Daniel and Jorge Explain the UniverseDaniel and Jorge explain why the two theories of physics are so at odds with each other.
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Hey Daniel, how smart do you have to be to be a physicist?
You know, it's not actually about being smart. 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. Wait, 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.
Well, are you saying math can be relaxing. It can be relaxing, and it can also be exciting. You know, sometimes you're like bush whacking 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 make, 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 are connecting yourself to the universe.
M 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, just like the mental kind.
Yeah, but you know, sometimes thinking really hard can make sweat. I've definitely perspired while doing intergals before m.
I see, have you ever wiped out doing integrals?
I've never injured myself doing math, that's for sure.
Well, I guess some people headaches. I guess that's sort of cow injury exactly. Migraines are a hazard of doing physic Yeah, no, I find math very relaxing. Actually 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 the call sign. There you go, that's very ASMR.
Yeah, and you will be subliminally learning many.
A billion dollar idea right here.
Yeah, 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 nineteen 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.
It'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 are placed in it is.
Sometimes these integrals are more annoying than mosquito bites, though never as dangerous as jaguars.
I don't think anything is this annoying as mosquito bites. Jaguars are pretty pesky too. Have you ever seen a jaguar in the wild in the wild? Uh no, no, thankfully not. Or do you mean like a car the car? I think plenty of jaguars around here in South Pasady.
That's true, and those drivers can be pretty dangerous.
Yeah, yeah, and annoying like moisquitoes.
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, Well, let me insult them on a podcast.
When you make a billion on your sleep math app then you can buy yourself a jaguar and you can be the same 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 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 do you think people in tana will live?
No, I'm asking paint us the picture.
I've seen a lot of coyotes around here in the while. Yes, yeah, those are pretty dangerous too. I guess if you're the if you're a small 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 is like spy? How dangerous are these spikes?
They're pretty big because the 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 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 that night, during the full daylight, they will hop in people's backyards and make off with their little Shitsues.
Sounds like maybe you just just make the coyotes your pets, and then that's also the problem, doesn't it. Well, Lass, then you get a problem of jaguars coming in here and eating your coyotes.
And somehow we have to connect all of this back to physics.
Yeah, no, there's a physics zero of sizes right right, yah 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 emotion 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 one 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 bush whacked our way into the mathematical jungle.
Yeah. Yeah, I think that's the problem, Daniel. You need to get off your couch an experiment with real gravity, not just like imagine ner gravity.
I'm waiting for my Indiana Jones had to come. You can't do it without the right cand of hat. You have to be prepared.
Oh that's right, Yeah, that's right. You don't want to get sunburned. 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 them into shape. But anyway, to be 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? Yeah?
But how is it going to fall without gravity?
Quantum mechanics?
I answered everything, it's going to fall and not fall. No.
I love that physics has a name for this, the 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 physicist name thinks. Well, anyways, we're wondering, as usual, how many people out there had thought about this question of why quantum gravity is so hard? As usual, Daniel went out there and God 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 me two questions at Danielandjorge dot com.
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 in a small as scale.
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, that 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, although maybe you should include a chat GPT answer every every time we do this. Yeah, that's right. Now, what'st say about quantum gravity being so hard.
Chat ChiPT says quantum gravity is consider 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 in our job? Does a chat GBT have like a 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 gipt just yet.
Oh right, you still need us to read the answer chat GPT gives out. I see.
Also, I would never rely on chatcheapt. I asked some hard physics questions sometimes and it just makes up bologny. Oh.
I think the news here is that you're asking chat GPT for answers.
Yeah. I like everybody else, I was curious when it came out, what you 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 gonna 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 one hundred years? So let's start with the basics, Daniel, what is quantum gravity?
So Chatchimititi got this bit right. Quantum gravity is an attempt to bring together are two great theories of physics, quantum mechanics that describes things like electromagnetism 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, job relativity for were the big stuff, quantum gravities, and attempt to have a single consistent theory that works for all the stuff.
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 photo electric effect, what happens when you shine a bright light at a piece of metal, could 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 and 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, they're 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 way like when a computer program runs 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 work 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 should 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, 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's 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, thinks 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 Newton described as a force of gravity is actually just objects moving through curved space. Newton imagined space was absolute as this backdrop of the universe, and then things with mass had a force between them. You know. He's famously explained that 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 spent by gravity, right, or gravity is the bending of space, right.
Bace gets spent by energy in a very complex way. Essentially, mass isn't 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, it's like, are you referring to the Newtonian force, which isn't really part of our picture anymore, or you're 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 a curve because of the presence of energy. Right, that's the basic and that effect basically you can sider lump it into the idea of gravity.
Yeah, exactly. And we had a whole podcast digging deep into 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 compar prison.
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 the 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, two theories breakdown or do they agree on their 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 curve 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 fifty percent chance to be at this altitude and fifty percent 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. General relativity requires that you know where the electron is, and so it ignores its quantum nature. So if the quantum nature of the electron is important to doing quantumy 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 dig into that scenario and dig into how exactly these two theories don't match up, and then we'll get into a 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 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 haven't 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 on your 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. Like 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 think 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're 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's 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 electrons. 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 treat it 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 is 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 a 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, it's quantum nature becomes important and those quantum effects dwarf gravity. They're 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? But 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 gravity. You know, they're like ten to the thirty times as powerful as gravity.
But like, what if it's like ten to thirty one 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're 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 it?
No, that's a cool idea. That's a scenario where gravity and conon 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. 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, are couch surfing and Danna Jones doesn't care.
The other way to tackle this is to say, well, what if you have a really really massive quantum particle 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 particle is going to do next, or we can't predict how it's going to interact. Can you're describing words what the problem is at the indec 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 like one theory says that the electron is going to turn right, and the other theory says the electric is going to turn left.
Yeah. For example, general relativity is a classical theory, and so it assumes electrons have very definitive location at every point in time, whereas quantum mechanics says no, there'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 doesn'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, the 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 say, like, 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.
Yes, 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 smooth 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 for one hundred 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 being curved here and the 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, I 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 ten 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 to even Einstein in a decade, and it takes a lot of people a lot of time to understand 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. It's like sixteen coupled equations which are really hairy, and if you add to those the probability that space is may be curved here and maybe 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 handsaw.
Yeah, Like you say, 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 theory is differential geometry had been developed like ten fifteen 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 felt 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 if 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 m I see.
I think what you're saying is like that each of these two theories work, but only if they ignore each other. Like quantum mechanics assume as that space doesn't bend and there is no gravity, basically gravity doesn't exist to quantum mechanics, 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, and 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, yeah, 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 wrap the comedy comes out.
Now there's exactly exactly right. Now you can't hang out with your friends anymore, right, boom the universe.
You gotta stop watching movies. You can stage your couch and just do physics and math all day. Has this ho already happened to you?
Ben?
I think Passengers? This was not a sci fi romantic comedy.
Oh, there you go. And that was very controversial, man, Exactly nobody likes that. If 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 trying to make a theory of quantum gravity that has a graviton. Say you know Einstein, that was cute. We like your idea of Kurt spacetime. It's 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'd 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, 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 one hundred and fifty percent, and the probability of that happening is one thousand percent, 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 gravitin 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 Eisin was right, And so you're sort of not really solving the problem, right, are you.
Well, you'd have to think about 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 ways 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 scatter against each other or when two black holes are being eaten, then this theory breaks down.
But generativity 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. We 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.
M I see, so graviton not a great ivan, you are 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, that 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 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, when you want pictures of roos, the 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 to 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, 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 styled radius, the distance from the singularity to the event horizon, just keeps growing as the black hole gets more massive. So somehow 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 handwavy to you, but it sort of tells.
You about that 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, hmm, 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 into 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 at 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. Who's 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.
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 mm.
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 relativity 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, at the smartest people in the universe. It 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, I know that 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 a 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 physics.
Right, I'm still waiting for that in the mail. It must have gotten lost. I guess you sent the right.
Oh, we sent it. Yeah, you should definitely get a physical copy. You do like a discount at dollar story.
This is an imaginary diploma again for 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 graph. But you 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 this 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. That's 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 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 field that's sitting inside some sort of meta space or superspace or subspace or some other kind of space. It tells us that this like 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, and 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 sit on top of a bendy stretch, Hey, space time. You know, sort of like you know, the planet Earth of 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.
M hmm. And I love how you just like casually suggesting these solutions, which are like whole areas of research work.
Daniel Kan, I mean, I just spent five twenty minutes talking about this, and I already know how to come up with the answer basically, so long.
No, I don't mean to marck 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 in 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 like 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 at 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 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 Kurve 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 don't Maybe we will talk to one of the smartest people in the universe who is tackling this problem directly.
That's right. So I reach down to Nathan, whose father is a listener of the podcast and listen to the podcast mostly so you can have a hope of understanding his son's research. Nathan is a physics bread 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 at Daniel's interview with Nathan Berwig, 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, So, 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.
That's a really good question. So I think part of it is just I always kind of grew up as a curious kid, 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 content 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 are most interacted with force at least sort of perceptually. You know, we talk about it a lot. You don't really talk about how electromagnet doesn't really plays a role in your life very often. But it's also very poorly understood in general, and there's still like 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 think I definitely should. Yeah, No, you definitely like fostered my curiosity in physics growing up. Both my parents did. 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 a bad 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 esthetic 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 esthetic 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 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 one hundred years you're willing to disregard. And you know that 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 probability distribution looks like. There's no like wave function understanding when you start getting into the smaller regimes for general relativity.
So that's a really big topic like general relativity and 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 gonna ask you 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 in there being eaten up. But I like to think that there's this sort of taboo in physics of natural infinities. They aren't there. 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 shooting up their straws as they enjoy and 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. Oh. I certainly hope that boba is it not at all comparable to singularities. But I suppose you never know what that would be the hell 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 John relativity and good luck figuring it all out.
Thank you very much, Thank you for having me.
All right, interesting chat there, pretty controver vircial about both boba and black holes.
You know, this is the cutting edge of food science and quantum gravity.
That's right, and sugary drinks. Yes, But it's interesting that he seems to follow pretty strongly on the general latuty 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 others a bunch of religious zillo its because your side is obviously right.
I mean they like boba, so what so you can't trust them at all?
Well, boba are like particles, so I would have thought they're fuzzy particles. I would have value you be a big fan.
No, they're macroscopic, man, They're dominated by gravity.
Oom. 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 certainties also get squished down.
Maybe we're gonna solve quantum gravity right here on this podcast today.
Yeah, yeah, let's do it. Why wait a hundred years, let's come up with that romantic comedy sci fi movie right now?
Why it's quantum gravity so hard because nobody asked a cartoonist.
There you go, boom, question answered, Boom done. Let's let's go tackle something harder like cancer or oba 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 bobba ball there to choke on it. It's boba relativity, that's right, The answer, the choking hazard of a boba is there. All right? Well, stay tuned. You hope you enjoyed that. Thanks for joining us. See you next time.
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. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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