Listener Questions 37: quantum cats, heat death and dark multiverses!

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Speaker 1: Hey, Daniel, you own a cat, right? You know we used to have a cat, but no longer. Oh no, what happened local pack of coyotes? I'm afraid? Whoa where do you live? And to the wild. We have colonized the wild and turned it into suburbs. Yeah, it is a concrete jungle out there. But I wonder is that really the whole story or is there more to it? What do you mean? What are you accusing me of? I don't know. I think cats and physicists. It never seems to go, well, did you do some kind of crazy experiment on the cat? I did have a cat, and I did have a box. Whether or not I did that experiment, I'd rather not say. Maybe you did and maybe you didn't both at the same time. Now I'm feeling boxed in. But you know, if we live in a multiverse, technically there are probably cat physicists out there, maybe experimenting on human pets or what did the aliens that come to visit Earth are cats? What are you gonna do? Maybe the coyotes will protect us. That's like the idea for an animated movie. Cats versus coyotes versus physicists. Hi I'm Jory mccartoonists and the creator of PhD Comics. Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I really do miss my cats. Oh you had multiple cats? Is it starting to feel like a trend here, like a string of clues? Not just one cat in multiple superpositions, but two cats at the same time, which we've lost both the same summer. Oh, no, to coyotes, both of them, I'm afraid. So we had indoor outdoor cats in Chicago, where it's pretty safe, very few coyotes, but when we moved to Irvine, they didn't really last very long. We tried to keep them inside for a while, but it just wasn't possible. You need to get coyotes for pets, you know, just move up the food chain. Coyotes and raccoons. Yeah, those are great ideas. I mean, that's kind of what dogs are, sort of. What's the difference between a kyote and a dog? I think several thousands of years of breeding that can come in handy. But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio in which we do our best to tame the wild universe out there with all of its crazy feral physics, everything that's happening out there in the universe, the crazy quantum frothing foam all the way up to the huge black holes sucking up enormous amounts of gas and dust. We want to tame all of that and try to bring it under human understanding. We want to build an explanation of it that makes sense to our human minds, and that lets us think about what else we could do in this universe. It's right, the universe is a gigantic jungle of amazing information and facts, of incredible things that are happening out there in the cosmos between the galaxies and the supercluster of galaxies, and also here on Earth inside of our molecules. Scientists are there to explore and to map out everything and us in it. That's right. We are hacking our way through the jungle of the universe, trying to figure out where everything is, what does the landscape look like underneath, how has everything worked together? Though our goal, of course, is not to pave over it with suburbs, but just to understand it, to describe it, to appreciate its magnificence and beauty. We're here to pave it over with knowledge, I guess right, and curiosity. I don't want to pave over anything, really, I mean, I don't want to come to some alien planet and just like cover it and concrete and Starbucks unless is pretty inhospitable in the first place. Isn't concrete and Starbucks better than like the serves of Venus right now? I suppose, But you know, let Venus be Venus. Maybe there's some kind of life form that loves sulfuric acid clouds and super high pressures and temperatures and would be pretty offended or even exterminated by the presence of concrete and Starbucks. Or maybe it's just because they haven't discovered a nice spice latting, you know, and they'd be like, oh, thank you so much for opening my eyes to the true wonders of the universe. Maybe it wasn't fosphine in their atmosphere, it was pumpkin spice, after all, the beginning of a downfall of any civilization. We've discovered Starbucks on Venus, But it is a pretty wild universe out there, full of undiscovered things that we can discover and learn about and ponder about to get a better perspective of how the universe works. And the way to get these answers is to explore, to go out there and try to find things that don't make sense to us, things that are new, things that our brains can't quite yet grapple with that our mathematical stories do not yet include, or sometimes we try to take those mathematical stories and apply them to things we thought we understood. But the common thread there is asking questions. In the end, it all comes down to people being curious and wanting to understand how the universe works, applying our understanding to it, and asking questions when it doesn't quite work out in their minds. That's right. We are all explorers in this jungle of a universe, and I guess there are our main tool for hacking our weights through all of this mystery, our questions, Right, questions are kind of the machete of physicists. Yeah, some people might say the experiments are the machete, but questions are the first step. Right. That's how you decide what you're going to look for, That's how you decide what experiments you're going to do. That's how you frame the possible answers you might get to your crazy scientific questions. So in the end, it's questions and curiosity that is driving all of human science and maybe alien science and coyote science. I guess you just have to make sure that your questions are sharp, right, Like, if you had dull questions, that wouldn't get you very far. Yeah, you wouldn't slice through very much science jungle with a dull question. But questions people ask, and questions scientists ask in questions we ask here on the podcast, and sometimes we even answer them for listeners. That's right. We want you to be an active participant in this podcast, not just sitting back and listening to me and Jorge joke about bananas and talk about science, but to engage your brain, because our goal is not just entertain you, but to really bring a level of understanding into your mind. We want to put these pieces into your brain and teach you how to manipulate them so you can use them to answer new questions, your own questions. And so we love when listeners do that, and then they come back to us with things that didn't quite make sense to them that let us unravel a little bit of misunderstanding or help them get a clear picture for how things work. So today on the podcast we'll be tackling listener questions number thirty seven. This is our thirty seventh listener question episode. Absolutely and if you have a question that you have been wondering about, please don't be shy right to us to questions at Daniel d Jorge dot com. We really do write back emails to all of our listener questions and some of them get put here on the podcast for us to talk about. Just don't ask about Daniel's cats. Don't ask too many questions about that, because this whole coyote theory is a little suspicious. You know, there's only so many answers the universe can provide. There's a quantum uncertainty to everything. Sounds like a great criminals there. It sounds like even practicing that that's right. On advice of counsel, I would like to invoke the Heisenberg on certainty principle. Oh that would be a funny courtroom scene. And then the lawyer, the opposing articles all drat, they got us, he got us. He can't do anything against the Heisenberg defense. Well, you have the Boeian attack. Right, I got a little too deep for me. Yeah, we'd like to answer listener questions here on the podcast at people send in and people still send you questions, right, Daniel, Oh, absolutely, I get dozens of questions every day, and I enjoy reading all of them because they show me what people understand and what people don't understand. You know, as a teacher and educator, the actual material that I'm teaching isn't always terribly exciting to me because I've been doing it for decades and decades. What's always a really fun puzzle is what confuses people, What got people to some point in their understanding that they ask a certain question. The question always reveals how they got there and the misstep they took. So for me, it's a fun puzzle to go from the question to understanding where they went wrong and then helping them find a path to clarity. You enjoy seeing their confus sort of like a cat plays with its praying. Well, that's very confrontational approach, But yeah, I do enjoy seeing where people got confused. It helps me sharpen my explanations. Yeah. Cool, Well, today we have three awesome questions from listeners. One of them is about Schrodinger's cat, another one is about the heat death of the universe, and the last one is about parallel universes and dark matter and are those two things tied together? So we'll jump right into our first question, and this one comes from Nils, who's fourteen years old and comes from Sweden. Hi, my name is Nieves Pasel. I'm a Swedish fourteen year old very interested in both physics and philosophy. And my name's Jacob and I'm Niel's dad. I'm an ICU nurse currently working on my PhD in clinical neuroscience and I share my son's passion for physics and philosophy. The other day we had a discussion about Schrodinger's cat and we would absolutely love it if you guys shed some light on this Shuliess cat is well a cat and therefore a quite intelligent and curious creature. Couldn't this cat itself be considered to be an observer inside the box? And if there is an observer inside the box, wouldn't that collapse the quantum uncertainty immediately upon being observed by this cat? We would love to hear your thoughts outside as well as inside the Box on this and thank you for a great podcast. All Right, that's an awesome question. I would say that's a pretty awesome question. Categorically speaking, it is awesome and especially awesome to hear parents and kids talking about science and philosophy together. That's wonderful. So thank you very much to all the parents out there encouraging your kids to think about science. Yeah. We get a lot of kids asking questions with their parents sometimes. Yeah, and we hear from people who listen to the podcast with their kids and talk about it. And while that's a dream come true for me, awesome. Well, the question is an interesting one. They're asking us to basically sort of explain what Shroudinger's cat is. Yeah, And Shroudinger's cat is a really fun thought experiment that puts its finger or I guess it's paw or its claw on a big problem in quantum mechanics and at the intersection of physics and philosophy. Yeah. And their question then gets very specifically about the nature of an observer in a quantum experiment and what that means and whether cats are smart. I guess that's where those part of the question that might take up the whole episode though our cats as smartest people, and does that solve the quantum mechanics measurement problem. In the end, you're gonna have to continue this with Katie to talk about cats. But Yees steps through Daniel. What is Schrodinger's cat Shortinger's Cat is a thought experiment that tries to expose a conflict in quantum theory. Right, So, for hundreds of years, we had a classical theory of the universe, one that said that you know, objects have locations and that they have velocities, that there is a reality out there that has infinite information and can be knowable, and that an object is in one place and in no other place. But then we developed quantum theory when we discovered that tiny little objects like photons and electrons don't follow those same rules. They don't have trajectories where they have a specific location and a velocity at every point in time. Instead, they can have multiple possibilities. The electron could be here and it could also be there at the same time, so we call this a superposition of possibilities. So we had new quantum rules for quantum stuff, and we had classical rules for classical stuff. Because like me and you and baseballs and cats don't seem to be able to do that thing that quantum objects can do have a possibility of being in multiple places at once. So there's this weird conflict between the classical rules and the quantum rules and where they overlap. So Schroedinger's cat is a thought experiment that tries to put its finger on this conflict and raise difficult questions. Okay, so then the basic conflict here between quantum mechanics and classical physics, it's kind of this idea of certainty, right like in classical there's nothing uncertain about the baseball flying through the air. You sort of know where it's going to land, and when you're holding in your hand, it's like it's there, there's no question that it's there. But in quantum mechanics there's no such thing kind of as certainty, right Like you can't hold a photon or an electron in your hand. You only can only sort of know where it could be. There's a lot of uncertainty about where it actually is. That's right, And in the classical case, we can't have uncertainty about where the baseball is. But that uncertainty is just our lack of knowing the information does exist. The baseball is in one particular location, it has one particular velocity, we just might not know it. In the quantum case, we think that information doesn't actually exist, Like when the particle is going through the double slit experiment, it really has the possibility to go through both slits at the same time, or the electron really could be here and it really also could be there, not just that we don't know the answer, but that it really isn't determined. And then the conflict is what happens when classical stuff and quantum stuff interact. Right, when you, as a classical object act look for an electron and say, hey, I want to know which slit the electron went through. What happens, right, and so at that interface is the really awkward part, because classical objects can't be in two different states, but quantum objects kind of canm Well, I guess, first of all, thanks for calling me classy. A lot of people do that. So then you were saying that Schrodinger's cat is sort of an example or like a thought experiment that doesn't it's not there to explain what happens between quantum and classical physics. It's more there to like kind of like a point a finger to it, or illustrate this conflict. Yeah, there are a lot of these great thought experiments in quantum mechanics that say, hey, look what you're saying about reality is kind of ridiculous. Here's a ridiculous outcome if what you're saying is true. And Shortinger's cat is a great example of it, and it tries to put its finger on this question of like what happens when something classical touches something quantum. If you're observing a quantum particle, you don't observe it to be in two places at once. Somehow it collapses into just one choice. If the electron could be spin up or could be spin down when you observe it, you just get one or the other. How does that work? What is the distinction between a quantum object and a classical object? When does the classical object know to collapse? So Shortinger thought this was fairly ridiculous, So he came up with this example where you have a box and inside the box is some quantum process like an atom that decays radioactively, and so it might decay sooner or might decay later. Based on quantum mechanics. But now that's linked to something classical like a cat. So when the atom decays, for example, it's linked to some mechanism that releases a poison gas and kills the cat. So if the atom has decayed, the cat is dead. If the atom has not decayed, then the cat is alive. And this is a fascinating thought experiment because before you look in the box, you can say that the atom is in a superposition of having decayed and not having decayed. It's not one or the other, has a probability of both, which means that the cat in that same sense, has a probability of being alive and the probability of being dead at the same time. I see. So then the cat is inside and it could be alive or it could be dead, depending on the outcome of some quantum process. But I guess the name question that Neils and Jacob were asking is that you know the cat is alive and dead only to us from the outside of the box, right, Yeah, that's exactly right. And what they're asking is why doesn't the cat collapse? The uncertainty, right, if the cat is a big, curious creature and a classical object. It can't be in multiple states. It can't be dead and alive at the same time. Then why doesn't it collapse the possibilities and force this atom to either decay or not decay. That's essentially their question, right because to us, technically the cat is alive and dead according to quantum mechanics. But to the cat, the cat knows if it's alive or dead. I think that's what they're kind of asking, right, Like, the cat is a somewhat conscious creature, and it knows whether it's sort of it's observing things, right, it's looking at things inside and it's it knows if it's breathing or not. So does that mean that the cat definitely is alive or dead for the cat, But it's alive and dead for us outside of the box. So the answer depends on which interpretation or quantum mechanics you prefer, right, Because we can't know without opening the box. So you can't make a definite statement about what really happens when you don't open the box. All we can do is say what we think might happen and what it means. So we're in the era of philosophy because we don't have experiments we can do to distinguish between these various ideas, And one of the key problems is that we don't have a good distinction between what counts as quantum, what is allowed to be described by quantum rules can have uncertainty, and what counts as classical. Right, And the Copenhagen interpretation is sort of standard interpretation or quantum mechanics that's taught to physics majors doesn't define these things. It leaves it uncertain, and so there's no clear distinction like when does a wave function collapse and when does it not? It's not defined or maybe it's only defined or respect to an observer, right, what counts is a classical observer that has classical interactions, and what counts is a quantum observer that can have quantum interactions without collapsing the wave function. That's not to I guess. I mean it's like it feels like, you know, it definitely collapse for the cat, but for us outside of the box where we have like zero information exchange, the cat is still alive and dead to us. So that's a specific interpretation of quantum mechanics. Carlo Rovelli would totally agree with you, because he's a proponent of relational quantum mechanics, which says that the collapse is relative, it depends on who's doing the observing. Carlo would say that the cat collapses the way function, but for us outside the box, it's not yet collapsed. That this question of whether or not you are collapsed is everything relative between an observer and the observed, and not everybody has to agree on it. The same way we don't have to agree about velocity, because you can have different velocities for different objects, he thinks, and relational quantum mechanics suggests that collapse is not a universal thing, that you don't have to collapse for everybody. You can only collapse for some observers and not yet for others. M I see. It seems so like philosophy Option one is that it's collapse for the cat. The cat knows it's alive or dead, but for us it's alive and did that's philosophy option one. Philosophy option B or two is that nobody knows. Right, it's like it's alive and dead for everybody, even for the dead cat. That's right. So relational quantum mechanics would say the cat has collapsed the wave function, but we have not. Copenhagen sort of orthodox quantum mechanics doesn't even really have an agreed upon answer. Essentially, how you define the difference between what collapses and what doesn't is something people agree on for every experiment. It's like, let's call this the observer or let's not call that the observer. It's even fuzzier than like not to find. It's like, you know, let's just have a consensus for everything and define where this cut is between quantum and classical. And so even people who believe in Copenhagen might disagree about how to interpret this experiment. Some might say, look, the cat is a classical object. It collapses the way of function, there's no question. And other people might say, I just view the whole box is a quantum object, and the cat is just like a big quantum object and it's not yet collapsed until we open it. So even people who believe in Copenhagen interpret, I think, would disagree about what's happening in this experiment. Well, I feel like maybe there's an option see for philosophy in the philosophy here, which is kind of this idea of the quantum multiverse, right, like maybe nothing ever collapses at all, Like maybe there's no such thing as collapse, and things just kind of stay uncertain for everyone forever, and you get all these different multiple universes, right, like maybe the cat is alive and dead inside the box, and we us outside of the box. There are two universes, one in which if I open the box, it's going to be alive, and one which if I open the box is going to be dead, and so we are also kind of in a state of superposition. Yeah. Absolutely, And that's the many worlds interpretation of quantum mechanics. So Sean Carroll would probably agree with you. The question there is when does the universe split. Does it split when the cat is looking at the atom with a Harry eyeball wondering if it's about to kill it, or when you open the box and observe it. But you're right, in this many worlds view, there is no collapse. It says. Collapse is nonsense. It's not consistent with the Shortener equation, it's not consistent with every anything we know about quantum mechanics. That information is preserved because it's collapsed, like violates the conservation of quantum information. It makes no sense mathematically, and so they say, let's just have the shorting your equation dictate everything. And what happens when you observe something as the wave function gets two branches, one for each slice of them multiverse, and now you're in one of them, so you only see one and not the other. I've always found that a little bit unsatisfying because it doesn't really answer the question of why you're in one of those branches and not the other branches. You're supposed to believe those other branches are at the same philosophical level as yours, even though you're in this one. But mathematically it's a very pretty view, all right. Then what's the answer for Niels and Jacob then, is that there's a superposition of answers here and they're all true and false at the same time. Yeah. Absolutely, And there's even philosophy option D, which is that there is no randomness. Right. Boemian mechanics believes the quantum mechanics isn't even random at all. It's deterministic, and either the cat is dead or it is alive, and it was determined before anything happened that went into the box. Maybe even by the Big Bang. So the answer for Neils and Jacob is that this is not something we're going to answer today on the podcast. Nobody knows where it really happens. None of our views of quantum mechanics really answered this question satisfactorily. No, no, no, Daniel, let's just collapse the question itself and let's just pick an answer, and that's the universe that we live in, and a quarter of the time will be right. Well, then I want the cat to be alive so that it can survive and then later get eaten by coyotes. I don't know what's worse to die in a physic experiment or to be eaten by coyotes. Well, you know, my cats had the option to be stuck in the box of my home or to live outside and enjoy the outdoor life of a cat, and that's what they chose, and they paid the price. Wow, now we're getting into free will and feline free will. We're gonna eat the not just katies, some kind of philosopher in here too. And for those of you worried about outdoor birds, we also worried about them, and we put very loud bells on the cats. Though that might not have been helpful when it came to the coyotes. All right, well, I think that's the as for Neils and Jacob, which is at it kind of depends and the very nature, the philosophical nature of the universe. It could be that they're right, that the cat collapses inside of the box and we just have to open it to find out what happened. It could be that the cat is alive and dead inside of the box and we just don't know it. And it could be that we're all in some kind of multiverse where every decision gets played out in a whole different plane of existence. That's right. If you want to go further down the philosophical rabbit hole, I recommend you read about Vigner's friends experiment, where you have people inside the box, and then the whole experiment inside another box, and then that whole experiment inside another box, etc. Etc. Oh, man, now you're gonna experiment with rabbits and humans. Let's not open that box. All right, Well, thank you Nielson Jacob for this question. Let's get to some of our other questions about the heat death of the universe and also dark matter and parallel universes. But first, let's take a quick break. All right. We are answering listener questions here today from curious listeners who have amazing questions. One question we didn't quite answer in the last segment, Daniel was about Truedinger's cat, which is, what's the origin of Trudinger's cat? The Truedinger really have a cat? Schroedinger is actually quite a colorful figure, and now a controversial one. It's not really that nice a dude. It turns out the kind of things he got into is not the kind of things we should dig into on a family friendly podcast. So I don't know what that says about the likelihood that he was nice to his cat. But I guess maybe what I'm asking is like, did he come up with this example of the Schulinger's cat maybe based on a cat you may or may not have had, or did somebody else come up with the cat in the box idea and just assigned it the Schroedinger. Oh no, it was very much Schroedinger's example. Yes, it is his idea of the cat in the box. M interesting, and he naended my cat. He's a here's an experiment, it's my cat. I don't know, maybe he hated cats, and that's why I put a cat in the boxing's trying to protect theoretical rabbits and theoretical dogs. I don't know, oh Man physicists or I just get to a second question here, and this one comes from Mattis. Hello, Daniel, and I have a question for the podcast. If all energy a movement stops at the heat that of the universe, which things still be moving at a quantum level? Thanks for answering, all right, nice and brief. I like that He's like, here's my question. Bam. He's got that very efficient Dutch attitude. He probably recorded this wall he was on his bicycle writing past some windmills, probably next to assume tulips eating at m cheese. I've seen the Dutch folks do things on bikes I've never imagined, like they bike away from the train station with a rolling suitcase next to them on the bicycle. It's amazing. Nice. Well, Mattie's question here is about the heat death of the universe, and I guess what it really means and whether or not quantum things still apply at the end of the universe if it dies by heat death. Yeah, this is a great question and one that I'm very hambid I talked about on the podcast today because there's a common misconception about what the heat death of the universe is. The heat death of the universe does not mean when everything is zero temperature and totally frozen at absolute zero. The heat death of the universe instead means when all the heat is smoothed out, when everything is the same temperature, when there's no more energy flow in the universe, basically when everything is in perfect thermodynamic equilibrium. Let's take a step back here and maybe talk about what is the heat death of the universe. It's it's kind of an idea about how the universe might end, right, and it comes from this idea that if the universe keeps expanding and expanding and further and further, eventually it's going to get to a point which, you know, some people might call it the end of the universe. Some people might just call it like the universe getting stuck, but they call it the heat death of the universe, even though it has nothing to do well, it won't be very hot, but it does have something to do with heat because it is a thermodynamic analysis and it's basically just trying to predict the future of the universe. You're right, because something we notice in the universe is that energy likes to even itself out. You have a hot spot on your counter, that energy is going to bleed out to the rest of your counter, and it's going to even out. You put an ice cube in a hot cup of coffee, you come back five minutes later, you don't still have an ice cube. You don't have half the coffee being hot and half of it being cold. You have it all about the same temperature. And so what we notice in the universe is that things tend to basically smooth themselves out in terms of temperature and in terms of energy. And so if you extrapolate that really far in the future, then you end up with universe that's sort of smooth that way, where there's no hot spots and no cold spots. And that would be a bummer because we kind of rely on energy flow to do most of the things we do, like living. I see. So it's kind of the idea that if you just leave the universe out on your coffee table, eventually, right now it has a lot of hot spots and cold spots. Right there's the Sun that's really hot. There's the center of the galaxy that's really hot, and some pots spots that are really cold. Like if you leave your universe out on the table for or you know, a few trillions of years, eventually it's all going to smooth out and just be like a room temperature universe. Yeah, exactly. It's sort of like the battery runs out on the universe, you know, batteries and any other sort of technique we have to extract energy from the universe relies on energy flowing, right, maybe water is flowing downhill, or steam is rising to turn a turbine or something that relies on energy flows, and so if the energy is not flowing anymore, you can't extract any more energy from the universe. So it's sort of like if you put ice in your drink and you come back later, it's all the same temperature. You can't do anything with that because there's no energy moving around. Even if there still is energy inside that drink, there's no useful energy. So there's a distinction there between useful energy and actual energy temperature in the universe. You can get to a place where the universe is totally smooth, everything is the same temperature, so you can't do anything without that temperature actually being zero, right, And maybe just for our listeners, maybe paint a picture of how that happens. Like right now, we have hot spots, like, for example, the Sun or like the core of the Earth. It's pretty hot. But I guess I should just like run the clock forward to what happens. I guess eventually the Sun will go out, they'll run out of fuel, and then it'll be sit there hot and then but then eventually all that heat will sort of radiate out into the universe, right, and it will just get cold. That's right. There's a lot of energy stored in the Sun, but that energy is not going to be stored there forever. It's getting released. That's what the Sun is doing. It's blasting its energy out into the rest of the universe. But the Sun will not last forever, and so that energy will fly out in terms of photons and get absorbed by other stuff. But energy likes to move around. If those photons heat something up, then that thing will also emit photons on its own. It will emit black body radiation to sort of heat up the stuff around it. So the energy in the whole universe just sort of spreads out. And this is a basic rule of thermodynamics. It's because entropy has to increase in the universe, and the way to increase entropy is to spread the energy out because it increases like the number of possible micro states you can have going on inside the thing, like for example, the Earth, Like the Earth is pretty warm at the center now, but eventually, over time it's just going to turn into a cold rock, and all that energy is going to go out into space and maybe hit other things. But eventually those think everything's going to kind of reach room temperature. Yeah, not quite room temperature, like comfortable temperature for us, but sort of like universe room temperature, which is going to be like a couple degrees above absolute zero. And you might wonder, like, how is the Earth going to lose that energy? It's not glowing like the sun, But actually it is right, everything is glowing. The Earth actually is glowing, just not in the visible light. If you looked at the Earth with an infrared camera, you would see it glowing just in a spectrum that's too long wavelength for our eyeballs to see. So we can see other planets out there glowing with a James Web infrared telescope. That's one way that they can discover exoplanets, for example. So everything out there in the universe is shedding its energy, and as time goes on and energy tends to smooth out more and more and more, and the heat death is just extrapolate that really really far out in the future where all the energy has managed to leak out and spread around. Yeah, so then now you kind of have to imagine the future of the universe where you know, the Earth is cold, the Sun has gone out and it's cold. Everything's cold. All the stars in the entire universe are cold, and now maybe even like pulverized, and it's all sort of this kind of basically super cold place. There's no energy stored in any one place for there to spark a new sun for example. Right, that's right, But even a cold Earth does have some energy stored inside of it. So you've got to go even deeper into the future when everything falls into the supermassive black holes at the hearts of galaxies and then is radiated back out by a Hawking radiation. So now everything has basically been converted to whatever is inside a black hole and then radiated back out sort of like fed through these cosmic shredder. Now the whole universe is just sort of bathed in Hawking radiation, so there's no like hard lumps of cold Earth or any thing I see. Eventually, even the cold Earth and the cold Sun will collapse, right because orbits don't last forever. Eventually everything's going to collapse, the galaxy of cold stars and planets, it's all going to collapse into a black hole. But then you're saying the black hole evaporates at some point, even if it's super gigantic massive, it's going to evaporate eventually right into what light. Well, Hawking radiation can be light, but it can also be other particles. It's really fascinating because it's sort of democratic, meaning it can create any kind of particle that's out there, which is a really awesome kind of tool because you want to study like what particles are out there in the universe. You can just go like observe the edge of a black hole and it'll basically show you all of nature's menu, which is amazing. Though that's technically quite difficult to do, and we've never actually observed hawking radiation. But in theory, all black holes, even the really really big ones, do emit hawking radiation because they have an entropy, they have a temperature, and everything in the universe with a temperature does emit some kind of radiation. And so then if you keep fast forwarding, these black holes will evaporate out, and so now universe is what just filled with flying photons and tiny matter particles. And the idea is that everything is perfectly spread out, so there's no like hot clumps of matter, there's no overdensities, there's no under densities. This is sort of like the very deep future. And we're not talking about like twenty fifty or five thousand years from now, or ten million years from now or ten trillion years from now. We're talking so far deep in the future it's hard to even really wrap your mind around the time we're talking about. It's like ten to the one hundred years in the future. But I guess maybe a question is if black holes do evaporate into matter particles, when of these matter particles flying around eventually like stick together or be attracted to each other by gravity, and then then you get more stuff, and then suddenly you have like new planets and new stars. So this is really the hard I think of Matthias's question, Right, it's like, what's going on with that stuff? Do you still have quantum frothiness or not? And you still do have particles and they are still flying around own and photons will still interact with electrons, right, Just like when your drink cools down to an even temperature because you put ice in it, you still have particles in that drink that are moving around. There's still motion, there's still interactions that are happening. It's not like everything is stuck and frozen. It's just that everything is smooth and even so nothing can get started, Like gravity in order to get started requires some overdensity. If you had a perfectly smooth universe, gravity couldn't do anything. Gravity can accentuate lumps, but it can't start lumps. Right, Well, you're assuming you start with like a universe where every particle standing still and it's a perfectly equal distance from all other particles. But that's not kind of like it. It seems very unlikely that we'll get to that point, you know what I mean? Like, if there are black holes evaporating, wouldn't they have you know, wouldn't the particles have some velocity coming out of the black hole, and then wouldn't those mixed with other things, and you know, wouldn't gravity eventually take hole somewhere? Like what's the scenario in which we suddenly everything suddenly freezes and is at the same distance from each other everywhere? Right, that's not the picture that we're painting. We are painting the picture where things are still flying around and still frothing. And so what thermodynamic says is that the most likely outcome is that things stay smooth. But this is just statistics. It just says it's the most likely. There's, of course, quantum randomness. Things can fluctuate. It's quantum mechanics, and so things can accidentally bump into each other and start to form a hot, spotted, denser spot that then collects stuff. Right, We actually talked about the probability of this kind of thing happening in our Boltspin Brain episode, Like, if you had a perfectly smooth universe, what's the possibily fluctuating an atom or a star or a brain out of that sort of quantum frothing void. Now in our universe, we got a head start, because we had inflation. We had tiny little quantum fluctuations that got blown up to much bigger fluctuations that then gravity you could grab a hold of. But yes, absolutely, even in this heat death scenario, you could have random quantum fluctuations that lead to overdensities and then do lead to the formation of structure. Thermodynamics doesn't say it's impossible. It just says it's un likely because that would effectively be decreasing entropy. Well, I wonder if in a way, gravity is kind of like the anti entropy. You know, like I understand this theory of entropy and that if you had a perfectly smooth liquid it wouldn't start collapsing. But I feel like maybe gravity is there to counteract that, right, Like, first of all, you need an infinitely sized universe for that to work, otherwise gravity would bring everything together. But I feel like, yeah, I feel like gravity somehow counteracts this idea that entropy always wins. Well, gravity definitely obeys the same laws of thermodynamics. I mean, gravity also wants things to like roll downhill. It doesn't like things having a lot of potential energy, for example. It likes to balance things. So I think it follows the same rules. Black holes, for example, which are gravitationally very dense objects, do contain an enormous amount of entropy. Entropy is not just like order versus disorder, right, It's about how many ways you can arrange the microphysical states and represent the same macroscopic object. Yeah, I guess it does get a little bit philosophical. But just to answer mat Tis this question, I guess the answer is. What you're saying is that, yes, even in a heat death of the universe, things are going to be moving at the quantum level, which maybe we'll end up kind of breaking the heat death of the universe exactly. There will always be quantum dancing. These quantum fields can never go down to zero energy. There will always be some energy and so some fluctuations, and so it's possible for the universe to get totally smooth and ten to the one hundred years and then fluctuate some new spot around which structure forms again. And some people even think maybe that's the story of our universe. Yeah. Well, I think you're saying two things. One is that they maybe things fluctuate and maybe they'll start clumping again together. But you're also saying maybe they'll fluctuates so crazily like suddenly the universe will just going to flip over and start over again. Yeah, randomly exactly, But we all have to wait ten to the one hundred years to find out. I think I'll just watch the recap episode for that. Who has time to binch that? Make sure you're not watching it while you're on your bicycle. All right, let's get to our last question here about dark matter and parallel universes. But first let's take a quick break. All right. We're answering listener questions about all kinds of things in the universe, including cats, rabbits, and ethically questionable physicists, at least in thought experiments. And our last question here comes from Craig, who hails from Ontario. Hi, Danielle and Horry, This is Craig from Ontario, Canada. Thanks for such an awesome pod. I was wondering about dark matter. Is there any chance that what we experience is dark matter in our universe could actually be stuffed with mass in an adjacent universe lending its gravity to us. Thank you guys for the pod, and I wish you both all the best. Whoa mind blown Craig just blue my mind. Also, I think I heard a cat in the background of Craig's recording. Did you really think I did? I think there's a meal there. Or maybe you're just hearing it in your head, Daniel, maybe your cats are haunting you. I'm just saying I'm looking for new subjects for thought experiments, and Craig, maybe you want to volunteer thought experiment or psychological experiments. Can't it be both? I guess all psychological experiments are thought experiments. That's right, And I'm gonna push back and defend physicist a little bit there. We do have a lot of crazy thought experiments, like a person in a box out in deep space and you got to wonder, how did this person sign a consent for him? Is somebody going to rescue them? Or they do they have a bathroom and water and this kind of stuff. But that's why these are just thought experiments and not experiments anybody's ever actually going to do right, right, Like you would never like build a giant ring Niva, and you know, you know, shooting things in incredible energy and then put people on top of it, or put an exco city or anything like that, right, that would be totally ethically you're RESPONSI, well, for example, we would never accelerate cats near the speed of light and collide them. Absolutely not. But protons don't have the same rights. You're categorically against them. But anyways, Craig, whether or not you have a cat, you have an interesting question here about dark matter adjacent universes, which is an interesting phrase, and so I think his question, and I have to say, I think I told you this theory a long time ago. I remember we were driving somewhere for our book tour and I was like, hey, Daniel, I have a theory about dark matter, and I think it was exactly this theory, which means Craig is a genius, or it means Craig should be a cartoonist. Yeah, maybe maybe you'll find more success in that like I did. But yeah, it's kind of an interesting question, like because we don't know there's dark matter in this universe. We notice it's there, but we don't know what it is. And so his idea, or I guess I should say our idea, the Craig cam theory about dark matter, that maybe what we feel as dark matter is actually like mass in a parallel universe at some how leaking over that somehow we can see through gravity. M Yeah, and I love this idea. It's really clever. But let's take a step back because you said there's dark matter in our universe. But I think Craig's question is asking us to reconsider that, because really, what do we observe. We observe gravity. We see something is contributing gravity to how galaxies spin, and how the large scale structure the universe formed, and how the ripples in the early universe plasma were created and propagated. What we see is gravity that we cannot explain. We attribute that to dark matter. We say there must be some missing mass in our universe we never observed before. It's basically what we say dark matter is. But what we actually observe is the gravity. So I think Craig's question is, like, how do we know that's mass in our universe and not just gravity leaking in from some other universe where that mass is right? Right, Although you're kind of parsing hare is here, because like if there is another universe and leaking into ours, then there's like overlap, right, I mean that universe is partly in our universe, right, and so it's technically kind of in our universe too, exactly. And that is the hair I think we are splitting here today, which is what does it mean to be in an adjacent universe? Right? If this mass is in another universe and we don't interact with it except through gravity, is it really in another universe? If we can interact with it, is it in another universe? I mean, I'm interacting with my chair and my desk right now. So we say that they're part of the universe. Could you say they're in another universe but we can still somehow interact with them. I don't know. I would say that things in another universe are things you cannot interact with. And so if we're interacting with this stuff, if we're feeling it's gravity, then it's in our universe. And that's not like a physics argument or philosophy argument. It's really just sort of like semantics. It's like, what do you call a universe? Yeah, and we never argue semantics here on this podcast. Well, I think there's kind of two interesting ideas here in Craig's question, right, Like one like maybe in our universe there's a whole other universe on top of us of particles that our particles don't interact with. There's like planets and suns and stars and photons and things like that that just don't like interact with us, our kind of particles. So that's why we don't see it, but they're there. They're like it's like a ghost universe on top of ours, but somehow, like the gravity, we do kind of feel its presence. It's gravity, and that's what maybe we see as dark matter. Then there's the other idea that maybe it is another universe, like maybe it's the multiverse or the quantum multiverse or you know these quantum foam bubble universes. It would be considered by some as a totally different universe that somehow leaks into hours. And so those are kind of two different ideas, right, that's like maybe we share the same space or maybe we don't share the same space. Yeah, those are two different ideas and they're really fun. The first one I think is beautiful because it's sort of our current conception of the universe, suggesting that there is all this matter out there that we can't interact with except through gravity. Gravity is like the great unifier because it interacts with everything. Anything that has mass or energy, regardless of any other quantum properties, it has gravity will talk to it. So if there's anything else out there in the universe, even if we can't interact with it, even if it has like weird particles and weird forces that are not overlapping at all with ours, we would know it because of its gravity. And you can sort of think of that as like another universe, as you say, like a ghost universe. You can imagine all these particles out there with new dark forces and dark charges and dark physics and doing their dark stuff, maybe making dark planets or completely different emerging dark phenomena that we can't even imagine because fundamentally they're very different physics at their core, and how that bubbles up to creating ice cream or not depends on that fundamental physics. So even that is fascinating to me. That's sort of like being another universe on top of ours, because it says that we are only experiencing a slice of this universe, and we already know that's true. We already know there's lots of stuff out there. We can't see neutrinos and stuff, though we can detect them through some quantum interactions. But gravity is this awesome way to say if there's something else, anything else in our space, then we will see it. And so the other idea which you bring up is like, maybe there's another universe that gravity can somehow interact with, Like maybe there is other matter, and if it were in our space, we would interact with it and we would see it. Right, it is sort of normalish matter. It's just like in some other space. But yet gravity is connecting our space and their space. It's sort of like the other idea you proposed, And that's a little bit theoretically problematic because remember gravity is about space. Gravity says when you have mass in space, it bends, and so if this mass is bending our space, then it's sort of in our space. There's no other way for that mass to bend our space without being in our space. You're saying like, if there is another universe out there that's separate from ours, and I like that this you made, like if it were in our universe, we would interact with it. But it is in a different kind of space or phase or whatever, and so therefore it's another universe. But you're saying, like, if we do feel it somehow, and there is a connection which means you should lump both of our universe together into one word called the universe. Yeah, exactly, if it's bending our space, then it's in our space. By definition, it can't both be bending our space and not be in our space, right, Like if I take a tunnel from my house to your house, suddenly it's my house too, it's our house. I guess, let's part the legalism of that put all the work into the tunnel, I suppose. So there are other interesting wrinkles here though, because, for example, it's possible that gravity acts in ways different from all the other interactions. Like it might be that the universe has multiple dimensions, right the ways that we can move x, y, and z, there might be more of those, And one theory is that gravity can operate in those other dimensions and the other forces cannot. So it's possible there are other quantum objects and quantum forces that can operate in those other dimensions that we can't operate in. Gravity would be sort of unifying. It would still be in our universe, right, but we wouldn't be able to see them or interact with them, but they'd be moving through sort of like other dimensions instead of the three that we have. So I guess maybe it all depends on how you define the universe, right, I mean, physics came up with the cause of the word multiverse for kind of a reason, right, to describe kind of like a collection of mini universes. And there's a whole set of different multiverses as like a multi multiverse, but in all of them there's no way to interact. Like there's the quantum multiverse that we just talked about in terms of the Shooting verse cat Well, there's two different branches of the universe reflecting different possibilities, but the branches can never interact with each other gravity and one doesn't leaked into the other. There's the bubble multiverse that suggests the bubbles of normal space were popped out of inflationary matter, but these would be separated by inflating matter, so that'd be so far away we can never interact with them. And all these different multiverse theories, you cannot interact with the other universes in the multiverse. That's only something in a Marvel cinematic universe that is possible, and that all that stuff is real too anyways, Right, Well, it's making real money, that's for sure. Well, I think maybe the answer for Craig here to this question of whether dark matter is just stuff that we feel from another universe. I feel like the answer is, yes, it's possible it could be the case. But then a physicist like you, Daniel will come in and say, well, technically, if we're feeling it, then it's part of our universe, and sort of try to cancel out the question. I would say, if we're feeling it, it's in our universe. Yeah, absolutely, But I guess the main main idea is that it could be that dark matter is just kind of part of this whole set of things that exists out there that we just don't mostly notice. And I think the spirit of Craig's question is that there's the possibility of a much wider and broader kind of reality than the one we experience and that we think about. And that's the cool thing about dark matter and about gravity is that it lets us explore. It gives us a poor to everything that's out there in every part of our universe because gravity is universal, and so that's the really exciting thing about it to me is not only the universe could be very different from the universe we experience and we've imagined, but that we have a way to discover it and explore it. So gravity is really this wonderful handle on everything that's out there. Interesting. Yeah, it's sort of like a universal currency for the multiverse exactly. It's pretty hard to exist without any gravity. All right, Well, I think that answers correct questions and all of our questions today. Those were really great questions. Thank you for taking them in. Yes, thank you everybody who's thinking about the universe and being curious. And if you have questions and have been too shy to send them in, please don't hold back or right to us two questions at Daniel and Jorge dot com. The only thing we ask is that you were ethical in your thought experiments, or at least to them in another universe. Then then you can do whatever you want, right, No, man, Other universes have feelings too, though. I guess if we're feeling them, then they're in our universe by my rule. Yeah, you just canceled your own question. I guess I did. All right. Well, we hope you enjoyed that. Thanks for joining us, see you next time. Thanks for listening and remember that. Daniel and Jorge Explain the Universe is a production of iHeartRadio. 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