Can Quantum Gravity Be Created in the Lab?

Quantum gravity is one of the biggest unresolved and challenging problems in physics, as it seeks to reconcile quantum mechanics, which governs the microscopic world, and general relativity, which describes the macroscopic world of gravity and space-time. Efforts to understand quantum gravity have been focused almost entirely at the theoretical level, but Monika Schleier-Smith at Stanford University has been exploring a novel experimental approach — trying to create quantum gravity from scratch. Using laser-cooled clouds of atoms, she is testing the idea that gravity might be an emergent phenomenon arising from quantum entanglement. In this episode of the Joy of Why podcast, Schleier-Smith discusses the thinking behind what she admits is a high-risk, high-reward approach, and how her experiments could provide important insights about entanglement and quantum mechanical systems even if the end goal of simulating quantum gravity is never achieved.

• 0:00Speaker 00

  Hey there. I'm Rachel Feldman, and I host a podcast from Popular Science called The Weirdest Thing I Learned This Week. Every other week, I circle up with guests like Bill Nye, Josh Gondalman, Mary Roach, and many more, to prove that the lofty and noble pursuit of science can also be profoundly weird. Ever heard about the time a bunch of doctors tried to turn George Washington into a zombie? Ever wondered what the people who climb Mount Everest do with all their poop? Ready to learn why snakes have two penises and humans may one day breathe through their butts? From flying Ford Pintos to the world's most illegal cheese, The Weirdest Thing I Learned This Week is the ultimate source for all things interesting, informative, and, most importantly, frickin' weird. So

• 0:43Speaker 00

  pause the podcast you're listening to right now and check out The Weirdest Thing I Learned This Week. It's available on Apple Podcasts, Spotify, or wherever you get your podcasts. Come on over whenever you're ready to get weird.

• 1:00Speaker 01

  I'm Steve Strogatz.

• 1:01Speaker 02

  And I'm Janna Levin.

• 1:02Speaker 01

  And this is The Joy of Why, a podcast from Quantum Magazine, exploring some of the biggest unanswered questions in math and science today. Here we are. Hey, Jana.

• 1:13Speaker 02

  Hey, Steve.

• 1:14Speaker 01

  I've got something really fun queued up for you today.

• 1:17Speaker 02

  Good. I'm looking forward to hearing about it.

• 1:18Speaker 01

  Okay, now, I think this should be kind of in your wheelhouse. It's about gravity.

• 1:22Speaker 02

  Hmm, that's definitely within my wheelhouse.

• 1:25Speaker 01

  And now let me add one more word: quantum gravity.

• 1:28Speaker 02

  Yeah, and now it's in no one's wheelhouse.

• 1:31Speaker 01

  That's interesting, isn't it? Because it's such a hard, open problem in physics. So I had the chance to speak to a great young physicist named Monica Schleier-Smith. She's at Stanford. And she is taking an approach I've never heard of anywhere else, which is to try to build a kind of toy model of quantum gravity in the laboratory.

• 1:53Speaker 02

  Wow.

• 1:54Speaker 01

  Funny thing, right? I mean, you think of quantum gravity as purely theoretical pencil and paper stuff.

• 1:59Speaker 02

  Yeah, absolutely. That seems maximally hard.

• 2:02Speaker 01

  Maximally hard, right? Do you ever hear this idea that gravity might emerge from entanglement?

• 2:09Speaker 02

  Oh, yes. It's one of my favorites.

• 2:11Speaker 01

  Is it?

• 2:11Speaker 02

  Actually, yeah. I find it really intriguing. I kind of think of it as the entanglement as like threads at the quantum level, and it embroiders a world that, from afar... Looks like it's smooth and continuous and you look up close and you realize it's really these entangled threads.

• 2:29Speaker 01

  Very poetic. I like it. Well, that is sort of the spirit of what we're doing in this episode with Monica. She's going to talk to us about the ways that she tries to entangle thousands of atoms.

• 2:43Speaker 02

  Hmm.

• 2:44Speaker 01

  That she has maintained at very low temperature so that they can express their quantum mechanical nature and get entangled. But it's many body quantum entanglement.

• 2:53Speaker 02

  So they're all entangled with each other as opposed to just pairs?

• 2:56Speaker 01

  Right. That's the new wrinkle here. I'm only used to the idea of, you know, you hear a lot about entangling two atoms or something like that in the old like Einstein, Podolsky, Rosen thought experiment and then later recreations of that in the lab. This is many body entanglement, thousands of atoms.

• 3:15Speaker 02

  Hmm.

• 3:16Speaker 01

  And I think she's trying to stitch together some kind of fabric of space and time like you just described.

• 3:21Speaker 02

  Wow. I mean, I'm not sure how she would get gravity out of that quite yet, but that's fascinating. I mean, I always thought there was a kind of monogamy of entanglement. So if one particle was maximally entangled with another, it had to be monogamous. It could not also be entangled with a third partner.

• 3:39Speaker 01

  That's interesting. I'd never heard that idea.

• 3:42Speaker 02

  So maybe each one's partially entangled with another, right? So they're not maximally entangled one to one.

• 3:49Speaker 01

  Yeah.

• 3:49Speaker 02

  It's a kind of polyamory of entanglement.

• 3:52Speaker 01

  Well, I knew you would get interested in this. And so I think we should just hear from Stanford physicist Monica Schleier Smith. Hey, Monica, welcome to the show.

• 4:04Speaker 02

  Thank you.

• 4:04Speaker 01

  Before we get rolling on the question of what you've been doing in quantum physics experiments, I read somewhere that you got an early start, even as a high school student doing nanotechnology in a lab.

• 4:16Speaker 03

  So yeah, I was very fortunate as a high school student to get to do summer research internships at a company called the Muyter Corporation. I wasn't yet working in the lab, but I was getting to really grapple with forefront issues of cutting-edge research in nanotechnology. And that was really remarkable, being a 16-year-old, getting to read scientific papers and be in a research group.

• 4:38Speaker 01

  Amazing. When you say you weren't in the lab, did they have you doing computer simulations? Or what kind of thing?

• 4:44Speaker 03

  That's right. I was doing computer simulations. I was developing some ideas that actually led to a patent. We were able to collaborate with a group at Penn State that had the expertise and was able to take it to the next level and led to actually publishable results. And after I left, they actually started a lab based on some of the ideas we'd been brainstorming about when I was there.

• 5:07Speaker 01

  What a great start to a scientific life. Should we picture you as a little kid with scientific parents or going out in the woods, looking at bugs? Or what was your deep background?

• 5:18Speaker 03

  Definitely going out into the woods, exploring. My mother, actually, her background was in linguistics and really more in the humanities. But she always wished she had gone into science. And so that was a very strong influence. And I have an older brother who took the first steps in studying physics in college. And I guess I was inspired and followed in those footsteps.

• 5:41Speaker 01

  Quite a family. Well, you're part of this giant enterprise of modern physics. And there's this so-called standard model. I mean, it's a super-successful theory, but even the most ardent proponents of the standard model would agree there are a few things that have to be filled in. Can you tell us a little about what is that model?

• 6:02Speaker 03

  Yeah. So the standard model describes a wide range of particles that make up matter in our universe: electrons, protons, and their constituent quarks, photons. And part of the standard model, for example, is also that some of these particles are responsible for electric forces between charged particles, electrons and protons. There are additional particles that are responsible actually for mediating those forces. A photon is actually responsible for mediating these electric forces, kind of bouncing between the charged particles that are interacting. One thing that's missing, actually, in the standard model, one significant omission is a particle that mediates the force of gravity.

• 6:43Speaker 03

  Right, so in the same way that charged particles can attract or repel depending on their charges, gravity seems, on the face of it, very analogous. The mass of an object is kind of the equivalent to the charge. In the case of electric forces, the mass determines how strong the attractive force is in gravity. But the standard model doesn't have that equivalent of the photon for electromagnetism. It doesn't have something like a graviton that would mediate the gravitational forces. It's possible to have this theory, the standard model, that is extraordinarily well-tested, but has this sort of glaring omission of gravity.

• 7:17Speaker 01

  Right, so you've mentioned it doesn't have the counterpart of the photon, the graviton. I mean, we talk about gravitons, but they're not part of the standard model. I hear there are a few other things, like neutrinos, these tiny neutral particles, shouldn't have any mass, but they do.

• 7:35Speaker 03

  Yeah, and there are big mysteries in our universe also about what we call dark matter and dark energy. You know, 70% of the energy that should be there is missing in the form of this dark energy that we can't account for. So lots of big mysteries in the universe, even though the standard model is experimentally extraordinarily well-tested.

• 7:53Speaker 01

  Okay, but you put your finger specifically on this missing part of gravity in the standard model, and so that's what we'll be talking about mostly. So, given that gravity is not in the standard model, even though we know it's a real important force, it's keeping us both in our seats at the moment, how is that hurting our current understanding of the universe? We do have a very excellent theory of gravity. Newton's theory, or then if we want to get fancier Einstein's general relativity.

• 8:21Speaker 03

  That's right, we do have an excellent theory. Einstein's general relativity is also extraordinarily well-tested. If I go back to the analogy with electromagnetism, the sort of classical picture is that electromagnetic forces, they're mediated by electromagnetic waves or light. And in gravity, we've by now even detected gravitational waves, right? So, in its own right, gravity is also very well-understood and tested. And at some level, the challenge is that the microscopic description is really Kind of quantum mechanical, quantum mechanics is a great theory for describing systems at very small scales. But gravity is a theory that works very well in the regime of massive objects. You know, the motion of planets.

• 9:05Speaker 03

  And these are tested in very different regimes. It's hard to get into a regime where actually both gravity and quantum mechanics matter. It's mostly so far in thought experiments that we realize we don't have a unified theory and that there's something missing when we can't connect the rules of quantum mechanics with the rules of gravity.

• 9:26Speaker 01

  That's interesting, this last point that you just raised, because there are parts of relativity that play nicely with quantum theory, right? Like, we do have special relativity.

• 9:38Speaker 03

  That's right. And again, I think it's partly this issue that one theory or the other applies well, or one can put in the minimal ingredients, let's say, special relativity of gravity and combine that with quantum mechanics. But somehow a full unified theory is still missing. I mean, just to give one other example that I find kind of puzzling. In gravity, space and time are treated on an equal footing. In quantum mechanics, we actually don't treat them on an equal footing. Systems evolve in time and space is thought of completely separately. And so somehow there are these two inconsistent ways of thinking about the universe. And one has to start doing thought experiments about things like what happens to information that falls into a black hole

• 10:22Speaker 03

  to start to realize that actually, to really fully understand our universe, we need to reconcile them.

• 10:28Speaker 01

  Hmm, I'm glad you put your finger on space and time, because that's really what we do want to be talking about here. I mean, they're all linked up, aren't they? Space, time, gravity. And then this other whole story of quantum mechanics. So let's talk about the possibility that space and time might not be as fundamental as we used to think.

• 10:46Speaker 03

  Yeah, and so one of the remarkable ideas that's emerged from theorists who think hard about this problem of reconciling quantum mechanics and gravity is the notion that perhaps the fundamental building blocks of gravity really are quantum mechanical. A number of years ago, I found a quote from, you might say, the father of the atom, Democritus, right? He was the Greek philosopher who recognized that matter is not just some smooth, continuous thing. It has actually fundamental building blocks that are atoms and molecules. And then he made this point that phenomena such as, you know, hot and cold, sweet or

• 11:29Speaker 03

  bitter, taste, temperature, colors, emerge from the microscopic configurations of individual atoms or molecules. I don't need to think about the positions of all the individual atoms to look at an object and say it's red, right? And so color is this kind of emergent phenomenon. So the question that has been explored in recent years in this effort to unify quantum mechanics and gravity is, could it be that gravity is actually also an emergent phenomenon? So the microscopic constituents are really quantum mechanical, and gravity emerges as this sort of simplified, smooth description of what fundamentally is really

• 12:08Speaker 03

  some complex, interacting quantum system. And I find that idea fascinating. And how might gravity emerge from quantum mechanics? The connection that's conjectured is a phenomenon called entanglement.

• 12:22Speaker 01

  Go on, I want to hear more because it is incredibly fascinating. The first time I heard it, my mind was blown. Tell us.

• 12:28Speaker 03

  Yeah, so entanglement is the idea that I can store information, not just in individual bits or particles, but actually in correlations. So, you know, in your computer, you have information that's stored in bits that are in like a one state or a zero state, but that information, it's really stored locally in an individual bit. So the quantum analog of a bit, we call it a qubit. And it's possible to have information that's not just stored in a single qubit. If you look at the state of a single qubit, it looks completely random. In fact, randomness is an inherent aspect of quantum mechanics. But if you look at the states of two of these qubits, you would find they're always either both 1 or they're always both 0, even though each one individually looks random. And so there is actually some order in the randomness,

• 13:12Speaker 03

  some information that can be stored in a way that you can only access if you look at both of these qubits. So this idea of correlations and information that are sort of hidden in this randomness, that's this notion of entanglement. And one of the sort of challenges this brings up is that describing a quantum system is actually much more complex than describing the bits in your classical computer, because you need to keep track not just of the states of the individual qubits, but of all of these correlations between them. So sometimes I like to sort of visualize a graph where I have my row of these qubits, but then I want to sort of draw some lines that indicate something about the structure of which ones are correlated with which ones. And that's still an overly simplified description.

• 13:57Speaker 03

  But roughly speaking, these correlations, I can visualize as some connections between the qubits. And now the idea is that perhaps actually this notion of gravity being an emergent phenomenon, the idea is actually describing those correlations. And I kind of think of it as there's this one additional dimension that allows me to capture extra information associated that describes the structure of correlations. There's some mapping from the quantum mechanical system to actually a geometrical description in which the distance between the qubits says something about how strongly they're correlated. This notion has also been given the name of holographic duality. So why holographic? A hologram is something that has two dimensions,

• 14:41Speaker 03

  but actually it looks like a three-dimensional image, right? It has this sort of additional dimension. So there's this notion that somehow, once one accounts for the entanglement between all these degrees of freedom, gravity may emerge as a description of those microscopic quantum building blocks, a sort of smooth macroscopic description in terms of spacetime curvature and geometry.

• 15:03Speaker 01

  Okay, so that is a lot, a lot, a lot going on there.

• 15:07Speaker 03

  I know.

• 15:08Speaker 01

  And that's fine, because you've given us a lot to chew on now. You said this really deep, interesting thing that, if I can paraphrase and correct me if I'm not hearing you right. It's sort of like saying distance is an illusion. What really is meaningful is correlation, right? That's sort of the idea, that things that look like they're a certain distance apart, that's our macroscopic way as big creatures of thinking about what microscopically is about strong correlations, or maybe weak correlations.

• 15:39Speaker 03

  Yeah, exactly. Like, a long distance would sort of correspond to a weaker correlation, roughly speaking. Exactly.

• 15:45Speaker 01

  Okay. So we'll have to come back to that, this idea that space and distance is really just an emergent way of talking about what's really going on under the hood, which is correlations of different strengths. But, so you spoke about that there can be information in the relationship between two things that are otherwise completely random on their own.

• 16:05Speaker 03

  Right. I kind of like to use the analogy of a coin toss, right? And so, like, imagine I'm here and I'm tossing coins, and every time I toss one, you'll also toss a coin. And when we look at the outcomes of those coin tosses, I'll see something completely random, you'll see a random sequence of heads and tails. And classically, that's all there is to it, and there's no correlation. But quantum mechanically, we could have a situation where every time I get heads, you get tails. And every time I get tails, you get heads, despite the fact that I'm here in California and you're...

• 16:38Speaker 01

  In Ithaca.

• 16:39Speaker 03

  Yeah, exactly. And so, that would be very weird, right?

• 16:43Speaker 01

  Right, especially where I'm far enough away that you couldn't possibly get a signal to me fast enough to influence me.

• 16:49Speaker 03

  Exactly. There were more and more experiments over the past 20 years or so, trying to really make sure that we verified entanglement in a setting where these two measurements were far enough apart that there couldn't be any information traveling between them and things like that.

• 17:04Speaker 01

  Okay. So, as it started, it was a very theoretical idea, going back to the 1930s or something, right? From Einstein and Podolsky and Rosen and Schrodinger and people like that. But now, fast-forwarding to almost a century later, it's not obvious to me how you would maintain the entanglement over great distances. Does it take tremendous care to keep them entangled?

• 17:24Speaker 03

  It's tremendously challenging and to bridge long distances. There are different choices you could make of photons because they travel at the speed of light across long distances. But even so, there's some possibility that the photon, if it's sent through an optical fiber, that it's lost along the way, or also if it's sent through free space, there's still some chance it'll get absorbed along the way. Sometimes there are tricks where you can do what's called heralding, that like, maybe you don't succeed every time, but there's a way to know actually whether you successfully created an entanglement

• 17:54Speaker 01

  Huh. Okay, but now it sounds like you and your students are doing this every day now. So maybe you should tell us, what are you entangling?

• 18:02Speaker 03

  In my lab, we work on smaller length scales, you know, the particles that we entangle are atoms. And an atom is an Angstrom scale object. Ordinarily, you would think that if I have two atoms that are, let's say, a millimeter apart, they won't interact, they won't become correlated. But that's actually a length scale where we are able to generate entanglement. And the way that we do it is in fact, actually using light. I use this notion of mediating interactions. We use a very engineered setup in the lab where a photon can bounce between two atoms or between two clouds of atoms and introduce correlations and entanglement between them. And that's not the only way that one can generate entanglement among atoms. I'll focus on this one because one of the nice things

• 18:47Speaker 03

  about photons is they can quickly bridge long distances, and they can give a lot of flexibility. Naively, you would think what will naturally happen is atoms will maybe bump into other atoms that are near them, and you'll sort of generate strong correlations between neighboring atoms. And what we like to be able to do is have some network where we can actually control the structure of correlations and decide by some knobs in the experiment, the atoms that are most strongly correlated. That's a way of using photons to

• 19:16Speaker 03

  Program the graph of correlations in, in our case, an array of clouds of atoms.

• 19:22Speaker 01

  So I do want to hear about the clouds of atoms and the programmable networks that you're building or engineering, but can you give us an oral picture? Like if we were standing behind you, looking over your shoulder, what would we see?

• 19:36Speaker 03

  In any given lab in my research group, you would see something like two to three optical tables. So each of these, something like four foot by eight foot, maybe even a bit bigger. There's usually one table that has a bunch of lasers because I mentioned we need laser light as our tool for manipulating atoms. So you'd see these lasers, you would see lots and lots of mirrors and various other optical elements to steer the lasers into the right places. And then all of these laser beams get steered into optical fibers, going from one table to another table, which carry that light to where our science experiments actually happen. And that second table has on it an ultra-high vacuum chamber, which we need in order to

• 20:20Speaker 03

  have particular atoms in one of our labs, that's rubidium atoms, that are sort of well-isolated from anything else in the lab, right? So we want to be operating in an ultra-high vacuum environment where I can just create a cloud or a few clouds of atoms that are at very low temperature and that are essentially suspended by laser beams in the middle of this, this vacuum chamber.

• 20:44Speaker 01

  These rubidium atoms are in what you're calling high vacuum, so meaning they're not bumping into any oxygen or nitrogen. There's no air in there.

• 20:52Speaker 03

  Right, yeah.

• 20:53Speaker 01

  They're just rubidium atoms, which I don't even really know how to think about rubidium. I've heard of rubidium.

• 20:58Speaker 03

  If you remember your periodic table.

• 21:00Speaker 01

  Yeah, no, I don't. What, tell me.

• 21:02Speaker 03

  I'll just say it's, it's in the first column. And what that means is basically there's one valence electron, so one outer electron that is relatively well-isolated from all of the other electrons. And that actually turns out to be convenient for making it a relatively simple atom to control and manipulate with lasers.

• 21:22Speaker 01

  I see. And why do you want to have a cloud of them?

• 21:25Speaker 03

  So I'll say that I might actually prefer not to have a cloud, but we work with a cloud.

• 21:29Speaker 01

  Oh.

• 21:29Speaker 03

  For the class of experiments I described where we use light photons as our means of generating some network of interactions between atoms. We can actually generate stronger interactions if we use many atoms rather than just one. If I go back to sort of the wave picture of light, there's constructive interference. A photon bouncing off one atom and hitting another atom to make them interact. If I have many atoms, the waves that they scatter can interfere constructively, and that can actually enhance the strength of interaction.

• 22:02Speaker 01

  Cool.

• 22:02Speaker 03

  And so at least for kind of first experiments, it's been convenient for us to work with, let's say, clouds where each cloud has a thousand atoms and then we have an array of such clouds. But there is a path that we're interested in actually going towards single atoms where each atom interacts more strongly.

• 22:19Speaker 01

  Okay, but so for now, we could think of a cloud of 10,000 atoms or something. But you mentioned low temperatures, so you want to tell us how low?

• 22:28Speaker 03

  Yeah, so typically, we work with temperatures that are on the scale of tens of microkelvin. A thousandth of a degree above absolute zero would be a millikelvin. We're often a factor of 20 or 50 below that in temperature.

• 22:43Speaker 01

  Wow.

• 22:44Speaker 03

  And so that sounds extraordinarily cold. One thing to keep in mind is actually, the room isn't cold, that vacuum chamber isn't cold. If you touch it, it's at room temperature, but it's just this cloud of atoms suspended by laser light that we are able to actually bring to very low temperature using tricks of laser cooling.

• 23:03Speaker 01

  There's some way of using the lasers to hit the atoms in just the right way that sort of knocks the wind out of them?

• 23:08Speaker 03

  Exactly, that's a great analogy.

• 23:09Speaker 01

  Okay, I see. So I'm getting the picture now. You've got these clouds, 10,000 atoms, you get them very cold, not to the point where they're a single quantum object in the sense of Bose-Einstein condensate, but still. They're constructively interfering in the wave picture enough that it's sort of like a strong edge in this network of interactions that you're trying to build.

• 23:31Speaker 03

  And maybe I can also just add, the reason we don't need to get them down to this state of matter of Bose-Einstein condensation, it turns out if the atoms are moving around a little bit, that's okay for the experiments that we do. There's still a sense in which we place all of our atoms in a given cloud into the same quantum state. So they're not all at the same position, but what we care about most in our experiments is some internal state of the atom. So there's that electron we talked about, and we control which state that electron is in, we control which way its spin is pointing, and so we have actually very good control over the internal states of these atoms, and those will all be identical in a given cloud.

• 24:13Speaker 01

  Uh huh. So is the entanglement that you're trying to set up at the level of spins, then? Because I mean, I know in the traditional old thought experiments about entanglement, they used to frequently talk about the spin of two different particles.

• 24:25Speaker 03

  Exactly. And so the spin is sort of the real physical implementation of what I talked about before with the heads and tails of the coin, right? These two possible states of the coin are like a spin that points up or down. And actually, in quantum mechanics, a spin that could point anywhere in three dimensions, but when we decide to do a measurement, we have to choose what we call a basis. We can measure, does it point up or down? Does it point right or left? But we actually can't determine both of those things at the same time. So these are examples of incompatible observables. A famous example is position and momentum of a particle. Two different components of the spin, the vertical or the horizontal, that would be another example.

• 25:08Speaker 01

  Okay, so now we've got the visual of you in your lab, and the optical table, and all the lasers, and mirrors, and cables. But then, now that you have this ability to entangle these clouds, you can make whatever networks you want, if I'm hearing you right. You know, you're doing this very fundamental research about what in the jargon might be called something like many-body entanglement.

• 25:33Speaker 01

  So that's sure to be important.

• 25:35Speaker 03

  Right, so having control over entanglement can be a resource for making better precision measurements when what limits you is quantum uncertainty, the sort of randomness inherent in quantum mechanics, or another one, quantum computation. The idea is if you have a sufficiently well-controlled quantum system, where you can really program in the interactions in the same way that you program your classical computer, but now the building blocks are quantum bits, then one place that seems very natural to give us an advantage is precisely in describing quantum mechanical systems, be it the behavior of electrons and materials, or be it actually problems from chemistry, for example.

• 26:18Speaker 02

  I'm kind of dumbfounded a little bit.

• 26:21Speaker 02

  I mean, you have these many-body systems, she's somehow managed to entangle them in this kind of a network. How do I get from there to gravity? Why would I think it's gravity and not some other complex system that emerges from the many-body problem?

• 26:36Speaker 01

  That's really a good question. I think she is looking for signatures of something that would be like a discrete analog of curvature of a continuous space. So networks can have properties like curvature, the way that smooth manifolds can have curvature.

• 26:54Speaker 02

  Amazing, so she's trying to make a spacetime, or like a manifold or something?

• 26:58Speaker 01

  It's something like you said, with your beautiful analogy of the embroidered fabric, that it might look like a nice, smooth dress, but you look up close, it's a lot of threads stitched together.

• 27:08Speaker 02

  Right, oh fascinating. So the network itself is an emerging spacetime, in some sense?

• 27:14Speaker 01

  Something like that, but it's controversial, what she's doing, and as she says, it's also possible that this won't lead to a deeper understanding of gravity, but maybe it will help with precision measurements, or maybe it will help with quantum computing. We're going to get right into that after the break.

• 27:47Speaker 01

  Welcome back to The Joy of Why, where we're speaking with Stanford physicist Monica Schleier-Smith, who's been telling us about her toy model of gravity. So, there is a certain amount of controversy attached to this idea that gravity and spacetime emerge from quantum entanglement. We haven't really said that out loud, but maybe we should.

• 28:10Speaker 03

  Right, absolutely.

• 28:11Speaker 01

  So like, even if that doesn't pan out, you're not wasting your time, I think, in the lab.

• 28:16Speaker 03

  I like to think not, again, it's not the only thing I'm working on, but also, for me, I'm fascinated by the idea that gravity in our universe might be an emergent phenomenon, where the building blocks are quantum mechanics. But there's also another way to think about this whole field, which is to say, there are certain cases where one sees this so-called duality. So there's a strongly interacting quantum system that has an equivalent description in terms of equations that look like gravity. And whether or not gravity in our universe is a manifestation of quantum mechanics, these theoretical tools of taking a strongly interacting quantum mechanical system and mapping them to a description in terms of curved space and gravity, maybe that gives you new ways

• 29:01Speaker 03

  of calculating things about the quantum mechanical system, or new insights into how it will behave.

• 29:06Speaker 01

  Okay, I'm sold. This is good. So, maybe you should tell me then about this kind of toy model of gravity. What do these networks have to do with quantum gravity and spacetime?

• 29:15Speaker 03

  Yeah, you know, this effort at understanding quantum gravity has been almost purely the domain of theory until recently. And so, my thought was, if gravity might be an emergent phenomenon, if curved space might be something that can emerge as a natural description of quantum correlations, can we build a system where we start to see this phenomenon of something that looks like curved space emerging as a description of the quantum correlations? Before we did an experiment, rather serendipitously, we began talking with a theorist at Princeton named Steve Gubser, who actually tragically passed away in a climbing accident a few years ago. But Steve was working on this effort to reconcile quantum mechanics and gravity.

• 29:59Speaker 03

  He had developed, I would say, a particular version of this holographic duality. Right, so holographic duality was this notion that I have a quantum mechanical system that I can think of as kind of living on the boundary where the higher dimensional space has gravity and the gravity gives rise to curvature in that space and the distances within that higher dimensional space say something about correlations in the quantum system on the boundary. What was really valuable to us about talking to Steve Gubser was that he was working on a formulation of this holographic duality where there's a very nice way of actually visualizing bulk geometry. He wanted actually a discretized theory. So what do I mean by that?

• 30:42Speaker 03

  He wanted there to be a shortest length scale in his theory, motivated actually by the fact that in our universe, there's a length scale called the Planck length, where you expect to start running into major problems in reconciling quantum mechanics and gravity. And anyway, so Steve had this discretized version of holographic duality where the bulk geometry is represented by a tree and the boundary where the quantum mechanical system lives, it lives on the leaves of the tree.

• 31:11Speaker 01

  Ah, nice.

• 31:12Speaker 03

  Yeah, so you can kind of imagine if you're at the trunk of the tree, that's somewhere in the middle of the bulk, somewhere in the middle of the part that's described by gravity. And then that trunk has two branches and each of those has two more branches and each of those has two more branches. In just a few steps, the number of leaves actually grows exponentially with the distance that you go out from the trunk. If you think of the tree as kind of radiating outwards so that the leaves end up on a circle, you have this weird thing where the circumference of that circle is actually exponentially larger than the distance measured in how many times you need to branch to get to the leaf. So we have this graph, and again, I kind of think of it as I can visualize it as the

• 31:54Speaker 03

  leaves live on the circumference of a circle, the circumference is exponentially larger than the diameter instead of being larger by a factor of pi. And so somehow like This isn't just a flat disk, it's curved, and it actually has negative curvature.

• 32:10Speaker 01

  Oh, I see. You're saying that's why it's curved, because if it were just a flat disk, it should only be the 2 pi times the radius, but this is way more circumference than that.

• 32:19Speaker 03

  Exactly. If anyone in the audience has seen some prints by Escher, so like he'll tile a disk with fish, and in the middle there are big fish, and around the circumference there are lots of really tiny fish, and that actually really is a representation of this hyperbolic geometry.

• 32:35Speaker 01

  Yeah, and why hyperbolic? I mean, there's also this nice picture of, like, if you're trying to flatten out a rug, it may be fine, but if you try to flatten out a piece of lettuce or something that looks like a saddle.

• 32:48Speaker 03

  Right, yeah.

• 32:49Speaker 01

  It keeps popping up, right? You can't flatten it out easily, and it's because it has too much circumference for its distance from the center. So that's what negative curved space is kind of like.

• 33:00Speaker 03

  Exactly. And so this tree graph is a discretized version of this negatively curved space, or what is called anti-disider space in the context of general relativity and gravity. So we kind of asked ourselves, is there a quantum mechanical system we can build that would have correlations that make it sort of look like it lives on the leaves of a tree graph? And we realized there is a toy model where a given site in my array can talk to its nearest neighbor, its second neighbor, its fourth neighbor, its eighth neighbor. It's not a lot of connections that you need to build, but they give you an efficient way of getting information from one point to any other. So we've actually been thinking

• 33:42Speaker 03

  about that, and then we got in touch with Steve Gubser, and he pointed out, if you tweak this model just a little bit, so that the longest range interactions are the strongest, like the atoms that are far apart physically actually have the strongest interactions, that might make it look like your system of atoms lives on the leaves of a tree graph.

• 34:01Speaker 01

  Mm-hmm. And so what would you have to measure to see a signature that the counterpart of space is being curved?

• 34:08Speaker 03

  Essentially, we measure spin correlations. If a given spin is pointing to the right, how likely is it that another site of our array also has the spin pointing to the right? So... My students started thinking about what's a clever way to plot these data. You have to put some constraints on how you plotted. It's nice to plot things on a page, so in two dimensions. And so he ended up with some plot where the sites ended up arranged around the circle. The ones where we measured the strongest correlations were close to each other. And the order of the sites is actually very different from what it is in our physical system, but it's a nice way of representing things as near each other if the correlations between them are strong.

• 34:50Speaker 03

  And then he also started drawing lines, so pairs of sites with the strongest correlations, he drew a line between them. And then he sort of treated those as a new bigger site, asked what's the average direction the spin is pointing on that bigger site. And by iterating that process, the picture that popped out was precisely this tree graph.

• 35:08Speaker 01

  Let me make sure I understand one aspect, though. Since you have so much ability to program, who's interacting with who? What part of the results is a surprise to you? How much is built in and how much is not built in?

• 35:21Speaker 03

  Yeah, so at some level, when we did the experiment, you know, we did know what we thought should come out.

• 35:26Speaker 01

  Right.

• 35:27Speaker 03

  We didn't know yet how we would analyze the data. And this way of actually visualizing the tree graph came up in discussions between me and my students, and that wasn't something we had thought about before we did the experiment. In general, we often start with something where we know what should happen. And the goal is eventually, and we're not there yet, to be honest, but the goal is eventually to get somewhere where we actually don't know what will happen. There are some proposals for more kind of agnostic methods of trying to go from a quantum mechanical system to determine, does it have an emergent description in terms of some curved higher dimensional space? What is the geometry? What is the metric on that higher dimensional space?

• 36:08Speaker 03

  So I think that's an important direction for future research, is to go beyond just we build a toy model where we know what should happen. There's lots more one can do quantitatively. And actually, since then, we've been developing tools where we actually do, not in the tree graph, but in simpler settings, really probe the spatial structure of entanglement. So I really think it's a first step to even start to connect. Part of it is even connecting two different communities, right? Figuring out a common language for even talking to theorists who think about gravity when all I know is quantum mechanics.

• 36:40Speaker 01

  Well, I hope this is not a too sensitive question, but you mentioned Steve Gupsor, who I didn't know. Was he alive to see the triumph of, you know, his insight about how to do the setup? That it did really work?

• 36:55Speaker 03

  He was not alive to see us do the experiment. Yeah, so we had a theoretical proposal published in physical review letters. And it was really shortly after we wrapped that up that he tragically passed away in this climbing accident. I'm sad that he didn't get to see the experiment. And I still find it really motivating to try to kind of push forward and continue.

• 37:18Speaker 01

  He clearly had some great insight because, as you say, it didn't seem obvious that that was the way to set things up.

• 37:23Speaker 03

  No, yeah.

• 37:24Speaker 01

  Well, the whole thing really seems very high risk to me. I have to say, high risk, high reward. I'm just wondering, is that kind of who you are? Are you that type of scientist?

• 37:35Speaker 03

  I'm somebody who looks for some of the problems I work on to be ones that everybody else isn't also working on. My sense is, you know, if everybody else is already doing a thing, like I might not need to do it. And yeah, maybe that does draw me into things that are a little bit more on the risky side. And this direction of simulating quantum gravity is one that I find high risk, high reward. But that same toolbox has applications in enhanced precision measurements, in quantum computation. Even if it helps, it gives some new insight to the theorists that then take that to the next level to learn something about gravity in our universe. That would be amazing. If it even just gives us new ways of thinking about quantum mechanical systems, right? That can help us in that effort to engineer and understand quantum antibody systems,

• 38:20Speaker 03

  be it for applications in precision sensing, computation, understanding the design of materials, right? I think there's a broad effort to control and understand entanglement that will benefit from research in this area. And if it answers questions about gravity in our universe, that's amazing, but even if it doesn't, we won't have wasted our time.

• 38:42Speaker 01

  I agree. It seems sort of surefire in a way. It can't possibly be as risky as it sounds because, as you say, how could it hurt to learn more about how to control quantum systems and manipulate them with entanglement? That has to be good.

• 38:55Speaker 03

  That's what I think. Yeah.

• 38:56Speaker 01

  Well, okay, so let me close with kind of an emotional question. What is it that really fires you up? What are you really trying to do? What's your motivation?

• 39:06Speaker 03

  I really think that it does help get me motivated to know that something we learn in our experiments may have practical application, but it's not important to me personally that that application is tomorrow. If it's 50 years down the line and actually it's not the application we thought it would be and it's a different one, that's fine with me. So I think I do believe deeply that fundamental science will ultimately have technological impact, but I enjoy it also just for the kind of thrill of being at the frontier of the unknown and trying to push that frontier forward.

• 39:40Speaker 01

  Well, it's so exciting to hear about this. I really appreciate you taking the time to talk to us today, Monica, and thanks for being on The Joy of Why.

• 39:49Speaker 03

  Thank you. It was a pleasure to chat.

• 39:53Speaker 02

  I love this high-risk statement. I absolutely love that she said she likes to at least do some of her work in an area which is not crowded, that nobody else is working on. I love both those things. It really appealed to me, and I think it's where a lot of the great stuff happens.

• 40:10Speaker 01

  But I wonder if your reaction is a little bit of a reflection of who you are.

• 40:16Speaker 02

  Oh, absolutely.

• 40:18Speaker 01

  You know, I don't think this is for everybody.

• 40:20Speaker 02

  It's not. It's not. And obviously, if you're gonna build the James Webb Space Telescope, you don't want to be a high-risk thinker. I mean, it's a very risky project, right? But it was clear and well-defined, and the reason it was late and over budget is because they were minimizing the risk. So you definitely don't want everybody being like that. You want people to collaborate on big projects together and move in the same direction. But yeah, absolutely. My whole life, people were telling me, "Don't work on that. Nobody's doing that."

• 40:50Speaker 01

  Well, I think the secret is sort of like in relationships where you're looking for the match.

• 40:56Speaker 02

  Yes.

• 40:57Speaker 01

  And so I feel like there's a lot of ways to be a good scientist or mathematician, and you have to know yourself. So Monica... Clearly is not scared of risk. She's thinking about the long game. You know, she reminds me of Gaudi, that Sagrada Familia in Barcelona. You know, he didn't get to live to see it finished, but he had a great vision and they're still building it as far as I can tell.

• 41:18Speaker 02

  Yeah, an interminable project.

• 41:21Speaker 01

  But it's not for everyone. Not everyone has that kind of nerve. Like she could hit the jackpot or she might, you know, get a nice little payout.

• 41:28Speaker 02

  Mm hmm. Yeah, it sounds like there's a lot of potential unanticipated consequences. I find that really inspiring. We throw something out in the world and we have to see who else is there to pick it up.

• 41:39Speaker 01

  Well, it's always a pleasure to chat with you about these things.

• 41:42Speaker 02

  And you, Steve.

• 41:42Speaker 01

  Thanks, Tiana. All right. Well, we'll see you next time.

• 41:44Speaker 02

  We'll see you next time.

• 41:49Speaker 02

  Thanks for listening. If you're enjoying The Joy of Why and you're not already subscribed, hit the subscribe or follow button where you're listening. You can also leave a review for the show. It helps people find this podcast, find articles, newsletters, videos, and more at quantamagazine.org.

• 42:08Speaker 01

  The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine. The Joy of Why is produced by PRX Productions. The production team is Caitlin Folds, Livia Brock, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzalez. Edwin Ochoa is our project manager. From Quanta Magazine, Simon France and Samir Patel provided editorial guidance with support

• 42:48Speaker 01

  from Matt Carlstrom, Samuel Velasco, Simone Barr, and Michael Canyongalo. Samir Patel is Quanta's editor in chief. Our theme music is from APM Music. The episode art is by Peter Greenwood and our logo is by Jackie King and Christina Armitage. Special thanks to the Columbia Journalism School and the Cornell Broadcast Studios. I'm your host, Steve Strogatz. If you have any questions or comments for us, please email us at [email protected].

• 43:30Speaker 02

  From PRX.