Speaker 1: Hey everyone, Daniel here.
Speaker 2: My kids were into fantasy novels a few years ago,
Speaker 2: and I remember one of them asking me if I've
Speaker 2: ever seen.
Speaker 1: Magic in real life. I knew what they had in mind.
Speaker 2: Sorcerers and spells, someone turning rocks into frogs or whatever,
Speaker 2: and so of course my first answer was no. My
Speaker 2: second thought, since that felt kind of boring and deflating,
Speaker 2: was to try to help them realize how amazing and
Speaker 2: wonderful and almost magical our world is. They don't need
Speaker 2: official magic to appreciate the awesome power of the sun,
Speaker 2: or the incredible feats of technology inside their phones, or
Speaker 2: the amazing biomechanics inside their bodies. It's an extraordinary universe
Speaker 2: out there, and we love to understand it. But that
Speaker 2: doesn't feel like magic. The scientific view of the world
Speaker 2: is appreciation through explanation. The thing that makes something magic
Speaker 2: is that it's not understood. And though it feels rare
Speaker 2: today to see something truly baffling with your own eyes,
Speaker 2: it used to be a very common experience. What did
Speaker 2: prehistoric people, biologically equivalent to us, and with just as
Speaker 2: much brain power, What did they think when they saw
Speaker 2: lightning or an eclipse or got sick. To them, it
Speaker 2: might as well have been magic, because none of it
Speaker 2: was understood. I think that's kind of what my kids
Speaker 2: were asking about, and it made me realize that actually,
Speaker 2: there's still a lot of magic in our lives, because
Speaker 2: there's plenty of the universe that remains unexplained, Not little
Speaker 2: details of scientific trivia, but things as basic and as
Speaker 2: every day as lightning and disease, things so fundamental to
Speaker 2: our existence that we sometimes don't even realize how magical
Speaker 2: they are. So here's a magic trick for you. Turn
Speaker 2: the future into the present, turn the present into the past.
Speaker 2: The universe is doing that all the time, unintended, and
Speaker 2: yet we don't really understand it or how it works.
Speaker 2: And it does it without any apparent lag or delay.
Speaker 2: So on today's episode, we'll dig deep into the nature
Speaker 2: of time and explore what physics can tell us about
Speaker 2: what it is, why we have it, and why it works.
Speaker 2: Today in the pod, we're asking what is time?
Speaker 1: Hi? I'm Daniel Whitson.
Speaker 2: I'm a particle physicist and a professor at UC Irvine,
Speaker 2: which means my.
Speaker 1: Time is not my own.
Speaker 3: Hi.
Speaker 4: I'm Kelly Wiener Smith and ever since having kids I
Speaker 4: never feel like I have enough time, but the time
Speaker 4: has been higher quality. And welcome to Daniel and Kelly's
Speaker 4: extraordinary universe where you'll have the time of your life.
Speaker 4: That was really corny. I should lose my job, but
Speaker 4: I'm going with it. I'm leaning in. So Daniel, speaking
Speaker 4: of corny, here is my question for you today. So
Speaker 4: last episode we talked about space, and this episode we're
Speaker 4: talking about time. So space time and space time are
Speaker 4: topics that sci fi deals with a lot. So what
Speaker 4: is the worst, most inaccurate representation of those concepts in
Speaker 4: any movie or TV show that you have experienced?
Speaker 1: Wow?
Speaker 2: How much time do you have? We should go a
Speaker 2: whole lot last episode just on that.
Speaker 1: Oh my gosh.
Speaker 2: I think one of my least favorite trips in science
Speaker 2: fiction is subspace or hyperspace that you've like dropped into
Speaker 2: some other kind of space where you can move faster
Speaker 2: than light or something. And I'm always wondering, like, you know,
Speaker 2: if space has multiple dimensions, you're still in that other
Speaker 2: space and you're in this space. What does this even mean?
Speaker 2: Where are we? What is the physics of this space?
Speaker 2: To me, it feels like you're just escaping rules and
Speaker 2: there's always gotta be rules, even in subspace or hyperspace.
Speaker 2: So yeah, it's pretty hard to watch science fiction as
Speaker 2: a physicist.
Speaker 4: So it actually like detracts from your enjoyment of it.
Speaker 1: Oh, absolutely it does.
Speaker 2: Yeah, And you know, I don't mind if the rules
Speaker 2: on the screen are different from the rules in our universe.
Speaker 1: That's awesome, that's cool, that's.
Speaker 2: What I want. But I want them to follow them.
Speaker 2: I want them to always be rules, because you know,
Speaker 2: without rules, there's no story, anything can happen, there's no stakes.
Speaker 2: It's not worth watching.
Speaker 4: So I know this movie was loved by many, but
Speaker 4: I kind of felt that way about like from dust
Speaker 4: till Dawn, like everything seemed normal and then vampires and
Speaker 4: you're like, what, this is not okay with me. Tarantino
Speaker 4: has more fans than I ever will have, so good
Speaker 4: for him.
Speaker 2: Maybe we should add vampires to this podcast.
Speaker 4: Oh sure, why not? I loved Anne Rice books when
Speaker 4: I was a kid. I don't know what that says
Speaker 4: about me.
Speaker 2: I worked with somebody in educational television who used to
Speaker 2: work in publishing, and she told me the day she
Speaker 2: had to leave publishing was the day she pitched a
Speaker 2: really great book. And the executive told her this is wonderful,
Speaker 2: but can you add sexy vampires to it? No, she
Speaker 2: was like, h we're done, yes.
Speaker 4: Up the end. Oh my goodness. Yeah, that's a good
Speaker 4: time to throw in your head.
Speaker 2: All right, But today on the podcast, we are not
Speaker 2: talking about sexy vampires or travel through subspace. We're talking
Speaker 2: about one of the slipperiest questions in all of physics,
Speaker 2: the kind of question that's hard to answer and hard
Speaker 2: to ask and hard to think about, which means it's
Speaker 2: one of the most important questions. It's a question you
Speaker 2: shouldn't shy away from. It's the kind of question you
Speaker 2: should dig into, you should confront, you should really try
Speaker 2: to tackle with everything that you have.
Speaker 4: And that's what we're gonna do today. We're gonna have
Speaker 4: another brain bending but absolutely critical discussion about something that
Speaker 4: feels like it makes sense but then doesn't when you
Speaker 4: dig a little deeper.
Speaker 2: And i'd encourage you to remember that in the history
Speaker 2: of physics and the history of science, there's been lots
Speaker 2: of times when a topic has gone from like weird
Speaker 2: and fuzzy and hard to grapple with to totally understood
Speaker 2: and like mathematically formulated and that's just the process of science.
Speaker 2: But the beginning steps, the first bites of the apple
Speaker 2: are hard, you know. When people were thinking about like
Speaker 2: what is everything made out of? Or like where did
Speaker 2: the earth come from? Or where are those shiny dots
Speaker 2: in the sky. Those were fuzzy questions, and people had
Speaker 2: pretty silly ideas and went down the wrong path lots
Speaker 2: of times. But hey, we got there, and we only
Speaker 2: got there because people took those first bites of the apple.
Speaker 2: People tried when it was still hard. So that's why
Speaker 2: we're asking these weird, fuzzy questions about something so basic
Speaker 2: as time.
Speaker 4: It's exciting. I remember when I was in undergrad and
Speaker 4: I was learning, you know, so much in my classes,
Speaker 4: but I started to think about grad school, which wasn't
Speaker 4: on my radar until I took like thiss ecology class
Speaker 4: that I loved, And I remember thinking, like a but
Speaker 4: we figured out so much already, Like what is there
Speaker 4: left to learn because it's all new when you're an undergrad,
Speaker 4: and it's like, no, there's so much left to learn
Speaker 4: and some of it is still fundamental, and I don't know,
Speaker 4: it's just exciting to know that there's still so much.
Speaker 2: To contribute, absolutely, and some of the things we have
Speaker 2: left to learn are things that we don't think about
Speaker 2: because they are just at the base level of our understanding.
Speaker 2: They're like the assumed context. And later, in one hundred
Speaker 2: years or a thousand years, when we understand these things better,
Speaker 2: people are going to look back and be like, Wow,
Speaker 2: what was it like to be human back then when
Speaker 2: they thought time worked this way? And now we know
Speaker 2: what actually is this completely other thing? And ha ha ha,
Speaker 2: how silly were they?
Speaker 1: Right?
Speaker 2: Yeah, it shapes our very existence and the way we
Speaker 2: think about our lives, and so it's definitely worth digging into.
Speaker 2: And if we don't do this work today, then they're
Speaker 2: not going to figure it out in a thousand years.
Speaker 4: All right, Well, I am hoping that by the end
Speaker 4: of today's episode I have a science based excuse for
Speaker 4: why I'm late all the time that has something to
Speaker 4: do with the confusing nature of time. So first, let's
Speaker 4: hear what our listeners had to say about what time
Speaker 4: really is.
Speaker 2: That's right, I went out there into the internet and
Speaker 2: asked a bunch of folks this hard question, what is time?
Speaker 2: If you would like to answer questions like this for
Speaker 2: the podcast. Please don't be shy. Right to us two
Speaker 2: questions at the day, and Kelly dot Org will hook
Speaker 2: you up and you can hear your voice on the podcast.
Speaker 2: In the meantime, here are fuzzy thoughts about a fuzzy
Speaker 2: question about time.
Speaker 5: Time is represented as its own dimension, like a column
Speaker 5: in an array. But I just think of it as
Speaker 5: one thing happening after the other.
Speaker 1: Time is a human experience. Time, I think is a
Speaker 1: measure of change.
Speaker 6: Time is change made measurable by quantification of entropy and
Speaker 6: by using some caesium oscillations.
Speaker 4: Time is an artificial.
Speaker 2: Construct humans have created to try and help explain.
Speaker 1: The universe.
Speaker 5: Is the second law of thermodynamics.
Speaker 1: Our perception of change.
Speaker 4: Time is a one way trip into the future.
Speaker 3: The human constructor rived from the sound cycles. I think
Speaker 3: that time is an abstraction. It's our capability to perceive
Speaker 3: changes occurring, to make connections between events and their consequences.
Speaker 3: But I don't believe that time is some sort of
Speaker 3: substance with an arrow of direction that can be flipped entropy.
Speaker 1: I guess the fourth dimension in Albert Einstein's theory.
Speaker 4: Oh wow, I would say it's how we experience events.
Speaker 1: Happening one after the other in a sequence.
Speaker 5: I'd have to say that it's the result of quantum processes, decisions,
Speaker 5: random interactions that occur between inside fields. It's kind of
Speaker 5: a direction of events in succession.
Speaker 4: So before we got to the listener questions, you said,
Speaker 4: these are fuzzy thoughts about a fuzzy question about time.
Speaker 4: And I gotta say, I was really impressed by how
Speaker 4: deep some of the these answers were and how seemingly
Speaker 4: thought out they were. And I guess how many like
Speaker 4: polysyllabic words they used in these explanations. That made me
Speaker 4: think you are kind of aware of the underpinnings of
Speaker 4: this question. And so yeah, I was impressed with the answers.
Speaker 2: And there's also an extraordinary breadth here. I mean, there
Speaker 2: are folks who are philosophizing about it it's an artificial construct,
Speaker 2: and then folks are just trying to describe it. You know,
Speaker 2: it's a one way trip into the future. People talk
Speaker 2: about how we measure it. It's really interesting how many
Speaker 2: different responses there are to this question. They tell you,
Speaker 2: like how many facets there are to it, and how
Speaker 2: hard it is to answer a basic question because we
Speaker 2: don't all necessarily even agree on, like what form the
Speaker 2: answer is. Like if you ask me what's one plus two?
Speaker 2: We all know the answer should be a number, right,
Speaker 2: and maybe we get it right or we can get
Speaker 2: it wrong. But the answer to the question like what
Speaker 2: is time? What does that answer look like?
Speaker 1: You know?
Speaker 2: Is it just a description of our experience? Is it
Speaker 2: a mathematical structure that tells us how time works? Is
Speaker 2: it a philosophical delve into like why it exists and
Speaker 2: why the universe needs it, or maybe why it doesn't
Speaker 2: need it, where it actually comes from, why we have it,
Speaker 2: and how it began. There's such a breadth of things
Speaker 2: to explore there.
Speaker 4: Yeah, And I feel like we had pretty much the
Speaker 4: same conversation when we were talking about space in the
Speaker 4: prior episode that, like, you can just come at it
Speaker 4: from so many different angles. There's so many absolutely fundamental
Speaker 4: things where it's even hard to figure out what kind
Speaker 4: of question we should be asking.
Speaker 2: That's right, And since this isn't a philosophy podcast or
Speaker 2: an engineering podcast, we're going to sort of take the
Speaker 2: middle road. We're not going to describe the details of
Speaker 2: how caesium atoms measure time or delve into like the
Speaker 2: philosophical underpinnings of a theory versus b theory. In philosophical
Speaker 2: questions of time, we're going to talk about the physics
Speaker 2: of time and what physics tells us about time, because
Speaker 2: obviously time is really important to physics, and it is
Speaker 2: deeply woven into how we have made sense of the
Speaker 2: world so far. And so what we can do is
Speaker 2: turn that around and say, all right, time is part
Speaker 2: of our understanding of how the world works. What does
Speaker 2: that tell us about what time is? And as usual,
Speaker 2: the answer is going to be a bunch of incoherent,
Speaker 2: inconsistent ideas that don't really come together because we don't
Speaker 2: understand it. And so we'll do our best to try
Speaker 2: to weave that all together into an excuse for Kelly
Speaker 2: for why she's always late.
Speaker 4: Yes, anyone who's thinking about working with me in the future,
Speaker 4: I'm actually usually five minutes early that I'm worried about
Speaker 4: my reputation here. But so for our space episode, the
Speaker 4: answer depended on whether you are in a quantum mechanics
Speaker 4: or a general relativity framework. Is that where we're going
Speaker 4: to get again, there's not just one answer. It depends
Speaker 4: who you talk to.
Speaker 1: Yes, exactly.
Speaker 2: There's a fascinating evolution of our understanding of time with
Speaker 2: respect to space and gravity, and that's super interesting and counterintuitive.
Speaker 2: It's also completely at odds with our understanding of time
Speaker 2: in a quantum mechanical universe. So, yeah, we're going to
Speaker 2: have a couple of threads here, we're going to show
Speaker 2: how they don't tie together. And then on top of that,
Speaker 2: there's statistical mechanics and particles iss which also have different
Speaker 2: views on time, and so we hope that one day
Speaker 2: somebody's being able to pull us all together into one answer.
Speaker 2: What we have right now are a bunch of threads,
Speaker 2: none of which are completely satisfactory. And you know, the
Speaker 2: thing that we're trying to describe in the end is
Speaker 2: something pretty intuitive, right, it's our daily experience. It's this
Speaker 2: kind of magic where like we have the present, like
Speaker 2: right now I'm experiencing something and five seconds ago you
Speaker 2: were saying something and I was laughing about it, and
Speaker 2: there's a difference between that. There's like what I'm doing
Speaker 2: right now and what I was doing and what I
Speaker 2: will be doing. And we know that there's a difference there.
Speaker 2: Right when we experience, when we remember when we anticipate,
Speaker 2: and that's just like our intuitive understanding of what time is.
Speaker 2: I think that's what we're trying to grapple with.
Speaker 4: I feel like, even in our day to day life, though,
Speaker 4: time can sometimes feel weird. And this is the biologist
Speaker 4: in me. You know, my daughter is not very coordinated.
Speaker 4: And once we were playing a game on a wooden
Speaker 4: floor where we were kicking a ball around, and she
Speaker 4: kicked it and one foot went up and landed on
Speaker 4: the ball, and she stood on top and she started
Speaker 4: falling forward and she didn't put her arms out to
Speaker 4: catch her. No, because she's my daughter. And I swear
Speaker 4: the world slowed down as her teeth hit the wood floor.
Speaker 4: Oh my gosh, something as easy as time. I feel
Speaker 4: like I've had jobs where I had like two weeks
Speaker 4: left and it felt like time had fundamentally changed. But anyway,
Speaker 4: this is the biologist's perspective and how brains work, I suppose.
Speaker 4: I guess we're not getting into that today.
Speaker 2: That is a really important angle. And if you've read
Speaker 2: folks like Daniel Dennett, he suggests that there is no
Speaker 2: present moment of consciousness, that the only experience we have
Speaker 2: is actually remembering the immediate past, that your brain assembles
Speaker 2: the present as an illusion. You never experience it, you
Speaker 2: just remember it. It's a weird concept. It makes a
Speaker 2: lot of sense when you're reading the book, and then
Speaker 2: later you're like, what, how does that possible? Like with
Speaker 2: a lot of philosophy. So, yeah, that's not something we're
Speaker 2: going to dive into today, but we will ask questions like,
Speaker 2: how does time work?
Speaker 1: What is it?
Speaker 2: Can we travel through time? Did time have a beginning?
Speaker 2: What does that even mean? Why does time seem to
Speaker 2: only go forward?
Speaker 1: Words, all this kind of stuff.
Speaker 4: All right, So we could have a whole podcast called
Speaker 4: Daniel and Kelly's Extraordinary Conversations about Time. We could take
Speaker 4: biology philosophy anyway. But today we're just doing physics, all right.
Speaker 4: So when we were talking about space, we went through
Speaker 4: sort of like the history of ideas about space? Where
Speaker 4: should our history of ideas about time start?
Speaker 1: This time?
Speaker 2: So, like so many things in physics, we got to
Speaker 2: start with Newton. I mean, people talk about time before Newton,
Speaker 2: but really Newton was the first person to systematize it
Speaker 2: and to incorporate it into his physics. And you know,
Speaker 2: he figured out gravity and the loss of motion and
Speaker 2: all this stuff so much depends on time. And for Newton,
Speaker 2: time was absolute. He imagined like a single clock for
Speaker 2: the whole universe. And the way Newton thinks about time
Speaker 2: is sort of like the way a movie works. You know,
Speaker 2: you imagine a movie is like some continuous motion, but
Speaker 2: really it's a bunch of snapshots.
Speaker 1: Right.
Speaker 2: If you slowed down a movie, you'd see it's a
Speaker 2: bunch of still frames and those are tied together in
Speaker 2: a sequence. Right, So you're watching like a whole movie
Speaker 2: of your daughter falling into the floor. You see her
Speaker 2: standing on the ball. The next frame she's falling a
Speaker 2: little closer to the floor, and the next frame, the
Speaker 2: next frame. Finally there is a frame where like the
Speaker 2: teeth make contact with the wooden floor.
Speaker 4: Right, yeah, yeah, why did I bring this up? I'm
Speaker 4: gonna have to relive it so many times?
Speaker 2: And it's a series of moments, right, still moments, and
Speaker 2: time is what ties those still moments together. In Newton's universe,
Speaker 2: time is what connects those as well, Like the laws
Speaker 2: of physics tell us if the universe was at this
Speaker 2: state in this organization, if your daughter's face was this
Speaker 2: far above the floor, then when is her face going
Speaker 2: to impact the floor. The physics predicts the future from
Speaker 2: the past. It connects all of these into a causal chain,
Speaker 2: and the whole universe ticks forward together like tic tick.
Speaker 2: You imagine the whole universe as a still frame, and
Speaker 2: then you tick it forward, and everything in the universe
Speaker 2: has taken one time step.
Speaker 4: So intuitively this makes sense to me who was the
Speaker 4: first person to make it? Confusing but more accurate, Confusing
Speaker 4: but probably more accurate.
Speaker 2: Let's not confuse people just yet. Let's marinate in how
Speaker 2: much sense it makes, just for what longer, because it
Speaker 2: is kind of wonderful, right imagining that far away in Andromeda,
Speaker 2: you know, the aliens are also rescuing their daughters from
Speaker 2: wooden floors or whatever. That the whole universe is following
Speaker 2: some laws of physics, and if you knew the state
Speaker 2: of the universe, you could predict its future, right. It's
Speaker 2: an incredible thing about physics. I remember learning it in
Speaker 2: high school and being like, oh my gosh, physics is
Speaker 2: the power to predict the future. You know, if I
Speaker 2: tell you the angle of my cannonball, I know whether
Speaker 2: it's going to go over that castle wall or not.
Speaker 2: Because it's described by the laws of physics. The past
Speaker 2: predicts the future. That's wonderful. It also tells you what
Speaker 2: can't happen right. It tells you that like your ball
Speaker 2: can't be here and then suddenly appear.
Speaker 1: On the other side of the wall.
Speaker 2: Or for example, you can't shoot a laser at those
Speaker 2: Aliens and Andromeda and fry them today because light takes
Speaker 2: time to get to Andromeda. It's like this sphere of
Speaker 2: influence you have over the universe. You can't affect things
Speaker 2: outside that sphere because light only travels at a certain
Speaker 2: speed and nothing can go faster than that. And so
Speaker 2: time is intimately connected with how the universe moves forward.
Speaker 2: And this is a beautiful vision to imagine we're all
Speaker 2: linked together in time. But Einstein was the first one
Speaker 2: to make this confusing because he realized that we don't
Speaker 2: actually have a single clock for everybody in the universe.
Speaker 2: His theory of special relativity messes that up.
Speaker 4: So for a second there you had me feeling like
Speaker 4: maybe I shouldn't have studied animal behavior, where everything is
Speaker 4: confusing and hard to predict, and you don't know if
Speaker 4: the person's going to catch the ball you were talking
Speaker 4: about But now you've made me feel like no, I
Speaker 4: made the right choice because physics is confusing too. Is
Speaker 4: it confusing in a predictable way or is it just yeah,
Speaker 4: what did Einstein say?
Speaker 2: So we're not doing quantu mechanics. Yeah, the universe isn't random.
Speaker 2: We're still dealing with the deterministic, predictable universe where the
Speaker 2: past perfectly predicts the future. It's just that it predict
Speaker 2: it differently than what Newton thought. The reason that time
Speaker 2: is all messed up is because of this amazing experimental
Speaker 2: fact that Einstein relied on about the speed of light.
Speaker 4: We are taking a break, and when we get back,
Speaker 4: we'll hear about how Einstein ruined Newton's beautiful idea of time.
Speaker 4: So we're back. One of the most exciting things to
Speaker 4: me about physics is how you have these predictions that
Speaker 4: are sort of counterintuitive but then actually help you really
Speaker 4: understand complex things and create new technologies and stuff like that. So,
Speaker 4: all right, you're about to tell us how Einstein came
Speaker 4: up with this complicated, counterintuitive idea, but it's correct and
Speaker 4: we use it all the time.
Speaker 2: Yeah, So Einstein's special relativity really messes up our concept
Speaker 2: of time. Newton's idea that everybody in the universe, can
Speaker 2: you share the same clock, that you can imagine like
Speaker 2: a still frame for the whole universe, and then take
Speaker 2: that forward to the next still frame for the whole universe,
Speaker 2: and that like physicist and Andromeda and physicists in the
Speaker 2: Milky Way can agree on how that all works. And
Speaker 2: this all comes out of a pesky little fact which
Speaker 2: initially doesn't sound all connected to time and watches and clocks,
Speaker 2: but it intimately is. And that's about how light works.
Speaker 2: In the late eighteen hundreds that people did experiments and
Speaker 2: discovered that light travels at the same speed for all observers.
Speaker 2: So like if you're holding a flashlight and you turn
Speaker 2: it on, you measure the speed of the photons will
Speaker 2: come out to be the speed of light. Now, if
Speaker 2: you're in a car and you're going sixty miles an
Speaker 2: hour and you turn the flashlight on, you measure those
Speaker 2: photons to be moving at the speed of light. Somebody
Speaker 2: on the ground who isn't in the car, they don't
Speaker 2: measure the photons to be moving at the speed of
Speaker 2: light plus sixty miles an hour. They still measure it
Speaker 2: to be moving at the speed of light anywhere, anybody, everywhere.
Speaker 2: Everybody always measures the speed of light to be the same,
Speaker 2: no matter how fast they're going.
Speaker 4: If you were in a car going sixty miles an
Speaker 4: hour and you took a ball and you threw it
Speaker 4: out your window, would you get a different answer or
Speaker 4: is it the same as the speed of light?
Speaker 1: Exactly.
Speaker 2: Balls work very differently than light does. If you're in
Speaker 2: the car and you throw the ball at sixty miles
Speaker 2: an hour, you measure it moving relative to you at sixty.
Speaker 1: Miles an hour.
Speaker 2: But if I'm on the ground and the car is
Speaker 2: moving at sixty miles an hour, I'm going to measure
Speaker 2: the ball to be going sixty plus sixty at one
Speaker 2: hundred and twenty miles per hour. That's also true of
Speaker 2: simpler things like sound. If you're in the car and
Speaker 2: you shout your daughter's name, like hey, watch out, you know,
Speaker 2: then that sound moves away from you at the speed
Speaker 2: of sound relative to the air. So I'm on the ground,
Speaker 2: I see that shout moving at the speed of sound
Speaker 2: relative to the air. If you're in the car, you
Speaker 2: could even catch up to that shout. The speed of
Speaker 2: that shout for you depends on how fast you're moving
Speaker 2: through the air. You could even catch up to that
Speaker 2: shout right, because the speed of sound is not that fast.
Speaker 2: You could have no speed with respect to your shout.
Speaker 2: Like if you're traveling at the speed of sound at
Speaker 2: mock one and you shout, your shout just stays there
Speaker 2: with you. Whereas photons, they're always moving at the speed
Speaker 2: of light relative to you. So light is very different
Speaker 2: from baseballs and from sound and all this stuff. And
Speaker 2: this says really important consequences for what time is and
Speaker 2: how we measure it.
Speaker 4: Why.
Speaker 2: Yeah, great question, And to answer it, let me give
Speaker 2: you an example. This is Einstein's classic example for why
Speaker 2: the speed of light misses up time. And Einstein isn't
Speaker 2: thinking about big clocks in the sky. He's just thinking about, like,
Speaker 2: when does stuff happen at the same time. So imagine
Speaker 2: instead of in a car, you're standing in a train
Speaker 2: and you have a flashlight, and you shine a flashlight
Speaker 2: forwards and backwards, or you turn on a light bulb
Speaker 2: so it shines in both directions. When is the light
Speaker 2: going to hit the front of the train and the
Speaker 2: back of the train. Well, you're in the train, You're
Speaker 2: in the middle of the car. It's going to hit
Speaker 2: the front and the back at the same time, right,
Speaker 2: No big deal. Okay, that's cool, But what about somebody
Speaker 2: on the ground. You know what, if you're in the train,
Speaker 2: you turn on your flashlight, you see the light hit
Speaker 2: the front of the train at the back of the
Speaker 2: train at the same time. I'm on the ground. I'm
Speaker 2: not in your train with you. To me, the train
Speaker 2: is moving past me at one hundred miles per hour
Speaker 2: or something.
Speaker 1: What do I see?
Speaker 2: I see the light moving forward at the speed of
Speaker 2: light and backwards at the speed of light. But the
Speaker 2: back of the train is moving towards the light bulb
Speaker 2: and the front of the train is moving away from it.
Speaker 2: So from my perspective, the light hits the back of
Speaker 2: the train before it hits the front of the train,
Speaker 2: because the back of the train is rushing to meet
Speaker 2: the light. So you see these things happening at the
Speaker 2: same time. The front and the back are simultaneous for you,
Speaker 2: but they're not simultaneous for me. And that's only because
Speaker 2: light travels at the same speed for me and for you.
Speaker 2: We did this example in sound or with baseballs, then
Speaker 2: we would agree about whether they're simultaneous or not. But
Speaker 2: because light always travels at the same speed for everybody,
Speaker 2: we don't agree about whether the events are at the
Speaker 2: same time.
Speaker 4: Okay, so the first this person turns on their flashlights
Speaker 4: and the answer is three seconds for forward and for back,
Speaker 4: and for somebody else the answer is four seconds for
Speaker 4: forward and two seconds for back.
Speaker 1: Yeah, exactly right.
Speaker 2: And if we did the same experiment with sound, right,
Speaker 2: if you stood in the center of the car and
Speaker 2: you shouted instead of turning on a light, then the
Speaker 2: shout would reach the front and the back of the
Speaker 2: train at the same time.
Speaker 1: For you, you're on the train, and.
Speaker 2: What would happen for me? For me, sound doesn't have
Speaker 2: to move at the same speed no matter what. So
Speaker 2: I would see the sound moving forward faster than I
Speaker 2: would see it moving backwards, because it's moving with the
Speaker 2: air inside the train. Right, sound doesn't have to move
Speaker 2: at the same speed for me, it doesn't have to
Speaker 2: follow Einstein's weird special rule, so I see it also
Speaker 2: as simultaneous. I see the front of the train racing away,
Speaker 2: but the sound is moving faster forward. And I see
Speaker 2: the back of the train rushing towards you, but the
Speaker 2: sound is moving slower in that direction, and so it
Speaker 2: all works out. It's all simultaneous for me and for
Speaker 2: you for sound. But because light breaks that, it says,
Speaker 2: I don't care how fast I'm going. Daniel's can to
Speaker 2: see me go in at the speed of light forward
Speaker 2: and backwards, And so Daniel doesn't see it simultaneous, and
Speaker 2: Kelly does.
Speaker 4: And this is something we got to a little bit
Speaker 4: in the last episode. It sounds like you can't really
Speaker 4: separate space and time. You need to understand them both.
Speaker 4: So we have two complicated concepts to try to define together.
Speaker 2: Exactly, and you put your finger on it. Whereas Newton said, look,
Speaker 2: there's one clock for the whole universe, Einstein says there's
Speaker 2: one clock at every point in space. Every location in
Speaker 2: space has a different clock, right, And so I see
Speaker 2: one clock running fast, you see another clock running slow.
Speaker 2: It's not just that there's a different clock at every space.
Speaker 2: How you see those clocks running also depends on your velocity,
Speaker 2: So there's no sense of universal time. There's not one
Speaker 2: time for the whole universe. This image we had where
Speaker 2: like the whole universe is frozen and then it ticks forward.
Speaker 2: According to physics, Like, if you're going to run a
Speaker 2: simulation of the universe, that's the way you might do it.
Speaker 2: That's broken. Now you can't have a single clock for
Speaker 2: the whole universe. We have to have a different clock
Speaker 2: at every location, and a different clock for people moving
Speaker 2: at different speeds. It really breaks Newton's idea that time
Speaker 2: is universal.
Speaker 4: Okay, but so on our planet we have people who
Speaker 4: travel in super fast jets, but we're all able to
Speaker 4: stay on the same clock without feeling confused. Are we
Speaker 4: just not going fast enough for this to be a problem?
Speaker 4: You know, the Andromeda Galaxy version of Daniel and Kelly's
Speaker 4: Extraordinary Universe. Could we schedule a time to interview them
Speaker 4: or no? Because we would never be able to figure
Speaker 4: out what time we meant.
Speaker 2: Well, first of all, it would take them millions of
Speaker 2: years to answer our email, right because and drama is
Speaker 2: still millions of years. So stay tuned, everybody. Let's hope
Speaker 2: this podcast lasts that long. That would be awesome. It
Speaker 2: also be a slow conversation the reason all.
Speaker 4: Right, so not fast paced storytelling.
Speaker 2: But you make a really good point, which is that
Speaker 2: we don't notice these effects our clocks. To get out
Speaker 2: of sync because somebody got on a train or an airplane.
Speaker 2: And that's because these effects depend on the velocity and
Speaker 2: they're mostly ignorable unless you get up to like seventy
Speaker 2: eighty ninety percent of the speed of light. It turns
Speaker 2: out that this is the way time actually works. This
Speaker 2: is the fundamental nature of time. But if you're going slow,
Speaker 2: those effects are very small, and you can basically just
Speaker 2: assume that time is the same for everybody. So Newton's
Speaker 2: idea works except when you're going really really fast. And
Speaker 2: because nobody's really ever going that fast, Newton didn't notice,
Speaker 2: and nobody noticed for a long long time that this
Speaker 2: was the true effect of time. So time is almost universal,
Speaker 2: but moving really really fast breaks it.
Speaker 4: So clearly, the most important question that we're going to
Speaker 4: tackle today is it possible to travel back in time?
Speaker 4: And have we learned anything yet that helps us think
Speaker 4: about that question.
Speaker 2: Yeah, this might give you the impression like, wow, time
Speaker 2: is sort of controllable. I can control whether I see
Speaker 2: things happening at the same time or not. I can
Speaker 2: even like reorder things. You know, if you're moving at
Speaker 2: one speed, you can see the light hit the front
Speaker 2: of the train first, and then the back of the train. Second,
Speaker 2: if you're moving at another speed, you can see the
Speaker 2: opposite order of events. Right, the light hits the back
Speaker 2: of the train first and then the front of the train,
Speaker 2: And that sort of boggles the mind. It makes you
Speaker 2: feel like I have control over the order of events
Speaker 2: in the universe. Newton was like A, then B, then C,
Speaker 2: then D in Einstein's like, no, if you're going fast enough,
Speaker 2: then you can have D before C. And you're like, what,
Speaker 2: that's crazy. And at least the questions like you just asked,
Speaker 2: like can we control the flow of time? And can
Speaker 2: we even make it go backwards? And so there are
Speaker 2: limitations to what Einstein lets you do. Einstein says, yes,
Speaker 2: the order events depends on your speed. But there's a
Speaker 2: limitation to how fast you can go in the universe, right,
Speaker 2: And you can only go the speed of light, and
Speaker 2: that limits how much you can reorder events.
Speaker 4: So, if I could move at the speed of light,
Speaker 4: could I have caught Ada before she fell? Less of
Speaker 4: a time question and more of a Kellius Sluss like question.
Speaker 2: No, that's a great question. If you could go faster
Speaker 2: than the speed of light, you would have all sorts
Speaker 2: of causal contradictions. If you go faster than the speed
Speaker 2: of light, then for example, you could see the light
Speaker 2: hit the end of the train before it even leaves
Speaker 2: the light bulb. Right, there's a limitation there. Light will
Speaker 2: let you reorder those events, but not in a way
Speaker 2: that breaks causality, So you can't make something happen first
Speaker 2: which is causally connected to something else in the future.
Speaker 4: That almost sounds like there's a little bit of time
Speaker 4: travel possible, but within some bounds of causality.
Speaker 2: But if I misunderstanding now, you can rearrange the order
Speaker 2: of events in the universe. You can make one thing
Speaker 2: happen before another thing, and somebody else can disagree, which
Speaker 2: is kind of crazy.
Speaker 6: You know.
Speaker 2: You can imagine like two people running a race and
Speaker 2: people having different opinions about like who wins the race,
Speaker 2: and you feel like, no, somebody is faster. Well the
Speaker 2: answer is there isn't really anybody faster. But yes, there
Speaker 2: are limited like you can't make the runners reach the
Speaker 2: finish line before they leave the starting line. And it
Speaker 2: might sound like, hey, I'm just adding an exception to
Speaker 2: fix physics so that it makes sense, but this really
Speaker 2: is systematized, like the things that you can reorder are
Speaker 2: things that are like outside of your sphere of influence.
Speaker 2: We talked earlier about how like, if you shoot a
Speaker 2: light rate at Andromeda, you can't hit the alien version
Speaker 2: of Daniel and Kelly and Anddrameda right now. You can't
Speaker 2: hit them for millions of years because it takes light
Speaker 2: to get there. So this is like sphere of influence.
Speaker 2: You have control over. Anything you have control over, you
Speaker 2: can't reorder, right because that would break causality. Anything outside
Speaker 2: your light cone you can reorder, so you can move
Speaker 2: stuff around that you don't have influence over because it's
Speaker 2: not constantly connected to you.
Speaker 4: All right, So what's the coolest thing you could do
Speaker 4: with that power? What is our superhero movie going to
Speaker 4: be about?
Speaker 1: Oh?
Speaker 2: Man, I think the coolest thing you can do is
Speaker 2: be younger than your twin. You know, you can actually
Speaker 2: like fly in a spaceship at a high speed, turn around,
Speaker 2: come back and find that a lot of time has
Speaker 2: passed on Earth and you're still young. And that's pretty cool.
Speaker 2: That seems so toxy. That's basically time travel into the future, right,
Speaker 2: and that you could actually do and they've done that.
Speaker 2: You know, there are twin astronauts, and one of them
Speaker 2: was in space.
Speaker 1: For a long time.
Speaker 2: It moved at a pretty high speed, and now his
Speaker 2: twin is like three minutes.
Speaker 4: Older, got it, And that doesn't break causality even though
Speaker 4: they were born at the same time, because they both existed.
Speaker 2: And because they were separated in space, okay, right, and
Speaker 2: so their time was flowing different and they were far
Speaker 2: apart from each other, so they couldn't break causality. They
Speaker 2: couldn't do things that influenced each other.
Speaker 1: Yeah, exactly.
Speaker 2: You can't have time to flow differently if you're in
Speaker 2: the same place. So if those twins like never separated,
Speaker 2: then their clocks would be inextricably linked.
Speaker 4: All right, Okay, all right, I think we could probably
Speaker 4: pitch that to Hollywood. So this is the general relativity
Speaker 4: understanding of things, right, because we're talking about Einstein.
Speaker 2: This is the special relativity. Einstein at two theories of relativity.
Speaker 2: One is special relativity has to do with the speed
Speaker 2: of light and clocks and all that kind of stuff
Speaker 2: that we've been talking about. But that's assuming that space
Speaker 2: is smooth and flat. There's no like weird curvature to it.
Speaker 2: There's no mass in the universe. Things get even weirder
Speaker 2: when Einstein allows us to add mass to the universe.
Speaker 4: And we talked last time about how space can bend.
Speaker 4: So if you're going around to space bends, how does
Speaker 4: that change time?
Speaker 2: So you're right earlier that we're connecting time and space
Speaker 2: in an important way.
Speaker 1: Right.
Speaker 2: The way that time flows depends on where you are
Speaker 2: in space, and we link them together like space time.
Speaker 2: We have this four dimensional construct and that's really important
Speaker 2: in general relativity, and sometimes it makes people think, well,
Speaker 2: time is part of space time, so maybe time is
Speaker 2: like a dimension the way space is, right, and that's
Speaker 2: very tempting because when you look at general relativity, you
Speaker 2: see that Einstein just treats time as like the fourth
Speaker 2: dimension of space. But we also know that time is
Speaker 2: different from the fourth dimension. It's not just like xyzt
Speaker 2: another fourth dimension operates the same way as you were
Speaker 2: saying earlier, like you can't go backwards in time, you
Speaker 2: can't revisit the same location in time, and general relativity
Speaker 2: respects this. There are a spacelike and timelike coordinates, and
Speaker 2: it tells us that time ticks very differently, but there
Speaker 2: are a lot of similarities for example, time is also
Speaker 2: affected by mass. Like you were saying you put mass
Speaker 2: in space, space bends and as we were saying last time,
Speaker 2: like photons curve around masses, Well, mass also bends time.
Speaker 2: It's not just space that gets bent, but time gets bent.
Speaker 1: Like, well, what does that mean.
Speaker 2: It's like a set of words that I don't know
Speaker 2: how to process right practically, what it means is that
Speaker 2: clocks tick slower near masses. So if you're near a
Speaker 2: black hole, for example, your clock will tick slower than
Speaker 2: someone who's further from a black hole.
Speaker 4: All right, well, I think the one thing my brain
Speaker 4: is having trouble with is like I'm picturing a clock
Speaker 4: from my childhood with the two hands, and I'm like,
Speaker 4: is it moving slower because the arms are getting pulled
Speaker 4: on it in a different way. But that's not the
Speaker 4: right way to think about it.
Speaker 2: Yeah, it would work with any clock exactly, because the
Speaker 2: laws of physics themselves slow down, and it depends here
Speaker 2: on the observer, Like if you're looking at that clock,
Speaker 2: you always experience time at the same rate, Like for you,
Speaker 2: it's one second per second, no matter what. But say
Speaker 2: you're orbiting near a black hole and I'm further away
Speaker 2: from the black hole, and I'm looking at you through
Speaker 2: a telescope. I'm comparing my clock to your clock. I'm
Speaker 2: going to see your clock ticking slower than mine. Every
Speaker 2: time your clock ticks one second, my clock is going
Speaker 2: to tick two. The amazing thing is in general relativity,
Speaker 2: you and I will agree on this. If you have
Speaker 2: a telescope and you're looking back at me, then you're
Speaker 2: going to say, oh, every time my clock ticks, Daniel's
Speaker 2: clock ticks twice. Right, So you and I agree that
Speaker 2: my clock is ticking faster than yours. The time is
Speaker 2: going slower for you, which means that the way to
Speaker 2: get into the future is like orbit a black hole
Speaker 2: and you'll see the future of the universe. Like all
Speaker 2: the clocks in the universe will fast forward. So you're curious,
Speaker 2: like what's going to happen in a billion years or
Speaker 2: a trillion years the universe? Boom, Just go orbit a
Speaker 2: black hole and you will be fast forwarded to the
Speaker 2: future of the universe.
Speaker 4: All right, we made it to my goal. Then okay, oh,
Speaker 4: I'm so sorry. You think I'm late. Your a clock
Speaker 4: must be taking faster than mine. I got here at
Speaker 4: the right time.
Speaker 2: For me, and this is something we actually can measure.
Speaker 2: You know, we haven't visited a black hole, of course,
Speaker 2: so we don't see really dramatic effects of this, but
Speaker 2: we can tell that this is happening. People done these
Speaker 2: amazing experiments with have super precise clocks, like one on
Speaker 2: the ground and one just like a couple of meters
Speaker 2: above the ground. And because the second clock is further
Speaker 2: from the Earth, it's experiencing less curvature of space and
Speaker 2: so it's not slowed down as much. And satellites in
Speaker 2: space also have super accurate clocks, and we can tell
Speaker 2: that those are going faster than the ones on the
Speaker 2: surface of the Earth. And in fact, our GPS is
Speaker 2: so precise that it has to take this into account,
Speaker 2: the fact that time clicks slower in space.
Speaker 1: I bet you didn't know that.
Speaker 4: I did know that. I learned that when I was
Speaker 4: an adult, and that was the first moment where I
Speaker 4: thought to myself, all right, these like esoteric conversations about
Speaker 4: space and time are important because there are practical things
Speaker 4: that have to change because of these things. It's not
Speaker 4: just like I smoked too many banana peels. It's like, no, no,
Speaker 4: this stuff is really happening.
Speaker 2: I always felt like maybe that was a good excuse
Speaker 2: for why space projects are so delayed, Like, hey man,
Speaker 2: everybody knows time ticks slower in space, right, So that's
Speaker 2: why this NASA mission is five billion over budget and
Speaker 2: ten years.
Speaker 4: Oh I don't think that's going to fly with Congress.
Speaker 4: All right, let's take a little break, and when we
Speaker 4: come back, we're going to tackle the big question about
Speaker 4: is there even a beginning to time? All right? So Daniel,
Speaker 4: time is different depending on like where you are, how
Speaker 4: fast you're going, is the space around you. What does
Speaker 4: all of this tell us, if anything, about whether or
Speaker 4: not there was a point where time began, or has
Speaker 4: the universe been around for like infinite time.
Speaker 2: Yeah, again, we can rely on the fact that time
Speaker 2: and space are connected to get some insight into this,
Speaker 2: because one thing we learned about one hundred years ago
Speaker 2: is that the universe is expanding, right, It's not just statics,
Speaker 2: not just galaxies hanging there in space. It's expanding. And
Speaker 2: that means we can run the clock backwards and say, oh,
Speaker 2: the universe used to be denser.
Speaker 1: We keep running the.
Speaker 2: Clock backwards and backwards and backwards, the universe gets to
Speaker 2: some like crazy high densities, like filled with plasma, and
Speaker 2: you keep pushing it further and further back in time,
Speaker 2: and you get to some moment when the universe is
Speaker 2: so dense, so jam full of stuff that our theories
Speaker 2: don't work anymore. Like quantum physics and gravity and all
Speaker 2: that stuff. We don't understand how to do those calculations.
Speaker 2: But if you ignore the quantum physics and you just
Speaker 2: said I'm just going to keep pushing and keep squeezing
Speaker 2: things down, that eventually what you get to is general
Speaker 2: relativity predicting a singular. This is not the Big Bang
Speaker 2: that a lot of people have in their minds. The
Speaker 2: big Bang is from that moment we can't understand when
Speaker 2: things are super dense and unique quantum gravity. It's that
Speaker 2: expansion forward before that we don't know what happens. But
Speaker 2: we can just say, oh, let's assume general relativity is correct,
Speaker 2: even though we know that probably isn't at this moment,
Speaker 2: and you can extrapolate back to this moment of incredible density,
Speaker 2: this early universe singularity. So general relativity tells us that
Speaker 2: there was a moment when the whole universe was a singularity,
Speaker 2: and that that was the birth of space and time.
Speaker 4: All right, That feels like a very satisfying answer. So
Speaker 4: of course it's got to get a little bit more complicated,
Speaker 4: which is a okay, all right, so we've got this set.
Speaker 4: One answer from general relativity is there's a point when
Speaker 4: time starts. That is like kind of confusing. If time
Speaker 4: had a moment where it started, what was before that?
Speaker 4: But I guess the point is there was nothing before
Speaker 4: we Yeah what does that mean?
Speaker 2: Yeah, it's really confusing, and even for people who like
Speaker 2: understand general relativity, it's very confusing and for the general public,
Speaker 2: like there's a lot of also mis explained ideas out there.
Speaker 2: You know, people think about the beginning of the universe
Speaker 2: as the singularity is a point in space. Really it's
Speaker 2: a singularity in time. You know, it's this moment when
Speaker 2: everything was incredibly dense and filling the universe. But nobody
Speaker 2: really knows what it means for time to begin there
Speaker 2: because beginning even means like now you're ordering events and
Speaker 2: there's a before and after, and how do you have
Speaker 2: a moment before time begins?
Speaker 1: Right?
Speaker 2: Like, without time, nothing can change. So if the universe
Speaker 2: was in some state before time begin how did it
Speaker 2: go from that state to a different state that requires
Speaker 2: change right, which seems to require time. Nobody understands it.
Speaker 2: And fundamentally the problem is, we know this is not
Speaker 2: the true story of the universe. This requires an extrapolation
Speaker 2: of general relativity beyond where we know it's true. You know,
Speaker 2: we think back into the early universe when things get
Speaker 2: denser and denser in death answer, and at some point
Speaker 2: we can't ignore quantum mechanics anymore. General relativity doesn't align
Speaker 2: with our quantum mechanical understanding of the universe, and we're
Speaker 2: just ignoring it, and we're extrapolating something beyond where we
Speaker 2: know it's reasonable. Like if you took your daughter's growth
Speaker 2: chart that you probably mark on the wall, and you say, oh, look,
Speaker 2: she's growing six inches a year. And I asked you,
Speaker 2: how tall is she going to be when she's eighty Kelly,
Speaker 2: And you said, oh, she grows six inches a year,
Speaker 2: so she's going to be forty feet high. Like that's nonsense.
Speaker 2: You and I both know that's nonsense. Doesn't make sense
Speaker 2: to dig into the philosophical implications of it. It's the
Speaker 2: same thing with the early universe. We know general relativity
Speaker 2: is wrong about what happened to the very early universe,
Speaker 2: and so to ask, like, what does it mean for
Speaker 2: time to begin? Is like asking what does it mean
Speaker 2: to be forty feet tall when you're eighty years old?
Speaker 4: Was this a bit of a like false setup because
Speaker 4: general relativity wasn't meant to be extrapolated back that far.
Speaker 4: And so if you asked a physicist, what does general
Speaker 4: relativity tell us about the start of time? You'd be like, no,
Speaker 4: not a relevant question.
Speaker 2: I'd say, can't answer that question. We don't know the answer.
Speaker 2: And we hope eventually to integrate general relativity with quantum mechanics,
Speaker 2: which has a very different concept of time, and figure
Speaker 2: it all out. But you can't use generativity by itself
Speaker 2: to answer this question. There just isn't an answer, even
Speaker 2: though it sounds sexy, and people talk about like the
Speaker 2: beginning of time is like the north pole. With this,
Speaker 2: you can't go further north. I think that's a distraction
Speaker 2: from the fact that we really don't know what happened
Speaker 2: in the early universe. And the reason is that we
Speaker 2: know there were quantum mechanical effects, and that again, quantum
Speaker 2: mechanics has a very different conception of time.
Speaker 4: Okay, so let's start talking about quantum mechanics. Then it
Speaker 4: just to jump to the chase. Is this going to
Speaker 4: give us an answer that we feel pretty good about
Speaker 4: when we get to the end here, or is this
Speaker 4: another quantum mechanics breaks down at that point.
Speaker 2: No, I think you basically cooked all the physics when
Speaker 2: you said, hey, this feels like a false setup. Most
Speaker 2: of physics is like, hey, let's understand the universe.
Speaker 1: Say.
Speaker 2: The answer is, we don't understand the universe. Sorry, guys,
Speaker 2: but maybe along the way we'll learn something. Okay, So
Speaker 2: quantum mechanics has a very different view of what time is. Right,
Speaker 2: put everything we've just been talking about aside. Imagine you
Speaker 2: had a completely different set of physics, group of people
Speaker 2: you locked in a completely separate room, came up with
Speaker 2: a very different idea about how the universe worked and
Speaker 2: what time is. And that's basically what quantum mechanics is.
Speaker 2: It's like a completely separate thread. It doesn't integrate nicely
Speaker 2: with anything we've been talking about, but we also know
Speaker 2: it's pretty accurate and in quantum mechanics, time is like
Speaker 2: a parameter. Quantum mechanics says there's space, and on top
Speaker 2: of that there are these weird quantum fields, and time
Speaker 2: is like a knob that you can turn and you
Speaker 2: can see things change, and we have equations that describe
Speaker 2: how things change. But for quantum mechanics, there's no connection
Speaker 2: between space and time, like space is where you put
Speaker 2: the things, and time is this parameter that tells you
Speaker 2: how things change. So whereas in relativity and Einstein tells
Speaker 2: the space and time are deeply connected and even interwoven
Speaker 2: because clocks move differently at different points, quantum mechanics is
Speaker 2: like nab or whatever. Let's just have space bio thing
Speaker 2: and time be its own thing.
Speaker 4: So because you can separate them, does quantum mechanics have
Speaker 4: a different answer to the time travel question.
Speaker 2: It doesn't have a different answer to the time travel
Speaker 2: question because there's still a sense of causality in quantum mechanics.
Speaker 2: People seeing like, oh, quantum mechanics is random and therefore
Speaker 2: the universe is nonsense. That's a bit too far right.
Speaker 2: Quantum mechanics says that the universe isn't deterministic. It's not
Speaker 2: that the present completely determines the future, but the presence
Speaker 2: still determines what's possible in the future. You know, it
Speaker 2: gives you a set of probabilities for what could happen
Speaker 2: in the future. So while the future isn't completely determined
Speaker 2: in quantum mechanics, it's still controlled by the past, and
Speaker 2: that means causality is still important and you can't go
Speaker 2: back in time and kill your grandfather. But it also
Speaker 2: tells us something else about the past, because in quantum
Speaker 2: mechanics is a really important principle that information is never destroyed,
Speaker 2: which means that the present determines what's possible in the future.
Speaker 2: Right That means that you can look at the future
Speaker 2: or any moment in the present, and you can tell
Speaker 2: what happened in the past, because every moment is uniquely determined.
Speaker 2: So this present, the moment we're having in the universe
Speaker 2: right now, encodes the entire history of the universe in
Speaker 2: it because there's only one way to get to this present.
Speaker 2: There's one unique past that leads to this present. So
Speaker 2: if you look at all the details of this present,
Speaker 2: you could tell how we got here.
Speaker 4: But that doesn't necessarily mean you personally can figure it out.
Speaker 4: Like if there's a ball in the middle of the room,
Speaker 4: you don't know if it rolled from the door to
Speaker 4: the middle of the room or from the opposite wall
Speaker 4: to the middle of the room. It's just there's only
Speaker 4: one way it could have happened, but you don't necessarily
Speaker 4: know which one it is.
Speaker 2: Yeah, in principle you could figure it out, but you
Speaker 2: need an extraordinary amount of information and infinite computing time.
Speaker 2: There's actually a pretty cool TV show called Devs that
Speaker 2: use this principle and they try to reconstruct like historical
Speaker 2: events to like view the crucifixion of Jesus from the
Speaker 2: air molecules outside, because in principle they're encoding everything that's
Speaker 2: ever happened, which is kind of cool, you know, but
Speaker 2: totally impractical. It's basically impossible. But what it means philosophically
Speaker 2: is that the universe can't destroy information. Information is constant,
Speaker 2: and that means that there has to be an infinite past.
Speaker 2: It means that time can't have a beginning because this
Speaker 2: information has to be propagated forward moment by moment by
Speaker 2: moment by moment. Is time is a continuous parameter, and
Speaker 2: so in quantum mechanics, it makes most sense for the
Speaker 2: universe to have always existed and to always exist because
Speaker 2: this is this continuous flow of information. It can't be destroyed.
Speaker 2: It can't just come from nowhere. It's created by the past.
Speaker 2: And so you can take the present moment and you
Speaker 2: can evolve it backwards using the equations of quantum mechanics
Speaker 2: to any moment in the past, right, And so there
Speaker 2: has to be those moments in the past. That's very
Speaker 2: much in contrast to what general relativity says that you
Speaker 2: can have this like singularity in time where space and
Speaker 2: time begin. Both of these are probably wrong, right, there's
Speaker 2: some future theory of quantum gravity where somebody is like,
Speaker 2: figured this out and will in this together and made
Speaker 2: sense of this, and both of these ideas quantum mechanics
Speaker 2: and general relativity will later see the way we look
Speaker 2: at Newton's idea of gravity, we're like, well, you know
Speaker 2: that mostly works under certain circumstances, but it's not the
Speaker 2: way things fundamentally operate. It's not the true story of
Speaker 2: the universe. It's just it works if you don't have
Speaker 2: the full.
Speaker 4: Picture quantum mechanics or general relativity. Does one of them
Speaker 4: give us a practical understanding of time that we can
Speaker 4: do things with better than the other, Or is that
Speaker 4: not a reasonable question, because it depends on the scale,
Speaker 4: and they both tell us practical things depending on what
Speaker 4: scale we're asking the questions.
Speaker 2: Yeah, that's exactly right. They both tell us practical things
Speaker 2: depending on the scale. You want to talk about particles,
Speaker 2: you just ignore general relativity, and you can make amazingly
Speaker 2: accurate predictions for what electrons are going to do milliseconds
Speaker 2: apter u zap them with a laser. You want to
Speaker 2: talk about asteroids and whether they're going to hit the Earth,
Speaker 2: you ignore quantum mechanics, and you can make very accurate
Speaker 2: predictions for when and that rock is going to swing
Speaker 2: around the Sun and hit the Earth or miss the Earth,
Speaker 2: and how much you have to deflect it. So, yeah,
Speaker 2: it depends on the question you're asking. And that's the
Speaker 2: frustrating thing is that these two things hardly ever talk
Speaker 2: to each other. They disagree vehemently about the way the
Speaker 2: universe works, but usually one is irrelevant, and so we
Speaker 2: never get to see which one is right which one
Speaker 2: is wrong, because they basically always one of them taps
Speaker 2: out and the other one steps in.
Speaker 4: Okay, they're like the best wrestling team ever. So if
Speaker 4: I'm understanding things correctly, both of the theories say that like,
Speaker 4: for the most part, time is moving forward, and you
Speaker 4: can move a little bit faster and tinker with some things,
Speaker 4: but you can't break causality, and so they both are
Speaker 4: like no time is like a forward thing.
Speaker 2: It's interesting because both theories have time in them and
Speaker 2: they're very different conceptions of time. But the reason why
Speaker 2: time moves forward is actually still kind of a deep mystery.
Speaker 2: Like if you look at the quantum mechanics view of things,
Speaker 2: a lot of the r quantum mechanics work the same way.
Speaker 1: Forwards and backwards.
Speaker 2: Like we have this Shortener equation that tells you, if
Speaker 2: you have the present, what's going to happen in the future.
Speaker 1: It also works backwards.
Speaker 2: That's why it's symmetric, Right, you can take the present
Speaker 2: and say, oh, what had to happen in the past
Speaker 2: in order to get here in the present. So the
Speaker 2: equations work the same way forwards and backwards. And in
Speaker 2: most situations, if you just like watched a video of
Speaker 2: particles interacting, you couldn't tell whether somebody was playing the
Speaker 2: tape forwards or playing the tape backwards just by looking
Speaker 2: at what happened, because the same rules apply the rules
Speaker 2: are symmetric in most cases.
Speaker 1: It's like if.
Speaker 2: Somebody's bouncing a ball and there's no air resistance and
Speaker 2: no friction, whatever ball is going to go down, hit
Speaker 2: the floor, come back up to your hand. You play
Speaker 2: that video backwards, it looks the same, right, And so
Speaker 2: if I play that video, you can't tell whether I
Speaker 2: flipped it or not. That's sort of a big puzzle
Speaker 2: about physics is that it works the same way forwards
Speaker 2: and backwards. So like, why is time go forwards or
Speaker 2: in one direction that we call forwards? Why isn't it
Speaker 2: going the other direction?
Speaker 4: So is answer to to my question I could tell
Speaker 4: someone like I'm sorry, I'm more in the quantum mechanics universe,
Speaker 4: where time sometimes moves backwards and it's the same and
Speaker 4: you're just confused time on time.
Speaker 2: It's okay, yeah, we made this appointment back in time.
Speaker 2: Actually you went to this time. That's right, exactly. No,
Speaker 2: it's a big puzzle in physics about why time moves forward,
Speaker 2: and so you can always tell your friend like even
Speaker 2: physicists don't understand how time works, so don't worry about.
Speaker 1: It so much.
Speaker 2: But there's a lot of talk in popular science about
Speaker 2: how the direction that time moves being explained by the
Speaker 2: second law of thermodynamics. We can outline that argument, but
Speaker 2: I don't think it works as powerfully as a lot
Speaker 2: of people think it does. The argument basically is like, yeah,
Speaker 2: at the particle level, everything seems symmetric, but then when
Speaker 2: you zoom out and you're like watching stuff happens, there's
Speaker 2: this thing we can calculate called entropy, which is kind
Speaker 2: of like a measure of the disorganization of the universe,
Speaker 2: the messiness of it. And as time goes on, this
Speaker 2: entropy increases. You know, so for example, you run a refrigerator.
Speaker 2: You're cooling the inside of the refrigerator. That decreases the entropy,
Speaker 2: but you got to also run the compressor to run
Speaker 2: the refrigerator. That creates heat on the outside, and it
Speaker 2: more than compensates. And so anytime you try to decrease entropy,
Speaker 2: you make things like more ordered. You're in the end
Speaker 2: increasing entropy, And if you look at the universe, this
Speaker 2: just keeps going. And so the argument is like, oh,
Speaker 2: maybe time is entropy increasing. This is the one part
Speaker 2: of physics that seems to prefer one direction to the other.
Speaker 1: That's the typical.
Speaker 4: Argument, and do you buy it.
Speaker 2: I don't really buy it for a couple of reasons.
Speaker 2: Number one is that the premise is wrong. You know,
Speaker 2: the argument that like, at the particle physics level, everything
Speaker 2: is perfectly symmetric isn't actually true. There are some particle
Speaker 2: physics processes that operate differently in time forwards and backwards.
Speaker 2: This has to do with violation of some fundamental symmetries
Speaker 2: amid a video actually with veritassium a few years ago.
Speaker 1: You can look it up.
Speaker 2: It's called this particle breaks time for some details about that.
Speaker 2: So so even at the particle physics level, it's not
Speaker 2: one hundred percent true the time is the same forwards
Speaker 2: and backwards. But my real complaint about this argument is that, yeah,
Speaker 2: it shows us that there's a connection between entropy and time.
Speaker 2: Entropy seems to go up as time goes forward. But
Speaker 2: to me, that doesn't tell you why time goes forward.
Speaker 2: Just says, look, if time is going to go forward,
Speaker 2: then entropy is going to go up. If time is
Speaker 2: going backwards, then entropy would be decreasing.
Speaker 1: Right.
Speaker 2: It doesn't tell you why time has to go one way,
Speaker 2: just says these two things are connected. That's not enough
Speaker 2: to drive the arow of time forward. It just says, yeah,
Speaker 2: the universe would be different if it went backwards.
Speaker 4: Are these the kind of things that physicists discuss at
Speaker 4: the bars late into the night at your conferences.
Speaker 2: I think most physicists honestly would run screaming from these
Speaker 2: kind of conversations. But you know, there's some category of
Speaker 2: physicists who like the philosophical implications of these things. For me,
Speaker 2: I'm into these questions in physics not just because hey,
Speaker 2: I want to build a faster iPhone and I want
Speaker 2: to be a master of the universe, but because they
Speaker 2: brush up against these deep questions about like what is
Speaker 2: the universe and how does it work?
Speaker 1: And is the whole thing an illusion?
Speaker 2: And in the one hundred years and we're going to
Speaker 2: pull back the veil and discover the things work fundamentally
Speaker 2: differently from the way we experienced, or are we going
Speaker 2: to meet aliens and they're gonna be like, ha ha ha.
Speaker 2: You guys still think that that's hilarious, how cute?
Speaker 1: You know?
Speaker 2: This is what I want And so to me, these
Speaker 2: philosophical questions the other reason I got into physics, and
Speaker 2: anytime the physics bumps up against those I get excited
Speaker 2: while everybody else runs screaming. But you know, I'm not
Speaker 2: alone in this. You know, folks like Sean Carroll and
Speaker 2: Carlo Ravelli and a bunch of folks that are interested
Speaker 2: and have done a really important work on what all
Speaker 2: these concepts and physics mean for time and the direction
Speaker 2: it flows and could we actually build a time machine?
Speaker 4: And so the physicists who aren't interested in these questions,
Speaker 4: what is their reason for not being interested? Or is
Speaker 4: that not fair? You're not in their heads, you don't know.
Speaker 2: No, that's a really fine question, you know. I feel
Speaker 2: like one of the wonderful things about science is that
Speaker 2: there's so many opportunities to get excited, and it's just
Speaker 2: very personal, you know.
Speaker 1: Like even within article.
Speaker 2: Physics, there are folks who like to build the detectors
Speaker 2: and they think it's really fun to get greasy and
Speaker 2: climb around on those things and like figure out how
Speaker 2: to line everything up and use a laser beam to
Speaker 2: calibrate everything. And there are other folks who like to
Speaker 2: write computer programs to analyze the data, and to them,
Speaker 2: that's the jam. And everybody finds a different thing to
Speaker 2: be excited about, which means that like science takes all kinds,
Speaker 2: you know, and everybody thinks that their bit is the
Speaker 2: most important, the most exciting bit, and everybody else is
Speaker 2: just like you know, box checking and stamp collecting. And
Speaker 2: that's fine because that means everybody gets to do the
Speaker 2: fun part, you know, and the most important part. The
Speaker 2: truth is, every part of it is important. The engineering,
Speaker 2: the physics, the philosophy, the biology. It's all important stuff.
Speaker 2: Everybody's contributing, and I just think it's great that everybody
Speaker 2: finds their own bit to be the best bit.
Speaker 4: As a biologist, you don't have to convince me that
Speaker 4: diversity is important. Intellectual diversity is important too, exactly.
Speaker 2: And so you know, the big picture to physics answer
Speaker 2: about what time is? Boy, we have a few ideas,
Speaker 2: and some of them seem to work pretty well, and
Speaker 2: let us do all sorts of things like design interconnected
Speaker 2: global position systems that work within centimeters and shoot laser
Speaker 2: beams across fast distances, all sorts of stuff, but we
Speaker 2: don't really understand what time is. We know that it flows,
Speaker 2: we know it's connected to space, we don't understand that
Speaker 2: in detail. We have a lot to learn about what
Speaker 2: time is.
Speaker 4: So for the kids who are sitting in a room
Speaker 4: their parents are listening to a podcast about what is time,
Speaker 4: you could answer this question and you could change the
Speaker 4: world if you could figure out what space or time are.
Speaker 4: There's got to be a Nobel prize in there.
Speaker 2: Oh yeah, there's lots of Nobel prizes for even the
Speaker 2: mini steps along the way. Absolutely, kids, there's so much
Speaker 2: left to discover about the universe, to unravel, to figure out.
Speaker 2: We are at the very beginning of understanding the way
Speaker 2: the universe works.
Speaker 1: We've just sort of.
Speaker 2: Like set the stage for the next generation to come
Speaker 2: in and figure it all out.
Speaker 1: That's you.
Speaker 4: We've done some cool stuff, but there's more to do.
Speaker 1: All right.
Speaker 2: Thanks everyone for taking the time to go on this
Speaker 2: journey to some of the deep questions about the nature
Speaker 2: of the universe on this podcast. You know, we think
Speaker 2: the universe is extraordinary and we love to share our
Speaker 2: joy of it with you. If you have questions about
Speaker 2: what you heard today, please don't be shy. I would
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Speaker 2: write to us to questions at Daniel and Kelly dot org.
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