Claudia de Rham: The search for quantum gravity

Gravity is something very fundamental. There is an interrelation between motion and gravity. This is really what Einstein understood. And so intrinsic in the notion of gravity is actually the structure of spacetime itself. Claudia, we've come to this quite mad location today. And it's all because of your book about your lifelong fascination with gravity. What I've been doing throughout my life in different ways is really to challenge it. But for that, first you need to understand it and have fun with it. That looked like it was super, super fun. But did you feel like you kind of got to experience gravity in a new way? Gravity, you don't actually feel because it acts on everything, on everyone in the same way. On every cell of your body in the same way. So your cell of the body don't move apart. You don't feel gravity per se. What you feel is the force of the pressure of the air. It's really remarkable how you can play with these two phenomena. And sometimes they compensate each other. Sometimes you can make them one win, the other one win. But it's really two very different ways of two different forces. But we can go back to the office and I'll show the equations. We've had a little go at virtual skydiving this morning, which I think has woken us both up so that we can talk about some hardcore physics now. I wonder if we can just start a little bit by going back to your earlier life. And tell me how did you first get interested in gravity? And how did you first start kind of prodding it and challenging it? It took some time to realise it was really, the fascination was about gravity. One aspect that really attracted me was the universal nature of some aspects of science. Some aspect of universal language and how you can use science, you can use mathematics, and you can use physics as a way to transcend some sort of miscommunication. It's very pure. There's a stability in it, which I found almost securing. You can always rely on it in a way that is going to be the same everywhere you go. It's going to be the same whoever you talk to. You did some other things as you were growing up. You learned to fly, I think, when you were doing your postdoc. Yeah, exactly. And you also decided to try to train to be an astronaut or be selected for an astronaut. Tell us a bit about that. It's funny to talk about it now and talk about it in the book. It has been a big part of my life for 20 years. Every single thing I would do was with trying to understand how can I go in space. I really wanted to have this embracing of understanding free fall, experiencing it, being part of it. You may think, well, I'm a theoretical physicist now. How can I learn about gravity in the theoretical point of view by flying, which is very more concrete, much more mechanical, and the laws underlying flying are very well known. The challenge is not in understanding how we fly. The challenge is more how to make the use of it. But when you give yourself to gravity completely and you know you can be in situations where you're flying, for instance, you're learning to fly and you're in a spin, and you know you can rely on a very scientific level on some laws that you learned and you can get yourself out of it and your life depends on it, then you can do this. This is simple in some sense. My life on a more intellectual level may depend on it, but not on an everyday level. No, you're not usually depending on it for your life. I made a few typos on purpose there. I like to see if anyone can spot them. But I don't think it's going to change the course of my life if I correct them immediately. That's also the beauty of falling. This is very intrinsic in gravity, this ability to fail and to come up from that and to learn from that. It's part of the scientific research, it's part of our lives. What's funny for me is that it is really part of gravity itself. We know that the laws of general relativity, for instance, that we use to describe gravity, they work with impeccable precision. They are the best laws you can imagine, the best description of reality you can imagine, and yet you know they will fail. I wonder if you can just give us a little crash course in general relativity. If you think of someone moving with respect to you, their speed will be relative to you. If you're at rest and they run past you, you will think that they run at a given speed with respect to you. But if you run along with them, then their speed with respect to you would be null because you're running at the same speed. So this is the relative motion of one person to another. Now, you would imagine the same thing should happen to light. If you think light is coming to you, whether you stay still or whether you're moving with respect to someone else, you would be measuring a different speed of light. That's what you would think. And in the late 19th century, Michelson and Morley set up actually the first interferometer to try to understand how to see if you can measure a different speed of light. And what they saw was no effect at all. The speed of light, as measured by the interferometer, appeared to be exactly the same in all directions, even though the Earth itself was moving, is moving, and even the experiment itself was spinning in some sort of mercury bath. Which is counterintuitive. It's extremely counterintuitive. And so that really tells you that if the speed of light is constant, there must be something we're not accounting for. And what it is we don't account with that picture is, while we consider the notion of space to be different when things are moving, we still assume that for light and for us and for the Earth and for anything moving with respect to us, we assume that we have an absolute notion of time, that we all agree on how time flows. And that would tell us that the speed of light should be different with respect to all different directions. That's our natural intuition. The observation telling us that the speed is the same with respect to everyone, irrespectively of our own motion, forces us to understand that it's actually what has to give way is the perception that time is absolute. Time has to be just as relative as space is. Just like the notion of space is different for different people as they move at different speeds, the notion of the flow of time is different for different people. And that is the basic of special relativity. So that was a huge step forward in understanding. But another thing that you need to put in for gravity is understand how not only objects move at a constant speed, but as they accelerate. And this is also related to the equivalence principle that there is an equivalence between acceleration and gravity. So, for instance, there's no distinction between being pushed on a spacecraft in outer space in what you would consider being the absence of gravity. You would feel the push from the acceleration. And that is the same thing as what we feel from the gravitational pull here on Earth. It's worth saying that there's not really any reason why that should be so, is there? There's not really any kind of reason to expect those things to be... A priori, no, there isn't. The reason why it is so is because gravity is actually something very fundamental, which is ubiquitous almost to space-time itself. There is an interrelation between motion and gravity. This is really what Einstein understood. And so intrinsic in the notion of gravity is actually the structure of space-time itself. And we have experienced part of that earlier today. We got as close as we could, didn't we? We had a lot of fun. One thing we have experienced with gravity is this equivalence principle. And the way we have experienced it is in not feeling anything. So when we were in this virtual skydive, you probably felt a lot of pressure on your belly, right? But did you feel anything on your back? No. I mean, I'm struggling to remember it now. Yeah, I don't think so. There's nothing on your back, because the way gravity acts on you is the same for everything in you, for every single cell of your body, for every single atom in your body, for every single fundamental particle in your body. Gravity acts on you and on me and on everybody in exactly the same way. It's the most inclusive thing you can imagine. I'd love to come on to talk a bit more about your own work. In order to do that, we need to introduce this idea of the graviton, I think. So tell us what a graviton is. I have one of my students made me this. I have a graviton. You made one earlier. Let me introduce you to the little graviton. In reality, they're a bit smaller than that, but that will do. So already in general relativity, as introduced by Einstein, that's a classical theory. But throughout the past century, we know that the fundamental laws of nature are quantum in nature. And since gravity interacts with everything completely in a universal way, if it interacts with quantum things, like light, like electrons, it has to be quantum itself. You can't have a quantum system interacting with something which is fundamentally just classical. And so there's a quantum description of gravity at the fundamental level. It can be described by a fundamental particle, which is a graviton. And even during the very beginning of the universe, we may think that there were some quantum fluctuations that occur, and that may lead actually to some signatures in the polarization of the cosmic microwave background. So I have actually, this is the temperature of the sky. If you subtract some homogeneous temperature throughout the sky, you really have to think at every point here on the sphere is you're looking at the sky in a different direction. So this is the cosmic microwave background, which is really some sort of temperature of the sky in different directions. If you looked at the, so this is, if you sense light from the sky, but light can be polarized. So if you look at, this is not represented here, but there are some experiments and missions are there trying to understand if we can see some patterns in the polarization of light as we receive it from every directions in the sky. And the quantum fluctuations of gravitons in the very beginning of the universe would lead to a particular signature in the polarization of the cosmic microwave background. Observing this particular pattern would signal the very nature of the quantum, the very quantum nature of gravity at the very beginning of the universe and therefore fundamentally. I wanted to talk though about gravitational waves because I always get a bit confused about this because in a sense, naively, you could think of those as being a sort of a way that gravity is transmitted. So what's the difference between a gravitational wave, which we have discovered, and a graviton? Yeah, so I think when we think of graviton, we think more of the particle, the quantum nature of gravity. Maybe to answer this question, it's easier to do the analogy with light. You receive a lot of actually photon or electromagnetic waves from the sun, from the light all around us. Saying that you observe a photon, you really need to be able to see the effect of one particular one of them among the huge lot of coherent wave of photons, of electromagnetic wave. So in the gravitational waves that we have observed, the same thing happens there. It's taken such a huge international endeavor to detect gravitational waves, such huge experiments over so many years with such a level of precision, and yet the gravitational waves you observe, the estimate is that it would roughly contain 10 to the 40 gravitons. So imagine together, working together, this 10 to the 40 gravitons have managed to move mirrors on interferometers located four kilometers apart by a distance which is smaller than the size of a proton. 10 to the 40 gravitons together, working together. So now you're going to say, well, let's not prove that there is a graviton, so you just want to see the effect of one of them. Can you imagine? Difficult. No, it's not only difficult. You can't engineer a thought experiment with an interferometer, no matter how large it would be. You can imagine an interferometer as large as the whole universe, if you got the funding for that. And still, the effect one graviton would have on it would be too small to be probable. It would be below Heisenberg Uncertainty Principle. So it's very challenging. So we're not going to find them that way. Not today. You got interested in this idea of whether the graviton could have mass. So in a sense, it was a question of, you could say, what does gravity weigh? So tell me about how you first got interested in that and what did you find? So I got interested in that in a roundabout way, but perhaps the best way to describe it is that's one of the most natural questions you can ask yourself. So you think of the force, the particle carrier of forces, like the photon carries the electromagnetic force and the W and the Z boson carry the weak force. And for every fundamental force of nature, the strong force, the weak force, the electromagnetic force, there's a particle carrier of that force. And some of them, they are massive. For the photon, it's massless. But for the W and the Z bosons, which carry the weak force, they are massive. Massive in the sense that, not that they're huge, but they have some mass. Yeah, they have some mass. They actually, they have a level of inertia, which means really literally, if you imagine you want to carry a big box, it'll take you more effort and more, it has more inertia. So you're not going to be able to drag it for as long and as far away than if you have something which is lighter. So if you think of a particle which is massless, like the photon, then it has no inertia in the same sense. And you actually can't stop it. It's never at rest. It always, it always splashes, literally, at the speed of light. But if you have a massive particle in the sense that it carries an inertial mass, then it's more happy to stay put and it's going to be harder to move it along, which means that the force associated with it will have a finite range in some sense. It's not going to be, you're not going to be able to spread or to communicate between two things through that force for as long a distance. So that's the case of the weak force. And you may not be familiar with the weak force because it's weak. And the reason it's weak is because the particles that are responsible for it are massive particles in the sense that they carry a very large mass, which weakens that force. So now for gravity, at the fundamental level, you can ask yourself the same question. We haven't observed a graviton, but you can ask yourself, if we did observe it, should we be looking for a massive or a massless graviton? And that has implications on how to look for it, for instance. But also in the way we understand the universe all around us, we see that the universe is expanding, but this expansion is accelerating. And this creates all sorts of challenges in our description of the universe and how to reconcile it with fundamental particles, with the quantum nature of the vacuum energy and all sorts of things that we can go into another day. But one way to think about it or to ask yourself the question is to see whether the way we're describing the universe in such a complicated way, is it because we're using the laws of general relativity beyond the regime of validity? And maybe if the graviton as described by general relativity wasn't massless as it is, but had a small mass, it could be that some of the observation could be more natural. We can reconcile them with some of the facts of nature that we know, for instance, about the effect of vacuum energy and how it affects very large distances. So this is more how I went in myself as a cosmologist, understanding whether we can think of the laws of gravity in a way that reconciles our understanding of the universe. And so my colleagues and I, we were trying to explore how to make sense of a theory of what we call massive gravity. And there's all sorts of different complications associated with that. In the equivalence principle, in the speed of gravity, for instance, you think of the speed of light. Probably we don't spend our time thinking of the speed of gravity, but that is a notion to think about. For instance, the gravitational waves that we observed, they have been propagating at a given speed, very close to the speed of light. But it could be that if you go and look at gravitational waves of a different color, of a much lower frequency, so more towards the red, much, much lower frequency, the effect of the mass would be different, and then they would have a different speed. Yeah, this is, I might be getting this wrong, but this is your idea of a gravitational rainbow? Oh, yes, exactly. Is that what you're getting at? Yeah, so if you think of gravitational waves, they come in with different frequencies, which in some sense you could think of different colors of gravitational waves. Of course, we're not going to observe them with our eyes and see different colors, but the different frequencies of gravitational waves, you can think of them as different colors. And if you have a small modification of gravity, for instance, you have a mass, then you would expect that the lower frequencies, so the colors, if you will, of gravitational waves more towards the red, very, very, very low frequencies, so much lower than the red even, would travel at a speed which could be smaller than the speed of light, smaller than the speed of gravitational waves at a higher frequency. And so you will have this separation in the speed of different frequency of gravitational waves, different colors of gravitational waves. So in some sense... We don't observe them slightly delayed from one another. That's right, that's right, that's right. So it's not going to be something beautiful in the sky unless we recompose it, but it is some sense of gravitational rainbow and how you can use that to say something about either the fundamental nature of gravity or what gravity has traveled through the universe to get to us. Just like if you see a real rainbow of light in the sky, you know that there's a mixture of shine and rain, you know that there must be a bit of humidity in the air, so we can do the same thing for gravitational rainbow, hopefully. We just need to observe them at a much, much lower frequency because the sort of color we observe them in now is still quite high as compared to the scale at which the modification of gravity would kick in. You would expect the modification of gravity as compared to GR to come in at very large distances, as large as the whole observable universe. So you can imagine this is extremely large distances, so very low, very low frequency, much lower than anything you can imagine with light itself. It would be imperceptible with our own eyes for light, but we can, hopefully, with our future detectors see that for gravitational rays. Is that the main way that you think that you could test this theory or are there perhaps easier options? Nothing is going to be easy because it has, this is one of those things in nature, it has a very powerful way to hide itself in some sense. Many of the different features that may arise in a theory of massive gravity are so that you recover general relativity in the limit where the mass is small enough. And since the mass is so small, the distinction is going to be very hard to perceive. But one thing that does happen with a theory of massive gravity, because you can have a mass and because the speed of gravitational waves doesn't need to be, strictly speaking, at the speed of light, it can be slightly different, you have, that leads to slightly different polarization of gravitational waves as well. So in some sense, it's like gravity was allowed to communicate with you and with the rest of us with new channels. There's new channels of communication through gravity. And you can think of this as, you know the gravitational waves, they come to us and they distort the notion of space as they travel through space and time to get to us in a way which, in general relativity, is always along the plane, perpendicular to the propagation. But if the speed was allowed to be slightly different than the speed of light, you could also have fluctuation which is along the line of propagation because they're not forced to travel at the speed of light, you have some fluctuations along that. So it's a slightly different way that gravitational waves could also manifest itself. Those haven't been detected and we have good reason to believe that they're actually quite shy, they would want to hide themselves. But they could, in principle, be there and so that, in principle, could be another way to see it. It's like you see a new animal out there, it's a new form of gravitational waves, a new form of polarisation of gravitational waves. And that can have all sorts of effects, either in the detection of gravitational waves themselves, but in also how gravitational waves are radiated, how a binary system loses energy through radiation of gravitational waves. If you have new channels of radiation, that opens up for new ways it can radiate and more power to be emitted and the force between different objects can be slightly different because of that. Very weakly so, very, very weakly so because the scale of the magnification is ridiculously small, really small. It's always going to be difficult to, this is a challenge, isn't it? It's always going to be difficult. In principle, there's a multitude of different features that emerge and perhaps the best thing is if there's actually a correlation between all of those things. It's not just one thing. Any new feature, you can probably explain it with different ways, maybe even simpler ways. But if you have different features coming up and all coming up around the same scale, then that would be a strong indication. Well, just to round us off, Claudia, you've explored gravity in all these different ways, both some that are more fun, some that are more intense theoretical physics. I'm wondering, what's next for you? What are you working on at the moment? What are you hoping for? What are you hoping to discover about gravity next? I guess what really, at the basis, fascinates me about gravity is that there's always going to be a next thing. We know gravity, as described by general relativity, the fact that we know it fails down, is something very powerful in itself because it gives me, it gives us an indication that we should be looking for something else. It's fascinating because it tells us there's so much more out there to be discovered. We're only ever seeing the tip of the iceberg. The deeper we go, and the deeper we see, we have to go. I think there'll never be an end to that. You may see this as a futile search, but in reality, this is how science works. You embrace this failure, and that's how you can go to the next level in discovering something new. Not me per se, but us as humans. We're going to keep on exploring. I can't tell you what's going to be next, but I can tell you there's going to be something next, and that's really something quite exciting, I think. Well, I think that was a really nice note to end on, Claudia. Thank you for giving us your time today. Thank you for coming virtual skydiving with us. It was fun. Thank you. I appreciate it. Thanks for the experience. It was fun. Not at all. I just wish you all the best with your continued forays into gravity.