Daniel and Kelly’s Extraordinary Universe - Why Does Time Go Forwards?

Episode Date: April 25, 2019

Why can't we change the direction of time? What are the past, present and future really?  Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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Do you think we have time for that? You know, there's no time like the present. Hmm, it could be a good time. Might be a fun way to pass some time. All right, I'll make time for it. It's about time. That's so corny. How many time puns can run fit into a single podcast?

That's right. How much time do you have? I've got time. I think it's a timely set of puns. Well, you use that pun time after time. I don't know. I've got nothing else. Now's the time to start the podcast. Now is the time. time to tackle this tricky topic.

Hi, I'm Jorge. And I'm Daniel. It's time for our podcast. Daniel and Jorge Explain the Universe, a production of IHeart Radio. In which we take everything about the universe, the now, the future, the past. the deep, deep, deep past, and try to explain it to you.

That's right. We take the time to take a little bit of your time and maybe get you to understand a little bit more about this amazing and spacious place that we live in called the universe. That's right. And sometimes we like to grapple with topics that are sort of obvious, you know, sort of things that are right in front of you and understand, like, does physics know how to make sense of it? Does physics have a good definition of it? Do we understand why it's this way and not that way?

That's right, and so today we're tackling a topic that is probably in everybody's mind all the time. That's right. This is something you think about every day, from when your alarm clock goes off to when it's time to go to bed. Today on the podcast, we'll be talking about... Time. What is time, anyway? Yeah, and why does it only go forwards? Why do you remember the past and not the future? Can you remember the future?

that would be pretty cool. Well, that's the whole question, right? On one hand, very intuitive understanding of time. We all know what time is, right? Yesterday was yesterday, tomorrow's tomorrow, right? Now is now. But when you get down to it, we don't really understand why we have it, right? Why is it like that?

Why does it only go forwards? Why is the future different from the past? Yeah, it's weird to think that we are in the present right now and there's a past going backwards in time and there's a future going forward in time, but what's really the difference between those two things? Why can't I see or feel or have any kind of memory about the future? Yeah, exactly.

What makes it different? Why is the past fixed, right, unchangeable, unless you believe in crazy science fiction, time travel stories, which I don't, and the future undetermined, right? What's the difference? Can physics reveal that? Is there some understanding of time from physics that tells us why one is different from the other? Yeah, why is there an arrow of time, like a one-way sign in the universe? in the highway of time of the universe. Yeah, and this is one of those wonderful questions

because on the surface of it, it seems kind of dumb. Like somebody asks you, what is time? You're going to tell them, oh, it's 3 o'clock. And if they dig deeper and ask you, they know, like, what does time even mean, man? Sometimes I say it's 2 o'clock, depending on the circumstances.

Oh, I always give the same answer, no matter what time is. That way, people stop asking me. No, my kids are always asking me, hey, what time is it? And they have, like, a watch on their hand. There's like two watches on the wall. Like, why are you asking me?

So I always just say it's 3 o'clock no matter what. So they stop asking me. It sounds like the same parenting strategy my wife has when we go on road trips. They always ask, how long are we till we get there? Are we there yet? And she always answers. Nope, it's going to take 20 more hours. No matter what.

You're like pulling into the place and she's like 20 more hours. Yeah, no matter what, she always says 20 hours. It's a great strategy. You know, me and your wife, we would really get along. It seems like we see eye to eye about how to handle these. approaches, how to handle these problems. You would have some very sarcastic kids, probably. Exactly.

Anyway, I love these questions that are like, on the surface so simple, but when you dig deep, they reveal, like, enormous gaps in our knowledge about the universe. Those questions are the best because those are the opportunities to reveal that the universe is different from the way we understood. Like, the way we think about the universe reflects just our experience, the way we've lived and grown up and not something fundamental, something universal, right? And that's the whole goal of physics. Yeah, it's one of those questions, like, it makes you kind of look almost inwards

or look in your, like, one day you just wake up and you don't even know where you're standing or how the house that you live in was built. Whoa, don't lose grasp of reality there, Jorge. We're trying to dig deeper. A little bit. We're not trying to make you go crazy. Doesn't it make you kind of question, like, the very nature of the universe? It does. It's sort of like if you say a word, like a hundred times, like say the word, marriage, a hundred times, it starts to turn into a really weird word, right? You take it apart, you look at it, you're like, that's really strange. It's the same thing with the concept of time. Like, you know, pick up your hand and rub your fingers together, right? You're feeling that right now. But what does now mean, right? Now sort of like is always, it's infinitely short and it's slipping constantly into the past. You can never grasp onto it. You can never hold it.

right it's always you're always losing it it's a really strange concept the concept of now yeah yeah although i don't know if that's what makes marriage weird but no there's lots of things that make marriage weird um saying it a hundred times is not it but um i remember reading a book about consciousness back when i was really interested in the science of consciousness which we can talk about another podcast but the basic idea in that book it's by dan it's called consciousness explained is that there is no now that you your the consciousness is basically an illusion that all you're ever doing is remembering the immediate past and i don't know if i believe that but it really made me think about what now is

and how you can never really grasp it it's always just like sliding away from you and you know whether it really exists whether there is a now right whether there's a special instant that that differentiates between the past and the future right right because sort of technically in physics of time could go both ways right like you could in time is kind of of just like an arrow you can flip back and forth in the equations that you guys use, right? Yeah, time is not really very central to physics. Yeah, I mean, on one hand it is and on one hand, it isn't.

You could write a lot of physics down without ever thinking about time. On the other hand, time is a central property of physics because we're trying to use physics to predict the future. So time is both not part of physics and deeply entwined in it at the same time. So it's an important topic. Yeah, no, and it's a topic that everybody experiences, right?

at every second of the day, every day of the year, every year of their life, they're experiencing time and they're moving forwards in it and they feel it, right? Yeah, they think about it, they think about the future, they remember the past, and so it kind of makes you wonder how many people actually think about or know what time actually is. Yeah, exactly. So I was wondering, do people know why time goes forward? Like, I mean, physicists don't really, but it's always fun to ask general public questions that scientists don't know the answer to, just to see what they come up with. So I walked around the UCERI Irvine campus and I asked people, why does time only go forwards? Here's what people had to say. Do you know why time only goes forwards?

Not really, honestly. I have like some sort of like intuition why it only goes forward, but I don't think we can't explain that. Yeah. Okay, cool. I have no idea. Okay. No, I don't. Any guess?

If I had to go, I'd just say gravity or something. Okay. No, I don't. Any idea of best guess? No, I'm sorry. I think time is like the first dimensional, right? Mm-hmm. So, actually, I don't know about it.

It's tricky, right? Yeah. Okay. I'm not sure. I'm not sure. Okay. Well, when I think of time, I think of it kind of almost as linear progression, meaning that it's a constant in the universe

and it's always kind of moving forward and that's something that we accept as being a fact of the universe that we say we understand it now. So in that regards, I think that's why we can only see it as moving forward because it doesn't kind of make sense to us or it's not even physically possible currently for us to move backwards in time. So we don't consider it in that way.

I think it might be a matter of observation. So relative to us, time moves forward in the way that we experience the world. biologically, it's designed to be in a manner that is sort of one-dimensional and a forward mind all the time. But in reality, it might be that the fourth dimension of time in space time might just be a way of describing the universe that the math predicts from physics and biology and perhaps other related science fields. Okay, cool. Yeah, so I got a pretty good mix of answers there, right, from the standard physics like gravity, which is a, you know, that's a good

answer you never know it might be right gravity that's somebody actually said gravity was the answer I don't know gravity it could be it could be yeah well gravity is like the answer to half the questions in physics the way like pie is the answer to half the questions in math right so if you're no idea you could just say gravity

and half the time you'll be right not the movie the force that's right yeah the fabric of space time itself that's right it's gravity yeah and then you know some other folks had some more elaborate discussions yeah but um some people had opinions. People did have opinions, yeah. Some of them were sort of longer word salad, right?

You know, some of them hit on some of the scientific concepts, time being a fourth dimension, et cetera. But yeah, a lot of people, when I asked them this question, you could just see in their faces that they had not ever considered this. They're not even a question they had imagined. But as soon as they thought about it, they realized they didn't know. And that's my favorite part of this topic, is that nobody has really thought about it in terms of the general public. but as soon as they do, they're very curious, right? Those are the best questions. It's amazing how little you actually kind of need to know about the universe

to live a full and happy life. You know what I mean? Like a full, productive, happy life. There's so much you don't need to know. Ignorance is bliss. You know, that does have a meaning. People have been living in this universe for hundreds of thousands of years, depending how you count the beginning of humanity.

And there's been a lot of happiness and joy without really any understanding. I mean, I would say people 500 years ago basically knew nothing about the way the universe actually works. But, you know, people had birthday parties and ate cakes and had moments of joy. Yeah, I mean, like only 100 years ago, right? In the 1900s, early 1900s, people didn't know the concept of atoms or quantum physics, right? Yeah, that was about 100 years old. We didn't know how long the universe had been around, right?

So pretty basic stuff about the way the universe works. is pretty modern, right? We didn't know there were other galaxies, right? No, they didn't. So what's your point here? Your point is you don't need physics to be happy. Is that what you're trying to say? Physics is irrelevant to happiness?

Think of how much of a richer life you can lead if you know physics. Okay, I like the way you spun that back, right? Using all this technology to talk to me that's based on physics. I mean, not like money rich, but, you know, like spiritually rich. Right, right. Isn't it your career based on technology that was developed, with the help of physicists. Where would you be without physicists?

Isn't your science dependent on technology to progress? Yes, exactly. Yes, I am a tiny little mite standing on the shoulders of giants, absolutely. But so are we all. So are we all. Yeah, yeah. Well, maybe it's time we got back on topic here. So let's break it down for people, Daniel.

How do physicists see time? Like, if you're at a conference and you guys start talking, and somebody said, hey, guys, what is time? What do you think people would generally say? I think there'd be a lot of disagreements because we don't really have a great description of time, one that really makes sense. And the way physics approaches a problem is,

first you try to build like a model of the problem. You try to describe it in a way that we can grapple using the language that we're familiar with, which is mathematics. You try to find an equation that tells you what's happening. Yeah, exactly. You know, something's happening, the real world. Maybe it's like a ball is being thrown in the air, and we try to understand it.

So we write down a bunch of equations to describe it, and that lets us build that model in our heads and manipulate it and then use information from the model to describe what's happening in the world. Cool. How do we do that with time, right? How do we ask this question about, like, why can I remember the past and not the future? Why does ice melt, but not unmelt? These kinds of questions about time. It's pretty hard to wrap your mind around. One approach is to think about the universe in terms of a bunch of snapshots. Like you think about a movie, right? What is a movie? It's not really a continuous experience in time. If you're looking at the screen, you're not really seeing smooth motion. You're seeing a bunch of snapshots. You're seeing like a little

cells in a film or like a bunch of screenshots. Yeah, it's a bunch of screenshots. And they're just packed so tightly together. that you don't notice that they're not actually smoothly varying, right? Your brain does the interpolation for you and tells you a story. So sometimes we think about the universe that way. We think about like all the universe at this moment and then all the universe at the next moment and all the universe at the next moment, like snapshots of the universe. By snapshots you mean like what is every particle doing and where is it going?

Yeah, exactly. And time then is a way to sort of order those snapshots, is to say this, one first, that one next, that one next, and to sort of put them together. This one can go in front of the other one. Yes, exactly. You know what I mean? Like it limits the ordering. Yeah.

And in that sense, physics, the job of physics is to take all the snapshots in the past and predict the ones in the future and to say, okay, according to the laws of physics, the next snapshot will look like this. Or if you want to talk quantum mechanically, there's a probability distribution of the next snapshots, these are likely, these are unlikely, these are impossible, right? So physics takes sort of that view of time. And there's a bunch of caveats there. Like you can't have a snapshot of the whole universe, right?

Because time is not universal. It means different things for different people. You can't know everything about the universe. But, you know, you take a small enough model or a single particle or something. You can also have quantum states, right? Like, isn't that prevent you from getting an exact snapshot of the universe? Yeah, exactly. So you can't really get an exact snapshot, but you could do a quantum mechanical version.

You could say, well, what's the quantum state of all the particles? Or, you know, what's the wave function? What's the state of the wave function right now? That you can talk about. And that determines probabilities. Oh, I see. So you're describing kind of how physicists see the universe. They don't see it as, they see it as this kind of sequence of snapshots of how things are arranged.

Yeah, for example, think about the Schrodinger equation. The Schrodinger equation, the famous equation, describes how a particle's wave function moves through time, right? And it tells you how it moves. It says, if you have this way function now and you experience this, then your way function will be that later, right? And so that's what physics tries to do. Or go back to the mechanical example of a ball going through the air, right?

It tells you, well, you shot your ball in this direction, where is your ball going to be in the future? So that's the role of time in physics, right? It's to predict the future snapshots. Oh, I see. So to you guys, time is like a like an, like an inch. input that tells you what the universe is likely to do. You know what I mean?

Like, do you, it's an input. It's not like an output or something that you can vary or that is affected by all the other variables. Yeah, exactly. And so, for example, you want to predict the flight of your ball. It's an input. You can dial that knob and say, I want to know where the ball is going to be in one second or five seconds or 8.29 seconds or whatever.

So it's sort of that knob. The thing we don't, the thing that that doesn't explain, at all is like why does it only go forwards and who's controlling how fast that knob is turning or why do we have time at all why don't we just have a static universe that's just sort of like there you know it's the first step of course is just try to describe it and then you can build from that and try to answer some of these questions what do you mean like how can we don't have a static universe like um why do we have time at all boom right i don't know you don't know Nobody knows.

Well, I know there's this idea that time is the fourth dimensions. We have three dimensions, up, down, left and right, backwards and forward. And maybe time is just another direction of the universe. And so I think the idea is that we do sort of exist in this state of being still in all four dimensions, but it's just so happens that we somehow feel one of the dimensions. Do you know what I mean? Yeah, I love the, it just so happens part of that explanation. Okay, here's the hardest part.

I'll just yada, yada, yada over that bit. That's my Nobel Prize winning paper. Officer, it just so happens that I was going 90 miles an hour. I can't explain it. No, you're right. Time is sometimes thought of as the fourth dimension. And so let's dig into that a little bit. What does that mean?

Why do we think of it that way? We think of it that way because it's helpful mathematically. Like when Einstein was developing his theory of special relativity, and we talked all about time dilation and stuff like that on a previous episode, he discovered that the equations have a certain symmetry. They look really nice, like that you can write them down really simply and compactly if you include time as the fourth dimension of this larger concept he called space time.

So he took three dimensions of space, he tacked on a fourth dimension, which is time, and he constructed this thing called space time. And because your perception of time depends on where you are in space, and how fast you are moving through space, the equations get much simpler if you think about all four dimensions in that way. If you think about time, it parallel to space in that way.

And that's a clue. When you write down equations and they become simpler if you think about them a certain way, that's a clue from the universe that it's maybe the right way to think about things. Okay, so that's time as a different dimension. Let's get more into it. Let's go down that rabbit hole.

But first, let's take a quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass. The injured were being loaded into ambulances, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay.

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Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor and they're the same age.

It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. The U.S. Open is here. Podcasts. Good Game with Sarah Spain. I'm breaking down the players from rising stars to legends

chasing history. The predictions will we see a first time winner and the pressure. Billy Jean King says pressure is a privilege, you know. Plus, the stories and events off the court and of course the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very fancy, wonderfully experiential sporting event. I mean, listen, the whole aim is to be accessible and inclusive for all tennis fans, whether you play tennis or not. Tennis is full of compelling stories of late. Have you heard about Icon Venus Williams' recent wildcard bids? Or the young Canadian, Victoria Mboko, making a name for herself?

How about Naomi Osaka getting back to form? To hear this and more, listen to Good Game with Sarah Spain, an Iheart women's sports production in partnership with deep blue sports and entertainment on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. So I think it's fun to think about time as the fourth dimension, but it's also kind of a trick, right? It doesn't really fit.

So the idea that it's a fourth dimension, you're saying, came from mathematical convenience by Einstein. Do you know what I mean? like it's not, it was just easier to treat it like a dimension. Yeah, and I wouldn't blow that off. Mathematical convenience is not a small thing. You know, the whole goal of physics, remember, is to like write down an equation of the universe in one line.

And so if you can write things more simply, that's a clue that it's probably more correct. It's more, and it's like a deeper understanding. So, yeah, he discovered that if you write things down with time as sort of the fourth column of your vector, then a lot of the equations are simpler to write down. There's a connection. There's a symmetry there, right?

We talked about symmetry in another episode. You can treat time the same way you treat space in many ways. And that's tempting because you think, oh, well, that answers the question. Time is just another dimension, right? But it doesn't answer the question because it's not just another dimension, right? You know time is not space, right? Time, what does that mean? Time is not space.

Well, for example, you can move forwards and backwards in space, right? You can go left, you can go right, you have some control over it. You can't do that with time. But isn't it just sort of a matter of perspective? Like in the grand scheme of the history of the universe, you know, I existed for this amount of time. But while I'm living it, I can only go forward. I can only get the experience of moving forward in time. Yeah, that's true.

Technically, I sort of existed all throughout my life in space time. You existed all throughout your life, yeah. but you don't exist all throughout time, right? There's no limit to where you can visit in space. You can go from here to there to the other places, but in time you can only visit between your birth and your death. And you can only go to each time once, right? That's not true for space.

I imagine there are lots of places you go to many, many times, like your refrigerator or your bed, right? You visit those places many times. So space is quite different from time, right? I can do loops in space, but I can't do loops in time. Exactly. And time has this special difference, right? The now, right?

There's no now in space. I mean, I'm here, I'm there, you're over here, but there's no special location in space, but time has this special location, this thing we call now, which exists weirdly, and slides forward weirdly. So I was saying earlier, why isn't the universe static? Why is time moved forward?

Why isn't it just stuck at t-equal zero and nothing happens? What's pushing it forward? What's turning the engine of the universe, right? I guess what I mean is, you know, if time is, you can treat it as a fourth dimension, then, you know, our experience of the universe is that time is constant and it moves at a steady pace and that we're only going forwards in it, right? Whereas in space, you're saying we can go backwards. But what if I just kind of take another dimension and use that as my kind of ticker?

Which dimension would you use as your clock? I don't know. Up, up. Okay, yeah, so you'd be like, hey, let's have lunch at X equals five meters or something. Yeah, I don't know. You know, kind of mathematically, what would that mean? They're different, right? Time and space are not the same.

You can't just swap out time for space because you don't have the same freedom in time as you do in space, right? If space moved always forward the way time did, like you could never go back to anywhere, right? You'd be like, oh, yeah, hey, I had lunch, and now it's sort of sliding slowly away from me. You know, it'd be pretty weird. But it's a very natural experience. Oh, I see. You're more limited in the time dimension.

Exactly. There's some extra rules that seem to apply to the time dimension that don't apply to the other dimensions. And that's weird. And any time we see symmetries, we're like, cool. But then when those symmetries are broken, we're like, okay, time is like the other dimensions, but it's different.

Then we ask, why is it different? How's it different? What makes it different? because that's the clue to solve the other puzzle, right? Okay, so time is, it is sort of a fourth dimension, but there are special rules that apply to it that makes us think that maybe it's not really a fourth dimension. Maybe it's just like a mathematical, um, uh, cluge, right?

Yeah, or maybe there's two kinds of dimensions. There are space dimensions and time dimensions, right? Oh, yeah. And maybe there are other kinds of dimensions we haven't imagined yet, you know? Wow. Yeah, that's the kind of dimensions and time dimensions, right? That's the kind of mind-blowing, like, look at the universe in a different way, discovery

that I always hope to make in science, you know, to, like, crack something open that's so deep that it, like, nobody gets it, but the stoners, man. Okay, so what are these rules? The rules are that you can only, you can't visit two times in your, you know, you can't go back in time,

you can't make loops, and it can only go forward. Are those the mainly the main, restrictions here with time? Yeah, exactly. Those are the main restrictions. And I think the goal of physics and the goal of our podcast is to try to understand where that comes from. Like can we look at the laws of physics and say, oh, that comes from this, you know, like the way we can look at the laws of quantum mechanics and from that understand the way hydrogen works or whatever. We want to look at the laws of physics and say, do they require this behavior? Is it necessary? What does it come from at

its deepest level, right? The problem is when we look at laws of physics, we We don't get those kinds of clues. When we look at the laws of physics, we don't... So the equations in your model of the universe don't tell you that there should be restrictions in time. Yeah, let's go back to those equations about, like, the ball moving, right? Imagine you throw a ball in the air. Like F equals MA, right?

You're talking about F equals MA. Basic stuff, right? You throw a ball in the air, it goes up, and it comes down. Let's start with the simplest case where there's no air resistance, right? It's path up, and it's path down are going to be very similar. In fact, if somebody took a video of that and then played it backwards, they couldn't tell whether the video is going forward or backwards, right? Physicists analyzing the motion of the ball couldn't tell you whether the video was going forwards or backwards. So if you fed it a time, like if you gave it time, it would give you the same output whether you put negative time or positive time.

Exactly. It's kind of how it breaks down, right? Exactly. Because the equations work in both directions, right? The equations have time in them. They tell you how things change with time, but they work the same forwards and backwards. So they don't tell you which way the universe should run. They tell you how the universe changes with time, but they don't say why it has to go forwards and not backwards,

or why it has to go anywhere at all, or I can't go forwards and then backwards, right? So if the equations of the universe really represent the universe, then the universe is okay with traveling backwards in time. you're saying exactly the equations sort of say like hey you want to go back in the time sure here is what would happen yeah exactly and it's sort of like you were saying earlier it's like an input you know imagine you're doing some calculation and the calculation is a function of position and time right you want to know how many balls are at this location at this time right you you drop a thousand balls into a box and you're wondering like how many balls are at this location

this time you can ask that question of any point in space and any point in time right You can take the current configuration and you can evolve it forwards or evolve it backwards. The laws allow for all of that. But for some reason, there's a difference in the way it actually works in the real world, right? Things slide forwards in time, and we don't know why. There's no restriction on the space parts, right? The equations are functions of space and time, but there's this, like, extra restriction on the time part of it when it comes to, like, actually implementing the universe. Right.

Right. It's kind of like we have the now, and if all we have was now, the equations would let us tell what happened before in negative time, and it would let us tell us what happens forwards in time, right? Exactly. The equations would, yeah, but somehow our experience, we only have experience of the stuff that happened in negative time. That's right. We only remember the past, right? That's the weird part, right? The past, we can remember, we have access to it. It affects us. the future we can't. And most of the laws of physics is an asterisk there. Most of the laws of physics are totally symmetric, right? They don't care whether the universe is going forward or backwards.

They're happy to do either one. Wow. So that's the mystery is the time would be sort of a fourth dimension, but there's some rules, but those rules are sort of not in the equations of the universe. Yeah, that's weird. But with an asteris, you're saying. There's an asteris. Right.

Yeah. So the game is, figure, out if the laws of physics that we know of, that we've discovered, that we've, you know, written down if those require things to move forwards. And if so, you know, does that give us a clue as to why it's happening? Or maybe there are other laws of physics out there that are much more sensitive to time that are the ones making time go forward. What do you mean? So you're looking for something in the equations that would require time to only move forwards. What does that mean? Like if you put negative time you should break or or yeah yeah exactly like let's think about that are there is there anything

in physics that does require time to move forward so let's go back to that ball example right i said if you throw a ball in the air um you can't tell whether the video of that ball in the air goes forwards and when we did that i said ignore air resistance and that's for an important reason right because air resistance changes the flight of the ball right slows it down so that in reality if you throw a ball in the air, right? It's going to slow down. And so you can tell the difference between going forwards and going backwards, right? Right. Because there's a, like, you lose some heat or you lose some energy in reality, right? Yeah, it's probably a simpler thing about, like, dropping a ball. You know, if you drop a ball and there's no friction, it's going to come all

the way up to where you dropped it from. If you drop a ball and it is air resistance, it's going to lose some energy to bumping against all the air molecules, right? It's not going to come up quite as high as you dropped it. You just keep letting it bounce, eventually it's just going to dribble down and stop, right? Right. And that one, if you played it backwards, you would be able to tell the difference. Exactly. That one, if you play it backwards, and that's the case for most things, right?

So where does that come from? Well, that comes from entropy, right? That comes from heat. The heat is really there is the key. It turns out that the universe likes to go from organized to disorganized, right? And that's this concept we call entropy. Yeah, I've heard of this before. But before we dive in, let's take a short break.

December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing. at the TWA terminal. Apparently, the explosion actually impelled metal glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene.

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on the IHart Radio app, Apple Podcasts or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. What do you mean when you say that the universe likes entropy or that it likes to go from order to disorder? What is it liking mean? It means that if the universe had a bedroom, it would be a mess, right? And it would never want to clean it up.

No, it's obviously... But that's here's our experience of it. Like, maybe what if there are aliens out there who experience time backwards? And they're like, why does the universe like order? Yes, everything we see seems to get ordered. Exactly. So there's a big flaw in this argument. But let's put a pin in that and come back to it once we've done constructing the argument.

Then we can take it apart. Got it, got it. Yeah, so you're right. What does it mean that the universe likes it? Well, it just means that we've noticed, we've observed, that disorder increases. Right? For example, the classic example is you have a box and you put a bunch of gas particles in the corner, right? That's very organized. It's like only one way to arrange or a few ways to arrange gas particles in the corner. What happens if you run the clock forward, if you let that go, well, if they're not totally frozen, they're going to spread out through the box, right? It's like you pour water onto a surface. It spreads out. Things like, things tend to go from organized configurations to disorganized. And that's just an observation. That's not a deep insight. side into the universe. It says, here's what we've seen. Right. You notice that there's a correlation between positive time and messiness.

Exactly. And that's called the second law of thermodynamics, right? It says that entropy always increases. Entropy is a measure of disorder, right? So things are basically always spreading out and getting more diffuse. And that's the whole universe is doing that, right? The universe, like on a grand scale, is spreading out and getting smoother and stuff like that. Is it a law law, like an equation or is it more like an observation right it's more like we've never seen something get unmessy with positive time yeah that's a great question it's a really deep question i love that question i think the answer is this um the answer is that it's an observation it's something we've noticed and we've never really seen it broken and so we thought well this must be important so let's

write that down let's call that a law because it's something we've noticed happens all the time It never gets broken. Why is that? Does that reveal something deeper about the universe? Can we explain that from something else? And if you dig into like statistical mechanics, you can derive the second law of thermodynamics from simpler assumptions. Like if you say every possible configuration of the universe has equal probability,

then it turns out there are more messy configurations than unmessy ones, right? Like there's a thousand ways to have gas particles spread out through a body. but there's only a couple ways to have them stuck in the corner. So it's just more likely to end up with the messy ones because there are more messy configurations. But that's sort of a cop-out. I mean, it's just sort of another way to state that, is to say, well, you can call this a deeper understanding,

but it's also just like a posture that we came up with so that we could derive the second law which we've observed, right? You're basically down to the same argument. You're saying, you're basically saying the reason we have entropy entropy is that when you move forward in times, you go to the most likely scenario. But then you can ask, why does when you go forward in time do you go to the most likely scenario? It's just a restatement, really. It's a sort of more fundamental statement of the same basic observation, that things go from organized to messy, right? And, yeah, exactly.

Okay, so we were looking for, like, something in the laws of physics that told us why going forward in time is preferred, and you're saying we have this observation that it always does, and it's somehow related to chaos and order and messiness. That's right. And so this is like the only thing we can really get our fingers on.

Everything else in physics is time symmetric. And there's another asterisk there. There are some things in particle physics, which are time asymmetric. Those are really, really small effects and we don't really think that that can explain the direction of time, but we can cover that in a whole other podcast topics.

sometime. But basically, entropy is the big one. Entropy is the only law that, like, clearly, as you say, has a correlation. But that's the key. You made this point, which I think is very insightful, was that there's a correlation. It's not causation, right? Right. We know they're related, but what's the connection between them? That's what you're looking for, right? Exactly. That would be something that explains why time can only go forward. Right, exactly. So if you just take the second law and you add it to the rest of physics, you could say, okay, there's a correlation between time and messiness. So either the universe has to go forward in time

and get messier, or the universe has to go backwards in time and get less messy, right? The second law of physics allows for both, right? So it doesn't actually tell you why time goes forwards. It just says, well, if you pick forwards, then this is going to happen.

If you pick backwards, that's going to happen, and there's a difference. So it breaks the symmetry of forward and backwards, but it doesn't pick one. Or have you assumed that messiness is kind of where the universe prefers to be, then that tells you that forward time is the preferred thing in the universe.

Yeah, and it doesn't even tell you that entropy determines time, right? It just tells you that they're connected. There could be something else, something deeper, which causes both time and entropy, right? That's the whole problem with, like, correlations, you know? You can find correlations between lots of things. It doesn't explain it, right?

What's some of those famous things? Like, you know, global warming is correlated with the, increase in the number of piracy events worldwide, right? Does that mean that pirates kept global warming at bay for hundreds of years? Certainly not, right? So we know time and entropy are correlated. We don't know if entropy causes time or the other way or if something else deeper

that we haven't even thought about yet. Or maybe it's just random. Like maybe it's a random connection like pirates and global warming. I think we've observed it in great enough detail that we know it better than pirates than global warming. but I love looking at these correlations you know there's like a correlation between the number of movies Nicholas Cage has done and the number of people per year that die because they're twisted in their own bed sheets

they're correlated those two things like when one goes up the other one goes up yeah exactly or they have right that doesn't mean they're going to continue in the future right that's the problem with these observational correlations they don't tell you is if you don't understand the mechanism of it you can't really argue that they're connected They could just be chance, right? I mean, sometimes there are connections.

Like, you know that the consumption of ice cream is very well correlated with the number of murders. What? Yeah, well, people kill people more in the summer, I guess, because they're grumpier and they're sweaty and hot. People also eat ice cream in summer, right? So some root cause is in charge of both ice cream consumption and murder rates, right? It doesn't mean that ice cream causes murder, right? That we know of. that we know of.

I mean, physicists don't understand everything. That's right. The fourth law of thermodynamics, ice cream causes murder. Okay, so entropy is our only kind of clue, you're saying. And there's three possibilities, either. Time and entropy are not at all connected. They just having to go in the same direction. Or they are both the result of something.

else deeper about the universe or maybe one causes the other or right yeah exactly directly connected and we don't know and we don't even know if there is much of a connection and if you know there's a lot of deep questions there like if time is because of entropy then like what happens when the universe reaches maximum entropy because you know if entropy is always increasing and time is going forwards then there's no rewind right the universe marches forward towards messier and messier and eventually you reach the heat death of the universe when everything is perfectly spread out, right, perfectly disorganized. There's no structures at all. Does time stop? Does time turn around and go the other way? Like, you know, just saying time is connected to entropy doesn't answer

most of the deepest questions. Does time quit? Exactly. Some people have theories that time stops and turns around, right? And the universe then goes towards more organized configurations. Like, Maybe. I love reading about theories of time because people really go bonkers. One of my favorites is that is a guy who asked the question, okay, we have three or more dimensions of space. Why should we have only one dimension of time, right? What if you had two dimensions of time?

That's pretty hard to think about, which is weird, because it's not so hard to think about two dimensions of space. You think of it like a plane instead of a line. But it's pretty hard to think about two dimensions of time. Like, I'll meet you at 3 o'clock in north and, you know, 415 east-west. Like, what does that even mean? What is before and after, right? Oh, you could have time coordinates.

Yeah, yeah. You could have multiple dimensions of time. Isn't that sort of like a multiverse argument then? Like, there's maybe parallel universes besides this one? Well, that's a bit of a cop-out, right? It's saying maybe there's every configuration and our universe just happens to have one coordinate of time, and that's the explanation. I think there must be a deep reason.

There must be something revealing about the structure of the universe as we've discovered it. It's got to be a clue as to the way the things actually work. And I suspect that it's going to be a really deep answer. That when we figure it out, it's going to show us that the universe works really differently from the way we imagined, that huge parts of our life and the way we live and the things we think are fundamental about the universe are accidents or just constructs. Right. They are timely accidents. It's right. It took a lot of time, but eventually we created this illusion of the universe.

Wow. All right. So then we haven't really answered the question. I mean, it sounds like there is no answer, right? Like, why does time only go forward? We know it's related to entropy, but we don't really know the connection. And that's kind of as far as we know. I mean, there are a lot of people out there thought about this much more deeply than I have, Sean Carroll, Carlo Revelli. these folks have written whole books on time. But in my view, the argument mostly boils down to, you know, either it's connected to entropy, which I don't find that convincing an explanation for why it goes forwards, right? Or that space and time itself are illusions and they come out of something deeper and, you know,

and we need to dig deeper to understand like the string theory of the universe or the quantum loops that make up the fabric of space and time itself. So we were pretty clueless as a feel. It's definitely an open topic. Well, until we figure it out, I hope that people out there don't lose it and that they make the best use of it, right? Yeah, but it's not a waste of time to think about time. It just takes a little bit of time. That's right.

But it's about time. And now it's time to end the podcast. That's right. So thanks for listening. And if you have questions about big, basic, fundamental questions of the universe, send them to us. We'd love to dig into them and explore our ignorance and the ignorance and the ignorance of modern. physics so send us your questions at feedback at danielanhorpe.com yeah thanks for listening see you next time or see you last time we should say oh see you last see you see you anytime see

see any time that's right thanks for listening if you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge, that's one word, or email us at Feedback at Danielandhorpe.com. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys.

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