Daniel and Jorge Explain the UniverseCould we reveal quantum gravity in a tabletop experiment?
Daniel and Jorge talk about how clever experiments might provide a breakthrough in the effort to understand the quantum nature of space-time.
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Hey, Daniel, who do you think would win in a fight theoretical or an experimental physicist?
That depends are we're talking arm wrestling or like integration competitions into what mathematical race?
Then I think I would put my money into theoretical physicist, I mean, no offense.
Maybe we have to do the experiment, or maybe.
You should keep this theoretical. I don't know if you want to pick a fight.
Well, maybe the two sides of the field just compliment each other beautifully.
Is that all. It takes us some compliments and you guys are back at friends.
Theories are cheap, right, They don't need money for experiments, they just need compliments in theory.
In my experience, Hi, I am more Headmaye, cartoonist and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, And back in the day, I did want to be a theorist.
Back in the day. How old were you.
When I started grad school? I wasn't sure if I wanted to do experimental or theoretical physics, so I guess I was in my early twenties, which by now is pretty far back in the day.
Did you actually get a choice, like they offer you an option of which way to go, or do you have to, like, I don't know, test into it.
You definitely have to kind of try out and work with the theorists if you want to be a theorist. But you have all the options when you start grad school. You could end up being an experimental particle physicist or a theoretical cosmologist or whatever. All those paths are available. You just got to like it enough and be good at it.
So what happened? Why didn't you pick the theory?
I discovered I just didn't like writing down equations as much as the theorists. They would sit there and like develop several different mathematical fonts to write their equations in, and I was like, Wow, I'm just not loving this as much as they're loving this.
It sounds like you were against the idea of it in theory.
My experience was the experiments were more fun.
But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we try to blur the line between theory and the experiment. We wanted to talk about all the concepts in theoretical physics that try to explain what's going on in our world, but we also try to touch back on the ground and understand what experiments are telling us about the nature of reality. What is nature actually saying to us as she spins the story of the universe, And then we try to explain all of it to you.
That's right, because it is a pretty storied universe, full of amazing little details and facts and things to discover out there that we are still puzzling over. And which require all kinds of scientists to figure out, theorists and experimentalists.
And in the history of physics we have made progress in lots of different ways. Sometimes the theorists have come up with a clever idea, a suspicion about how the universe might work, with lots of cool directions for experimentalists. Go out and check this thing. Measure how light bends around the sun. See if you can find the Higgs boson. Those can be wonderful directions to help unravel the mysteries of the universe. But sometimes the experimentalists lead the way, turning on particle smashers and discovering gobs and gobs of new particles that nobody Expectedestion.
And danially is why can't you be both? Why can't you be a theoretical and an experimental physicist.
I'm doing my best. Actually, i'm doing my best. But the reality of academia these days is to get one of these jobs, you have to be the world's expert in some subfield, and that makes it really hard to sort of live between two fields, because you have to be like the top person in that field that year. And so if the theorists aren't sure if you're a theorist and the experimentalist aren't sure. If you're an experimentalist, nobody's going to give you that job. So you've got to sort of get the job in one category and then inch your way over to the other one if you're interested.
That's kind of clickish, it's.
Definitely very cliquish. Absolutely, these fields form and then they protect themselves and it can be hard for new kinds of sub fields to emerge. Like right now we have the emergence of physicists who are experts in machine learning, and people aren't sure is that theoretical is it experimental? Because you're running a bunch of calculations. Nobody's really sure. Everybody knows that it's valuable, but we aren't quite sure where to put with them.
That's because they're robots? Are they?
In disguise? We're all just biological robots?
Man?
Oh, there you go, aren't you the expert in squishy robots?
I am, yeah, Well I used to be at least a lifetime ago, or a couple of lifetimes ago.
Now back in the day. Is there such a thing as a theoretical roboticist?
Uh? Yeah, there's a lot of theory in robotics as well. But no, as we I guess we're not as cliqu as you're just a roboticist. If you're into robots, you're just a roboticist.
New York because you just build your own friends. You're like, hey, look I don't need people's friends. I can build my own.
Yeah. But as you said, I guess you need both kinds of endeavors or research. You need experimental research and you need theoretical research in order to figure out how things work in the universe, because I guess you need to come up with a theory so that you can prove it with an experiment, and you need an experiment to prove the theories. Otherwise there's no science.
That's sort of a theoretical way of thinking about it, that we come up with the theories and improve them with experiment. Remember that sometimes experiments don't just prove theories, they blow up theories and tell us that the universe is different from the way we understand it and operates in some other way we don't yet understand. Like the photo electric effect was a demonstration that boy, we really don't understand light and how it works, and it took a few years before the theorist came up with any sort of explanation for it.
Yeah, but I guess experimenters blunches kind of experiment blindly, right. You usually have some sort of theory at hand when you design your experiments, when you go out there and turn stuff on.
It's a bit of a raging debate right now in experimental physics whether we should be focused on searching for the ideas that theoretical physicists are suggesting, or whether we should be developing strategies that are more just exploratory that leave us open to surprises. Like when you turn on the Hubble Space telescope and look out into space. Sure, you want to see the things that you had in mind to look at, but you're also open to like seeing aliens waving at you, or seeing new kinds of stuff you didn't even expect to see.
But I guess also at the same time, we're getting to a spot where you know, things are so complex and so subtle and so hidden that you kind of need to know what you're looking for in a way, Right, it's kind of hard to just like look for everything.
It is really hard to look for everything. You really put your finger on it, especially when your data is very statistical. If you do like a single experiment and you get some weird result, you might be able to say, hey, look, there's definitely something new here. But if the data are subtle, if the new things appear as like trends in your data, then you're right. It can be hard to know how to find them. So then you have to play some clever statistical arguments and say, well, you're the kinds of things that we could see, and here are the ways that we could search for them. So you have to do a little bit more work to define the kinds of things you might be able to see, even if you aren't sure which specifically might pop up in your data.
Well, sometimes there are cases where both the theories and the experimental lists are stumped. And that is the case where non physics. There's kind of a big hole in physics in terms of our knowledge of how things work in the universe.
That's right, at the most fundamental level, we still don't really understand the basic rules of physics. We have two pillars of modern physics relativity that tells us about space time and gravity, and quantum mechanics that tells us about particles and forces, and we just don't know how to bring them together. And it's important because it has to do with one of the most basic questions in physics, which is what is the universe made out of? What is the fundamental fabric of reality? After all?
Yeah?
And is it soft and comfortable? Is what I want to know.
It seems to have a little bit of spandex in it.
Here, you guys, long is a stretchy that can accommodate all sizes.
Because my waste is not the size it was back in the day.
Yeah, you want the universe to kind of expand with you, your mind and your waste. But yeah, there's kind of a big hole in our understanding of the universe. And it has to do with gravity. We're not quite sure where gravity falls, whether it falls where it fits with quantum mechanic skill theory, or whether it works the way that Einstein envisioned it in special relativity.
Right, that's right. Einstein and special relativity tells us about light and how it propagates. His theory of general relativity tells us about space time and how it bends. And these two theories are in conflict and tell us very different stories about the nature of the universe, but so far we haven't been able to figure out a way to test them without building a solar system sized particle collider or peering inside a black hole. So experimental physicists have not really been able to contribute to this conversation until now.
So the deal in the podcast, we'll be asking the question, can we test quantum gravity in a tabletop experiment? And right here the word tabletop I think of board games. Is this what we're talking about? Like a little cardboard unfolding thing with pieces, and then you test quantum gravity exactly.
You can download the schematics from the internet and print out your own Nobel Prize winning experiment.
There you go. Is it called Settlers of Park or quantum ton.
I'll leave you to do the branding of it. But when we say tabletop experiment in physics, we basically mean something not like the large hadron collider or something that doesn't require a ten billion dollar facility staffed by thousands of people. We mean the kind of thing a single physicist could do in their laboratory in the basement of your nearby university.
I see, you're talking about a million dollar table talk, not a billion dollar table.
Talk exactly, just like everybody has a million dollar table in their kitchen. No, it's really like a single physicist experiment, something you can do in a reasonable physics lab, not something people are going to be doing on their kitchen table.
Well, as usual, we were wondering how many people out there had thought about this question or perhaps have any ideas about how to do it.
So thanks very much to everybody who participates in this segment of the podcast. If you've been listening for years and would like to hear your voice speculating about the topic of the day, please write to us two questions at Danielandjorge dot com. Everybody's welcome them.
So think about it for a second. Do you think we can test quantum gravity on somebody's table? Here's what people have to say.
Well, since quantum gravity is, you know, with the gravity of the really small, I don't see why the experiments with it couldn't be done on a tabletop. I just have no idea what those experiments would even begin to look like.
Though.
If yes, then it will come to our table soon.
But till then, I don't think it is possible at all.
Uh.
Yeah, you probably could, but probably not today.
I do not feel like we could test quantum gravity in a tabletop experiment because you need a lot of gravity for it to work, and I don't think the Earth has that kind of gravity.
All right, not a lot of optimism. I like the person who said the tabletop, I don't think so. But maybe a desktop or on the floor or on a shelf, maybe mountaintop maybe, yeah, tabletop on a mountain. There you go, lower gravity.
Well, we talked recently about how to measure big g and that experiment was definitely done on a mountain side swinging pendulums next to a big mountain in Scotland. So yeah, you can do funny gravity experiments on tops of mountains.
And I like the person said, probably but not today? Like is today a bad day for that? Were they busy that day? How about next week? Next week? Work?
Please fill out this doodle pole for when we will win a Nobel prize. There you go.
Yeah, I guess people didn't feel like it could work, but let's find out. Daniel step us through this. What is quantum gravity?
So when we say quantum gravity, what we mean is a theory that explains both the quantum mechanical behavior of super tiny particles, the way like electrons and photons do things that baseballs and basketballs and mountaintops don't do. You know, they don't move in smooth paths. They have weird quantized energy levels. They can be in a superposition of different states, like maybe they're here, maybe they're there. They can interact with each other and interfere in all sorts of complicated ways described by their wave function. And we want a theory that explains gravity as we know it. That things seem to move in these inertial paths through curved space time, and that mass in space tends to bend the path, which affects the motion of other mass. So we have these two very different theories of the universe, and so far we can't bring them together. So quantum gravity would be a theory that explains both these things somehow harmoniously. But it's not a theory that we have today.
Well, I guess maybe step us through a little bit of what we haven't been able to bring these two things together. As far as I understand it, it's kind of due to two things, right, Like one is that we haven't measured the gravitational force at the level of the quantum particles, right, that's one thing. And also we don't know what happens to general relativity when you get down to that small level.
Too exactly, I think you put your finger on it. Really, we don't know what the gravity is for little particles. The gravity for a baseball or for a moon, we think we understand and we've been able to test that.
Right.
We see moon's orbiting planets, we see planets orbiting suns. We see how gravity works. But that's all really really big stuff. What we don't know is what happens when you have gravity for particles, because particles are super duper tiny, which makes it really complicated for two reasons. One is that they have almost no gravity. Remember that gravity is like the weakest force in the universe, and so the other forces overwhelm it. You try to do experiments with electrons, then their charge is much more powerful than their mass. Right. The electromagnetic force is much more powerful than the gravitational force on an electron. So it's basically impossible to measure the gravitational force on an electron.
Can I ask why that is?
Though?
Like, couldn't I shoot an electron from here? To London and see if it curves with the curvature of the Earth.
You could try that, absolutely, I think you probably shouldn't shoot beams across the surface of the Earth without getting signatures from everybody who might live in between. But say you did that, the electors would be affected by all sorts of charge particles between here and London. Right, There'd be lots of other effects on the electron which would swamp out any gravitational effects.
But I guess maybe, like from a satellite, I'm thinking, you know, I just shoot a whole bunch of them, and wouldn't the effects from other things kind of even out if you shoot a bunch of them out, Like don't we have like quantum drives or like electron cannons. What happens if I just shoot them out there in space? Do they keep going straight or do they bend?
Yeah, you could build an electron gun and put it in space and shoot them out, but still it would be dominated by the effects of other particles. Remember space is not totally empty. There's cosmic microwave background photons there, there's other charge particles from the Sun, and all of these would dominate the fate of that electron. Really the problem is that the charge is more powerful than the mass. We talked about this once, and this is either because gravity itself is just weaker than the other forces for reasons we don't understand, or because electrons are just packed with a lot of charge compared to how much mass they have. You can think about it sort of either way. But that just means that the effect of gravity is tiny compared to the effect of electromagnetism. So to do that experiment, you'd need to isolate those particles from any sort of effect. And today we'll talk about an experiment that's going to try to do that, all.
Right, So then that's where quantum gravity comes in. It's kind of a is it a theory or an idea that tries to bring these two big ideas together.
It's not a theory. It's like a category of theories. It's like a dreamt of theory. What we want is a theory that bring these two things together. We don't have one. We don't know what the theory of quantum gravity is. You know, sometimes you have like ten different theories that describe the universe, and the experiment has to go off and tell you which one is correct. Right now, we have zero We have zero theories that explain quantum mechanics and gravity at the same time. So we sort of need an experimental result to be like, hey, this is the right direction, no where, Hey here's something to grab on to, here's a clue. But there's the second reason why these experiments are difficult that we didn't get to yet. One is just that gravity is so weak, and the other is that these particles do things that we don't know how to explain with gravity. Like particles don't have smooth paths. It's not like the electron is always somewhere, has some velocity. You know, you want to calculate the gravity of an electron, you have to know where it is, so you know how far away it is you can calculate it's gravity. But electrons don't have specific locations that have probabilities, So we don't know. For example, if an electron when it has probabilities to be in multiple places, does it mean it has like multiple different possible gravityes. We just don't know how to do gravity for things that have uncertainties in their locations.
You mean, we don't know how to do that. If gravity was not a quantum force, right, Like you're sort of assuming that you I guess you want gravity to be like a quantum force like the other forces that we know about, right.
Yeah, that's sort of one of the basic questions when you want to build the theory of quantum gravity, like is it a quantum force. If so, then two electrons interact I think gravitationally wouldn't like collapse each other's wave functions. Some bits of one wave function would interact with some bits of another wave function, and they could do all sorts of weird quantum interactions. But if gravity's actually a classical force and not a quantum force, then it would collapse the wave function, sort of like when you use a detector in a double slit experiment, it forces the particle to pick one of the options instead of the other one. So we just don't know, like is gravity classical is it quantum mechanical? We just don't even know where to begin.
And when you say classical, you mean like basically not quantum mechanical, like not fuzzy, not uncertain.
Yeah, exactly, we mean not quantum. Classical is sort of an overused word. Some people say classical to mean not relativistic like Newtonian, but today we mean not quantum mechanical, So we don't know if gravity like really is just classical the way Einstein described it, thinking about space as smooth and continuous and everything having passed, or if it is a quantum effect, in which case it could either be a force like you suggested medi by weird gravitons, or maybe like space itself is quantum mechanical and uncertain. If gravity is the curvature of space, maybe space itself can be like maybe bent here and maybe bent there in some weird quantum mechanical way. There's so many possible directions for quantum gravity, nobody really knows which one is going to build a viable theory that even can do calculations.
All right, well, let's get a little bit deeper into quantum gravity and whether or not we can test it, and test it for under a billion dollars, because I guess the cheaper the better. We'll dig into that be first, let's take a quick break.
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All right, we're talking about quantum gravity and whether or not that is a thing at all, whether it will bring together quantum mechanics in general relativity to give us one theory of the universe, and whether or not we can even design experiments to test such a theory.
I like your threshold of a billion dollars.
Yeah, well, though these days with inflation, maybe that's more like ten billion.
Though.
You know, if we could spend a billion dollars and get the answer to quantum gravity, I'm pretty sure we would do it. The truth is, the experiments might cost a lot more than one billion dollars.
All right, well, let's dig into the cost of these experiments. How can we test quantum gravity and figure out whether or not it's a real thing or not?
Well, you had sort of the right idea, which is like, let's just zoom in on a quantum particle and look at its gravity somehow. But remember the scale of things we're talking about here, like these particles are super duper tiny, and the effects we're talking about what happened on really really short distance scales, Like gravity gets more powerful when things get closer together. In order for gravity to be powerful enough for us to really test it, you need to get things together to like the Plank scale distances we're talking about, like ten to the minus thirty five meters. So until recently, it seemed like, well, the only way to test quantum gravity is to have like a microscope that can see effects at the scale of ten to the minus thirty five meters, which felt almost impossible.
Now, I guess, pain me a picture here of what it is that you would be trying to do. Like, for example, what if I just take a bunch of hydrogen atoms. Like a hydrogen atom is just an electron and a proton, so it's perfectly balanced in terms of charge. And I know that if I stick a bunch of them in a container, they'll sort of tend to fall down because of gravity. Right, those sort of acumulate the pressure of the hydrogen tank will be higher at the bottom than at the top. That means gravity is working on them and it is pulling them down. Why can't I build some sort of model or theory that kind of models or tells me how it's working at the quantum level.
Well, there's the theoretical difficulty, and then there's the experimental difficulty. On the theoretical side. Like we've tried to build those theories, they just don't work. Gravity is complicated because everything is affected by it. It's not like electromagnetism where you can like shoot out photons and those photons themselves don't feel electromagnetism, right, Photons don't interact with other photons. Gravity interacts with everything with energy. So when you try to build a quantum theory of gravity, like including the exchange of gravitons, those gravitons amid other gravitons which feel those gravitons, and it gets very hairy, very quickly. We talked once about the strong nuclear force, which has a similar property that it's gluons amid other gluons which affect other gluons and it's a nightmare to do any calculations. Gravity's even more complex than that, and that's sort of one of the reasons why it's been so difficult to build a theory. So anytime people build a theory of quantum gravity, it just sort of predicts nonsense. We just can't mathematically make it work. And then there's the experimental challenge. And what you're talking about is like trying to build a setup where you can see the gravitational effects on particles. But the experiment that you describe like a bunch of hydrogen, you know, those are classical effects. The fact that those hydrogen atoms are quantum particles is irrelevant to the fact that they have more pressure on the bottom of the tank than the top of the tank.
Oh, I see, you're trying to kind of like see what happens to gravity at the quantum distance level. Right, that's kind of the problem, right, Like you might be able to design a hydrogen gun something that shoots hydrogen atoms and you can track how the gravity affects its path, maybe, but that doesn't necessarily tell you whether or not there's like uncertainty or whether the there's fuzziness at the you know, really small distance.
Exactly in the same way that like every time you toss a baseball, and principle, you're tossing quantum objects, right, a baseball just a bunch of quantum objects, and definitely they're feeling gravity. We're not asking like, do electrons and protons feel gravity? We're pretty sure they do. We're asking is, how does their quantum mechanicalness interact with their gravitational attraction? You know, when they're doing their weird quantum stuff, how does gravity play a role with that? You know, if you have a particle that like has a possibility to be here and they're simultaneously, what is its gravity? So you've got to get something to be showing as quantum effects, which means really small distances and revealing its gravitational interactions, which requires really really large masses, Which is why some people are excited to see inside black holes, because that's where you have like really really really big masses squeeze down to quantum distances, and so what's going on inside a black hole would really tell us about the nature of quantum gravity and therefore the deepest nature of space time itself. Of course, we can't see inside black holes, so those secrets are hidden from us.
Yeah, you might want to let that one go. It seems like we're never going to find out what's inside of a black hole.
There's even a theory called cosmic censorship that suggests will never be able to answer this because the answers are always going to be hidden behind some weird horizon. It's sort of a pessimistic approach.
WHOA, I didn't know there was a ratings board for the universe.
And there are even some theorists that suggests this whole enterprise is a waste of time. Like Freeman Dyson, the guy who thought of Dyson's fears. He likes to think that we live in a dualistic universe, that quantum mechanics and gravity just sort of like rule in different regimes and they never actually overlap at any place where they come into contact is hidden from us by these event horizons.
Like maybe gravity is classical and it's not quantum. But you're saying, or he's saying that at the quantum level, there's things that are happening that you will never find out.
Yeah, exactly that maybe you don't have a single theory at the universe. You like two theories and each have their own REGI and they never overlap anywhere we could test them. But a lot of people don't really like that theory. I really don't like that theory because I want there to be one theory of the universe, one thing that explains everything. And I'd love to see these two different concepts battle it out. I want to force the universe to show us what the answer is.
But I wonder couldn't they Couldn't he be right though, Like, couldn't it just be the gravity, you know, bend space and quantum fields and quantum particles exist in that band space.
Yeah, he could be right. But if we can come up with some experiments that force quantum mechanics and gravity to speak at the same moment, to say, like, all right, here's what happens when you have a particle that has two possibilities and it has some gravity, then we'll know. And maybe he's right. Maybe gravity really is classical And what happens when particles interact gravitationally is that their wave functions collapse, because that's what happens when classical objects interact with quantum objects. But it sure would be nice to know.
Yeah, gravity is pretty cool. So talk to us a little bit about how we've been trying to study this or get answers to this question.
So other than like wishing we could see inside a black hole, the other typical tool in our toolkit is a particle collider. You build a big particle smasher, you pour a lot of energy into one tiny little spot. Then you can like break open bonds, you can see how the pieces interact. But in order to see gravitational effects, you would need so much energy. You'd basically need like the Plank energy. It would require building a collider that's like the size of the galaxy in order to get enough energy into it. Or some calculations suggest if you build a particle collider that big, it would collapse into a black hole.
Wait, why do you need so much energy?
Because gravity is really really weak, which means it operates on really small distance scales. In order to get to small distant scales, you need a lot of energy. It's sort of like the de Burglely wavelength, right, Like the wavelength of a particle is inversely proportional to its momentum, and so the more momentum an object has, the smaller its wavelength. And you want to see like really really short distance effects, you need really really high energy probes. So you need like super duper high energy particle collisions in order to see things happening at really short distance scales.
Why because I guess the more energy two particles have when they smash into each other somehow, that gives you more resolution in space.
Yeah, exactly, The more momentum the particle has, the smaller the wavelength of their wave function. You can think of the motion of every particle is described by a little wave function that determines what happens to it, the same way you can think of like light as a wave. Right, it's wiggling around. And if you're using light to see things, you only really see things that are the wavelength of that light or larger. Anything smaller than that wavelength the photon sort of can't interact with it. And so you want to see really really small effects, you need really really high energy photons or in our case, we need really really high energy particle beams to see very very short distance interactions.
Right, Because I guess you need things with mass right to test the quantum gravity or gravity at the quantum level. And so that's also true for things with mass, Like the faster they're going, the smaller they are.
Is that what you're saying, effectively, the smaller their wavelength is. Another way to think about it is that you need enough energy in those collisions to make gravity stronger, Like you want to overcome the electromagnetic force and the strong force and make gravity as powerful as those other forces so that you can see its effects. You need to pour a lot of energy into those collisions because the power of gravity is linked to the mass and to the energy of these things. So you pour enough energy into one little location, you'll get a very strong gravitational interaction. So if we want to see the gravitational effects on quantum particles, you need to pour a lot of energy into one little spot.
And is that the only way to do it through particle colliders? Isn't there some like I don't know, like aim your beams better approach or make smaller wavelength particles. I don't know, Like can we do this without making a black hole in our solar system?
Short answer is no. I mean, we do our best with particle beam aiming already, but really the limitation is the energy of the particles, and we have them going as fast as we can, and the wavelength of the particle is determined by its energy, so really sort of at the limit there. We talked recently about other strategies for accelerating particles that might make it smaller, faster, cheaper. So there might be a breakthrough in accelerator technology which could leap us up like a factor of ten or one hundred. But we are like a factor of a trillion away from being able to test quantum gravity in particle colliders. So really, nowhere in the near future will particle colliders be able to answer this question.
All right, Well, I think part of what we're going to be talking about here today are experiments that have kind of ideas about how to test this without destroying the Solar system. And they involve diamonds and lasers and space lasers.
No tabletop lasers in space. No, no tabletop lasers in Pasadena.
Oh that take up space in Pasadena a table near you? All right, well, if it's stay into it, Daniel, What is the first of these experiments?
So the first of the experiments involves falling diamonds, and the goal here essentially is to create a situation where a particle has the probability of being in two places at once, and then you test its gravity. You see if it's gravity really is sort of like split between its two possible locations, or if when you probe it with gravity, it somehow collapses into just having one possible location. Because remember this, quantum particles can do this weird thing. They can be in a superposition, like if there's two possibilities for an electron, it doesn't have to choose A or B. It can maintain both possibilities until something interacts with it classically, which forces it to choose. That's the weird thing about quantum mechanics and something we don't understand. So this is very hard, of course, because particles are very small and they're very delicate. But they've come up with a clever way that they think might be possible, and it involves electrons embedded in falling diamonds.
Sounds like a rap video where there's like money and diamonds falling from the sky. Break it down for a How does his experiment work?
So what you do is you take a very tiny diamond and has a nitrogen ated inside of it, like embedded inside the diamond, and this has a cool property, which is that if you zap it with a laser, the electrons have a probability to absorb that photon, which case they flip their spin to be up, or to ignore that photon and flip their spin to be down. So you shoot a laser at this diamond, and now it's in a quantum superposition of two possibilities. Maybe the electron inside there on the nitrogen is spin up, and maybe it's spin down. So you have your particle now in a quantum superposition. But what you need is for it to be in a quantum superposition of two locations rather than two spins. So then you pass it to a little magnetic field. The magnetic field will push it left or right based on the spin. So you have this falling diamond which passes through a magnetic field and it either moves left or it moves right. Now, if it's in a quantum superposition, then has both possibilities to move left and to move right. So now it's location depends on this quantumness. Now you do the same thing for another diamond nearby. Now you have this pair of falling diamonds, each of which has the possibility to be in two slightly different locations, and you see how they interact. Do the possibilities for one diamond interact with the possibilities for the other diamond, or do the two diamonds like collapse each other's wave functions?
I see. So you embed a little nitrogen atom into the diamond ezacly with a laser, and now the nirogen atom has quantumn certainty, which kind of extends to the whole diamond. Is basically what you're saying, right like if I don't know, there's quantum certainty about the electron into nitrogen, and that means there's quantum certainty about the whole diamond, because it could be swinging right or left. And now if you put two of them together really close, they should interact with gravity. And so now you have the system where you have two quantum objects interacting with gravity exactly.
And they have some really clever mathematical way to tell if the two diamonds interacted in a quantum way or if the two diamonds interacted in a classical way, like if they interacted in a quantum way, when you measure the spins of those electrons after they fall far enough in your experiment, they'll have some cool correlation to them, and if they interacted in a classical way, then they'll be uncorrelated, like whether they're spin up or down will just be random. And so because of the weird rules of quantum mechanics, you can tell whether quantum mechanics has been at play in the gravitational interaction, Like did gravity cancel out the quantum mechanic effects because it's really just a classical force, or did it allow the quantum uncertainty to be maintained, meaning that gravity would be a quantum mechanical effect not a classical effect.
Well, I guess quantum mechanics aside. Can you measure gravity the force of gravity by just dropping two diamonds together and seeing if they attract each other? Is that like a real thing you can do.
It's a real thing you can try to do. That's very very difficult because little diamonds have very very gentle gravity, and so this is it's not something we think we can do today. There's a group in the UK that thinks that they can figure out how to do this, and there's lots of complicated steps involved, and they're hoping to maybe pull this off sometime in the next ten years. But there's a lot of really complicated moving parts involvement getting the nitrogen inside the diamond, flipping its spin with a laser beam, getting two pairs of diamonds to fall simultaneously close enough each other that maybe they have a gravitational interaction. Now you don't actually have to see any sort of like gravitational pull. You're not measuring like how far did the diamond move because of gravity. You're just bringing them close enough together that you think gravity is at play. That gravity like wakes up and says, ooh, there's something going on here. You don't have to measure the gravity. You just have to see if gravity messes up the quantum state. I see.
But to measure the quantum state at the end, wouldn't you be doing something like measuring that whether or not the two diamonds were attracted to each other gravitationally or not.
No, all you need to do is measure the spins of those electrons embedded inside the nitrogen atoms in the diamonds. You don't see the gravitational effects directly. It's sort of like in the double slit experiment when you add a detector and that ruins the interference. We're adding gravity to a quantum interaction and seeing if it ruins the interference or not.
But would that necessarily tell you anything about quantum gravity or gravitons.
It wouldn't tell you that much, but it would be a powerful clue. It would tell you if gravity is classical or quantum mechanical. Like, if gravity is classical, it'll act like a detector and it'll collapse those wave functions and destroy this interference. If gravity is quantum mechanical, it won't, and everything quantum mechanical will stay quantum mechanical, and you get all sorts of weird interference. So that just tells you if gravity is classical or quantum mechanical. It doesn't tell you like, oh, space is quantized, or oh there are gravitons. It doesn't tell you which theory of quantum gravity. But it is a powerful clue. It would mean, for example, if we know gravity is quantum mechanical, the Freeman Dyson is wrong about classical gravity and quantum mechanics being able to play together.
Yeah, he could be wrong, in which case he might need to stick to making vacuum cleans. All right, Well, that's one experiment, and I guess it's in progress. I guess they're designing it or making it. Where are they with that?
This physicist at University College London who's leading a team of researchers who are trying to make this work, and there's folks in Santa Barbara as well, and they're trying to work on this. But you know, there's a lot of complicated steps and making this thing do its stance and being sure you know, what they're doing is a lot of pieces involved, lots of complicated experimental cleverness really required just to be able to do this test. So they're hoping sometime in the next ten years to be able to pull this off.
All right, Well, let's get to the second of these potential experiments to measure quantum gravity. We'll dig into that, but first let's take another quick break.
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All right, we're talking about quantum gravity and whether or not it's a thing, whether gravity is quantum mechanical or is it pretty class and classical and doesn't care about quantum mechanics and this weirdness of things be uncertain, And so we talked about one possible experiment that it might look at that using falling diamonds. And there's another interesting potential experiments happening also, right.
That's right, And this one is being developed and built in your.
Backyard, like literally my backyard.
Look at your window.
Man.
You ever wonder what those people want? Told you what? No, it's at cal Tech. Both the theorists and the experimental list are at Caltech and it's a really cool idea and what they're trying to do in this experiment, it's completely different from the other one, is try to see if space itself is quantum mechanical. Like, if gravity is quant mechanical and there are gravitons, then that would mean that graviton should be like popping out of the vacuum all the time, the same way that other quantum particles are. Like if you go out into the middle of empty space, there's nothing there, there's still always a little bit of energy in the quantum fields, which means that like those fields can turn into particles briefly and then back into potential energy. So if space itself is quantum mechanical, if gravity is quantum mechanical, then gravitons should also be popping out of the vacuum. There should be like effectively tiny little ripples in space making quantum size gravitational waves.
WHOA wait, I think you just confused me a little bit. So I thought there were two possibilities. Either gravity is quantum mechanical or space is quantized. Which one are you talking about here?
Here we're talking about gravity being quantum mechanical, that there exists gravitons which mediate the force of gravity, which in this theory would be a quantum force like the other forces in the universe.
Okay, so we're not talking about quantizing space itself.
That's right. We're not talking about quantizing space and like a space foam, but we're talking about space being filled with a quantum force of gravity, which would have fluctuations in it, right, And those fluctuations would be like quantum gravitons popping in and out of the quantum gravitational field.
I see. So anything that is quantum or has a quantum field by its nature, by its kind of statistical random nature, has these particles popping out of nothingness. But doesn't it need some sort of like energy to it.
It does, but quantum fields always have energy to them. They can never relax down to zero because the uncertainty principle, the minimum energy level of a quantum field is always above zero energy, which is why there's always energy in quantum fields, which is why there's energy in all of space because of this quantum nature.
Well, that's kind of an odd idea, Like what happens if a graviton appears out of nothingness, well.
Mostly almost nothing, because gravitons would be super duper tiny. Gravity is super duper weak, and so it would be basically impossible to see these things. What have effects on super tiny distance scales we typically can't probe. But a theorist at Caltech, Catherine Zurich, came up with this idea that maybe gravitons can all work together. Instead of just looking for one graviton, maybe she can look for like larger effects of graviton, sort of working together to make something else emerge from this quantum craziness. And she designed an experiment to maybe see that. Hmm.
Interesting. Well, we actually have an interview of Daniel talking with professor Catherine Zurich from cal Tech about her idea for this experiment.
That's right. Kathy and I have known each other since we were postos, and so I called her up and asked her about her crazy idea to not build a black hole in Pasadena.
I feel like it's a little says. You had to throw that disclaimer in there. It's like, what are you guys doing? I am not destroying your town if that's what you're asking. First of all, let's get that clear.
That didn't make you feel any better.
Nobody asked I wasn't something I was concerned about before, all.
Right, in that case, I'm also not testing any nuclear weapons in Pasadena.
Oh good, thank you? What else are you not doing in Pasadena? Let's go down the list. All right, Well, here is Daniel's interview with Professor Catherine Zurich from cal Tech.
All right, so it's my pleasure to welcome Professor Katherin Zurich the podcast. Thank you very much for chatting with us.
It's my pleasure to join you.
So help me stand first of all, how it's possible at all to see effects of quantum gravity. We understood for a long time that these things were just on the plank scale. How do they sort of work together to emerge to some signal we can see experimentally.
So it's just like smoke. So if you ask yourself the question how to smoke spread? So there are interactions of molecules on very short distance scales, much shorter than what we can observe, and yet you can see the effects of those short distance interactions simply by waiting a while for the effects of those short range interactions to accumulate over time. So that's a physical analogy for what we're interested in doing. So we have these quantum fluctuations on very short distance scales. So in this case, it's the plank length, which is about ten to the minus thirty five meters. And the idea is that if those quantum fluctuations accumulate over long times, then we can observe them. They're still very small, but we can observe them then with sufficiently precise instruments.
So what makes quantum fluctuations add up to make a microscopic effect and what makes them not because sometimes they don't. Right, you have like a bunch of electrons in a baseball, they have all such fluctuations, those average out to nothing. You can see what makes these guys add up over longer distance scales.
So it's really the fact that you're losing information. Any measurement that you make is over a finite time. So you know, I turn on my instrument. Let's say it's in an intraferometer, and the light goes out, it comes back, and I make a measurement of it. And so what it does. What an instrument does is it defines what we call a horizon. So there's a region of the space time that I measure and there's a region of the space time that I don't, And so that leads to a quantum mismeasurement effect that accumulates over time. So You're absolutely right that, you know, normal systems, where we can confine all of our information to a particular region, there's no information that's going to accumulate over time. But in this case, we can't actually confine quantum fluctuations. There's just part of the space time that we can't measure. And so what we're doing now is quantifying how much of that information is lost over the period of time that I make that measurement.
Very cool, and so what kind of models of quantum space time is as sensitive to generally any kind of model where space time has quantum fluctuations or only specific sort of kinds of ideas.
So what we're trying to show is that this effect occurs very generally the space of theoretical ideas that people explore, you know, commonly. So what do I mean by that exactly? So we're still trying to understand a precisely what are the minimal sets of requirements that you need. At minimum, we need quantum fluctuations at the plank scale, so that has to be there, and those quantum fluctuations have to accumulate into the infrared. And there are various ways that we can see that we can see it actually coming out in a quite broad range of theories where we can just write down some general properties of the theory and then crank through it and we see this effect come out. So we think it's pretty generic.
Wonderful, And so why can't existing in a pometers like LEGO, which is already very very precise, why can't it see signatures of this quantum fluctuation.
So we actually think that LEGO is not very far from being able to see it. But one of the reasons why LIGO is not optimally sensitive to this signal is because they recycle their light by which I mean the light beam goes out and it comes back, and they don't make a measurement of it after one round trick. It actually goes out and comes back many times before they make a measurement of it. And so for the signals that they're interested in, which come from you know, let's say black holes merging, that's fine because there's a classical source that generates a wave at some frequency. And this case, we're also interested in gravitational waves, but they're gravitational waves that come from the vacuum fluctuating, and they're uncorrelated. If I measured the system over time scales that are long in comparison to the light crossing time. So the fact that ligo weights and their beam goes out and comes back many times before they measure it means that they're actually average down their signal, and so they have a reduced sensitivity to it in comparison to if they had this same that they could measure the same space time fluctuation. But they did it after the light just went out and came back. Then we claim that you can actually see this signal.
Do these space time fluctuations look different from a gravitational way you would get from black hole collisions for example?
Yeah, they do. So one thing that's different about this signal in comparison to what you would get from let's say, black hole mergers is in that case, the signal is the signal. It doesn't depend on my measuring apparatus. If I have a gravitational wave coming in at some frequency, it's like your radio station is broadcasting something and it has a frequency, and that's just you know, you tune it to some station and that's what it is. In this case, what you measure actually depends on your apparatus, Like your interferometer. So if I have a smaller apparatus, my signal is going to be coming in at a higher number radio state. Then if I have a bigger apparatus, then it comes in at a lower frequency station. The reason for that is because it's the quantum mismeasurement. And of course how much you're mismeasuring the space time depends on how big you know, the volume of space time.
You're measuring affects your horizon.
Yeah, it depends on the size of your horizon. That's another way of saying it. It depends on the size of your horizon, depends on how many quantum degrees of freedom or fluctuating inside your volume, which depends on how big your horizon is. And so as a result, you know, you would really know about this signal. First of all, it would have a very particular shape, but it would depend on your measuring apparatus, so you could compare between different instruments and then start to tell what the source of it would be.
And can you also see things unexpected, like if there's a general enough detector that you might see things that aren't these quantum fluctuations, then aren't gravitational wave Some black holes, but something else, you know, surprising.
Yeah. Sure, So these instruments that were interested in building, they can be sensitive to anything that's generating gravitational waves in that same frequency range. So the signal definitely has to be predictive enough to be able to tell apart different sources. And our claim is that the signal has very particular you know, frequencies that it's peaked at. It has angular correlations, like it depends on the angle between the arms and your interferometer. So therefore you'll be able to tell what the source of these gravitational waves.
Are and what's the sort of timeline like best case scenario when you guys can build this thing and discover quantum gravity.
Yeah.
Yeah, So we've got the first bit of funding to come in, and my colleague Lemacullor, who's spearheading this effort here at Caltech, you know, his lab is ramping up on this. There are some technological objectives that they have to demonstrate. They have to do R and D because they're proposing a novel readout scheme for these interferometer. What we have proposed is to have a demonstrator apparatus that would kind of scrape the signal. Okay, we're talking about two sigma kind of sensitivity in five years, so I think to really start to see this, you know, like five sigma, you're just really confident you can start to test various aspects of it. I think we're probably talking the ten year timescale.
So I've read your paper. There's a lot of nice theoretical maneuvers in there. My question to you is, do you believe this is going to be real? Like we turn this thing on in ten years? Nature tales? You answer, what's your confidence that this is out there that you're going to see it?
Yeah, so it doesn't seem to be going away. Let's put it that way. When you see something in a calculation, you know, you try to test it by doing a different calculation that behaves differently. You know, it has different theoretical systematics, and the kinds of things that you could mess up in the calculation are different, so on and so forth, and then you also check for whether it's in conflict with anything that you know. And through the process of doing this, you know, based on my experience, when you try to build a theory, oftentimes it'll fail, and then you try to fix it up. By adding other things to it. This has not been like that. If it seems like it's going to fail for some reason, it means that you should just stop and wait and try to understand what's there better, because it fixes itself. So to me, that's an indication that there's something there. It hangs together in a very self consistent way, and so from that point of view, I find it theoretically very attractive, very interesting. It smells right now. I don't want to tell nature what to do. Right. Nature gets to decid. You know, there are some things that go in right, there's this fundamental fluctuations, and then spacetime needs to remember right, So there needs to be the sense in which you're losing information. And if those two things are there in nature, and we certainly know lots of analogous physical systems where that happens, then we'll see it. But at the end of the day, nature decides. And that's one of the things I really like about this problem is I can write these things down on paper and they're beautiful, and I'm understanding more things about it from a mathematical perspective. But at the end of the day, nature gets to the side.
All right, well, we look forward to hearing nature's side of the story. Thanks very much for joining us today.
All right, pretty interesting. I'm super impressed you can talk to a theorist. I thought you guys spoke different languages and didn't like each other.
They mostly speak in Greek symbols exactly, but sometimes I can translate. These days, I'm trying to move a little bit in the direction of theoretical physics, so it's really fun for me to talk to these folks. But yeah, they think on a whole different plane of existence. But what's really cool are theorists who propose experiments, who develop new techniques and new ideas that allow experimentalists to maybe force the universe to reveal something about its nature. And the story of this one is similar to the story of a very similar experiment, which is LIGO, the interferometer that looked for classical gravitational waves. That was originally just a theoretical idea, and experimentalists were like, all right, let's try to build it, see if we can find it, and they did. This is like the quantum version of it, which would look for little quantum ripples in space time, basically little quantum gravitational waves, and the experiment itself is similar. It's a little interferometer, like shoot laser beams back and forth, see how they overlap, and see if you can catch a graviton interfering with those laser beams.
Hmmm, because the gravit times would be sort of like bending space. Is that the idea? Because gravity can't interact with photons unless or candy.
Gravity doesn't interact with photons in a sort of Newtonian way because toons have no mass, but gravity does bend space, and photons move through that bend space. And so yeah, you're exactly right. Like a little gravitational quantum fluctuation the kind she's looking for, would affect the shape of space for one of these beams and would sort of knock a photon out of the path. And that's what they're looking for.
The idea is that like a graviton would pop out of nowhere, it pops out, it bends space around it and maybe able to deflect the photon. Is that the idea.
That's the idea. But it's not one single graviton that would be totally invisible. It's this effect where a lot of gravitons are working together and the super duper weird thing is that this effect only happens when you're making a measurement. It's a quantum effect. It comes from not being able to see the whole universe. So she's imagining space filled with all these gravitons, and when you make this measurement, it can only be affected by like a certain bubble of the universe, a bubble of the universe that's like close enough to you that light can travel to you. Because you create this information horizon, you limit like the links of these gravitons, and so only some of them can talk to your experiment, and that's what creates this weird effect. And I'll be totally honest, there's a lot of math there that I just don't even understand. But she's been trying to prove to herself that this works or that this doesn't work, and the math just keeps holding together no matter how she probes it. So, as you heard maybe in the interview, she really believes this is real.
And so the idea is that you could maybe build this experiment on a tabletop like it could be, you know, a small experiment to prove a huge thing like quantum gravity.
Exactly, Ligo. The classical gravitational wave experiment is like kilometers long and cost billions of dollars. This would be like meters long. You literally could build it in a lab in the basement at Caltech, and if it works, they could see quantum gravitational effects on these beams of light, and they could prove that gravitons are out there and that they're dancing together to make these little tiny ripples in space time.
Cool. Well, she's welcome to hang out in my backyard and do the experiment here. That could be exciting.
I don't think she wants her experiment it's sprayed by the hose or like doused with water.
Balloons, yeah, or have screaming kids running all around it that you usually it tends to make gravitons shy.
I tend to dampen the effects of your experiment.
All right, Well, pretty exciting. Thank you to doctor Catherine Zurich for talking about her research. What does this all mean, Daniel? Are we far or near proving the idea of quantum gravity?
I think we're still pretty far from figuring anything out. The theorists are working hard and making progress all the time about building their theories. But now it's exciting that we have experimental efforts which maybe in the next five, ten, fifteen years, could provide us with really valuable clues to tell us, Oh, gravity is classical or nope, gravity is quantum mechanical. You better figure it out. That would be really powerful indication for sort of which direction to go theoretically. And I love this dance between experimental and theoretical physics. You know, the ideas flourish and then experiments kill them, or sometimes experiments discover something weird which inspires lots of new theoretical ideas. It's really beautiful to see the interplay of these two different avenues of exploration.
It's like a theoretical tango exactly.
Even the physicists don't really know how to flirt, and I think the tango is pretty flirtatious.
All right, well, it sounds like the answer is stay tuned. In theory, it might be ten to fifteen years, but in reality, who knows. It could be that we may never answer this question, or it could be that we'll answer it within our lifetimes.
That's right. We could be flirting with understanding or confusion.
We hope you enjoyed that thanks for joining us, see you next time.
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