Speaker 1: Hey, you, welcome to Stuff to Blow your Mind. My
Speaker 1: name is Robert Lamb and I'm Joe McCormick. And, uh,
Speaker 1: this week, Rob and I are out on breaks, so
Speaker 1: we've got some great Vault episodes for you. This one
Speaker 1: originally aired December ninth, and it's our interview with Daniel
Speaker 1: Whitson from the podcast Daniel and Jorge Explain the Universe.
Speaker 1: I think we talk all kinds of time travel and
Speaker 1: stuff like that. Welcome to Stuff to Blow your Mind,
Speaker 1: production of My Heart Radio. Hey, you, welcome to Stuff
Speaker 1: to Blow your Mind. My name is Robert Lamb and
Speaker 1: I'm Joe McCormick. And for today's episode, we're going to
Speaker 1: be chatting with Daniel Whitson, who is a particle physicist
Speaker 1: and science communicator and one of the hosts of the
Speaker 1: podcast Daniel and Jorge Explain the Universe. This is Enniel's
Speaker 1: third time hopping on the show with us. The previous
Speaker 1: episodes were in September of twenty nineteen and April of
Speaker 1: And for this episode, we're gonna be talking about a book.
Speaker 1: Daniel and his co host and co author Jorge him
Speaker 1: have a new book called Frequently Asked Questions About the universe.
Speaker 1: So it was a real pleasure to have Daniel on
Speaker 1: the show for the hat trick, and I guess without
Speaker 1: any further delay, we will go right into the interview. Daniel,
Speaker 1: welcome back to the show. We're so glad you're here.
Speaker 1: Thanks very much for having me back. I always fun
Speaker 1: to talk to you guys about things that blew my mind. Awesome. So,
Speaker 1: um the podcast Daniel and Jorge explain the Universe still
Speaker 1: going strong? Um, how how far are you into explaining
Speaker 1: the universe in its entirely. We have explained zero point
Speaker 1: zero zero zero zero zer zero zero zero one percent
Speaker 1: of the universe so far. Nice. I uh, actually I
Speaker 1: was looking at your recent episodes and I saw did
Speaker 1: you recently do one that was an interview with Shawn
Speaker 1: Carroll about the uh, the many world's interpretation of quantum mechanics.
Speaker 1: I know, I know he favors that, right. Yeah. We
Speaker 1: actually have a series where I interview an expert on
Speaker 1: each of the interpretations of quantum mechanics. We did one
Speaker 1: on Copenhagen interpretation with Anna Becker, we did one on
Speaker 1: the relational interpretation of quantum mechanics with Carlo Rovelli, and
Speaker 1: then we talked to Sean about many world's interpretation, and
Speaker 1: just a couple of weeks ago we did one about
Speaker 1: the pilot wave theory of quantum mechanics, which totally blew
Speaker 1: my mind. Really much overlooked and unnecessarily maligned interpretation of
Speaker 1: quantum mechanics, in my opinion, malign, like people are being
Speaker 1: mean to it. Well, there's this famous proof by John
Speaker 1: von Neumann like seventy years ago demonstrating that it was
Speaker 1: essentially impossible, and because von Neuman is such a giant
Speaker 1: of the field, everybody thought, well, that's that. Turns out
Speaker 1: he was wrong, though, and it took people years to
Speaker 1: figure it out. It was Belle actually who figured out
Speaker 1: that Nouman was wrong, and that it's possible to have
Speaker 1: a theory of quantum mechanics with hidden variables that's deterministic,
Speaker 1: that's not random at all um. But still to this day,
Speaker 1: nobody really takes pilot wave theory seriously, to Bell's great restoration,
Speaker 1: and I think it's because Neuman sort of through shade
Speaker 1: on it decades ago and it never really recovered. I
Speaker 1: guess that's always dangerous when there's like a famously smart
Speaker 1: person who has an opinion, absolutely, And I find that
Speaker 1: physics Nobel Prize winners are especially guilty of this imagining
Speaker 1: that they are experts in every corner of everything and
Speaker 1: opining on economics or you know, social politics or whatever.
Speaker 1: If you have Nobel Prize winner in front of your name,
Speaker 1: you're an expert. Uh So, today we wanted to talk
Speaker 1: about a couple of chapters that are in a book
Speaker 1: of yours. Did that come out earlier this year? Tell
Speaker 1: us a bit about the book. Yeah. So the book
Speaker 1: is called Frequently Asked Questions about the Universe that I
Speaker 1: wrote together with my co host on the podcast and
Speaker 1: longtime collibrator Orge h. Cham who's also famous for being
Speaker 1: the genius behind PhD Comics. And the book comes from
Speaker 1: noticing that people who write into our podcast often ask
Speaker 1: similar types of questions. There are a few things that
Speaker 1: seems like everybody just wants to know about or understand,
Speaker 1: or the things that people grapple with. You know, I'm
Speaker 1: a professional particle physicist in my day job, um, and
Speaker 1: so I like asking questions about, you know, the deep
Speaker 1: nature of the universe and how our space and time
Speaker 1: really related. But you don't have to be a professor
Speaker 1: of physics to find these things interesting. And we feel like,
Speaker 1: in a sense, you know, curiosity is democratic. Everybody wonders
Speaker 1: about these things, so we wanted to try to attack
Speaker 1: some of these really big questions that everybody wonders about
Speaker 1: in an approachable way, in the way that doesn't require
Speaker 1: you to really have any knowledge of modern physics at all. Yeah.
Speaker 1: I've really been enjoying the chapter as I was reading.
Speaker 1: One thing I like that you do in this book, um,
Speaker 1: is that you know, it's not like a continuous narrative
Speaker 1: that has to you have to have read everything that
Speaker 1: came before in order to understand. Like, the chapters can
Speaker 1: be consumed pretty much on their own, right. Yeah, we figured,
Speaker 1: you know, each chapter should be like one long bathroom break.
Speaker 1: So I mean, I'm not telling you where to read it,
Speaker 1: but looking you're reading while you're busy sitting down doing
Speaker 1: something else. Each chapter, you know, should entertain you while
Speaker 1: you're doing your business, just you know, don't get so
Speaker 1: distracted that you forget to flush right now. Obviously this
Speaker 1: edition would lack the wonderful illustrations that are in the
Speaker 1: print and the Kindle version, but um, but you guys
Speaker 1: put together an audio version as well. Right, yes we did.
Speaker 1: We got to record the audio version of the book,
Speaker 1: which is out now also, and the chapters are read
Speaker 1: by me and by Jorge alternating, which is a lot
Speaker 1: of fun just sort of hear your words come to life.
Speaker 1: But yes, the audio book does miss some other the
Speaker 1: real genius of Jorge's drawings, Um. Jorge and I started
Speaker 1: working together on science communication and more than ten years
Speaker 1: ago when I reached out to him because I thought
Speaker 1: that cartoons would be a really great medium for communicating science,
Speaker 1: because they don't take themselves seriously, you know, They're a cartoon.
Speaker 1: Is different from like a figure into science paper, you know,
Speaker 1: which is very official and formal. A cartoon like makes
Speaker 1: fun of itself and is easy to you know, hang
Speaker 1: out with and accessible. And Jorge was great at that
Speaker 1: kind of stuff. So he and I started working together
Speaker 1: on explaining science using cartoons a long time ago. Um,
Speaker 1: And one thing I really value about his cartoons is
Speaker 1: not just that they are good visual explainers. He has
Speaker 1: a real visual skill for explaining something simply on the page,
Speaker 1: but also that there's sort of a second voice there
Speaker 1: you can hear, like in the text, is the voice
Speaker 1: of me as a physicist, and then in the cartoons
Speaker 1: you can hear sort of his response to some of
Speaker 1: the crazy ideas um and that sort of mirrors the
Speaker 1: way the podcast works. On the podcast, I'm talking about
Speaker 1: physics and Jorges you know something like that doesn't make
Speaker 1: any sense, or how could that possibly be? Or what
Speaker 1: you got to explain that again, So it sort of
Speaker 1: tries to capture those two voices. Yeah, I really liked
Speaker 1: that the illustrations almost seem kind of riffing on the
Speaker 1: written contents of the book. Well, so the parts of
Speaker 1: the book that we wanted to focus on today, I
Speaker 1: think we're mostly centered around the idea of time and
Speaker 1: so maybe maybe a good place to start is you
Speaker 1: have a chapter in the book where you talk about
Speaker 1: time travel and you make some arguments about which types
Speaker 1: of time travel are plausible from a physics perspective and
Speaker 1: which are not. So maybe that would be a good
Speaker 1: place to start. Give us the way of the land,
Speaker 1: like what types of time travel are the least consistent
Speaker 1: with the known laws of physics and which are the
Speaker 1: most consistent. Yeah. Sure, so for those of your listeners
Speaker 1: who are busy building their time travel devices, of the
Speaker 1: usable advice well, you know, the kind of time travel
Speaker 1: that's most inconsistent with the law of physics is the
Speaker 1: kind that most people want to do, you know. It
Speaker 1: is I want to go back in time and change
Speaker 1: age something. I want to not spill my coffee on
Speaker 1: my lap, or I wanted to go, you know, not
Speaker 1: make a mistake, or I want to go ask that
Speaker 1: person out in high school, which I was too timid
Speaker 1: to do, and now I realized I should have that
Speaker 1: kind of thing. It's not just that it's ruled out
Speaker 1: by the laws of physics. In my view, it's not
Speaker 1: even sort of internally self consistent. What it means um,
Speaker 1: you know, and and a lot of people think about
Speaker 1: time travel is like I want to go back in time,
Speaker 1: as if time was a place, like if it's a
Speaker 1: somewhere you can go. It's just sort of like along
Speaker 1: a different direction or something. And it's tempting to think
Speaker 1: about it that way because we we hear a lot
Speaker 1: about modern physics telling us that space and time are
Speaker 1: related and time is like a fourth dimension of space,
Speaker 1: and so it makes you want to think about time
Speaker 1: as a direction in which you can move and maybe
Speaker 1: you could just rewind it somehow, right, But the problem
Speaker 1: is that time. You know, first of all, we don't
Speaker 1: understand time like at all. You can dig into that
Speaker 1: in a minute if you like. Um, but the problem
Speaker 1: is that time sort of reflects how the universe changes,
Speaker 1: and so you know, I think about time is like
Speaker 1: you have a timeline. That timeline is the universe changing,
Speaker 1: Like you have the universe at one moment, and you
Speaker 1: have the universe at another moment. The next moment comes
Speaker 1: later in time, and things can't change without time. Time
Speaker 1: is that change. So the self consistency problem is that
Speaker 1: going back in time to change it changes the timeline itself.
Speaker 1: So like, how does the timeline change? If the timeline
Speaker 1: is the change, how does the timeline itself change? It
Speaker 1: would need like its own time, Like the timeline is
Speaker 1: now moving through time because it was a time before
Speaker 1: you changed in a time after you changed it, So
Speaker 1: it needs like a second dimension of time. I mean,
Speaker 1: it just sort of all becomes very complicated and falls
Speaker 1: apart as soon as you start thinking about it carefully.
Speaker 1: So going back and changing something in the past really
Speaker 1: just makes no sense from a physics point of view. Yeah,
Speaker 1: I love this because I have long kind of been
Speaker 1: skeptical about the idea of time travel into the past.
Speaker 1: And one of the reasons I had doubts about this
Speaker 1: is that wouldn't we expect to have already encountered lots
Speaker 1: of time travelers at some point in history, And there's
Speaker 1: no unambiguous evidence of that. I mean, obviously some people,
Speaker 1: you know, they're weird little things people think or time travel,
Speaker 1: but nothing that looks really clear. So it kind of
Speaker 1: makes me think that if if time travel into the
Speaker 1: past ever happens in the future, it will be of
Speaker 1: a very limited nature. Yeah, I love that as an
Speaker 1: experimental proof, you know, like, if time travel exists any
Speaker 1: time in the future, then you would expect to see it. Now.
Speaker 1: I love that. It's just such a powerful argument. It
Speaker 1: sort of reminds me of Stephen Hawking's famous invitation to
Speaker 1: time travelers, where he threw a party and then he
Speaker 1: posted the invitation later after the party. The idea of
Speaker 1: being that time travelers, you know, they should be able
Speaker 1: to get there anyway, but of course nobody should have
Speaker 1: to his party, well that we know of. He might
Speaker 1: have dispensed with them, or maybe he is a time traveler.
Speaker 1: Oh that's a good premise. For a sci fi movie
Speaker 1: like the Time Traveler Hunters trying to eliminate all evidence
Speaker 1: of the time travelers. Well that that kind of plays
Speaker 1: into um you know, some of what you're talking about
Speaker 1: about it being if it if it does exist in
Speaker 1: the future, then it must be limited in scope. And
Speaker 1: I guess you could look at it a couple of
Speaker 1: different We could basically just sci fi the hell out
Speaker 1: of it in multiple directions. But you know, you could say, like, well,
Speaker 1: maybe travel into the past. It has a range and
Speaker 1: we haven't reached the point to where time machines of
Speaker 1: the future can reach us, or it's just so tightly
Speaker 1: policed that nobody can make it back. You know, we
Speaker 1: have time cops or or something that are that are
Speaker 1: keeping people from making too much of a show of
Speaker 1: the whole thing. There's so many of those science fiction
Speaker 1: depictions of like a b time bureaucracy, you know that's
Speaker 1: managing the time flow, like you saw that in Loki
Speaker 1: and in Umbrella Academy, and and and in um that
Speaker 1: book recently, this is how you win the time war.
Speaker 1: And those can be a lot of fun, but also
Speaker 1: I feel like they're they just make no sense at all.
Speaker 1: You know, how do you have this weird administration that's
Speaker 1: separated from time and also weirdly frozen in like nineties bureaucracy.
Speaker 1: It's you know, it's fun, but not not if you
Speaker 1: think about it really at all. What also reminds me
Speaker 1: a lot of something I was actually chatting with you
Speaker 1: about a couple of weeks ago when I interviewed you
Speaker 1: for a short freelance piece for how stuff works dot
Speaker 1: com about UM about the zoo hypothesis. You you spoke
Speaker 1: about about that for the interview, and uh, you mentioned
Speaker 1: that one of the strong arguments against it is that
Speaker 1: if there is actually this, um, this conspiracy of of
Speaker 1: aliens to avoid contact with humans and and and keep
Speaker 1: us in the dark about the uh you know, the
Speaker 1: Galack tick, civilizations just outside of our view. UM, the
Speaker 1: main argument against it is that that governments as we
Speaker 1: know them, by the only model that we know the
Speaker 1: the the human model, are not really good at keeping secrets.
Speaker 1: They're not good at managing secrets. And it seems like
Speaker 1: you could also apply that to the idea of intelligent
Speaker 1: beings or humans and the future managing the timeline and
Speaker 1: so forth. Exactly, you know, some version of Elon Musk
Speaker 1: in the future is get to get his hands on it,
Speaker 1: and then he's going to launch a bunch of crazy,
Speaker 1: you know missions, and somebody's gonna mess something up. So
Speaker 1: it's hard to imagine that people in the future having
Speaker 1: time travel and somehow keeping it a secret or slipping
Speaker 1: into the past unnoticed and nobody ever, you know, breaking
Speaker 1: the protocol or something. It's it just becomes totally implausible
Speaker 1: the more you think about it. Picking up off that,
Speaker 1: I mean, this is another one of the weird things
Speaker 1: about time is it seems like time is actually one
Speaker 1: of the arguments against the idea of a coherent galactic civilization,
Speaker 1: if this makes any sense, because like you think, a
Speaker 1: civilization in order to organize itself has to have some
Speaker 1: pretty close to synchronous, uh, you know, thing going on,
Speaker 1: Like things have to be happening pretty close to around
Speaker 1: the same time for them. But does it even make
Speaker 1: sense for I don't know, one planet in a galactic
Speaker 1: civilization to be part of a civilization with one on
Speaker 1: the other side of the galaxy? I mean, is there
Speaker 1: you know, can they say, uh, is there such a
Speaker 1: thing as what's happening right now on a planet on
Speaker 1: the other side of the galaxy. Yeah, you make a
Speaker 1: great point, because there's a speed limit to information moving
Speaker 1: through the universe, which puts an effective limit on like
Speaker 1: how well you can coordinate and organize things. Makes you
Speaker 1: think about this in cosmology all the time, because there's
Speaker 1: a like the largest thing that can exist in the universe.
Speaker 1: Just from very simple arguments like the aged the universe
Speaker 1: and the speed of light. You can't have an object
Speaker 1: that's like ten thousand billion light years wide that's like coordinated,
Speaker 1: it has like a structure that's like gravitationally bound on
Speaker 1: itself because there hasn't been time for like a photon
Speaker 1: to even cross over the entire size of that object.
Speaker 1: So there's like a limit to how big the universe
Speaker 1: can even build like a thing, not to mention like
Speaker 1: the close coordination required like organize a galactic empire. And
Speaker 1: so yeah, absolutely, Um, I think that the sheer size
Speaker 1: of space definitely limits our ability to explore it unless
Speaker 1: it breaks down into you know, lots of different unorganized entities,
Speaker 1: like maybe we send humans in an arc off to
Speaker 1: another star and they start their own human colony and
Speaker 1: we're not in touch and we're not part of some
Speaker 1: you know, political nation state. But at least we are
Speaker 1: humans here and there are humans there. Yeah. I think
Speaker 1: that's a great way to conceptualize it. So, I guess
Speaker 1: coming back to time travel for a minute, I wanted
Speaker 1: to talk about some of the specifics you offer about
Speaker 1: physically plausible ways of traveling into the past. Uh. So
Speaker 1: you mentioned a couple of things. You mentioned the idea
Speaker 1: of wormholes, and then you also mentioned one that might
Speaker 1: be less familiar to people, the idea of an infinitely
Speaker 1: long cylinder of spinning dust, which could potentially, at least
Speaker 1: maybe depending on something about whether something about relativity is
Speaker 1: true or not, could potentially allow time travel into the
Speaker 1: past through something called time loops. Could could you explain
Speaker 1: how this would work? Like, what would this experience be
Speaker 1: like for the time traveler? Yeah, Well, the short answer is,
Speaker 1: we just don't know. Uh. This is a realm where
Speaker 1: we are like on the cutting edge. Theoretically, people are
Speaker 1: looking at the rules of how space and time bend
Speaker 1: and twist because you know, the general relativity our theory
Speaker 1: for space and time itself essentially tells us that space
Speaker 1: and time bend in response to mass, and then tell
Speaker 1: mass is how to move. So, for example, you have
Speaker 1: an empty universe and you put a star in it,
Speaker 1: it bends the space around the star, and then the
Speaker 1: bending of that space tells things how to move, and
Speaker 1: not just through space but also through time. So you
Speaker 1: go near a black hole, for example, time is slowed down.
Speaker 1: So there's definitely some deep connection there between space and time.
Speaker 1: And what people have done is trying to explore extreme
Speaker 1: scenarios of that what happens if you do this, what
Speaker 1: happens if you do that? Is this allowed? Is that allowed?
Speaker 1: And so it's sort of like exploring the universe, but
Speaker 1: just inside our own heads. We can't necessarily yet go
Speaker 1: out there and build these things in space and say,
Speaker 1: let's see what happens experimentally, But we can do similar
Speaker 1: like fought experiments, where we say, what would happen if
Speaker 1: you did this, and let's just let's assume the equations
Speaker 1: are correct and see what happens. And so there's a
Speaker 1: couple of fund scenarios there. One, as you said, is wormholes.
Speaker 1: These aren't really crazy because they are like connections between
Speaker 1: different points in space, and when you think of space,
Speaker 1: you probably think of like just sheer emptiness, you know,
Speaker 1: the back drop the stage on which the universe happens.
Speaker 1: But now we know that space is more complex. It
Speaker 1: can bend and it can twist, and that might be
Speaker 1: something that you can put in your head. You can
Speaker 1: imagine like space bending around the Sun. But because space
Speaker 1: is like a thing with an arrangement, it could also
Speaker 1: do other really weird things, like be connected non trivially.
Speaker 1: So you have like a chunk of space over here,
Speaker 1: it can be directly connected to a chunk of space
Speaker 1: over there. What does that mean. Well, you're used to
Speaker 1: the space around you being connected to the space right
Speaker 1: next to it. That's what it means to be right
Speaker 1: next to it. Right, you take a step to the left,
Speaker 1: you move to the next sort of piece of space.
Speaker 1: Think of it sort of like pixels on a screen. Right, Well,
Speaker 1: a wormhole is a connection between two points in space
Speaker 1: that are otherwise really distant. And so you take a
Speaker 1: step from a from one pixel and now you're in
Speaker 1: a pixel on the other side of the screen. And
Speaker 1: so that seems weird and impossible, but remember space can
Speaker 1: have all sorts of strange connections, and according to the
Speaker 1: equations of general relativity, the ones that define how space
Speaker 1: is organized that is allowed, it is possible, and so
Speaker 1: a couple of folks at cal Tech we're thinking about, well,
Speaker 1: you know what about time? Is it possible for one
Speaker 1: end of the wormhole to be in one place and
Speaker 1: the other end to be in another time because, as
Speaker 1: you were mentioning earlier, like the notion of simultaneity, like
Speaker 1: when is now depends really on where you are. Also,
Speaker 1: so they have this idea to take one end of
Speaker 1: the wormhole and you accelerate it near the speed of light.
Speaker 1: That effectively it can be sort of back in time.
Speaker 1: And this all works theoretically, but it also sort of
Speaker 1: contradicts other things we know, like if you go through
Speaker 1: this wormhole and you come out in the past, you know,
Speaker 1: doesn't that break things like causality? And you come out
Speaker 1: in the past and kill yourself before you um do
Speaker 1: the experiment, then you don't do the experiment, you don't
Speaker 1: come out in the past. So it appears to create paradoxes,
Speaker 1: and nobody knows like how to resolve that. Does that
Speaker 1: mean that these things are impossible? Does that mean if
Speaker 1: you did that the universe would like disappear in a
Speaker 1: puff of logic? Nobody really knows what would happen, so
Speaker 1: that it's a bit of a contradiction in the theory
Speaker 1: itself that it predicts something which seems to be disallowed
Speaker 1: by other parts of the theory. And it's a similar
Speaker 1: idea for these closed timelike curves. People said, if you
Speaker 1: create these infinitely long cylinders of spinning dust, which doesn't
Speaker 1: sound easy to do, then it bends time in this
Speaker 1: way that the time then as you move forward in time,
Speaker 1: you're actually moving sort of like sideways through space time
Speaker 1: in a way that's similar to the experience of going
Speaker 1: into a black hole. Outside a black hole, time always
Speaker 1: moves forwards. Inside a black hole, space has bent so
Speaker 1: much that space only moves towards the center of the
Speaker 1: black hole. It's like one direction to space. So if
Speaker 1: you imagine space being distorted, not quite as much as
Speaker 1: a black hole, but sort of in a similar direction
Speaker 1: that it sort of bends space sideways, then you can
Speaker 1: create these paths where something can move in a loop
Speaker 1: through time. Um, but you would be trapped on that loop,
Speaker 1: so you wouldn't be able to like change anything. It's
Speaker 1: like a fixed loop, sort of like Harry Potter style
Speaker 1: loop through time where every time you go through, it's
Speaker 1: exactly the same thing happening. And these are really fun
Speaker 1: because nobody knows like if these are actually possible, and
Speaker 1: what would happen if you actually went through them. So
Speaker 1: so we don't really know, for instance, like what conceivable
Speaker 1: reason there would be for a civilization to conceivably construct
Speaker 1: one of these, Yeah, because we don't know practically what
Speaker 1: you could achieve. And also, an infinite cylinder spitting dust
Speaker 1: sounds like an expensive project, you know, the word infinite
Speaker 1: seems to raise some doubts. And when it comes to wormholes,
Speaker 1: people know how to calculate whether a wormhole is allowed
Speaker 1: by the theory of general activity. Nobody knows how to
Speaker 1: build a wormhole. You know. It's sort of like saying, Okay,
Speaker 1: it's possible to have an apple pie, but but we
Speaker 1: don't have a recipe for making one. Right. It's a
Speaker 1: different thing to say, like I know how to put
Speaker 1: it together than to say it's technically allowed to exist
Speaker 1: in the universe. You know, It's like if you say, well,
Speaker 1: the sun is allowed by the laws of physics, but
Speaker 1: I don't know how to make it happen. If I
Speaker 1: just start from a cloud of gas, for example, and
Speaker 1: so that's a big puzzle. Nobody really knows how to
Speaker 1: build a wormhole or even keep one open. Um if
Speaker 1: you did manage to build one, Are there any reasons
Speaker 1: to suspect that wormholes exist naturally? Oh? Great question? Not
Speaker 1: yet know. Um. Some people wonder if there are wormholes
Speaker 1: that connect the super massive black holes at the hearts
Speaker 1: of all of our galaxies, but there's not like any
Speaker 1: evidence out there anything that can't be explained without wormholes
Speaker 1: that you would need wormholes to explain. Um, that would
Speaker 1: be super cool, though, Um, I'm not aware of any
Speaker 1: evidence like that. Well, Daniel, you also have a chapter
Speaker 1: in the book that I really liked on the question
Speaker 1: of will time ever stop? And I think this is
Speaker 1: one of those great questions because it's a yes or
Speaker 1: no question, and like many big questions in physical cosmology,
Speaker 1: it's a binary. But no matter which answer it is,
Speaker 1: it's mind boggling, like it is impossible to imagine time
Speaker 1: either stopping or going on forever. Uh So, so, what
Speaker 1: are your thoughts here about whether time will ever stop?
Speaker 1: I go, you're feeling there. And I also think it's
Speaker 1: really fascinating to go back through history and read about
Speaker 1: which concept felt more natural to people. Initially, it felt
Speaker 1: to people like time should go on forever. Obviously, that
Speaker 1: was like a hundred fifty years ago, before we knew
Speaker 1: that the universe was expanding. People looked out in the
Speaker 1: stars and they looked like they were just sort of
Speaker 1: hanging out, and they thought, maybe the universe is just
Speaker 1: sort of there, and so obviously it's been there forever, right,
Speaker 1: And that was like this, you know, de facto assumption
Speaker 1: in science until Hubble discovered that the universe is expanding,
Speaker 1: and that gave the universe sort of like a direction.
Speaker 1: It's like things are changing, and as you look back
Speaker 1: in time, that suggests, you know, something a moment when
Speaker 1: the universe was like crazy infinitely dense. So it suggested
Speaker 1: a beginning. And that must have been an incredible sort
Speaker 1: of mind bending mental gymnastics to execute to go from thinking, oh,
Speaker 1: it makes sense for the universe to be infinite in
Speaker 1: time to going to like, oh, the universe had a beginning,
Speaker 1: and now let's trying to figure out what that beginning was.
Speaker 1: Um So, I think that's really interesting, And you know,
Speaker 1: I think the thing that's really cool about this question
Speaker 1: is not just that it's tangible because it makes you wonder,
Speaker 1: like am I going to go on forever? As the
Speaker 1: universe always going to be here? But because it really
Speaker 1: gets at the heart of the deepest problem in physics
Speaker 1: right now, the like the fundamental conflict we have between
Speaker 1: two idea he is in physics, which are quantum mechanics
Speaker 1: and general relativity. You know, we have a quantum mechanical
Speaker 1: description of how like particles bounce off each other, and
Speaker 1: we you know, have a lot of questions about how
Speaker 1: that works, but we have a pretty good theory for
Speaker 1: you know, understanding quantum particles, and we've been talking about
Speaker 1: general relativity, you know, how space bends and how it
Speaker 1: affects time and what happens in black holes and all
Speaker 1: that stuff also very successful. The problem is that these
Speaker 1: two theories nobody knows how to bring them together, and critically,
Speaker 1: they have very different stories to tell about what time is.
Speaker 1: They treat time totally differently with huge consequences for the
Speaker 1: answer to this question will time ever stop? And so
Speaker 1: to me, this is a fun question because it puts
Speaker 1: its finger on right on that conflict. Yes, so there
Speaker 1: is there's a concept that you introduce in this chapter
Speaker 1: about uh sort of time as we experience it, being
Speaker 1: a sort of special case or special circumstance of a
Speaker 1: hypethetical substance you refer to as meta time. Can you
Speaker 1: explain something, well, like what are you getting at here? Well,
Speaker 1: one of the basic questions is is time fundamental or
Speaker 1: is it emergent? You know, a deep question in modern
Speaker 1: physics is like, what are the essential ingredients to the universe?
Speaker 1: What did it start with? And then what sort of
Speaker 1: arises out of that out of the complexity of the
Speaker 1: possible interactions. You know, for example, if you're playing with legos,
Speaker 1: the fundamental ingredients are the basic pieces, and from that
Speaker 1: you can make complicated things dinosaurs or pirates or spaceships
Speaker 1: or whatever. But those spaceships they're emergent, you know, They're
Speaker 1: not necessary. They don't have to exist. You can take
Speaker 1: it apart and just have the legos. In the same way,
Speaker 1: in our universe there are complicated things like ice cream
Speaker 1: and hurricanes, but those don't have to exist in the universe,
Speaker 1: right You can imagine a universe without hurricanes or without
Speaker 1: ice cream, as sad as that is. So, then the
Speaker 1: question is what are the basic elements of the universe
Speaker 1: and for a long time, you know, people like Newton
Speaker 1: thought that, well, obviously space and time are fundamentals of
Speaker 1: the universe. They're just like, you gotta have that, right,
Speaker 1: And now people are wondering, like, well, is that really true?
Speaker 1: Is it possible to have a universe without space or
Speaker 1: without time? You know, we gotta when you're really digging
Speaker 1: deep into the nature of the universe, you gotta push
Speaker 1: hard on the fundamental assumptions. So there are a lot
Speaker 1: of ideas now about how space could be emergent, you know,
Speaker 1: how it could be that the universe itself, that space
Speaker 1: is not a natural thing, that like ice cream, you
Speaker 1: could have a time in the universe where there wasn't
Speaker 1: any space. The space is like just briefly, the stitching
Speaker 1: together of these um separated pixels of space using quantum
Speaker 1: entanglement to sort of weave together this idea of space,
Speaker 1: these relations between different locations that we experience, and we
Speaker 1: could talk about that for an hour um. But even
Speaker 1: moving beyond that, now folks are wondering, like, is time
Speaker 1: also emergent? Is it possible that time is not a
Speaker 1: fundamental property of the universe, but it just sort of
Speaker 1: something that exists now and it's really hard to even
Speaker 1: think or talk about it because, like I just said,
Speaker 1: it exists. Now I'm using time to talk about when
Speaker 1: time is. It's very complicated and confusing. But there are
Speaker 1: some theories that tell us that time might be not
Speaker 1: an illusion, right, not in the sense that it doesn't exist,
Speaker 1: but it might not be fundamental, that it might arise
Speaker 1: from complex interactions of smaller, more fundamental elements of the universe.
Speaker 1: And so that's this idea of meta time. You have
Speaker 1: to imagine some like deeper laws of physics that control
Speaker 1: those fundamental bits that I'm being vague about because we
Speaker 1: have no idea what they would be or what they are,
Speaker 1: what the rules are. And And if this seems sort
Speaker 1: of like frustrating, it's because we're at the very beginning
Speaker 1: of even talking about the answers, because we're just formulating
Speaker 1: the questions. You know, sometimes it takes like a hundred
Speaker 1: years to figure out, Okay, the question to ask is
Speaker 1: is there always time in the universe? What does that mean?
Speaker 1: And how do you even think about a universe without time?
Speaker 1: Then you can start to make progress on the crazy
Speaker 1: ideas that might explain it. Uh, this may be kind
Speaker 1: of a tangent. But this actually makes me wonder about
Speaker 1: a question that's come up on the show before. Do
Speaker 1: you have a view on what the present is, on
Speaker 1: whether something special is actually happening in the present. Uh, Like,
Speaker 1: does only the present exist? Or does all of time exist?
Speaker 1: It's a really great question. We don't understand that at all.
Speaker 1: I don't understand it. I don't even know if it's
Speaker 1: a question of science or if it's a question of philosophy,
Speaker 1: because it goes into the nature of consciousness. You know,
Speaker 1: does the whole timeline exist and we only experience part
Speaker 1: of it? Or you know, does only this moment exist? Um.
Speaker 1: Physics doesn't have a great way to even define what
Speaker 1: the present is um, and so it's it's pretty hard
Speaker 1: to put your finger on it um. And I love
Speaker 1: because these are questions that like, we don't even really
Speaker 1: know how to attack these questions. And what that suggests
Speaker 1: is that there's something wrong and the way we're organizing
Speaker 1: our thinking. You know, it's like if you're asking a
Speaker 1: question and you're just using the wrong language, we're using
Speaker 1: the wrong notation, then your question seems really complicated and confusing.
Speaker 1: And if you learn a new perspective and then suddenly
Speaker 1: would make sense. You know, I'm reminded of that Far
Speaker 1: Side cartoon where the scientists are trying to understand dolphins
Speaker 1: and they're writing down phonetically what the dolphins are saying,
Speaker 1: and they're saying things like you know, obla Espanol and
Speaker 1: the you know, the scientists don't speak Spanish, so tho
Speaker 1: to them, it's just nonsense. My bodies if they if
Speaker 1: they knew the language, it would all click together. And
Speaker 1: I feel like that's the problem we have sometimes that
Speaker 1: was just not speaking the right language at the universe yet,
Speaker 1: and that's why some of these questions are awkward and
Speaker 1: really hard to grapple with. I thought you were gonna
Speaker 1: say today's physicists are only equipped with cow tools only
Speaker 1: for spherical cows. You know, all of this um also,
Speaker 1: it reminds me a bit of the Copernican principle to UM.
Speaker 1: But but going beyond just the idea of like, you know,
Speaker 1: there being some uh I we should not not see
Speaker 1: that there's something privileged about about our planet or about humans.
Speaker 1: But but could you could you even apply that based
Speaker 1: on what you're saying to to the present moment, to
Speaker 1: this time that in which from which we are viewing
Speaker 1: the universe. Yeah. Probably, And I think that's why a
Speaker 1: lot of progress could be made if we ever did
Speaker 1: get to talk to alien scientists, because I think we
Speaker 1: would learn a lot about, um, you know, the biases
Speaker 1: that creep into our questions and our reference frame for
Speaker 1: answering those questions because of our human experience, and alien
Speaker 1: intelligence that might have a very different relationship with the
Speaker 1: concept of time, might have a very different treatment of
Speaker 1: it mathematically and physically, and might make a lot more sense.
Speaker 1: You know. The problem with alien intelligence, of course, is
Speaker 1: you know, finding them, talking to them, decoding their language,
Speaker 1: and then if they are so fundamentally different that they've
Speaker 1: made that they avoid human biases, they might be impossible
Speaker 1: to understand. And so while it's tantalizing to imagine that
Speaker 1: like aliens are out there with the answers the deep
Speaker 1: questions about the universe, it might also be that, uh
Speaker 1: that we could never understand what they have to say.
Speaker 1: I have long thought we should outsource all of our
Speaker 1: physics research to like a seventeen dimensional octopus. Um, if
Speaker 1: you know one, I'd like to meet it, because I
Speaker 1: got questions than uh so, but to come back to
Speaker 1: the idea of will time ever stop? You talk about
Speaker 1: a couple of possibilities for what that would look like.
Speaker 1: Say that you know the far future of our own universe,
Speaker 1: at least at least what we can reason from what
Speaker 1: we know today, and and a couple of these options
Speaker 1: are are the Big Crunch and the heat death of
Speaker 1: the universe. Do you do you want to talk about
Speaker 1: what those would means as best we can guess for
Speaker 1: time itself. Yes, So remember that there are two paths
Speaker 1: to go down if you're asking questions about the deep
Speaker 1: future of the universe, and one is quantum mechanical and
Speaker 1: the other one is general relativity. And quantum mechanics is
Speaker 1: pretty straightforward about this. It says that, look, time always
Speaker 1: existed and time will always exist. And there's a pretty
Speaker 1: simple argument there because according to quantum mechanics, quantum information
Speaker 1: can't be destroyed, like when something happens um, you know,
Speaker 1: the information about what used to happen is encoded into
Speaker 1: the future, and so it suggests that time has always existed.
Speaker 1: There's no mechanism in quantum mechanics for time to start.
Speaker 1: It should always have existed, and you flip it around
Speaker 1: the other direction, it should always exist, so they should
Speaker 1: always be a universe, and clocks should always tick forwards
Speaker 1: according to quantum mechanics. But that assumes you know that
Speaker 1: space is flat, and bole and general relativity, the other
Speaker 1: pillar of modern physics, tells us that space is not simple.
Speaker 1: It's not flat, it's complicated. It's expanding, and you know,
Speaker 1: the mechanism by which space is expanding is not something
Speaker 1: that we understand. Hub will discover a hundreds something years
Speaker 1: ago that the universe is expanding and things are moving
Speaker 1: away from us, And then twenty something years ago we
Speaker 1: discovered even more mind boggling lee that that expansion is accelerating. Right,
Speaker 1: It's not like stuff is moving through space and gradually
Speaker 1: slowing down and maybe eventually gonna stop and turn around
Speaker 1: and come back um and collapse, but that it's speeding up,
Speaker 1: which means that there's some massive, incredibly powerful force in
Speaker 1: the universe that's literally tearing it apart. Because we don't
Speaker 1: know the mechanism for it, though, we can't predict what
Speaker 1: it's gonna do, Like it turned on about five billion
Speaker 1: years ago, started tearing the universe apart. Will it do
Speaker 1: that forever? If so, you end up with like a
Speaker 1: universe with everything is super far apart. It's just like
Speaker 1: a bunch of black hole holes from collapsed galaxies, separated
Speaker 1: by you know, unthinkably vast distances. Even compared to the
Speaker 1: distances we see between our galaxy and other galaxies today,
Speaker 1: you know, these galaxies would be so far apart that
Speaker 1: they could never even see each other. You know, light
Speaker 1: would never reach one from the other. On the other hand,
Speaker 1: dark energy could change its direction, it could stop, it
Speaker 1: could turn around, it could cause the universe to collapse
Speaker 1: back down into an incredible moment of singularity at the
Speaker 1: end of the universe um, and then we can ask
Speaker 1: questions like, well, what happens then, you know, does the
Speaker 1: universe stop when you reach another singularity, another moment of
Speaker 1: incredible density. We just don't know because general relativity describes
Speaker 1: that process. But when you actually get to the singularity,
Speaker 1: people think of singularities is like a feature of general relativity.
Speaker 1: Really they're like a failure of general relativity. It can't
Speaker 1: predict anything that happens there. Doesn't know what to do.
Speaker 1: It's like, well, that's the direction you're going, but once
Speaker 1: you get there, I can't tell you what's going to
Speaker 1: happen next. So if that happens, we just really don't
Speaker 1: know what the fate of the universe would be in
Speaker 1: that scenario, but you know, it wouldn't be pleasant for
Speaker 1: humans or for seventeen seventeen dimensional octopi. But I guess
Speaker 1: with with the other option, with like you know, the
Speaker 1: heat death of the universe, everything just expanding and cooling
Speaker 1: and reaching some kind of equilibrium where um, where there's
Speaker 1: no there's no imbalance to distribute any further. I think
Speaker 1: in the book you raised the idea that this could
Speaker 1: in a way represent a threat to to our concept
Speaker 1: of time because time would maybe in itself, time has
Speaker 1: something to do with entropy, and this would be a
Speaker 1: state of maximum entropy. Yeah, we see the universe proceeding
Speaker 1: through time and we see entropy increasing, and entropy is
Speaker 1: a really tricky topic. You hear people talk about it
Speaker 1: a lot, but it's really hard to sort of grapple
Speaker 1: with intellectually, and people try to think about it in
Speaker 1: terms of like amount of disorder in the universe, but
Speaker 1: that can be pretty misleading. Technically, it's really relates to
Speaker 1: the number of different ways you can arrange the microscopic
Speaker 1: nature of the universe to be consistent with the macroscopic
Speaker 1: nature that you observe. That's a little bit more subtle,
Speaker 1: but it's actually a more accurate guide to what entropy is.
Speaker 1: And what we notice is that entropy seems to be
Speaker 1: increasing through the universe, like there's something we've observed, and
Speaker 1: a lot of places in physics seem to be sort
Speaker 1: of like ambivalent about time. The laws will run the
Speaker 1: same forward or backwards. It doesn't really matter, if you know,
Speaker 1: without friction or air resistance. For example, you can throw
Speaker 1: a ball up in the air and it lands back
Speaker 1: in your hands. If you played a movie of that backwards,
Speaker 1: it would look exactly the same again without air resistance,
Speaker 1: because that increases entropy. Um. But entropy is the one
Speaker 1: place where in the laws of physics there seems to
Speaker 1: be a preference for things moving forwards. So it's often
Speaker 1: claimed that entropy might be the reason time moves forwards,
Speaker 1: and I think that's a bit of a step too far.
Speaker 1: You know, we see that entropy increases as time moves forward,
Speaker 1: so there's a connection between them. That doesn't mean necessarily
Speaker 1: the time has to move forward. I mean, if time
Speaker 1: moved backwards. It just means that maybe entropy would decrease,
Speaker 1: right It um creates this connection between entropy and time.
Speaker 1: It doesn't necessarily imply a direction, but some people wonder
Speaker 1: what would happen when you reach a state of maximum entropy,
Speaker 1: and maximum entropy would be as you say, everything progresses
Speaker 1: forward and the universe so of spreads out and it
Speaker 1: becomes maximumly even there's no like hot spots and cold spots,
Speaker 1: because that allows you to rearrange the microscopic state as
Speaker 1: many ways as possible, so the most freedom to rearrange
Speaker 1: the microscopic state and so the most entropy. And in
Speaker 1: that state, it's called the heat death of the universe
Speaker 1: because you have no hot spots and no cold spots,
Speaker 1: so no way for like energy to flow. Nobody do anything.
Speaker 1: The way that you operate as a human being is
Speaker 1: through energy flows, and the way that computation happens this
Speaker 1: through energy transfers, and so you can't really do anything
Speaker 1: if there's no energy ingredients. So that's why it's referred
Speaker 1: to as the heat death of the universe. And people
Speaker 1: who think of that time is deeply connected to entropy
Speaker 1: wonder if when entropy reaches its maximum point, if time
Speaker 1: then somehow stops, or maybe time stops and then turns around,
Speaker 1: and entropy starts to decrease like a bounce in time.
Speaker 1: And nobody knows the answer to these questions. Nobody's gonna
Speaker 1: be around to know the answer to these questions, even
Speaker 1: if you're optimistic about the length of human civilization. But
Speaker 1: they're really fun to think about because they make you
Speaker 1: think about what time is and you know, and how
Speaker 1: it relates to the whole universe. Well, though, on the
Speaker 1: question of nobody being around this this may also be
Speaker 1: a tangent. But this makes me wonder do you have
Speaker 1: opinions on the alleged Boltzman brain problem. I know we
Speaker 1: talked about this on an episode a few years back,
Speaker 1: and um so maybe kind of fuzzy on the details,
Speaker 1: but if I recall, it has been used to argue
Speaker 1: against some types of future eternity ees. But basically the
Speaker 1: the argument is, if the universe were to go on
Speaker 1: existing literally forever, with certain types of properties in play,
Speaker 1: eventually people whose brains randomly formed from fluctuations in space
Speaker 1: would outnumber people who exist through biological evolution on a
Speaker 1: rocky planet, and thus we would expect to be those
Speaker 1: brains instead of these biological brains. Is that roughly right? Yeah.
Speaker 1: Essentially it's arguing that if the universe reaches heat death
Speaker 1: and then goes on forever, that most of the time
Speaker 1: in the universe is in during heat death, right, that
Speaker 1: really basically randomly sampled moment in universe should be when
Speaker 1: the universe is spread out and boring and gray. So
Speaker 1: then Boltman said, well, what if you just had a
Speaker 1: quantum fluctuation while aims before quantum mechanics, but what if
Speaker 1: you had a random fluctuation? Because you know, the law
Speaker 1: of entropy statistical it's not exact, allows for fluctuations. And
Speaker 1: so he said, well, what's the chances of the whole
Speaker 1: universe then being like a fluctuation in some vast or
Speaker 1: heat dead universe that already has existed for unknown millions
Speaker 1: of years. So he was trying to fluctuate an entire
Speaker 1: universe out of basically nothing. And so as a counterpoint,
Speaker 1: people are like, well, you know, there are smaller but
Speaker 1: more ridiculous things that you could fluctuate out of the universe,
Speaker 1: like a galaxy or even just like one brain. And
Speaker 1: so at the point was made actually to criticize those
Speaker 1: kinds of cosmological models, because if your cosmological model seems
Speaker 1: less likely than you know, brains forming spontaneously in space
Speaker 1: and thinking that they're people, then it seems pretty unlikely. Um.
Speaker 1: And so I don't think anybody really takes it seriously.
Speaker 1: Is like a theory of the universe. It's sort of
Speaker 1: just more like a mental exercise to wonder, like, how
Speaker 1: likely is your theory of the universe? Um, you know,
Speaker 1: is it less likely than this absurd scenario. So there's
Speaker 1: another thing you bring up in your chapter. Will time
Speaker 1: ever stopped? Is an idea I was instantly captivated by,
Speaker 1: which is you point out that technically, um, time could
Speaker 1: be stopping and restarting all the time, frequently without us
Speaker 1: ever realizing it, because how would we know, right, Like,
Speaker 1: our consciousness, our experience of the world is through time,
Speaker 1: So if time were to stop, uh and then restart,
Speaker 1: that might just be invisible to us. So you know,
Speaker 1: maybe there are just uh, these huge gaps in our life. Though.
Speaker 1: That makes me wonder if time we're stopping, would it
Speaker 1: be possible to measure how long it stopped for? Oh,
Speaker 1: that's really interesting. You know, this sort of presupposes some
Speaker 1: sort of like meta time, some you know, other rules
Speaker 1: of the universe that's controlling our time. And because time
Speaker 1: itself controls how our universe changes, then strictly speaking, if
Speaker 1: time does pause you know, core to this meta time
Speaker 1: and then pick up again, nothing should change because that
Speaker 1: would require our time to tick forward. Like no particles
Speaker 1: can move, no galaxies, no space can be created. You know,
Speaker 1: no expansion can happen without time our time ticking forward,
Speaker 1: and that means that there's nothing in our universe that
Speaker 1: should change if time doesn't take forward, which means that
Speaker 1: there should be no way to tell. So it could
Speaker 1: be like a near infinite amount of time between every
Speaker 1: tick of our universe could be passing in sort of
Speaker 1: like the meta universe. I think the easiest way to
Speaker 1: imagine this is in the simulation hypothesis, the idea that
Speaker 1: the universe is like a computer program running on some
Speaker 1: mega computer. And you know, if the aliens or super
Speaker 1: beings running that simulation pause the simulation to go to
Speaker 1: the bathroom and come back, then we don't know that
Speaker 1: they've paused it. Right. It's just like the characters in
Speaker 1: your video game. They're not like, hey, buddy, that was
Speaker 1: a long number two. You know what you're doing everything okay?
Speaker 1: When you come, they have no idea, and to them,
Speaker 1: you know, the experience is completely smooth. So I think, No,
Speaker 1: there's no way to know how long time has been
Speaker 1: paused for if it does get paused. I always wondered
Speaker 1: with that hypothesis, would we notice if the resolution on
Speaker 1: our simulation was downgraded, you mean, if they lost their
Speaker 1: funding and had to had to go to a more
Speaker 1: course resolution decreased render distance. Yeah, I don't know. Sometimes
Speaker 1: it does feel like the resolution decreases or increases in
Speaker 1: depending on what's going on. I'm gonna blame my failing
Speaker 1: memory for that, Like, you know what the aliens have
Speaker 1: just been like cleaning up the cash, and that's why
Speaker 1: I can't remember, you know, what happened last week, or
Speaker 1: when I agreed to clean to the garage or whatever.
Speaker 1: But there are ways that we do think we might
Speaker 1: be able to probe the resolution of the simulation of
Speaker 1: the universe under the assumption that we live in that
Speaker 1: crazy scenario. Because these simulations the way we do them,
Speaker 1: at least as we tend to like divide the universe
Speaker 1: into huge cubes and stimulate each cube separately, assuming that
Speaker 1: like the interactions between cubes are pretty small, which works
Speaker 1: pretty well. You know, if you're in your if you're
Speaker 1: simulating like a single galaxy at the time, because mostly
Speaker 1: you're dominated by what's going on inside the galaxy and
Speaker 1: not stuff from other galaxies. But we have these particles,
Speaker 1: these crazy high energy particles that whizz through space at
Speaker 1: velocities nobody's ever seen before, or energies nobody's ever seen before,
Speaker 1: much much higher energy than anything like created by our
Speaker 1: particle accelerators. And they might be like tripping up that
Speaker 1: simulation because they skip through several of these simulation pixels
Speaker 1: faster than anything you should expect. And you know, there
Speaker 1: are some things about those particles we see out there
Speaker 1: in space that we don't understand, and so that opens
Speaker 1: the door to like maybe you could explain those particles
Speaker 1: as being like a glitch in the simulation. Now, speaking
Speaker 1: of simulations and going back to time travel, does anyone
Speaker 1: out there like make any out of an argument for
Speaker 1: time travel into the past by saying, well, if we
Speaker 1: are living within a simulation, then time travel into the
Speaker 1: past and the ability to change the past would be
Speaker 1: possible within the confines of that simulation. Yeah, you know,
Speaker 1: if you're living in a simulation, then the rules are
Speaker 1: essentially arbitrary, and then yeah, you could wind time backwards.
Speaker 1: I think this goes to the heart of, like, I think,
Speaker 1: a basic confusion about time travel, because people imagine, like
Speaker 1: you get in a time machine and you and the
Speaker 1: time machine does something to you, and then you end
Speaker 1: up back in the past. I don't really see how
Speaker 1: that could possibly work. What you really want in time
Speaker 1: travel is for the whole universe to travel back in
Speaker 1: the past and for you to not. So you gotta
Speaker 1: like get in the time machine and it's got to
Speaker 1: like rewind the clocks of the rest of the universe, right,
Speaker 1: You don't want to be like, Okay, it's still today,
Speaker 1: but now I'm ten years younger. I mean maybe some
Speaker 1: people want that. That's a whole different thing to look for.
Speaker 1: If you want to like unspill your coffee, but you
Speaker 1: still want like the ideas you want to remember having
Speaker 1: spilled it on yourself so you cannot just repeat it.
Speaker 1: Then you need to rewind the whole rest of the
Speaker 1: universe somehow, which seems like a much bigger job that. Yeah,
Speaker 1: that's a great point. Yeah, that so the time machine
Speaker 1: would have to change the universe, not you. Yeah, I
Speaker 1: never thought of it that way. Yeah. And a lot
Speaker 1: of our listeners right in when we talk about time
Speaker 1: travel and raise a similar point and a criticism of
Speaker 1: science fiction levels, which is that, you know, if you
Speaker 1: do go back in time somehow, how do you know
Speaker 1: where you're going to be? You know, because the Earth
Speaker 1: and the Sun and the Milky Way they're all moving, Um,
Speaker 1: so how do you know where you're going to end up?
Speaker 1: And it's a fun question. Um though, I think if
Speaker 1: you're gonna posit like, okay, you can travel through time,
Speaker 1: then ostensibly probably you can travel travel through space time,
Speaker 1: so you can appear wherever you want. But the problem is, like,
Speaker 1: as I'm not even really necessarily well defined, because what
Speaker 1: does it mean to be here at a point in
Speaker 1: space or there at a point in space right now?
Speaker 1: This point in space? Now where is that point in
Speaker 1: the future, Because there is no like marker to space.
Speaker 1: Space is all relative. It's not absolutely you can't like
Speaker 1: grasp this point of space and give it a name
Speaker 1: and say where does this bit go? There's only stuff
Speaker 1: moving through space relative to each other, so it turns
Speaker 1: out that's not even really well to find, Like, where
Speaker 1: was the Earth, you know, a million years ago in
Speaker 1: our space doesn't actually have a meaning? That is when
Speaker 1: I had thought of before that always seemed an insurmountable problem. Yeah,
Speaker 1: but this, this actually reminds me of another thing I
Speaker 1: wanted to talk about briefly, which is relating time to
Speaker 1: the history of the universe and the Big Bang, a
Speaker 1: thing people often ask, and I know there are theories
Speaker 1: to address this is is what happened before the Big Bang,
Speaker 1: But if you have an understanding that, you know, you
Speaker 1: have a singularity at the origin of the Big Bang,
Speaker 1: that was the beginning of time itself as we know it.
Speaker 1: What are physicists talking about exactly when they try to
Speaker 1: envision causes leading to the first instant of the Big Bang?
Speaker 1: Mostly they're trying to avoid that singularity because that singularity
Speaker 1: is a problem. You know, we don't see things like
Speaker 1: singularities in the universe. We don't see infinities, we don't
Speaker 1: see things with infinite density, we don't see things of
Speaker 1: infinite size. I mean, maybe the universe is itself infinite,
Speaker 1: but there's nothing that's like infinitely smooth or perfectly circular.
Speaker 1: These are sort of abstractions in our mind, and so
Speaker 1: most physicists who are working on the very early universe
Speaker 1: are trying to avoid that singularity because, as I said earlier,
Speaker 1: general relativity breaks down. That's what it means like, if
Speaker 1: your theory predicts something infinite, it doesn't know how to
Speaker 1: do any calculations beyond that. So instead of having like
Speaker 1: a moment of singularity, which is sort of like the
Speaker 1: naive general relativistic prediction of increasing density, instead they're going
Speaker 1: back and saying, well, maybe big Bang was just like
Speaker 1: a rapid expansion of space from a previously dense kind
Speaker 1: of universe that we don't understand at all. So the
Speaker 1: basic sketches, like you have some kind of weird state
Speaker 1: the universe is filled with like inflotons, some particle we
Speaker 1: don't know if it existed, but maybe it did. And
Speaker 1: then those inflotons they are causing me rapid expansion of
Speaker 1: space and decay then into normal matter. So that so
Speaker 1: now the Big Bang is that moment when the in
Speaker 1: photons are expanding and then decay into like our universe.
Speaker 1: That's how our universe is sort of created out of
Speaker 1: these in photons, and that avoids this moment of singularity.
Speaker 1: It's never like a moment when the universe is infinitely dense.
Speaker 1: But you know, again, this is very speculative stuff. We
Speaker 1: think inflation happened this crazy expansion in the very beginning, um,
Speaker 1: and this is like a way to avoid having to
Speaker 1: put before that this dot, this singularity that breaks all
Speaker 1: of the mathematics, instead of replacing it with like some
Speaker 1: other weird kind of sub So we don't even really
Speaker 1: know what it's like or what it's about. Um, we're
Speaker 1: just really beginning to know how to ask questions about it.
Speaker 1: And you know, that suggests a really interesting question, which
Speaker 1: is like, if there is something before the Big Bang,
Speaker 1: what was it? And was there something before that? It
Speaker 1: seems like, in one hand, super frustrating because you're just
Speaker 1: kicking the can down the road, Like, all right, so
Speaker 1: the early universe was this expansion, and before that came
Speaker 1: something which caused the expansion, and before that came something
Speaker 1: which caused that, which caused the expansion. But is there
Speaker 1: in the end something original which caused it. We don't know.
Speaker 1: And there's two possibilities. One is that we just keep
Speaker 1: digging forever and dig further and further and further and
Speaker 1: further back and never get to anything which seems like
Speaker 1: could have caused itself. Or it could be that we
Speaker 1: get to some state where we're like, hmm, this makes
Speaker 1: sense to have to be a beginning. It's like it's
Speaker 1: sort of the only way things could have happened to me.
Speaker 1: It's it's hard to grapple with this idea, so it's
Speaker 1: easier to think about it sort of in a parallel way,
Speaker 1: which is like, what is the smallest thing in the universe.
Speaker 1: We don't know if as we tear apart, particles will
Speaker 1: keep finding things that are smaller and smaller and smaller
Speaker 1: and smaller, or if eventually we'll get to one where, like,
Speaker 1: you know, what, this one it makes sense to be
Speaker 1: a fundamental ingredient to the universe. We can just start
Speaker 1: from here and build up. You know, maybe it's like
Speaker 1: the smallest fundamental thing. It's at the plank length or something.
Speaker 1: We don't know if we'll ever get there, or if
Speaker 1: it will be self evident, or if there will be
Speaker 1: always people who say, like, I don't know, I want
Speaker 1: to dig deeper. In the same way, it might be
Speaker 1: that we're doomed to keep digging deeper and deeper back
Speaker 1: into the history of the universe, never finding out if
Speaker 1: there was an original cause. All right, so, I think
Speaker 1: we're probably getting close to the end of our time.
Speaker 1: But I gotta come back to time travel before we do,
Speaker 1: because I'm wondering what what do you think You mentioned
Speaker 1: Stephen Hawkings party where the invitations were sent out after
Speaker 1: it happened. But what is your personal favorite way to
Speaker 1: hunt for a time traveler? What would you do if
Speaker 1: you wanted to find evidence of people from the future. Wow,
Speaker 1: I've I've never given that any thought about evidence for
Speaker 1: people from the future. Um. I try to think about
Speaker 1: what people would want to do, Like if I were
Speaker 1: a time traveler, why would I come to one? Uh?
Speaker 1: You know the obvious answers of like change history. Uh,
Speaker 1: in which case, you know, I guess you can blame
Speaker 1: those time travelers for you know, the reason things have
Speaker 1: gone the way they are. Maybe there because time travelers
Speaker 1: have come back and tweaked election results or you know,
Speaker 1: or something like that. Um. So, I guess the best
Speaker 1: way to find time travelers with then to be present
Speaker 1: at critical hinge moments in history and look around for
Speaker 1: suspicious behavior. I suppose I don't really know. That's a
Speaker 1: great question. Yeah, I was thinking about all this in
Speaker 1: terms of of ancient aliens as well, because both you
Speaker 1: have you have people of course who obsess about the
Speaker 1: idea of of aliens having visited during ancient times and
Speaker 1: so forth, and and you also have I guess, a
Speaker 1: more recent phenomenon of people looking back at old pictures
Speaker 1: and painting and you know, playing this game of basically
Speaker 1: misinterpreting um things and paintings and photos, like looking back
Speaker 1: in an old picture and saying, oh, well, that person,
Speaker 1: their their style of dress does not look archaic enough.
Speaker 1: They must be traveler, yeah, or just painting. She's holding
Speaker 1: something that looks like an iPhone. Obviously this is a
Speaker 1: renaissance painting of a time travel No. I think that
Speaker 1: just says a lot about us, you know, and who
Speaker 1: we are, you know, the same way that like photos
Speaker 1: of UFOs seem to be constantly grainy. As you know,
Speaker 1: imaging technology improves, it's always on the edge of the
Speaker 1: our ability to capture it. So I think it says
Speaker 1: something about our desire to discover weird things and reveal
Speaker 1: the truth, which I'm totally sympathetic to I also want
Speaker 1: to peel back a layer of the universe and wake
Speaker 1: up to its true nature. I remember Carl Sagan, and
Speaker 1: I forget which which book this was, but it was
Speaker 1: in one of the books where he talks a little
Speaker 1: bit about the idea of ancient aliens, and I remember
Speaker 1: him him basically outlining the sort of ancient account, the
Speaker 1: sort of myth that one might look to as as
Speaker 1: the sort of ancient astronaut account that could exist, have
Speaker 1: such things were possible? And I wonder if anyone's ever
Speaker 1: taken a similar approach to the concept of time travel,
Speaker 1: like like basically like boiling it down, saying, okay, if
Speaker 1: there is actually evidence, say in you know, the historical
Speaker 1: record all of people having traveled back in time, you know, well,
Speaker 1: what exactly would we be looking for? What exactly would
Speaker 1: they would they have been doing? Um and uh and
Speaker 1: and and how would and I guess it would come
Speaker 1: down to, like you'd have to imagine, like how truthful
Speaker 1: are they going to be? Are they just gonna lie
Speaker 1: about themselves being time travels? Because that's then you can
Speaker 1: basically point to any pivotal individual or any person in
Speaker 1: a pivotal period of time, right, Yeah, and would they
Speaker 1: even be humans? Right, Like we fantasize about going back
Speaker 1: to see the dinosaurs. So if now we're putting ourselves
Speaker 1: back in the past and imagining time travelers, we might
Speaker 1: have to imagine some like post human apocalyptic, newly intelligent
Speaker 1: species of you know, who knows what, penguins or or
Speaker 1: something coming back in time to investigate humans, you know,
Speaker 1: to understand what happened before the apocalypse or whatever. Um.
Speaker 1: But I think machines, machines. Most of the most of
Speaker 1: our space exploration is uncrewed probes. Now you would have
Speaker 1: to imagine the same would hold true for time. Yeah,
Speaker 1: that's probably true, or you know, after the machines have
Speaker 1: killed us all and they just have myths about those
Speaker 1: weird meat creatures that used to uh roam the earth
Speaker 1: or something. Um. That's fun, but it's fundamentally is limited
Speaker 1: by our imagination. It's the same problem with trying to
Speaker 1: look for aliens. We look for aliens in the way
Speaker 1: we expect to see them, although we're pretty sure that
Speaker 1: if aliens exist, they're not anything that we expected. So
Speaker 1: we need to like push really hard on all the
Speaker 1: boundaries of our imagination. To make sure we're looking for
Speaker 1: aliens as broadly as possible so we don't miss them.
Speaker 1: We don't just like come by, We're like, oh, that's
Speaker 1: not aliens. So it's the same problem with imagining future
Speaker 1: time travelers, like who these these people or things or
Speaker 1: entities are are well beyond I think even our most
Speaker 1: creative science fiction UM writers, well, like you said, like
Speaker 1: what's interesting? What would be interesting about to someone from
Speaker 1: the future, And we instantly think, too, oh, well, the
Speaker 1: you know, the coronavirus or something going on in you know, geopolitics,
Speaker 1: or even in the environment. But it could be something
Speaker 1: entirely different. It could be you know, the very beginning
Speaker 1: of something um that doesn't matter at all today, right,
Speaker 1: but but matters say in exactly which we could never
Speaker 1: possibly imagine. I mean, think about people a thousand years
Speaker 1: ago trying to anticipate was important to us today. We
Speaker 1: couldn't even do that from twenty years ago, not to
Speaker 1: mention a thousand Okay, last question, Daniel, what's your favorite
Speaker 1: time travel movie? Oh, my favorite time travel movie has
Speaker 1: to be Primer So I think that most clearly sets
Speaker 1: out rules, rules that make sense, and then follows those
Speaker 1: really carefully with lots of fascinating and unexpected results. Good answers. Yeah,
Speaker 1: that's a good one. I often gravitate towards the ones
Speaker 1: that have really goofy time travel rules, but they but
Speaker 1: if they still stick to those rules, then I tend
Speaker 1: to forgive them. They're always boundary cases, though they're always cased.
Speaker 1: They were like I'm not sure what would happen in
Speaker 1: this scenario or that scenario. So I like Primary because
Speaker 1: it has really clear crisp rules and those and it
Speaker 1: has cost. You can't just like pop back in time.
Speaker 1: You have to like spend time going backwards um, which
Speaker 1: has really interesting consequences. So I found it to be
Speaker 1: really creative totally. All right, Well, thanks so much for
Speaker 1: joining us today, Daniel. This has been a lot of fun.
Speaker 1: Thank you very much. Always a pleasure to talk to
Speaker 1: you guys. All right, well, thanks once more to Daniel
Speaker 1: Watson for jumping on the old podcast machine and lead
Speaker 1: us uh Pokemon Produm with various questions about time travel
Speaker 1: and wormholes and what have you. Yeah, if you want
Speaker 1: to learn more, so if you're not subscribed to Daniel
Speaker 1: and Jorge Explain the Universe, you can find that wherever
Speaker 1: you get your podcast, but you can also go to
Speaker 1: www dot Daniel and Jorge dot com. And you can
Speaker 1: also find the website for their new book. Again. The
Speaker 1: book is called Frequently Asked Questions about the Universe, and
Speaker 1: the website for that is www dot Universe f a
Speaker 1: Q dot com. And if you'd like to check out
Speaker 1: other episodes of Stuff to Blow Your Mind, well you
Speaker 1: can find our show wherever you get your podcasts. Just
Speaker 1: look for the Stuff to Blow Your Mind podcast feed.
Speaker 1: We run multiple episodes per week, with core episodes dealing
Speaker 1: with science and culture on Tuesdays and Thursdays. On Monday's
Speaker 1: we do listener mail, on Wednesday's we do a short
Speaker 1: form artifact titled the Artifact, and on Friday's we do
Speaker 1: something called Weird House Cinema, which is our time to
Speaker 1: set aside most serious matters and just discuss aus strange film. So,
Speaker 1: of course thanks again to Daniel for joining us today,
Speaker 1: and as always, a big thank you to our excellent
Speaker 1: audio producer Seth Nicholas Johnson. If you would like to
Speaker 1: get in touch with us with feedback on this episode
Speaker 1: or any other, to suggest topic for the future, or
Speaker 1: just to say hi, you can email us at contact
Speaker 1: at stuff to Blow Your Mind dot com. Stuff to
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