Can we simulate the Universe?

24:16And so let's take a break. And when we get back, we will talk more about simulations.

24:30We'll be right back.

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27:41And we're back. Okay, so we are talking about simulations, and Daniel, I have heard that quantum computers are going to help us simulate things in better ways. Quantum computers, I think when you listen to pop articles about quantum computers, they're able to do all kinds of amazing things, and I'm not actually sure that they're able to do all of the things that they are said to be able to do. You don't think they can quantum balance your checkbook? I do a fine job at – you know what? Actually, I don't have a checkbook. I just every once in a while open up my bank account and I'm like, that's what's happening there.

28:15You're like, oh, I hope the number's not negative. Yay, it's not negative. That's okay. Still not negative. We don't buy a lot of stuff. But anyway, so would quantum computing make it easier for us to simulate the universe or is it essentially no different than a classical computer in this regard? I mean, yes and no. The no is because anything a quantum computer can do, a classical computer can also do, right? Okay. So there's no problems that quantum computers can solve that classical computers can't. Can they do it faster? And so not in general. It's not like quantum computers are faster at everything than classical computers.

28:50You know, there are a few cases where we think quantum computers could be faster than classical computers. But so far, those are like really, really specialized cases, you know, like one factoring algorithm here or another mathematical algorithm there. So, so far, it's still like at the, ooh, this would be cool, and maybe it's the harbinger of things to come, and eventually everybody will have a quantum computer on their wrist. That would be cool, and it's exciting because it's like another way to do computing, which is fascinating from a philosophical point of view. But practically, it's not going to speed up our simulations of the Large Hadron Collider.

29:26It's not going to help us predict the weather. It's not going to balance your checkbook. There is one way in which it's cool. And this is one of the initial, I think, impetuses for quantum computing, which is, look, a lot of the physics we do is quantum, right? And so doesn't it make more sense to have a quantum computer to model quantum processes rather than a classical computer? You know, in some sense, the universe is quantum. So why are we building classical computers onto it to then model quantum processes? And so, you know, there are ideas about building mini quantum systems to describe those quantum systems.

30:01The same way like you build a mini river delta with real water to describe the real water in the actual river delta. You could build a little mini quantum system that has quantum properties and help you model a bigger or more complicated quantum system you're interested in. But it's not a general overall simulation. It's not going to be instantaneous or anything like that. Okay. Okay. And just to sort of bottom line the problem that we're trying to solve here, when someone says we're going to simulate the universe, do they mean like at the quantum level, like everything at the quantum level and up?

30:34Is that what you would need to create a simulated universe? Or could you create a simulation at like a higher level, but just let the quantum stuff happen to produce the higher level stuff without modeling it? Does that make sense? Yeah. Well, the answer is always you're doing a simulation to answer a specific question. Nobody's out there being like, let's simulate the whole universe, man. Just because. I mean, it would be massively complicated and expensive. But isn't that what we're talking about today? We're not talking about trying to simulate the whole universe? I mean it. No. Well, we are because we ask questions about the universe.

31:09Like we ask, how old is the universe? Or how did galaxies form? And will they continue to form? And why are galaxies distributed in this way and that way? And those are questions about the universe. And so to answer those questions, we need a simulation of the universe. That simulation doesn't have to include everything in the universe because some details about the universe don't change the answers to that. If you bought your kid a red bicycle or a blue bicycle for their ninth birthday, it doesn't change the answer to, is Andromeda going to smash into the Milky Way? Or why is this galaxy bigger than the other galaxy?

31:43Your kid's bike is awesome, but really it's kind of irrelevant on the cosmic scale. And in the same way, lots of stuff is irrelevant. And so we don't include it in the simulation because we don't need it to answer that question. So there's no general purpose simulation. So just like simulate everything just because we're always building simulations to answer science questions. And often it's the only way we can answer a question. Like you want to know what's going on inside the sun or at the heart of neutron stars? We can't yet observe that stuff. And so the only way to think about it is using simulations.

32:18Also because we don't have equations that describe it. There's no like F equals MA emergent physics that describes the turbulence inside the sun. And so we can simulate it and build models of it, but we don't have any exact solutions. Right, but that's the human limitation. If we were living in a simulation created by aliens much smarter than us, would their simulation have to include everything from the quantum scale to your son's bike? If we're living in a simulation run by super intelligent or just hilarious aliens, then whatever their simulation includes is our reality. And so really then you're asking me, are they modeling everything down to the quantum level?

33:03Well, if that exists, then they're modeling it, assuming our universe is a simulation, right? Okay. Yeah, I think so. So the answer to that is yes. But we can dig into that question at the very end after we talk about the Flamingo Project. What? All right, we're back to animals. No, I know. You told me it's a tortured acronym. What is the tortured acronym here? So the Flamingo Project stands for Full Hydro Large Scale Structure Simulations with All-Sky Mapping for the Interpretation of Next Generation Observations. Holy cow. Wait, but I thought you said full hydros. Was it the Flamingo Project?

33:40I lost track of the rest of it after that. No, no. You can just pick letters from your name to make an acronym in physics, apparently. F from full hydro. L from large scale structure simulations with A from all sky. M from mapping for the I from interpretation of G from generation. O from observations. Flamingo. I told you it has nothing to do with birds. Wow. Wow. Okay. Nothing to do with birds. And I think that actually wins the most tortured acronym that I have come across awarding. But anyway, I'm glad that they had animals on the brain. I love an acronym that's tortured, but then also manages to actually be related to the topic at hand.

34:21You know, like if it's just totally irrelevant, then I don't think it's very impressive. But to be clear, flamingo doesn't have anything to do with the topic at hand, right? No. Absolutely nothing other than there are flamingos in the universe and they are simulating the universe. but that's a stretch. Okay, fair enough. And they're probably one of the better parts of this universe because who doesn't love a flamingo? All right. So what is this project working on? This project wants to understand how did the universe get the way that it is? Like we look out in the universe and we see there's structure there, right?

34:50There are stars, there are galaxies, there are clusters of galaxies, and there's dark matter laying behind all of it, right? Everywhere there's a galaxy almost, there's a big halo of dark matter surrounding it. There's dark matter between the galaxies. There's streams of matter and dark matter between clusters of galaxies. How did everything get into this configuration? Well, we have an idea about that. We have these theories that 14 billion years ago-ish, the universe was filled with a hot, dense plasma with small fluctuations in density. And we say, well, gravity. Gravity clumped those things together, a little bit of density here, and a little bit of density there had more gravity, which pulled on more things, dot, dot, dot, you get galaxies.

35:34Well, what we really want to do is flesh out the dot, dot, dot part and understand, does our physics predict that when you go from a universe filled with plasma with slight over densities and under densities, that you get the large scale structure that we see? And if we run the simulation and it disagrees, that tells you that either something is wrong with our picture of the initial conditions of the universe, or something is wrong with our understanding of how things evolve in time. Super duper useful, right? Because it gives you a handle on something that's going on in the universe. Yeah, but just to interrupt, what does agreement look like?

36:12Like, would agreement mean, like, every star, for example, needs to be in exactly a predictable place? Or just, like, sort of, it generally has the same shape? Like, what would we consider to be a model that had simulated the universe? Great question. And I think a lot of people imagine that when you simulate in the universe, you're predicting the outcome for our exact universe, that you should be able to predict exactly the stars in the sky and this galaxy and that galaxy. And in principle, you might be able to do that if you knew the exact initial conditions of our exact universe, like where every particle was 14 billion years ago and where all the densities were, et cetera.

36:47You might be able to do that. We don't know that specifically. We only know it statistically. Like we know on average what was the density of stuff. How big were the fluctuations, right? We can't see the whole early universe. We can look out in the sky and we can see light from the very early universe, but we're just getting one slice of that light. You know, the universe is 14 billion years old, and so we're getting the light that has just now arrived to us from that plasma in every direction. Imagine like a huge but very thin shell of plasma really far away 14 billion years ago, all emitting light towards where we are now.

37:26We're here. It arrives here. So we see that little slice of the universe. And as time goes on, we see a different slice. That shell grows as the time goes on. But we never see the whole initial slice of the universe. So we don't know enough to predict our specific universe. Instead, what agreement looks like is, well, here's what we think the distribution of stuff looked like statistically. So do we get galaxies of the same size? Do we get galaxies in the same kind of arrangements? Not specifically this galaxy and that galaxy, but like, what's the distribution of galaxy sizes? What's the typical distance between galaxies?

38:00This kind of stuff. So it's more a statistical comparison than a specific one, which is kind of unfortunate. It would be awesome to simulate our exact universe and be like, look, that's where Kelly appears. Yeah, but, you know, it would not make it more useful to know where Kelly appears, even if it would make it more awesome to know where Kelly appears. But it does allow you to ask really cool questions like, what would happen if you had a universe without dark matter? Like, take the universe as we know it, fill it with the photons and the protons and the neutrons that we think were there, but don't put in the dark matter.

38:34What happens? Well, we can answer that question. You do that simulation and you don't get galaxies. Like 14 billion years is not enough time for gravity to pull protons and neutrons together to make stars and galaxies without the help of the gravity from dark matter. So that's already like a really interesting result. It says that the structure we see, the fact that we have galaxies and stars is itself evidence for dark matter or evidence for something out there helping pull stuff together because the visible matter doesn't have enough gravity to do it on its own. Okay. And so I think that's really cool because it shows you the power of simulations.

39:17It says, if your initial conditions and the rules to evolve it are inconsistent with what we see, right, then something is wrong with one of those two. And so if you put in no dark matter and you run the rules, you get something which looks very much not like our universe. So that's probably wrong. But if you put dark matter in, then you do see formation of galaxies. Dark matter itself clumps. Remember, there's a lot more dark matter than anything else. And it pulls those stars together and it pulls those galaxies together. And so if you put in the dark matter we think was there and it's measured independently in like nine other ways, then you get the formation of structure just like the structure that we see.

39:56And I assume this is something we've simulated before the Flamingo Project. Is that right? Yes. Okay. That's right. And people have played with it. They're like, well, what if we put in more dark matter? Let's double the amount of dark matter. What happens? Well, the universe collapses. There's too much gravity and things get squished and it doesn't look like our universe. So you can use this to measure the amount of dark matter in our universe by tuning the simulation, the initial conditions, until you get an outcome that matches what we're seeing today. It's like if you didn't know how much salt Zach put in dinner, but you ate the dinner and you were like, okay, let me try it.

40:32I'm going to try it without salt. No, it comes out bland. I'm going to try it with, you know, a pound of salt. No, that comes out too salty. And you refine it, comparing your experiments to your memory of dinner that Zach made until you figure out how much salt Zach put in his dinner. It's just like that with dark matter in the universe. Got it. Okay, let's take a break. And when we get back, we will find out what the Flamingo Project did in particular to create a newsworthy simulation.

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44:18All right, and we're back. And Daniel, in closing, I said that this was a newsworthy simulation, simulation but maybe it only appears newsworthy to me because it's got the word flamingo in it and so I feel like it should be in the news because that's awesome did this uh simulation tend to make the news well I mean the news that I read of course and the emails I get from listeners I definitely heard a lot about it it was in sky and telescope and a bunch of other pop site places so okay cool it was a step forward I don't always understand why a particular piece of science gets a bunch of attention and another piece of science I think is really cool doesn't but you know that's It's just the way the pop-si cookie crumbles.

44:55But this is legitimately a really cool piece of science. One, because it's huge. Like, they took on a really big task. They wanted to simulate a big chunk of the universe. So not the whole universe. But they described a cube of universe with sides 10 billion light years on a side. Wow. So, like, that's a lot of stuff. And in that are a lot of particles. They couldn't model all of them. With their computing budget, what they could do is describe particles with masses like 130 million times the mass of our sun. So basically, they can zoom into mini galaxies. They couldn't describe individual stars or individual planets or flamingos or podcasters.

45:36They can only have granularity down to mini galaxies. But in a cube of universe that big, there's like 300 billion of those. So that's what they described, like 300 billion clumps of matter, each about as massive as a mini galaxy. And for reference, do you have like a sense for how much of an advance in like specificity this is relative to prior models? This is a big chunk of compute, but the real step forward is not that they zoomed in more or they had a bigger slice of the universe. The specificity is that they made the rules much more realistic. So previous simulations were like, let's model the dark matter and we'll also put the protons in, but we won't let the protons interact with each other because, ooh, that's really complicated.

46:23Like dark matter is very simple. It doesn't really interact with itself. It's just gravity and it flows around and it clumps and we can handle that. Protons are hard because they have the strong interaction and they have the weak interaction and they generate other kinds of particles, which are really complicated. And we know because we eat them every day and that's why they're delicious because of all their complexity. And so most simulations have the atoms, the baryons in them, but don't let them interact because that's too complicated. And so what the Flamingo Project did was they included the baryons and their physics.

46:56So they have things like gas flows and hydrodynamics and, you know, how do winds get generated? All these things are really important for the structure of galaxies. You know, like when there's massive radiation from the central black hole that blows out stars in the centers of galaxies, and it changes how stars form in galaxies. And you have like active galactic nuclei that can shape the structure of nearby clusters. So they included all of those details in the simulation and they included neutrinos. So previous simulations at this scale hadn't included this baryonic physics, like all the interactions between protons or the neutrinos.

47:36Okay, awesome. That seems like quite a big step forward. Did they have access to a hyper-super-duper computer that we didn't have access to before? They got time on a very fragilistic computer at Durham University. They ran it on 30 ,000 CPUs using 50 million compute hours. So this is a computer that existed. It wasn't built specifically for them, but they got a bunch of time on it. So, you know, people always want time on these computers. It's very valuable. And you're competing with folks who want to simulate hurricanes or want to simulate nuclear weapons or want to simulate other stuff. So it was awesome that they got all this compute time to answer this question.

48:15Because there are really interesting open puzzles. Like we talked about modeling how dark matter changes the structure of the universe. And we described it as if it sort of works. And it does mostly work. And it's cool. But the more we study it, the more we see discrepancies. discrepancies. Just like when we were talking about throwing a ball and you use your first model and it mostly works, but then you notice, hmm, it's not exactly right. What are we missing? As we get more and more specific, we can look more and more carefully at the discrepancies and wonder, what are we missing? And you've probably heard of the Hubble tension, you know, this discrepancy between how rapidly the universe was expanding at early times, like 14 billion years ago, as measured in the cosmic microwave background radiation, and how rapidly it seems to be expanding today as measured by like supernova and other things, there are other tensions in cosmology, places where our models of the universe don't quite agree with our observations.

49:08And one of them is called the S8 tension. It's not as sexy or doesn't get as much press as the Hubble tension, but it's really important. This S8 is a measurement of like how clumpy is the universe? Has stuff pulled together into big clumps or is it still mostly spread out. And just like with the Hubble Tension, we have two measurements from the CMB and from late times, and those disagree. So the CMB says the universe was clumpier, and the late times says, no, the universe was less clumpy. And we want to understand which of these measurements is wrong. Can you build a model of the universe that's compatible with both of them?

49:43So far, we haven't been able to. And so this simulation thought, well, let's study this question. Let's include as all the physics that we can, because we know that protons and neutrinos affect clumpiness. And they ran 28 slightly different simulations, tweaking cosmic parameters like the dark matter fraction, or how massive is the neutrino, or how important are these active galactic nuclei, this hydrodynamics of the baryons we were talking about, how big are stars when they're formed typically, which percentage of stars fall in which mass ranges. They ran 28 different versions to see if they can understand this problem.

50:21I bet they agonized over the 28 different simulations. When you're doing, I remember when I used to do simulations, the hardest part was figuring out what part of the parameter space am I going to explore? And yeah, trying to figure out like the, you know, values of the variables for those different simulations was probably painstakingly done. Oh, you know, they argued, they had meetings, they had disagreements. More meetings. Yeah. People suggested stuff. Other people asked their suggestions. and there was grumbling and teeth gnashing. And these are important decisions because later when you get the results, you're like, oh, that's interesting.

50:54I wish we had another point with this other tweaked value, but you used up all your time, sorry. Yep, yep. Or one answer comes out really boring. You're like, well, that was a big waste of a zillion hours of computing. Thanks, Joe. I know, great idea, Joe. So yeah, exactly. It's definitely very painful when that happens. But they ran these simulations, and what they found is that the baryonic effects, including all these protons, does change the clumpiness, but not enough to explain this S8 tension. And so the question they went in with was, does adding the protons, making the simulation more realistic, does this solve the problem?

51:36Is the problem just an artifact of our simulation being too simplistic and not including all the physics we love and know? And the answer to the question is no, it's not an artifact of the simulation being too simplistic in this one way. So that would have been very cool if they had solved that problem. Like, oh, look, the universe now makes sense. But now it's very cool because it means, oh, the universe doesn't fully make sense, which means there's something to learn, right? Either maybe there's some kind of dark matter that has self-interaction that's not being included in these models, right?

52:07We describe usually dark matter just as having gravity and no other kind of interaction. But maybe dark matter is complicated and there's lots of different kinds of it. And some of it has interesting interactions with other kinds. And that's not being included in the model. And that's why the models don't agree with reality. That's just like one hypothesis. So there's a lot still to learn here. It's such a roller coaster doing science because, like, you know, obviously you set out to do an experiment. You kind of think you know what the result's going to be, or at least that's what you say in your grant.

52:36Yeah. And if you get the result, you're like, what? Yay! But then if you don't get the result, you're like, but this could be equally as interesting or maybe even more interesting, especially if it sets you on like gives you a little bit of a hint for what you were maybe missing that you should think more about. But like, yeah, so this sounds like something where the result would have been fascinating no matter what. Yeah, exactly. And I think it made news because it's a big, awesome piece of computing. Like, wow, what a cool project. But unfortunately, it didn't get the answer they were hoping for.

53:08But, you know, we learned something. we crossed something off of our list of hypotheses and we're still on the hunt to understand the universe and figure out why it is exactly as clumpy as it is today. I think it's wonderful. I don't think it's too clumpy. I don't think it's overly clumped. I don't think it's under clumped. I think it's just right. What do you think, Kelly? I think you are a very accepting universe partner. And, you know, I haven't thought too much about the universe's clumpiness. I like it for its personality. and so I'm totally okay with the universe as its clumpy self, you know?

53:41You're making me sound superficial here. I'm only into the universe because of its clumpiness. I mean, you were the one who was focusing on the clumpiness. I'm just saying. I didn't say it's the only thing I like about the universe, Kelly. The universe is multifaceted. Yes, it is. So I think that answers Joe's question and we'll send this to him and hear if he has any follow-ups. But, you know, the episode as posed sort of asks a bigger question. Like, all right, this simulates a bunch of stuff in the universe, but it glosses over a lot of details. If the smallest thing in your simulation is a particle with 130 million solar masses, then you're not describing little Timmy's bicycle.

54:21Is it red or is it blue? And I think it's an interesting question that the listeners were responding to and that you were asking about earlier. Like, is it possible to describe the whole universe? And I think that the answer to that question has to be no, because a simulation containing the complete quantum state of the universe would require at least as much information as the universe itself contains, right? And so, like, a smaller computer cannot contain a lossless description of a larger physical system. And if that computer is in the universe, then it has to be smaller than the universe. You can't use the whole universe to simulate the universe.

54:58And even if you did, there are parts of your computer which are not being described in the simulation. And so I think there's this cool recursive issue. Your simulation would have to conclude a description of the simulation. And so I don't think it's possible to describe the whole universe within the universe. The simulation hypothesis is another question. It's like, is our universe a simulation? That implies that our universe is running on a computer in some larger universe. The same way that like, you know, we describe Super Mario Brothers in a simulation. We create a universe and it has its own rules and that exists within our universe.

55:37And in our universe is a little piece of the universe we call a Nintendo system, which is complicated enough to describe the Super Mario universe. And so if our universe is a simulation, then that implies that there's a computer out there capable of describing all of these details that must be within another universe that's at least as big. Yeah, so I was going to ask, why would anybody want to simulate our universe? But now that you've used Super Mario Brothers as an example, our universe is clearly better with it in it. And so now maybe we are the fun part of some other creature's universe. I dig.

56:13Yeah, but I want to point out some fun loopholes here. People talk about looking at the way our universe operates and looking for evidence that it is a simulation. I think that's really dangerous and misleading because what you're looking for is evidence that the universe operates the way our simulations operate. But if our universe is a simulation in some other universe, we have no idea what the physics is of that universe. The same way that Mario has no idea what the rules of physics as Newton and Einstein have developed are. His universe has different physics. And so if he's looking for hints in the Mario universe as to how our universe operates, he's not going to find anything.

56:57And so if we're looking in our universe for evidence that operates like our computers, we're on the wrong track. We need to look in our universe for evidence that operates like the meta-universe's computers, about which we have no information because they follow other laws of physics we don't know anything about. And so I think that's a little bit hopeless. Okay, so just to be clear, Daniel is being the wet blanket today and is essentially saying there's nothing in this universe that can answer the question, are we living in a sim or not? I think that's true, yeah. Oh, man. But there are some other fun loopholes like, you know, can you simulate a whole universe?

57:33I don't think you can, but you can simulate large parts of the universe, right? Like you could, a big chunk of the universe, really high fidelity, and that can answer a lot of physics questions. Often you don't need to simulate the whole universe. like to answer this question about dark matter, you don't need to add Timmy's bicycle on whether it's red or blue. It doesn't change the answer. And so the fact that the universe breaks into these levels of emergence where you don't need to know all the lower level details to make chicken soup, for example, like you can make chicken soup without knowing quantum gravity or string theory.

58:05That's great. Chicken soup is delicious, but it also means we can do science at various scales without knowing all the details. That's a blessing. So it's good, actually, that our simulations don't have to include everything in the universe. And then the other thing is that sometimes you can get away with not describing the universe in all of its gory detail because there are patterns. So we have laws of physics, we have symmetries that tell us, constrain what can happen. So for example, say you're simulating a particle and it decays into two other particles. Well, conservation of momentum tells you that if the first particle was at rest, the next two particles have to be back to back.

58:43They have to have momentum that's exactly opposite each other so that the total momentum is still zero. What does that mean? How does it help you? It means you only have to store in your simulation one of the momenta. You don't have to store both because the other one you can derive instantly from your rule. So symmetries and rules mean you don't have to store the full state of the simulation in your computer. You only have to store enough information to derive it when you need it. And so these symmetries, these constraints essentially are like a way to compress your knowledge of the universe more compactly and losslessly.

59:20So there are some loopholes here too. You have to have a computer the size of the universe in order to describe the universe. Yes. Like what if we are one run out of 10 ,000 of a simulation that's testing five rules and we are the result of testing some of those rules? Would that work? Yeah, that could work. And I wonder if our universe is one that somebody fought for in inclusion. That's right. Thank you, Cosmic Joe, for including our universe. Our Joe totally messed it up. I hope he's pleased you. Yeah, but Cosmic Joe. Thank you, Cosmic Joe. All right. So we'll send this episode to Joe from Ann Arbor to hear if we've answered his question about the Flamingo Simulation and about simulating the universe in general.

1:00:07Here's Joe's answer. I totally forgot that the listener's name was Joe when I picked Joe as the, maybe that was in the back of my brain subconsciously the whole episode. Sorry, Joe, we didn't mean to blame you for the bad run of the simulation. It just goes to Joe you. Thank you so much. I really loved that episode. Daniel, I know that you would especially like to contact the being running the simulation, but I can neither confirm nor deny whether I am Cosmic Joe. I can confirm, however, that the universe is overly clumpy in the vicinity of my desk. But seriously, thank you both for the thorough explanation of simulations and the Flamingo Project.

1:00:46Emergent properties at different scales seem to be a major theme on the podcast lately, and I am fascinated of the idea of simulations being a way to probe from one layer of abstraction to the next. I don't have a follow-up question, but I do want to urge everyone to ask their own questions. Listening to the raw audio was a treat, especially right after listening to the episode with Matt Kesselman. Danielle and Kelly are somehow even more charming in real time. My only complaint is that I didn't hear a single goat. Thanks for answering my question. I love getting smarter with you. Thanks very much, everybody, for taking this trip down the simulated universe to understand how we think about the universe, how we understand it, and how simulation is an absolutely vital part of modern science.

1:01:32May your simulation be a good one.

1:01:40Thanks, everybody, for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky, Instagram, and X, D &K Universe. Come engage with us. You can email us at questions at danielandkelly.org. We really do want to hear from you. And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing universe.

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