Into the Impossible With Brian Keating - Did This NEW Theory of Gravity Solve the Expanding Universe? Claudia de Rham [Ep. 419]

Episode Date: May 26, 2024

Join my mailing list https://briankeating.com/list to win a real 4 billion year old meteorite! All .edu emails in the USA 🇺🇸 will WIN! The acceleration of the expansion of the universe is one of the fundamental questions in cosmology.  Scientists believe that dark energy is driving and accelerating the expansion; however, there is a discrepancy between the predicted amount of dark energy in the universe and our theoretical calculations based on the properties of fundamental particles. To get to the bottom of this, today's guest, Professor Claudia de Rham, proposed a new theory of "massive gravity" that could solve this “impossible” riddle.  Claudia de Rham is a Swiss theoretical physicist at Imperial College London. She is known for her work at the intersection of gravity, cosmology, and particle physics. She won the 2020 Blavatnik Young Scientist Award for reviving the theory of massive gravity and has made significant contributions to understanding dark energy, dark matter, and gravity. Her research focuses on modified theories of gravity, including massive gravity and Galileon gravity. Join us as we explore the true nature of gravity and the beauty of falling! Key Takeaways: 00:00:00 Intro 00:01:19 Mass of the graviton  00:08:20 Judging a book by its cover 00:15:20 Could AI experience the joy of falling? 00:20:07 The speed of gravity  00:22:55 How would a breathing mode manifest itself? 00:34:08 Extra dimensions   00:40:00 Do we need a quantum theory of gravity?  00:45:00 Nothing happens at the Planck length 00:50:27 Quantum nature of gravity  00:55:34 Outro — Additional resources: 📝 Get one month of Snipd Premium for free with this link: https://get.snipd.com/Cx7S/brianSnipd Snipd lets you take Smart Notes 🧠 with AI 💡 — it’s my favorite podcast player 😀 ! ➡️ Connect with Claudia de Rham: 💻 LinkedIn: https://www.linkedin.com/in/claudia-de-rham-b2b4b3274/  ➡️ Follow me on your fav platforms: ✖️ Twitter: https://twitter.com/DrBrianKeating  🔔 YouTube: https://www.youtube.com/DrBrianKeating?sub_confirmation=1  📝 Join my mailing list: https://briankeating.com/list  ✍️ Check out my blog: https://briankeating.com/cosmic-musings/  🎙️ Follow my podcast: https://briankeating.com/podcast Into the Impossible with Brian Keating is a podcast dedicated to all those who want to explore the universe within and beyond the known. Make sure to subscribe so you never miss an episode! Learn more about your ad choices. Visit megaphone.fm/adchoices

What if there was an idea, a new theory of gravity that could bridge the gap between our predictions and observations of the most mysterious substance in the universe? Dark energy. Dark energy is a force that scientists believe is causing the expansion of the universe to not only increase, but to do so at an accelerating rate. But there are discrepancies between the predicted amounts of dark energy in the universe and are theoretical calculations based on the properties of the fundamental particles that we know and love. We're trying to get to the bottom of this for years, almost 30 years without success. That is, until today's guest, Professor Claudia Duram proposed a new theory of massive gravity that could explain why the universe is accelerating and also solve the impossible dark energy puzzle. Join Claudia and me as we fall head first into the dark side of the universe

to uncover its most seemingly impossible mysteries and turn the impossible into the inevitable. Let's go. Any sufficiently advanced technology is indistinguishable from magic. Open the pod bay doors, Howard. Claudia, how are you doing today? Well, I'm great. Thank you. How are you, Brian? I'm excellent now that I get a chance to talk to you. We have a lot in common, but I'm going to start first with a rather provocative question, if you'll indulge me.

A lot of this book and a lot of your work corresponds to the desire to find a solution. to the so-called mass of the graviton. I want to start by asking you, why should we care about finding the mass or attributing mass to a particle whose existence we don't even know for sure about? So that's many, many different reasons. The first one I would say, which is going to convince the few people to place the truth, is because we as human care about understanding nature at the most fundamental level. And so we should really always dive deeper and deeper.

That's how we make progress. And sometimes it's really in asking ourselves provocative questions, just like you do, that we make progress. And questions that may not have a specific outcome for tomorrow, but it ultimately has shaped us, shape the future of humanity. That is really the real reason. But then there's many other reasons I can tell you of why I went into that specific. question. First, let me just say that it's about understanding whether a fundamental particle, which we may never detect per se, has a mass, but it has an implication also on gravitational waves, which we do have, we have observed. And actually, from the very first detection of,

direct detection of gravitational waves here on Earth in 2015, the very one thing, the first thing that they were able to do, is put an upper limit on how massive the graviton was, because it has an effect on the propagation of gravitational waves. So now that limit is still quite high as compared to what we ideally would like it to be, for the reasons I'll explain, but still, in principle, it has some real consequences that we can observe here on Earth through gravitational waves and through many different test of gravity, test of the universe, test of why we're here, where we're going, our origin and all of those questions.

Why I am interesting, why I do spend my time looking at that as opposed to all sorts of other fundamental questions is because I think it has a potential to tackle the most challenging question of fundamental physics, the biggest discrepancy of the whole history of science, which is the cosmological constant problem. In other words, why are we living in an universe that looks the way it does, and it's not actually expanding at an accelerated rate which is way faster by at least 56 orders of magnitude, if not 120 orders of magnitude, as should be the case if the quantum fluctuations from all the fields that we know exist, like the electrons and the hicks and all of those fields,

all of those other fundamental particles that made us so really exist, we would expect them to lead to what we call an energy in the vacuum, an energy of nothingness that fills the whole universe itself and should lead to a hugely faster, accelerating rate of expansion of the universe. The discrepancy is simply the biggest discrepancy in the whole history of science, and that is at the interface of the two pillars of modern surgical physics, a modern science, which on one side is gravity, on the other side is quantum field theory,

putting the two together, which is really the foundation of the standard model of particle physics, as we know it today. And so it's very much at the interface between the two of them that we have the biggest discrepancy we can ever imagine. And so it seems natural that the resolution is very much at the interface between the two of them, thinking of gravity, thinking of gravity as well, as a quantum fuel theory and understanding what are the properties of gravity more fundamentally.

So recently the DESE collaboration made an announcement that there seems to be evidence for the variation of the cosmological constant from constant to perhaps a contestant-like behavior. Can you tell me how would that impact the statement you just made? Would it affect the search or the motivation to look for a massive graviton? So it is within the same framework that something has to be slightly different as compared to the standard I'll call Lambda CDM of a vanilla model for the cosmological paradigm, which we would just think until now, everything seemed to fit beautifully with having cold dark matter and a simple cosmological constant, so very natural. in that evolution sense. In a parameter sense, it's very simple in that sense, but actually in understanding how that fits in in the bigger picture

is that's where the real naturalist question is coming. And so if there's any departure from this Lambda CDM, it really opens up the opportunity for there to be something else out there. And any modification of gravity could feed into that, any dynamical dark energy could fit into that. And sometimes there's a little bit of a mentioness on how much you consider modify gravity and dark energy. Some models of modification of gravity can be recaptured in effectively looking for cosmological purposes as a dynamical, cosmological constant, but dynamical dark energy.

So, for instance, if you're in a theory of massive gravity and you want to tackle the cosmological constant problem with that, you would want to accept that the universe is filled with vacuum energy as expected from particle physics, but this huge level of vacuum energy doesn't lead to a huge accelerated rate of the universe today. It did in the past, but then through the weakening of gravity due to the mass of the graviton, through the evolution of the universe, you would have a relaxation of the effect of this vacuum energy on the evolution of the universe. So in some sense, it does look like a dynamical, cosmological constant, and that seems paradoxal, but the source would be constant in some sense. It would be, with a vacuum energy, it would be a cosmological constant,

but then its effect on gravity, its effect on the evolution of the universe would be dynamical. It would be diluted, relaxed away through the evolution of the universe. And so we would expect, indeed, this type of departure from the standard Lambda CDM, the standard cosmological constant. So some of your work in this book, which we will now play my favorite game about called Judging Books by Their Cover, is about a topic near and dear to my heart, the cosmic microarray background, and the subject of polarization. But before we get there, I want us to indulge ourselves in this game that I call judging books by their covers. So, Claudia, please, if you would, take us through the beautiful artwork, the title and the subtitle of this wonderful book. Okay, so the title is The Beauty of Falling and Subtitle, A Life in Pursuit of Gravity. And I'm really happy to guess what I mean by all those things, which is a lot. But I'm very happy with the cover, actually, because it's very symbolic of the story. There is a real story in the book.

So on the cover, you start seeing on the top an astronaut, an astronaut floating in need air, experiencing, some level of free-falling. and some freedom, very much some freedom in that. And then in the middle you see the same person, I think, L.A. each. Falling now, you can imagine she's falling. She's falling on earth. She's really embracing some level of free fall. There's probably some friction from the air because she's falling on earth. But she's enjoying it.

There's a beauty in the fall itself. The fall itself is beautiful. Maybe what comes next thing. is what was a bit more worrying. But luckily for her, the last picture of the bottom, is her diving in the sea and really enjoying this last part of the free fall and seeing how this channel, how this falling has been channeling through a different perspective, now enduring, playing with gravity as she dives in the ocean and enjoying this new environment.

The subtiles is beautiful. And of course, a lot of these things on the cover that does, characters experiencing are relevant to you. The first one being astronaut, which you have attempted to be from the European Space Agency sign. The middle one is you, flying, maybe doing a dive,

but there's an airplane in the back. You and I are both pilots. I don't know how much you fly nowadays, but I have flown a diamond katana before. The catamination. Amazing. Exactly. I know that.

I have a picture of, of it there. I'm not going to tell the screen right now, but it's a love that happening. It just wants to fly. It's amazing. It just pull the nose up and it wants to fly. It's incredible. And then there's a woman ends up underwater wearing a scuba diving suit. And that takes me to my first question from my guest, from a, not a guest, an audience member who's a very special person to both of us. His name is Professor Greg Gabadadzai, who is not only a friend and a brilliant scientist, an innovator, truly just one of my heroes in science. He's also an administrator at NYU, a dean. I don't know what masochistic desire he had to do that. He's from Georgia, not the state, but the country of Georgia.

So, you know, they make tough, tough people there, brilliant and tough. But Greg asked me to ask you when I solicited a question. He said, thanks for reaching out. Claudia wrote a wonderful book with a clear and modern perspective on physics and also connected to remarkable personal experiences. And I would have a fun question for Claudia. I'd ask her if there might be any advantage. to thinking about physics while floating in the cosmos or underwater. And I want to get your answer to that, Gregory's question. In both ways, I think. Being a theoretical physicist and thinking about quite abstract concepts,

I think there is a beauty in doing that when you fly, when you dive, float in the cosmos, in embracing this understanding of nature at a deeper level, there is a deep connection that gets there. But also the other way around, of course, I would like to tell you that I'm an astronaut and I'm doing experiences about gravity in the microgravity on freefall, an orbit around the Earth. It hasn't happened, and I'm not doing that when I'm scuba diving. I'm not doing some explicit state-of-the-art test of gravity when I do that. But I do it with my body, and I've seen there is, as you say, Greg is a very, he's a very tough guy.

And I'm not saying that we should all be very tough, but there is some level in which learning to have this balance between judging your instinct, judging your intuition in very challenging situations, but also seeing how much you can judge intuition and how much you have to rely on your confidence to follow a pattern to follow the logic, to follow scientific roles in situations where actually your instinct,

naively would tell you to do something which would be quite catastrophic. And you're kind of, you know that, for instance, if you're flying and you're stalling, naturally, if you hadn't had any training, the first thing you would try to do is pull the nose up because you're saying you're going down, you're going to pull the nose up so that you pick up altitude. but actually that decreases the lift and so the plane sinks much more and that can actually lead to an accident. So, following too much instinct in these situations can lead to catastrophic outcomes. So if you learn to put yourself in uncomfortable situations and have this balance between knowing what to do, there's a level of instinct that you learn after having practiced it for hundreds of times,

And, but also it's very much based on scientific understanding. You understand what you have to do logically. That makes complete sense. And then you train yourself to learn the process in that. With your body, a situation where your life depends on it, then you can do that on the Black Book. It's so much easier. Hey there. While Dark Energy is one of the most mysterious concepts in the known universe,

another mystery I'm trying to solve is why only 50% of you out there are are subscribed to my YouTube channel or following the podcast on your favorite podcast app. Are you one of them? Please, don't be in the silent majority. Do me a favor. Subscribe, follow, and leave a comment or review. It really helps me out, and I respond to every single one. And when you do it, it helps me get great guests like Claudia, Dan Dennett, Sam Harris.

It's all a numbers game, unfortunately, and we have to make sure the algorithm respects the matter that matters that we discuss on the Into the Impossible podcast. So before we get back to the episode, just take a second and hit that subscribe button, leave a comment or a thumbs up. Anything will help out. Appreciate it. Now back to the episode. And that brings me to my next question, which is related to this gentleman who features very prominently in this book, Albert Einstein. And as you know, I have no one.

Oh, you do? Okay. We can have a dueling one. So I'm going to do something mean, which looks cruel, but actually it was one of his most exciting breakthroughs. In fact, he called this the happiest thought of his life, which was that if he was falling, he would experience no gravitational force. And that led to the Einstein Equivalence principle. And I want to ask this as a first detour into your life as a professor and say that nowadays,

we hear a lot about artificial intelligence and machine learning and all sorts of things. I think they're very powerful. But as a professor, I think about, well, how could these things aid us? How could they be better researchers? could such an entity, artificial made of silicon and glass and steel and whatever else, could it experience the joy of falling? Could it experience this most exhilarating feeling of Albert Einstein? And then if it could experience that viscerally with a body, or if it could not, rather, experience because it doesn't have a body, how could it make a breakthrough equivalent to what Einstein,

the greatest scientist of the last hundred years, perhaps, how could it even be considered to be intelligent if it can't even do what Einstein did. I mean, it's a high bar, but do you think that physicists can be replaced or mimicked even by artificial intelligence? I think there's no replacement. I think we've done so well at using things to our advantage. And there's clear advantages to using national learning. It can speed up a lot of the things.

Now, when it comes to, can it feel for us? That's a deep question. I can't do I really feel something or am I in a matrix and believe that I feel something and how will I know the difference? I don't know. I think related to your question is also how we model the world all around us. If a computer is able to model the world, the universe, and nature all around itself to incredible precision, does it make it that this model is the reality or not? And there's a theme line there sometimes. So your question makes me reminded of that.

If a computer can reproduce all the signs of what it means to feel something, does it make it feel it or not? You think not because we are experiencing something deeper than that, but how can you tell? I don't know. Very philosophical is way beyond. But I do think that there are, it's a difference between speeding things up. and reproducing, relying on past understood patterns

or past patterns that haven't necessarily been understood, but have been identified and seeing them and being able to get some sense and logic and organizing all of that, particularly when it comes to a huge level of data, to speed up some connection between some patterns and some model and some values that you want to. want to take out of these patterns or these data. There's a huge difference between doing that and actually coming up with a breakthrough in

itself, which requires a huge level of creativity. It's like all of a sudden thinking you're going to paint the world, but not with pencils and paint, but using another dimension. And it's something I can't even capture in words. And so if you haven't taught the machine to think in that language, to use tools, that will rely on the extra dimension. Let me just be crazy out there. How could it?

I don't think you will, out of the blue, come up with things we haven't ourselves yet invented with a completely new language. And so that level of creativity, I think, we will always have. And it's a little bit different. I should say that the response to that, some people say that computers through machine learning,

they can come up with new songs, And so that shows some level of creativity. But those new songs are still within the same remit of previous songs, still with musical notes, still within the same sense of that. It will be quite different to all of a sudden think of a new channel of communication, which is not through sound, it's not through light, it's not even through gravitational waves. That would be something yet completely different. And that's what we scientists are very good at to think about new ways to connect with

old. And in a 2020 paper published in PRD, you described the speed of gravity. I always tell my students to their great horror, you know, the sun could disappear right now. And God forbid that happens, but we wouldn't know for eight minutes. But is that really true? I mean, how do we know if that's actually true? So your paper, the speed of gravity with Andrew Talley at Imperial as well, makes a prediction that the speed of gravity could differ from lowercase C. And I'm surprised. Why isn't this made a bigger impact? Why aren't you even more famous than you are?

You need to do that. You can imagine gravitational rainbows where the speed of gravity does depend on the frequency, just like light depends on the frequency. So not in the vacuum, not in empty space per se. for light, we can understand, I can sort of conceive what empty space is, is the absence of interactions between light and anything else. So we are used to the fact that light in space travels at the speed of light, but if you shine light through water, for instance, then light will start interacting slightly differently with a medium, with the water, with the water molecules,

depending on the frequency of light. And so you see that, and because of this interaction, light doesn't travel at the speed of light in the vacuum in water. And the way it travels, the speed of which it travels depends on its color. So we all used to that phenomenon. We all see rainbows, and that's the proof that light doesn't always travel at the speed of light. So you can ask yourself the same question for gravity. And that's, you know, it's quite subtle,

Because now what does it mean in empty space where actually everything connects with gravity? Everything lives in your space time. And if a universe is filled with dark energy, then is there such a thing as empty space? I don't know. And it could be that gravitational is do interact every so slightly with anything living in that space time,

including dark energy. And if that were the case, then it would modify the speed of gravitational work. it would do so at very, very low frequencies. So if one of a sudden you were to take off the sun, that would be kind of a high frequency, a high frequency thing. And so the propagation of this information, you would expect it to take very close to eight minutes to reach to us.

Because at those frequencies, you would expect the speed of gravitation was to be very, very close to that of light. And another thing that would come concomitantly with such a phenomenon would be additional polarization modes in addition to the so-called plus and cross modes that my colleagues and I are trying to study in the cosmic microwave background. Can you explain these types of new polarization states? There's a beautiful illustration in the book about them, the so-called breathing one. That's very evocative. Can you explain what these polarizations would do? and why you manifest, you have to have new polarization states of gravity if gravitational waves,

or you called it glight, glight that will maybe catch on and supplement your salary there. I hope so. I hope you get the trademark on that. But how would they manifest themselves? How would a breathing mode manifest itself? And what would it look like in the power spectrum of the CMB that my colleagues and I in the Simons Foundation have supported to the tune of over $110 million so far? First of all, I think we all sort of accepted that, for instance, light itself comes in two polarizations and you put some very cool sunglasses and then you see maybe one polarization and the other one gets filtered out. And we can ask a question already for gravitational waves. Can you put some cool sunglasses for gravitational waves and all? You see one polarization or the other one.

That would be a bit more challenging. But already, yes, gravitational waves, what I like to call glide, because in some level is quite similar to light, comes already in two different flavors. And that's imposed by symmetry according to Einstein's theory of general relativity. Now, imagine that the gravitational waves weren't traveling at the speed of light, very close to the speed of light, but not necessarily weren't forced anymore to travel at the speed of light.

then you could in principle imagine some sort of analog to sand waves, some longitudinal waves, where you could have some gravitational waves speeding up a little bit and slowing down a little bit. So these would be modes which would not look like the standard gravitational waves, the cross and the plus which are transfers to the line of propagation of the gravitational waves. They would actually be along the line of propagation of the gravitational waves. So that mode, we typically don't like it, but that's a different story, is just to illustrate the fact that you open up our whole set of other possibilities once you align yourself to sink beyond the box,

not that I want to put Einstein in a box, but beyond the theory of Einstein, the original activity. And so in addition to this large genome modes, you also have some modes which are in between, so they are a little bit, they're halved through the transverse direction, and halve through the longitudinal direction. And also related to that, you could have, you can now start imagining quite different things where rather than just have the plus and the cross mode

transfers to the propagation, you could have a breathing modes. Really just think about breathing. And if you think of some beads distributed along a circle, as the mode, that mode goes through, they would sort of become, seem to just as a big, circle and it comes to a smaller circle and bigger circle and smaller circle. There's something quite pacific about that.

I like this picture. It makes me feel meditating as that mode goes through. That mode is actually very important in any theory of gravity beyond an star theory of gravity because that is typically expected to be the smoking gun signature for a theory beyond an instant theory of generality. That mode, the real question is why isn't it there non-star theory of general relativity? You could expect it to be there in principle, and it's not because of symmetry. Anastan theory of general relativity was based on the equivalence principle and on some other

principles that nowadays we understand them as being symmetry driven, and that symmetry simply prevents this formal or breathing mode from appearing. Now, you can think if the graviton had a mass, you can always make that mass sufficiently small so that you don't see any departure. And that is true. That is true in principle. However, because now there are additional polarization for gravitational waves, you can think about having different ways to communicate through gravity.

There's different channels of communication through gravity. and that has a big impact in how we're testing gravity. As you say, that could mean, for instance, that at the very beginning of the universe, you could have production of not only primordial gravitational waves as the one you're thinking about, which would lead to polarizations in the CMB, but you could also think of production

of primordial gravitation waves in this breathing mode. Now, this breathing mode in itself, from a cosmological perspective, it wouldn't be during inflation, it wouldn't necessarily look so dissimilar to the inflaton itself or to other field that are already present and whose effect has already been seen imprinted

in the CMB through the temperature fluctuations. So actually there are models out there of massive gravity for the very beginning of the universe where this additional polarization of gravitation with do act potentially as a substitute for the inflotone. And so the fluctuations in those, those primordial breathing mode polarizations would lead to the temperature fluctuations that we do see in the cosmic microwave background, for instance. So would there be a modification to the BB mode power spectrum, the autocorrelation, or would it induce new forbidden correlations, e-b correlations, would it change the shape of the auto-correlation? Is there some way to detect it,

you know, because these are the only, this is all you have, right? You make the case in the, in the, in the book that beyond the, beyond the, you know, realm of the galaxy or even clusters, there's no hope of discovering physics of gravity at the plank scale other than inflation, if inflation did take place. We don't know that it did. Some of us hope that it did, but we've had that, we've had that dream before. But the question is, are there new observable signatures that we can use to detect the presence or absence of a massive graviton or and the concomitant breathing modes that would also be induced in novel polarization states. Yeah, so that's a great question. One of the thing that would happen is if the mass of the graviton is a little bit too large,

if you imagine you have graviton being too massive, then it would prevent the production of palmaudial rapidational waves whose frequency are comparable, um, as. the mass of the graviton during the period where they should have been produced. So, in effect, what you would expect in the power spectrum of primordial polarized, of perundal gravitational waves, let me say, of remote polarization, you would expect a plateau. You wouldn't expect any production of primordial gravitational ways at very low L, because those would be forbidden by the fact that the graviton, prevents the production of primordial gravitational waves within those scales.

So seeing actually the production of primordial gravitational ways, a C&B polarization, B mode spectrum at very low L, that would be a sign against massive gravity, against a massive graviton or against massive polarizations during the period of inflation. One thing that people don't talk about as much, I mean, people don't talk about this massive gravitons very much either, but comparatively, even if you can imagine, even less attention is paid to the existence of spin three halves particles.

We have a spin zero particles. We have abundance of those. We have a lot of spin one halves. Thank God. Or you and I wouldn't be here talking. We have plenty of light, you know, with an L and no G in front of it. We even have hopes of glite with spin. But the absent, you know, the so-called black sheep, maybe the missing, you know, missing

term, missing spin, statistic term is spin three halves. Why? What did nature do to so alienate one of the most prominent types of fundamental and intrinsic spin modes that could possibly exist? Yeah, it's a good question. I don't know. I don't know. I don't know. I mean, those particles, not at the fundamental level, but at composite level, they do exist. So we know they can be represented to some level in nature. but having them as fundamental particle, we know that then their interactions with other fields is very difficult. And their implementation within a theory of gravity is also very, very challenging. I know that for Spin 2, and it's just the same case for Spin 3-5 as well. The framework in which you construct even the non-inarities,

the interactions for spin up to 1 is relatively, straightforward, I would say, and the connection with gravity as well is relatively straightforward. But when you go beyond that, it becomes extremely complicated. And Annie, you have to be very subtle in how you try to connect those modes with others, how they communicate with others. You very easily open new channels of instabilities. So it's not typically you can just add some level of interactions and you'll care with that. as soon as you open up and up a smallest level of interaction, you need to reinvent the wheel,

if so says, reinvent a whole framework to see how they embed into a deeper one. So that we see from the theoretical perspective how challenging it is. And so maybe because of everything that comes along with it is much harder for them to manifest themselves in nature. Hey there, dark matter is mysterious. But ordinary matter can be had in the form of a meteorite. If you just do me the favor of joining my free Monday Magic mailing list, where I send out the coolest, most interesting information in all the known universe. I don't sell anything, nothing else to do, just share my passion with the cosmos.

And all my guests also subscribe to this newsletter. So you're a great company. You go to Brian Keating.com slash list, and you'll be entered to win an actual piece of space dust, a meteorite from the early solar system. And if you have a dot edu email address, you automatically win. Go to Brianking.com slash list

or Brianking.com slash edu if you have an EDU email address. Now, let's get back to the dark side of the universe with Claudia Duram. Very interesting. I think another question that my listeners and viewers might have is this concept of extra dimensions, which plays a big role in your research. Is there any difference between

when you speak about extra dimensions and when string theorists or fundamental particle physicists speak about it. They both require or mandate the existence of these things which are as yet undetected as you say in the book. So what is it? Are these extra dimensions

related to the extra dimensions of string theory and if not, what are they? The way I think about them doesn't preclude them from coming from string theory. But actually the concept of extra dimension is a very, very old one and within the framework of general relativity, the same modern framework is in which we understand them today, they were already

proposed by Kaluza and Klein, so very, very shortly after the proposal of general relativity. They have very natural extension of general relativity. They have sort of taken a comeback since string theory, because string theory needs them, whereas if you don't think of string theory, you can ask yourself, as to why we live in a world where there is three-dimensional space and one of time. You can ask yourself, why is that the case, but you don't necessarily believe that there needs to be an additional one, extra one. Whereas in string theory, there needs to be extra one.

There's no choice on the matter. And then they are typically expected to be confined around themselves, compactified, very small, very small extra dimensions, or there could be some larger ones, and there we would be confined on a membrane and a brain, deep brain, in those extra dimensions. But a lot of my research in the way I've been thinking about it and looking at the implications for gravity and for cosmology, that was very much some of those ideas were motivated by string theory.

I don't necessarily need to have string theory to think about them, but a lot of them nowadays, they are motivated again for, for one or another. In themselves, actually, very natural. One of the question, I think to me, one of the most fundamental questions there is, is why three dimensions? And you mentioned Robert Brunenberger earlier.

He's an incredible physicist, but he's one of the physicists that has come up with the most ideas on why it is that we are experiencing three dimensions. Even in string theory, we understand there are extra dimensions, but precisely what is the mechanism

that led to us living in a universe that experiences three dimensions is something quite challenging and it's not natural at all in string theory. It's very hard to make that work. There's no natural process.

He and collaborators of him, also Anne Davis and with Dura have been people that have been exploring the idea of how dynamically you can have a sort of a selection mechanism that either picks out membranes in extra dimensions which are three plus one dimensional. It's almost an evolution selection mechanism where those are the ones we end up living in or some other types of mechanisms in why the other dimension would be confined, compactified,

to be very small and we only experience the ones we do. But even in both case scenarios assumes the existence of other dimensions are there, and that could have consequences on us. To see the consequences of that, typically you expect you have to go to very high energy scale to be able to probe those extra dimensions, and that corresponds to going, again, back to the very early universe, again, to the type of research that you're doing. And it is possible, and I would hope very much so, that it could be imprints on the very early universe of those extra dimensions. And that could be imprints on the power spectrum of the CMB and B mode polarization, possibly.

That's for you to tell me, actually. They can be in some other models of extra dimensions. They can be some effect, actually, not at very high energy, but at very low energy, that things will look very similar to a four-dimensional three-plus-weensual. one, and a four-dimensional, I mean three of space and one of time, a dimension for everyday life. But as you go to larger and larger distances, some cosmological distances, then you're able to start probing the extra dimension. And that's why the universe, the evolution of the universe nowadays seems kind of counterintuitive. And that's because we're starting possibly to probe

the extra dimension, the extra dimensional space. So it's sort of a relic, or a shadow of the higher dimensions that we perceive it. Yeah, yeah, yeah. Yeah, there's a lot of models out there where our cosmological evolution, what we live as a universe, is really, as you say,

a remnant or a shadow, a consequence of our motion through time, yes, but really through the extra-dimensional space. We are traveling through this extra-dimensional space and that has an effect on our universe as a whole with just a consequence of that. Question I have for you, perhaps, to elucidate, and I've asked it to many scientists, but I never really get an answer that I can try.

And I've asked even, you know, Roger Penrose and there's multiple occasions of being on the podcast. And then you hope I have on that. Yeah. I hear what he says, but I don't know if I understand what he says, although that might be due to my failure to, you know, have consciousness, according to him. But my question for you is, first, do we need a quantum theory of gravity? I mean, the only two scenarios where quantum gravity seems to pertain in any kind of important, impactful, relevant role is not in our daily lives, but are at the singularities of black holes. And perhaps the origin of time, if indeed time had an origin, and many people don't believe that it did or it was a classical bounce or different cosmological models, past guest, Paul Steinhart, Anna EGis, Neil Turak, many others, don't believe in the singular quantum gravity. necessity. So why do we need a theory of quantum gravity? If these only two realms that we know about

are shielded by event horizons, what's the point? We'll never be able to witness this anyway. So actually, I am working on some understanding how to make that connection. So you probably want concretely to understand how to compute those things in a situation that we will likely never experience ourselves. That is true. But at the same time, we want to understand why we live in the world we are living in. I think we all ask ourselves those questions. It's all part of the origin, why we're here, why is life here, why universe is here? But even more fundamentally, why is a representation of nature, a representation of space time here?

And that the answer is, I think, ceded in a theory, a more fundamental theory, a more fundamental theory, a more fundamental theory of something that, unifies, again, not only the fundamental forces of nature, as we know them, with weak and strong forces and natural monotism, but also gravity and this unification could have some consequences of what is our deep down, our origin, in terms of what is the origin of space time itself? I think we talked about that at the beginning already. Why are we pursuing some such fundamental questions? will never be able to really experience the consequences of that. And history has told us that often we think that would be the case. We saw Einstein thought that gravitational ways will never have any consequences to have no, no connections to reality. And we do, we do use them now to communicate in some sense.

Maybe communicates a big word because that incinerates a two-way communication, which is not the case so far. But we do communicate with them with the universe. We use them. We use them as a new window as a new way to see our universe, see gravitational waves. That's an example where 100 years ago we thought there's never any use to it. And yet we're here.

They are real. We have detected them. So the fact that I can't tell you right now whether we'll ever be able to experience a situation where quantum gravity is needed to predict what happens precisely. Doesn't mean that there wouldn't be one, but who is to answer that? If we don't know what it is, we'll never be able to even answer that.

I think we need to explore beyond what we know to be able to see how much more there is out there and make sense of all of those things. Let me go back, though, to a point because we do say, for instance, I'm not a string theorist, so I can say that. I'm not a string theorist. I'm not.

Nobody's perfect, Claudia. Nobody's perfect. I'm not a straight theorist. I'm quite agnostic on whatever is going out at very high energy. And some of my colleague would not talk to me anymore for saying that. By the way, I have to interrupt you. And pardon my interruption. But part of the mission of this podcast is to educate young scientists and how to be a good scientist.

And I just want to highlight what Claudia just said, that it's her job and my job as an experimentalist, her job as a theorist to be agnostic. I was asked by the New York Times recently, you know, what do I hope for? What do I really want to find with the Simon's Observatory? And I said, you know, our job is not to prove things. My job is to prove people like you wrong. And your job is to keep refining what you know about the universe so that eventually I can't prove you wrong, but that won't prove you right. And so what Claudia just did is incredibly important, right? That's right. That's right. It's not about, it's never about proving someone right or wrong in some sense. It's more about deepening our understanding of nature.

That's where we're going. And Annie, this is why the book is called The Beauty of Falling, because I will fall every day. I will come up with things which are wrong every day. I will fall. And that is important. This is the way we make progress. Right.

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GoogleFi Wireless is not subject to data traffic deprioritization during times of high network usage. In fact, past these pictures that that is part of the beauty is that not that just that you fall and that, you know, that can be, you know, awesome. But part of the journey is how you get back up, right, and how you respond to these challenges. And I think, again, just from my young students and postdocs and young faculty and old faculty who might need to learn this lesson, but the concept of quantum gravity is a beautiful one because we have this innate need to quantize everything we see. It's done so well. But the question I have even goes, you know, you talk about pie in the sky and it reminded me of this past guest Elon Musk, who I did talk to on the podcast, you know, tangentially, you can find it on the videos and audio. But, but he once said,

you know, the, the plank, you know, length sets this fundamental limit beyond which we don't even need to know the digits of pie. In other words, there's some minimum radius, you know, circle, which would be the plank length. And I, I responded to him. He didn't report right back, but, but that's sort of nonsensical, right? Nothing really happens at the plank length, right? Am I wrong about that? It is a, it is a correspondence, a new. a logical coincidence, but nothing more. Or am I wrong? So what we know is that there comes a point, it's not about a distance. It's never was about a distance in itself. It's about when the curvature, some measurable effect of the curvature

becomes big enough. And that is not so much about what distance you can measure, but for instance, going within regions of space time, where you end up having, for instance, close to a black hole or at the Big Bang, as you were mentioning, that it will seem to suggest that the curvature there becomes very large. So that are physical region where these things can happen. But understanding how we transition from a semi-classical model of gravity through general relativity, which is embedded in an effective quantum field theory,

how we transition between that and something which is, takes over when this effective quantum fuel theory breaks down, I think it is important to make communication between the two. And a lot of the work I do at the moment, which is not a massive gravity, is to understand what are the key ingredient that we have to keep beyond our understanding of reality, beyond general activity. You don't even think of those concepts anymore. You don't think of space and time anymore.

you think of probabilities and you think of the notion of causality is extremely important. And it's embedded not so much into return to the future kind of cartoon line. But it's encoded in deep statements in how we can think of probabilities and how we can think of also the notion of probabilities in a quantum sense, whether they respect some notion of unitarity, where if you sum up the probabilities of a given outcome, whether they will sum up to one, or whether they sum up to something imaginary or negative,

which would make no sense. And so these are very, very basic criteria that are satisfied by string theory, for instance. And you like to ask yourself, without necessarily even relying on string theory, are those concepts even correct? and is it okay for me to rely on them when all now the notions of reality have fallen apart,

is it okay for me to rely on those specific, just hang on onto them to see how this very basic principle cascade their way down to something concrete at low energy and something which is consistent with what we observe. And we can actually make connections. We can actually come up. I can give you a model today. That seems completely meaningful.

I can come up with a model that can describe most of, if not everything that I observed today. And then I know that if I were to go back up and make a connection with how it will be realized at very much more fundamentally, it will have to break some of those assumptions. You will have to break some of this assumption. or some of the way I think about these much more basic amplitudes. And so I think it is important in some sense to see what is it we relying on at the end of the day? What are we aiming for? And we know that the way we're going to end up describing things nature,

once we no longer can rely on even the notion of space and time to describe them, it's going to be very challenging. We're going to need to invent new tools, new way to thinking, to represent them. And I don't even know precisely what is the right question for me to ask, to even be able to express that. But I want to hang on on very basic principles. And I think it is a meaningful question for me to ask myself,

are those okay to rely on? Because if they're not, then I have to give up even more than that. Yeah, and I guess the question I keep coming back to is what would constitute a detection of a quantum nature of gravity and how could we do it in the case we have to be open to the prospect at the Simon's Observatory

and every other observatory may fail to detect gravitational waves in primordial nature but that doesn't mean we couldn't rule out quantum gravity and you and I go back in another way in addition to Robert Brandenberger in addition to being pilots in addition to I'm not interested in scuba diving but we were both at Case Western

where the very first Michelson interferometer was done. And I understand from your book that, you know, Michelson got into a little bit of a palaver, as you Brits might say, you're not British. But the point being, they used the Michelson Interferometer, and now we have these LIGO machines. And my friend Eric Weinstein, you know, came up with a beautiful thought experiment that I want to run by you as we wrap up.

Imagine the double slit experiment. The double slit experiment is the most quantum of all quantum mechanical experiments in terms of manifesting truly, irreproducible quantum behavior. Eric asked the question to me, and I couldn't answer it, so I'll ask it to you. Hopefully you can answer it. But if you had a double slit experiment, you know, the electron goes through, you know, these, goes through both slits, the wave function goes through both slits in some sense.

But in a classical sense, could you not use the gravitational effects of the electron, just as a thought experiment? And could you not probe the, use the gravitational waves produced by this electron as it flies through, you know, presumably one of the two slits, at least in Einstein's conception. So could you use these interferometers, these Michelson interferometers of LIGO, just as a thought experiment, to reveal which segment of the, or which slit the electron went through and therefore, you know, kind of simultaneously be able to localize its position and momentum in a sense. But from the gravitation waves produced by the electron itself?

Yeah, you use the gravitational waves to then measure the, which slit they'll electron went through. Yeah. So you need to look at what is the magnitude of a gravitational weight the electron would produce. I'm not saying you could do it. Yeah, that was my thought too. And that is very weak. That is going to be very weak in amplitude. And the issue with that is that if you have a very weak gravitational wave that goes through, then in the case of the interthermators here on Earth, then they will move the position of the mirrors, which will be below Heisenberg-unergedy principle. Now, I am a theorist, so I have no problem saying and applying for funding to say, let's build an interthermiter, which is of the size of the animals, let's do that.

But even if you get the funding, you know, from the king. You write the proposal then. So let's do that. I'll talk to Greg. You help me sweet talk Greg. Yeah, okay. I'm going to be blacklisted, not. That's right.

The issue is if you think about the gravitational wave produced by an electron in that process, it's way too weak to being able to detect its effect. But there are some experiments out there where I think one of the experiments you should try to think about is having electrons absorbing a gravitational wave and a quantum of gravitational waves. So absorbing a graviton. That's really what you want to do to test the quantum nature of gravity in a way which would be very similar to that applied. We know that electrons can absorb or emit unique quanta, discrete quanta of light or photons. And as they do that as a change in energy level around the atoms.

So you want to do the same thing with gravitons. And in principle, that would happen. The probability would be very, very small. But in principle, you can imagine that would happen. The issue is always the reality of the world in which we live in. It's just so sad because it's, it's a very small. because it's not the process that is quite so, I mean, it is challenging itself. It's not just that.

It's the fact that there's a lot of noise out there. There's a lot of other things that would lead to electrons changing energy levels, for instance. And so if you want to shield your whole experiment from light and neutrinos and all sorts of other cosmic rays, that would have the same effect on your electrons. You're in trouble. that it's the reality of life in which we live in. If we were living in a much simpler universe where there was nothing other than electrons and gravitons and there was no cosmic rays and even light,

then it may have been easier to obstruct yourself from any other noise and prove maybe the quantum nature of gravity. Well, I think that would be a wonderful place to wrap up. there's so many interesting things we could explore in this wonderful new book. We'll have to do it perhaps in person someday. The beauty of falling is a beautiful book, a life in pursuit of gravity. And it's endorsed by some of the past guest on the podcast, including Sarah Seeger and Sean Carroll. Both of them give it their most high encomium, as do I. Claudia Durham, I hope you have a great deal of success with this wonderful book. And I think your students are lucky to have you and your colleagues are lucky to have you. And it's wonderful to have an iconoclastic thinker that can operate as you do in the mainstream. You could talk to string theorists. You can talk to people that study loop quantum gravity or causal set theory.

And you're not afraid to kind of push back respectfully as I think is so refreshing. So this book is a wonderful addition. It's incredibly readable. I listened to it. Princeton University sent me the audio copy as well, which is not read by you, but is a delightful voice, whoever the narrator is. So I cannot recommend this highly enough. Thank you. Thank you, Brian. It was so much fun to talk to you. Thanks. Thanks. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank.