Daniel and Kelly’s Extraordinary Universe - What is post-quantum gravity?

Episode Date: January 16, 2025

Daniel and Kelly talk to Jonathan Oppenheim about his unusual theory of unpredictable gravitySee omnystudio.com/listener for privacy information.

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Physics, general relativity that describes space and time and how stuff moves and quantum mechanics that describes probabilities and how tiny particles behave when you're not looking. Bringing these two ideas together into one harmonious explanation of everything has been a real puzzle that has stumped some of the greatest thinkers in history. We have some famous ideas that try to tackle it, string theory, loop quantum gravity. Then there's some fringe ideas that very few people take very seriously from Wolfram. or Weinstein. But what if everybody's taking the wrong approach? What if instead of attacking the problem, the right strategy is to actually sidestep it and reframe the question. That's what

we'll do today when we talk about post-quantum gravity. Welcome to Daniel and Kelly's extraordinary universe. Hello, I'm Kelly Wiener-Smith. I'm a parasitologist, and today we are going way out of my comfort zone to discuss post-quantum gravity. Hi, I'm Daniel. I'm a particle physicist, and I don't even understand pre-quantum gravity, not to mention post-quantam gravity. You know, Daniel, one thing I appreciate about my field of study that I don't think I really appreciated enough before this conversation that we had is how great it is that I know where a fish are and I can count them. that's because you've never met a post quantum fish yet kelly they're very unpredictable uh you know

no i think that the fish are going to be way more predictable and zach always likes to joke that kelly's job is to figure out what the fish are up to and i think that's way easier than figuring out what the electrons are up to so what kind of personality type do you think ends up in both of these fields yeah that's a great question you know i think people who like to deal with the tiniest bits of the universe or people have to give up the concreteness of being able to see what you're working on, you know, being able to like look at your experiment and say, oh, I see what it's doing or I can take pictures of it at least. So this is like leap into the abstract realm where you just have this like mathematical scaffolding. You have to just sort of trust. And you got to be sort

into that, you know, mental puzzles and mathematical mazes. But it's often unsatisfactory, right? Like we build these huge machines. We collide these particles. We can't even really look at what's happening at their core. So I agree with you and I disagree. So like I study fish, but what I'm really interested are the parasites that are inside of them that I can't see and that I have to like try to study in indirect ways. Maybe I split the difference. Still, I guess at the end of the day, I humanely euthanize those fish and open them up to see

the parasites, which is much easier than seeing the particles. I have this fantasy sometimes about solving most of science just by having like perfect visualization. Like, if you could watch anything, anywhere in the universe at any time, you could, like, zoom in arbitrarily and just look at stuff, you would understand what happens. Like, how do viruses kill bacteria? We'll just watch it, you know? But most of our science is so limited by not having the instrumentation, but not being able to see what we want to see and having to have all these indirect probes, growing bacteria, seeing if they die, speculating about the mechanisms. It'd be amazing to be able to just, like, x-ray the whole universe and know what's happening. Because then the explanations, I feel like they would just fall out.

Well, so to be the wet blanket we've all come to know and love. Hopefully, no and love. When we were doing the human genome project, we felt like as soon as we could, like, get down to that level, suddenly we'd be able to like solve all cancer. And now we're working on the human connectome project where we're going to like know about all the connections in our brains, like all of our neurons connect. But I don't feel like immediately being able to see things at a higher level gives us the answer. And I guess you're saying once we can see everything,

I still feel like there would be decades of us trying to be like, well, what the heck does it all mean? I just need one more level of Zoom, bro. Just one more level of Zoom and I'll figure it all out. Well, the biologists have been saying that for a long time. And so far, we're discovering it just makes things more complicated. But I hope you're right. All right. Well, a big group of physicists have been zooming in on the fundamental nature of the universe,

trying to understand how it all works. What is the universe really made out of at the smallest scale? What is the bedrock foundation of the universe itself? and they've all run into a problem trying to understand how gravity and quantum mechanics come together. So we've had a few episodes about various solutions to one of the biggest puzzles in modern physics, quantum gravity. And today we have another episode talking about the theory of post quantum gravity, which is an intriguingly compact name for a theory. Yeah. And I've really been enjoying looking at this problem of quantum mechanics and general relativity and how we make them work together from a couple different angles. and, you know, it's exciting to get to hear yet another angle today.

Soon we'll be interviewing fish about it, right, to see what is the fish perspective on quantum gravity. I mean, I hope. But, you know, as an experimentalist, I love that at the end, there are actually experiments that can be tested to figure out if this one is correct or not. So everyone's just going to have to, you know, hold their breath. That's right. And so at the beginning, as usual, we ask people what they think about post-quant gravity or if they even know what it is. Thank you very much to everybody who sends in their thoughts. If you'd like to be part of this crew, just email us to questions at danielandkelly.org. You can play along at home. It's very easy and fun. So think about it for a minute.

What do you think the theory of post-quantum gravity is? Here's what our listeners had to say. Post-quantum gravity, I would guess, is the products of finding out theory that explains quantum gravity, so what the benefits of such a theory would be? The name suggests that there is two types of gravity, one that is before quantum effects and one that is after quantum effects and post-quantum gravity would be that it only appears if quantum effects are done with their work. I would guess that's the gravity that happened after the Big Bang turned everything from actual energy into matter. When you spill your cereal in the morning for your coffee.

The reality inside a black hole where the quantum realm is affected by gravity at its most extreme. Gravity that happens when particles drink their milk and grow up from being quantum particles to regular-sized particles. And then they have regular-sized gravity. Pop quiz. Trisha, what is post-quantum gravity? Gravity after children. I think that's postpartum. Post-quant gravity is like post-punk rock.

The notion that the idea is not working, we should just move on. I thought that gravity was basically irrelevant at the quantum level, so I can't say I have any idea. I would say the new physics that we find after we find quantum. gravity. Post quantum gravity is the conceptualization of what we think of is gravity taking into account that neither general relativity nor quantum mechanics completely explains the phenomenon that we experience. Maybe post quantum gravity is the state of society after we figure out what quantum gravity

is and we're bored because we don't have any more problems to solve. David Letterman's theory of gravity. you start looking for when you give up on the idea of quantum gravity? I believe it is a term that refers to the effects of gravity beyond this scale of particles. Maybe a theory that tries to explain gravity in an even smaller scale. Post-quantum gravity is a theory of gravity that finally reveals that gravity actually does not obey quantum rules. I think post-quantum gravity might be the effects of gravity at quantum scales down at the

particle level, where it's got to fight it out with all the other forces down there. All right, Kelly, do these responses align with your thoughts of post-quantum gravity? That we were all apparently comparably confused, yes. I feel like we have a particularly intelligent audience. It made me feel better that I did not know what post-quant gravity. was when I heard these answers. So go team. Well, it's sort of a niche idea in a niche field of quantum gravity,

but it's one that a listener wrote into me and said, hey, can you explain what this is? I was trying to read an article about it, and I couldn't understand it. So as an invitation to everybody out there, if you're reading about some niche theory and physics and it doesn't make sense to you, please write to us.

We will break it down for you. And we're lucky enough on this episode to have as a guest, the guy who invented post quantum gravity and does a pretty solid job of explaining it. Totally solid. Yes, it's so nice when you can have the experts come in to tell you what's up. And he's got a beautiful voice as well. So here's our interview with Jonathan Oppenheim. So it's my pleasure to welcome to the podcast, Professor Jonathan Oppenheim, a physics professor at University College London and proponent of the theory of post quantum gravity. Jonathan, thank you very much for talking to us.

All right. Thanks for having me. So first, I'd like to set the stage and understand what is it we are trying to solve? Why is everybody after quantum gravity? Why can't we just have general relativity to describe big stuff and quantum mechanics to describe small stuff and be happy with that? Why do we need one unified theory of the universe? Well, the two theories are frameworks, general relativity, and quantum mechanics, they're inconsistent. So they can't really live together in a mathematically consistent way. And so we know that either one of them is wrong or both of them are wrong.

And so we know that they cannot be a correct description of nature. What if they always describe different domains? Do they need to give us a single unified sort of conceptual understanding of the universe? Or do they actually disagree about what happens in the universe? Do they conflict in terms of their predictions? Yeah, they conflict. I mean, the regimes that we're used to, it doesn't matter so much. But now that we're exploring the quantum realm and we're exploring the quantum realm when it comes to larger and larger particles, then it does start to matter.

If you think of like a very small gold atom, which is put in a superposition of two places at once, which is something we can do with gold atoms, then gold atoms gravitate in a very small way, but we don't have a theory which would describe that. And maybe that's not a realm that we care that much about, but we know from the history of physics that when we have these inconsistencies and contradictions in our theory, that once we start to try and fix them, everything just starts to unravel. I mean, if you look at the history of quantum mechanics, there was only one or two little places where things seemed strange. and once we started really looking at how we could reconcile and explain certain phenomena back in the beginning of the 1900s, we realized that all of our laws of physics were wrong and there was the whole quantum revolution. So we know from history that when there are these contradictions, they usually spell the beginning of a new era of physics.

Could you, for the biologist, dig in a little bit more to the gold superposition atoms? Yeah. So quantum mechanics is strange. And in the world that we inhibit, we think of things as being in definite places. So a coffee mug is in one place or it's in another place. But as you go to smaller and smaller particles in smaller and smaller systems, then things behave very differently. So gold atom, which is not like a coffee cup,

we say that it's in a superposition of being in many places at the same time. So in some sense, it doesn't have a definite position, it doesn't have a definite velocity. As we get smaller and smaller, things just behave. very weirdly. And that's what quantum mechanics is. And so quantum mechanics can explain the weirdness of what the gold atom is doing, but general relativity cannot. Is that right? That's right. That's right. I mean, general relativity is our theory of gravity and it describes planets and the solar system and the way the universe evolves and the big bang. And so we used to explain those very big

things, but what contradicts what's happening on the small scale. So we have this theory of the very large general relativity, which requires us to know where things are and when they're there in order to describe how space is bending and how things gravitate. But then we have quantum mechanics, which says things can have an uncertainty in their location, and these two things are fundamentally in contradiction. That's a good way of putting it. And maybe just to say that you mentioned space time, and the thing to say maybe is that our theory of gravity, what Einstein, one of his massive contributions to science, is he taught us that gravity is really space, time bending. So we know that large objects bend space time. That's what gravity is and very

small objects because they don't have a definite position, as you said, we don't know how space time should bend because space time doesn't even know where they are. So that's why you get these contradictions. And you have to cook up this example of a gold atom because typically general relativity applies to the very large things, planets and galaxies and quantum mechanics to the very small so you need something sort of on the edge there where it's large enough where we can measure its gravity but it's small enough that quantum mechanical effects are still relevant is that why you come up with this idea of a gold atom and why gold in particular would it work also for a lead atom? I mean I love gold because it's shining but it's actually because gold is very dense

and so it turns out that you run into trouble actually with very dense objects rather than very heavy objects that's one of the things we found out but you know it's true that we generally think of the gravitational field as being caused by the moon and the earth and things like that, but one kilogram mass will bend. We can feel that gravitational pull of a one kilogram mass. And now we're doing experiments where we can feel the pull of gold, which is, say, a millimeter sphere. We can feel the gravitational pull of that kind of an object. But when they get much smaller than that, then we don't know what happens. And we haven't even been able to really perform the experiments that will tell us what happens at that scale. Right. So we need some sort of

unified picture of gravity and quantum mechanics. Why don't we just do that? I mean, we did it for electromagnetism. We had classical theory of electromagnetism, and then folks quantized it and gave us a theory of quantum electrodynamics. Why can't we also just do that for gravity? Take space time, consider that a field, quantize it, Bob's your uncle. Yeah. I mean, that's where everyone has thought, and that's what we've been thinking for the last 100 years, which is probably the amount of time that we've been failing to quantize gravity. So yeah, you have this contradiction. You have gravity, which is not quantum. You want to make a quantum to fit in with everything else. That would be, I think, the thing that everyone thought, that's what we should be doing. But we're finding that really

troublesome. And there's reasons for that. And one of the, I think, big reasons is that gravity is really different from the other forces. So we're used to electromagnetism, which is what keeps my two fingers from being able to push through each other. They're repulsed by the electromagnetic force. gravity just behaves very differently and what Einstein taught us about gravity is that unlike the other forces gravity is space-time bending so the reason that the earth

goes around the sun is that the sun causes space-time to bend in just such a way that the earth instead of rolling past it orbits around it so it's slightly weird than that actually I mean you're often in science centers

and stuff there'll be a little demonstration where you see a big planet like the sun sitting in the middle of a cone and then the earth goes around the sun as if it's caught in this vortex or funnel. It's the kind of demonstration we often see to kind of give us a sense of why gravity as space-time bending is causing the earth to orbit the sun. Yeah, since you brought that up, do you find that to be an accurate description, like a useful mental model of how gravity works? Because I find it to be very confusing. You have like a 2D situation and then you're adding curvature in a third dimension where general relativity has the

curvature to be intrinsic in the 3D space. Do you find those demonstrations to be misleading or do you find them to be a useful description of what's happening in general relativity? All our descriptions are misleading. What we do is we tell ourselves lies and each other lies and they're useful for a bit and then we replace it with a better lie. And I feel like the funnel is a reasonable lie because gives you a partial sense of what's happening. And like when I say a gold atom can be in a superposition of two places at once, that's also a bit of a lie, because you could just as well say it's in a superposition of being in neither place at once. This is a mental picture that we're using, and it has some truth to it, also there's parts where it fails. And the funnel that you

see at many science centers, which is meant to explain why space-time bending causes the Earth to around the sun. Well, a better lie is that what's really happening is that time is slowing down as you get closer to the sun. And the reason that the earth is going around the sun is because time slows down closer to the sun and the earth wants to travel in a path which causes clocks to tick the least number of times. And so it's actually time that's bending and traveling at a different rate, at different places in space. And that's why. the earth goes around the sun. Why does Earth want that?

Earth is laid on its deadlines, Kelly. That's why. I hear you, Earth. I hear you. Yeah, Earth is traveling the shortest distance between two points. And why it wants to do that? I don't know. And there's lots of principles in physics where things want to do,

the principle of least action, which is how we derive most of our physical laws. And was famous, you know, the movie, Arival is about that principle. And we don't really know why that is, but that just seems to be the way it is. I think it's at the level philosophically of if you make this assumption, you get a model which works and describes the universe very well, and then you can come back to, well, what does that mean about the universe? Well, we're still shrugging your shoulders over that one. I think the thing you learn in physics when you're in grade school, which is that, you know, when you are in outer space and there's no force acting on you, you move in a straight line.

That's an example of that. why do we move in a straight line when there's no force applied to it? Well, we're just taking the shortest distance in some sense. We're taking the shortest path, the easiest path in some sense. And that's a particular example of that. But things could be different, but they're not. It's also interesting to me how some things when you say them people will just accept them because they sound intuitive and other things demand an explanation. Why do clocks tick slower when you see them moving at high speeds? That needs an explanation. Why would clocks always tick at the same speed. Why doesn't that need an explanation? So there's an imbalance there

because something's confront our intuition and something's supported. I mean, maybe this is not for this podcast, but I could give you a good reason why we think that the Earth is traveling the shortest distance in curved space. Please do. If you're willing to accept Newton's law, which is that in empty space, my velocity should stay the same if no force is acting upon me. And if I should just move in a straight line in empty space, if you accept that, then you can now ask the question, well, if there's no push or pull on me and I'm in a curved space, how should I move? I think there's one answer, which is you should take the shortest path because that's what the satellite is doing an empty space when it's going around the earth or when it's going in a straight line. It's just taking the shortest path.

I see. So you're making the argument that in flat space, a straight line is the shortest path. And so if you generalize to any space, including curved space, you should always still take the shortest path. which in this case is not a straight line. That's right. Fascinating. So I think we interrupted you as you were explaining why gravity is weird and different and why it's harder to quantize than electromagnetism.

And you were explaining how gravity is actually the curvature of space. Why does that make it harder to quantize it? Why can't we take the same tricks we applied to the electromagnetic field and apply them to the metric of space-time or the curvature of space-time? I mean, there's a bunch of technical reasons

why we run into trouble, I think a way to put it conceptually is this idea that gravity is about time flowing at a different rate in different points in space. And it's about the flow of time, how fast it flows. And if you think about it, it's almost like, what are we doing as physicists? What we're doing as physicists is we're describing how things change relative to our clock. So we have a clock, and it's telling the time, and we say, okay, at 9 a.m., the particles, we're in this configuration and then I predicted some later time they're going to be in some

other configuration. And so we're predicting the future based on the past. And in order to do that, we need to know how our clocks and our rods are all behaving. And if we're going to quantize that, if we're going to make our clocks and our rods and the speeds at which the clock ticks and the length of our rods, if that is going to be quantum, then personally I don't know, how to describe physics anymore because you've cut my legs out from underneath me and I no longer am able to really I think talk about how things evolve relative to some time if the rate at which time is flowing if that itself becomes the thing I have to quantize and doesn't have a definite value because I guess remember that the thing that distinguished classical things and quantum things

was that classical things have definite values. The coffee mug is at a definite position. And quantum things don't. The gold atom that is quantum can be as if it's in many places at once. And so if your time can be running at many different speeds, how do I tell you what is happening relative to the time if I can't even really tell you what time my clock is running out? I see.

So for an electron, if we just ignore gravity and we think about it quantum mechanically, we can handle its uncertainty and we can propagate that forward in time. We have the shorteninger equation. And we can even allow for that uncertainty to create new uncertainties and more uncertainties because we always agree on our clock. And we can say what time is and we can propagate things forward and calculate how this uncertainty envelope is going to change.

But if I'm now talking about something that has gravity, the additional complexity, the time itself is changing in a way that depends on what's happening. And so time is bent by the object and, its motion then depends on time. So there's this back and forth interaction between the motion and time itself. Is that the complexity?

That's definitely part of it. Yeah, that's a big part of it. It's not even clear what I mean by the past and the future. It's not even definite that something is to the past or to the future of some other event. If I have two events that are happening, you know, the mere act of setting initial conditions, which is what we do in physics.

We say, here are the initial conditions. This is what's going to happen in the future. The mere act of doing that, I think, becomes problematic if you're trying to quantize the space time itself, or at least the flow of time and the distances. If those are being quantized, then I think it's very difficult to even ask the questions that we're used to asking as physicists. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place or even a relationship. I'm Emily Tish Sussman and on she pivots, I dive into the inspiring

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Can you help us understand where the infinities come from and also why they're a problem? Why can't we have infinities in our theories? Well, so this is something which is called renormalization. And what that really just means is that most of our physical theories, they break down at very short distances. With most of the theories that we have at the moment, they are valid at very short distances. But gravity is not. And actually, one of the things which gives me some faith in this post-quantum theory is that it turns out, we've just recently shown that it's formally renormalizable.

So even though quantum gravity has the property that it's not valid to very short distances, this post-quantum theory does seem to be valid at very short distances. And that's important because this description of gravity in terms of geometry, if you think that that really is what gravity is, like gravity really is geometry, then you believe that this picture that's geometry should hold up to the very smaller scales. And so that's one of the reasons why we want to have our theories be predictable at short distances. So renormalizable means it doesn't matter what scale you're looking at, the theory still works. Would that be a fair?

Okay, thanks. The biologist needs simplification every once in a while. And what do you mean when you say it doesn't work? I mean, we have a theory of particles that allows us to smash electrons together at tiny distances to describe, you know, the creation of very heavy particles. Why doesn't that work for gravity? Why does it break down? What happens? Well, there are two breakdowns that happen with gravity. One is something called the black hole singularity, which is that black holes, which are the strongest gravitational fields we know and cause light to bends to the point where they can't get out. those have something called a singularity which means we just don't know what happens

in the center of a black hole. That is maybe related but maybe not related to another kinds of infinity that we get. The reason that gravity has this problem and our other forces don't have this problem, it's for a reason that I don't know

that I have a good podcasty way of saying other than that's saying that the Newton's content has the wrong dimension. But there is a technical reason why the usual thing you would do for other forces just doesn't work with gravity. The gravity we say has these infinities which you cannot get rid of and therefore the quantum

theory of gravity, at least in the three spatial dimensions that we live in or appear to live in, in three spatial dimensions it has these infinities and we don't know what to do. And that's actually the reason why we have approaches like string theory and loop quantum gravity, theories in which you imagine that space, time lives on a lattice, or, you know, instead of having point particles, we have these extended strings, those are solutions to the problem that gravity has these infinities. And that's why these other approaches have been born. So your background is in string theory, right? You were once a string theorist. Is that a fair description? Well, no, I hang out with a lot of string theorists. I have some good string theorists.

friends. And I've written some papers that are string theory adjacent. So you don't want to be described as a string theorist, but you know something about string theory. And you decided to take a different path than the string theory crew. Why did you not follow the crowd? What is it about string theory that doesn't satisfy your desire to unify quantum mechanics and gravity? Well, I think one of the things is that I think we should take this geometric picture of gravity very seriously. I mean, maybe that's just a description which breaks down at some scale. But let's just try and assume that that is actually what's happening, that gravity is actually space-time bending.

And I think if you have that as your picture as to what is happening, then quantizing it becomes a lot more problematic. And string theorists, for example, or I would say almost everybody else, the various approaches to quantizing gravity, they somehow need to have it at some small scale, that picture is not true and breaks down. The geometrical picture, you mean?

The geometrical picture of gravity. And so if you want to hold true to this geometrical picture, then I don't think you can quantize it. Now, maybe that's the wrong approach. It could be that the geometric picture does break down and we get to the smallest scale and space time is emergent in some way. Or, you know, we're all just fuzzy dots in a lattice of space and time. All those things are possible and I'm attracted to them. They sound sci-fi and great. But I don't know how to make them work.

I don't know how to think about them, and I think it's worth trying the kind of conservative imagines geometry all the way down approach. And so of all of the things that you could have decided, let's assume that this is true and then work around that. Why did you pick the geometry thing? Yeah, I mean, I guess there's an aesthetic element to it, but I think also I have a feeling that I don't think we should all be doing the same thing. I think that's really important for science.

I applaud and cheer my string theory friends when they make breakthroughs and my new point gravity friends, et cetera, et cetera, I think we should be supporting each other. But I also think we should be diversifying and trying as many approaches as possible. And so I'm going to pick this kind of slightly lonely route, but I think it's important that we try these different things. And I think it would be dangerous to put all our eggs in one basket. We're looking for a needle in a haystack, and we shouldn't all be looking in the same place. Thank you. And I think that's a very valuable contribution. to science more broadly.

So let's talk about more deeply your idea, post-quantum gravity. How is it that we can avoid quantizing gravity to take the geometry seriously all the way down and still somehow handle the uncertainty of quantum mechanics? I mean, classical gravity, geometric gravity says we need to know where a particle is to know how it bends space time. But quantum mechanics says sometimes that knowledge doesn't exist,

not just that it's not known, but the gold atom is partially here and partially there has a probability to be here. here or there or neither. So in your picture where you take geometry very seriously, what happens to space in that situation? How do you avoid quantizing it? You have to give up something.

And the thing you give up is predictability. Didn't you say earlier that you needed predictability? That was the problem you were trying to solve. Yes, I did. Yeah, I mean, it's very strange that people are willing to accept that you can't predict exactly where the particle is and has a certain probability of being. found in a certain place. So people accept that. Or maybe they don't accept that. You could do a whole show on the different interpretations of quantum theory that people have and maybe you've done

the show on this. But quantum mechanics does have this lack of predictability. Whereas classical physics doesn't, right? Like in classical general relativity, in Einstein's theory of classical general relativity, there is only one space time. Right. And it is definite. And space time has a definite configuration. So if you're going to wed these two theories together and you want to keep space time as classical in the sense that it has definite features, the only way to do it is, and this is a difficult concept to kind of get your head around, it has to be both definite and unpredictable. That was definitely unpredictable. What do you mean about that? What happens when you have a particle and it's potentially in two different places?

Does it bend space time in both places? Is space time probably bent in both places? Or is it random where it gets bent? Well, space time has to be undergoing these random fluctuations. Some people have said it's like it's wobbly. It's undergoing these random fluctuations. The particle then will kind of bend it in all the places that it's in. But you won't be, able to really tell where it's bending it. So if I were to look at space time, I wouldn't be able to tell where the particle is because space time will be undergoing all these random fluctuations. It's kind of wobbly and jumping around all over the place. And so if I looked at the space time, even though it was being bent by the particle, I wouldn't be able to tell where the particle

was. And that's what you need in order to reconcile quantum theory with general relativity. if you're going to keep general relativity as a really a theory of definite geometry. I see. So space time is not emerging from some deeper string theory. It fundamentally is the geometry of the universe, but it's also fundamentally random. And when you say that, you mean that it's random and unknowable

the way it is in quantum mechanics, where the information just does not exist, it's not determined until you measure it, or that it's random, but it's unknown, like in classical physics, You know, if I flip a coin and I don't look at the answer, in principle, I could have calculated the outcome of that coin. The information exists, and it is either heads or tails under my hand, even if I haven't looked at it. Is your spacetime random and unknowable or random and unknown?

It's knowable. In principle, I could know exactly what configuration it is at the present time, but it's unpredictable in that a second later I will not be able to predict. Oh. It will evolve, too. So it's more like a breakdown in predictability rather than a breakdown in unknowing. And I think you're really right to make that distinction because, you know, when people first learn quantum mechanics, they might learn the Heisenberg Uncertainty principle where they say, you know, you don't know the position of the particle. You don't know where the gold atom is. But I think as you're hinting, it's weirder than that.

The gold particle doesn't have a position. Yeah. It does not exist at all. And so that's maybe the difference between a kind of a classical. breakdown in predictability and a quantum one. A classical object can be definite but unpredictable

whereas a quantum system is not knowable but it can be predictable because that's the strange thing about quantum mechanics I can actually predict with certainty how something called the wave function will evolve but

the position of the particle doesn't have definite value. So this notion of predictability, definiteness and knowable that they're all kind of tied in a very strange not they're quite complicated anyway in both classical mechanics and quantum mechanics and they really get jumbled around here so let me see if I understand the distinctions so in a classical theory we have something which is definite space time has values and locations and bending and it's also predictable in that the past controls the future the future is determined by the past

whereas in quantum mechanics we can predict the possible outcomes very precisely Shortinger equation tells us how to describe the possible outcomes, but we don't know which one will actually be selected when we interact with it. But in your theory, you have something which is definite but unpredictable. Does that mean that the past doesn't completely control the future? That space time in one moment is not determined by space time in the past? That's right. Does that allow time travel? Let's get to the important part here. Kelly working on her deadlines again. That's the question on everyone's mind.

Can we get somewhere faster? Does a lot of time travel? So if I imagine like an empty universe, right? No mass, no radiation, nothing. Space is completely flat? In your conception, is that fluctuating? Even though there's nothing happening to it, nothing is being inserted, nothing is moving.

Is space time still fluctuating, just like randomly changing? Yeah, but it's not that there's nothing there. There's always something there. There's the vacuum. I mean, in quantum mechanics, nothing's complicated. It might look like there's nothing there, but at the small scale,

there are actually in the quantum realm, also these fluctuations. But in some weird way, in the quantum realm, they look random, but they're not. They're only random if you make a measurement. You know, if you don't make a measurement, then they're not that random. You can kind of predict how things evolve precisely.

It's a bit why this whole theory is very tied up with this whole measurement problem in quantum mechanics. because in this theory, you don't need something called the measurement postulate. In quantum mechanics, you know, you don't know where the atom is. It could be anywhere. And then you make a measurement and you find that it's here or there with certain probabilities. That's called the measurement postulate. It says that the particle, when you measure it, will appear to be somewhere with some probability.

And in this theory, you don't need the measurement posture. That's where unpredictability comes in in quantum theory. That's the only place it comes in. And it's a very artificial way that it comes in. And we don't really understand it. We kind of put that on top of the rest of quantum theory. We put in this measurement posture, which tells us that, okay, you might not know where the particle is, but then you measure it and you see it there with probability you have.

Because when we measure things, we don't get answers that are like, it's half here and it's half there. That's right. We get it's here or it's there. So we have to somehow reconcile the spread of predictions of quantum mechanics with the specific observations. That's right. And that's where the unpredictability comes in in quantum theory. But if you remove the measurement postulate, and in many interpretations of quantum theory, they don't like the measurement postulate and they remove it. If you remove the measurement postulate, then there is no unpredictability in quantum theory. And that's what I've done here. So in this postponum gravity, there is no measurement postulate. So quantum theory by itself would be predictable, but not definite. So the particle doesn't have a definite position. But Everything's predictable. The particle is predictably in a superposition of being in many places at the same time. Right.

Right. I feel like I might understand it better if there were like an experiment that we could describe for how we would test this. Or is this one of those things that can't be, I think string theory, we can't do an experiment to test either? Yeah, where are we here with experiments? Yeah. So at the moment, there's a few experiments that people are doing to test theory. So it turns out that you can test both the theory specifically and then just the proposition more generally about whether space time is quantum. So maybe we must quantize gravity

and we now have actually experiments which can test whether space time should be quantum. And then those experiments will also test this particular theory because this is a particular theory in which space time is not quantum. So there are experiments to do that. And so these gold atoms you were talking about earlier, do we have enough control of gold atoms so that we could potentially measure their gravity and understand how their superposition affects their bending of space time? Are we still years away from being able to do that actual experiment? We can measure the gravity produced by at least millimeter-sized gold spheres. But strange enough, you can test these theories in different ways, which don't involve having to measure gravity.

They do require us to measure gravity very precisely, but we do have that already. So we have these satellites in empty space, which have done very precise measurements of gravity. And so we can use that and then we can also use, you know, people are taking gold atoms and they're putting them in the superposition of being in many places at once. And we can use those experiments. And then the combination of those two experiments can be used to rule out a theory in which gravity is not quantized. So could you have an answer about whether or not you're right, like in the next five years or something? That would be exciting. Yeah. So I think it's possible in the next five years. So we're still doing calculations to figure out how close we are.

I think five years is probably reasonable. And then there's another set of experiments which my colleague Zugato Boza has proposed, as well as a number of other people, which is about producing something called entanglement using gravity. And those are going to require probably, you know, a decade, maybe two decades. Who knows? I mean, those are very difficult experiments. They're as difficult as building a quantum computer and that's another test that will require a huge effort but which exciting we can use to determine the quantum nature of space time. So there are experiments and I think that's what's exciting about this field. Well, are you excited or terrified to see the outcome of those experiments? I mean, I think there's a sense that you're going to spend a lot of

time doing something. You want to find out as quickly as possible if it's right or wrong. It's not really personal in the sense that, okay, you're going to spend some time working out of theory. And so because you've invested yourself in it, you kind of wanted to be true. And I think that's true of everyone that is string theorist, one string theory to be true. But at the end of the day, we're making an assumption. We're seeing what the theory predicts and then we're testing it. And I think that's a good thing regardless of the outcome. Yeah, absolutely. Even if the answer is no, that's a good answer. Then people can stop looking down that particular path. Like, I think it's still worthwhile no matter what the answer is. Yeah. Yeah.

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Like, why our last names would make the worst hyphen ever. My last name is Cummings. I have sympathy for nobody. Yeah, mine's brown-olar, but with an H. So it looks like brown holer. Okay, that's, okay, yours might be worse. We can never get married. Yeah.

Listen to this episode with Whitney Cummings and check out new episodes of bananas every Tuesday on the exactly right network. Listen to bananas on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. we can do those experiments, we can try to tackle some like stubborn and conceptual problems that arise from interactions of quantum mechanics and gravity. I was reading your comments on solutions to the black hole information paradox and how your approach might untangle that problem. Can you tell us briefly how post quantum gravity solves the black hole information paradox? What happens to information as it falls into a black hole? I wouldn't go so far as to say that what I would say is that the paradox, it kind of loses its bite. And it's this weird thing

that probabilities seem to emerge in physics in a few different places. As physicists, we don't like it because we believe everything should be predictable. So the measurement problem that we've discussed is an example of that. You know, physics only lets us predict probabilities of the particle being in certain places. And that is kind of information destruction, right? Because we start off with something which is in a definite state and we end up with some indeterminism like where the particle is and that seems that's a kind of information loss and so probabilities come up in quantum theory

in terms of this measurement problem that we talked about and then the other place where these probabilities come up and information loss comes up is in black hole so in a black hole you throw something into the black hole which is in a definite state the black hole sucks that information in and then it slowly evaporates away. And when it finishes evaporating,

it appears that we're left with just a bunch of noise, a bunch of thermal radiation. And so predictability seems to be lost. And physicists don't like that. I'm okay with that. I think we've just lost predictability in measurements. So why are we so unhappy about losing predictability in black holes? And in fact, I think they're probably related.

And in this theory, they are related. So this theory allows for information loss because it has this probabilistic nature. And so I spent a lot of my time previously trying to construct a theory which allowed for information loss. I wasn't able to do it within quantum theory, and I don't think it's possible to do it within quantum theory. But within this theory, it's possible. And in fact, it's a feature of the theory that you have this information loss. So if you have information loss, then there is no black hole paradox. The paradox arises in black holes.

if you insist that information is preserved, then you get the paradoxes. So if you insist that information is not lost, then you run into a whole bunch of paradoxes. And so in this theory, information is lost, no paradox. It sort of sounds like you solve the problem by saying it isn't actually a problem. Like information is lost by a black hole, but that's okay. Well, that was always the case for the information loss paradox. So there was always this debate in the community where usually in general relative. versus to people who think space-time geometry is really what's going on, who study space-time geometry. They were always or generally tended to be okay with information loss. Information is lost,

get over it, not a problem. It was the string theorists usually or the high-energy physicists who insisted that information should be preserved. And that's when you get a paradox. So it's only in that case that you run into various paradoxes. If you're willing to accept information loss, then you probably don't call it the Black Hole Information Paradox. You call it something else like the Black Hall information and annoyance or something. Sounds like a better title anyway. Not as catchy. So to accept that, you have to accept something kind of weird about the universe,

that there is a randomness, that it's not predictable, that one moment is not determined by the previous moment, or even the probabilities aren't determined, that there's something fundamentally random about gravity itself. you must hear a lot of sort of philosophical objections, even if the mathematics of your concept work and you can make predictions and you can describe experiments, you must hear philosophical objections to having a universe that works this way. Do you hear those objections? And what are your answers to them?

So I think as business, we believe we can predict everything. And so it's really, how can we not predict something? It must be. So it's interesting. People are okay with a breakdown in predictability as we discussed when it comes to the measurement problem in quantum mechanics. but for some reason, and I think it's partly sociological, they're not willing to accept it in black holes. I think there's a good reason that you might not accept it in black holes, and the good reason is that you say, well, okay, but I can just imagine that there is this randomness somewhere else,

which determines, you know, I can imagine that there's this hidden environment. We call it a hidden variable, you know, a hidden system. And if I knew what that system was, then everything would be predictable. So everything is predictable. there's just something that I'm not looking at. And that's, I think, the philosophical objection.

But, you know, we know that doesn't work for the measurement problem in quantum theory. So we know that there could be no hidden variables, no hidden environment, which you could explain randomness in quantum theory, at least not a local hidden variable theory. There's a very famous theorem called Bell's theorem, which tells us that. And in this theory, although we haven't proven it, I think there is no way to construct a hidden environment or a hidden system which would explain everything and make everything predictable.

You know, I think the philosophical objection is you could always imagine some other system, which if you knew about it, you would be able to predict things. And I think that we know from quantum theory, and I think this is also true in this theory, that that may not be true. But yeah, I would say that philosophically, that's probably the biggest stumbling block. If you're going to have a problem with this theory, I think that's the reason you're going to be skeptical. So then my last question is a philosophical one.

Imagine that aliens have arrived on Earth and they're scientific, and we get to talk to them, and you get to meet with their physicists. What do you think the chances are that they have a community that does string theory and a community that does loop quantum gravity in a community that does post-quantum gravity, or that they even think quantum gravity is a hard problem or an interesting problem? Do you think that we're probing something about the universe here, or exploring questions that have to do more with the way humans organize our thoughts? I feel like we understand so little about the universe.

I guess I like to think of ourselves as like these little single-cell organisms that are kind of slowly moving towards something they think is light. If my dog met my cat, they would have some similar theories about the world, but they would be pretty different, I guess. I mean, maybe my dog and my cat would have similar theories, But maybe my dog and my amoeba would have very different conceptions of the universe. And I imagine that's how it would be. I mean, maybe they're more advanced civilizations, we would look quite foolish to them.

We hope not. Let's hope when they arrive that they don't treat us like amoeba and just try. Well, they would tweet, okay, because you ignore an amoeba. That might be the best outcome. Being ignored aliens. All right. Well, thank you very. much for your clear and cogent explanations of your theory post quantum gravity. My actual

last question is, why did you call it post quantum gravity? Oh, I think because quantum theories is modified in this theory, so you have to modify it in order to make it consistent with geometry. I mean, there's a whole kind of literature of post quantum theories, modifications to quantum mechanics, and this fits in there, but probably in the most gentle way. You can imagine, I mean, people, myself included spend a lot of time trying to imagine theories that were really different, you know, that would go well beyond quantum theory. And they're actually almost impossible to construct, and they may not exist. So this may be the only kind of modification to quantum theory that we can make. I don't know.

Wonderful. Well, thanks again very much for your time and your explanations. I really appreciated it. Thank you. Yeah, thanks very much. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you. We really would.

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