Quantum mechanics, people say "Oh, it's our most wonderful theory and most amazing description of the universe." Yes, that's true. But it doesn't give you a description of the universe which involves significant mass displacements. Quantum theory says that any object, even a mountain, can exist in two places simultaneously. However, you never see this. Why? Today, I have a huge treat as this was months in the making: a new experiment which until today was in the pre-publishing phase and is now published. Sir Roger Penrose and Professor Ivette Fuentes are here to talk about the controversial consequences it
has for understanding the relationship between quantum theory, collapse, and gravity. My name's Curt Jaimungal, and on this channel I interview researchers regarding their theories of reality with rigor and technical depth. Today, I finally have permission to show you a conversation that spans how Roger sees mass displacements as causing collapse, The difference between active and passive gravity, and the intricacies of this new Ron Folman T-cubed experiment. We also go over Penrose’s speculation about dark matter and cyclic cosmology. I traveled from Toronto to film live at Oxford University Math Institute. I truly hope you enjoy this. I'm
joined here with Roger Penrose and Ivette Fuentes. Welcome. Thank you all for coming again. Thanks. Great to be here. So there's an experiment that's making the rounds. It's pre-published, not published yet. Maybe as of this recording it's published, but it's called the Ron Folman phase experiment. You both see it as extremely important. However, there's disagreement in the field about its interpretation. So before we get to the importance of the experiment, how about you explain what the experiment is, Ivette, and then we'll hear, Roger, why you think it's so important. Okay, yes. So it's a very
special atom interferometer. So maybe I should start by explaining what an atom interferometer is. Well, in a regular interferometer, you have a Photon and you pass it through what we call a beam splitter, which lets the photon go through one trajectory and then another. It gets reflected by some mirrors and then into another beam splitter, which is a crystal that allows the two paths to interfere. And then at the output of the interferometer, you see interference fringes with a certain contrast and so on. So black and white interference fringes typical from a wave propagation in
the interferometer. So an atom interferometer is kind of the same thing, but instead of using photons, it's using atoms. And one of the things that I find really beautiful about light and matter interactions is that if you have the atoms being interfered, instead of one using like a crystal to interfere photons, you use laser fields—so a light field—to play the role of the beam splitter for the atoms and so on. So in a regular atom interferometer, you would have atoms arriving to the laser that splits the atom. So the atom goes itself in a superposition
of two different trajectories. It gets reflected again; instead of mirrors, they’re fields that reflect it back so That they can interfere again at another laser, and then you can detect the output and so on. So that's kind of one version of this super famous experiment that was done in the early times of quantum mechanics with electrons, right? That the interference fringes of electrons were observed, confirming that they behave quantum mechanically—this particle-wave duality experiment. So that's now almost 100 years since the Nobel Prize was given to Pauli for that and so on. But since then, people
have been doing interference with atoms and even with bigger systems. The record is by Markus Arndt where he can put molecules in the interferometer and see the fringes and the contrast. Now what Ron did was a very special type of interferometer because, well, usually there's two different versions of it. One is the atoms are in free fall. So you take like an atom and you beam split it and you let it fall. But it's in such a way that the atom follows two different trajectories. And then with fields, you make It interfere. And actually, that's
one way in which we measure gravity. That's a gravimeter, because the different sort of trajectories, due to that, the atom picks up information of the local gravitational field. And that's how gravimeters are made. But they're in free fall. Another type of atom interferometer would be in which it's called a guided interferometer, in which you have fields to trap the atoms. They're not in free fall. And then you could separate them and make them interfere again. So what Ron Folman did is that he did a hybrid version of that, in which he takes the atom—so he
starts with a Bose-Einstein condensate made of rubidium-87 atoms, and he cools them down to something like 3 nanokelvin. So this is super cold. It’s as cold as we can get things in the lab in the experiment. And then he makes the atoms go through the beam splitter. But one of the atoms—well, one of the arms of the interferometer—is such that the atoms are at rest. So he basically acts with fields on the atoms such that he cancels gravity and the atom is just levitating there. And in the other arm, he uses again fields to kick
the other branch of the interferometer and let then this atom go in free fall. So this is done with what we call an atom chip. So this is a chip where you can build like a little experiment inbuilt in the chip that produces... You do like wires and they produce magnetic fields. And the whole system is controlled by this chip through the magnetic fields. So you use a magnetic field to make one of the branches levitate and the other one to give a kick to the atom. So the magnetic fields sort of change the internal
states of the atoms to some states that see magnetic fields and others that don't, to produce this sort of hybrid interferometer where one of the atoms is at rest in the lab and the other one is in free fall. And then he interferes them and he sees the fringes. And what he finds in the fringes is that there is an oscillation, which has a very special type of oscillation because, well, oscillations obviously always depend on time, but this type of oscillation depends on the cube of time. Okay. Then maybe Roger can explain the significance of
that. Yes, well, the importance of the experiment, as far as I can make out—I'm not an experimentalist, so I have to judge for what other people say about it—but I gather the idea is, you see, there's a very famous principle called the principle of equivalence. It actually goes back to Galileo, so it's a very ancient principle, which is that you can get rid of gravity by free fall. So Galileo imagined dropping a big rock and a little rock from the Leaning Tower or something like that, and he was well aware that air resistance... if you
drop a feather and a rock, the feather will go slower, but that was because of air resistance. He was perfectly aware of that. But if you could remove the air, then the two would fall together. That is to say, if you were in a falling frame, it would look as though they're simply hovering there and gravity has disappeared. So the principle of equivalence basically is to say that locally, a gravitational field is just like an acceleration. So you can get rid of it by freely falling. Now that's well known for classical physics from Galileo, and
it's a basic principle that when you Go to relativistic situations, looking at fastly moving things like that, then Einstein used this as the basis for his general theory of relativity. So Einstein's general relativity was based on this very principle, principle of equivalence, that you can get rid of a gravitational field by simply, locally, simply falling. Of course, if you want to say that you can get it freely falling in Pisa, for example, but that doesn't get rid of it in New York. So if you want to have a theory which encompasses the principle of equivalence,
that's where Einstein comes in and needs to have curved space-time in order to make sense of this principle of equivalence. But it is the basic principle of Einstein's general theory of relativity. Now, the question is, to what extent is this also consistent with quantum mechanics? And the basic principle is this principle of equivalence. Is the principle of equivalence respected by quantum mechanics? Now, theoretically, you can look at this and see if you have an accelerating frame. Does it look the same as having a force? And it almost does. When I say almost, It's quite surprising
how almost it works, because you can consider two cases. One is that you can consider a quantum mechanical situation where you put a term in the Hamiltonian. That’s a technical term considering you have a force and you describe that force by putting a term in the Hamiltonian. That’s basically what you do. Then you can try it again a different way, which is to consider a freely falling frame and you do freely falling coordinates. And you see, is the answer the same? Well, it's almost the same. When I say almost, it's a bit hard to explain
without knowing a bit about quantum mechanics, that you find that your wave function, the thing that describes the system, differs by a phase factor. That's a number which multiplies your wave function by a number which you can ignore normally because the phase factor doesn't come into calculations of probabilities and things like that. But if you look carefully at the phase factor and you compare it, the free fall case with the gravitational force case, you see that there is a phase factor which is a little bit peculiar because it involves an Exponential of the cube of
the time. $T$ is the time, $T^3$ in that expression. And you might say, well, who cares? Because when you're working out probabilities and things like that, this doesn't enter into it. However, it means you really are looking at something a bit different. The term, to be a little bit technical, it means that your quantum theory is a little bit different. In quantum field theory, you have to start with what you call the vacuum, and you build up your states by putting things into the vacuum. Now, the vacuum state is different in these two cases because
of this T-cubed term. So it's interesting to see whether quantum mechanics really respects the principle of equivalence in this particular way. And to see that, I mean, it's purely theory that you'd expect to see this. Now, does nature really respect this theory? And what's, to me, important about this experiment is you actually see this T-cubed effect in the experiment. So you can look at the phase by comparing one branch with the other, and apparently this experiment seems to see exactly what you would hope to See if the principle of equivalence is respected by quantum mechanics.
And to me this is very important because it’s combining these two great theories of 20th century physics: one general relativity, which is a purely classical theory and it’s based on the principle of equivalence being true—that's the basic principle upon which the whole theory is based. And when you think about quantum mechanics, you want that to fit in with this framework. And we do see that the way in which it has to fit in was with this curious exponential of a T-cubed term. And you see this in the phase which is observed in the experiment. So
it seems to me, although one expects it from theoretical grounds... But it's important to see that it's really true. And it's not a surprising result in the sense that you'd expect the principle of equivalence to hold in quantum systems too, but the fact that it actually does, and it shows up in this peculiar term, is very important. It also has an importance when you go a little further. And this is not something which... See, the T-cubed term apparently has Been seen about 100 years ago in other... I looked up the old papers and I never
could quite see why it's actually doing the same. It's just calculations when you're looking at things in accelerating frames and so on. So you could say, well, it's not that exciting. But that's just the mathematics. We can see you need this term in order to make the acceleration be equivalent to, and the acceleration due to gravity be equivalent to something you can get rid of with free fall. But the importance of the experiments, I think, is just to verify that this is true. But it's also important, if you take this a little further—this is not
part of the experiment, but it's part of what I looked at and what Ivette and I looked at in papers which we looked at, collaborated with later on. And the thing is that if you consider going a little further, by considering some body and its own gravitational field is important. You've got the field of the Earth and you've got some other body which may be putting into a superposition. And you look at it in the field of Earth and then you think, Well, the correct way of looking how gravity is dealt with in quantum mechanics
is you say, well, you can get rid of it locally by the principle of equivalence. Now, if you have a body which is put into a superposition of two different locations, is that still true? And the trouble is you can't really do it with an individual body because the acceleration is different all the way around the body. So you have to sort of look at it a little bit more. But what we do is we say, well, we know that getting rid of the gravitational field by free fall is the correct way of doing it,
but you can't do that with a body in superposition because the free fall is different in different places and all that stuff. So what you've got to do is to try and see, if you do it, thinking of gravity as a force and saying, well, that's not quite the right way of doing it, but think of it as a force and see whether there is an error that you can calculate. And then you work out this error and you see that tells you this thing has a lifetime. And that's important because the lifetime of a
superposition is that something, if you have A body here and here at the same time—quantum mechanics, you can have a body here or a body here, and the state with it being here and here at the same time is part of quantum mechanics. It can be two places at once. That's a well-known puzzle about quantum mechanics. But if you have a big body which gravitates, does that give you problems? And yes, it does. You find it gives you, because of all this business with the T and all that stuff, and you see that it does
give you a problem. And that problem indicates that this superposition maybe is unstable in a sense that it will decay into one or the other in a certain lifetime, which would be very exciting to see. I mean, that's going way beyond what one can do at the moment in experiments. But do we see in experiments—and this is the experiment that Ivette is involved in trying to do—can you see effects that show up, well sort of comparing the gravitational field in effect of the body itself, and does that lead to effects that you can experiment and
observe? I really can't go into it because it's rather technical. In fact, I'm not sure I understand it completely, But this is... Ivette's experiment is to see whether these effects that should come about from this experiment, whether these effects are there. Now, you see, it's important because quantum mechanics, I mean, people say it's a most wonderful theory and most amazing description of the universe. Yes, that's true. It is a wonderful theory, and it gives you an amazing description of the universe, but it doesn't give you a description of the universe which involves significant mass displacements.
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It arrives rolled up in a box, and the setup is just trivial. Unbox it and let it expand. You get a 120-night sleep trial, so you have over three months to make sure that you love it. Plus, there's a limited lifetime warranty with financing options available. If you're looking for a new mattress, head to nolamattress.com/TOE and use code TOE for an extra $50 off. This is an additional discount on top of what's already being offered on their site. So check them out while you can. So you want to have an experiment where you're beginning to
see this effect. And this was telling us where, you see, quantum mechanics has this problem that it gives you ridiculous answers if you think of it for macroscopic bodies. I mean, why couldn't a rock sitting on the table here be in these two places at once? And according to the framework of quantum mechanics, sure, they could be in two places at once, but you never see a rock in two places at once. And that depends Upon going a little further, and you have to say, well, if you consider the gravitational field of the rock, and
can that be consistent with quantum mechanics, you lead into certain problems. And these problems seem to indicate that that lifetime of a rock being in two places at once, that's a finite lifetime. And for an actual rock, it would be a ridiculously tiny fraction of a second. You try and put it in this place and this place at the same time, it wouldn't last, would be absolutely instantaneous, it would become one or the other. And that's what we would experience in actual life. We don't see things in superpositions of two places at once. People often
say, well, the reason you don't is because the environment has got involved in the thing. That's not a real answer. You have to say, why does the environment actually make a difference? And if you go into the calculations, you look at it, no, no, it doesn't answer the question. It’s because the environment is also... it sort of moved around, yes, and the environment can be most of the movement in the system. And that's why the Thing can't be in two places at once, because the environment has been jiggled too much. But there's no experiment yet
that has looked at this effect of why quantum mechanics, if you like, gets into trouble with macroscopic bodies. And when you have mass displacements, well, you see, it's a long way from the experiment that has been done, that Ivette was just describing, the one where you have the bodies and free fall and compare it with the one on the table, so to speak, and you see this T-cube factor. That's just the first hint of what is, why macroscopic bodies don't behave, as you might expect, as a quantum mechanical system. And it’s the first little clue.
That’s why it’s important. And I think that it’s only a little clue, but it’s important as being the first little clue as far as I can see. So it sounds to me like this Ron Folman T-cubed experiment is testing whether the equivalence principle holds in quantum mechanics. Okay, seems like the answer is yes. It seems to me like it would be more interesting if the answer was no, because that would indicate there's Some inconsistency there. And it sounds like what you're saying, Roger, is that Ivette's experiment takes this further to test a collapse model, a
certain type of collapse model, namely yours. Okay, so let me clarify a few things and talk about the experiments that are taking place by other groups, and so to put the whole thing in context. I mean, the reason why Roger says that Ron's experiment is very far away is because effectively the experiment of Ron is with one atom. And well, maybe it's good to also make the distinguish between passive gravity and active gravity that you usually make. And I think this sort of clarifies things. So you have the Earth's active mass, let's say, makes the
atoms fall. And then you would say, okay, but then the Earth has a passive mass, but the Sun has the active mass that makes the Earth move around. So that was given by Newton, this distinction. So what Ron has is that he has the atom as a passive mass in a superposition in the active mass of the active gravitational field of the Earth. But the mass of the System itself is just in superposition in the presence of the field of the Earth. And what Roger's calculations and ideas and so sort of he argued is that
under that circumstance, the equivalence principle and the superposition principle, they're not in conflict with each other. So that's exactly what Ron shows. And what Roger was just explaining now, just put in active, passive terms, is that when you have... Now forget about the Earth. There’s no Earth. It’s just the superposition of the atom with itself. So in that case, it’s the active mass of the atom in a superposition that Roger pointed out that in that case, there is a conflict between the equivalence principle and the superposition principle that should lead to the collapse. Now, why
in the experiment by Ron you cannot measure that is because for one atom, the effect is tiny. So you would have to wait, I think it's $10^{20}$ seconds or something like this to see the effect for one atom. So basically, you can just neglect that and forget about it. But the point is that when you start having heavier systems, then this becomes Relevant. So Roger and I wrote a paper where we were exploring the possibilities of doing an experiment with a Bose-Einstein condensate. And in that paper, we gave numbers. So usually this atom interferometry experiment,
that's a good reference that we were talking about for Ron. The experiment takes milliseconds. That’s how long he can hold this superposition. You would need, let’s say, if Ron could keep a superposition for a second or two, which is still really far away from what experiments are, but not impossible. But in order to see the effects of self-gravity, you would need something like $10^9$ atoms in a superposition. So a mass, for example, could be a lump or a silica bead with the equivalence of, let's say, very roughly speaking, with $10^9$ atoms in it. And that's
very different to the one atom that Ron had in his impressive... I mean, his experiment is impressive, but tests something different. Now, there has been progress in testing the superposition with massive systems. And like I mentioned before, the record is currently by Markus Arndt at the University of Vienna, Where he puts these molecules that have like 2,000 atoms each. Well, but that’s the molecule in superposition with itself. But that’s the comparison with... I think now he managed to do this with an order of magnitude more, but I don't know. I heard, so I don't know
if that’s like published yet or not. But last thing I heard was that he managed to take it one order of magnitude further. So let's say something like $2 \times 10^4$ compared to the... we found $4 \times 10^9$, really far away. Yes. And it's very difficult to take those steps by making things more and more precise. And there's many groups around the world working in that direction. Ron Folman himself, he wants to do the experiment with diamonds, which have a lot more mass. But it's very difficult to put solids in a superposition because they're very
hot. So they can, for example... Well, Markus can cool down the molecules to, I think it's microkelvin temperatures. And those temperatures are still too Hot to see the gravitational effects. But still, there's a lot of activity worldwide because of the importance of this experiment to our understanding of the interplay of quantum mechanics and gravity, that there's many people trying. So they use nanodiamonds, silica beads, silica rods, membranes, cantilevers. There's a number of possibilities. But if you see all of these involved solids, and that's kind of the state of the art and where things are at.
So now a Bose-Einstein condensate, we were talking about that before, you can cool down to half a nanokelvin. So from millikelvin to half a nanokelvin, that's like really long. And then you would say, well, why? No, that was kind of the idea. Why not use a Bose-Einstein condensate? So they have another problem. Maybe before I go into Bose-Einstein condensate, I would also like to mention work from two other people that I am impressed. Sure. And another one is Markus Aspelmeyer, who is also at the University of Vienna. So I spent three years there because of
these amazing people. You know, because I like to propose experiments and so on. So having the opportunity to talk to the two Markuses—Anton Zeilinger is also there. It's just an amazing place to do experiments. Markus Aspelmeyer can cool down a bead to quantum scales. And these beads are pretty big. They have like $10^8$ atoms. So they have kind of getting the mass right, but he hasn't been able to put the bead in a superposition. So he can cool this big system to the ground state, to quantum scales of a harmonic oscillator, but he’s still working
towards doing the superposition. And another person that I would like to mention is Hendrik Ulbricht, who's at the University of Southampton. Okay. And he's done some really beautiful experiments measuring gravity with these nanobits. So these are like the smallest sort of systems that you can still measure Gravity between two different—not just one, it's like two different systems. It's not quantum yet, but it's really like their technology is really getting quite impressive. But still very far away from being able to test if gravity collapses the wave function. So we wrote this paper on what could you
do with a Bose-Einstein condensate because could we use as an advantage that you can cool it down to those temperatures. But now here comes the big problem is that if you take a Bose-Einstein condensate, what you want to do to test gravity is that you want to create a superposition of the atoms left and right. Actually I didn't say what a Bose-Einstein condensate is. Maybe I should start there. So take an atom in a well and cool it down to the ground state. So that you do in second year quantum mechanics is the typical example.
And it's really beautiful because what you see when you cool down the atom to the ground state is that it becomes completely delocalized in the potential. So it's not just like left or right. It's like everywhere. Now take $10^5$. That's like the typical size of a Bose-Einstein condensate for most experiments. $10^5$, $10^6$, rubidium, sodium, something like this, atoms, into the ground state. So the system, because atoms are bosons, the system behaves like a big macroscopic system behaving quantum mechanically. So that's very nice, and people use them for many things. But now the challenge is take
the $10^6$—we need $10^9$ actually—to create a superposition of left and right. And that is very difficult because a Bose-Einstein condensate is not a solid, it’s a fluid. So in particular the atoms are not bounded together. And the moment that you lose one single atom from this superposition—all of them left plus all of them right—the whole thing collapses. So you would do this in what we call a double well potential. So people in the lab have already done these double well potentials. Like, for example, Markus Oberthaler in Heidelberg, several people, but I remember him in particular,
that you can have these atoms in these double wells. But nobody has been able to do a superposition of left plus right in that way, because if you lose one, The whole thing collapses. And actually, they just haven't. I think the record is two atoms or something like this by Chris Westbrook in this situation. So whereas the temperature seemed to be kind of promising, well, then the fact that the atoms, if you lose one, is so fragile, then again made the possibility look very unlikely. But then what people actually do in the lab is that
they don't prepare these states, which are actually called NOON states because our $N$ on the left, nothing on the right, plus nothing on the right and $N$ on the left, so they're called NOON states, is that they've been able to prepare another type of state that are very interesting. So in a double well potential, again, because the atoms are not bounded, you can have the atoms tunnel from one well to the other. Now the Nobel Prize a few days ago on the 7th of October was given to tunneling in a different system, right? But the
fact that atoms can tunnel through a potential. So what you have in a Bose-Einstein condensate is that in a double potential is that the atoms can tunnel From one well to another. And that gives you like a variety of quantum states that you cannot get in a solid because in a solid they're all bounded together so you can only do a superposition here. But in a double well you could do more like a whole family of very rich quantum states. One type are called like two-mode squeezed states. And this is like superpositions of 1, 1,
2, 2, 3, 3, like more sophisticated states. But some of these generalized states have already been produced in the lab with quite a few number of particles. So those are accessible to the experiment. So then you would say, why don't you test gravitational effects, active gravity, in those systems that people have already produced that sort of states? Well, because there was no formalism to study the gravitational self-energy for that kind of states. And actually, well, we tried for a while, but now I have developed like a new formalism that allows you to study self-gravity for
these new states. And then you can use these easier, more accessible states that don't have the super strong requirement that you would have on One hand on solids or on the other hand on the NOON states to test the gravitational effects. And this is the experiment that Roger sometimes mentions. And I think I should say that the experiment... I'm a theoretician, you know, I figured out how to calculate the self-gravity for this state and proposed the experiment. But the experiment is being done by Philippe Bouyer at the University of Amsterdam and Chris Westbrook who’s in
Paris. So the team are us and with the advice always of Roger. And of course, we have students and so on. So it's a very nice team. I love it. I love working with Philippe and Chris and, of course, with Roger. And well, let's see if this alternative route gives us some results hopefully in the near future. Roger, why is this T-cube test, why is it with Ron Folman, why is it causing such a hubbub in the physics community among the people who know about it? I'm a bit confused myself so I don't think I
can answer your question. I think I get the impression that Ron was not quite so... I mean what I regard as important about his Experiment he was not regarding perhaps as the main feature of the experiment. I'm not sure. I don't think so. I think Ron is very much in agreement with the importance. I was a bit puzzled because he was trying to remove the term principle of equivalence from his... he was suggesting... I mean he changed his mind. I said, “That's ridiculous,” you see. So he seemed to have a somewhat different view about the
importance of his experiment. I don't know. Maybe I'm going wrong. No, I don't think so. I do think that he sees it in the same light as you do. I think there's been some confusion about it because the point that Roger makes is a subtle one. It's a subtle point. And I think not always is that maybe people working in atom interferometry and quantum experiments are not that familiar with the subtleties of it. And somehow I think it goes somehow overlooked. That's why I think it's great that Roger explains his point of view and so
on in this alternative way. He wasn't removing the principle of equivalence, but I just thought the fact that he was... puzzled me, that's all. Yeah. It seems to me it's an important experiment and it ought to be... I mean, it's not unexpected in the sense that when you look at the subtleties of the principle of equivalence and connection with quantum mechanics. But that's the case like with every proposal. You propose something, right, that you prove mathematically using the theory. Like, you know, an example that I gave about, I took the theory of Berry phase and
put a vacuum state. And then for me, it was like, well, the fact that this was shown was not a surprise because the mathematics and the theory showed already. I don't know very well why there was a controversy about it. Sometimes there is a controversy on like the assumptions that you might make in a given proposal or in a different result. But in that way, an experiment confirming like a theory sometimes... oh well it was not expected. But it's always like in a way a surprise because the theory can have places where it goes wrong,
right? Like in... well I mean hopefully not in the mathematics but sometimes or more likely I think in the assumptions made. So it’s always for me a big thing when an experiment confirms a piece of theory. But this time, it's been like a long time that the theory is quite well established, and finally the experiment confirms it. Yeah, yeah. The Economist covers math, physics, philosophy, and AI in a manner that shows how different countries perceive developments and how they impact markets. They recently published a piece on China's new neutrino detector. They cover extending life via
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special discount. Head over to economist.com/TOE to subscribe. That's economist.com/TOE for your discount. It seems like we've covered the ground when it comes to the Ron Folman experiment. I'll end with a question that is... I think you all agree or you all disagree but for similar reasons: does the graviton exist? Oh. Gosh. I'd hope so. I'm not sure whether that's relevant to any... that's something... a different question. Yes, yes. We're changing gears. No, well I would certainly think that's... it has to have... There would be such a thing as a graviton, yes. But this is, you
see, gravity in normal experience is so weak. I mean, to try and see quantum effects in gravitation are extremely difficult. As a force it’s very, very weak. Also it’s not really a force even, it behaves differently from other standard forces. So I’m not quite sure what the question is here. I mean, why is gravitons not observable as such? No. What I'm wondering is, in our first conversation, there's plenty of talk about, well, let's not focus so much on quantizing gravity, let's quote unquote “gravitize” or “gravitationalize” the quantum. In that, in quantum gravity, the graviton makes
appearances, but not in all quantum gravities. For instance, loop quantum gravity doesn't have a graviton. We also should have talked about spin networks, actually. That's another conversation. Anyhow, I wanted to know if, because of the way that you view the world, as in gravitizing the quantum, it doesn't seem like there's room for the graviton to exist. But I think that's a bit misleading. That's not my point of view. The trouble... I mean, gravitons should exist. It's just that we're so far away from anything which would see the... I mean, gravitation is such a... you see
it as a macroscopic thing. I mean, the fact that we're sitting down on the Earth here, sitting down on our chairs rather than floating around, is because there's an enormous Earth there. You have to have something that big in order to see the effect. And so if you want to try and do an experiment in a lab, which is looking for the quantum effects of gravitation, it's hugely far off. It doesn't mean I don't think there are such things as gravitons. It just means that the effects of the... the particle effects of gravitation are so
far away from anything one could see in an experiment. It's... it's alright to talk about them, and I do sometimes, but what we see is gravitational fields. And the Fields are... are... But you see, gravity is different in many respects. It doesn't even have an energy momentum tensor in the same way that ordinary things do. I mean you can force it into it but it’s not... I’m not sure. So I’m not answering your question really. You see, I think there's a whole subject which doesn't really exist, and I haven't quite thought of a good name
for it, which is, I would say, “big physics.” But big is the wrong word. I'm trying to think of the right word. Penroseology. No, that's not the right word. No, it's something to do with it being on a huge scale. You see, what is the biggest stuff in the universe, in a sense, including the biggest mass? Well, it's dark matter. All these particles that we talk about, which are so important to our existence and experiments that are done and all that stuff, they're a trivial correction, if you like, to the big stuff in the world.
Now, what's the big stuff? Well the big stuff is dark matter which in my view is a form of gravity and gravitons. Now they’re big stuff. So to treat them quantum mechanically is a way, way, way, way off, you see. I mean it may be that the dark matter particles decay in a way that conceivably could be observed. That would be very exciting if that's the case. But it's a different world almost. I mean, it's not a different world because we're sitting down here in chairs, which we're not floating around because of gravity. But it's
a different world from the particles which behave very quantum mechanically when you go discuss them, but they're a trivial modification to the big stuff which I'm trying to say. I need a better word for it than that. And the big stuff would be gravitation and dark matter, basically, and cosmological constant in some form comes in. And there's a whole world of how to describe all that stuff, and the matter is a sort of perturbation. But it of course very important to us and our lives are dominated by the small stuff. But I don’t know the
right way to talk about that. You could say dominating stuff, no? Well, it's certainly dominated. You see, the dark matter is the main stuff in the solar system, in the galaxy. It's the dark matter. Now, we don't have a proper theory. It's very far from the small stuff which we're made of. And we're a perturbation. If you consider the overall effect of it, we're a little perturbation to what's happening to the big stuff. So it needs a theory of big stuff. And that's really what I'm trying to think about, mainly, at least on the physics
side, because cosmology is very much driven by that kind of thing. Of course, you do have matter playing a role as well, but it's more like a perturbation to what the big stuff is doing. I don't think I'm going to call it big stuff. That's not a very good term. I would need a more... what do we need, something? A grander term. Grand, grand. You could call it the grand stuff. I don't know. It's Not quite soft. Ivette, does the graviton exist? I think so, yes. Well, I think gravity should be quantized. So I would
disagree with my colleague, Jonathan Oppenheim, on that. By the way, I love his work. I am a fan of what he does, although I disagree that gravity is classical, like he proposes. But what I really love about his work is that he comes with his own idea on how would you unify, let's say, a gravitational theory, which is more like stochastic. Who's work are we talking about? Jonathan Oppenheim from UCL. Oh, I see. Yeah, well, I mean, what he's done that I like is that he proposes an experiment. So that's what I admire of his
work. And also that he comes up with his own idea on how to unify gravity and quantum mechanics. But he does it... He says that gravity should not be quantized but It’s more like a stochastic thing. But then he proposes an experiment. And that's why I love his work so much, because that's where I want things to go. That we start being creative, we come up with our ideas, we propose things that can be tested in the experiment. And then since experiments are really getting so, you know... So, I mean, they are amazing and they
can go... well, when people would do entanglement on tabletop experiments. Now Anton Zeilinger has been able to do, you know, demonstrate entanglement across thousands of kilometers using satellites. I mean, you see the progress has been really amazing in many different directions. So I think that we have to make use of quantum technologies and these improvements in order to find ways of testing the theory. And like we were talking before about how some things maybe look impossible, but if you find the right angle and the right way to pose things, then maybe they're more at reach.
I also maybe gave you an example of how to deal with these states. There are kind of the states that already people do in the lab or another example of thinking things in a different perspective, no? So I do think that gravity should be quantized. And I do think that there is a particle, you know, that mediates. I don't know if the graviton is, as in other, you know, candidate theories proposed, but I do think that there might be such a thing, yeah. Something that interests me about you is that you propose interesting experiments. Popper
is often misquoted as saying that if your theory is falsifiable, then it's scientific. It's actually the opposite. If your theory is scientific, then it's falsifiable. It's a necessary condition, but it's not a sufficient one. I could imagine a theory or a theorist who says, “Okay, I've predicted supersymmetry is going to come on at 14 TeV.” And then it doesn't. And then they say, “Okay,” they work away. Then they say, “It's going to come on at 14.5 TeV.” And so they're making predictions and perhaps they're even proposing experiments. And then it doesn't show up. And then
they'll say “15 TeV.” I'm just imagining right now. Yes. If that's a word. I’m imagining right now. Okay so what makes a good experiment? What makes a good theorist who proposes good experiments? Because what I just said I imagine isn’t great science. I can tell when an experiment is going to work or not. I mean, that's business. I mean, sure, it's got to be testable. I mean, a theory which isn't... But you see, it doesn't have to be experiments. When I talk about big stuff, the biggest stuff pretty well that's ever, not the whole universe,
but is these wonderful observations due to Alexia Lopez. Huge rings of galaxies. Absolutely enormous. They're so big that there wasn't enough time in the age of the universe to make them that big. When I say the age of the universe, I'm talking about the normal view about the age of the universe, which is starting with the Big Bang. My view is that there has to have been something prior to the Big Bang which would cause this. Now, this is observational. I mean, they're not experiments in the sense that Ivette's doing. I mean, you've got your
lab and you observe, testing certain things that you can test in a lab. These are out there in the world. And you're observing what's there. And you have to take what's given to you. But some of these effects tell you maybe something different about the universe from what you thought previously. And I think these rings that Alexia Lopez has seen, when she's seen... I mean, the techniques are very important, which she happened to use in order to make these observations. That apparently, I should say, there are several times the diameter of the moon, you see.
You're looking at, if you could actually see these rings, they would make a big thing in the sky. Really enormous. And she's found three of them, apparently. The ones that were in the news more recently. Well, first it was a ring. Actually, she found the arc. There was an arc and then a ring. And then I was in email contact with her, and she said she thinks the Arc is actually another ring. So there are two rings, and I heard recently that she's found a third one. Now these huge things were not predicted by anybody,
not even by me I should say. So although I have a... when I heard about them I thought they were very exciting because they seemed to Confirm the fact that there was something prior to what we believe to be our universe. And they would be something like the collisions between enormous supermassive black holes. You see, you expect to see this in the remote future. You've got galactic clusters of galaxies. And these clusters of galaxies that we see in our universe, they're so big... Well, I should say first that the galactic cluster does not expand with
the universe. They remain bound. So as the universe expands, these clusters remain more or less bound. But then the stars in them gradually get swallowed by black holes and the black holes get bigger and bigger and bigger and gulp down most of the stars. And then occasionally you get a few big black holes which will run into each other sometimes, absolutely enormous black holes, and they will send out a signal of gravitational waves, and these gravitational waves, according to the view I'm trying to promote, will come through from the previous eon into ours and could
well trigger the seed the galaxies which we now see. It would be an enormous effect which would trigger the creation of new galaxies. And these galaxies Could be in the form of what we see as a ring. So I found her observations very exciting, because although I hadn't thought of this as an observational test, thinking about it later, it's a very good indication that there was something before the Big Bang. I'm only saying this really in the context of our conversation. There's a whole area of not so much experiment but observation. I mean, okay, the
telescopes they make and the techniques that I use to see these rings, for instance, there's a new technique where you look at magnesium lines and you're looking at absorption lines in magnesium and that tells you the presence of galaxies. I don't understand it fully, but they tell you the presence of the galaxy. So you don't see these rings. You see them only by the absorption lines and the magnesium. And you look at more distant quasars and the light from then so on so on. But all I mean is there are a lot in the way
of observation. Okay, it’s experiments to some degree because you're maybe sending a satellite out there which can see effects that you wouldn't see just from sitting on the Earth. It strikes me that there's a whole other area of observational physics which tells us something about the structure of the universe and about the contents of the universe. Because these dark... Well, I think it's the dark matter. I have a view, which I'm still not quite formalized, which has to do with how gravitons... You asked me about gravitons. Yes, I do believe that gravitons should be there.
On the whole, when it looks at overall effects, so you don't see individual gravitons, but there's another particle, which would be the dark matter particle, which I refer to as an Erebon. This is, I think, well, I've used this term in papers. I like that name very much. Wait, sorry, repeat that name. Erebon. Well, you see, there's Erebus or Erebus. He was the god of darkness. He's a very ancient, way before... He's not even a god because the gods were more recent, you see. I don't know about... It was a good idea to call the
party. He was a pre-god, I think. Well, he was, I think, What was it? Chaos, yes. You see, chaos was... This isn't physics at all. It's just a nice word. Yes, it's a nice word. So I thought that Erebon was a good term because it is the god of darkness. And he was way there with chaos right at the beginning. As a student, I found how sometimes physicists are very good at finding very beautiful names for horrible to calculate things. I was so much looking forward to learning what's charm and what's strange. It sounds like
wonderful. And then when I actually had to do some calculations, I was like... Yes. Some of the names are dreadful, I think. I mean, dark energy to me is a dreadful name because it's neither dark nor is it energy. It's certainly not energy. It’s the wrong place when you put the energy momentum in business. Energy was a certain spot in that thing and this is not that at all. That’s certainly not energy. It’s not dark, it’s invisible. The dark matter suffers from The same thing, it's not dark, it's invisible. I mean, if you look at
galaxies, you see dark, if you see a galaxy edge on, you see maybe there will be a black line along the middle. Now that's dark. That's dark stuff. But that's not dark matter. The dark matter you can't see at all. It's invisible. That's a quibble. I think dark energy is worse than a quibble. I think that really is a bad name. I just heard somebody on the radio saying, what your wonderful name it was, and he'd only thought of this, and it just fits so well, and all that stuff. I don't know. I wanted to
say something about the experiments. I wanted to mention two experiments. I think the comment I wanted to make is how difficult sometimes it's to rule something out. So, Diósi came up with the idea as well. Roger and Lajos Diósi came up with this idea independently that gravity collapses the wave function, but Diósi took it a step forward and wrote down a stochastic model that predicts more the detail of the collapse, but it does not conserve energy. So one of the predictions of the model is this radiation that Should be observed. And there's been a really
beautiful experiment done underground by Catalina. Now, can you remind me how to pronounce her name? She's done this experiment underground. Well, with a big deal. Oh, this is the, yes, yes, the heating. Yeah. Spontaneous heating. Yes, yes, yes. So she's done an amazing experiment to test not only Diósi's model, but a whole bunch of collapse models. And they haven't seen the signature of these models, right? So up to certain parameters, these models have been ruled out. But, you know, models usually depend on a parameter. So then you could say, well, I mean, you can't really
say, “That's it, the models are dead,” because we don't see, because there could be some other scales. And I so that’s a bit of a difficult thing, but you were asking me like what would be like good proposals or or no, and I think a little bit that Um that at least you can test a big part of the parameter space in a realistic way. But yeah, there you we have to live with the fact that sometimes ruling out things can be very difficult. And I'm just now let me mention one other example because I
find this very relevant to the discussion and also like a beautiful proposal. So Sugato Bose, a colleague of mine, who actually I used to work with him when I was here at Oxford in a junior research fellowship, he proposed an experiment that is to test quantum gravity. So I think he builds on something that Feynman proposed before, but in a time that you didn't have the advances of the quantum technologies that we have now. And then Sugato takes it, and Sugato and colleagues take it further by kind of stating how would you do this test
nowadays, right? So the idea is that you have one particle in a superposition, but not only one, you need a second particle, in another superposition. So you see already the challenge, we've been talking for a long time, of getting one in a superposition, and here you need two in a superposition. But anyway, So this side of the superposition, you bring close to the other side of the superposition. And the idea is that if gravity entangles these particles, then gravity is quantum. Because what the work done by them claims is that you need a quantum mediator
to entangle. Now, I'm not going to comment on that because there are discussions and some people agree on if that's the case or some people disagree on what the case and there's a discussion. I don't want to chip into that discussion. What I want to talk about is the course in relevance to the question that you asked me about the experiments. Also, we were talking about how I put my bar like really high sometimes. That experiment is much more difficult to do than other experiments that I proposed in the past, like the gravitational wave detector
and things like that. And that's where I say like, oh, you know, like I've kind of said I'm maybe not going to like push on this because that is really far away the line. And this is an example of an experiment that is really difficult to do but you have a huge community working on it. And why not? I think they should Be working on it because you find creative ways to overcome hurdles and then you’re successful. Example is LIGO, right? At the beginning, there were so many sources of noise, and then the community comes
together, works together, you come up with new ideas on how to solve some of the problems, and there you go, they detect the gravitational waves. So I'm very supportive of the experiment that they proposed and with the community following it. But it is a difficult one. But let's say you have this situation. Now let's say that Diósi and Roger are right, and gravity collapses the wave function, right? So they're never... that doesn't mean that gravity is not quantum, but in my opinion, but they're never going to be able to test that if gravity collapses the
wave function, because actually it happens at similar scales. So if Roger and Diósi are correct and gravity collapses the wave function, boom, boop, boop, they're going to, you know, collapse that superposition. And then they won’t see the effect proposed by Sugato Bose and others. But that doesn’t mean that what they’re proposing is not there. What it just would Mean is that the scales at which this effect exists are still pushed to scales where it’s even more difficult to see. Right? Because the scales where Sugato says that these things would get entangled are the same as
the ones we would expect to see the collapse of the wave function. So if collapse happens, then you don't get entanglement because the state collapsed. But the state could still be getting entangled before the collapse at other scales that are maybe more difficult to access in the experiment. So you're not ruling out. So that's the thing is that what we're looking for doesn't rule out the other experiment. So I think that's kind of an interesting thing. When is an experiment, when is a theory completely ruled out or not? Thank you both for coming on. No,
thank you very much. It's been a big pleasure talking to you as always, and especially with Roger. No, it's always been great fun. Thank you. Curt here. I’m glad you enjoyed that. I’m inferring that you enjoyed that because you’re continuing to watch all the way up until this point. Now it takes a huge amount of time to prepare for interviews like this. I study the guests' papers. I study adjacent fields. I construct quizzes for myself and then perform those or test myself for weeks prior. I then also talk to the guests' colleagues often so that
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