From the Vault: Daniel Whiteson, Time Traveler

to Blow your Mind. My name is Robert Lamb and I'm Joe McCormick. And for today's episode, we're going to be chatting with Daniel Whitson, who is a particle physicist and science communicator and one of the hosts of the podcast Daniel and Jorge Explain the Universe. This is Enniel's third time hopping on the show with us. The previous episodes were in September of twenty nineteen and April of And for this episode, we're gonna be talking about a book.

Daniel and his co host and co author Jorge him have a new book called Frequently Asked Questions About the universe. So it was a real pleasure to have Daniel on the show for the hat trick, and I guess without any further delay, we will go right into the interview. Daniel, welcome back to the show. We're so glad you're here. Thanks very much for having me back. I always fun to talk to you guys about things that blew my mind. Awesome. So, um the podcast Daniel and Jorge explain the Universe still

going strong? Um, how how far are you into explaining the universe in its entirely. We have explained zero point zero zero zero zero zer zero zero zero one percent of the universe so far. Nice. I uh, actually I was looking at your recent episodes and I saw did you recently do one that was an interview with Shawn Carroll about the uh, the many world's interpretation of quantum mechanics. I know, I know he favors that, right. Yeah. We actually have a series where I interview an expert on

each of the interpretations of quantum mechanics. We did one on Copenhagen interpretation with Anna Becker, we did one on the relational interpretation of quantum mechanics with Carlo Rovelli, and then we talked to Sean about many world's interpretation, and just a couple of weeks ago we did one about the pilot wave theory of quantum mechanics, which totally blew

my mind. Really much overlooked and unnecessarily maligned interpretation of quantum mechanics, in my opinion, malign, like people are being mean to it. Well, there's this famous proof by John von Neumann like seventy years ago demonstrating that it was essentially impossible, and because von Neuman is such a giant of the field, everybody thought, well, that's that. Turns out he was wrong, though, and it took people years to

figure it out. It was Belle actually who figured out that Nouman was wrong, and that it's possible to have a theory of quantum mechanics with hidden variables that's deterministic, that's not random at all um. But still to this day, nobody really takes pilot wave theory seriously, to Bell's great restoration, and I think it's because Neuman sort of through shade

on it decades ago and it never really recovered. I guess that's always dangerous when there's like a famously smart person who has an opinion, absolutely, And I find that physics Nobel Prize winners are especially guilty of this imagining that they are experts in every corner of everything and opining on economics or you know, social politics or whatever. If you have Nobel Prize winner in front of your name,

you're an expert. Uh So, today we wanted to talk about a couple of chapters that are in a book of yours. Did that come out earlier this year? Tell us a bit about the book. Yeah. So the book is called Frequently Asked Questions about the Universe that I wrote together with my co host on the podcast and longtime collibrator Orge h. Cham who's also famous for being the genius behind PhD Comics. And the book comes from noticing that people who write into our podcast often ask

similar types of questions. There are a few things that seems like everybody just wants to know about or understand, or the things that people grapple with. You know, I'm a professional particle physicist in my day job, um, and so I like asking questions about, you know, the deep nature of the universe and how our space and time really related. But you don't have to be a professor of physics to find these things interesting. And we feel like,

in a sense, you know, curiosity is democratic. Everybody wonders about these things, so we wanted to try to attack some of these really big questions that everybody wonders about in an approachable way, in the way that doesn't require you to really have any knowledge of modern physics at all. Yeah. I've really been enjoying the chapter as I was reading.

One thing I like that you do in this book, um, is that you know, it's not like a continuous narrative that has to you have to have read everything that came before in order to understand. Like, the chapters can be consumed pretty much on their own, right. Yeah, we figured, you know, each chapter should be like one long bathroom break. So I mean, I'm not telling you where to read it, but looking you're reading while you're busy sitting down doing

something else. Each chapter, you know, should entertain you while you're doing your business, just you know, don't get so distracted that you forget to flush right now. Obviously this edition would lack the wonderful illustrations that are in the print and the Kindle version, but um, but you guys put together an audio version as well. Right, yes we did.

We got to record the audio version of the book, which is out now also, and the chapters are read by me and by Jorge alternating, which is a lot of fun just sort of hear your words come to life. But yes, the audio book does miss some other the

real genius of Jorge's drawings, Um. Jorge and I started working together on science communication and more than ten years ago when I reached out to him because I thought that cartoons would be a really great medium for communicating science, because they don't take themselves seriously, you know, They're a cartoon. Is different from like a figure into science paper, you know, which is very official and formal. A cartoon like makes fun of itself and is easy to you know, hang

out with and accessible. And Jorge was great at that kind of stuff. So he and I started working together on explaining science using cartoons a long time ago. Um, And one thing I really value about his cartoons is

not just that they are good visual explainers. He has a real visual skill for explaining something simply on the page, but also that there's sort of a second voice there you can hear, like in the text, is the voice of me as a physicist, and then in the cartoons you can hear sort of his response to some of the crazy ideas um and that sort of mirrors the way the podcast works. On the podcast, I'm talking about physics and Jorges you know something like that doesn't make

any sense, or how could that possibly be? Or what you got to explain that again, So it sort of tries to capture those two voices. Yeah, I really liked that the illustrations almost seem kind of riffing on the

written contents of the book. Well, so the parts of the book that we wanted to focus on today, I think we're mostly centered around the idea of time and so maybe maybe a good place to start is you have a chapter in the book where you talk about time travel and you make some arguments about which types of time travel are plausible from a physics perspective and which are not. So maybe that would be a good

place to start. Give us the way of the land, like what types of time travel are the least consistent with the known laws of physics and which are the most consistent. Yeah. Sure, so for those of your listeners who are busy building their time travel devices, of the usable advice well, you know, the kind of time travel that's most inconsistent with the law of physics is the kind that most people want to do, you know. It is I want to go back in time and change

age something. I want to not spill my coffee on my lap, or I wanted to go, you know, not make a mistake, or I want to go ask that person out in high school, which I was too timid to do, and now I realized I should have that kind of thing. It's not just that it's ruled out by the laws of physics. In my view, it's not

even sort of internally self consistent. What it means um, you know, and and a lot of people think about time travel is like I want to go back in time, as if time was a place, like if it's a somewhere you can go. It's just sort of like along

a different direction or something. And it's tempting to think about it that way because we we hear a lot about modern physics telling us that space and time are related and time is like a fourth dimension of space, and so it makes you want to think about time as a direction in which you can move and maybe you could just rewind it somehow, right, But the problem is that time. You know, first of all, we don't understand time like at all. You can dig into that

in a minute if you like. Um, but the problem is that time sort of reflects how the universe changes, and so you know, I think about time is like you have a timeline. That timeline is the universe changing, Like you have the universe at one moment, and you have the universe at another moment. The next moment comes later in time, and things can't change without time. Time is that change. So the self consistency problem is that going back in time to change it changes the timeline itself.

So like, how does the timeline change? If the timeline is the change, how does the timeline itself change? It would need like its own time, Like the timeline is now moving through time because it was a time before you changed in a time after you changed it, So it needs like a second dimension of time. I mean, it just sort of all becomes very complicated and falls apart as soon as you start thinking about it carefully. So going back and changing something in the past really

just makes no sense from a physics point of view. Yeah, I love this because I have long kind of been skeptical about the idea of time travel into the past. And one of the reasons I had doubts about this is that wouldn't we expect to have already encountered lots of time travelers at some point in history, And there's no unambiguous evidence of that. I mean, obviously some people, you know, they're weird little things people think or time travel,

but nothing that looks really clear. So it kind of makes me think that if if time travel into the past ever happens in the future, it will be of a very limited nature. Yeah, I love that as an experimental proof, you know, like, if time travel exists any time in the future, then you would expect to see it. Now. I love that. It's just such a powerful argument. It sort of reminds me of Stephen Hawking's famous invitation to time travelers, where he threw a party and then he

posted the invitation later after the party. The idea of being that time travelers, you know, they should be able to get there anyway, but of course nobody should have to his party, well that we know of. He might have dispensed with them, or maybe he is a time traveler. Oh that's a good premise. For a sci fi movie like the Time Traveler Hunters trying to eliminate all evidence

of the time travelers. Well that that kind of plays into um you know, some of what you're talking about about it being if it if it does exist in the future, then it must be limited in scope. And I guess you could look at it a couple of different We could basically just sci fi the hell out of it in multiple directions. But you know, you could say, like, well,

maybe travel into the past. It has a range and we haven't reached the point to where time machines of the future can reach us, or it's just so tightly policed that nobody can make it back. You know, we have time cops or or something that are that are keeping people from making too much of a show of

the whole thing. There's so many of those science fiction depictions of like a b time bureaucracy, you know that's managing the time flow, like you saw that in Loki and in Umbrella Academy, and and and in um that book recently, this is how you win the time war. And those can be a lot of fun, but also I feel like they're they just make no sense at all. You know, how do you have this weird administration that's separated from time and also weirdly frozen in like nineties bureaucracy.

It's you know, it's fun, but not not if you think about it really at all. What also reminds me a lot of something I was actually chatting with you about a couple of weeks ago when I interviewed you for a short freelance piece for how stuff works dot

com about UM about the zoo hypothesis. You you spoke about about that for the interview, and uh, you mentioned that one of the strong arguments against it is that if there is actually this, um, this conspiracy of of aliens to avoid contact with humans and and and keep us in the dark about the uh you know, the

Galack tick, civilizations just outside of our view. UM, the main argument against it is that that governments as we know them, by the only model that we know the the the human model, are not really good at keeping secrets. They're not good at managing secrets. And it seems like you could also apply that to the idea of intelligent beings or humans and the future managing the timeline and

so forth. Exactly, you know, some version of Elon Musk in the future is get to get his hands on it, and then he's going to launch a bunch of crazy, you know missions, and somebody's gonna mess something up. So it's hard to imagine that people in the future having time travel and somehow keeping it a secret or slipping into the past unnoticed and nobody ever, you know, breaking the protocol or something. It's it just becomes totally implausible

the more you think about it. Picking up off that, I mean, this is another one of the weird things about time is it seems like time is actually one of the arguments against the idea of a coherent galactic civilization, if this makes any sense, because like you think, a civilization in order to organize itself has to have some pretty close to synchronous, uh, you know, thing going on, Like things have to be happening pretty close to around

the same time for them. But does it even make sense for I don't know, one planet in a galactic civilization to be part of a civilization with one on the other side of the galaxy? I mean, is there you know, can they say, uh, is there such a thing as what's happening right now on a planet on the other side of the galaxy. Yeah, you make a great point, because there's a speed limit to information moving through the universe, which puts an effective limit on like

how well you can coordinate and organize things. Makes you think about this in cosmology all the time, because there's a like the largest thing that can exist in the universe. Just from very simple arguments like the aged the universe

and the speed of light. You can't have an object that's like ten thousand billion light years wide that's like coordinated, it has like a structure that's like gravitationally bound on itself because there hasn't been time for like a photon to even cross over the entire size of that object. So there's like a limit to how big the universe can even build like a thing, not to mention like

the close coordination required like organize a galactic empire. And so yeah, absolutely, Um, I think that the sheer size of space definitely limits our ability to explore it unless it breaks down into you know, lots of different unorganized entities, like maybe we send humans in an arc off to another star and they start their own human colony and we're not in touch and we're not part of some you know, political nation state. But at least we are

humans here and there are humans there. Yeah. I think that's a great way to conceptualize it. So, I guess coming back to time travel for a minute, I wanted to talk about some of the specifics you offer about physically plausible ways of traveling into the past. Uh. So

you mentioned a couple of things. You mentioned the idea of wormholes, and then you also mentioned one that might be less familiar to people, the idea of an infinitely long cylinder of spinning dust, which could potentially, at least maybe depending on something about whether something about relativity is true or not, could potentially allow time travel into the past through something called time loops. Could could you explain how this would work? Like, what would this experience be

like for the time traveler? Yeah, Well, the short answer is, we just don't know. Uh. This is a realm where we are like on the cutting edge. Theoretically, people are looking at the rules of how space and time bend and twist because you know, the general relativity our theory for space and time itself essentially tells us that space and time bend in response to mass, and then tell

mass is how to move. So, for example, you have an empty universe and you put a star in it, it bends the space around the star, and then the bending of that space tells things how to move, and not just through space but also through time. So you go near a black hole, for example, time is slowed down. So there's definitely some deep connection there between space and time. And what people have done is trying to explore extreme scenarios of that what happens if you do this, what

happens if you do that? Is this allowed? Is that allowed? And so it's sort of like exploring the universe, but just inside our own heads. We can't necessarily yet go out there and build these things in space and say, let's see what happens experimentally, But we can do similar like fought experiments, where we say, what would happen if you did this, and let's just let's assume the equations are correct and see what happens. And so there's a

couple of fund scenarios there. One, as you said, is wormholes. These aren't really crazy because they are like connections between different points in space, and when you think of space, you probably think of like just sheer emptiness, you know, the back drop the stage on which the universe happens. But now we know that space is more complex. It can bend and it can twist, and that might be

something that you can put in your head. You can imagine like space bending around the Sun. But because space is like a thing with an arrangement, it could also do other really weird things, like be connected non trivially. So you have like a chunk of space over here, it can be directly connected to a chunk of space over there. What does that mean. Well, you're used to the space around you being connected to the space right next to it. That's what it means to be right

next to it. Right, you take a step to the left, you move to the next sort of piece of space. Think of it sort of like pixels on a screen. Right, Well, a wormhole is a connection between two points in space that are otherwise really distant. And so you take a step from a from one pixel and now you're in

a pixel on the other side of the screen. And so that seems weird and impossible, but remember space can have all sorts of strange connections, and according to the equations of general relativity, the ones that define how space is organized that is allowed, it is possible, and so a couple of folks at cal Tech we're thinking about, well,

you know what about time? Is it possible for one end of the wormhole to be in one place and the other end to be in another time because, as you were mentioning earlier, like the notion of simultaneity, like when is now depends really on where you are. Also, so they have this idea to take one end of the wormhole and you accelerate it near the speed of light. That effectively it can be sort of back in time.

And this all works theoretically, but it also sort of contradicts other things we know, like if you go through this wormhole and you come out in the past, you know, doesn't that break things like causality? And you come out in the past and kill yourself before you um do the experiment, then you don't do the experiment, you don't come out in the past. So it appears to create paradoxes, and nobody knows like how to resolve that. Does that

mean that these things are impossible? Does that mean if you did that the universe would like disappear in a puff of logic? Nobody really knows what would happen, so that it's a bit of a contradiction in the theory itself that it predicts something which seems to be disallowed by other parts of the theory. And it's a similar

idea for these closed timelike curves. People said, if you create these infinitely long cylinders of spinning dust, which doesn't sound easy to do, then it bends time in this way that the time then as you move forward in time, you're actually moving sort of like sideways through space time in a way that's similar to the experience of going into a black hole. Outside a black hole, time always

moves forwards. Inside a black hole, space has bent so much that space only moves towards the center of the black hole. It's like one direction to space. So if you imagine space being distorted, not quite as much as a black hole, but sort of in a similar direction that it sort of bends space sideways, then you can create these paths where something can move in a loop through time. Um, but you would be trapped on that loop,

so you wouldn't be able to like change anything. It's like a fixed loop, sort of like Harry Potter style loop through time where every time you go through, it's exactly the same thing happening. And these are really fun because nobody knows like if these are actually possible, and

what would happen if you actually went through them. So so we don't really know, for instance, like what conceivable reason there would be for a civilization to conceivably construct one of these, Yeah, because we don't know practically what you could achieve. And also, an infinite cylinder spitting dust sounds like an expensive project, you know, the word infinite seems to raise some doubts. And when it comes to wormholes, people know how to calculate whether a wormhole is allowed

by the theory of general activity. Nobody knows how to build a wormhole. You know. It's sort of like saying, Okay, it's possible to have an apple pie, but but we don't have a recipe for making one. Right. It's a different thing to say, like I know how to put it together than to say it's technically allowed to exist in the universe. You know, It's like if you say, well, the sun is allowed by the laws of physics, but

I don't know how to make it happen. If I just start from a cloud of gas, for example, and so that's a big puzzle. Nobody really knows how to build a wormhole or even keep one open. Um if you did manage to build one, Are there any reasons to suspect that wormholes exist naturally? Oh? Great question? Not

yet know. Um. Some people wonder if there are wormholes that connect the super massive black holes at the hearts of all of our galaxies, but there's not like any evidence out there anything that can't be explained without wormholes that you would need wormholes to explain. Um, that would be super cool, though, Um, I'm not aware of any evidence like that. Well, Daniel, you also have a chapter in the book that I really liked on the question

of will time ever stop? And I think this is one of those great questions because it's a yes or no question, and like many big questions in physical cosmology, it's a binary. But no matter which answer it is, it's mind boggling, like it is impossible to imagine time either stopping or going on forever. Uh So, so, what are your thoughts here about whether time will ever stop? I go, you're feeling there. And I also think it's really fascinating to go back through history and read about

which concept felt more natural to people. Initially, it felt to people like time should go on forever. Obviously, that was like a hundred fifty years ago, before we knew

that the universe was expanding. People looked out in the stars and they looked like they were just sort of hanging out, and they thought, maybe the universe is just sort of there, and so obviously it's been there forever, right, And that was like this, you know, de facto assumption in science until Hubble discovered that the universe is expanding, and that gave the universe sort of like a direction.

It's like things are changing, and as you look back in time, that suggests, you know, something a moment when the universe was like crazy infinitely dense. So it suggested a beginning. And that must have been an incredible sort of mind bending mental gymnastics to execute to go from thinking, oh, it makes sense for the universe to be infinite in time to going to like, oh, the universe had a beginning, and now let's trying to figure out what that beginning was.

Um So, I think that's really interesting, And you know, I think the thing that's really cool about this question is not just that it's tangible because it makes you wonder, like am I going to go on forever? As the universe always going to be here? But because it really gets at the heart of the deepest problem in physics right now, the like the fundamental conflict we have between two idea he is in physics, which are quantum mechanics

and general relativity. You know, we have a quantum mechanical description of how like particles bounce off each other, and we you know, have a lot of questions about how that works, but we have a pretty good theory for you know, understanding quantum particles, and we've been talking about general relativity, you know, how space bends and how it affects time and what happens in black holes and all

that stuff also very successful. The problem is that these two theories nobody knows how to bring them together, and critically, they have very different stories to tell about what time is. They treat time totally differently with huge consequences for the answer to this question will time ever stop? And so to me, this is a fun question because it puts

its finger on right on that conflict. Yes, so there is there's a concept that you introduce in this chapter about uh sort of time as we experience it, being a sort of special case or special circumstance of a hypethetical substance you refer to as meta time. Can you explain something, well, like what are you getting at here? Well, one of the basic questions is is time fundamental or

is it emergent? You know, a deep question in modern physics is like, what are the essential ingredients to the universe? What did it start with? And then what sort of arises out of that out of the complexity of the possible interactions. You know, for example, if you're playing with legos, the fundamental ingredients are the basic pieces, and from that you can make complicated things dinosaurs or pirates or spaceships or whatever. But those spaceships they're emergent, you know, They're

not necessary. They don't have to exist. You can take it apart and just have the legos. In the same way, in our universe there are complicated things like ice cream and hurricanes, but those don't have to exist in the universe, right You can imagine a universe without hurricanes or without

ice cream, as sad as that is. So, then the question is what are the basic elements of the universe and for a long time, you know, people like Newton thought that, well, obviously space and time are fundamentals of the universe. They're just like, you gotta have that, right, And now people are wondering, like, well, is that really true? Is it possible to have a universe without space or

without time? You know, we gotta when you're really digging deep into the nature of the universe, you gotta push hard on the fundamental assumptions. So there are a lot of ideas now about how space could be emergent, you know, how it could be that the universe itself, that space is not a natural thing, that like ice cream, you could have a time in the universe where there wasn't

any space. The space is like just briefly, the stitching together of these um separated pixels of space using quantum entanglement to sort of weave together this idea of space, these relations between different locations that we experience, and we could talk about that for an hour um. But even moving beyond that, now folks are wondering, like, is time

also emergent? Is it possible that time is not a fundamental property of the universe, but it just sort of something that exists now and it's really hard to even think or talk about it because, like I just said, it exists. Now I'm using time to talk about when

time is. It's very complicated and confusing. But there are some theories that tell us that time might be not an illusion, right, not in the sense that it doesn't exist, but it might not be fundamental, that it might arise from complex interactions of smaller, more fundamental elements of the universe.

And so that's this idea of meta time. You have to imagine some like deeper laws of physics that control those fundamental bits that I'm being vague about because we have no idea what they would be or what they are, what the rules are. And And if this seems sort of like frustrating, it's because we're at the very beginning of even talking about the answers, because we're just formulating the questions. You know, sometimes it takes like a hundred years to figure out, Okay, the question to ask is

is there always time in the universe? What does that mean? And how do you even think about a universe without time? Then you can start to make progress on the crazy ideas that might explain it. Uh, this may be kind of a tangent. But this actually makes me wonder about a question that's come up on the show before. Do you have a view on what the present is, on whether something special is actually happening in the present. Uh, Like, does only the present exist? Or does all of time exist?

It's a really great question. We don't understand that at all. I don't understand it. I don't even know if it's a question of science or if it's a question of philosophy, because it goes into the nature of consciousness. You know, does the whole timeline exist and we only experience part of it? Or you know, does only this moment exist? Um. Physics doesn't have a great way to even define what the present is um, and so it's it's pretty hard

to put your finger on it um. And I love because these are questions that like, we don't even really know how to attack these questions. And what that suggests is that there's something wrong and the way we're organizing our thinking. You know, it's like if you're asking a question and you're just using the wrong language, we're using the wrong notation, then your question seems really complicated and confusing. And if you learn a new perspective and then suddenly

would make sense. You know, I'm reminded of that Far Side cartoon where the scientists are trying to understand dolphins and they're writing down phonetically what the dolphins are saying, and they're saying things like you know, obla Espanol and the you know, the scientists don't speak Spanish, so tho to them, it's just nonsense. My bodies if they if

they knew the language, it would all click together. And I feel like that's the problem we have sometimes that was just not speaking the right language at the universe yet, and that's why some of these questions are awkward and really hard to grapple with. I thought you were gonna say today's physicists are only equipped with cow tools only for spherical cows. You know, all of this um also, it reminds me a bit of the Copernican principle to UM.

But but going beyond just the idea of like, you know, there being some uh I we should not not see that there's something privileged about about our planet or about humans. But but could you could you even apply that based on what you're saying to to the present moment, to this time that in which from which we are viewing

the universe. Yeah. Probably, And I think that's why a lot of progress could be made if we ever did get to talk to alien scientists, because I think we would learn a lot about, um, you know, the biases that creep into our questions and our reference frame for answering those questions because of our human experience, and alien intelligence that might have a very different relationship with the concept of time, might have a very different treatment of

it mathematically and physically, and might make a lot more sense. You know. The problem with alien intelligence, of course, is you know, finding them, talking to them, decoding their language, and then if they are so fundamentally different that they've made that they avoid human biases, they might be impossible

to understand. And so while it's tantalizing to imagine that like aliens are out there with the answers the deep questions about the universe, it might also be that, uh that we could never understand what they have to say. I have long thought we should outsource all of our physics research to like a seventeen dimensional octopus. Um, if you know one, I'd like to meet it, because I got questions than uh so, but to come back to

the idea of will time ever stop? You talk about a couple of possibilities for what that would look like. Say that you know the far future of our own universe, at least at least what we can reason from what we know today, and and a couple of these options are are the Big Crunch and the heat death of the universe. Do you do you want to talk about what those would means as best we can guess for

time itself. Yes, So remember that there are two paths to go down if you're asking questions about the deep future of the universe, and one is quantum mechanical and the other one is general relativity. And quantum mechanics is pretty straightforward about this. It says that, look, time always

existed and time will always exist. And there's a pretty simple argument there because according to quantum mechanics, quantum information can't be destroyed, like when something happens um, you know, the information about what used to happen is encoded into the future, and so it suggests that time has always existed. There's no mechanism in quantum mechanics for time to start.

It should always have existed, and you flip it around the other direction, it should always exist, so they should always be a universe, and clocks should always tick forwards according to quantum mechanics. But that assumes you know that space is flat, and bole and general relativity, the other pillar of modern physics, tells us that space is not simple. It's not flat, it's complicated. It's expanding, and you know, the mechanism by which space is expanding is not something

that we understand. Hub will discover a hundreds something years ago that the universe is expanding and things are moving away from us, And then twenty something years ago we

discovered even more mind boggling lee that that expansion is accelerating. Right, It's not like stuff is moving through space and gradually slowing down and maybe eventually gonna stop and turn around and come back um and collapse, but that it's speeding up, which means that there's some massive, incredibly powerful force in

the universe that's literally tearing it apart. Because we don't know the mechanism for it, though, we can't predict what it's gonna do, Like it turned on about five billion years ago, started tearing the universe apart. Will it do that forever? If so, you end up with like a universe with everything is super far apart. It's just like a bunch of black hole holes from collapsed galaxies, separated

by you know, unthinkably vast distances. Even compared to the distances we see between our galaxy and other galaxies today, you know, these galaxies would be so far apart that they could never even see each other. You know, light

would never reach one from the other. On the other hand, dark energy could change its direction, it could stop, it could turn around, it could cause the universe to collapse back down into an incredible moment of singularity at the end of the universe um, and then we can ask questions like, well, what happens then, you know, does the universe stop when you reach another singularity, another moment of incredible density. We just don't know because general relativity describes

that process. But when you actually get to the singularity, people think of singularities is like a feature of general relativity. Really they're like a failure of general relativity. It can't predict anything that happens there. Doesn't know what to do. It's like, well, that's the direction you're going, but once you get there, I can't tell you what's going to

happen next. So if that happens, we just really don't know what the fate of the universe would be in that scenario, but you know, it wouldn't be pleasant for humans or for seventeen seventeen dimensional octopi. But I guess with with the other option, with like you know, the heat death of the universe, everything just expanding and cooling and reaching some kind of equilibrium where um, where there's

no there's no imbalance to distribute any further. I think in the book you raised the idea that this could in a way represent a threat to to our concept of time because time would maybe in itself, time has something to do with entropy, and this would be a state of maximum entropy. Yeah, we see the universe proceeding through time and we see entropy increasing, and entropy is

a really tricky topic. You hear people talk about it a lot, but it's really hard to sort of grapple with intellectually, and people try to think about it in terms of like amount of disorder in the universe, but that can be pretty misleading. Technically, it's really relates to the number of different ways you can arrange the microscopic nature of the universe to be consistent with the macroscopic

nature that you observe. That's a little bit more subtle, but it's actually a more accurate guide to what entropy is. And what we notice is that entropy seems to be increasing through the universe, like there's something we've observed, and a lot of places in physics seem to be sort of like ambivalent about time. The laws will run the same forward or backwards. It doesn't really matter, if you know,

without friction or air resistance. For example, you can throw a ball up in the air and it lands back in your hands. If you played a movie of that backwards, it would look exactly the same again without air resistance, because that increases entropy. Um. But entropy is the one place where in the laws of physics there seems to be a preference for things moving forwards. So it's often claimed that entropy might be the reason time moves forwards, and I think that's a bit of a step too far.

You know, we see that entropy increases as time moves forward, so there's a connection between them. That doesn't mean necessarily the time has to move forward. I mean, if time moved backwards. It just means that maybe entropy would decrease, right It um creates this connection between entropy and time.

It doesn't necessarily imply a direction, but some people wonder what would happen when you reach a state of maximum entropy, and maximum entropy would be as you say, everything progresses forward and the universe so of spreads out and it becomes maximumly even there's no like hot spots and cold spots, because that allows you to rearrange the microscopic state as many ways as possible, so the most freedom to rearrange

the microscopic state and so the most entropy. And in that state, it's called the heat death of the universe because you have no hot spots and no cold spots, so no way for like energy to flow. Nobody do anything. The way that you operate as a human being is through energy flows, and the way that computation happens this through energy transfers, and so you can't really do anything if there's no energy ingredients. So that's why it's referred

to as the heat death of the universe. And people who think of that time is deeply connected to entropy wonder if when entropy reaches its maximum point, if time then somehow stops, or maybe time stops and then turns around, and entropy starts to decrease like a bounce in time. And nobody knows the answer to these questions. Nobody's gonna be around to know the answer to these questions, even

if you're optimistic about the length of human civilization. But they're really fun to think about because they make you think about what time is and you know, and how it relates to the whole universe. Well, though, on the question of nobody being around this this may also be a tangent. But this makes me wonder do you have

opinions on the alleged Boltzman brain problem. I know we talked about this on an episode a few years back, and um so maybe kind of fuzzy on the details, but if I recall, it has been used to argue

against some types of future eternity ees. But basically the the argument is, if the universe were to go on existing literally forever, with certain types of properties in play, eventually people whose brains randomly formed from fluctuations in space would outnumber people who exist through biological evolution on a rocky planet, and thus we would expect to be those brains instead of these biological brains. Is that roughly right? Yeah.

Essentially it's arguing that if the universe reaches heat death and then goes on forever, that most of the time in the universe is in during heat death, right, that really basically randomly sampled moment in universe should be when the universe is spread out and boring and gray. So then Boltman said, well, what if you just had a quantum fluctuation while aims before quantum mechanics, but what if you had a random fluctuation? Because you know, the law

of entropy statistical it's not exact, allows for fluctuations. And so he said, well, what's the chances of the whole universe then being like a fluctuation in some vast or heat dead universe that already has existed for unknown millions of years. So he was trying to fluctuate an entire universe out of basically nothing. And so as a counterpoint, people are like, well, you know, there are smaller but more ridiculous things that you could fluctuate out of the universe,

like a galaxy or even just like one brain. And so at the point was made actually to criticize those kinds of cosmological models, because if your cosmological model seems less likely than you know, brains forming spontaneously in space and thinking that they're people, then it seems pretty unlikely. Um. And so I don't think anybody really takes it seriously. Is like a theory of the universe. It's sort of just more like a mental exercise to wonder, like, how

likely is your theory of the universe? Um, you know, is it less likely than this absurd scenario. So there's another thing you bring up in your chapter. Will time

ever stopped? Is an idea I was instantly captivated by, which is you point out that technically, um, time could be stopping and restarting all the time, frequently without us ever realizing it, because how would we know, right, Like, our consciousness, our experience of the world is through time, So if time were to stop, uh and then restart, that might just be invisible to us. So you know, maybe there are just uh, these huge gaps in our life. Though.

That makes me wonder if time we're stopping, would it be possible to measure how long it stopped for? Oh, that's really interesting. You know, this sort of presupposes some sort of like meta time, some you know, other rules of the universe that's controlling our time. And because time itself controls how our universe changes, then strictly speaking, if time does pause you know, core to this meta time and then pick up again, nothing should change because that

would require our time to tick forward. Like no particles can move, no galaxies, no space can be created. You know, no expansion can happen without time our time ticking forward, and that means that there's nothing in our universe that should change if time doesn't take forward, which means that there should be no way to tell. So it could be like a near infinite amount of time between every tick of our universe could be passing in sort of

like the meta universe. I think the easiest way to imagine this is in the simulation hypothesis, the idea that the universe is like a computer program running on some mega computer. And you know, if the aliens or super beings running that simulation pause the simulation to go to the bathroom and come back, then we don't know that they've paused it. Right. It's just like the characters in your video game. They're not like, hey, buddy, that was

a long number two. You know what you're doing everything okay? When you come, they have no idea, and to them, you know, the experience is completely smooth. So I think, No, there's no way to know how long time has been paused for if it does get paused. I always wondered with that hypothesis, would we notice if the resolution on our simulation was downgraded, you mean, if they lost their funding and had to had to go to a more

course resolution decreased render distance. Yeah, I don't know. Sometimes it does feel like the resolution decreases or increases in depending on what's going on. I'm gonna blame my failing memory for that, Like, you know what the aliens have just been like cleaning up the cash, and that's why I can't remember, you know, what happened last week, or when I agreed to clean to the garage or whatever.

But there are ways that we do think we might be able to probe the resolution of the simulation of the universe under the assumption that we live in that crazy scenario. Because these simulations the way we do them, at least as we tend to like divide the universe into huge cubes and stimulate each cube separately, assuming that like the interactions between cubes are pretty small, which works

pretty well. You know, if you're in your if you're simulating like a single galaxy at the time, because mostly you're dominated by what's going on inside the galaxy and not stuff from other galaxies. But we have these particles, these crazy high energy particles that whizz through space at velocities nobody's ever seen before, or energies nobody's ever seen before, much much higher energy than anything like created by our

particle accelerators. And they might be like tripping up that simulation because they skip through several of these simulation pixels faster than anything you should expect. And you know, there are some things about those particles we see out there in space that we don't understand, and so that opens the door to like maybe you could explain those particles

as being like a glitch in the simulation. Now, speaking of simulations and going back to time travel, does anyone out there like make any out of an argument for time travel into the past by saying, well, if we are living within a simulation, then time travel into the past and the ability to change the past would be

possible within the confines of that simulation. Yeah, you know, if you're living in a simulation, then the rules are essentially arbitrary, and then yeah, you could wind time backwards. I think this goes to the heart of, like, I think, a basic confusion about time travel, because people imagine, like you get in a time machine and you and the time machine does something to you, and then you end up back in the past. I don't really see how

that could possibly work. What you really want in time travel is for the whole universe to travel back in the past and for you to not. So you gotta like get in the time machine and it's got to like rewind the clocks of the rest of the universe, right, You don't want to be like, Okay, it's still today, but now I'm ten years younger. I mean maybe some people want that. That's a whole different thing to look for.

If you want to like unspill your coffee, but you still want like the ideas you want to remember having spilled it on yourself so you cannot just repeat it. Then you need to rewind the whole rest of the universe somehow, which seems like a much bigger job that. Yeah, that's a great point. Yeah, that so the time machine would have to change the universe, not you. Yeah, I

never thought of it that way. Yeah. And a lot of our listeners right in when we talk about time travel and raise a similar point and a criticism of science fiction levels, which is that, you know, if you do go back in time somehow, how do you know where you're going to be? You know, because the Earth and the Sun and the Milky Way they're all moving, Um, so how do you know where you're going to end up?

And it's a fun question. Um though, I think if you're gonna posit like, okay, you can travel through time, then ostensibly probably you can travel travel through space time, so you can appear wherever you want. But the problem is, like, as I'm not even really necessarily well defined, because what does it mean to be here at a point in space or there at a point in space right now? This point in space? Now where is that point in the future, Because there is no like marker to space.

Space is all relative. It's not absolutely you can't like grasp this point of space and give it a name and say where does this bit go? There's only stuff moving through space relative to each other, so it turns out that's not even really well to find, Like, where was the Earth, you know, a million years ago in our space doesn't actually have a meaning? That is when

I had thought of before that always seemed an insurmountable problem. Yeah, but this, this actually reminds me of another thing I wanted to talk about briefly, which is relating time to the history of the universe and the Big Bang, a thing people often ask, and I know there are theories to address this is is what happened before the Big Bang, But if you have an understanding that, you know, you have a singularity at the origin of the Big Bang, that was the beginning of time itself as we know it.

What are physicists talking about exactly when they try to envision causes leading to the first instant of the Big Bang? Mostly they're trying to avoid that singularity because that singularity is a problem. You know, we don't see things like singularities in the universe. We don't see infinities, we don't see things with infinite density, we don't see things of infinite size. I mean, maybe the universe is itself infinite, but there's nothing that's like infinitely smooth or perfectly circular.

These are sort of abstractions in our mind, and so most physicists who are working on the very early universe are trying to avoid that singularity because, as I said earlier, general relativity breaks down. That's what it means like, if your theory predicts something infinite, it doesn't know how to

do any calculations beyond that. So instead of having like a moment of singularity, which is sort of like the naive general relativistic prediction of increasing density, instead they're going back and saying, well, maybe big Bang was just like a rapid expansion of space from a previously dense kind of universe that we don't understand at all. So the basic sketches, like you have some kind of weird state the universe is filled with like inflotons, some particle we

don't know if it existed, but maybe it did. And then those inflotons they are causing me rapid expansion of space and decay then into normal matter. So that so now the Big Bang is that moment when the in photons are expanding and then decay into like our universe. That's how our universe is sort of created out of these in photons, and that avoids this moment of singularity. It's never like a moment when the universe is infinitely dense.

But you know, again, this is very speculative stuff. We think inflation happened this crazy expansion in the very beginning, um, and this is like a way to avoid having to put before that this dot, this singularity that breaks all of the mathematics, instead of replacing it with like some other weird kind of sub So we don't even really know what it's like or what it's about. Um, we're just really beginning to know how to ask questions about it.

And you know, that suggests a really interesting question, which is like, if there is something before the Big Bang, what was it? And was there something before that? It seems like, in one hand, super frustrating because you're just kicking the can down the road, Like, all right, so the early universe was this expansion, and before that came something which caused the expansion, and before that came something which caused that, which caused the expansion. But is there

in the end something original which caused it. We don't know. And there's two possibilities. One is that we just keep digging forever and dig further and further and further and further back and never get to anything which seems like could have caused itself. Or it could be that we get to some state where we're like, hmm, this makes sense to have to be a beginning. It's like it's sort of the only way things could have happened to me.

It's it's hard to grapple with this idea, so it's easier to think about it sort of in a parallel way, which is like, what is the smallest thing in the universe. We don't know if as we tear apart, particles will keep finding things that are smaller and smaller and smaller and smaller, or if eventually we'll get to one where, like, you know, what, this one it makes sense to be a fundamental ingredient to the universe. We can just start from here and build up. You know, maybe it's like

the smallest fundamental thing. It's at the plank length or something. We don't know if we'll ever get there, or if it will be self evident, or if there will be always people who say, like, I don't know, I want to dig deeper. In the same way, it might be that we're doomed to keep digging deeper and deeper back into the history of the universe, never finding out if there was an original cause. All right, so, I think we're probably getting close to the end of our time.

But I gotta come back to time travel before we do, because I'm wondering what what do you think You mentioned Stephen Hawkings party where the invitations were sent out after it happened. But what is your personal favorite way to hunt for a time traveler? What would you do if you wanted to find evidence of people from the future. Wow, I've I've never given that any thought about evidence for people from the future. Um. I try to think about what people would want to do, Like if I were

a time traveler, why would I come to one? Uh? You know the obvious answers of like change history. Uh, in which case, you know, I guess you can blame those time travelers for you know, the reason things have gone the way they are. Maybe there because time travelers have come back and tweaked election results or you know, or something like that. Um. So, I guess the best way to find time travelers with then to be present at critical hinge moments in history and look around for

suspicious behavior. I suppose I don't really know. That's a

great question. Yeah, I was thinking about all this in terms of of ancient aliens as well, because both you have you have people of course who obsess about the idea of of aliens having visited during ancient times and so forth, and and you also have I guess, a more recent phenomenon of people looking back at old pictures and painting and you know, playing this game of basically misinterpreting um things and paintings and photos, like looking back

in an old picture and saying, oh, well, that person, their their style of dress does not look archaic enough. They must be traveler, yeah, or just painting. She's holding something that looks like an iPhone. Obviously this is a renaissance painting of a time travel No. I think that just says a lot about us, you know, and who we are, you know, the same way that like photos of UFOs seem to be constantly grainy. As you know, imaging technology improves, it's always on the edge of the

our ability to capture it. So I think it says something about our desire to discover weird things and reveal the truth, which I'm totally sympathetic to I also want to peel back a layer of the universe and wake

up to its true nature. I remember Carl Sagan, and I forget which which book this was, but it was in one of the books where he talks a little bit about the idea of ancient aliens, and I remember him him basically outlining the sort of ancient account, the sort of myth that one might look to as as the sort of ancient astronaut account that could exist, have

such things were possible? And I wonder if anyone's ever taken a similar approach to the concept of time travel, like like basically like boiling it down, saying, okay, if there is actually evidence, say in you know, the historical record all of people having traveled back in time, you know, well, what exactly would we be looking for? What exactly would

they would they have been doing? Um and uh and and and how would and I guess it would come down to, like you'd have to imagine, like how truthful are they going to be? Are they just gonna lie about themselves being time travels? Because that's then you can basically point to any pivotal individual or any person in a pivotal period of time, right, Yeah, and would they even be humans? Right, Like we fantasize about going back

to see the dinosaurs. So if now we're putting ourselves back in the past and imagining time travelers, we might have to imagine some like post human apocalyptic, newly intelligent species of you know, who knows what, penguins or or something coming back in time to investigate humans, you know, to understand what happened before the apocalypse or whatever. Um. But I think machines, machines. Most of the most of our space exploration is uncrewed probes. Now you would have

to imagine the same would hold true for time. Yeah, that's probably true, or you know, after the machines have killed us all and they just have myths about those weird meat creatures that used to uh roam the earth or something. Um. That's fun, but it's fundamentally is limited by our imagination. It's the same problem with trying to look for aliens. We look for aliens in the way we expect to see them, although we're pretty sure that

if aliens exist, they're not anything that we expected. So we need to like push really hard on all the boundaries of our imagination. To make sure we're looking for aliens as broadly as possible so we don't miss them. We don't just like come by, We're like, oh, that's not aliens. So it's the same problem with imagining future time travelers, like who these these people or things or entities are are well beyond I think even our most creative science fiction UM writers, well, like you said, like

what's interesting? What would be interesting about to someone from the future, And we instantly think, too, oh, well, the you know, the coronavirus or something going on in you know, geopolitics, or even in the environment. But it could be something entirely different. It could be you know, the very beginning of something um that doesn't matter at all today, right, but but matters say in exactly which we could never possibly imagine. I mean, think about people a thousand years

ago trying to anticipate was important to us today. We couldn't even do that from twenty years ago, not to mention a thousand Okay, last question, Daniel, what's your favorite time travel movie? Oh, my favorite time travel movie has to be Primer So I think that most clearly sets out rules, rules that make sense, and then follows those really carefully with lots of fascinating and unexpected results. Good answers. Yeah,

that's a good one. I often gravitate towards the ones that have really goofy time travel rules, but they but if they still stick to those rules, then I tend to forgive them. They're always boundary cases, though they're always cased. They were like I'm not sure what would happen in this scenario or that scenario. So I like Primary because it has really clear crisp rules and those and it has cost. You can't just like pop back in time. You have to like spend time going backwards um, which

has really interesting consequences. So I found it to be really creative totally. All right, Well, thanks so much for joining us today, Daniel. This has been a lot of fun. Thank you very much. Always a pleasure to talk to you guys. All right, well, thanks once more to Daniel Watson for jumping on the old podcast machine and lead us uh Pokemon Produm with various questions about time travel

and wormholes and what have you. Yeah, if you want to learn more, so if you're not subscribed to Daniel and Jorge Explain the Universe, you can find that wherever you get your podcast, but you can also go to www dot Daniel and Jorge dot com. And you can also find the website for their new book. Again. The book is called Frequently Asked Questions about the Universe, and the website for that is www dot Universe f a

Q dot com. And if you'd like to check out other episodes of Stuff to Blow Your Mind, well you can find our show wherever you get your podcasts. Just look for the Stuff to Blow Your Mind podcast feed. We run multiple episodes per week, with core episodes dealing with science and culture on Tuesdays and Thursdays. On Monday's we do listener mail, on Wednesday's we do a short form artifact titled the Artifact, and on Friday's we do something called Weird House Cinema, which is our time to

set aside most serious matters and just discuss aus strange film. So, of course thanks again to Daniel for joining us today, and as always, a big thank you to our excellent audio producer Seth Nicholas Johnson. If you would like to get in touch with us with feedback on this episode or any other, to suggest topic for the future, or just to say hi, you can email us at contact at stuff to Blow Your Mind dot com. Stuff to

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