What is time?

00:00:06
Speaker 1: Hey everyone, Daniel here.

00:00:07
Speaker 2: My kids were into fantasy novels a few years ago, and I remember one of them asking me if I've ever seen.

00:00:14
Speaker 1: Magic in real life. I knew what they had in mind.

00:00:18
Speaker 2: Sorcerers and spells, someone turning rocks into frogs or whatever, and so of course my first answer was no. My second thought, since that felt kind of boring and deflating, was to try to help them realize how amazing and wonderful and almost magical our world is. They don't need official magic to appreciate the awesome power of the sun, or the incredible feats of technology inside their phones, or the amazing biomechanics inside their bodies. It's an extraordinary universe out there, and we love to understand it. But that doesn't feel like magic. The scientific view of the world is appreciation through explanation. The thing that makes something magic is that it's not understood. And though it feels rare today to see something truly baffling with your own eyes, it used to be a very common experience. What did prehistoric people, biologically equivalent to us, and with just as much brain power, What did they think when they saw lightning or an eclipse or got sick. To them, it might as well have been magic, because none of it was understood. I think that's kind of what my kids were asking about, and it made me realize that actually, there's still a lot of magic in our lives, because there's plenty of the universe that remains unexplained, Not little details of scientific trivia, but things as basic and as every day as lightning and disease, things so fundamental to our existence that we sometimes don't even realize how magical they are. So here's a magic trick for you. Turn the future into the present, turn the present into the past. The universe is doing that all the time, unintended, and yet we don't really understand it or how it works. And it does it without any apparent lag or delay. So on today's episode, we'll dig deep into the nature of time and explore what physics can tell us about what it is, why we have it, and why it works. Today in the pod, we're asking what is time?

00:02:40
Speaker 1: Hi? I'm Daniel Whitson.

00:02:41
Speaker 2: I'm a particle physicist and a professor at UC Irvine, which means my.

00:02:45
Speaker 1: Time is not my own.

00:02:47
Speaker 3: Hi.

00:02:47
Speaker 4: I'm Kelly Wiener Smith and ever since having kids I never feel like I have enough time, but the time has been higher quality. And welcome to Daniel and Kelly's extraordinary universe where you'll have the time of your life. That was really corny. I should lose my job, but I'm going with it. I'm leaning in. So Daniel, speaking of corny, here is my question for you today. So last episode we talked about space, and this episode we're talking about time. So space time and space time are topics that sci fi deals with a lot. So what is the worst, most inaccurate representation of those concepts in any movie or TV show that you have experienced?

00:03:29
Speaker 1: Wow?

00:03:30
Speaker 2: How much time do you have? We should go a whole lot last episode just on that.

00:03:34
Speaker 1: Oh my gosh.

00:03:35
Speaker 2: I think one of my least favorite trips in science fiction is subspace or hyperspace that you've like dropped into some other kind of space where you can move faster than light or something. And I'm always wondering, like, you know, if space has multiple dimensions, you're still in that other space and you're in this space. What does this even mean? Where are we? What is the physics of this space? To me, it feels like you're just escaping rules and there's always gotta be rules, even in subspace or hyperspace. So yeah, it's pretty hard to watch science fiction as a physicist.

00:04:08
Speaker 4: So it actually like detracts from your enjoyment of it.

00:04:11
Speaker 1: Oh, absolutely it does.

00:04:13
Speaker 2: Yeah, And you know, I don't mind if the rules on the screen are different from the rules in our universe.

00:04:18
Speaker 1: That's awesome, that's cool, that's.

00:04:20
Speaker 2: What I want. But I want them to follow them. I want them to always be rules, because you know, without rules, there's no story, anything can happen, there's no stakes. It's not worth watching.

00:04:29
Speaker 4: So I know this movie was loved by many, but I kind of felt that way about like from dust till Dawn, like everything seemed normal and then vampires and you're like, what, this is not okay with me. Tarantino has more fans than I ever will have, so good for him.

00:04:43
Speaker 2: Maybe we should add vampires to this podcast.

00:04:45
Speaker 4: Oh sure, why not? I loved Anne Rice books when I was a kid. I don't know what that says about me.

00:04:52
Speaker 2: I worked with somebody in educational television who used to work in publishing, and she told me the day she had to leave publishing was the day she pitched a really great book. And the executive told her this is wonderful, but can you add sexy vampires to it? No, she was like, h we're done, yes.

00:05:08
Speaker 4: Up the end. Oh my goodness. Yeah, that's a good time to throw in your head.

00:05:13
Speaker 2: All right, But today on the podcast, we are not talking about sexy vampires or travel through subspace. We're talking about one of the slipperiest questions in all of physics, the kind of question that's hard to answer and hard to ask and hard to think about, which means it's one of the most important questions. It's a question you shouldn't shy away from. It's the kind of question you should dig into, you should confront, you should really try to tackle with everything that you have.

00:05:37
Speaker 4: And that's what we're gonna do today. We're gonna have another brain bending but absolutely critical discussion about something that feels like it makes sense but then doesn't when you dig a little deeper.

00:05:48
Speaker 2: And i'd encourage you to remember that in the history of physics and the history of science, there's been lots of times when a topic has gone from like weird and fuzzy and hard to grapple with to totally understood and like mathematically formulated and that's just the process of science. But the beginning steps, the first bites of the apple are hard, you know. When people were thinking about like what is everything made out of? Or like where did the earth come from? Or where are those shiny dots in the sky. Those were fuzzy questions, and people had pretty silly ideas and went down the wrong path lots of times. But hey, we got there, and we only got there because people took those first bites of the apple. People tried when it was still hard. So that's why we're asking these weird, fuzzy questions about something so basic as time.

00:06:33
Speaker 4: It's exciting. I remember when I was in undergrad and I was learning, you know, so much in my classes, but I started to think about grad school, which wasn't on my radar until I took like thiss ecology class that I loved, And I remember thinking, like a but we figured out so much already, Like what is there left to learn because it's all new when you're an undergrad, and it's like, no, there's so much left to learn and some of it is still fundamental, and I don't know, it's just exciting to know that there's still so much.

00:06:57
Speaker 2: To contribute, absolutely, and some of the things we have left to learn are things that we don't think about because they are just at the base level of our understanding. They're like the assumed context. And later, in one hundred years or a thousand years, when we understand these things better, people are going to look back and be like, Wow, what was it like to be human back then when they thought time worked this way? And now we know what actually is this completely other thing? And ha ha ha, how silly were they?

00:07:22
Speaker 1: Right?

00:07:22
Speaker 2: Yeah, it shapes our very existence and the way we think about our lives, and so it's definitely worth digging into. And if we don't do this work today, then they're not going to figure it out in a thousand years.

00:07:33
Speaker 4: All right, Well, I am hoping that by the end of today's episode I have a science based excuse for why I'm late all the time that has something to do with the confusing nature of time. So first, let's hear what our listeners had to say about what time really is.

00:07:48
Speaker 2: That's right, I went out there into the internet and asked a bunch of folks this hard question, what is time? If you would like to answer questions like this for the podcast. Please don't be shy. Right to us two questions at the day, and Kelly dot Org will hook you up and you can hear your voice on the podcast. In the meantime, here are fuzzy thoughts about a fuzzy question about time.

00:08:09
Speaker 5: Time is represented as its own dimension, like a column in an array. But I just think of it as one thing happening after the other.

00:08:20
Speaker 1: Time is a human experience. Time, I think is a measure of change.

00:08:27
Speaker 6: Time is change made measurable by quantification of entropy and by using some caesium oscillations.

00:08:40
Speaker 4: Time is an artificial.

00:08:45
Speaker 2: Construct humans have created to try and help explain.

00:08:50
Speaker 1: The universe.

00:08:51
Speaker 5: Is the second law of thermodynamics.

00:08:54
Speaker 1: Our perception of change.

00:08:57
Speaker 4: Time is a one way trip into the future.

00:09:00
Speaker 3: The human constructor rived from the sound cycles. I think that time is an abstraction. It's our capability to perceive changes occurring, to make connections between events and their consequences. But I don't believe that time is some sort of substance with an arrow of direction that can be flipped entropy.

00:09:22
Speaker 1: I guess the fourth dimension in Albert Einstein's theory.

00:09:26
Speaker 4: Oh wow, I would say it's how we experience events.

00:09:30
Speaker 1: Happening one after the other in a sequence.

00:09:33
Speaker 5: I'd have to say that it's the result of quantum processes, decisions, random interactions that occur between inside fields. It's kind of a direction of events in succession.

00:09:50
Speaker 4: So before we got to the listener questions, you said, these are fuzzy thoughts about a fuzzy question about time. And I gotta say, I was really impressed by how deep some of the these answers were and how seemingly thought out they were. And I guess how many like polysyllabic words they used in these explanations. That made me think you are kind of aware of the underpinnings of this question. And so yeah, I was impressed with the answers.

00:10:15
Speaker 2: And there's also an extraordinary breadth here. I mean, there are folks who are philosophizing about it it's an artificial construct, and then folks are just trying to describe it. You know, it's a one way trip into the future. People talk about how we measure it. It's really interesting how many different responses there are to this question. They tell you, like how many facets there are to it, and how hard it is to answer a basic question because we don't all necessarily even agree on, like what form the answer is. Like if you ask me what's one plus two? We all know the answer should be a number, right, and maybe we get it right or we can get it wrong. But the answer to the question like what is time? What does that answer look like?

00:10:51
Speaker 1: You know?

00:10:51
Speaker 2: Is it just a description of our experience? Is it a mathematical structure that tells us how time works? Is it a philosophical delve into like why it exists and why the universe needs it, or maybe why it doesn't need it, where it actually comes from, why we have it, and how it began. There's such a breadth of things to explore there.

00:11:11
Speaker 4: Yeah, And I feel like we had pretty much the same conversation when we were talking about space in the prior episode that, like, you can just come at it from so many different angles. There's so many absolutely fundamental things where it's even hard to figure out what kind of question we should be asking.

00:11:26
Speaker 2: That's right, And since this isn't a philosophy podcast or an engineering podcast, we're going to sort of take the middle road. We're not going to describe the details of how caesium atoms measure time or delve into like the philosophical underpinnings of a theory versus b theory. In philosophical questions of time, we're going to talk about the physics of time and what physics tells us about time, because obviously time is really important to physics, and it is deeply woven into how we have made sense of the world so far. And so what we can do is turn that around and say, all right, time is part of our understanding of how the world works. What does that tell us about what time is? And as usual, the answer is going to be a bunch of incoherent, inconsistent ideas that don't really come together because we don't understand it. And so we'll do our best to try to weave that all together into an excuse for Kelly for why she's always late.

00:12:16
Speaker 4: Yes, anyone who's thinking about working with me in the future, I'm actually usually five minutes early that I'm worried about my reputation here. But so for our space episode, the answer depended on whether you are in a quantum mechanics or a general relativity framework. Is that where we're going to get again, there's not just one answer. It depends who you talk to.

00:12:38
Speaker 1: Yes, exactly.

00:12:39
Speaker 2: There's a fascinating evolution of our understanding of time with respect to space and gravity, and that's super interesting and counterintuitive. It's also completely at odds with our understanding of time in a quantum mechanical universe. So, yeah, we're going to have a couple of threads here, we're going to show how they don't tie together. And then on top of that, there's statistical mechanics and particles iss which also have different views on time, and so we hope that one day somebody's being able to pull us all together into one answer. What we have right now are a bunch of threads, none of which are completely satisfactory. And you know, the thing that we're trying to describe in the end is something pretty intuitive, right, it's our daily experience. It's this kind of magic where like we have the present, like right now I'm experiencing something and five seconds ago you were saying something and I was laughing about it, and there's a difference between that. There's like what I'm doing right now and what I was doing and what I will be doing. And we know that there's a difference there. Right when we experience, when we remember when we anticipate, and that's just like our intuitive understanding of what time is. I think that's what we're trying to grapple with.

00:13:43
Speaker 4: I feel like, even in our day to day life, though, time can sometimes feel weird. And this is the biologist in me. You know, my daughter is not very coordinated. And once we were playing a game on a wooden floor where we were kicking a ball around, and she kicked it and one foot went up and landed on the ball, and she stood on top and she started falling forward and she didn't put her arms out to catch her. No, because she's my daughter. And I swear the world slowed down as her teeth hit the wood floor. Oh my gosh, something as easy as time. I feel like I've had jobs where I had like two weeks left and it felt like time had fundamentally changed. But anyway, this is the biologist's perspective and how brains work, I suppose. I guess we're not getting into that today.

00:14:22
Speaker 2: That is a really important angle. And if you've read folks like Daniel Dennett, he suggests that there is no present moment of consciousness, that the only experience we have is actually remembering the immediate past, that your brain assembles the present as an illusion. You never experience it, you just remember it. It's a weird concept. It makes a lot of sense when you're reading the book, and then later you're like, what, how does that possible? Like with a lot of philosophy. So, yeah, that's not something we're going to dive into today, but we will ask questions like, how does time work?

00:14:53
Speaker 1: What is it?

00:14:54
Speaker 2: Can we travel through time? Did time have a beginning? What does that even mean? Why does time seem to only go forward?

00:15:00
Speaker 1: Words, all this kind of stuff.

00:15:02
Speaker 4: All right, So we could have a whole podcast called Daniel and Kelly's Extraordinary Conversations about Time. We could take biology philosophy anyway. But today we're just doing physics, all right. So when we were talking about space, we went through sort of like the history of ideas about space? Where should our history of ideas about time start?

00:15:21
Speaker 1: This time?

00:15:21
Speaker 2: So, like so many things in physics, we got to start with Newton. I mean, people talk about time before Newton, but really Newton was the first person to systematize it and to incorporate it into his physics. And you know, he figured out gravity and the loss of motion and all this stuff so much depends on time. And for Newton, time was absolute. He imagined like a single clock for the whole universe. And the way Newton thinks about time is sort of like the way a movie works. You know, you imagine a movie is like some continuous motion, but really it's a bunch of snapshots.

00:15:56
Speaker 1: Right.

00:15:56
Speaker 2: If you slowed down a movie, you'd see it's a bunch of still frames and those are tied together in a sequence. Right, So you're watching like a whole movie of your daughter falling into the floor. You see her standing on the ball. The next frame she's falling a little closer to the floor, and the next frame, the next frame. Finally there is a frame where like the teeth make contact with the wooden floor.

00:16:14
Speaker 4: Right, yeah, yeah, why did I bring this up? I'm gonna have to relive it so many times?

00:16:19
Speaker 2: And it's a series of moments, right, still moments, and time is what ties those still moments together. In Newton's universe, time is what connects those as well, Like the laws of physics tell us if the universe was at this state in this organization, if your daughter's face was this far above the floor, then when is her face going to impact the floor. The physics predicts the future from the past. It connects all of these into a causal chain, and the whole universe ticks forward together like tic tick. You imagine the whole universe as a still frame, and then you tick it forward, and everything in the universe has taken one time step.

00:16:57
Speaker 4: So intuitively this makes sense to me who was the first person to make it? Confusing but more accurate, Confusing but probably more accurate.

00:17:08
Speaker 2: Let's not confuse people just yet. Let's marinate in how much sense it makes, just for what longer, because it is kind of wonderful, right imagining that far away in Andromeda, you know, the aliens are also rescuing their daughters from wooden floors or whatever. That the whole universe is following some laws of physics, and if you knew the state of the universe, you could predict its future, right. It's an incredible thing about physics. I remember learning it in high school and being like, oh my gosh, physics is the power to predict the future. You know, if I tell you the angle of my cannonball, I know whether it's going to go over that castle wall or not. Because it's described by the laws of physics. The past predicts the future. That's wonderful. It also tells you what can't happen right. It tells you that like your ball can't be here and then suddenly appear.

00:17:52
Speaker 1: On the other side of the wall.

00:17:54
Speaker 2: Or for example, you can't shoot a laser at those Aliens and Andromeda and fry them today because light takes time to get to Andromeda. It's like this sphere of influence you have over the universe. You can't affect things outside that sphere because light only travels at a certain speed and nothing can go faster than that. And so time is intimately connected with how the universe moves forward. And this is a beautiful vision to imagine we're all linked together in time. But Einstein was the first one to make this confusing because he realized that we don't actually have a single clock for everybody in the universe. His theory of special relativity messes that up.

00:18:34
Speaker 4: So for a second there you had me feeling like maybe I shouldn't have studied animal behavior, where everything is confusing and hard to predict, and you don't know if the person's going to catch the ball you were talking about But now you've made me feel like no, I made the right choice because physics is confusing too. Is it confusing in a predictable way or is it just yeah, what did Einstein say?

00:18:56
Speaker 2: So we're not doing quantu mechanics. Yeah, the universe isn't random. We're still dealing with the deterministic, predictable universe where the past perfectly predicts the future. It's just that it predict it differently than what Newton thought. The reason that time is all messed up is because of this amazing experimental fact that Einstein relied on about the speed of light.

00:19:17
Speaker 4: We are taking a break, and when we get back, we'll hear about how Einstein ruined Newton's beautiful idea of time. So we're back. One of the most exciting things to me about physics is how you have these predictions that are sort of counterintuitive but then actually help you really understand complex things and create new technologies and stuff like that. So, all right, you're about to tell us how Einstein came up with this complicated, counterintuitive idea, but it's correct and we use it all the time.

00:20:02
Speaker 2: Yeah, So Einstein's special relativity really messes up our concept of time. Newton's idea that everybody in the universe, can you share the same clock, that you can imagine like a still frame for the whole universe, and then take that forward to the next still frame for the whole universe, and that like physicist and Andromeda and physicists in the Milky Way can agree on how that all works. And this all comes out of a pesky little fact which initially doesn't sound all connected to time and watches and clocks, but it intimately is. And that's about how light works. In the late eighteen hundreds that people did experiments and discovered that light travels at the same speed for all observers. So like if you're holding a flashlight and you turn it on, you measure the speed of the photons will come out to be the speed of light. Now, if you're in a car and you're going sixty miles an hour and you turn the flashlight on, you measure those photons to be moving at the speed of light. Somebody on the ground who isn't in the car, they don't measure the photons to be moving at the speed of light plus sixty miles an hour. They still measure it to be moving at the speed of light anywhere, anybody, everywhere. Everybody always measures the speed of light to be the same, no matter how fast they're going.

00:21:11
Speaker 4: If you were in a car going sixty miles an hour and you took a ball and you threw it out your window, would you get a different answer or is it the same as the speed of light?

00:21:19
Speaker 1: Exactly.

00:21:19
Speaker 2: Balls work very differently than light does. If you're in the car and you throw the ball at sixty miles an hour, you measure it moving relative to you at sixty.

00:21:27
Speaker 1: Miles an hour.

00:21:28
Speaker 2: But if I'm on the ground and the car is moving at sixty miles an hour, I'm going to measure the ball to be going sixty plus sixty at one hundred and twenty miles per hour. That's also true of simpler things like sound. If you're in the car and you shout your daughter's name, like hey, watch out, you know, then that sound moves away from you at the speed of sound relative to the air. So I'm on the ground, I see that shout moving at the speed of sound relative to the air. If you're in the car, you could even catch up to that shout. The speed of that shout for you depends on how fast you're moving through the air. You could even catch up to that shout right, because the speed of sound is not that fast. You could have no speed with respect to your shout. Like if you're traveling at the speed of sound at mock one and you shout, your shout just stays there with you. Whereas photons, they're always moving at the speed of light relative to you. So light is very different from baseballs and from sound and all this stuff. And this says really important consequences for what time is and how we measure it.

00:22:26
Speaker 4: Why.

00:22:27
Speaker 2: Yeah, great question, And to answer it, let me give you an example. This is Einstein's classic example for why the speed of light misses up time. And Einstein isn't thinking about big clocks in the sky. He's just thinking about, like, when does stuff happen at the same time. So imagine instead of in a car, you're standing in a train and you have a flashlight, and you shine a flashlight forwards and backwards, or you turn on a light bulb so it shines in both directions. When is the light going to hit the front of the train and the back of the train. Well, you're in the train, You're in the middle of the car. It's going to hit the front and the back at the same time, right, No big deal. Okay, that's cool, But what about somebody on the ground. You know what, if you're in the train, you turn on your flashlight, you see the light hit the front of the train at the back of the train at the same time. I'm on the ground. I'm not in your train with you. To me, the train is moving past me at one hundred miles per hour or something.

00:23:14
Speaker 1: What do I see?

00:23:16
Speaker 2: I see the light moving forward at the speed of light and backwards at the speed of light. But the back of the train is moving towards the light bulb and the front of the train is moving away from it. So from my perspective, the light hits the back of the train before it hits the front of the train, because the back of the train is rushing to meet the light. So you see these things happening at the same time. The front and the back are simultaneous for you, but they're not simultaneous for me. And that's only because light travels at the same speed for me and for you. We did this example in sound or with baseballs, then we would agree about whether they're simultaneous or not. But because light always travels at the same speed for everybody, we don't agree about whether the events are at the same time.

00:23:59
Speaker 4: Okay, so the first this person turns on their flashlights and the answer is three seconds for forward and for back, and for somebody else the answer is four seconds for forward and two seconds for back.

00:24:09
Speaker 1: Yeah, exactly right.

00:24:10
Speaker 2: And if we did the same experiment with sound, right, if you stood in the center of the car and you shouted instead of turning on a light, then the shout would reach the front and the back of the train at the same time.

00:24:19
Speaker 1: For you, you're on the train, and.

00:24:21
Speaker 2: What would happen for me? For me, sound doesn't have to move at the same speed no matter what. So I would see the sound moving forward faster than I would see it moving backwards, because it's moving with the air inside the train. Right, sound doesn't have to move at the same speed for me, it doesn't have to follow Einstein's weird special rule, so I see it also as simultaneous. I see the front of the train racing away, but the sound is moving faster forward. And I see the back of the train rushing towards you, but the sound is moving slower in that direction, and so it all works out. It's all simultaneous for me and for you for sound. But because light breaks that, it says, I don't care how fast I'm going. Daniel's can to see me go in at the speed of light forward and backwards, And so Daniel doesn't see it simultaneous, and Kelly does.

00:25:06
Speaker 4: And this is something we got to a little bit in the last episode. It sounds like you can't really separate space and time. You need to understand them both. So we have two complicated concepts to try to define together.

00:25:20
Speaker 2: Exactly, and you put your finger on it. Whereas Newton said, look, there's one clock for the whole universe, Einstein says there's one clock at every point in space. Every location in space has a different clock, right, And so I see one clock running fast, you see another clock running slow. It's not just that there's a different clock at every space. How you see those clocks running also depends on your velocity, So there's no sense of universal time. There's not one time for the whole universe. This image we had where like the whole universe is frozen and then it ticks forward. According to physics, Like, if you're going to run a simulation of the universe, that's the way you might do it. That's broken. Now you can't have a single clock for the whole universe. We have to have a different clock at every location, and a different clock for people moving at different speeds. It really breaks Newton's idea that time is universal.

00:26:09
Speaker 4: Okay, but so on our planet we have people who travel in super fast jets, but we're all able to stay on the same clock without feeling confused. Are we just not going fast enough for this to be a problem? You know, the Andromeda Galaxy version of Daniel and Kelly's Extraordinary Universe. Could we schedule a time to interview them or no? Because we would never be able to figure out what time we meant.

00:26:38
Speaker 2: Well, first of all, it would take them millions of years to answer our email, right because and drama is still millions of years. So stay tuned, everybody. Let's hope this podcast lasts that long. That would be awesome. It also be a slow conversation the reason all.

00:26:52
Speaker 4: Right, so not fast paced storytelling.

00:26:55
Speaker 2: But you make a really good point, which is that we don't notice these effects our clocks. To get out of sync because somebody got on a train or an airplane. And that's because these effects depend on the velocity and they're mostly ignorable unless you get up to like seventy eighty ninety percent of the speed of light. It turns out that this is the way time actually works. This is the fundamental nature of time. But if you're going slow, those effects are very small, and you can basically just assume that time is the same for everybody. So Newton's idea works except when you're going really really fast. And because nobody's really ever going that fast, Newton didn't notice, and nobody noticed for a long long time that this was the true effect of time. So time is almost universal, but moving really really fast breaks it.

00:27:41
Speaker 4: So clearly, the most important question that we're going to tackle today is it possible to travel back in time? And have we learned anything yet that helps us think about that question.

00:27:54
Speaker 2: Yeah, this might give you the impression like, wow, time is sort of controllable. I can control whether I see things happening at the same time or not. I can even like reorder things. You know, if you're moving at one speed, you can see the light hit the front of the train first, and then the back of the train. Second, if you're moving at another speed, you can see the opposite order of events. Right, the light hits the back of the train first and then the front of the train, And that sort of boggles the mind. It makes you feel like I have control over the order of events in the universe. Newton was like A, then B, then C, then D in Einstein's like, no, if you're going fast enough, then you can have D before C. And you're like, what, that's crazy. And at least the questions like you just asked, like can we control the flow of time? And can we even make it go backwards? And so there are limitations to what Einstein lets you do. Einstein says, yes, the order events depends on your speed. But there's a limitation to how fast you can go in the universe, right, And you can only go the speed of light, and that limits how much you can reorder events.

00:28:54
Speaker 4: So, if I could move at the speed of light, could I have caught Ada before she fell? Less of a time question and more of a Kellius Sluss like question.

00:29:06
Speaker 2: No, that's a great question. If you could go faster than the speed of light, you would have all sorts of causal contradictions. If you go faster than the speed of light, then for example, you could see the light hit the end of the train before it even leaves the light bulb. Right, there's a limitation there. Light will let you reorder those events, but not in a way that breaks causality, So you can't make something happen first which is causally connected to something else in the future.

00:29:32
Speaker 4: That almost sounds like there's a little bit of time travel possible, but within some bounds of causality.

00:29:39
Speaker 2: But if I misunderstanding now, you can rearrange the order of events in the universe. You can make one thing happen before another thing, and somebody else can disagree, which is kind of crazy.

00:29:48
Speaker 6: You know.

00:29:48
Speaker 2: You can imagine like two people running a race and people having different opinions about like who wins the race, and you feel like, no, somebody is faster. Well the answer is there isn't really anybody faster. But yes, there are limited like you can't make the runners reach the finish line before they leave the starting line. And it might sound like, hey, I'm just adding an exception to fix physics so that it makes sense, but this really is systematized, like the things that you can reorder are things that are like outside of your sphere of influence. We talked earlier about how like, if you shoot a light rate at Andromeda, you can't hit the alien version of Daniel and Kelly and Anddrameda right now. You can't hit them for millions of years because it takes light to get there. So this is like sphere of influence. You have control over. Anything you have control over, you can't reorder, right because that would break causality. Anything outside your light cone you can reorder, so you can move stuff around that you don't have influence over because it's not constantly connected to you.

00:30:44
Speaker 4: All right, So what's the coolest thing you could do with that power? What is our superhero movie going to be about?

00:30:52
Speaker 1: Oh?

00:30:52
Speaker 2: Man, I think the coolest thing you can do is be younger than your twin. You know, you can actually like fly in a spaceship at a high speed, turn around, come back and find that a lot of time has passed on Earth and you're still young. And that's pretty cool. That seems so toxy. That's basically time travel into the future, right, and that you could actually do and they've done that. You know, there are twin astronauts, and one of them was in space.

00:31:15
Speaker 1: For a long time.

00:31:15
Speaker 2: It moved at a pretty high speed, and now his twin is like three minutes.

00:31:19
Speaker 4: Older, got it, And that doesn't break causality even though they were born at the same time, because they both existed.

00:31:25
Speaker 2: And because they were separated in space, okay, right, and so their time was flowing different and they were far apart from each other, so they couldn't break causality. They couldn't do things that influenced each other.

00:31:35
Speaker 1: Yeah, exactly.

00:31:36
Speaker 2: You can't have time to flow differently if you're in the same place. So if those twins like never separated, then their clocks would be inextricably linked.

00:31:43
Speaker 4: All right, Okay, all right, I think we could probably pitch that to Hollywood. So this is the general relativity understanding of things, right, because we're talking about Einstein.

00:31:54
Speaker 2: This is the special relativity. Einstein at two theories of relativity. One is special relativity has to do with the speed of light and clocks and all that kind of stuff that we've been talking about. But that's assuming that space is smooth and flat. There's no like weird curvature to it. There's no mass in the universe. Things get even weirder when Einstein allows us to add mass to the universe.

00:32:15
Speaker 4: And we talked last time about how space can bend. So if you're going around to space bends, how does that change time?

00:32:25
Speaker 2: So you're right earlier that we're connecting time and space in an important way.

00:32:29
Speaker 1: Right.

00:32:29
Speaker 2: The way that time flows depends on where you are in space, and we link them together like space time. We have this four dimensional construct and that's really important in general relativity, and sometimes it makes people think, well, time is part of space time, so maybe time is like a dimension the way space is, right, and that's very tempting because when you look at general relativity, you see that Einstein just treats time as like the fourth dimension of space. But we also know that time is different from the fourth dimension. It's not just like xyzt another fourth dimension operates the same way as you were saying earlier, like you can't go backwards in time, you can't revisit the same location in time, and general relativity respects this. There are a spacelike and timelike coordinates, and it tells us that time ticks very differently, but there are a lot of similarities for example, time is also affected by mass. Like you were saying you put mass in space, space bends and as we were saying last time, like photons curve around masses, Well, mass also bends time. It's not just space that gets bent, but time gets bent.

00:33:33
Speaker 1: Like, well, what does that mean.

00:33:34
Speaker 2: It's like a set of words that I don't know how to process right practically, what it means is that clocks tick slower near masses. So if you're near a black hole, for example, your clock will tick slower than someone who's further from a black hole.

00:33:49
Speaker 4: All right, well, I think the one thing my brain is having trouble with is like I'm picturing a clock from my childhood with the two hands, and I'm like, is it moving slower because the arms are getting pulled on it in a different way. But that's not the right way to think about it.

00:34:01
Speaker 2: Yeah, it would work with any clock exactly, because the laws of physics themselves slow down, and it depends here on the observer, Like if you're looking at that clock, you always experience time at the same rate, Like for you, it's one second per second, no matter what. But say you're orbiting near a black hole and I'm further away from the black hole, and I'm looking at you through a telescope. I'm comparing my clock to your clock. I'm going to see your clock ticking slower than mine. Every time your clock ticks one second, my clock is going to tick two. The amazing thing is in general relativity, you and I will agree on this. If you have a telescope and you're looking back at me, then you're going to say, oh, every time my clock ticks, Daniel's clock ticks twice. Right, So you and I agree that my clock is ticking faster than yours. The time is going slower for you, which means that the way to get into the future is like orbit a black hole and you'll see the future of the universe. Like all the clocks in the universe will fast forward. So you're curious, like what's going to happen in a billion years or a trillion years the universe? Boom, Just go orbit a black hole and you will be fast forwarded to the future of the universe.

00:35:05
Speaker 4: All right, we made it to my goal. Then okay, oh, I'm so sorry. You think I'm late. Your a clock must be taking faster than mine. I got here at the right time.

00:35:13
Speaker 2: For me, and this is something we actually can measure. You know, we haven't visited a black hole, of course, so we don't see really dramatic effects of this, but we can tell that this is happening. People done these amazing experiments with have super precise clocks, like one on the ground and one just like a couple of meters above the ground. And because the second clock is further from the Earth, it's experiencing less curvature of space and so it's not slowed down as much. And satellites in space also have super accurate clocks, and we can tell that those are going faster than the ones on the surface of the Earth. And in fact, our GPS is so precise that it has to take this into account, the fact that time clicks slower in space.

00:35:53
Speaker 1: I bet you didn't know that.

00:35:54
Speaker 4: I did know that. I learned that when I was an adult, and that was the first moment where I thought to myself, all right, these like esoteric conversations about space and time are important because there are practical things that have to change because of these things. It's not just like I smoked too many banana peels. It's like, no, no, this stuff is really happening.

00:36:12
Speaker 2: I always felt like maybe that was a good excuse for why space projects are so delayed, Like, hey man, everybody knows time ticks slower in space, right, So that's why this NASA mission is five billion over budget and ten years.

00:36:24
Speaker 4: Oh I don't think that's going to fly with Congress. All right, let's take a little break, and when we come back, we're going to tackle the big question about is there even a beginning to time? All right? So Daniel, time is different depending on like where you are, how fast you're going, is the space around you. What does all of this tell us, if anything, about whether or not there was a point where time began, or has the universe been around for like infinite time.

00:37:11
Speaker 2: Yeah, again, we can rely on the fact that time and space are connected to get some insight into this, because one thing we learned about one hundred years ago is that the universe is expanding, right, It's not just statics, not just galaxies hanging there in space. It's expanding. And that means we can run the clock backwards and say, oh, the universe used to be denser.

00:37:29
Speaker 1: We keep running the.

00:37:30
Speaker 2: Clock backwards and backwards and backwards, the universe gets to some like crazy high densities, like filled with plasma, and you keep pushing it further and further back in time, and you get to some moment when the universe is so dense, so jam full of stuff that our theories don't work anymore. Like quantum physics and gravity and all that stuff. We don't understand how to do those calculations. But if you ignore the quantum physics and you just said I'm just going to keep pushing and keep squeezing things down, that eventually what you get to is general relativity predicting a singular. This is not the Big Bang that a lot of people have in their minds. The big Bang is from that moment we can't understand when things are super dense and unique quantum gravity. It's that expansion forward before that we don't know what happens. But we can just say, oh, let's assume general relativity is correct, even though we know that probably isn't at this moment, and you can extrapolate back to this moment of incredible density, this early universe singularity. So general relativity tells us that there was a moment when the whole universe was a singularity, and that that was the birth of space and time.

00:38:34
Speaker 4: All right, That feels like a very satisfying answer. So of course it's got to get a little bit more complicated, which is a okay, all right, so we've got this set. One answer from general relativity is there's a point when time starts. That is like kind of confusing. If time had a moment where it started, what was before that? But I guess the point is there was nothing before we Yeah what does that mean?

00:39:00
Speaker 2: Yeah, it's really confusing, and even for people who like understand general relativity, it's very confusing and for the general public, like there's a lot of also mis explained ideas out there. You know, people think about the beginning of the universe as the singularity is a point in space. Really it's a singularity in time. You know, it's this moment when everything was incredibly dense and filling the universe. But nobody really knows what it means for time to begin there because beginning even means like now you're ordering events and there's a before and after, and how do you have a moment before time begins?

00:39:33
Speaker 1: Right?

00:39:33
Speaker 2: Like, without time, nothing can change. So if the universe was in some state before time begin how did it go from that state to a different state that requires change right, which seems to require time. Nobody understands it. And fundamentally the problem is, we know this is not the true story of the universe. This requires an extrapolation of general relativity beyond where we know it's true. You know, we think back into the early universe when things get denser and denser in death answer, and at some point we can't ignore quantum mechanics anymore. General relativity doesn't align with our quantum mechanical understanding of the universe, and we're just ignoring it, and we're extrapolating something beyond where we know it's reasonable. Like if you took your daughter's growth chart that you probably mark on the wall, and you say, oh, look, she's growing six inches a year. And I asked you, how tall is she going to be when she's eighty Kelly, And you said, oh, she grows six inches a year, so she's going to be forty feet high. Like that's nonsense. You and I both know that's nonsense. Doesn't make sense to dig into the philosophical implications of it. It's the same thing with the early universe. We know general relativity is wrong about what happened to the very early universe, and so to ask, like, what does it mean for time to begin? Is like asking what does it mean to be forty feet tall when you're eighty years old?

00:40:44
Speaker 4: Was this a bit of a like false setup because general relativity wasn't meant to be extrapolated back that far. And so if you asked a physicist, what does general relativity tell us about the start of time? You'd be like, no, not a relevant question.

00:40:59
Speaker 2: I'd say, can't answer that question. We don't know the answer. And we hope eventually to integrate general relativity with quantum mechanics, which has a very different concept of time, and figure it all out. But you can't use generativity by itself to answer this question. There just isn't an answer, even though it sounds sexy, and people talk about like the beginning of time is like the north pole. With this, you can't go further north. I think that's a distraction from the fact that we really don't know what happened in the early universe. And the reason is that we know there were quantum mechanical effects, and that again, quantum mechanics has a very different conception of time.

00:41:31
Speaker 4: Okay, so let's start talking about quantum mechanics. Then it just to jump to the chase. Is this going to give us an answer that we feel pretty good about when we get to the end here, or is this another quantum mechanics breaks down at that point.

00:41:42
Speaker 2: No, I think you basically cooked all the physics when you said, hey, this feels like a false setup. Most of physics is like, hey, let's understand the universe.

00:41:48
Speaker 1: Say.

00:41:48
Speaker 2: The answer is, we don't understand the universe. Sorry, guys, but maybe along the way we'll learn something. Okay, So quantum mechanics has a very different view of what time is. Right, put everything we've just been talking about aside. Imagine you had a completely different set of physics, group of people you locked in a completely separate room, came up with a very different idea about how the universe worked and what time is. And that's basically what quantum mechanics is. It's like a completely separate thread. It doesn't integrate nicely with anything we've been talking about, but we also know it's pretty accurate and in quantum mechanics, time is like a parameter. Quantum mechanics says there's space, and on top of that there are these weird quantum fields, and time is like a knob that you can turn and you can see things change, and we have equations that describe how things change. But for quantum mechanics, there's no connection between space and time, like space is where you put the things, and time is this parameter that tells you how things change. So whereas in relativity and Einstein tells the space and time are deeply connected and even interwoven because clocks move differently at different points, quantum mechanics is like nab or whatever. Let's just have space bio thing and time be its own thing.

00:42:58
Speaker 4: So because you can separate them, does quantum mechanics have a different answer to the time travel question.

00:43:06
Speaker 2: It doesn't have a different answer to the time travel question because there's still a sense of causality in quantum mechanics. People seeing like, oh, quantum mechanics is random and therefore the universe is nonsense. That's a bit too far right. Quantum mechanics says that the universe isn't deterministic. It's not that the present completely determines the future, but the presence still determines what's possible in the future. You know, it gives you a set of probabilities for what could happen in the future. So while the future isn't completely determined in quantum mechanics, it's still controlled by the past, and that means causality is still important and you can't go back in time and kill your grandfather. But it also tells us something else about the past, because in quantum mechanics is a really important principle that information is never destroyed, which means that the present determines what's possible in the future. Right That means that you can look at the future or any moment in the present, and you can tell what happened in the past, because every moment is uniquely determined. So this present, the moment we're having in the universe right now, encodes the entire history of the universe in it because there's only one way to get to this present. There's one unique past that leads to this present. So if you look at all the details of this present, you could tell how we got here.

00:44:21
Speaker 4: But that doesn't necessarily mean you personally can figure it out. Like if there's a ball in the middle of the room, you don't know if it rolled from the door to the middle of the room or from the opposite wall to the middle of the room. It's just there's only one way it could have happened, but you don't necessarily know which one it is.

00:44:37
Speaker 2: Yeah, in principle you could figure it out, but you need an extraordinary amount of information and infinite computing time. There's actually a pretty cool TV show called Devs that use this principle and they try to reconstruct like historical events to like view the crucifixion of Jesus from the air molecules outside, because in principle they're encoding everything that's ever happened, which is kind of cool, you know, but totally impractical. It's basically impossible. But what it means philosophically is that the universe can't destroy information. Information is constant, and that means that there has to be an infinite past. It means that time can't have a beginning because this information has to be propagated forward moment by moment by moment by moment. Is time is a continuous parameter, and so in quantum mechanics, it makes most sense for the universe to have always existed and to always exist because this is this continuous flow of information. It can't be destroyed. It can't just come from nowhere. It's created by the past. And so you can take the present moment and you can evolve it backwards using the equations of quantum mechanics to any moment in the past, right, And so there has to be those moments in the past. That's very much in contrast to what general relativity says that you can have this like singularity in time where space and time begin. Both of these are probably wrong, right, there's some future theory of quantum gravity where somebody is like, figured this out and will in this together and made sense of this, and both of these ideas quantum mechanics and general relativity will later see the way we look at Newton's idea of gravity, we're like, well, you know that mostly works under certain circumstances, but it's not the way things fundamentally operate. It's not the true story of the universe. It's just it works if you don't have the full.

00:46:20
Speaker 4: Picture quantum mechanics or general relativity. Does one of them give us a practical understanding of time that we can do things with better than the other, Or is that not a reasonable question, because it depends on the scale, and they both tell us practical things depending on what scale we're asking the questions.

00:46:39
Speaker 2: Yeah, that's exactly right. They both tell us practical things depending on the scale. You want to talk about particles, you just ignore general relativity, and you can make amazingly accurate predictions for what electrons are going to do milliseconds apter u zap them with a laser. You want to talk about asteroids and whether they're going to hit the Earth, you ignore quantum mechanics, and you can make very accurate predictions for when and that rock is going to swing around the Sun and hit the Earth or miss the Earth, and how much you have to deflect it. So, yeah, it depends on the question you're asking. And that's the frustrating thing is that these two things hardly ever talk to each other. They disagree vehemently about the way the universe works, but usually one is irrelevant, and so we never get to see which one is right which one is wrong, because they basically always one of them taps out and the other one steps in.

00:47:22
Speaker 4: Okay, they're like the best wrestling team ever. So if I'm understanding things correctly, both of the theories say that like, for the most part, time is moving forward, and you can move a little bit faster and tinker with some things, but you can't break causality, and so they both are like no time is like a forward thing.

00:47:46
Speaker 2: It's interesting because both theories have time in them and they're very different conceptions of time. But the reason why time moves forward is actually still kind of a deep mystery. Like if you look at the quantum mechanics view of things, a lot of the r quantum mechanics work the same way.

00:48:01
Speaker 1: Forwards and backwards.

00:48:03
Speaker 2: Like we have this Shortener equation that tells you, if you have the present, what's going to happen in the future.

00:48:08
Speaker 1: It also works backwards.

00:48:09
Speaker 2: That's why it's symmetric, Right, you can take the present and say, oh, what had to happen in the past in order to get here in the present. So the equations work the same way forwards and backwards. And in most situations, if you just like watched a video of particles interacting, you couldn't tell whether somebody was playing the tape forwards or playing the tape backwards just by looking at what happened, because the same rules apply the rules are symmetric in most cases.

00:48:34
Speaker 1: It's like if.

00:48:34
Speaker 2: Somebody's bouncing a ball and there's no air resistance and no friction, whatever ball is going to go down, hit the floor, come back up to your hand. You play that video backwards, it looks the same, right, And so if I play that video, you can't tell whether I flipped it or not. That's sort of a big puzzle about physics is that it works the same way forwards and backwards. So like, why is time go forwards or in one direction that we call forwards? Why isn't it going the other direction?

00:49:00
Speaker 4: So is answer to to my question I could tell someone like I'm sorry, I'm more in the quantum mechanics universe, where time sometimes moves backwards and it's the same and you're just confused time on time.

00:49:12
Speaker 2: It's okay, yeah, we made this appointment back in time. Actually you went to this time. That's right, exactly. No, it's a big puzzle in physics about why time moves forward, and so you can always tell your friend like even physicists don't understand how time works, so don't worry about.

00:49:28
Speaker 1: It so much.

00:49:29
Speaker 2: But there's a lot of talk in popular science about how the direction that time moves being explained by the second law of thermodynamics. We can outline that argument, but I don't think it works as powerfully as a lot of people think it does. The argument basically is like, yeah, at the particle level, everything seems symmetric, but then when you zoom out and you're like watching stuff happens, there's this thing we can calculate called entropy, which is kind of like a measure of the disorganization of the universe, the messiness of it. And as time goes on, this entropy increases. You know, so for example, you run a refrigerator. You're cooling the inside of the refrigerator. That decreases the entropy, but you got to also run the compressor to run the refrigerator. That creates heat on the outside, and it more than compensates. And so anytime you try to decrease entropy, you make things like more ordered. You're in the end increasing entropy, And if you look at the universe, this just keeps going. And so the argument is like, oh, maybe time is entropy increasing. This is the one part of physics that seems to prefer one direction to the other.

00:50:29
Speaker 1: That's the typical.

00:50:30
Speaker 4: Argument, and do you buy it.

00:50:32
Speaker 2: I don't really buy it for a couple of reasons. Number one is that the premise is wrong. You know, the argument that like, at the particle physics level, everything is perfectly symmetric isn't actually true. There are some particle physics processes that operate differently in time forwards and backwards. This has to do with violation of some fundamental symmetries amid a video actually with veritassium a few years ago.

00:50:55
Speaker 1: You can look it up.

00:50:56
Speaker 2: It's called this particle breaks time for some details about that. So so even at the particle physics level, it's not one hundred percent true the time is the same forwards and backwards. But my real complaint about this argument is that, yeah, it shows us that there's a connection between entropy and time. Entropy seems to go up as time goes forward. But to me, that doesn't tell you why time goes forward. Just says, look, if time is going to go forward, then entropy is going to go up. If time is going backwards, then entropy would be decreasing.

00:51:24
Speaker 1: Right.

00:51:24
Speaker 2: It doesn't tell you why time has to go one way, just says these two things are connected. That's not enough to drive the arow of time forward. It just says, yeah, the universe would be different if it went backwards.

00:51:36
Speaker 4: Are these the kind of things that physicists discuss at the bars late into the night at your conferences.

00:51:42
Speaker 2: I think most physicists honestly would run screaming from these kind of conversations. But you know, there's some category of physicists who like the philosophical implications of these things. For me, I'm into these questions in physics not just because hey, I want to build a faster iPhone and I want to be a master of the universe, but because they brush up against these deep questions about like what is the universe and how does it work?

00:52:04
Speaker 1: And is the whole thing an illusion?

00:52:06
Speaker 2: And in the one hundred years and we're going to pull back the veil and discover the things work fundamentally differently from the way we experienced, or are we going to meet aliens and they're gonna be like, ha ha ha. You guys still think that that's hilarious, how cute?

00:52:17
Speaker 1: You know?

00:52:17
Speaker 2: This is what I want And so to me, these philosophical questions the other reason I got into physics, and anytime the physics bumps up against those I get excited while everybody else runs screaming. But you know, I'm not alone in this. You know, folks like Sean Carroll and Carlo Ravelli and a bunch of folks that are interested and have done a really important work on what all these concepts and physics mean for time and the direction it flows and could we actually build a time machine?

00:52:40
Speaker 4: And so the physicists who aren't interested in these questions, what is their reason for not being interested? Or is that not fair? You're not in their heads, you don't know.

00:52:49
Speaker 2: No, that's a really fine question, you know. I feel like one of the wonderful things about science is that there's so many opportunities to get excited, and it's just very personal, you know.

00:52:58
Speaker 1: Like even within article.

00:53:00
Speaker 2: Physics, there are folks who like to build the detectors and they think it's really fun to get greasy and climb around on those things and like figure out how to line everything up and use a laser beam to calibrate everything. And there are other folks who like to write computer programs to analyze the data, and to them, that's the jam. And everybody finds a different thing to be excited about, which means that like science takes all kinds, you know, and everybody thinks that their bit is the most important, the most exciting bit, and everybody else is just like you know, box checking and stamp collecting. And that's fine because that means everybody gets to do the fun part, you know, and the most important part. The truth is, every part of it is important. The engineering, the physics, the philosophy, the biology. It's all important stuff. Everybody's contributing, and I just think it's great that everybody finds their own bit to be the best bit.

00:53:47
Speaker 4: As a biologist, you don't have to convince me that diversity is important. Intellectual diversity is important too, exactly.

00:53:56
Speaker 2: And so you know, the big picture to physics answer about what time is? Boy, we have a few ideas, and some of them seem to work pretty well, and let us do all sorts of things like design interconnected global position systems that work within centimeters and shoot laser beams across fast distances, all sorts of stuff, but we don't really understand what time is. We know that it flows, we know it's connected to space, we don't understand that in detail. We have a lot to learn about what time is.

00:54:26
Speaker 4: So for the kids who are sitting in a room their parents are listening to a podcast about what is time, you could answer this question and you could change the world if you could figure out what space or time are. There's got to be a Nobel prize in there.

00:54:39
Speaker 2: Oh yeah, there's lots of Nobel prizes for even the mini steps along the way. Absolutely, kids, there's so much left to discover about the universe, to unravel, to figure out. We are at the very beginning of understanding the way the universe works.

00:54:53
Speaker 1: We've just sort of.

00:54:54
Speaker 2: Like set the stage for the next generation to come in and figure it all out.

00:54:58
Speaker 1: That's you.

00:54:59
Speaker 4: We've done some cool stuff, but there's more to do.

00:55:03
Speaker 1: All right.

00:55:03
Speaker 2: Thanks everyone for taking the time to go on this journey to some of the deep questions about the nature of the universe on this podcast. You know, we think the universe is extraordinary and we love to share our joy of it with you. If you have questions about what you heard today, please don't be shy. I would love to answer them. Everybody gets an email back. Just write to us to questions at Daniel and Kelly dot org.

00:55:25
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00:55:28
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00:55:36
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00:55:47
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00:55:59
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