,
"text": " And so the same thing would be true for massive gravity and gravitational waves could also speed up every so slightly or slow down every so slightly. And so that means that in addition to just having polarization which are transfers to the line of of propagation you could also play with the longitude on our direction a little bit like sound waves if you're thinking of how you."
},
{
"end_time": 3452.039,
"index": 140,
"start_time": 3423.575,
"text": " Not for you and me right now but but how you hear each other it is a sound wave and is a compression of the air pressure and more pressure and less pressure and the along the line of propagation of the wave or just if you drop a stone and on a pond you'll see some waves traveling along the surface of the pond and those are what we call longitudinal waves because they go along the line of propagation of the wave."
},
{
"end_time": 3472.807,
"index": 141,
"start_time": 3452.773,
"text": " And so massive gravity have this additional freedom as well in how gravitational waves can evolve and this additional freedom missing like okay it's great you can do that as well it also mean that in terms of the force of gravity because it carries additional channels."
},
{
"end_time": 3503.183,
"index": 142,
"start_time": 3473.49,
"text": " which gravity can be mediated you would expect gravity in massive gravity on short distances to actually be stronger and that's counterintuitive we came up with a theory or not we but overall when people think of massive gravity in the sense that the particle has a mass one wants to do that because it weakens the behavior of gravity at large distances but what seemed other time price to pay for that would be to have additional"
},
{
"end_time": 3530.452,
"index": 143,
"start_time": 3503.541,
"text": " channels of communications for gravity, so an additional way to transmit the force of gravity, which through longitudinal polarizations, which would also mean that gravity on shorter distances would seem at the time to be stronger. And so this is what V, D, V and C discovered in 1970, that because of this additional channel, which seems to be present, even when the gravitational mass is as small as you want,"
},
{
"end_time": 3559.019,
"index": 144,
"start_time": 3530.794,
"text": " That means that there's a discontinuity between what happens in term relativity where the mass is exactly zero as compared to what happens in the massless limit of massive gravity. Yes. So that was the original issue. Controversies set up by VD and VDV and Z in 1970. But then two years later in 1972, Weinstein"
},
{
"end_time": 3577.21,
"index": 145,
"start_time": 3559.565,
"text": " I understood what happened i can't even stand came along and realize that actually the remit within which this understanding was done this calculations were done and how you think of a notion of force in terms of being mediated by this different channels different polarizations."
},
{
"end_time": 3607.363,
"index": 146,
"start_time": 3577.756,
"text": " Does make sense in some limits, but when you really try want to understand what happens in a very small masses, actually, you can't just neglect a whole sorts of other things that should otherwise be present. So actually, while it is true that those additional polarization are present when the mass is finite, right? Actually, when the mass is becoming very, very small, it becomes extremely hard to excite them. They actually themselves, they freeze in some sense."
},
{
"end_time": 3630.247,
"index": 147,
"start_time": 3607.824,
"text": " But you understand this freezing mechanism, what we now call actually the van stein mechanism. It's a screening mechanism. You need to understand how the self interactions of gravity allow for specific polarizations, which otherwise would not be there in general activity to freeze themselves."
},
{
"end_time": 3652.142,
"index": 148,
"start_time": 3630.503,
"text": " so it's almost if i'm taking too much what is meant by this freezing it's yes so it's almost as if i'm putting too much luggage on myself that i'm no longer able it completely inhibits my emotion and my ability to communicate so it is a it is it is a little bit as if um so we"
},
{
"end_time": 3673.251,
"index": 149,
"start_time": 3652.363,
"text": " When we're thinking of the force being mediated by gravity at the level of this different polarization we have a very simple picture in mind but when the mass of the garage becomes very very small this additional polarization interact with itself so it plays it's almost playing the role of a honey."
},
{
"end_time": 3701.323,
"index": 150,
"start_time": 3673.456,
"text": " In which it prevents its own dynamics it's no longer free to move at wish and it does that by itself through its own interaction so it is it is what we call a phenomenon of strong coupling the self interactions of the gravity on a particular sector become so important that they were sent to something which is very different as compared to what one would have expected in the first place."
},
{
"end_time": 3729.718,
"index": 151,
"start_time": 3702.363,
"text": " So i understood that a phenomenon like that had to be the case but you understand how this is implemented in practice how it happens in practice you need to have a fully non linear theory of gravity where all of the nonlinearities all of the interactions of the gravity on coma long just like would happen in general activity now you can think of"
},
{
"end_time": 3760.009,
"index": 152,
"start_time": 3730.418,
"text": " You can think of what happens for general activity when the interactions become very strong. When the curvature becomes too strong, you can imagine of having a black hole. You can imagine that if you have general activity and you have a regime where actually not necessarily the curvature, but the nonlinearities of gravity become important. That's where you actually quite far away from Newton and gravity. And this is precisely what happened at the onset of a black hole horizon."
},
{
"end_time": 3787.637,
"index": 153,
"start_time": 3760.606,
"text": " A black hole is precisely where things will be very different as compared to what you would have expected in Newton gravity because you no longer in the week gravity regime you starting having important interactions for gravity. This is the whole realm of black holes testing some new aspects of of general activity in a regime that would otherwise be the case in the solar system for instance where even though we do"
},
{
"end_time": 3815.623,
"index": 154,
"start_time": 3787.91,
"text": " Understand the subtle difference between Newton and gravity and general relativity in the solar system. They're still very subtle differences. They're not all the one difference. But when you get close to a black hole, actually the difference between Newton and gravity and general relativity, they are very big. They're very noticeable. Now for massive gravity, in addition to what would seem to be this distance associated with the size of the horizon of a black hole,"
},
{
"end_time": 3844.036,
"index": 155,
"start_time": 3816.101,
"text": " You have an additional distance scale related to where the non-linearity is now for the additional polarizations become important. So you have two distance scales. You have the non-linearities are important for the additional modes of the graviton. And then you have a very much smaller distance, which your standard Schwarzschild radius, if you want in, uh, Einstein theory of relativity. Okay. I don't know how familiar people are with the Schwarzschild radius and the idea of horizon."
},
{
"end_time": 3870.162,
"index": 156,
"start_time": 3845.162,
"text": " So what we need to understand is how to make this transition transition between what happens a very, very far, very big distances where there we understand gravity should be weaker and it is in a linear regime. But as you go to look in a theory of massive gravity and you start looking at some other distances, you need to start kicking in the nonlinearities for some sector of gravity."
},
{
"end_time": 3896.544,
"index": 157,
"start_time": 3870.503,
"text": " Which will then suppress the effect of the additional polarization and the departure of massive gravity as compared to general activity within that radius. We call that the branched on radius. Right. And when, and when the, when the gravity mass becomes very, very small, this function radius becomes larger and larger. And as the gravity mass becomes zero."
},
{
"end_time": 3918.968,
"index": 158,
"start_time": 3896.817,
"text": " This function radius becomes infinite and so the whole universe is within its own function radius which means that it looks identical to g.r. interest so we need to understand the non-linearities to make that happen so it exactly smoothed it out didn't just temper the discontinuity that's right it's smoothed it out and and so now we have exact realizations."
},
{
"end_time": 3941.903,
"index": 159,
"start_time": 3919.189,
"text": " Massive gravity where we can see precisely this transition where we have the we can think of the forces of force between the earth and the moon in a theory of massive gravity and we know precisely what happens when the mass of the gravity is smaller and smaller and smaller and we cover precisely the same result as in general relativity when the mass is exactly zero."
},
{
"end_time": 3965.418,
"index": 160,
"start_time": 3942.125,
"text": " So we understand that and therefore in a massive gravity if the mass is sufficiently small as what we would want anyways what we know is that the prediction for gravity in massive gravity would be extremely similar to what they are in general activity the departure would be extremely small it doesn't mean that we may not never see them in the solar system."
},
{
"end_time": 3972.79,
"index": 161,
"start_time": 3965.725,
"text": " Because actually we have very very precise tests of gravity in the solar system but they are very very suppressed."
},
{
"end_time": 4002.5,
"index": 162,
"start_time": 3973.78,
"text": " This episode is brought to you by State Farm. Listening to this podcast? Smart move. Being financially savvy? Smart move. Another smart move? Having State Farm help you create a competitive price when you choose to bundle home and auto. Bundling. Just another way to save with a personal price plan. Like a good neighbor, State Farm is there. Prices are based on rating plans that vary by state. Coverage options are selected by the customer. Availability, amount of discounts and savings, and eligibility vary by state."
},
{
"end_time": 4031.732,
"index": 163,
"start_time": 4003.66,
"text": " Okay, so let's see if I can do a summary so far. So the VDVZ says that if you have a massive gravity theory, sorry, not VDVZ, but generally, you should have your theory. As you take some parameter and you deform it down to zero, agree with a theory where it has zero if you're trying to recover that theory. So for people who are familiar with quantum mechanics, there's h bar and the correspondence principle."
},
{
"end_time": 4060.23,
"index": 164,
"start_time": 4032.108,
"text": " So what that means is as you set H bar to zero, if H bar is supposedly supposed to measure the quantumness and you set H bar to zero, then you should recover classical mechanics. There's a way you can do that with a theorem, although there's some hand waviness, I believe. But it doesn't matter. The point is there's no discontinuity there. And then you think, OK, well, if I have a massive theory of gravity, if I have a massive graviton, it should be straightforward to just put that mass down to zero and recover GR."
},
{
"end_time": 4084.497,
"index": 165,
"start_time": 4060.725,
"text": " But it turns out you don't. And that's quite odd. But then the reason why it's odd is because this this van stein guy realized that we weren't taking into account the nonlinearities and the other mode of polarization. Exactly. So you got exactly that exactly right. It is a there's a lot of subtleties going on. But this is this is exactly the gist of the story so far."
},
{
"end_time": 4111.886,
"index": 166,
"start_time": 4084.991,
"text": " So we still in nineteen seventy two and then the story is not over because still in nineteen seventy two. The same year as what i came along and resolved this video is discontinuity and said hang on a second you can't just be the calculations that were done so far what they were much more similar to what happens in newton and gravity as what happens in."
},
{
"end_time": 4141.476,
"index": 167,
"start_time": 4112.227,
"text": " In a fully nonlinear theory of gravity, like general activity. And he said, you need to account for all of those subtleties. In reality, already in general activity, we need to account for many subtleties that arise for nonlinear interactions. We need to account for them because that's, that's precisely how we understand black holes. This nonlinear effect of gravity are important there. So, so it is the case that general activity is also something that has very important interactions."
},
{
"end_time": 4162.927,
"index": 168,
"start_time": 4141.749,
"text": " Manifest itself in specific frameworks and so for massive gravity it also has to be the case that it's interactions have to be accounted for and we understand how accounting for this non-trivial interactions help us understanding how to take the small mass limit of gravity smoothly to zebra."
},
{
"end_time": 4189.889,
"index": 169,
"start_time": 4163.677,
"text": " I should say it's not a doc is not something we're putting by hand at the end of the story to fudge things so that they work with observation or so that they satisfy a principle that we had in impose on ourselves in advance it is something that comes in naturally you're thinking of a theory of gravity naturally has to be something which carries non-trivial interactions and when you account for these non-trivial interactions."
},
{
"end_time": 4208.933,
"index": 170,
"start_time": 4190.213,
"text": " Naturally you see this function mechanism emerging and a smooth limit to general activity when the mass is small so it's not something a doc is something fully fledged in the model itself understood but but then what in the same year what happened in nineteen seventy two."
},
{
"end_time": 4235.213,
"index": 171,
"start_time": 4209.48,
"text": " These are sunny days are and ball were so two physicists realize that when you account for this non-trivial interactions in massive gravity. This seems to always come in hand in hand with what we call a ghost and it is a ghost which seems to be present in the physical sector. Please explain what ghost particles are."
},
{
"end_time": 4265.691,
"index": 172,
"start_time": 4235.776,
"text": " So I ghost, I ghost should not exist. People who are not physicists, they just think that this is a different channel now. I think the new Ghostbusters movie came out. That's basically what you did in 2010. That's right. And I have a Ghostbusters t-shirt. So I'm not going to go too crazy, but it sounds, it sounds"
},
{
"end_time": 4286.664,
"index": 173,
"start_time": 4266.34,
"text": " The word sounds slightly science fiction but it's all really based in the in the rim of a. Very very scientific and i didn't come up with a time ghost myself it's something that was known since a hundred years as well since the nineteen thirties so yeah i'm also hundred years already so i ghost is."
},
{
"end_time": 4314.684,
"index": 174,
"start_time": 4287.858,
"text": " Are the most basic level a particle with negative energy what we mean by negative energy there is negative kinetic energy which means that if you have a ghost and you make it run around at a given speed it's energy will decrease so it will release energy to the system and the faster it goes the more energy it releases to the system."
},
{
"end_time": 4338.763,
"index": 175,
"start_time": 4314.974,
"text": " And the more it does that the more other particles which are normal particle can absorb that energy and that means that the universe as we see it would be completely unstable because you would have this possibility to trade up positive and negative energy so we all of us would be allowed to be as excited as we want and take over the energy"
},
{
"end_time": 4366.852,
"index": 176,
"start_time": 4339.189,
"text": " That the ghost is releasing and by making the ghost go as fast as they as they want so this seems like a very unstable process it is different it is different from having a particle. Who's you may have heard having a particle which has a negative potential negative mass square like some of the phase of the expo zone may have had that in the history that is."
},
{
"end_time": 4387.892,
"index": 177,
"start_time": 4367.073,
"text": " Just a transition phase when that happens is that action is just a transition phase where for a while princess you go up the hill with your bike and when you are the top of the of the hill you seem like you can very quickly speed down the hill but that's just a transition phase up to the point where you find again yourself at the bottom of the hill."
},
{
"end_time": 4401.903,
"index": 178,
"start_time": 4387.892,
"text": " And then there'll be no way to go from there are you just when you're the top of the hill you're not on the most stable position but then you always can go and decay to something which is stable and that's around the stable point the point is that is bounded from below."
},
{
"end_time": 4429.326,
"index": 179,
"start_time": 4402.619,
"text": " it's exactly that yeah it's bounded from below because some people will say the problem is negative energy but would it be more accurate to say the problem is that it's not bounded from below you can have negative energy that's bounded exactly that's exactly you're absolutely right so the problem is that it's not bounded from below so it's you're absolutely right so a tachyon is a particle with negative energy but it will still be bounded from below um it's in that case for the tachyon it has negative mass squared"
},
{
"end_time": 4458.951,
"index": 180,
"start_time": 4429.326,
"text": " And it will still be bounded from below once you find a right vacuum was for a ghost it's unbounded from below and you know it's unbounded for below because you can decrease its energy. By going faster and faster and there's no limit to how close to the speed of light you wanted to go so it can go as close to the speed of light as you want and that would lead it to as a negative energy as you want it to have so that's the real issue with the ghost you."
},
{
"end_time": 4489.121,
"index": 181,
"start_time": 4459.258,
"text": " You're exactly right the real issue is that it's unbounded from below and so there's no sense in which we can start our life around a stable vacuum where the particles don't go all crazy and where we can build a model from the ground up from that stable vacuum because there's nothing to start from there's no grand basis there's no grand zero this the energy is unbounded from below in the case of the existence of a ghost of a ghost particle."
},
{
"end_time": 4512.09,
"index": 182,
"start_time": 4489.872,
"text": " That's for any type of ghost. So yeah, so this is, this is impressive for any type of ghost. So if, if the ghost is really there in your theory as seem to be the case, um, as observed by, uh, Dazer and Bulwar, Bulwar and Dazer in 1972 for, for massive gravity, that it is an issue. And then that's it."
},
{
"end_time": 4538.541,
"index": 183,
"start_time": 4513.251,
"text": " This is a bit different from the idea of ghosts which are not really ghost which are present in your theory for the fadda fadda of ghosts. They are a mathematical trick they are trick in when you try to quantize some. Some fields which have some nice symmetry embedding into them sometimes it's easier to."
},
{
"end_time": 4559.957,
"index": 184,
"start_time": 4538.968,
"text": " Use an extended framework where you have the impression that they have additional ways to excite themselves which is not the case and then to cancel them directly one by one with fadif pop of ghost so the fadif pop of ghost don't really exist and the additional modes that of the."
},
{
"end_time": 4584.224,
"index": 185,
"start_time": 4560.452,
"text": " Are you trying to contact i'm not really there but you promote them to being there because it makes it easier to look at all this formulation so it is a mathematical trick in some sense but you do so that is an exact constellation between two pairs two pairs etc. I'm afraid of ghost and what would otherwise have been a mode that you are officially introduced in your theory."
},
{
"end_time": 4606.749,
"index": 186,
"start_time": 4584.65,
"text": " So it's not something physical it's a it's a mathematical way to shortcut some of the framework that you try to establish when you try to quantize it. So the fire pop of ghost event though the word ghost is present in this case it's a good ghost in the sunset specifically engineer to patch."
},
{
"end_time": 4636.049,
"index": 187,
"start_time": 4607.142,
"text": " I know the mode that you artificially introduced so you're everything is under control the terminology seems to be the same but in this case is is doing surgery in a way that you know precisely what you include and like with like in such a way that everything is is fine but for massive gravity we never no one ever came along and say oh again i introduce a ghost so that you can patch something up no he just comes up by itself"
},
{
"end_time": 4661.135,
"index": 188,
"start_time": 4636.254,
"text": " And that is really the issue because it is there it is physically there and he had as soon as it is present it leads to energy be unbounded from below. It's very existence means that you can reduce the whole of the energy of the universe to as much as what i did once by simply going as fast as it wants and that's that's the real issue with that."
},
{
"end_time": 4688.285,
"index": 189,
"start_time": 4661.698,
"text": " Now I'd like you to take us through to 2010 when you and your colleagues circumvented that ghost and what that experience was like. Yeah. Yeah. So, so you busted that ghost. It was exactly like in the movies, exactly like that. Um, so yeah, so that was in 1972. Uh, ball winders are short that in a theory of massive gravity or in a theory of gravity that has a finite range."
},
{
"end_time": 4716.049,
"index": 190,
"start_time": 4688.695,
"text": " It seems to be always the case that they come hand in hand with the existence of a ghost whether you want it or not it is present and many different people we explore that analysis but why does are we explore this in different languages actually you can think of it at the level of particles at level of energy various different type of levels and then in the middle of the eighties etc people had given up on the idea of massive gravity because"
},
{
"end_time": 4742.363,
"index": 191,
"start_time": 4716.323,
"text": " It seemed to be always the case that you can't make it work. To have a smooth limit to general activity where the graviton is massless, you need to implement these non-linearities, but these non-linearities always seem to lead to an excitation of a ghost, which seemed to be impossible. So that was up to mid-80s, but in 1998,"
},
{
"end_time": 4764.445,
"index": 192,
"start_time": 4743.49,
"text": " There was the confirmation from different groups that from supernova observations that the expansion rate of the universe was not slowing down as well as was expected rather it was going fast and faster so 1998 was when it was confirmation that the universe is accelerating."
},
{
"end_time": 4790.725,
"index": 193,
"start_time": 4764.77,
"text": " And so from there on, there was this whole emergence of new ideas in understanding whether it is the vacuum energy, whether it is something like the cosmological constant that also plays the role of the vacuum energy, whether it's dark energy and all this tuning and self tuning issues. And so along with all of this, we came the idea that gravity could possibly be modified. It could have a finite range."
},
{
"end_time": 4806.391,
"index": 194,
"start_time": 4790.725,
"text": " So as to tackle what we call this cosmonautical constant problem or the vacuum energy problem related to the accelerated rate of expansion of the universe and this is not where i came in i started thinking about those ideas a few years later."
},
{
"end_time": 4829.701,
"index": 195,
"start_time": 4806.698,
"text": " I was doing my undergrads at the time i started my phd in two thousand and two and there it was already well established people had tried again to understand whether you can have a fear of massive gravity which would have no ghost and you seem to be always coming up with the same impossibility to understand how to make the algebra."
},
{
"end_time": 4857.125,
"index": 196,
"start_time": 4829.701,
"text": " Of the interactions work out in a way that there was no ghost and actually throughout the notice i think you call this the notice from two thousand yeah okay got it is that the notice yeah there's arrows there's been a whatever that period was called lady gaga period that's all i remember okay that's a good name that's a good name um so during that period there's been quite a few very systematic papers being published"
},
{
"end_time": 4877.159,
"index": 197,
"start_time": 4857.125,
"text": " Going precisely through the different proves in different languages in different with different logics and showing how for every single one of these way to thinking about it that we always end up with the same issue that you can't have a theory of massive gravity with this nonlinear it is without also coming up with a ghost."
},
{
"end_time": 4897.5,
"index": 198,
"start_time": 4877.517,
"text": " And so it concluded in a set of what we call no go theorems and no go theorem is exactly what the name indicates it tells you that it is a rigorous math mathematical theorem which the answer is no there's a no go you can't do that and what was this theorems name."
},
{
"end_time": 4924.633,
"index": 199,
"start_time": 4897.927,
"text": " The no-go theorem or the no-go theorems in this case. Yes, there was no-go theorems for massive gravity. So there were, you can look it up, no-go theorem for massive gravity or for finite range gravity where you go through an analysis. They wouldn't have had a better name than that in themselves. Okay. So it's not like the Coleman-Mendula theorem. No, no, no, no, no, no, it wouldn't be like that. Yeah."
},
{
"end_time": 4949.684,
"index": 200,
"start_time": 4925.333,
"text": " So this is this was the situation where i was a researcher i in fact at the pyramid institute and not my master and then in geneva in late two thousand nine two thousand ten. And instead of thinking of theory of massive gravity because of the time i was convinced that all of this made sense."
},
{
"end_time": 4971.664,
"index": 201,
"start_time": 4950.128,
"text": " We were trying to start to look for a model of gravity which is based on having extra dimensions extra dimensions were very big at the time that could have some of the properties of massive gravity without this issue with the ghost and we did come up with a model a model based on extra dimension."
},
{
"end_time": 4999.855,
"index": 202,
"start_time": 4972.346,
"text": " In retrospect it wasn't fully finite so it still has some issues at some level but when we looked at how it was consistent with the current framework in simply four dimensions it seemed to be leading to a theory from a four-dimensional perspective that would have looked like a theory of massive gravity with a finite range"
},
{
"end_time": 5027.09,
"index": 203,
"start_time": 5000.64,
"text": " And yet by going through this extra dimension we seem to be able to implement something which implement a framework which was free of the ghost so when we were going through the proof for why they should be a ghost we were going through every stage. And somehow everything would agree up to a given level but then at some point we saw that the outcome for our theory was."
},
{
"end_time": 5048.558,
"index": 204,
"start_time": 5028.166,
"text": " To be absent of the ghost was in the no go serum the ghost should have been there. And even though the model we we were working on was a model that was based in a structure in the way that we engineered it was based on extra dimensions and really what i did is that."
},
{
"end_time": 5070.094,
"index": 205,
"start_time": 5049.036,
"text": " The type of bricks are the type of legos that we used to build it were within the logic of the extra dimension as opposed to the logic of four dimensions so we were using a language was which was more appropriate for the symmetries in five dimensions as compared to what we would have used had we been in four dimensions from the outset."
},
{
"end_time": 5090.998,
"index": 206,
"start_time": 5070.418,
"text": " Have a event that we did that it doesn't matter we can still think of a theory based on extra dimension and ask yourself the question of what would a four dimensional observer see in that model what would be the four dimensional characteristics of the theory of gravity from a purely four dimensional perspective."
},
{
"end_time": 5119.855,
"index": 207,
"start_time": 5091.476,
"text": " And on the one hand it was leading to a theory that would look like a massive gravity that would have a finite range gravity where the would be four-dimensional gravity would be massive. And on the other hand we couldn't see any sign of that ghost at least not to the level where it was indicated in all of these no-go theorems. And this is really where pushed us to understand where was the discrepancy between the model that we were having"
},
{
"end_time": 5145.503,
"index": 208,
"start_time": 5120.111,
"text": " And all of those no-go theorems. And to be honest, I was certain I had made a mistake. I really, I remember going through it over and over again. I said, it simply doesn't make sense. There must be, the ghost must be there. And where is it? Where I must have hidden it somewhere. It's very easy to do that. It's very easy to convince yourself that things are fine. But then the problem is hidden much deeper. So I spent a year trying to understand"
},
{
"end_time": 5171.476,
"index": 209,
"start_time": 5146.408,
"text": " Where this pathologies in my model should be hidden because surely is be should be there somehow but i can't quite see it at the first site and understanding whether i made a mistake or whether things were actually more subtle but the pathology would manifest itself in a given way until i realize. Going back to all of the proofs that actually they had been some shortcut being used."
},
{
"end_time": 5201.613,
"index": 210,
"start_time": 5171.92,
"text": " I'm in some cases there had been some implicit assumption being used in some case there had been. Too many shortcuts being used in such a way that the answer the result wasn't as general and i wasn't applicable to all possible situation as compared to what people thought and the example we had found was precisely almost by miracle one that fit precisely in the box of things that could work out that could make the ghost disappear."
},
{
"end_time": 5224.599,
"index": 211,
"start_time": 5202.142,
"text": " You evaded those assumptions. Yeah, sorry, what would be an example of one of those implicit assumptions? And by the way, when you say that you mean to say that the paper itself didn't make clear or explicit the assumption, it was just embedded in the ethos of their argument, but it was subtle. That's right. That's right. So it's a little bit how what do you mean by a ghost in some sense?"
},
{
"end_time": 5250.828,
"index": 212,
"start_time": 5224.94,
"text": " When you try to understand what a ghost is, it's very clear what you mean by that when you can identify different particles and when you can identify the energy, the kinetic energy of every single particles. And that is something we all know how to do very well in the simplest scenario when we're in flat space time and when things are relatively simple."
},
{
"end_time": 5279.531,
"index": 213,
"start_time": 5251.561,
"text": " This is not what we're interested in. We were interested in having a theory of gravity where you want to think of it in potentially quite different geometries where you're not in Minkowski flat space-time where you understand precisely what is your notion of energy, what is your notion of pressure. We want to understand it in curved space-time where things become much more murky on precisely how to separate out your notion of energy with the other notions."
},
{
"end_time": 5307.261,
"index": 214,
"start_time": 5279.94,
"text": " And that's where you can have some mixing between what you think is energy what you think is pressure is adjust. Examples and the different modes actually the different modes of the gravitational waves can start be interacting with one another in a very subtle way so that the way you identify what you would have traditionally called as the standard gravitational polarizations along the transfers mode."
},
{
"end_time": 5336.237,
"index": 215,
"start_time": 5307.261,
"text": " Not in america space time starts getting mixed up with what you would have called being the logical directions and what you would have called the ghost so we needed to formulate a new framework which would allow you to. Separate out this different characteristics if you want if you want imagine you're in a pond is a clear beautiful day and you see you drop you drop a stone and you see this beautiful waves going on the surface."
},
{
"end_time": 5358.933,
"index": 216,
"start_time": 5336.681,
"text": " We can all agree those are the waves on the flat surface of the pond and it's beautiful. Now imagine this you're not on a pond you're in the middle of the ocean is the the huge storm of the century with waves which are bigger than your boat. Who is to say why start the small fluctuation from the little"
},
{
"end_time": 5388.234,
"index": 217,
"start_time": 5359.565,
"text": " Stone that you dropped and where are the underlying huge fluctuations huge waves which are bigger than anything else that anyone is very hard it's very hard on first sight to distinguish one for another and yet you need to do that to separate out what you mean by the different polarizations of the gravity tone in that situation. And how they interact with one another and what type of energy they carry out you need to be able to separate all of those things so there was some."
},
{
"end_time": 5411.971,
"index": 218,
"start_time": 5388.882,
"text": " Implicit level of assumption of how you separate those things out there a little bit technical in how to do that but but it's a little bit as if you imagine for your for your ocean you say okay i'm gonna say this is the zero uh depth of the ocean and i'm gonna just calibrate them in the way i would have sort of doing it if i were on a pond"
},
{
"end_time": 5438.114,
"index": 219,
"start_time": 5412.278,
"text": " Sometimes that's not the right way to do it you really need to reach change your perspective change completely the way you gonna characterize all of those things so that you end up with something where you can separate out the different modes of the gravity. So when people were identifying the existence of a ghost for the gravity on actually what they were identifying was one of the normal modes of the gravity on."
},
{
"end_time": 5466.049,
"index": 220,
"start_time": 5438.49,
"text": " Because it's difficult to really understand how what type of energy carries. They were just thinking that this was corresponding to the ghost when in reality it was just one of the healthy modes of the Graviton. Okay, so you spent a year checking this over and over. Yeah, it's like that in research. Yes. I mean, I spent I spent a lot of time a lot of a lot of nights going through through all of this. And even even when we understood"
},
{
"end_time": 5495.606,
"index": 221,
"start_time": 5467.039,
"text": " That this could be a possibility it's not enough that you understand it's a possibility that you need to understand better how these things work you you still need to understand what is the best way to frame all of these things and to have an explicit realization so that you see whether it can be fully fledgling in a full in a full theory so the model we had in the model of extra dimension that was a model where we could have a first insight of how you could work out in principle"
},
{
"end_time": 5511.254,
"index": 222,
"start_time": 5496.254,
"text": " In a limited context but it wasn't the full story because it was still breaking down at some point so but that was enough because we understood how we had the seed of the idea of how things could work out in principle."
},
{
"end_time": 5540.128,
"index": 223,
"start_time": 5511.698,
"text": " And then what you have to do is engineer a model which fits precisely in that box satisfies precisely what was falling between the cracks. Feel those cracking precisely the right way so that you can end up with a massive gravity that evades all of these no-go theorems. So to the young theorist who's watching this sounds inspirational because they may have some theory and their advisor may push against it because it violates some"
},
{
"end_time": 5567.671,
"index": 224,
"start_time": 5540.469,
"text": " No go theorem or it produces an anomaly. What would be your advice then to them? So I mean, one of the things is that, of course, that's, that's the beauty of falling, right? Sometimes things don't work. And then it is in the beauty of understanding why they don't work, why they don't work precisely, that you may discover something, something new, so something beautiful, even if that thing is not useful for precisely what you wanted, even in a fear of massive gravity may not be"
},
{
"end_time": 5587.841,
"index": 225,
"start_time": 5568.251,
"text": " For the discussion of the universe as we wanted it still has some structure in itself it still has some level of inner beauty which i think is worth in in and of itself but so absolutely sometimes and i have happened to my career multiple times where there's a consensus of what is accepted what are the theorems."
},
{
"end_time": 5615.981,
"index": 226,
"start_time": 5588.951,
"text": " And you should in reality take nothing for granted have in most of the case it is true that this is a deep reasons for why things have been fledged in a particular way so it's not like i just woke up in the morning and i thought okay i just gonna go against the norm and come up with all sorts of different ideas that don't fit the box i think you always need to go back to what has been proven what has been understood so far"
},
{
"end_time": 5643.37,
"index": 227,
"start_time": 5615.981,
"text": " And and really understand this to that depth it's by understanding the work that has been done so far to its depth and then you can also understand how what you want to do can come along and how i can complement how i can even contradict what has been presented. What's right trying to understand whether you can have a theory of gravity which is goes beyond general activity i think i'm in a very good position to tell you how."
},
{
"end_time": 5672.739,
"index": 228,
"start_time": 5644.241,
"text": " How incredible general activity is how you may think that it's set up like that because of some assumptions of the pillars of Einstein and some particular assumption that were preset in advance but actually for me it's much more self-consistent in itself and i can see that all of the beauty that is present in general activity is completely self-consistent from the outset it's extremely hard to just challenge a tiny little thing"
},
{
"end_time": 5699.718,
"index": 229,
"start_time": 5672.739,
"text": " You need to understand all of this beautiful structure first before you understand how you can start dismantling maybe a small little piece without everything falling apart. Yes. Okay. So it's not throwing away the history of physics. It's deeply understanding it in order to snake your way through a narrow path. That's right. So this is an extremely bold new theory of gravity that in some ways shouldn't exist."
},
{
"end_time": 5729.906,
"index": 230,
"start_time": 5700.128,
"text": " Does it solve any other problems like the information paradox or tell you what happens in the inside of a black hole near the singularity? So by design, the theory of massive gravity is a theory of gravity that generalizes or goes beyond Einstein's theory of general relativity on very large distances or very small curvature regions of the universe, which is precisely the opposite end of the spectrum as compared to what we're interested in"
},
{
"end_time": 5743.251,
"index": 231,
"start_time": 5730.401,
"text": " In black holes for instance the horizon of black holes are in black holes themselves so in it's very construction the theory of massive gravity i would say from the outside shouldn't have anything to say about those aspect of."
},
{
"end_time": 5765.247,
"index": 232,
"start_time": 5743.507,
"text": " I am completely agnostic and I think it's good to have a spectrum of different point of views because I think it's good to understand how to connect between those"
},
{
"end_time": 5792.517,
"index": 233,
"start_time": 5765.247,
"text": " So far i have no particular preference necessarily on how to which one is right or wrong i think it is useful to understand them all it is useful actually to very much understand them all and very much understand what are the common points and what are the differences so that then we can try much more to distinguish them at different levels let me let me just say that for the same gravity."
},
{
"end_time": 5816.408,
"index": 234,
"start_time": 5792.875,
"text": " Even though i don't have a preference to say what is the ultimate what we call high energy completion of it because it has some some of these additional modes some of those additional polarizations and a radius associated with it which we call the bench and radius which is much bigger as compared to the horizon i would be black hole."
},
{
"end_time": 5845.623,
"index": 235,
"start_time": 5816.817,
"text": " And so in this they are massive gravity even though from the outside that's not the reason it was engineered. It comes in as well with some features where you start needing to understand some phenomenon about let me call them strong coupling or quantum nature of gravity of some of this polarization of gravity are ready at the scale of the balance and radius and not at the scale of the horizon so some of the."
},
{
"end_time": 5862.978,
"index": 236,
"start_time": 5846.425,
"text": " Mysteries of gravity that are present in general activity they are very much present in massive gravity and actually they manifest themselves in some of the modes of the graviton before they would have manifested themselves for general activity now that this."
},
{
"end_time": 5893.439,
"index": 237,
"start_time": 5863.439,
"text": " On one side is going to be very very complicated it is it is very very complicated but on the other hand you can also see it as a play field to explore some of these ideas that you would want to explore for quantum general activity from quantum gravity in a framework of massive gravity where this comes in for just one of the modes so you can just explore the effect for one of these modes and maybe understand some of the phenomenology of quantum gravity in a simplified model."
},
{
"end_time": 5922.398,
"index": 238,
"start_time": 5893.439,
"text": " Now, there's something called the Higuchi bound, which imposes a limit on the mass of a spin two particle in decider space. Yes. So are you within that bound? Are you above it? Are you below it? Yes. Yes. So the Higuchi bound is absolutely right if you are in decider space. So if you're in an accelerated expanding universe, let's say we close to decider space. Now in the universe, you it tells you that the mass of the graviton"
},
{
"end_time": 5940.964,
"index": 239,
"start_time": 5922.875,
"text": " Has to be either zero or it has to be larger than twice the herbal parameter or actually the square root of the herbal parameter. It's interesting. It's like a Yang-Mills mass gap. There must be a mass gap. There must be a mass gap. Exactly. Square root two herbal parameter."
},
{
"end_time": 5970.247,
"index": 240,
"start_time": 5941.323,
"text": " Otherwise, you end up with a ghost, which is not the ball, what does it goes to this time? It's called the Gucci ghost. It's not an additional mode, which is a new ghost mode. It's actually one of the mode of the gravity tone, which we discussed before, which itself starts becoming has a negative mass. So this is okay. If you want your theory to take off even further, you should have called it ghost gravity. That's such a catchy name."
},
{
"end_time": 5985.794,
"index": 241,
"start_time": 5970.674,
"text": " Many books can be written about that."
},
{
"end_time": 6014.821,
"index": 242,
"start_time": 5986.596,
"text": " Going back to your question of whether this is okay, it's fine. It's certainly fine today because the herbal parameter today is actually of the same order of the mass of the graviton we would like it to have today. We want the mass of the graviton to be slightly larger than the herbal parameter today, so we would have expected the graviton to be within the realm of mass which satisfies the Iguchi bound."
},
{
"end_time": 6036.766,
"index": 243,
"start_time": 6015.35,
"text": " I have a if you want to understand how this occurred throughout the history universe so if you are the very beginning of the universe you were close to another the city region in universe when are the hubble parameter was way higher and so you would have expected to have the mass of the gravity on the time to be much"
},
{
"end_time": 6054.974,
"index": 244,
"start_time": 6037.142,
"text": " Do you need to satisfy any good she found which is much stronger or the gravity mass would have needed to be much higher to satisfy the good she found at the time what what we think of the moment is actually there is some sort of redressing mechanism and that comes in naturally with sad examples."
},
{
"end_time": 6083.609,
"index": 245,
"start_time": 6055.213,
"text": " Where we see that naturally occurring from the environment. So when we are in the very early regions of the universe, the effective mass or what you call the effective mass of the Graviton, you identify as the mass of the Graviton is actually redressed by its environment. So it leads to effects on the Graviton mass, which means that the Graviton mass is actually carried by the environment and carried by the herbal parameter other time. So it satisfies the Gucci bound other time."
},
{
"end_time": 6112.483,
"index": 246,
"start_time": 6084.087,
"text": " And as you have an evolution of the universe, the mass of the graviton also effectively evolves with the curvature and with the evolution of the universe so that it becomes very small today, but it could have been much larger in the earlier parts of the universe. And that in itself is also consistent with with observations. Interesting. So are you suggesting that just like the Higgs so-called gives mass to the fermions,"
},
{
"end_time": 6136.049,
"index": 247,
"start_time": 6112.944,
"text": " That there's something that couples like a Yukawa coupling for the Graviton that gives mass to the Graviton. So I'm certainly not suggesting a Higgs mechanism for Graviton. That mechanism is not so much a mechanism where you're in the same way as the Higgs mechanism where it's in the interplay between the Higgs bath and the"
},
{
"end_time": 6150.367,
"index": 248,
"start_time": 6136.459,
"text": " Is it is something slightly different where."
},
{
"end_time": 6178.097,
"index": 249,
"start_time": 6150.503,
"text": " Because of the existence of an environment being present, it does carry along with the graviton. So the graviton itself carries along the environment in which it is living. But you don't need an extra particle like the Higgs to lead to this environment in some sense. It is something that you can have at the completely classical level."
},
{
"end_time": 6199.753,
"index": 250,
"start_time": 6178.916,
"text": " There's an article by the economist I'll put on screen called the dominant model of the universe is creaking. And it's about the DESI results from the past couple months or so. Can you go over either the DESI results or any new data that validates that you're on the right track? So"
},
{
"end_time": 6217.722,
"index": 251,
"start_time": 6200.009,
"text": " There's been in the past almost ten years or so while all observations completely agree that the accelerate and the universe is accelerated within a given right there's been a deepening in the."
},
{
"end_time": 6240.725,
"index": 252,
"start_time": 6218.217,
"text": " The level of which different type of observation leach slightly different rate for the hubble parameter which we call the hubble tension if you want the rate of accelerated expansion of the universe and this seems to be some slight discrepancy between the rate you seem to be observing depending on the on the type of observation whether you dealing with observation which are."
},
{
"end_time": 6256.869,
"index": 253,
"start_time": 6241.015,
"text": " I'm late on in the age of the universe as compared to early on in the age of the universe and depending on the scale of the surface of the nation so far since between supernovae or what you would have from other observation of form from observation of the cosmic microwave background."
},
{
"end_time": 6278.729,
"index": 254,
"start_time": 6257.21,
"text": " So the result which i'm more recent and we're still expecting much more from the results they see the extremely extremely interesting. I personally think it's a little early to reach too much into the result i think they will have a full."
},
{
"end_time": 6299.855,
"index": 255,
"start_time": 6279.07,
"text": " Many years of observations where they can consolidate summer some of the result but taking that face value today they seem to suggest that one in principle having. Accelerated expansion of the universe and evolution of the universe which is consistent with it being driven simply by pure cosmetical constant."
},
{
"end_time": 6328.848,
"index": 256,
"start_time": 6300.094,
"text": " It seems to be slightly favored to have a dynamical dark energy, so the equation of state parameter that changes every so slightly over time. And what seems to be also very interesting, if correct, is that the equation of state parameter for the would-be dark energy is not within a regime we would have anticipated should be based on typical scalar field models a priori."
},
{
"end_time": 6351.203,
"index": 257,
"start_time": 6329.258,
"text": " And so all of this missing to suggest that there's more much more to the picture of dark energy and to the accelerated expansion of the universe as compared to what the most vanilla pure constant cosmo to call a constant model we seem to suggest i mean it is possible that there's some systematic effects between different type of observations."
},
{
"end_time": 6381.169,
"index": 258,
"start_time": 6351.459,
"text": " Which are playing a role into that. I am, I'm not at all in within this data. I can't say anything about that. But if it is all entirely correct, it seems to suggest that there is some dynamics within the evolution of the accelerated expansion of the universe. And so something else than simply a pure cosmological constant or a pure vacuum energy with the right order of magnitude should be at play to explain those observations."
},
{
"end_time": 6405.623,
"index": 259,
"start_time": 6381.664,
"text": " To me that's really is fascinating because it's it's a sign pose for potentials science of new physics and whether it is done i make all dark energy or modification of gravity or anything in between is is is very much something we have to better understand but it tells us that there's something which is beyond the most simple the simplest possible model."
},
{
"end_time": 6433.643,
"index": 260,
"start_time": 6405.964,
"text": " And so massive gravity where you could have or other models of modified gravity, where you can have a behavior of gravity, which is every so slightly different throughout the ages of the universe is something which could in principle help with understanding some of those questions. Does massive gravity have any implications for anti-gravity? For intergravity, as you, as you think of it, in the sense of having two masses,"
},
{
"end_time": 6459.735,
"index": 261,
"start_time": 6433.933,
"text": " The reality is massive gravity is so anchored within the framework of general relativity with very small departure that you're not going to end up with a result which is so radically different as compared to general relativity. It's not going to make something flip sign in itself."
},
{
"end_time": 6487.21,
"index": 262,
"start_time": 6460.077,
"text": " This very notion that you could have anti-gravity or that things could start really fundamentally becoming repulsive as opposed to attractive as in two masses being repulsive that as much in massive gravity than in general activity leads to some instability is also related to negative mass if you want a negative energy which is which is unstable so"
},
{
"end_time": 6513.695,
"index": 263,
"start_time": 6487.773,
"text": " A lot of what we do in Massive Gravity is still following very much the same rules as in general relativity. So those type of things would not be directly applicable for Massive Gravity. And does the Witten-Weinberg no-go theorem about the massless spin-2 particle, does it apply to yours or do you see it as evading it? Okay, so what the Weinberg"
},
{
"end_time": 6542.039,
"index": 264,
"start_time": 6513.985,
"text": " Sorry, let me go through all that. I knew it."
},
{
"end_time": 6571.288,
"index": 265,
"start_time": 6543.183,
"text": " So some people saw that as suggesting that the graviton shouldn't exist in four dimensions. One of the ways around it is to exceed or shorten your dimensions or to say that the graviton is a composite particle. But it sounds like massive gravity is an evasion of this theorem. Yeah. Yeah. Yeah. Yeah. Yeah. So the Weinberg-Witten theorem tells you that you"
},
{
"end_time": 6600.776,
"index": 266,
"start_time": 6571.578,
"text": " A massless graviton in four dimensions cannot be a composite particles. So that's okay for the graviton in general relativity, which is a massless particle, but it would be a fundamental particle. Now, if it is a massive particle, then that evades the theorem altogether. So in principle, it could be a composite particle. So professor, what are you working on now? What are you most looking forward to?"
},
{
"end_time": 6616.391,
"index": 267,
"start_time": 6601.186,
"text": " Yes so what i do at the moment is some of these aspects are quite different as compared to massive gravity or they started with massive gravity to some extent i'm trying very much to understand how to make connection with the series."
},
{
"end_time": 6646.817,
"index": 268,
"start_time": 6617.415,
"text": " that we use on a daily basis to describe the world around us, still as a theorist, as a quantum field theorist. So we have the framework of effective field theories. For instance, the standard model of particle physics is an effective description for all of the standard model and all of the constituents of matter and the other forces of nature, aside from gravity. Or general relativity is an effective description of gravity, which works extremely well, we believe,"
},
{
"end_time": 6658.302,
"index": 269,
"start_time": 6647.159,
"text": " I'm on low energy scales but we know because of the issues with related to embedded general activity in a quantum world at high energy."
},
{
"end_time": 6686.442,
"index": 270,
"start_time": 6658.677,
"text": " That at some point we need to have a better description of gravity. And so that can be string theory. It can be look on gravity. It can be causal sets. It can be all sorts of different alternatives. It is possible that it is another UV high energy ultraviolet high energy completion of gravity, which we haven't yet come across, which we haven't yet envisioned. So there's all sorts of different possibilities are high energy."
},
{
"end_time": 6706.8,
"index": 271,
"start_time": 6687.056,
"text": " And myself i don't want to be too specific on which particular completion i want to commit into dimension before i like to remain agnostic on the type of completion that i will allow for myself being a string theory etc but i still want physics to make sense ultimately."
},
{
"end_time": 6733.114,
"index": 272,
"start_time": 6707.295,
"text": " So for instance i don't need to know precisely what the laws of physics are at infinitely high energy but it still is meaningful for me to ask that whatever they are the satisfy what we call unitarity so they satisfy some laws of quantum probability so that i think sums up to one that's what i mean by unitarity to some extent."
},
{
"end_time": 6757.705,
"index": 273,
"start_time": 6733.507,
"text": " It's a bit more than that but to some extent I can think of it like that. I can also ask for instance for causality we understand which that is probably not controversial although how you formulate this is probably more controversial but at the bare level the notion of causality is that I would like"
},
{
"end_time": 6776.152,
"index": 274,
"start_time": 6758.097,
"text": " The consequence of an effect to happen after the effect not before so if i were to to kick this table i want to be hurting after i kick it and not and not feel it before i kick it this is my notion of of causality."
},
{
"end_time": 6797.193,
"index": 275,
"start_time": 6776.152,
"text": " We can we can stay that in more formal more rigorous terms in saying that i want to have no support of my propagator outside my light cone and i can state it like that because if i also think of fundamentally laurence invariance and then i'm different boosted up servers should be."
},
{
"end_time": 6824.48,
"index": 276,
"start_time": 6797.5,
"text": " Equivalent with respect to one another and so if i have something which seems to be outside my light cone then find a server which is boosted respect to me that may seem to be perceived as something that goes backwards in time. So that there are some relatively easy to formulate or relatively general statement about physics. Which are not too controversial in themselves."
},
{
"end_time": 6853.541,
"index": 277,
"start_time": 6824.735,
"text": " And I still want them to be satisfied in physics in general. So even within a realm of physics for which I don't have direct access, neither theoretically nor observationally, I have no access to it. And yet I want to make sure that physics satisfies those notions because they make sense. Conceptually, they make sense. If things were starting to become a causal, I will need to rethink about everything from the ground up. If things were"
},
{
"end_time": 6881.527,
"index": 278,
"start_time": 6853.899,
"text": " Not satisfied unity then i would need to rethink completely about the laws of quantum mechanics so it's not many of them there's a few set of properties of physics i want to make sure they be satisfied at very high energy but those them in themselves whether they are realized in the way that string theory realize them or look on the gravity realizes them or other type of uv completion realizes them"
},
{
"end_time": 6911.937,
"index": 279,
"start_time": 6882.176,
"text": " It doesn't matter in which one they still have consequences for the laws of physics in the way that I observe them at the moment and in the way that I can actually probe with my observations not mine but with observation at our disposal or experiments in a particle colliders for instance so there are some features some imprints of high-energy physics based on those assumption"
},
{
"end_time": 6933.131,
"index": 280,
"start_time": 6912.244,
"text": " Which should be present on the low energy framework that i'm using to describe the world around me and we used to those actually we used to knowing that the notion of causality at very high energy has for a fact that no one can travel faster than the speed of light at low energy the speed of light in the vacuum."
},
{
"end_time": 6948.677,
"index": 281,
"start_time": 6933.831,
"text": " The notion of causality is something which is actually embedded at very high energy, at infinite energy, because it's related to what we call the front velocity, the infinite frequency limit of the phase velocity."
},
{
"end_time": 6974.531,
"index": 282,
"start_time": 6948.916,
"text": " The front velocity is something I think we all hear but maybe we don't all remember. We all hear for instance that the phase velocity can be superluminal so long as the group velocity can be subluminal because we are thinking of the group velocity as generally carrying the information with the phase velocity something more artificial."
},
{
"end_time": 6993.353,
"index": 283,
"start_time": 6974.531,
"text": " That is that is actually incorrect they are experiments for which the group velocity is superluminal and it doesn't actually violate causality because the very notion of causality is actually set up in the infinite frequency limit."
},
{
"end_time": 7012.398,
"index": 284,
"start_time": 6993.353,
"text": " Off the face velocity so if you're thinking of national causality i want almost to have a discontinuity i want to send you a signal so it cannot be the case that i was sending you a signal since the beginning of time there needs to be a time where i'm not sending your signal."
},
{
"end_time": 7041.271,
"index": 285,
"start_time": 7012.756,
"text": " And then it starts kicking in or sending you a signal. This, this is what has to happen. And so there's a discontinuity there. This is me with no signal sending to you and now I'm sending you a signal. And so it is in this discontinuity that most of the notion of causality starts kicking in, but because it's discontinuous is something if I were to do a Fourier transform, I don't know. Or if you want to think of, of the end, the frequency associated to that, that's something that leaves that infinite frequency."
},
{
"end_time": 7057.824,
"index": 286,
"start_time": 7041.681,
"text": " infinite energy so i don't know how familiar your audience is with with those terminology but it is actually something that doesn't i can't realize this exactly within the really what i have i have."
},
{
"end_time": 7078.148,
"index": 287,
"start_time": 7058.507,
"text": " i have contact right you can imagine a real discontinuity is something that would require so much precision that i can't do it exactly it's the same reason that in heisenberg's uncertainty you can't reduce the position down to delta function down to just one point exactly is exactly the same thing so"
},
{
"end_time": 7098.029,
"index": 288,
"start_time": 7078.882,
"text": " Is something which is not within my realm to achieve it is something that would require me an infinite energy to being able to achieve so really to prove the notion of causality to very prove the notion of what happens if i make the transition between not sending your signal and starting."
},
{
"end_time": 7128.046,
"index": 289,
"start_time": 7098.882,
"text": " t zero i start sending your signal then i need to be living at infinite frequency and infinite energy so this is something within the realm of the uv completion of everything in the grand theory of everything that's where the notion of causality really resides but it doesn't mean that it's completely disconnected to how we experience it in our everyday life and we still know that this very notion of causality as embedded at very high energy"
},
{
"end_time": 7142.858,
"index": 290,
"start_time": 7128.387,
"text": " Has consequences for everyday life and we know that as a consequence it means we cannot be traveling faster than the speed of light in the vacuum that has a consequence unless there's some."
},
{
"end_time": 7171.135,
"index": 291,
"start_time": 7143.285,
"text": " Different features that emerge from a small violation of unitarity in some particular fluids that we can engineer locally or other things like that. This is this is how this is being played out in all of this engineer system where they managed to achieve having a good velocity, which is faster than light in the in the vacuum. So that's just one example, but there's actually an infinite number of consequences that can be explored."
},
{
"end_time": 7195.367,
"index": 292,
"start_time": 7171.357,
"text": " That we can use to better understand how physics get implemented at high energy so it's almost a two-way street where a inputting some assumption of high energy it can guide us on how to think about the physics in the way we describe it at low energy and you can guide us where to look for signals"
},
{
"end_time": 7225.452,
"index": 293,
"start_time": 7195.845,
"text": " What's up what's up what's up what's up what's up what's up"
},
{
"end_time": 7246.715,
"index": 294,
"start_time": 7225.452,
"text": " Searches for new physics at low energy that's one way to think about it another way to think about it is in exploring how high energy physics imprints itself at low energy in exploring high energy looks like we may also be able to get a better understanding of how high energy physics."
},
{
"end_time": 7274.309,
"index": 295,
"start_time": 7247.022,
"text": " Looks like and whether some of the assumption that we think we should impose on ourselves at high energy whether they are justified or not maybe we observing low energy physics in such a way that some of those assumption at high energy should be violated and so maybe that could guide us to understanding where we should be in a real of high energy completion is it more to a string theory is it more towards something else."
},
{
"end_time": 7277.756,
"index": 296,
"start_time": 7275.367,
"text": " One of the beauty"
},
{
"end_time": 7306.544,
"index": 297,
"start_time": 7278.166,
"text": " way of trying to make connection with the high high energy world which is not specific to string theory is very much in addressing this notion that you may think you may have heard that string theory is not a theory because he doesn't have a specific observable and actually that that may not be true there may be some ways that you can falsify string theory because it comes in with some assumptions which have consequences"
},
{
"end_time": 7332.79,
"index": 298,
"start_time": 7306.544,
"text": " For physics that we can observe that we can test within our realm today and so we could come up with a result of an experimental observation that would falsify string theory the very underlying assumptions of string theory. Can you talk briefly once more about how is it that if you were to exceed the speed of light it wouldn't break causality because in the traditional model that you learn"
},
{
"end_time": 7357.654,
"index": 299,
"start_time": 7333.114,
"text": " In university, you have the light cone and as soon as you tilt past that, then you can transform your vector in any which way and you would violate causality. It's, it's complicated. So let me see if I can. So, so when we go through the standard explanation of, um, if you have a wave, let's imagine you have a wave, um, which is your signal. And depending on, on,"
},
{
"end_time": 7386.084,
"index": 300,
"start_time": 7358.029,
"text": " The velocity of the wave if the group velocity of the wave exceeds the speed of light the traditional picture isn't to say that this is signaling that something is outside your light cone and then you can go into a frame of reference when observer is boosted with respect to you and something which is outside your light cone for them looks like it's traveling backwards in time according to their frame of reference."
},
{
"end_time": 7413.865,
"index": 301,
"start_time": 7386.357,
"text": " So this is how it seems typically to suggest that traveling faster than the speed of light in the sense that if you have a group velocity for a signal away with a particular frequency going faster than the speed of light, this seems to suggest a violation of causality. But this argument is what is mainly true. The reality is this is an idealized scenario where you imagine a signal being emitted by a wave."
},
{
"end_time": 7444.002,
"index": 302,
"start_time": 7414.224,
"text": " Can i wave which is being there since the beginning of time there's no beginning or end to the wave because if they were beginning or end to the wave then you wouldn't correspond to a wave with just a particular frequency you will need to in how this wave dies out or what you need to include the frequency associated with the physics of this dying in and dying out so when we really think much more about the notion of causality."
},
{
"end_time": 7460.23,
"index": 303,
"start_time": 7444.804,
"text": " I want to think of what it means to send you a signal and go from the transition between me not telling you anything. It's embargoed to me starting to tell you something and so I need to switch on my signal."
},
{
"end_time": 7487.346,
"index": 304,
"start_time": 7460.435,
"text": " Which means i can't just be communicating this information with a single frequency wave which has been there since the beginning of time it won't just be one frequency wave and just switching on my signal will lead to a spectrum which also include infinite frequencies so i pretty sharp wavelength in the signal that i'm trying to send you and so it is very much in this very very sharp."
},
{
"end_time": 7515.145,
"index": 305,
"start_time": 7487.927,
"text": " Very very small wavelength part of the signal that information about the high energy physics is encoded and it causality is therefore very much encoded at high energy is not something that i can simply diagnose at low energy but that's an example where high energy physics actually has an imprint in how we think about physics at low energy and how it still tells us that"
},
{
"end_time": 7530.811,
"index": 306,
"start_time": 7515.879,
"text": " We shouldn't be able to travel faster than the speed of light in the vacuum."
},
{
"end_time": 7556.135,
"index": 307,
"start_time": 7531.493,
"text": " And I recommend it. It's an honor to be able to speak with you. Thank you. You're great at explaining concepts extremely simply. And you have an effervescence about you that I appreciate and the audience can relate to, I'm sure. Thank you, Kurt. Thank you. That was very, very, that was great. Thanks. Really, really fun. Yeah, feels a lot were very, very technical. I hope your audience likes it. Yeah."
},
{
"end_time": 7573.37,
"index": 308,
"start_time": 7556.476,
"text": " Thank you. Thanks for the questions. They were really great. So you did a PhD in Toronto? No, I did a bachelor's in University of Toronto. Wow, but you know everything. It's incredible. Wow. I do my homework."
},
{
"end_time": 7599.582,
"index": 309,
"start_time": 7573.899,
"text": " Also, thank you to our partner, The Economist."
},
{
"end_time": 7616.783,
"index": 310,
"start_time": 7601.834,
"text": " Firstly, thank you for watching, thank you for listening. There's now a website, curtjymungle.org, and that has a mailing list. The reason being that large platforms like YouTube, like Patreon, they can disable you for whatever reason, whenever they like."
},
{
"end_time": 7642.346,
"index": 311,
"start_time": 7617.039,
"text": " That's just part of the terms of service. Now, a direct mailing list ensures that I have an untrammeled communication with you. Plus, soon I'll be releasing a one-page PDF of my top 10 toes. It's not as Quentin Tarantino as it sounds like. Secondly, if you haven't subscribed or clicked that like button, now is the time to do so. Why? Because each subscribe, each like helps YouTube push this content to more people"
},
{
"end_time": 7660.691,
"index": 312,
"start_time": 7642.346,
"text": " like yourself plus it helps out Kurt directly aka me. I also found out last year that external links count plenty toward the algorithm which means that whenever you share on Twitter say on Facebook or even on Reddit etc it shows YouTube hey people are talking about this content outside of YouTube"
},
{
"end_time": 7689.974,
"index": 313,
"start_time": 7660.691,
"text": " which in turn greatly aids the distribution on YouTube. Thirdly, there's a remarkably active Discord and subreddit for theories of everything where people explicate toes, they disagree respectfully about theories and build as a community our own toe. Links to both are in the description. Fourthly, you should know this podcast is on iTunes. It's on Spotify. It's on all of the audio platforms. All you have to do is type in theories of everything and you'll find it. Personally, I gained from rewatching lectures and podcasts."
},
{
"end_time": 7709.872,
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"text": " I also read in the comments"
},
{
"end_time": 7736.169,
"index": 315,
"start_time": 7709.872,
"text": " and donating with whatever you like there's also paypal there's also crypto there's also just joining on youtube again keep in mind it's support from the sponsors and you that allow me to work on toe full time you also get early access to ad free episodes whether it's audio or video it's audio in the case of patreon video in the case of youtube for instance this episode that you're listening to right now was released a few days earlier every dollar helps far more than you think"
},
{
"end_time": 7756.169,
"index": 316,
"start_time": 7736.357,
"text": " Either way, your viewership is generosity enough."
},
{
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"text": " Jokes aside, Verizon has the most ways to save on phones and plans where everyone in the family can choose their own plan and save. So bring in your bill to your local Miami Verizon store today and we'll give you a better deal."
}
]
}No transcript available.