Daniel and Kelly’s Extraordinary Universe - Why does gravity slow down time?

Episode Date: October 28, 2021

Daniel and Jorge explore why time slows down when space is curved Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

This is an I-Heart podcast. Get fired up, y'all. Season two of Good Game with Sarah Spain is underway. We just welcomed one of my favorite people, an incomparable soccer icon, Megan Rapino, to the show, and we had a blast. Take a listen. Sue and I were, like, riding the lime bikes the other day,

and we're like, we're like, we're like, people ride bikes because it's fun. We got more incredible guests like Megan in store, plus news of the day and more. So make sure you listen to Good Game with Sarah Spain on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Brought to you by Novartis, founding partner of IHeart Women's Sports Network. And here's Heather with the weather. Well, it's beautiful out there, sunny and 75, almost a little chilly in the shade. Now, let's get a read on the inside of your car.

It is hot. You've only been parked a short time, and it's already 99 degrees in there. Let's not leave children in the back seat while running errands. It only takes a few minutes for their body temperatures to rise, and that could be fatal. Cars get hot, fast, and can be deadly. Never leave a child in a car. A message from Nitsa and the Ad Council. Have you ever wished for a change but weren't sure how to make it?

Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Wittmer, Jody Sweetie. Monica Patton, Elaine Welteroff. Learn how to get comfortable pivoting because your life is going to be full of them. Listen to these women and more on She Pivots. Now on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Tune in to All the Smoke Podcast, where Matt and Stacks sit down with former First Lady, Michelle Obama.

Folks find it hard to hate up close. And when you get to know people and you're sitting in their kitchen tables and they're talking like we're talking. You know, you hear our story, how we grew up, how Barack grew up, and you get a chance for people to unpack and get beyond race. All the Smoke featuring Michelle Obama. To hear this podcast and more, open your free iHeartRadio app. Search all the smoke and listen now. Hey, it's Jorge and Daniel here, and we want to tell you about our new book. It's called Frequently Asked Questions About the Universe. Because you have questions about the universe, and so we decided to write a book all about them. We talk about your questions.

we give some answers, we make a bunch of silly jokes. As usual, and we tackle all kinds of questions, including what happens if I fall into a black hole, or is there another version of you out there? That's right. Like usual, we tackle the deepest, darkest, biggest, craziest questions about this incredible cosmos. If you want to support the podcast, please get the book and get a copy, not just for yourself, but, you know, for your nieces and nephews, cousins, friends,

parents, dogs, hamsters. And for the aliens. So get your copy of Frequently Asked Questions. about the universe is available for pre-order now coming out in November 2nd. You can find more details at the book's website, universefaq.com. Thanks for your support. And if you have a hamster that can read, please let us know. We'd love to have them on the podcast.

Hey, Daniel, I've noticed something pretty strange about how time works. Oh, yeah? Did you have time to make your own theory of time? Well, that's the thing, right? Like, I noticed that time seems to speed up as you get closer to a deadline. That's true. Deadlines seem to be infinitely far in the future until all of a sudden they're on top of you. Right? I think the only thing that can explain it is that time itself bends when it's close to some procrastination. Well, time does slow down near a massive object like a black hole. I think maybe I just need less massive deadlines. maybe? Or maybe your to-do list is so dense, it's becoming a black hole.

Definitely sucks you in. Hi, I'm Jorge. I'm a cartoonist and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm just part of of Jorge's to-do list. That sounds kind of inappropriate, Daniel. You mean I have to be like Daniel or have to be Daniel? I mean that I just contribute elements to your to-do list. Oh, I see.

You add to my things to-do list. I'm on your to-do list. But anyways, welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of IHeart Radio. In which our to-do list is to explain the entire universe, bit by bit, concept by concept, puzzle by puzzle to you. Our wonderful listeners who deeply and desperately want to understand the nature of the universe we find ourselves in, the whole project of physics, starting with ancient man looking up at the sky and extending to the Greeks trying to understand the nature of reality ends here on the podcast when we try to break down the nature of the universe and explain all of it to you. And hopefully a timely matter, right?

Because, you know, time is money and money is gravity. money is dough and everybody needs it yeah sometimes we eat too much dough but yeah it is a big beautiful universe and we like to talk about all the things in it even time itself because time is part of the universe right time is part of the universe though we don't understand

is it a fundamental element of the universe is it something that bubbles up and just sort of appears from other fundamental things like ice cream and lava and hurricanes or is it really deeply ingrained and on this podcast we don't shy away from asking those really big, deep, and difficult to grapple with questions. You know, like, what is time anyway?

How do you even define it? How do you even ask crisp and precise questions about this slipperiest, but most essential of concepts? Yeah, because time always seems to slip by you, no matter what you do. And asking these questions is not just in our to-do list, but it's also in our want-to-do list, right, Daniel? I mean, that's kind of what you're getting paid for. That is what I get paid for, not on a day-to-day basis.

I mostly am cracking open particles at the large Hedron Collider. But these are the big goals of physics, is to understand the nature of our experience. You know, some people think physics is sort of like abstract and separated from humanity, but it's taken me a long time to realize that it's the most human of sciences because it asks these questions at the very core of the context of what it's like to be alive.

You know, you notice things slipping into the past through this weird instantaneous slice called the present. It's definitely something we love. like to understand. Are you saying physics out humanities to humanities, Daniel? You out philosophize the philosophers. Yeah, you know, I used to think that physics was the most interesting science because it was probably the most fundamental, the most universal that somehow it escaped humanity, that if aliens came, they wouldn't be interested in our biological advances because they wouldn't be relevant to them, but they would be interested in what we've learned about physics.

I'm not so sure about that anymore. I can hear all those mathematicians out there laughing at you right now. You're like, you're pure. We're the purest. Even mathematicians, man. Mathematics is just codification of the logic in human brains. And I suspect that aliens might not even do math. And if they do, we might not even recognize it. Oh, man.

You're saying physics supersedes math from a human point of view. Oh, boy. We just started an academic war here. You just did. That's not what I'm saying at all. I'm saying that everything we do is based in human thought and contextualized by human questions. But that doesn't make it worthless. It means that we get to ask.

really interesting questions about what it's like to be alive. Yeah. And so sometimes those questions get into the idea of time itself because I think maybe most people have a conception of the universe that's sort of existing outside of time or like it's a universe moving through time. But actually physicists think of time as part of the universe, right? It's like another thing in it. Yeah, we don't really understand. Physicists like to divide elements of the universe into things that are fundamental, meaning they're like basic, they're essential. they have to be there.

They define what the universe is. And then other things that are emergent that sort of come out of the interplay of those fundamental objects. And we don't know if time is fundamental, if it's like absolutely essential. It's part of the nature of the universe or if it bubbles up from something else.

We don't know if space and matter sort of sit in a framework of time, which is external to them, or if the whole thing is just, you know, comes up from some other weird, deep nature we haven't even imagined. Like, is it hard-coded into the circuitry of the universe or is it like a program running on top of the chip of the universe? That's kind of what you mean, right?

Or is it just the way the humans think? You know, it might be that our tendency to think in terms of stories and narratives which have cause and effect might bias us to see things as flowing forwards in time when in reality the truth might be much more complex. We just did an episode about how causality, cause and effect might not. not even be an essential element of the universe. And so it might be that time itself could be something that's just sort of very human. Yeah, it's sort of like this podcast. We have no narrative, right? We are just jumping around in time to whatever comes into our minds here. But it's these kind of basic questions that give us a launching off point for asking questions and doing studies and trying to like find a way to grapple with these things scientifically. Because we could sit here

and smoke banana peels and talk about the nature of time forever without making progress. But we want to do experiments. We want to use science to add to our body of actual knowledge about time. Yeah. So we'll be exploring a little bit about this weird concept of time and especially about how time behaves according to our theories about the universe. So today on the podcast, we'll be asking the question. Why does gravity?

Slow down time. Or should I ask it, slow down time? Or maybe you should use your gravest voice, you know, your James Earl Joan voice. You're trying to make a gravity joke? That's kind of heavy, man. Don't make light of the situation. I'm a massive fan of that movie, you know. Now, this question kind has two components to it, like you just said.

It has gravity and time in it because we know from physics that somehow gravity affects time or is related to time. because and the time can be affected. It's not like some sort of absolute thing. That's right. We do not have like a single clock for the universe. The simplest model of how time works might be imagining that the universe all over the place is in one state. You know, particles are going in some direction or they're in some location. And then things sort of tick forwards and everything takes a step forward in unison.

And in that picture, the whole universe has like a single clock. But we've learned over the last hundred years that that conception of time is not very very. The time flows differently in different parts of the universe and differently for different observers and moving at different speeds. So time is much weirder, more local, and less universal than we ever imagined. In addition, it's also weirdly bent by heavy objects.

Yeah, things that are really heavy, like black holes or even just our planet, they have gravity and that somehow makes time slow down. Now, that's a pretty weird concept. I guess that was in that movie Interstellar. It was exactly in that movie Interstellar. Every time they visited a really heavy planet or came near a black hole when they left,

they found that the rest of the universe had experienced a lot more time than they had. And so I think a lot of people are maybe familiar with this part of relativity where like if you're moving fast, time slows down. But also it also happens when you're near a heavy object. That's right.

These are two completely separate effects with different sources and importantly different mechanisms and different consequences. Yeah. So as usually, we were wondering how many people out there even knew that gravity slowed down time

or much less I have thought about why it slows down time. So Daniel went out there into the internet to ask people why does gravity slow down time? I like the way you say I went out there into the internet. It makes me feel like I got sucked into my computer and went and like visited these people. Like Tron, like you pulled the 80s movie plotline and you got sucked into your CRT monitor.

Yeah, I'm writing those Tron cycles around the internet. gathering information from our listeners. Dodging crulls and tweeting left and right. But if you out there would like me to beam myself into your inbox with crazy questions about the nature of the universe so you can hear yourself speculate about them on the podcast, please don't be shy. Write to me to questions at Danielanhorpe.com. Yeah. And you actually answer every email, right? I do answer every email with every question from everybody.

You take the time. It slows me down, but I love it. All right. Well, here's what people had to say. Well, gravity is basically the same as acceleration. And when you're accelerating, the speed of light stays the same for everybody. And so it must be time that's slowing down. And when we're going at our normal speeds, you don't really notice it.

So gravity being equivalent to acceleration causes the same effect. Gravity slows down time because as particles with mass move through the Higgs field, they're slowed down. The stronger the Higgs field the gravity is, the slower the quarks and the leptons move, causing quantum interactions to take place at a different rate, depending on the strength of the field. Scaled up into the macro world, that's what we experienced as time. We would never notice if our particles were moving at a faster, slower speed, if everything we observed was experiencing the same Higgs fields along with us. I guess that the reason is because gravity bends down space time.

And since the length of spaces, space changes, in order for the speed of light to remain constant, also time must flow in a different way. First of all, time is like space dimension, so the gravity can affect time. But how I'm thinking right now, like the gravity, thus to also the objects, then contracts them. And this is what I'm thinking, that it contracts time.

It squeezes it like the space. And doing this, it's slowing it down. I guess gravity could slow down time because gravity makes those wells in like the space-time fabric. And so as that material of the universe gets stretched out, the time would slow down as it gets deeper, sort of like if you're driving over hills versus driving over flat things. I can only imagine it's got something to do with space time. Like gravity isn't just a force, but as Einstein says, it's a warping of space

time. And if you've got a ton of gravity, say in a black hole warping space, space is not really a thing, but space time is a thing. So you can't affect one without affecting the other is what I'm guessing. I know time moves slower, relatively speaking for someone that travels faster. So maybe gravity slows down time because it also necessarily makes an object or person move fast. All right. Some pretty good answers here. A lot of people sort of knew that time can slow down. And I guess they assumed that gravity is somehow related to relativity and moving fast. And so why not? Yeah, a lot of really interesting and insightful and thoughtful questions here. Some confusion about the role of the Higgs field, but also a lot of good concepts about the connections between space and time and how masses have to bend both of them. All right. Well, let's get into this idea of time slowing down. And I guess let's recap maybe. the one people are most familiar with, maybe, which is the one that when you're moving fast

and especially close to the speed of light, time slows down for you. So maybe walk us through that and then we'll get into the one about gravitation. That's right. So the one you're probably more familiar with, we call velocity time dilation. This is something that happens in special relativity. And to think about this, you should imagine an empty universe, one without really massive objects that are going to make space curvy and all sorts of weird stuff. In this universe, light travels in straight lines and spaceships zoom around and all the time dilation just comes from the relative velocity of objects. And the most important thing to remember is that moving clocks run slowly. So people often make the mistake of saying, oh, I'm going fast in a spaceship so my

time should slow down, right? Well, it's only moving clocks that slow down. So if you're in a spaceship and you're holding the clock, then the clock's not moving relative to you. So you're not going to see it's slow down. So you never experience velocity time dilation because you're never moving relative to yourself. Somebody else out there on a planet that you're zipping by could see your clock running slowly because your clock is moving for them. And so moving clocks run slowly, meaning that your clock would run slow. You don't experience it, but they see your clock moving slowly. Right. Like if your time is moving slowly, you don't notice it because, you know, your brain is also sort of moving slowly in a way, right?

So everything about you is moving slowly as well. And so you don't notice that you're actually moving slowly. But even that suggests there's some sort of like universal picture of what really happened. And in that universal picture, it makes sense for you to feel like time move normally, even though it actually moved slowly. But there is no like what actually happened. Some observers seeing you go by the planet at 95% of the speed of light, they see your time as moving slowly.

You see your time as moving normally. You can try to unify those into one picture of what. actually happened, but there is no what actually happened. There's just what different observers observe. And what you see depends on where you are and how fast things are moving relative to you. For example, if you're on the spaceship and you're looking at the clock on Earth, you see Earth moving past you at high speed. And so you see Earth's clock running slowly. So Earth sees your clock running slow. You see Earth's clock running slow. What actually happened? Well, both of those things happen is just what happened depends on where you are.

I guess maybe you step me through it. So I'm here on Earth. I'm watching you on a spaceship go by at 95% of the speed of light. Like, what does it mean for me to see your time slow down? Like, I see the clock inside of your spaceship ticking, but it's not ticking as fast as my clock. Exactly. So you get a telescope. It's super powerful so you can look at a clock that's inside my ship and you watch it and you see its ticks going.

You compare it to a clock that's right in your hand, not moving relative to you. And so every time my clock ticks on the spaceship, you see the clock in your lap ticking 10 times. So time is moving faster for you in your lap than you see it moving for me on the ship. And this, of course, already takes to an account the fact that it takes light time to get to you to the telescope from the ship. We sort of factor that out already. So what does it mean? So I have a telescope and I'm pointing it at you, but you're zooming by. So I have to kind of track you.

I have to move my telescope. And so I track you as you're going by and I'm moving my telescope. and I'm measuring your ticks and they're not ticking as fast as my clock. That's right. You see my clock as running slowly. So you see me aging slowly. You see me moving slowly. You see me like waving back to you in super slow motion.

So you see my clock is running slow relative to your clock. And that's the sort of a consequence of just how the universe is or somehow the limitations of the speed of light. Yeah, it's really interesting. You can start from lots of different places to derive this. You can say like, well, we've seen. that nothing can move faster than the speed of light. That's a hard limit on the speed of information. And from that, you can derive these things,

these time dilation effects. And you can walk yourself through an example. We actually have it worked out in detail in our book. We have no idea, a guide to the unknown universe. And you can think about how a photon clock would tick in a spaceship as it moves up and down. And if it's moving really fast, then it has to go like a little bit diagonal, for example.

And because light can't move faster than the speed of light, When it moves on a diagonal, it takes longer to get from one side of the clock to the other. So it's basically the constancy of the speed of light and the fact that nothing, even light, can move faster than the speed of light, directly lead to this consequence. The time goes slower for moving clocks. Right. And so there's this sort of famous scenario called the twin paradox or the twin experiment where, like, you take a pair of twins here on Earth and you put one of them in a spaceship that goes out into space at the speed of light and then comes back. and supposedly when they come back, they're a different age than the one that stayed on Earth.

Yeah, this is a wonderful paradox because it gets to the heart of like what actually happened. Because in the example we're talking about, it feels awfully symmetric, right? Like, I'm looking at you through my telescope and I'm seeing your clock tick slowly, but maybe on the ship, you're also looking at me on Earth

and you see my clock ticking slowly. So you want to feel like, well, what actually happens? And the way to like bring that to a point is to like bring those two people back together. So if one twin goes off on their spaceship, twin on Earth sees the spaceship twin aging slowly, and the spaceship twin sees the Earth twin aging slowly. And so you want to feel like, well, which one is actually younger? And so you turn the spaceship twin around, bring it back to Earth, and you ask like, well,

which one is younger? And what you discover is that the spaceship twin is younger. And so that sort of breaks this symmetry. And you wonder, like, hold on a second, if this is supposed to be symmetric, if it just depends on relative velocities, why is it? it that one of them is now younger than the other? So that means it's not just like a perception thing. It's like time actually slowed down for the space twin.

In that case, it is because they've broken special relativity. One of the rules of special relativity is no acceleration. You can fly at high velocity and then you can make all these measurements and do these calculations and things are just as we described. But as soon as you accelerate, then you're out of the bounds of special relativity. It's something you can do. You can do it. We can calculate it.

but it makes things more complicated. And the simple rules we described earlier of moving clocks run slow get much more tricky. So here, for example, when the spaceship twin turns around to come back to Earth so that he or she can compare her age

with her twin, then she's accelerating because changing your direction means accelerating. And what happens when you accelerate is you break the symmetry. Now, one of the twins is accelerating, the other one is not.

And when you accelerate, weird things happen to time. Specifically, when the twin that in the spaceship turns around and accelerates, time jumps forward for the rest of the universe. So time runs a little slower for the accelerating twin, jumping forward for the rest of the universe, which is why the twin in the spaceship now is younger when they arrive at Earth than the twin that stayed home.

You just kind of accelerate it a little too fast there for my brain. I guess one question is, but isn't acceleration also relative? Like if I'm accelerating away from you, I see you accelerating away from me. So why is an acceleration also like kind of symmetric? And the second question is you're saying it's acceleration that causes time to slow down. So that's sort of a different scenario? Yeah, these are great questions.

Acceleration is actually absolute. Velocity can only be measured relative to other stuff. Like if you're an empty universe, you can't measure your velocity because there's nothing to move past, right? Velocity is only defined, only has meaning relative to other objects. That's not true for acceleration. Acceleration is something you can measure even in an empty universe. So, for example, put yourself in a spaceship, you're in an empty universe. If you're moving, your motion has no meaning.

There's no experiment you can do inside your spaceship to measure your actual velocity because there's nothing outside to measure it relative to. That's not true for acceleration. You can measure inside your spaceship whether or not you're accelerating. For example, you can tell, do I feel a force? Am I being pressed by one. side of the spaceship. You'll feel those G forces if you're accelerating. So acceleration is different from velocity. You can't have an absolute acceleration. You can measure it. And so that's

why the rules are different for acceleration and for velocity. And so then you're saying that the twin who went out into space will actually be younger when they come back. They will actually be younger. Exactly. So when the twin comes back to Earth and is now in the same reference frame moving at no velocity relative to the other twin, then you can ask real questions about in this reference frame who is younger and who is older. And because the twin that went into space also did some acceleration, their time slowed down a lot during that acceleration or equivalently time for the rest of the universe jumped forward during that acceleration. So that breaks the symmetry because only one twin accelerated and so the twin that stayed home actually is older. And you know,

this isn't just like a thought experiment. There actually are a pair of twin astronauts. One of them went and spent a lot of time up in space and the other one didn't. And they've compared the two. Really? All right. Well, let's get into that real life experiment and then let's talk about how gravity changes time. But first, let's take a quick break. I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these skills and I get, eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face.

When you think about emotion regulation, like, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome as a result of it, if it's going to be beneficial to you. Because it's easy to say like, like go you, go blank yourself, right? It's easy. It's easy to just drink the extra beer. It's easy to ignore, to suppress, seeing a colleague who's bothering you and just like walk the other way.

Avoidance is easier. Ignoring is easier. Denials is easier. Drinking is easier. Yelling, screaming is easy. Complex problem solving. Meditating. You know, takes effort. Listen to the psychology podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. And here's Heather with the weather. Well, it's beautiful out there, sunny and 75, almost a little chilly in the shade. Now, let's get a read on the inside of your car. It is hot. You've only been parked a short time, and it's already 99 degrees in there. Let's not leave children in the back seat while running errands. It only takes a few minutes for their body temperatures to rise, and that could be fatal.

Cars get hot, fast, and can be deadly. Never leave a child in a car. A message from Nitsay and the Ad Council. I'm Simone Boyce, host of the Brightside podcast, and on this week's episode, I'm talking to Olympian, World Cup Champion, and podcast host, Ashlyn Harris. My worth is not wrapped up in how many things I've won, because what I came to realize is I valued winning so much that once it was over, I got the blues, and I was like, this is it. For me, it's the pursuit of greatness, it's the journey, it's the people, it's the failures, it's the heartache. Listen to The Bright Side on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

Hello, Puzzlers, let's start with a quick puzzle. The answer is Ken Jennings' appearance on The Puzzler with A.J. Jacobs. The question is, what is the most entertaining listening experience in podcast land? Jeopardy-truthers who say that you were given all the answers believe in... I guess they would be Kenspiracy theorists. That's right. Are there Jeopardy-truthers? Are there people who say that it was rigged? Yeah, ever since I was first on, people are like,

they gave you the answers, right? And then there's the other ones which are like. They gave you the answers, and you still blew it. Don't miss Jeopardy legend Ken Jennings on our special game show week of The Puzzler podcast. The Puzzler is the best place to get your daily word puzzle fix. Listen on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast.

All right. All right. We're asking the question. Why does gravity slow down time? And we were talking first about the twin experiment where you send the twin out into space. They go really fast. They come back. And they've aged less. And this age and you're saying is actually due to the acceleration. It's not actually due to the speed, right? There are two effects there. There's the velocity time dilation, which is the one you're very familiar with, where moving clocks appear to be slow. But that's not a universal phenomenon. It depends on who is doing the observing and their relative velocity. Acceleration, however, is different. And the acceleration does cause an actual slowing of time, which can be measured by everybody. Because it's not symmetric.

It's absolute. You can measure somebody's absolute acceleration, and that makes them different. So it breaks the symmetry. So really, when people say going fast slows, down time, really we should be saying accelerating fast causes time to slow. And is that something that just somehow changes time or is it because you're pushing all of the particles and somehow that slows how they interact? Or how do you explain acceleration changing time? Well, first of all, it is still correct to say that moving fast slows down time. It's just that

it slows down time only for observers, right? Observers moving fast relative to those clocks. It's still true. It still happens. It's not like just a perception issue. It's a true thing about the universe. Acceleration slows down time in a different way. It's a different mechanism. It's much harder to understand in terms of these like ticks on the photon clock, but you can see it also comes as a consequence of the speed of light. All right, well then, so that's acceleration and time dilation because of acceleration and because of going close to the speed of light. But the one we're talking about today is the one due to gravity. So whenever you're next to something that's really heavy or massive, time also slows down. But does it

slow down in the same way that acceleration slows time down or does it slow down in the same way that going at a constant speed slows time? Do you know what I mean? Like is it observer base or is it actually like time slowing down? Yeah, great question. And so gravity slows down time or the curvature of space slows down time. And this effect on time is the same as acceleration slowing down time. And in fact, it's sort of a deep idea here because in general relativity, one of the whole inspirational ideas, of general relativity is that gravity is equivalent to acceleration. You know, the experiment we talked about a minute ago, like if you were in space, could you tell if your spaceship was accelerating? You could. And in fact, it would feel like you were being pressed against one wall of the ship. Or equivalently, it would feel like you were standing on a planet with gravity, right? You can in fact make artificial gravity on a spaceship by providing acceleration, either by spinning or by zooming off in one direction. So the whole idea that gave Einstein the inspiration for general relativity was this one.

It's called the equivalence principle that says that there's no difference between gravity and acceleration. And so we just went through the details of how acceleration can cause time to move slowly. And this is exactly the same effect. Gravity also makes time move slowly. The curvature of space around you makes time move more slowly. It's exactly the same effect. And so it's an absolute effect. not a relative one like for velocity.

So you're saying it's really sort of acceleration that causes time to slow down and gravity is sort of like an acceleration or is it the other way around? But I guess you can have acceleration without gravity. So it's more like gravity is kind of like an acceleration. Yeah, gravity is essentially like a geometrical interpretation of acceleration. Or, you know, said another way, what happens when you have mass in space? Well, it changes the curvature of that space. And so what happens then is that things move differently.

And they can appear, for example, to be accelerating. If you aren't aware of the curvature of space, then it seems like there's a force there which provides an acceleration towards those masses. And so that's really what gravity is. Gravity is the bending of space in a way that appears to provide acceleration. And so gravity and acceleration really are exactly the same phenomenon. Either acceleration in empty flat space gives exactly the same effects as curving of space itself. You remember we did recently a fun episode about how if you're accelerating, there are times that photons cannot catch you.

You have essentially an event horizon if you're accelerating constantly. And the explanation there was the same as here is that accelerating constantly is sort of the same as curving space. And we know that if you curve space, weird things happen like you can be inside a black hole and photons cannot escape. And so the core idea here is to understand that accelerating is really the same thing. as gravity. And so if you think of gravity is causing time dilation, it's really the concept of acceleration causing time dilation that's sort of mentally equivalent. There are a lot of leaps here, I feel like, and it's kind of hard to keep track of, because I feel like you're saying gravity is acceleration and gravity is also the curvature of space. Does that mean that all

acceleration is also the curvature of space? Or can you have acceleration in not bent space? Or can you think of all acceleration, even by like electromagnetic forces? as some sort of curvature of space. Yeah, that's a deep question. And there are people out there trying to interpret all acceleration in terms of the curvature of space or, you know, like all forces as being the product of the curvature of space. But that's not necessary. You can think about acceleration in flat space, you know, just like put on a rocket ship,

accelerate your spaceship, now you're going fast. But the effects of that on your time and the way you perceive the universe are equivalent to if space was curved around you. So you can think of acceleration separately from the curvature of space and from gravity, but it has exactly the same effect because that's really kind of what gravity is. So meaning I feel like then that you're saying that it's not really gravity that's slowing down time, it's really the acceleration caused by gravity or the bending of space caused by gravity, which is the same as acceleration.

Yeah, there's lots of different ways to think about it. You can think of acceleration caused by gravity is really just like motion through curved space. And one of the other impacts of curved space is that time also slows down. All right. Well, then maybe let's try it. Let's see. Why does acceleration slow down time? Because that seems to be the bigger problem, right?

That seems to be the bigger question. Yeah. So I guess you can say that gravity slows down time because it's equivalent to acceleration slowing down time. Why does acceleration slow down time? That gets back to the speed of light as the limiting piece of information. You can derive this in a flat universe using the twins. as an example, it's a little bit more complicated mathematically than like normal special relativity where you can do these observing frames with Lorentz transformations.

It gets a little hairy and mathematical, but the core concept that it comes from is this maximum speed of light. Everything comes out of that, both time dilation from velocity and also time dilation from acceleration, which really is equivalent to time dilation from gravity. So I guess if I'm in a spaceship and I'm accelerating the limitations of the speed of light, Does that mean that, are you sort of saying that it somehow limits how the things can evolve inside of that spaceship, you know, move from, you know, information between molecules and things like that? So things sort of evolve slower or they have a limit to how fast they can evolve. Yeah, I would say it a little bit differently.

I would say that requiring that the speed of light is constant and that everybody observed the speed of light always to be the speed of light restricts the kinds of universes that we can have. It puts a lot of restrictions on the way space and time have to work in that universe. And these effects that we're talking about, the slowing down of time by velocity and by acceleration, are consequences of the structure of that space and time. They sort of come out of that. All right. So you're saying it's just the way it is. When things accelerate, there's a limit in the speed of light.

And so that makes time sort of slow down. Makes everything slow down. And I think it's super cool because it's not symmetrical. You know, like two people can agree on who is accelerating more. And so that means that they can agree on whose time is moving more slowly. Or, you know, said another way in terms of gravity, like you and I can agree that if you're near the black hole, then you're in a part of space that's curved more than my part of space.

And so we can agree that your time should be moving more slowly. That's not true for the spaceships, right? If we're in two spaceships passing each other, it feels symmetric because it is symmetric. I say you're moving past me. you say, I'm moving past you, everybody's right. In the case of the black hole, we can agree. It's not symmetric. So we should agree that your time is moving more slowly.

So then acceleration causes time to slow down. And definitely when you're near a black hole, you are being accelerated probably a lot because black holes are very massive. They're pulling you in. And so if I'm near a black hole, then I'm going to be moving slower through time than you who is out way far from the black hole. Exactly. And so another cool thing is that I see.

see your time moving slowly, it means that you see my time moving more quickly. This is the real difference with velocity time dilation. In velocity time dilation, we both see each other's time moving more slowly. Here, if you're near a black hole and you're looking out into the universe, you see the rest of the universe running forward in time very quickly. And as you get closer and closer to the black hole and more and more curvature, you see the universe's clock speeding up into the future. So then, like if I'm falling into a black hole, like all the stars will suddenly start speeding up around me. Like I'll see the universe kind of in fast forward. Exactly. And some people imagine, well, does that mean that you'll see like the end of the universe, the end of time?

Or you'll know like the final fate of the universe just as you fall into the black hole. Well, that would be super cool. But unfortunately, it takes a finite amount of time from your perspective to fall into a black hole. So there isn't time for all that information from the future universe to get to you. So you see the fast forwarded universe for a while, but you don't see like all the way into the infinite future. Well, we talked about this last time. Like when you actually get to the surface of the black hole, then time actually stands still, right? Like it slows down more.

The closer you get to the black hole and then it sort of stands still at the surface. It sort of does, but that's only for somebody far away. They see your time moving slowly and they see you sort of smeared across the event horizon. But for you, you actually fall into the black hole. You don't notice anything different changing, right? You notice the rest of the universe is clock speeding up. But from your perspective, you fall into the black hole, you pass the event horizon, you get sucked into the singularity. So your time is definitely finite from your perspective.

And this is not something that we understand very well. There's all sorts of weird paradoxes and contradictions here about how one person sees you not falling into the black hole into the end of time. And you see yourself actually falling in. It's not something that we know how to reconcile. I think what you're saying is that the person falling in, you're saying they'll see themselves falling through. But we don't actually know if that's true, right? like they might just actually freeze at the edge. We just don't know.

Yeah, we don't know. It's true because nobody's done it and come to report back. It's possible they actually just freeze the edge. And they think that they're inside, but it's actually that the inside of a black hole is a hologram projected from the surface of the black hole. We just don't really know what's going on inside a black hole. And so this effect of gravity on time doesn't just happen in black holes. I mean, black hole is sort of the extreme example.

But it actually happens like every day and everywhere. Like here on Earth, the Earth is slowing down time and even like, I'm slowing down time sort of for the things around me, right? Yeah, absolutely. Everywhere there is curvature, time is slowed down. And the Earth curved space, right? Because the Earth has a lot of mass. That's why, for example, you don't fall off the Earth. You're feeling it's gravity.

So anywhere you're in a situation where you're feeling gravity, you're also having your time affected. And because gravity is stronger as you get closer to the Earth and weaker as you move away from it, that means that the clocks are. are variable. Time flows in a variable way as a function of the distance from the center of the earth. And this is something you can measure. Like over your life, your feet will age one second more than your head. Only if you spend a lot of time standing up, Daniel, which the cartoonists don't do a lot of. So I guess our feet are still young. We are still light on our feet. Yeah, exactly. That's why you lay down all the time, just to keep your body like in sync. Just to keep my feet young. All right. Well, let's get into what this all,

means? Does that mean that we're all moving slower through time than we should? And whether does that mean that also there's no universal clock actually to measure time in the universe? So let's get into that. But first, let's take another quick break. I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these and I get eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face. When you think about emotion regulation,

like you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome as a result of it if it's going to be beneficial to you because it's easy to say like go you go blank yourself, right? It's easy. It's easy to just drink the extra beer. It's easy to ignore, to suppress, seeing a colleague who's bothering you and just like walk the other way. Avoidance is easier. Ignoring is easier. Denial is easier. Drinking is easier. Yelling, screaming is easy.

Complex problem solving, meditating, you know, takes effort. Listen to the psychology podcast on the IHartRadio app, Apple Podcasts, or wherever you get your podcasts. How serious is youth vaping? Irreversible lung damage serious. One in 10 kids vape serious, which warrants a serious conversation from a serious parental figure. Like yourself, not the seriously know-at-all sports dad or the seriously smart podcaster. It requires a serious conversation that is best had by you. No, seriously, the best person to talk to your child about vaping is you. To start the conversation, visit talk about vaping.org, brought to you by the American Lung Association and the Ad Council. Have you ever wished for a change but weren't sure how to make it?

Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive in. into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweeten, Monica Patton, Elaine Welteroff. I'm Jessica Voss. And that's when I was like, I got to go. I don't know how, but that kicked off the pivot of how to make the transition. Learn how to get comfortable pivoting because your life is going to be full of them.

Every episode gets real about the why behind these changes and gives you the inspiration and maybe the push to make your next pivot. Listen to these women and more on She Pivot. Now on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. I'm Simone Boyce, host of the Brightside Podcast, and on this week's episode, I'm talking to Olympian, World Cup Champion, and podcast host, Ashlyn Harris. My worth is not wrapped up in how many things I've won, because what I came to realize is I valued winning so much that once it was over, I got the blues, and I was like, this is it.

For me, it's the pursuit of greatness. It's the journey. It's the people. It's the failures. It's the heartache. Listen to the bright side on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. All right, Daniel, my favorite question in all these topics. What does it all mean?

Man. So anything with gravity, bends space around it, which causes acceleration and accelerating things slow down in time. So things are always slowing down in time everywhere all the time. Yeah, everywhere there's a gravitational field. Clocks are being slowed. So if you're in a gravitational field, then your sense of now is moving forward differently than other people who are like out in deep space. And so if you spend a lot of time near gravitational objects, then you are younger. than you otherwise would be. Right, but it's not just gravity too, right?

It's like if I get in my car and I accelerate up to the freeway, I somehow slowed down time for myself. Yeah, gravity and acceleration. They're equivalent. And so both of them will slow down time. Every time you accelerate, the universe sort of leaps forward a little bit relative to you. If you accelerated a lot for a long time,

you would notice clocks around you seeming to move forward faster than one second per second on your clock. And so I guess that means, first of all, that there's no real age to the universe? Is that really true? Does that mean that, you know, there's no, like, absolute time? Yeah, there's no absolute time,

which is really frustrating if you like to have a sense that, you know, there's truth, that there's something really going on out there outside of our skulls. It's frustrating to imagine that, like, different people can tell different stories and they can both be correct. But there's an even deeper problem

if you try to extrapolate back to time equals zero. You know, we say this thing. The universe is 13. 8 billion years old. Well, according to what clock, right? Is that clock been moving on a spaceship? Has that clock spent a lot of time near a black hole? Because if so, it's going to have a different answer. And so because different parts of the universe have different curvature, right, like near black holes or near suns or whatever, then different parts of the universe have aged differently since its beginning. So the universe does not have one single age, just like your feet and your head

are not the same age, assuming you haven't spent your whole life in bed. The parts of the the universe have different ages. I do a lot of handstands, so I'm trying not to go bald, so I'm keeping my head. I think we all need a video of you doing a handstand. Let's see that. Let's do it. But I guess that confuses me because you told me earlier that acceleration is absolute, so I can measure acceleration absolutely, and time is actually bent by acceleration.

So couldn't I, I don't know, find a spot in the universe that's never been accelerated and say that, like, that's the absolute age of the universe? Well, that's the age of that part of the universe. And that would be the oldest part of the universe. That part would have experienced the most time. But, you know, if you had put a clock somewhere else in the universe and let it run since the beginning, it would have a different number. So, like, different parts of the universe have different ages.

And you might reasonably say, well, the oldest part of the universe, I'm going to use that as the age of the universe. Yeah. Nobody cares how young my feet are. I care, man, I care. You care. It sounded a lot like I care, though, right? That sounded sincere, didn't it?

But yeah, so there's sort of an age limit to the universe, right? Like you're saying that there is an absolute age of the universe by which we can measure all other ages. Yeah, there's a maximum age to the universe, right? There is a number beyond which no part of the universe could have experienced more time than that. And if you wanted to find that as the age of the whole universe, I guess that makes sense. But I think more conceptually it makes sense to imagine like how many clock ticks have there been in a given part of the universe.

And that's not equal all over the universe. It depends on how much gravity there is nearby. Right. Because I guess even our solar system is being accelerated around the Milky Way. So therefore our time is sort of being slowed down in that way too. Yeah. The curvature of the center of the Milky Way and that super massive black hole does affect the motion of the sun and the curvature nearby, which slows down our time. And I guess you were trying to tell me earlier.

that there is sort of a philosophical question here, which is like, does acceleration change time or does time change acceleration? Yeah, it's familiar to think about general relativity is saying that mass causes space to bend and then the curvature of space tells masses how to move, right, that you get an appearance of acceleration and this effective force of gravity

because space itself is bent. There's a missing component. there, right, which is that time is also curved by mass. So you have mass somewhere, it doesn't just curve space. It also curves time, which is what we've been talking about today. And the curving of time also contributes to this force of gravity. So gravity is an apparent force that comes not just from the bending of space, but also from the bending of time. The two work together because space time really is sort of one thing to create this effect of gravity. You're saying time, you can also bend time just like you can bend space and somehow that's where gravity comes from.

Yeah, exactly. It's familiar to use like a rubber sheet analogy where parts of space are bent and that changes how an object moves sort of naturally. And you know, that's a little confusing because in what direction is the rubber sheet bending? It's bending in some sort of like external direction. You can measure it in terms of like another dimension. In reality, in our space, it's intrinsic curvature. It just changes the relationship between points in space, the relative distances. So that's a familiar way to think about how the curvature of space affects the motion of an object. You can do something similar for a time. You can imagine like different parts of the universe flowing with different time, right? It's sort of like you're moving down a river and different

parts of the river are moving faster than others. And that will affect the motion of objects. Like you have a really big object in a river and the river is flowing faster on the left side than on the right side. It's going to change the way that object moves. It'll like tug it towards the slow-moving part of the river. And that's part of how the curvature, space, and time creates this effect of gravity that objects no longer move in what we perceive to be straight lines. I see. You're saying, like, you can think of it the other way around.

Like, me falling to the Earth or me being in orbit around Earth is actually a consequence of the differences in time. And, like, differences in time cause me to move from one place to another. Exactly. It's space and time being occurred. Both affect your trajectory. So in a way, asking why does gravity slow down time, you could also maybe ask why does time slowing down time cause gravity? Yeah, exactly. Another way to think about it is that the effect of gravity we observe is coming from the curvature of space and the curvature of time.

And that gravitational time dilation is just another aspect of the curving of space time in respect to mass. So I guess in the end, you just have to say that it's all sort of kind of the same thing. It's all sort of related and it's all, you know, you can look at it from one way or you can look at it upside down. But at the end of the day, it all comes down to really, I think, acceleration, right? Like things that accelerate have to slow down in time because of the speed limit of the universe. Yeah, that's a consistent way to think about it. I prefer the sort of geometrical way to think about it that we're living in a universe that's courage and warp, but we don't perceive it directly.

And so the way things move through space and time are affected by these like invisible warping. That sort of geometrically makes the most sense to me. But you can also think about it just in terms of acceleration in flat space, yeah. All right. Well, I guess that answer is sort of the question. Why does gravity slow down time? The answer is because of acceleration. And why does acceleration slow down time?

Well, we probably need a whole new podcast episode about it. Yeah. Why does gravity slow down time? Because time is bent in the presence of mass just like spaces. I think you just went all the way around. Why does acceleration slow down time? because slowing down time causes acceleration. Sounds like a great answer.

And it causes podcasts to go in circles. Yeah, and now you can just hit replay and listen to this episode all over again. And it should make sense, right? It's all pretty tricky stuff. In the end, it all comes down to consequences from our observation that the speed of light is the maximum speed of the universe. That just happens to be the universe we live in. And when we build in those constraints into our theories, these are all the consequences that come out of it. And maybe that points to what you were saying earlier that, you know, we have this speed limit.

it and it causes time to do weird things. So maybe time is not a fundamental thing, right? Like it's not outside of the universe. Maybe time is something that comes out of how the universe works. Yeah, or how we are perceiving it or measuring it. And you know, there's a whole universe of crazy ideas about time, even ideas that like time is not an essential part of the universe but comes out of something else or that there are multiple dimensions of time the way there are

multiple dimensions of space. and we're just moving on like a 1D line through three-dimensional time, man. There's a whole like crazy, crazy set of really fun ideas to dig into. Unfortunately, we are out of time for this episode. We'll have to find more time to get into it more. But we hope you enjoyed that. And maybe got you to think a little bit more about how young your feet are and how you should maybe do more handstand.

I want to see that video of you doing a handstand. I can do it actually. But now it's not the time. All right. Well, thanks for joining us. See you next time. 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.

Get fired up, y'all. Season two of Good Game with Sarah Spain is underway. We just welcomed one of my favorite people, an incomparable soccer icon, Megan Rapino, to the show. And we had a blast. Take a listen. Sue and I were, like, riding the lime bikes the other day. And we're like, we're like, people ride bikes because it's fun. We got more incredible guests like Megan in store, plus news of the day and more.

So make sure you listen to Good Game with Sarah Spain on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Brought to you by Novartis, founding partner of IHeart Women's Sports Network. And here's Heather with the weather. Well, it's beautiful out there, sunny and 75, almost a little chilly in the shade. Now, let's get a read on the inside of your car. It is hot. You've only been parked a short time, and it's already 99 degrees in there. Let's not leave children in the back seat

while running errands. It only takes a few minutes for their body temperatures to rise and that could be fatal. Cars get hot, fast, and can be deadly. Never leave a child in a car. A message from Nitzai and the Ad Council. I'm Simone Boys, host of the Brightside podcast,

and on this week's episode, I'm talking to Olympian, World Cup champion, and podcast host, Ashlyn Harris. My worth is not wrapped up in how many things I've won, because what I came to realize is I valued winning so much that once it was over, I got the blues, and I was like, this is it. For me, it's the pursuit of greatness. It's the journey. It's the people. It's the failures. It's the heartache.

Listen to The Bright Side on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast. Thank you.