Daniel and Kelly’s Extraordinary Universe - Why is quantum gravity so hard?

Episode Date: February 27, 2024

Daniel and Jorge explain why the two theories of physics are so at odds with each other.See omnystudio.com/listener for privacy information.

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Therapy for Black Girls podcast, Dr. Angela Neal-Barnett and I discuss flight anxiety. What is not a norm is to allow it to prevent you from doing the things that you want to do, the things that you were meant to do. Listen to therapy for black girls on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. It's more about thinking that these kind of particular challenges are really fun. So if you like having fun, you shouldn't be a physicist? What do you mean? I mean, you know, science is a very personal thing.

So some people might think doing integrals is really boring and somebody else might do them to relax. Are you saying math can be relaxing? It can be relaxing and it can also be exciting. You know, sometimes you're like bushwhacking through the math and you make amazing discoveries and you don't even have to risk your life to Jaguars. Well, you do have to worry about paper cuts, right? Yeah, you know, I think there's a reason they didn't make Indiana Jones a physicist. Yeah, I don't think physicists could pull off the Indiana Jones hat. And we all have daddy issues.

Hi, I'm Jorge, I'm a cartoonist and the author of Oliver's Great Big Universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I always wanted to have physics adventures. Ooh, like real adventures? Like in your couch? Like, oh, no, I spilled my coffee. I don't think you have to risk your life and, like, get into a spaceship or even become an astronaut to have physics adventures. You know, you can explore the universe in your mind, make amazing discoveries, and feel like you're connecting yourself to the universe. I guess technically aren't like real adventures, physical adventures. But I guess maybe you don't want physical adventures. You want physics adventures.

Yeah, exactly. Physics versus physical. You don't want to make any physical exertions or efforts. You just like the mental kind. Yeah, but you know, sometimes thinking really hard can make you sweat. I've definitely perspired while doing integrals before. I see. Have you ever wiped out doing integrals?

I've never injured myself doing math, that's for sure. Well, it gives some people headaches. I guess that sort of counts. Is it an injury? Exactly. Migraines are a hazard of doing physics. Yeah, no, I find math very relaxing, actually. It puts me right to sleep.

In fact, you should make an app for that. Like a sleep relaxation app. And now we're going to do integrals. People do like to fall. There you go. That's very ASMR. And you'll be subliminally learning math. The billion dollar idea right here.

I can finally quit this podcast job. But anyways, welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of IHeartRadio. In which we do our best to make the complexities and the challenges of physics accessible to you and to everybody. because we think that the whole point of physics is to understand the universe and not just by a select few group of people who can understand 19-dimensional string theory integrals but by everybody because in the end science is a bunch of stories mathematical or intuitive or in English and we want to tell those stories to you that's right we take you through the

adventures of science the wipeouts the close calls and the headaches of trying to find out how the universe works and what our place in it is Sometimes these integrals are more annoying than mosquito bites, though never as dangerous as jaguars. I don't think anything is as annoying as mosquito bites. Jaguars are pretty pesky too. Have you ever seen a jaguar in the wild? In the wild? No.

No. Thankfully not. Oh, do you mean like a car? The car? I see plenty of jaguars around here in South Pasadena. That's true. And those drivers can be pretty dangerous. Yeah, yeah.

And annoying like mosquitoes. You just want to swat all the Jaguar drivers? No, no. That's a, you can get arrested for that kind of thing. Yeah, exactly. I just like to insult them on a podcast. When you make a billion dollars on your sleep math app, then you can buy yourself a Jaguar, and you can be the only one.

I can buy several Jaguars. And a mosquito repellent. Well, I was just wondering, because I know you grew up in Panama, if you'd ever seen a Jaguar, or maybe just Jaguar. sized mosquitoes. Oh, I see, I see. That's your perception of how I grew up. I grew up in the hut, in the jungle, barefoot with my little pet jaguar next to me. Is that how you think people in Panama live? No, I'm asking. Paint us the picture. I see a lot of coyotes around here in the wild. Yes. Yeah, yeah. Those are pretty dangerous too, I guess, if you're the, if you're a small,

if you're a small baby or something. In our neighborhood, everybody with a small dog puts these spiky vests on them so the coyotes can't just snatch them up and have a snack. Wait, what? You turn your dog into a weapon? What do you mean? Like, how dangerous are these spikes? They're pretty big because there's a whole epidemic of coyotes jumping into people's backyards and like grabbing these little dogs.

Oh, wow. Maybe just not keep your dog out at night. Don't turn it into a dangerous weapon. Like what if the dog runs at a little kid or something wearing this killer jacket? Yeah, these coyotes are pretty brazen. It's not just at night. During the full daylight, they will hop in people's backyards and make off with their little shih Tzu's. Sounds like maybe you just make the coyotes, your pets, and then that solves the problem, doesn't it?

Unless then you get a problem of jaggors coming in here and eating your coyotes. And somehow we have to connect all of this back to physics. No, there's a physics here of sizes, right? Right. Yeah, sizes, yes. Some problems are hard, like how to protect your little pet from neighborhood coyotes, and other problems are hard like, how do you figure out the mathematics of the universe? It's all connected. Yeah, because I guess figuring out the math of the universe has been one of the goals of physics to understand what's underlying everything that we see around this and all of the mechanics and the motion and the energy that is swirling around us.

What is at the core of the universe? physics has two great theories two incredible ideas that have been very very successful quantum mechanics and general relativity but bringing them together into an idea of quantum gravity has been a challenge that has stood for over a century the greatest minds in physics have tried to take a bite out of it but it seems to be protected by a spiky vest that's right it's been one of the hardest problems to solve in physics for the last hundred years which makes you wonder, why is it so hard? What's taking so long to solve this fundamental problem in physics? It's because us physicists haven't just gotten off our couch

and bushwhacked our way into the mathematical jungle. Yeah, I think that's the problem, Daniel. You need to get off your couch and experiment with real gravity, not just like imaginary gravity. I'm waiting for my Indiana Jones hat to come. You can't do it without the right kind of hat. You had to be prepared. Oh, that's right.

Yeah, that's right. You don't want to get sunburn. It's not like there are other ways to protect yourselves from the UV rays. I mean, I want to understand the universe, but I'm not willing to make physical sacrifices. Do you also need a whip also? I want to whip those integrals into shape for sure. Yeah, there you go. Crack him into shape.

But anyway, today on the podcast, we'll be tackling the question. Why is quantum gravity so hard? Now, is this like hard, like difficult or hard like tough? If your floors are made of quantum gravity and you drop your glass, man, is it going to shatter? Well, how is it going to fall without gravity? Quantum mechanics. The answer to everything. It's going to fall and not fall at the same time.

No, I love that physics has a name for this theory, quantum gravity, but it's just kind of like a placeholder. We don't know what it is or how it works or what the mathematics of it are. People argue about different approaches. We already have a name for it, but we haven't even figured it out yet. Sounds on brand for how physicists name things. Well, anyways, we're wondering, as usual, how many people out there had thought about this question of why quantum gravity is so hard. This is as usual.

Daniel went out there and got answers from real people. Thank you to all the real people and definitely not made up chat GPT inspired bots who answered this question. If you are a real person and you'd like to answer future questions, please write to me to questions at Daniel and Horace. so think about it for a second why do you think quantum gravity is so hard to solve here's what people have to say i know that quantum mechanic is very good explaining things that happen in very small scale and gravity is not very strong in a small scale uh i don't know i think that's the

reason but i don't really i don't really know I honestly don't have an intelligent answer for that, but I will say this. If Albert Einstein was afraid of it, then I am too. It's hard because we are trying to apply our comparatively super massive vantage point to these tiny particles in the quantum realm that probably don't even know the difference. They probably don't even know that gravity is a force. Did you mean real people as opposed to chat GPT or real people as opposed to physicists? I'm just now realizing that was a jab. I mean like not paid actors.

Okay. Although maybe you should include a chat GPT answer every time we do this. Yeah, that's a cool idea. Interesting. Yeah. It's right now. What does chat GPT say about quantum gravity being so hard? ChatGPT says quantum gravity is considered challenging because it involves the attempt to

reconcile two fundamental theories of physics, quantum mechanics, and general relativity. Each of these has been incredibly successful on its own, but becomes problematic when combined. Whoa. It just did the podcast for us. Done. AI did replace our inner job. Does chat GPT have like a voice output? Can you have it read the answer?

I think the free version that I have access to can't do images or voices. So no, you cannot be replaced by chat GPT just yet. Oh, right. You still need us to read the answer chat GPT gives out. I see. Also, I would never rely on chat GPT. I ask some hard physics questions sometimes, and it just makes up baloney. Oh, I think the news here is that you're asking chat GPT for answers.

Yeah, like everybody else, I was curious when it came out. What's it like to talk to the average of the internet, wisdom and follies? And the answer is, it's not very reliable. Well, technically, neither are we, Daniel. I don't think we're going to have a hard answer here today. No, but we're not going to make stuff up. Well, so let's dig into this question. Why is quantum gravity so hard?

Why is it so difficult? Why has it puzzled physicists for over 100 years? So let's start with the basics, Daniel. What is quantum gravity? So Chachapiti got this bit right. Quantum gravity is an attempt to bring together our two great theories of physics. Quantum mechanics that describes things like electromagnets, magnetism and how particles work and gives us a probabilistic picture of the universe, and general

relativity that describes space and time and gravity and explains things like the expansion of the universe and how things move through space and time. And both of these work really, really well in their own regime. Quantum mechanics for the small stuff, general relativity for the big stuff. Quantum gravity is an attempt to have a single consistent theory that works for all this tough. And these were developed independently sort of, right? Like while Einstein was coming up with general relativity, other people were thinking about things at the smallest level and why they were quantized, right? Exactly. They were developed independently, though around the same time. And both were actually sparked by Einstein. Quantum mechanics really got its kickoff from Einstein's realization

that the photoelectric effect, what happens when you shine a bright light at a piece of metal, can only be explained by the fact that photons were little packets. They were quantized. They weren't just continuous beams of energy. Because what you saw was as you turned up the energy of that beam of light, you didn't get electrons with more energy boiling off. You got more electrons because each one just gets one serving, one photon. That was explained by saying the beam of light had more photons in it, not just a brighter beam. So Einstein kicked off quantum mechanics around the turn of the century. And at the same time, he developed his theory of special relativity and general relativity that explained the apparent force of gravity. And these two have been in parallel

development over the last hundred years, but nobody's been able to bring them together into one picture of how the universe works. I guess maybe the question is, why do you want to bring them together? Like, if you have one that works really well for some things and the other one works well for other things, what's the need to unify them? Yeah, it's a fair question. You know, sometimes in life we have things that are separate like you got one group of friends and another group of friends you bring them together it's awkward you don't do it again right yeah that's usually a bad idea and i guess in this case there are two answers one is philosophical and the other really is experimental philosophically we just think that the universe probably does have a single set of

laws you know there should be one explanation for why something happens you know the same with like when a computer program runs it's running with one source code it's not like there's two codes there battling it out. There should be one explanation. And this is just sort of like a philosophical preference. It would be nice if the universe had a single unified theory. It would sort of make sense to our brains. That doesn't mean it has to happen.

It's just sort of like a philosophical preference. Well, maybe explain to folks how they're separate. So for example, quantum mechanics works to describe what exactly, like the motions or of little tiny particles or their interactions or what exactly does quantum mechanics do? Quantum mechanics describes everything about tiny little particles. Their motion, their interactions, what's going to happen, what's not going to happen. If you have, for example, two electrons and they're interacting with each other, quantum mechanics tells you what's going to happen.

You make two electron beams, you shoot them at each other. Quantum mechanics tells you the probabilities of what will come out of those collisions. or if you replace an electron with a muon, or you put in a quark or a proton or whatever, quantum mechanics is the rules of all of those interactions. And the standard model of particle physics, what we talk about on this podcast all the time, that has been super successful in explaining the structure of matter deep down inside the atom and why everything is bound together and how that all works, that's all quantum mechanics.

It's all fundamentally quantum mechanics. Every little bit of it is quantum mechanical. And it's quantum mechanical because it paints this picture. of how the universe works, that's very different from the way that our universe seems to work, the one on our level, you know, about baseballs and planets and basketballs where things have like smooth paths. It tells us that fundamentally the universe follows very different rules, that quantum objects, tiny little bits only have probabilities to go places. They don't have smooth paths. Right. Things are kind of fuzzy down at the microscopic level. What does

general relativity do exactly? So general relativity explains space and time and gravity. So it says that what Newton described as a force of gravity is actually just objects moving through curved space. Newton imagined space was absolute. It was his backdrop of the universe and that things with mass had a force between them. You know, he famously explained an apple dropping and also the moon orbiting the earth unified in his law of gravity as an attraction between mass. But Einstein tells us that that's not the case. General relativity tells us that actually things are just moving through the curvature of space. Space itself is curved when mass is nearby, and that changes the natural inertial path of objects. Objects will move in what looks like

curves, even without accelerating. And space gets bent by gravity, right? Or gravity is the bending of space, right? Space gets bent by energy in a very complex way. Essentially, mass is a kind of energy, so it helps bend space, but it's not the only way you can bend space. And gravity is sort of a fuzzy term now. Are you referring to the Newtonian force, which isn't really part of our picture anymore? Or are you talking about the whole theory of general relativity as an explanation for it? But, you know, what we describe as gravity things seeming to fall down is explained by Einstein is things just following the curvature of space. Which gets occurred because of the presence of energy, right? That's the basic.

And that effect, basically you can sort of lump it into the idea of gravity. Yeah, exactly. And we had a whole podcast digging deep into. be like why gravity isn't the force and how if you're moving along with the curvature of space time, you don't feel any acceleration, even if other people see you like moving in circles or moving towards the center of the earth, all that kind of stuff. It's a really fascinating different way to think about how the universe works. It tells a very different story from Newton's, but it mostly describes really, really big stuff because you need a lot of mass to curve space

and that curvature is kind of gentle. So the effect of that curvature is hard to measure, especially compared to these quantum forces which are extraordinarily powerful in comparison. All right, so now maybe paint us a picture of how they are not unified. Like, can I just have some quantum particles interacting in a gravitational field? Or can I just have the path of a quantum particle bent

by the bending of space and time? What doesn't these two things do together? Like, what are scenarios in which they exclude each other? Yeah, great. And so this is sort of like number two reason why we want to unify them, because in some situations, they disagree. Like we talked earlier about having separate theories of the universe. Maybe that's cool, but it's not cool if they're talking about the same phenomenon.

If you're asking them the question, what happens here? Most of the time, you can keep them separate because for tiny little particles, you can ignore gravity. Gravity is very, very weak for little particles. And for really big stuff, quantum effects mostly average out. You don't need to know quantum mechanics to predict the path of a baseball. But in some scenarios, they do disagree. Things like what happens inside a black hole.

That's a scenario where you have really powerful gravity. So gravity can no longer be ignored. And things are very, very small because we think that things are super compressed inside a black hole. So quantum effects are important. So super duper massive, very, very tiny objects, quantum mechanics and general relativity disagree about what happens there. And so the universe can't have a contradiction. Two theories tell different stories. they can't both be right.

But I guess I mean in like an everyday scenario, do they work together? Sort of like if I'm imagining, say, a microscopic particle like an electron out there in a near Earth orbit and it's floating out there in space close to the Earth, does it get pulled by gravity? Is it going to fall down to Earth? Is there a problem with me trying to use quantum mechanics to model how it falls to Earth? Yeah, so you might think, can't we just test quantum gravity and figure out like what the answer is, which one's right? by looking at the gravity of a tiny quantum particle. Right.

So that's what you're asking. What happens for the Earth's gravity on an electron? Yeah, like, do the two theories break down or did they agree under normal conditions that are not inside of a black hole? So the two do not agree about what happens to an electron in the Earth's gravitational field. They don't? They don't, but they're not both relevant at the same time. It's a little tricky. And the issue is, how do you calculate the gravitational field of the electron or even the gravitational force on the electron or the

effective curved space, however you want to say it, because that depends on where the electron is. If the electron is a little quantum particle with quantum effects, then maybe it has like a 50% chance to be at this altitude and 50% chance to be at that altitude, in which case it would feel different amounts of gravity. And so how do you calculate the gravity on a quantum particle? We don't know. The theory of quantum gravity would tell us how to do that, but general relativity doesn't tell us how to do that.

generally relativity requires you know where the electron is. And so it ignores its quantum nature. So if the quantum nature of the electron is important, it's doing quantum-y stuff, then we don't know how to calculate the force of gravity on it. But also, we can't measure the force of gravity on a tiny little object because the force is so small, because its mass is so small. Sounds like a great situation and maybe one that we need a little bit more time on. So let's take into that scenario.

and dig into how exactly these two theories don't match up, and then we'll get into the little bit of the math that makes it so hard to integrate the two. So let's do that. But first, let's take a quick break. All right, we're talking about why quantum gravity, a theory that unites quantum mechanics and general relativity, it's so hard to come up with

and to make these two theories play well together. Daniel, we're talking about a scenario in which I have an electron in near Earth orbit. It's out there in space above the atmosphere and I'm trying to figure out what's going to happen to this electron. Is it going to fall to Earth?

What path is it going to take as it falls to Earth? And you're saying that it's hard to theoretically predict what's going to happen, right? Because it's definitely going to fall if I put an electron near Earth. Right? Like it's going to do something, but we don't really have a good theory to predict what it's going to do.

Is that what you're saying? We can't be very, very precise about its predictions. We can be approximate. There's two approaches you can take. You can say, I'm going to ignore the quantum mechanical part of it. I'm just going to treat the electron like it's a tiny little rock or a tiny little ball. I'm going to calculate its gravity and I'm thinking about how it's basically in orbit around the Earth. And you can do that and you get very good predictions.

and you can calculate how things boil off the top of the atmosphere or they fall to Earth or whether they're in stable orbits or not. So basically ignore the quantum nature of the electron, treat it like a tiny classical object, and do gravity on it. That's one approach. But then you were saying it's hard to know how much gravity is applied to the electron because of quantum mechanics, or is it hard? Can you just say like the electron has this much mass and it's a little tiny rock and so that's how much gravity is going to feel? or is that at some level wrong? Well, that's at some level wrong because you're ignoring the quantum nature of the electron.

You're treating it like a tiny rock, and it's not a tiny rock. Yeah, but I guess I mean like if you do treating like a rock, do you get something wildly off or do you get something that seems to be pretty exact? You get something that works pretty well as long as it doesn't have any interactions. As long as that electron's not interacting with any other particles, it's mostly just ignoring them, then yeah, you get something that's correct. Like it's going to follow the same path as it. little rock. Yes, as long as it's not interacting. But if it's in a soup of other electrons and

charged particles and it's interacting with those, then boom, its quantum nature becomes important. And those quantum effects dwarf gravity. They completely take over. So you can either ignore the quantum effects and just do the gravity, or you can ignore the gravity and just do the quantum effects. For an electron, only one of those is relevant at a time. And that's why you don't need quantum gravity to think about electrons. You can do either quantum mechanics or gravity. They're never both relevant at the same time. But I guess to an approximation, so then when do you get into trouble? Like, when is it a problem that these two are not unified? What's the scenario? Like, it's in gravitational orbit around the earth, the electron is, and it's sort of a little bit

interacting with another electron, then it's like we don't know what to do. As long as it has a quantum interaction, that's just going to dominate because the quantum forces are so much more powerful than You know, they're like 10 to the 30 times as powerful as gravity. But like what if it's like 10 to 31 times further away? Wouldn't it be at the same level of gravity? I mean, gravity falls with distance just like quantum forces do, right? So it's not a matter of distance. It's a matter of the mass to charge ratio.

Like if you have two electrons, they feel a very strong electromagnetic repulsion because of their charge. They don't feel a very strong gravitational attraction because of their mass. The electromagnetic force there is always more powerful at any distance. I thought maybe the scenario you were trying to paint was like I have an electron out there in space and it's been attracted by gravity to the giant Earth. But maybe it's also sort of being repelled by another electron that's nearby. And so then we don't know what's going to happen.

You have a scenario. We have a single electron orbiting the Earth and then some very distant electron is very gently pushing on it with the same power as the gravity of the entire Earth. Yes. Is that like the scenario that you run into problems, or can you still handle that? No, that's a cool idea. That's a scenario where gravity and quantum forces are at the same level, and so you can't ignore one of them.

You have to take both into account, and we don't know how to do that prediction. That kind of experiment is also pretty hard to realize because you need an isolated electron affected by only one other electron that's super far away. So it's not like practically something we could set up. Otherwise, that would be really awesome. I mean, it would tell us something about quantum gravity. Yeah, yeah, but that's a physical problem, about a physics problem. So we don't, you know, our couch surfing, Indiana Jones doesn't care.

The other way to tackle this is to say, well, what if you have a really, really massive quantum particle, a particle that is feeling quantum forces, but actually has enough mass that its gravity can't be ignored. And that's when you end up in a black hole. All right, so can you be more specific about what the problem is? Like, we don't know how to tell what the particles are going to do next, or we can't predict how it's going to interact. Can you describe in words what the problem is in these scenarios? The problem is that our two theories, general relativity and quantum mechanics, make different

predictions about what's going to happen. What do you mean? Like one theory says that the electron is going to turn right and the other theory says the electron is going to turn left? Yeah. For example, general relativity is a classical theory and so it assumes electrons have a very definitive location at every point in time, whereas quantum mechanics says, no, that's probabilistic and you can get things like interference.

General relativity says, I'm ignoring all that interference stuff. And it's going to make a prediction based on treating the electron like it's a little rock flying through space. So they're going to come up with very different predictions for what's going to happen. General relativity isn't allowed for like entanglement or any of the other important quantum effects that totally control what happens to an electron. I see. Because quantum mechanics also might change which direction the electron goes. Absolutely. Yeah.

Electromagnetism is a quantum effect, right? Those forces, all the forces in the universe that cause acceleration, weak force, electromagnetism, the strong force. These are all quantum effects. They all operate on the probabilistic wave functions that control these particles. General relativity ignores those. So you ask quantum mechanics and GR, what's going to happen to this electron? They disagree about what's going to happen. Well, maybe this is the point where we have to get more into the math.

Because, you know, as a lay person, I might just say, like, why can you just add these two things? Like, why can't just have a particle that gravity pulls on its average position? and so the average position curves according to gravity, but then where it is exactly might be fuzzy due to quantum mechanics. Yeah, so what you're trying to do right now is come up with a theory of quantum gravity. You're trying to say, can I get all the good bits of general relativity

and all the good bits of quantum mechanics and smush them together to make a theory of quantum gravity that makes one prediction, right? And so that's the topic of the episode. Why is that so hard? And people have been working on it for 100 years. It sounds straightforward, but there's a bunch of reasons why it's actually quite tricky.

One of them is the problem you just mentioned, which is this question of like space and time and probability. You know, quantum particles don't have definitive locations. And general relativity doesn't allow for probabilities in space time. What you just described is like, well, what if the electron is allowed to have a probability being here or there? And so we just say like space has a probability of being curved here and a probability of being curved there. Right. That's like a pretty deep change to how general relativity works. and the mathematics of it breaks down.

Like general relativity doesn't allow for those probabilities. What do you mean like doesn't allow it? Like you just don't know how to write it down or like you get nonsensical answers or it's like trying to fit a square peg in a round hole. You know, am I trying to use fractions to, you know, compute things to a certain decimal point or something? Yeah, the mathematics of general relativity is hard. You know, it took Einstein like 10 years to figure out how to wrangle these equations to make any sense of them. He had this idea that maybe space was curved, and that was explaining what gravity really was. But to make the math work took even Einstein a decade, and it takes a lot of people, a lot of time to understand it and to wrestle with the equations, which turn out to be really, really complicated.

It's not like one equation for a general relativity that says more mass means more curvature. It's a matrix of equations. There's like 16 coupled equations, which are really hairy. And if you add to those, the probability that space is maybe curved here and may be curved here. and may be curved there, it increases the complexity exponentially. And it makes those equations impossible to even write down. We don't know how to write down equations that both describe space as a curvature of this differential geometric manifold and allow for probabilities.

Like, we just don't have the tools for it. It might be that somebody out there is developing some cool theory of probabilistic manifolds that later will be able to slip in to build the theory of quantum gravity. But it's like we need a power tool and all we have is a hand. saw. Yeah, like you said, like if we just don't have the right tools that will both fit a square peg and a round hole. And we don't know if we're missing the right tool. Maybe that's just the wrong direction, right? It could be that that's not the right way to try to build the theory of quantum gravity. But a lot of times, it is the case that progress in mathematics is

limiting progress in physics. Einstein was only able to build general relativity because the theories of differential geometry had been developed like 10, 15 years earlier by mathematicians who didn't care at all about gravity or physics, they just thought it was cool to think about like wiggly shapes in their mind. And so this kind of stuff happens all the time. I see. You're saying it's all the mathematicians fault. Like they're holding you back, man. Math is the language of physics. And in the end, it's mathematical problems that are preventing us from building theories of quantum gravity. And what you described is basically trying to make space quantum mechanical. You can also go the other direction and you can say, well, what if we try to make gravity itself quantum

mechanical what we try to describe the theory of gravity as a quantum force instead of this whole crazy curvature of space and time and then you run into a completely different kind of mathematical I see I think what you're saying is like they each of these two theories work but only if they ignore each other like quantum mechanics as soon that space doesn't bend and there is no gravity basically gravity doesn't exist to quantum mechanics and general relativity assumes that things are not fuzzy at any level exactly which works for a lot of situations, but in some situations, you have to take them into account both at the same time.

Exactly. And it's amazing that both of them work. They tell very different stories about what's really happening in the universe. And in almost every scenario, you can ignore one, right? It's like you have two friends that are like different kinds of movies or something. And most of the time, you can just ignore one friend and listen to the other friend. But sometimes they have opinions about the same kind of movie. You're like, well, is this movie good or bad?

I don't know who to listen to. And for general relativity and quantum mechanics, currently they only overlap in places we cannot see inside black holes. So we don't know who's fundamentally right or if either of them are right. I see. It's like having a friend who only watches sci-fi movies

and then having another friend who only watches romantic comedies and usually you can have perfectly good conversations with either of them, but let's say science fiction romantic comedy comes out, now there's trouble. Exactly, exactly, right. Now you can't hang out with your friends anymore, right? boom the universe explodes you got to stop watching movies you can stay in your couch and just do

physics and math all day exactly has this already happened to you dan i think passengers wasn't that a sci-fi romantic comedy oh there you go and that was very controversial yeah exactly nobody liked it we don't have the math tools or framework or theories that let us tackle these two things at the same time what are some other ways that make it hard to unify these two things. So a really popular approach is to try to make a theory of quantum gravity that has a graviton. Say, you know, Einstein, that was cute. We like your idea of current space time. Pretty, but maybe it's just fundamentally the wrong direction. Maybe if you zoom in, what really is happening is that gravity is a force and it's exchanging gravitons. And I mean,

then we can describe the whole theory of gravity back sort of as like a quantized version of Newton's theory. And people have tried to do this because that would be pretty, right? If we could just like, add gravity to the standard model and have another particle and ignore this whole curvature business. That would be cool. So that would be going back to the idea that gravity is a force and not a bending of space and time. Exactly. So to tell philosophically a very different story from what Einstein is telling us about how the universe works. They would say there is no curvature, there are these tiny little invisible gravitons being passed back and forth. The same way that like electromagnetism, we think about in terms of electric fields that are sort of like virtual photons being

passed back and forth. We can think of gravity in terms of gravitons being passed back and forth. So that's one direction. The problem is nobody can make that math work either. When you do the calculations there and you ask like, well, what happens if you try to collide two black holes together or even two protons together, you get nonsense answers. You get answers like, well, the probability of this happening is 150%. And the probability of that happening is 1,000%, just like numbers that do not make sense. You can't have probabilities greater than one, but that's what these calculations spit out. Like if you assume a graviton exists, then you get these weird answers. But more fundamentally, like you're ignoring general relativity, right? You're ignoring things that a lot

of experiments have verified that Einstein was right. And so you're sort of not really solving the problem, right? Are you? Well, you'd have to think of it as an upgrade to general relativity. You'd have to reproduce all the predictions of general relativity. So you need to develop a theory of gravitons, which when you zoom out, looks a lot like general relativity. And that people actually can do. There are theories of quantum gravity that involve graviton exchange that when you zoom out look a lot like general relativity. So space time is not being bent? Yeah, they tell a different story, but they make the same predictions about like the motions of objects. Including things like gravitational waves and frame dragging and all that. All that kind of stuff. The problem is what

happens at the small scale when you try to think about like when two particles scatters, are against each other, or when two black holes are being eaten, then this theory breaks down. But gener relativity still works for those. General relativity still works for those, but we think it's wrong, right? General relativity, we think, is giving the wrong answer for what happens when two protons collide or what's at the heart of a black hole because it's ignoring the quantum mechanical effects. You try to build a theory of quantum gravity that has gravitons and explains all of general relativity

and gives you a gravitational, quantum mechanical prediction for what happens when two particles collide or two black holes collide, then you get all sorts of nonsense. You get all sorts of infinities that we don't know how to wrangle. I see. So graviton, not a great idea or one that hasn't worked so far. Yeah, exactly. There are also sort of fundamental problems. We just don't know how to solve for quantum gravity, like deep inconsistencies between the

picture of the universe we get from general relativity and the picture of the universe we get from quantum mechanics about how physics should work that we just don't know how to reconcile. What do you mean? Like what? Like a deep principle in physics is this idea of locality, that things should be near each other to affect each other. You shouldn't have like an electron over here affecting something really, really far away in the universe, especially in quantum mechanics. And in quantum mechanics, we have this deep connection between the distances between things and their energies. Like the reason that we use the large Hadron Collider to study really, really, really tiny things is you need really high energy to study really high energy to study really. really small distance scales, like things that have a lot of energy interact with each other very,

very closely. Because if things have low energy, then they don't interact with the things around them. If things have low energy, then they can interact with stuff that's further away. Another way to think about it is in terms of the wavelength of stuff. You know that you need like really high energy photons to see really, really small stuff. With lower energy photons, they have a longer wavelength. You can't like resolve small details. That's why, for example, example, when you want pictures of really, really tiny stuff, you use high-frequency photons. You go beyond that to use like electrons to take pictures of atoms, for example. So you want to see the universe on a really small scale.

You need to use really high-energy probes. Maybe just say high-frequency instead of high-energy. Yeah, energy and frequency, very closely connected in quantum mechanics. You need very high-frequency stuff to see really short distances. Okay. Okay. In general relativity, they have the opposite relationship. As you add energy to something in general relativity, then its influence grows to longer distances.

So quantum mechanics, higher energy means shorter distances. In general relativity, higher energy means larger distances. Like think about what happens to a black hole's radius. As you add energy to a black hole, black hole gets bigger. You add more energy, black hole keeps getting bigger. The short-style radius, the distance from the singularity to the event horizon, just keeps growing. as the black hole gets more massive. So somehow general relativity doesn't have the same relationship

between energy and frequency and the distances involved. They have like this deeply opposite relationship. This might sound like sort of weirdly philosophically hand-wavy to you, but it sort of tells you about... Not at all. I don't know what you mean. But the reason it's important is that it tells us that these two theories have like a fundamentally different sort of philosophical foundation.

Like one of them is very, very local. the other one is very non-local. So when we go to make a theory of quantum gravity, we're like, these two things are kind of like very different. How do we bring them together? It's like how do you get your science fiction fan friend and your rom-com fan friend together to a single movie

if they have just like really opposing needs for pacing and jokes and whatever in the movie? It might be fundamentally impossible if these two things are so deeply in conflict. It sounds like you're just kind of maybe saying the same thing we talked about before, which is like, you know, quantum gravity is good for things that are small and general relativity is good for things that are really big.

But there is a certain overlap between them and that's where you get into trouble. Yeah, exactly. But I think there's one more layer there. Like imagine you have a singularity. So a tiny little spot with a huge amount of mass, right? So it's quantum mechanically important but also has a huge amount of mass. General relativity says it affects things really, really far away because it creates a black hole whose event horizon can be really, really far away.

Quantum mechanics says, no, it can only interact with stuff really, really nearby because that's really, really tiny frequency. Well, I would say that it can only interact quantum mechanically with things that are close by, but then it can interact through gravity or general relativity for things that are far away. Like, are we back to the same spot? Yeah, exactly. We're back to the same spot that if you have quantum gravity, then you don't know. can it only interact nearby in its vicinity or can it interact far away as well.

Gravity says far away. Quantum mechanics says nearby. We don't know what quantum gravity says. It seems like maybe impossible to come up with a theory that satisfies both. Just like it's impossible to come up with a good sci-fi rom-com. Exactly. They're fundamentally opposed. All right.

Well, let's dig into some of the other ways that make it hard to unify general relativity and quantum mechanics. But first, let's take another quick. break. All right. We're talking about quantum gravity. Can we unite quantum mechanics and general activity, which has gravity in it? So far, Daniel, you're saying it's really hard. It's really hard.

Some of the smartest people in the universe have tried for decades and failed. In the universe. that's a big claim well the smartest people in the universe there could be aliens out there much smarter than us I don't know are they people though are aliens people well you're assuming that the smartest people on earth have become physicists oh that's a good point yeah no there people out there who are like hedge fund bros yeah or cartoonist maybe you know I'm just saying you're sort of making a general assumption here are you saying you're not a physicist I think years in basically you're physicist by now oh if that's

true, then I have a diploma for you to sign. Yeah, I think we gave you that podcast diploma of physics. Right. I'm just waiting for that in the mail. It must have gotten lost, I guess. You sent it, right? Oh, he sent it. Yeah, you should definitely get a physical copy. You can do like a discount at dollar

store and stuff like that. This is an imaginary diploma again. An imaginary field of study. All right, so it's hard to combine these two big theories. We talked about how the math makes it really difficult. the philosophy of them make it really difficult?

What are some other ways that make it hard? People argue about the fundamental story of space and time that quantum gravity will have to tell us because quantum mechanics and general relativity really do tell very, very different stories here. In quantum mechanics, we have kind of like a Newtonian view of space and time. We say space and time, they're backdrops. They're absolute. They're fixed.

And we put quantum fields on top of that space and time. We assume that space and time already exists somehow. And we say that there's quantum fields in that space and time and those fields operate and they're part of space, but we don't ask about where space comes from or what it is, this kind of stuff. It's like a fixed background, we call it. But in general relativity, space time is dynamical. It's not like fixed. You can bend and twist and it doesn't like exist inside some other kind of space. We have this way to like calculate the relative distance between points, but space itself is not like some new space.

feel that's sitting inside some sort of meta space or super space or subspace or some other kind of space, it tells us that there is no fixed background. So people describe general relativity as background independent. Like the universe, space itself is not sitting inside some sort of deeper box, which is kind of weird because it makes you feel a little like unmoored from the foundations of reality. But I guess maybe couldn't you just have quantum mechanics? it on top of a bendy, stretchy space time, you know, sort of like, you know, the planet Earth or the sun are moving through space time and they're getting, they're flowing through the curvature

and all the bending. Couldn't you have quantum fields also kind of right on top of the curves of space time? And I love how you're just like casually suggesting these solutions, which are like whole areas of research that people are working on for 20 years. What's so hard about this, Daniel? Come on. I mean. I mean, I mean, I just spent five, 20 minutes talking about this, and I already know how to come up with the answer. You're basically a business. It's so long. No, I don't mean to mock it at all.

I mean that these are actually really clever ways forward. And these are exactly the things that people are working on at the cutting edge. It turns out to be complicated. You can do quantum mechanics on curved space. You can put these fields on curved space time. And mostly it works as long as the curvature is small. So like if they're a little bit curved, things are. fine. As soon as the curvature becomes large, you get back to all those crazy infinities that we

can't wrangle and all sorts of nonsense predictions. So the quantum mechanical theory breaks down if the curvature is really, really high. So we just don't know how to do those kinds of calculations. And that includes time, right? Because in general relativity, time can slow down and time can speed up. And so you're saying you don't know how to do that in quantum mechanics? Exactly. We don't know how to do that in quantum mechanics, again, when the curvature is high. for what they call a weak field gravity, where it's like a very small effect, gravity, then we can do those calculations

and even feel the influence of gravity, not just negligible gravity, just weak gravity. But when gravity gets strong, which is what it does near a black hole, where we think these two things are both important, we don't know how to do those calculations. That's when quantum mechanics on curved space breaks down because of all these infinities we can't grapple with.

All right. Well, as we mentioned, we're not quite experts in this topic. But we thought maybe we would talk to one of the smartest people in the universe who is tackling this problem directly. That's right. So I reached down to Nathan, whose father is a listener at the podcast and listen to the podcast mostly so he can have a hope of understanding his son's research. Nathan is a physics grad student at Arizona State University,

a well-known department with experts in cosmology and theories of quantum gravity. And he was kind enough to spend five minutes talking to me about his research. And so here is Daniel's interview with Nathan Berwick, quantum gravity researcher. So then it's my pleasure to welcome Nathan to the podcast. Nathan, please introduce yourself and tell the listeners who you are. Yes, hello. I'm Nathan Berwick. I'm currently a first-year PhD student at Arizona State University studying physics and cosmology and hoping to continue doing so and move into ideally.

stuff along the lines of quantum gravity awesome well we often talk on the podcast about how science is just a bunch of people following their curiosity and pushing forward the forefront of knowledge so tell us what is it about physics and gr that drew you in that made you decide you want to spend your life on this question uh that's a really good question so i think part of it is just i always kind of grew up as a curious kid um and you know when i was younger i always knew i wanted to be like a scientist in air quotes. But eventually I sort of narrowed in on physics. And I think as soon as I learned about like the most basic concepts in general relativity, sort of the idea that space time is one great big thing. And gravity is just the curvature of that. It was just immediately alluring,

especially when you start to consider how gravity is like our most interacted with force, at least sort of perceptually. You know, we talk about it a lot. You don't really talk about how electromagnetism really plays a role in your life very often, but it's also very poorly understood in general. And there's still a lot of really big open questions to be answered for general relativity. And I think that sort of openness is very much an invitation to explore, which I really like. So you're not going to give your dad any credit for encouraging you to study physics? I think I definitely should.

Yeah, no, he definitely, like, fostered my curiosity in physics growing up. Both my parents said, but my dad was always fascinated in physics. And so from just, like, really small things, you know, we'd always find questions on the internet, you know, whether it be about string theory, which is always like a very big, you know, media topic and whatnot. But he definitely played a very big role in me wanting to do physics. So a lot of people say that general relativity, once you fully understand it, is deep and beautiful. do you have that kind of aesthetic reaction to it?

Yeah, I think a lot of the beauty of it comes from the idea alone that it's all just curvature, that gravity is in essence just curvature of this big manifold, which you may or may not, well, which you can't really visualize. But that for me is the part of the aesthetic that general relativity has is just like a breakdown into a more fundamental concept. So if GR is so gorgeous, And it works so well, all these experiments confirming Einstein all the time.

How can it be wrong? I mean, is it wrong in the way that, like, Newton's gravity is wrong, where it was telling fundamentally the wrong story about what was happening, but we didn't really notice until we dug into the details? Or is it mostly telling the right story just needs some, like, corrections and band-aids here and there? Yeah, so that's a really good question. And I think it really depends on how much of the last hundred years you're willing to disregard. And, you know, that has certain consequences but also benefits. But I think the most

generally accepted perspective right now is that it needs band-aids because general relativity as this idea of gravity being caused by space-time curvature is so, so rigorously tested at the moment. You know, we keep trying to knock Einstein down and find errors in his theory somewhere, but every single time, like, he just was right. And so that's a difficult thing to try and find errors in. But there's obviously still big questions. So quantum gravity is probably the largest area of questions regarding general relativity. And a lot of that has to do with just quantum mechanics not playing nicely when you try and think about how gravitational fields work with it. General relativity doesn't have any fundamental understanding of what a probability distribution looks like.

There's no wave function understanding when you start getting into those smaller regimes for general relativity. So that's a really big topic, like general relativity in quantum mechanics. What are you actually researching right now? Like what are you working on today? Today, so throughout the last semester, I've been working on what are called scalar tensor theories, which I actually think you guys did an episode fairly recently on no hair theorems for black holes. And that's very much what I'm working on at the moment. Just showing that there's no sort of scalar hairs for certain types of theories.

Cool. So then I'm going to ask you to speculate unscientifically. What do you think is inside a black hole? Oh, boy. What do I think is inside a black hole? speculative answer is, well, if it's a big black hole, there's probably stuff sort of floating around and they're being eaten up. But I like to think that there's this sort of taboo in physics of natural infinities. They aren't, usually when there's an infinity, it's something horribly, horribly wrong.

And that was sort of the big hub-up when they figured out that black holes could be a thing that actually existed, was that it just felt wrong, that there could be such a dense object that's just so infinitely packed. I like to think that it is just, you know, a perfectly infinite density, singularity, just because I think it'd be very wild and wacky and cool to have this very strange yet natural, like, divergence or infinity just existing in the universe. Do you think the singularity inside a black hole is as dense as Boba, you know, these chewy blobs of death that people inexplicably like?

Like shooting up their straws as they enjoy an otherwise relaxing beverage? I happen to know your father agrees with me on this. He sent me a clip of this just the other day. I certainly hope that boba is it not at all comparable to singularities, but I suppose you never know. But that would be the hill that my dad would die on, I think. Well, I think maybe your dad should open up a black hole boba shop that sells black hole boba super dense super dense little chunks of destruction all right nathan thanks very much for taking some time to talk to us about your work on the forefront of understanding of general relativity and

good luck figuring it all out thank you very much and thank you for having me all right interesting chat there um pretty controversial about both boba and black holes you know this is the cutting edge of food science and quantum gravity that's right and um and um sugary drinks, yes. But it's interesting that he seems to fall pretty strongly on the general relativity side, he thinks inside of black holes are singularities. Yeah, I've seen this in a lot of quantum gravity theorists that they are intoxicated by the beauty of general relativity.

It's sort of hard to overstate the impact this has on people when they are able to import Einstein's equations into their mind and they can see the universe in terms of this differential manifold. They feel like the scales have fallen. from their eyes, and they're seeing the universe the way it really is, and it has to be true. So they want to preserve this vision of the universe as having curved space time. It's not exactly religious, but it's almost like a spiritual experience. Oh, my goodness.

This is coming from, of course, a quantum mechanics particle researcher, right? You're trying to say that the other side is a bunch of religious zealots, because your side is obviously right. I mean, they like Boba, so you can't trust them at all. Well, Bobar like particles. So I would have thought, they're fuzzy particles. I would have thought you'd be a big fan. No, they're macroscopic, man.

They're dominated by gravity. Well, I wonder if maybe there is a singularity, you know, and maybe it's also fuzzy at the center of it. It's just that maybe in a singularity space is so compressed or so squished together that maybe the quantum uncertainties also get squished down. Maybe we're going to solve quantum gravity right here on this podcast today. Yeah, yeah, let's do it. Why wait 100 years?

years. Let's come up with that romantic comedy sci-fi movie right now. Why is quantum gravity so hard? Because nobody asked a cartoonist. There you go. Boom. Question answered. Boom. Done. Let's go tackle something harder like cancer or democracy. All right. Well, you're on one side. People like Nathan are on the other side. Sounds like we still have a long way to go. We do have a long way to go. And there are deep philosophical

and mathematical hurdles to overcome. We want to have a single theory of the universe, one that gives us a complete picture of what's really happening, the source code for the universe, but so far, none of our attempts actually compute. And so it's an active area of research,

maybe waiting for the next person to tackle this and possibly solve it. And that could be you out there, asking these questions, listening to this podcast, wondering how the universe works. I hope that future genius doesn't suffer an unfortunate boba accident choking to death

before they can share their insights with humanity. Oh my gosh. It's a little bit dark there, a little bit dark. I think you need a pretty large boba ball there to choke on it. It's boba relativity. That's right. Yeah, sorry, the choking hazard of a boba is relative. All right, well, stay tuned.

We hope you enjoyed that. Thanks for joining us. See you next night. For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discord, Insta, and now TikTok. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. Friday, and cancer-free the next Friday. No chemo, no radiation, none of that.

On a recent episode of Culture Raises Us podcast, I sat down with Warren Campbell, Grammy-winning producer, pastor, and music executive to talk about the beats, the business, and the legacy behind some of the biggest names in gospel, R&B, and hip-hop. Professionally, I started at Death World Records. From Mary Mary to Jennifer Hudson, we get into the soul of the music and the purpose that drives it. Listen to Culture Raises us on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Welcome to Pretty Private with Ebeney, the podcast where silence is broken and stories are set free. I'm Ebeney, and every Tuesday I'll be sharing all new anonymous stories that would challenge your perceptions and give you new insight on the people around you.

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