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

0:01
Speaker 1: Hey, Jorhan Daniel here, and we want to tell you
0:03
Speaker 1: about our new book. It's called Frequently Asked Questions about
0:07
Speaker 1: the Universe because you have questions about the universe, and
0:09
Speaker 1: so we decided to write a book all about them.
0:12
Speaker 1: We talk about your questions, we give some answers, we
0:14
Speaker 1: make a bunch of silly jokes as usual, and we
0:17
Speaker 1: tackle all kinds of questions, including what happens if I
0:20
Speaker 1: fall into a black hole? Or is there another version
0:22
Speaker 1: of you out there that's right? Like usual, we tackle
0:25
Speaker 1: the deepest, darkest, biggest, craziest questions about this incredible cosmos.
0:29
Speaker 1: If you want to support the podcast, please get the
0:31
Speaker 1: book and get a copy, not just for yourself, but
0:34
Speaker 1: you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
0:39
Speaker 1: and for the aliens. So get your copy of Frequently
0:42
Speaker 1: Asked Questions about the Universe is available for pre order now,
0:46
Speaker 1: coming out November two. You can find more details at
0:49
Speaker 1: the book's website, Universe f a Q dot com. Thanks
0:53
Speaker 1: for your support, and if you have a hamster that
0:55
Speaker 1: can read, please let us know. We'd love to have
0:57
Speaker 1: them on the podcast. Hey Daniel, I've noticed something pretty
1:10
Speaker 1: strange about how time works. Oh yeah, you have time
1:13
Speaker 1: to make your own theory of time. Well that's the thing, right, Like,
1:17
Speaker 1: I noticed that time seems to speed up as you
1:19
Speaker 1: get closer to a deadline. That's true. Deadlines seem to
1:23
Speaker 1: be infinitely far in the future, until all of a
1:25
Speaker 1: sudden there on top of you. Right. I think the
1:28
Speaker 1: only thing that can explain it is that time itself
1:31
Speaker 1: bends when it's closed to some procrastination. Well, time does
1:37
Speaker 1: slow down near a massive object, like a black hole.
1:40
Speaker 1: I think maybe I just need less massive deadlines maybe,
1:43
Speaker 1: or maybe your two do list is so dense it's
1:45
Speaker 1: becoming a black hole. Definitely sucks you in. Hi am Jorge.
2:04
Speaker 1: I'm a cartoonist and the creator of PhD comics. Hi
2:07
Speaker 1: I'm Daniel. I'm a particle physicist and a professor you
2:10
Speaker 1: see Irvine, and I'm just part of Jorges to do list.
2:14
Speaker 1: That sounds kind of inappropriate, Daniel, I mean I have
2:17
Speaker 1: to be like Daniel, or have to be Daniel. I
2:20
Speaker 1: mean that I just contribute elements to your to do list. Oh,
2:23
Speaker 1: I see you add to my things to do. I'm
2:26
Speaker 1: on your to do list. But anyways, welcome to our
2:29
Speaker 1: podcast Daniel and Jorge Explain the Universe, a production of
2:32
Speaker 1: My Heart Radio, in which our to do list is
2:35
Speaker 1: to explain the entire universe bit by bit, concept by concept,
2:40
Speaker 1: puzzle by puzzle to you, our wonderful listeners who deeply
2:44
Speaker 1: and desperately want to understand the nature of the universe.
2:47
Speaker 1: We find ourselves in the whole project of physics, starting
2:51
Speaker 1: with ancient man looking up at the sky and extending
2:53
Speaker 1: to the Greeks trying to understand the nature of reality.
2:56
Speaker 1: Ends here on the podcast when we try to break
2:59
Speaker 1: down on the nature of the universe and explain all
3:02
Speaker 1: of it to you and hopefully a timely matter, right,
3:05
Speaker 1: because you know, time is money and money is gravity.
3:11
Speaker 1: Money is dough and everybody needs it. Yeah, sometimes we
3:14
Speaker 1: eat too much dough. But yeah, it is a big,
3:17
Speaker 1: beautiful universe and we like to talk about all the
3:19
Speaker 1: things in it, even time itself, because time is part
3:23
Speaker 1: of the universe. Right. Time is part of the universe
3:26
Speaker 1: that we don't understand. Is it a fundamental element of
3:28
Speaker 1: the universe? Is it something that bubbles up and just
3:31
Speaker 1: sort of appears from other fundamental things like ice cream
3:34
Speaker 1: and lava and hurricanes? Or is it really deeply ingrained?
3:38
Speaker 1: And on this podcast we don't shy away from asking
3:40
Speaker 1: those really big, deep and difficult to grapple with questions,
3:44
Speaker 1: you know, like what is time anyway? How do you
3:47
Speaker 1: even define it? How do you even ask crisp and
3:50
Speaker 1: precise questions about this slipperyst but most essential of concepts. Yeah,
3:55
Speaker 1: because time always seems to slip by you no matter
3:58
Speaker 1: what do you do, and asking these questions, it's not
4:01
Speaker 1: just in ourder to do list, but it's also in
4:03
Speaker 1: our want to do list, right, Daniel, I mean, that's
4:06
Speaker 1: kind of what you're getting, kid. That is what I
4:07
Speaker 1: get paid for, not on a day to day basis,
4:09
Speaker 1: and mostly I'm cracking open particles at the large age
4:12
Speaker 1: on collider, But these are the big goals of physics,
4:15
Speaker 1: is to understand the nature of our experience. You know,
4:19
Speaker 1: some people think physics is sort of like abstract and
4:22
Speaker 1: separated from humanity. But it's taken me a long time
4:24
Speaker 1: to realize that it's the most human of sciences because
4:27
Speaker 1: it asked these questions to the very core of the
4:29
Speaker 1: context of what it's like to be alive. You know,
4:32
Speaker 1: you notice things slipping into the past through this weird
4:35
Speaker 1: instantaneous slice called the present. It's definitely something we'd like
4:39
Speaker 1: to understand. Are you saying, physics out humanities? The humanities?
4:43
Speaker 1: Daniel out philosophies the philosophers. Yeah, you know, I used
4:48
Speaker 1: to think that physics was the most interesting science because
4:51
Speaker 1: it was probably the most fundamental, the most universal, that
4:53
Speaker 1: somehow it escaped humanity, that if aliens came, they wouldn't
4:56
Speaker 1: be interested in our biological advances because they wouldn't be
5:00
Speaker 1: relevant to them, but they would be interested in what
5:02
Speaker 1: we've learned about physics. I'm not so sure about that anymore.
5:05
Speaker 1: And can hear all those mathematicians out they're laughing at
5:07
Speaker 1: you right now. You're like, you're pure, We're the purest,
5:11
Speaker 1: even mathematicians. Man, Mathematics is just codification of the logic
5:15
Speaker 1: and human brains. And I suspect that aliens might not
5:18
Speaker 1: even do math, and if they do, we might not
5:20
Speaker 1: even recognize it. Oh man, you're saying physics supersedes math
5:25
Speaker 1: from a human point of view. Oh boy, we just
5:27
Speaker 1: started in an academic warrior. You just did that. That's
5:30
Speaker 1: not what I'm saying at all. I'm saying that everything
5:32
Speaker 1: we do is based in human thought and contextualized by
5:35
Speaker 1: human questions. But that doesn't make it worthless. It means
5:38
Speaker 1: that we get to ask really interesting questions about what
5:41
Speaker 1: it's like to be alive. Yeah, and so sometimes those
5:44
Speaker 1: questions get into the idea of time itself, because I
5:47
Speaker 1: think maybe most people have a conception of the university's
5:50
Speaker 1: sort of existing outside of time, or like it's a
5:53
Speaker 1: universe moving through time, but actually physicists think of time
5:56
Speaker 1: as part of the universe, right, it's like another thing
6:00
Speaker 1: in it. Yeah, we don't really understand. Physicists like to
6:02
Speaker 1: divide elements of the universe into things that are fundamental,
6:06
Speaker 1: meaning they're like basic, they're essential, they have to be there.
6:10
Speaker 1: They define what the universe is, and then other things
6:13
Speaker 1: that are emergent that sort of come out of the
6:14
Speaker 1: interplay of those fundamental objects. And we don't know if
6:18
Speaker 1: time is fundamental, if it's like absolutely essential it's part
6:22
Speaker 1: of the nature of the universe, or if it bubbles
6:24
Speaker 1: up from something else. We don't know if space and
6:27
Speaker 1: matter sort of sit in a framework of time which
6:30
Speaker 1: is external to them, or if the whole thing is
6:33
Speaker 1: just you know, comes up from some other weird, deep nature.
6:36
Speaker 1: We haven't even imagined, like, is it hard coded into
6:39
Speaker 1: the circuitry of the universe or is it like a
6:41
Speaker 1: program running on top of the chip of the universe
6:44
Speaker 1: that's kind of what you mean, right, Or is it
6:47
Speaker 1: just the way the humans think, you know? It might
6:49
Speaker 1: be that our tendency to think in terms of stories
6:52
Speaker 1: and narratives which have cause and effect, might bias us
6:57
Speaker 1: to see things as flowing forwards in time, when in
7:00
Speaker 1: reality the truth might be much more complex. We just
7:04
Speaker 1: an episode about how causality, cause and effect might not
7:07
Speaker 1: even be an essential element of the universe, and so
7:11
Speaker 1: it might be that time itself could be something that's
7:13
Speaker 1: just sort of very human. Yeah, it's sort of like
7:16
Speaker 1: this podcast. We have no narrative, right, We are just
7:19
Speaker 1: jumping around in time to whatever comes into our minds here.
7:23
Speaker 1: But it's these kind of basic questions that give us
7:25
Speaker 1: a launching off point for asking questions and doing studies
7:29
Speaker 1: and trying to like find a way to grapple with
7:32
Speaker 1: these things scientifically, because we could sit here and smoke
7:35
Speaker 1: man in appeals and talk about the nuture of time
7:37
Speaker 1: forever without making progress. But we want to do experiments.
7:40
Speaker 1: We want to use science to add to our body
7:43
Speaker 1: of actual knowledge about time. Yeah, so we'll be exploring
7:47
Speaker 1: a little bit about this weird concept of time and
7:50
Speaker 1: especially about how time behaves according to our theories about
7:55
Speaker 1: the universe. So today on the podcast, we'll be asking
7:58
Speaker 1: the question why does gravity slow down time? Or should
8:08
Speaker 1: I should I ask it slow down time? Or maybe
8:12
Speaker 1: you should use your gravest voice, you know, your James
8:15
Speaker 1: Earl Jones voice. You're trying to make a gravity joke.
8:18
Speaker 1: That's kind of heavy, man, don't make light of the situation.
8:20
Speaker 1: I'm a massive fan of that movie. You know. Now,
8:23
Speaker 1: this question kind has two components to it, Like you
8:25
Speaker 1: just said, it has gravity and time in it. And
8:27
Speaker 1: because we know from physics that somehow gravity affects time
8:31
Speaker 1: or is related to time, because in the time can
8:34
Speaker 1: be affected. It's not like some sort of absolute thing.
8:37
Speaker 1: That's right. We do not have like a single clock
8:40
Speaker 1: for the universe. The simplest model of how time works
8:43
Speaker 1: might be imagining that the universe all over the place
8:46
Speaker 1: is in one state. You know, particles are going in
8:48
Speaker 1: some direction or they're in some location, and then things
8:51
Speaker 1: sort of tick forwards and everything takes a step forward
8:54
Speaker 1: in unison and in that picture, the whole universe has
8:57
Speaker 1: like a single clock. But we've learned over the last
8:59
Speaker 1: time of years that that conception of time is not valid.
9:02
Speaker 1: That time flows differently in different parts of the universe,
9:05
Speaker 1: and differently four different observers and moving at different speeds.
9:09
Speaker 1: So time is much weirder, more local, and less universal
9:13
Speaker 1: than we ever imagined. In addition, it's also weirdly bent
9:17
Speaker 1: by heavy objects. Yeah, things that are really heavy, like
9:21
Speaker 1: black holes or even just our planet. They have gravity,
9:25
Speaker 1: and that somehow makes time slowed down. Now, that's a
9:29
Speaker 1: that's a pretty weird concept. Bread. I guess that was
9:31
Speaker 1: in that movie Interstellar. It was exactly in that movie Interstellar.
9:34
Speaker 1: Every time they visited a really heavy planet or came
9:37
Speaker 1: near a black hole. When they left, they found that
9:40
Speaker 1: the rest of the universe had experienced a lot more
9:42
Speaker 1: time than they have. And so I think a lot
9:44
Speaker 1: of people are maybe familiar with this part of relativity,
9:47
Speaker 1: where like, if you're moving past, time slows down. But
9:49
Speaker 1: also it also happens when you're near a heavy object.
9:52
Speaker 1: That's right. These are two completely separate effects with different
9:55
Speaker 1: sources and importantly different mechanisms and different consequences. Yeah, so
10:00
Speaker 1: as usually when we were wondering how many people out
10:01
Speaker 1: there even knew that gravity slowed down time or much less,
10:06
Speaker 1: I have thought about why it slows down time. So
10:09
Speaker 1: Daniel went out there into the internet to ask people
10:12
Speaker 1: why does gravity slow down time? I like the way
10:14
Speaker 1: you say I went out there into the internet. It
10:16
Speaker 1: makes me feel like I got sucked into my computer
10:18
Speaker 1: and went and visited these people like tron like the
10:22
Speaker 1: m eighties movie plotline and you got, you know, salkt
10:26
Speaker 1: into your CRT monitor. Yeah, I'm writing those tron cycles
10:29
Speaker 1: around the Internet, gathering information from our lisitors, dodging calls
10:33
Speaker 1: and tweeting left and right. But if you out there
10:36
Speaker 1: would like me to beat myself into your inbox with
10:39
Speaker 1: crazy questions about the nature of the universe so you
10:42
Speaker 1: can hear yourself speculate about them on the podcast, Please
10:45
Speaker 1: don't be shy. Right to me two questions at Daniel
10:48
Speaker 1: and Jorge dot com. Yeah, and you actually answer every email, right,
10:52
Speaker 1: I do answer every email with every question from everybody.
10:55
Speaker 1: You take the time. It slows me down, but I
10:58
Speaker 1: love it all. Right. Well, here's what people had to say. Well,
11:01
Speaker 1: gravity is basically the same as acceleration, and when you're accelerating,
11:11
Speaker 1: the speed of light stays the same for everybody, and
11:15
Speaker 1: so it must be time that's slowing down. And when
11:20
Speaker 1: we're going at our normal speeds, you don't really notice it. Um.
11:24
Speaker 1: So gravity being equivalent to acceleration causes the same effect.
11:30
Speaker 1: Gravity slows down time because as particles with mass moved
11:33
Speaker 1: through the Higgs field, they're slowed down. The stronger the
11:36
Speaker 1: Higgs field, the gravity is the sole of the quarks
11:38
Speaker 1: and leptons move, causing quantum interactions to take place at
11:41
Speaker 1: a different rate depending on the strength of the field.
11:44
Speaker 1: Scaled up into the macro world, that's what we experienced
11:47
Speaker 1: this time. We would never notice if our particles were
11:49
Speaker 1: moving at a faster slower speed, if everything we observed
11:52
Speaker 1: was experiencing the same Higgs field along with us. I
11:54
Speaker 1: guess that the reason it's because of gravity bench down
11:58
Speaker 1: of pace time and thinks the length of space faith
12:03
Speaker 1: changes in order for the speed of life we're being constant.
12:07
Speaker 1: Was the time of flowing differently. First of all, time
12:11
Speaker 1: is like space dimensions, so the gravity can affect time.
12:19
Speaker 1: But how I'm thinking right now, like the gravity does
12:24
Speaker 1: to also the objects that contracts them. And this is
12:31
Speaker 1: what I'm thinking, That it contracts time. It squeezes it
12:36
Speaker 1: like the space and doing this it is slowing it down.
12:41
Speaker 1: I guess gravity could slow down time because gravity makes
12:45
Speaker 1: those wells and like the space time fabric, and so
12:51
Speaker 1: as that material of the universe gets stretched out, the
12:56
Speaker 1: time would slow down as it gets deeper, sort of
13:02
Speaker 1: like if you're driving over hills versus driving over flat things.
13:06
Speaker 1: I can only imagine it's got something to do with
13:10
Speaker 1: space time. Like gravity isn't just a force, but as
13:15
Speaker 1: Einstein says, it's a warping of space time. And if
13:22
Speaker 1: you've got a ton of gravity staying a black hole,
13:24
Speaker 1: warping space space is not really a thing, but space
13:28
Speaker 1: time is a thing. So you can't affect one without
13:31
Speaker 1: affecting the other, is what I'm guessing. I know time
13:36
Speaker 1: moves slower relatively speaking for someone that travels faster, So
13:41
Speaker 1: maybe gravity slows town time because it also necessarily makes
13:46
Speaker 1: an objectile person move faster. All right, some pretty good
13:50
Speaker 1: answers here. A lot of people sort of knew that
13:54
Speaker 1: time can slow down, and I guess they assume that
13:57
Speaker 1: gravity is someone related to really activity and moving fast
14:01
Speaker 1: and so why not. Yeah, a lot of really interesting
14:03
Speaker 1: and insightful and thoughtful questions here, some confusion about the
14:06
Speaker 1: role of the Higgs field, but also a lot of
14:08
Speaker 1: good concepts about the connections between space and time and
14:12
Speaker 1: how masses have to bend both of them. All right, well,
14:14
Speaker 1: let's get into this idea of time slowing down, and
14:17
Speaker 1: I guess let's recap maybe the one people are most
14:20
Speaker 1: familiar with, maybe, which is the one that when you're
14:22
Speaker 1: moving fast and especially close to the speed of light,
14:25
Speaker 1: time slows down for you. So maybe walk us through that,
14:28
Speaker 1: and then we'll get into the one about gravitation. That's right,
14:31
Speaker 1: So the one you're probably more familiar with. We call
14:33
Speaker 1: velocity time dilation. This is something that happens in special relativity.
14:38
Speaker 1: And to think about this, you should imagine an empty universe,
14:41
Speaker 1: one without really massive objects that are gonna make space
14:44
Speaker 1: curvy and all sorts of weird stuff. In this universe,
14:47
Speaker 1: light travels in straight lines, and spaceships zoom around and
14:51
Speaker 1: all the time. Dilation just comes from the relative velocity
14:55
Speaker 1: of objects. And the most important thing to remember is
14:59
Speaker 1: that movie clocks run slowly, so people often make the
15:03
Speaker 1: mistake of saying, oh, I'm going fast in a spaceship,
15:06
Speaker 1: so my time should slow down, right, Well, it's only
15:09
Speaker 1: moving clocks that slow down. So if you're in a
15:11
Speaker 1: spaceship and you're holding the clock, then the clock is
15:14
Speaker 1: not moving relative to you, so you're not going to
15:17
Speaker 1: see it slow down. So you never experience velocity time
15:20
Speaker 1: dilation because you're never moving a relative to yourself. Somebody
15:24
Speaker 1: else out there on a planet that you're zipping by
15:27
Speaker 1: could see your clock running slowly because your clock is
15:30
Speaker 1: moving for them, and so moving clocks run slowly, meaning
15:33
Speaker 1: that your clock would run slow. You don't experience it,
15:37
Speaker 1: but they see your clock moving slowly. Right, Like, if
15:40
Speaker 1: your time is moving slowly, you don't notice it because
15:43
Speaker 1: you know your brain is also sort of moving slowly
15:46
Speaker 1: in a way, right, So like everything about you is
15:48
Speaker 1: moving slowly as well, and so you don't notice that
15:51
Speaker 1: you're actually moving slowly. But even that suggested some sort
15:53
Speaker 1: of like universal picture of what really happened. And in
15:57
Speaker 1: the universal picture, it makes sense for you to feel
15:59
Speaker 1: like time move normally even though it actually moved slowly.
16:03
Speaker 1: But there is no like what actually happened. Some observers
16:06
Speaker 1: seeing you go by the planet at the speed of light.
16:10
Speaker 1: They see your time as moving slowly. You see your
16:13
Speaker 1: time as moving normally. You can try to unify those
16:16
Speaker 1: into one picture of what actually happened, but there is
16:18
Speaker 1: no what actually happened. There's just what different observers observe,
16:23
Speaker 1: and what you see depends on where you are and
16:25
Speaker 1: how fast things are moving relative to you. For example,
16:29
Speaker 1: if you're on the space ship and you're looking at
16:30
Speaker 1: the clock on Earth, you see Earth moving past you
16:33
Speaker 1: at high speed, and so you see Earth clock running slowly.
16:37
Speaker 1: So Earth sees your clock running slow. You see Earth
16:40
Speaker 1: clock running slow. What actually happened, Well, both of those
16:44
Speaker 1: things happened. Is just what happened depends on where you are.
16:47
Speaker 1: I guess maybe stem me through it. So I'm here
16:49
Speaker 1: on Earth, I'm watching you on a spaceship go by
16:52
Speaker 1: the speed of light, Like, what does it mean for
16:55
Speaker 1: me to see your time slow down? Like? I see
16:57
Speaker 1: your the clock inside of your spaceship take but it's
17:00
Speaker 1: not taking as fast as my clock exactly. So you
17:03
Speaker 1: get a telescope. It's super powerful. So you can look
17:06
Speaker 1: at a clock that's inside my ship and you watch
17:09
Speaker 1: it and you see it's ticks going and you compare
17:11
Speaker 1: it to a clock that's right in your hand, not
17:13
Speaker 1: moving relative to you. And so every time my clock
17:17
Speaker 1: ticks on the spaceship, you see the clock in your
17:19
Speaker 1: lap taking ten times. So time is moving faster for
17:22
Speaker 1: you in your lap. Then you see it moving for
17:24
Speaker 1: me on this ship. And this, of course already takes
17:27
Speaker 1: to an account the fact that it takes light time
17:29
Speaker 1: to get to you to the telescope from the ship.
17:32
Speaker 1: We sort of factor that out already. So what does
17:34
Speaker 1: it mean. So I have a telescope and I'm pointing
17:36
Speaker 1: it at you, but you're zooming by, so I have
17:38
Speaker 1: to kind of track. You have to move my telescope,
17:42
Speaker 1: and so I track you as you're going by, and
17:44
Speaker 1: I'm moving my telescope and I'm measuring your ticks and
17:47
Speaker 1: they're not taking as fast as my clock. That's right.
17:50
Speaker 1: You see my clock as running slowly. So you see
17:53
Speaker 1: me aging slowly, You see me moving slowly. You see
17:56
Speaker 1: me like waving back to you in super slow motion.
17:59
Speaker 1: So you see my clock is running slow relative to
18:02
Speaker 1: your clock. And that's the sort of a consequence of
18:05
Speaker 1: just how the universe is or or to the somehow
18:07
Speaker 1: the limitations of the speed of light. Yeah, it's really interesting.
18:11
Speaker 1: You can start from lots of different places to derive this.
18:13
Speaker 1: You can say like, well, we've seen that nothing can
18:16
Speaker 1: move faster than the speed of light. That's a hard
18:18
Speaker 1: limit on the speed of information. And from that you
18:21
Speaker 1: can derive these things, these time dilation effects, and you
18:24
Speaker 1: can walk yourself through an example. We actually have it
18:26
Speaker 1: worked out in detail in our book We have No Idea,
18:29
Speaker 1: a Guide to the Unknown Universe. And you can think
18:32
Speaker 1: about how a photon clock would tick in a spaceship
18:36
Speaker 1: as it moves up and down, and if it's moving
18:38
Speaker 1: really fast and it has to go like a little
18:39
Speaker 1: bit diagonal for example, And because light can't move faster
18:44
Speaker 1: than the speed of light, when it moves on a diagonal,
18:46
Speaker 1: it takes longer to get from one side of the
18:48
Speaker 1: clock to the other. So it's basically the constancy of
18:52
Speaker 1: the speed of light and the fact that nothing, even
18:54
Speaker 1: light can move faster than the speed of light directly
18:56
Speaker 1: lead to this consequence. The time goes slower for moving
18:59
Speaker 1: clock right. And so there's this sort of famous scenario
19:03
Speaker 1: called the twin paradox for the twin experiment where like
19:06
Speaker 1: you take a pair of twins here on Earth, and
19:08
Speaker 1: you put one of them in a spaceship that goes
19:10
Speaker 1: out into space at the speed of light and then
19:12
Speaker 1: comes back, and supposedly when they come back there are
19:15
Speaker 1: a different age than the one that stayed on Earth.
19:18
Speaker 1: This is a wonderful paradox because it gets to the
19:20
Speaker 1: heart of like what actually happened. Because in the example
19:24
Speaker 1: we're talking about, it feels awfully symmetric, right like I'm
19:27
Speaker 1: looking at you through my telescope and I'm seeing your
19:29
Speaker 1: clock take slowly. But maybe on the ship you're also
19:33
Speaker 1: looking at me on Earth and you see my clock
19:36
Speaker 1: taking slowly. So you want to feel like, well, what
19:38
Speaker 1: actually happens? And the way to bring that to a
19:40
Speaker 1: point is to like bring those two people back together.
19:43
Speaker 1: So if one twin goes off on their spaceship twin
19:46
Speaker 1: on Earth sees the spaceship twin aging slowly, and the
19:49
Speaker 1: spaceship twin sees the Earth twin aging slowly, and so
19:52
Speaker 1: you want to feel like, well, which one is actually younger?
19:55
Speaker 1: And so you turn the spaceship twin around bring it
19:57
Speaker 1: back to Earth, and you ask like, well, which and
20:00
Speaker 1: is younger? And what you discover is that the spaceship
20:02
Speaker 1: twin is younger, and so that sort of breaks this symmetry.
20:06
Speaker 1: And you wonder, like, hold on a second, if this
20:08
Speaker 1: is supposed to be symmetric, if it just depends on
20:11
Speaker 1: relative velocities, why is it that one of them is
20:14
Speaker 1: now younger than the other. So that means it's not
20:16
Speaker 1: just like a perception thing, it's like time actually slowed
20:19
Speaker 1: down for the space twin. In that case, it is
20:22
Speaker 1: because they've broken special relativity. One of the rules of
20:25
Speaker 1: special relativity is no acceleration. You can fly at high
20:29
Speaker 1: velocity and then you can make all these measurements and
20:31
Speaker 1: do these calculations, and things are just as we described.
20:34
Speaker 1: But as soon as you accelerate, then you're out of
20:37
Speaker 1: the bounds of special relativity. It's something you can do.
20:40
Speaker 1: We can you can do it, we can calculate it.
20:41
Speaker 1: But it makes things more complicated, and the simple rules
20:44
Speaker 1: we described earlier of moving clocks run slow get much
20:48
Speaker 1: more tricky. So here, for example, when the spaceship twin
20:51
Speaker 1: turns around to come back to Earth so that he
20:54
Speaker 1: or she can compare her age with her twin, then
20:58
Speaker 1: she's accelerating because changing your direction means accelerating, And what
21:02
Speaker 1: happens when you accelerate is you break the symmetry. Now
21:05
Speaker 1: one of the twins is accelerating, the other one is not,
21:08
Speaker 1: And when you accelerate, weird things happen to time. Specifically,
21:12
Speaker 1: when the twin in the spaceship turns around and accelerates,
21:14
Speaker 1: time jumps forward for the rest of the universe. So
21:18
Speaker 1: time runs a little slower for the accelerating twin jumping
21:21
Speaker 1: forward for the rest of the universe, which is why
21:24
Speaker 1: the twin in the spaceship now is younger when they
21:27
Speaker 1: arrive at Earth than the twin that stayed home. You
21:29
Speaker 1: just kind of accelerated a little too fast there for
21:31
Speaker 1: my brain, I guess. One question is, but isn't acceleration
21:35
Speaker 1: also relative? Like if I'm accelerating away from you, I
21:39
Speaker 1: see you accelerating away from me, So why is an
21:41
Speaker 1: acceleration also like kind of symmetric. And the second question
21:46
Speaker 1: is you're saying it's the acceleration that causes time to
21:49
Speaker 1: slow down, so that's sort of a different scenario. Yeah,
21:52
Speaker 1: these are great questions. Acceleration is actually absolute. Velocity can
21:56
Speaker 1: only be measured relative to other stuff, like if you're
21:59
Speaker 1: an empty universe. You can't measure your velocity because there's
22:03
Speaker 1: nothing to move past. Right, velocity is only defined only
22:07
Speaker 1: has meaning relative to other objects. That's not true for acceleration.
22:12
Speaker 1: Acceleration is something you can measure, even in an empty universe. So,
22:16
Speaker 1: for example, put yourself in a spaceship. You're in an
22:18
Speaker 1: empty universe. If you're moving, your motion has no meaning.
22:21
Speaker 1: There's no experiment you can do inside your spaceship to
22:25
Speaker 1: measure your actual velocity because there's nothing outside to measure
22:29
Speaker 1: relative too. That's not true for acceleration. You can measure
22:32
Speaker 1: inside your spaceship whether or not you're accelerating. For example,
22:35
Speaker 1: you can tell do I feel a force by being
22:38
Speaker 1: pressed by one side of the spaceship. You'll feel those
22:41
Speaker 1: g forces if you're accelerating. So acceleration is different from velocity.
22:46
Speaker 1: You can't have an absolute acceleration, you can measure it.
22:48
Speaker 1: And so that's why the rules are different for acceleration
22:51
Speaker 1: and for velocity. And so then you're saying that the
22:53
Speaker 1: twin who went out into space will actually be younger
22:56
Speaker 1: when they come back, they will actually be younger. Exactly,
22:59
Speaker 1: So when the twin comes back to Earth and is
23:01
Speaker 1: now in the same reference frame moving no velocity relative
23:05
Speaker 1: to the other twin. Then you can ask real questions
23:07
Speaker 1: about in this reference frame, who is younger and who
23:10
Speaker 1: is older. And because the twin that went into space
23:13
Speaker 1: also did some acceleration, their time slowed down a lot
23:17
Speaker 1: during that acceleration, or equivalently, time for the rest of
23:20
Speaker 1: the universe jumped forward during that acceleration. So that breaks
23:24
Speaker 1: the symmetry because only one twin accelerated, and so the
23:27
Speaker 1: twins stay at home actually is older. And you know
23:30
Speaker 1: this isn't just like a thought experiment. There actually are
23:33
Speaker 1: a pair of twin astronauts. One of them went and
23:35
Speaker 1: spent a lot of time up in space and the
23:37
Speaker 1: other one didn't, and they've compared the two. Really all right, well,
23:41
Speaker 1: let's get into that real life experiment and then let's
23:44
Speaker 1: talk about how gravity changes time. But first let's take
23:47
Speaker 1: a quick break. All right, we're asking the question why
24:02
Speaker 1: does gravity slow down time? And we were talking first
24:05
Speaker 1: about the twin experiment where you send a twin out
24:08
Speaker 1: into space, they go really fast, they come back and
24:11
Speaker 1: they've aged less. And this age and you're saying is
24:15
Speaker 1: actually due to the acceleration, it's not actually due to
24:17
Speaker 1: the speed. Right, there are two effects there. There's the
24:19
Speaker 1: velocity time dilation, which is the one you're very familiar
24:22
Speaker 1: with where moving clocks appear to be slow. But that's
24:25
Speaker 1: not a universal phenomenon. It depends on who is doing
24:28
Speaker 1: the observing and their relative velocity. Acceleration, however, is different,
24:32
Speaker 1: and the acceleration does cause an actual slowing of time,
24:36
Speaker 1: which could be measured by everybody because it's not symmetric,
24:40
Speaker 1: it's absolute. You can measure somebody's absolute acceleration and that
24:44
Speaker 1: makes them different, so it breaks the symmetry. So really,
24:46
Speaker 1: when people say going fast slow is down time, we
24:48
Speaker 1: really we should be saying accelerating fast causes time to slow.
24:52
Speaker 1: And is that something that just somehow changes time or
24:56
Speaker 1: is it because you're pushing all of the particles and
24:59
Speaker 1: somehow that's blows how they interact, or how how do
25:02
Speaker 1: you explain exploration changing time? Well, first of all, it
25:05
Speaker 1: is still correct to say that moving fast slows down time.
25:08
Speaker 1: It's just that it's slows down time only for observers, right,
25:11
Speaker 1: observers moving fast relative to those clocks. It's still true,
25:15
Speaker 1: it still happens. It's it's not like just a perception issue.
25:17
Speaker 1: It's a true thing about the universe. Acceleration slows down
25:20
Speaker 1: time in a different way. It's a different mechanism. It's
25:23
Speaker 1: much harder to understand in terms of it means like
25:25
Speaker 1: ticks on the photon clock. But you can see it
25:27
Speaker 1: also comes as a consequence of the speed of light.
25:30
Speaker 1: All right, well then, so that's acceleration and time dilation
25:33
Speaker 1: because of exceleraation and because of going close to the
25:37
Speaker 1: speed of light. But the one we're talking about today
25:39
Speaker 1: is the one due to gravity. So whenever you're next
25:42
Speaker 1: to something that's really heavy or massive, time also slows down.
25:47
Speaker 1: But does it slow down in the same way that
25:50
Speaker 1: acceleration slows time down, or does it slow down in
25:53
Speaker 1: the same way that going at a constant speed slows time?
25:56
Speaker 1: Do you know what I mean? Like is it observer
25:57
Speaker 1: base or is it actually like time slowing down? Great question.
26:01
Speaker 1: And so gravity slows down time where the curvature of
26:04
Speaker 1: space slows down time, and this effect on time is
26:08
Speaker 1: the same as acceleration slowing down time. And in fact,
26:12
Speaker 1: it's sort of a deep idea here because in general relativity,
26:16
Speaker 1: one of the whole inspirational ideas of general relativity is
26:20
Speaker 1: that gravity is equivalent to acceleration. You know, the experiment
26:23
Speaker 1: we talked about a minute ago, like, if you were
26:25
Speaker 1: in space, could you tell if your spaceship was accelerating,
26:28
Speaker 1: You could, and in fact it would feel like you
26:30
Speaker 1: were being pressed against one wall of the ship, or equivalently,
26:34
Speaker 1: it would feel like you were standing on a planet
26:37
Speaker 1: with gravity. Right, you can in fact make artificial gravity
26:40
Speaker 1: on a spaceship by providing acceleration, either by spinning or
26:44
Speaker 1: by zooming off in one direction. So the whole idea
26:47
Speaker 1: that gave Einstein the inspiration for general relativity was this
26:51
Speaker 1: one is called the equivalence principle that says that there's
26:53
Speaker 1: no difference between gravity and acceleration. And so we just
26:58
Speaker 1: went through the details of how acceleration can cause time
27:02
Speaker 1: to move slowly, and this is exactly the same effect.
27:05
Speaker 1: Gravity also makes time move slowly. The curvature of space
27:09
Speaker 1: around you makes time move more slowly. It's exactly the
27:13
Speaker 1: same effect. And so it's an absolute effect, not a
27:16
Speaker 1: relative one like for velocity. So you're saying it's really
27:19
Speaker 1: sort of acceleration that causes time to slow down, and
27:22
Speaker 1: gravity is sort of like an acceleration or is it
27:25
Speaker 1: the other way around? But I guess you can have
27:27
Speaker 1: acceleration without gravity, So it's more like gravity is kind
27:30
Speaker 1: of like an acceleration. Yeah, gravity is essentially like a
27:32
Speaker 1: geometrical interpretation of acceleration. Or you know, said another way,
27:37
Speaker 1: what happens when you have mass in space, Well, it
27:40
Speaker 1: changes the curvature of that space, and so what happens
27:44
Speaker 1: then is that things move differently and they can appear,
27:47
Speaker 1: for example, to be accelerating. If you aren't aware of
27:50
Speaker 1: the curvature of space, then it seems like there's a
27:53
Speaker 1: force there which provides an acceleration towards those masses. And
27:57
Speaker 1: so that's really what gravity is. Gravity is the bending
28:00
Speaker 1: of space in a way that appears to provide acceleration.
28:03
Speaker 1: So gravity and acceleration really are exactly the same phenomenon.
28:07
Speaker 1: Either acceleration in empty flat space gives exactly the same
28:11
Speaker 1: effects as curving of space itself. You remember, we did
28:14
Speaker 1: recently a fun episode about how if you're accelerating there
28:19
Speaker 1: are times that photons cannot catch you. You have essentially
28:22
Speaker 1: an event horizon if you're accelerating constantly, And the explanation
28:26
Speaker 1: there was the same as here is that accelerating constantly
28:30
Speaker 1: is sort of the same as curving space. And we
28:33
Speaker 1: know that if you curve space, weird things happen, like
28:35
Speaker 1: you can be inside a black hole and photons cannot escape.
28:39
Speaker 1: And so the core idea here is to understand that
28:41
Speaker 1: accelerating is really the same thing as gravity, And so
28:45
Speaker 1: if you think of gravity is causing time dilation, it's
28:48
Speaker 1: really the concept of acceleration causing time dilation the sort
28:51
Speaker 1: of mentally equivalent. There are a lot of leaps here,
28:54
Speaker 1: I feel like, and it's kind of hard to keep
28:56
Speaker 1: track of because I feel like you're saying gravity is
28:59
Speaker 1: acceleration and gravity is also the curvature of space. Does
29:03
Speaker 1: that mean that all acceleration is also the curvature of space?
29:06
Speaker 1: Or can you have acceloration in not bend space, or like,
29:10
Speaker 1: can you think of all acceleration even by like electromagnetic forces,
29:14
Speaker 1: as some sort of curvature of space. Yeah, that's a
29:16
Speaker 1: deep question, and there are people out there trying to
29:19
Speaker 1: interpret all acceleration in terms of the curvature space, or
29:23
Speaker 1: you know, like all forces as being the product of
29:26
Speaker 1: the curvature space. But that's not necessary. You can think
29:29
Speaker 1: about acceleration in flat space, you know, just like put
29:32
Speaker 1: on a rocketship, accelerate your spaceship. Now you're going fast.
29:36
Speaker 1: But the effects of that on your time and the
29:39
Speaker 1: way you pursue you the universe are equivalent to if
29:42
Speaker 1: space was curved around you. So you can think of
29:45
Speaker 1: acceleration separately from the curvature space and from gravity, but
29:48
Speaker 1: has exactly the same effect, because that's really kind of
29:50
Speaker 1: what gravity is, so meaning I feel like then that
29:53
Speaker 1: you're saying that that it's not really gravity that slowing
29:56
Speaker 1: down time. It's really the acceleration caused by gravity, or
30:00
Speaker 1: the bending of space caused by gravity, which is the
30:02
Speaker 1: same as exceleration. Yeah, there's lots of different ways to
30:05
Speaker 1: think about it. You can think of acceleration caused by
30:08
Speaker 1: gravity is really just like motion through curved space. And
30:12
Speaker 1: one of the other impacts of curved space is that
30:14
Speaker 1: time also slows down. All right, well, then maybe let's
30:18
Speaker 1: try it. Let's see why does acceleration slow down time?
30:22
Speaker 1: Because that seems to be the bigger problem, right, that
30:24
Speaker 1: seems to be the bigger question. Yeah, so I guess
30:26
Speaker 1: you can say that gravity slows down time because it's
30:29
Speaker 1: equivalent to acceleration slowing down time. Why does acceleration slow
30:34
Speaker 1: down time? That gets back to the speed of light
30:37
Speaker 1: as the limiting piece of information. You can derive this
30:39
Speaker 1: in a flat universe using the twins as an example.
30:42
Speaker 1: It's a little bit more complicated mathematically than like normal
30:45
Speaker 1: special relativity, where you can do these observing frames with
30:48
Speaker 1: Lorentz transformations. It gets a little hairy and mathematical, but
30:52
Speaker 1: the core concept that it comes from is this maximum
30:56
Speaker 1: speed of light. Everything comes out of that both time
31:00
Speaker 1: dilation from velocity and also time dilation from acceleration, which
31:03
Speaker 1: really is equivalent to time dilation from gravity. So I
31:07
Speaker 1: guess if I'm in a spaceship and I'm accelerating the
31:09
Speaker 1: limitations of the speed of light, which does that mean that?
31:13
Speaker 1: Are you sort of saying that it somehow limits how
31:15
Speaker 1: the things can evolve inside of that spaceship, you know,
31:18
Speaker 1: move from you know, information between molecules and things like that,
31:22
Speaker 1: So things sort of evolve slower or they have a
31:25
Speaker 1: limit to how fast they can evolve. Yeah, I would
31:28
Speaker 1: say a little bit differently. I would say that requiring
31:31
Speaker 1: that the speed of light is constant and that everybody
31:33
Speaker 1: observes the speed of light always to be. The speed
31:36
Speaker 1: of light restricts the kinds of universes that we can have.
31:40
Speaker 1: It puts a lot of restrictions on the waste space
31:43
Speaker 1: and time have to work in that universe. And these
31:46
Speaker 1: effects that we're talking about, the slowing down of time
31:49
Speaker 1: by velocity and by acceleration, are consequences of the structure
31:53
Speaker 1: of that space and time the sort of come out
31:56
Speaker 1: of that all, right, So you're saying it's just the
31:58
Speaker 1: way it is saying when things accelerate, there's a limit
32:01
Speaker 1: in the speed of light, and so that makes time
32:04
Speaker 1: sort of slow down, makes everything slow down. And I
32:07
Speaker 1: think it's super cool because it's not symmetric. You know,
32:10
Speaker 1: like two people can agree on who is accelerating more,
32:14
Speaker 1: and so that means that they can agree on whose
32:16
Speaker 1: time is moving more slowly, or you know, said another
32:19
Speaker 1: way in terms of gravity, like you and I can
32:22
Speaker 1: agree that if you're near the black hole, then you're
32:25
Speaker 1: in a part of space that's curved more than my
32:27
Speaker 1: part of space, and so we can agree that your
32:30
Speaker 1: time should be moving more slowly. That's not true for
32:33
Speaker 1: the spaceships, right, If we're in two spaceships passing each other,
32:36
Speaker 1: it feels symmetric because it is symmetric. I say you're
32:38
Speaker 1: moving past me, You say I'm moving past you. Everybody's right.
32:42
Speaker 1: In the case of the black hole, we can agree
32:44
Speaker 1: it's not symmetric, So we should agree that your time
32:47
Speaker 1: is moving more slowly. So then a coloration causes time
32:49
Speaker 1: to slow down. And definitely, when you're near a black hole,
32:52
Speaker 1: you are being accelerated, probably a lot, because black holes
32:55
Speaker 1: are very massive. They're pulling you in. And so if
32:59
Speaker 1: I'm near a black hole, then I'm going to be
33:01
Speaker 1: moving slower through time than you, who is out way
33:04
Speaker 1: far from the black hole exactly. And so another cool
33:08
Speaker 1: thing is that I see your time moving slowly. It
33:11
Speaker 1: means that you see my time moving more quickly. Right,
33:14
Speaker 1: This is the real difference with velocity time dilation and
33:17
Speaker 1: velocity time delation. We both see each other's time moving
33:20
Speaker 1: more slowly. Here, if you're near a black hole and
33:22
Speaker 1: you're looking out into the universe, you see the rest
33:25
Speaker 1: of the universe running forward in time very quickly. And
33:27
Speaker 1: as you get closer and closer to the black hole
33:29
Speaker 1: and more and more curvature, you see the universe's clock
33:32
Speaker 1: speeding up into the future. So then like if I'm
33:35
Speaker 1: falling into a black hole, like all the stars will
33:38
Speaker 1: suddenly start speeding up around me, like I'll see the
33:40
Speaker 1: universe kind of and fast forward exactly. And some people imagine, well,
33:43
Speaker 1: does that mean that you'll see like the end of
33:45
Speaker 1: the universe, the end of time. You'll know, like the
33:48
Speaker 1: final fate of the universe just as you fall into
33:51
Speaker 1: the black hole. Well that would be super cool, but unfortunately,
33:53
Speaker 1: it takes a finite amount of time from your perspective
33:57
Speaker 1: to fall into a black hole, so there isn't time
33:59
Speaker 1: for all that information from the future universe to get
34:02
Speaker 1: to you. So you see the fast forward universe for
34:05
Speaker 1: a while, but you don't see like all the way
34:07
Speaker 1: into the infinite future. Well, we talked about this last time,
34:09
Speaker 1: Like when you actually get to the surface of the
34:11
Speaker 1: black hole, then time actually stands still, right, Like it
34:16
Speaker 1: slows down more the closer you get to the black hole,
34:18
Speaker 1: and then it sort of stands still at the surface.
34:20
Speaker 1: It sort of does, but that's only for somebody far away.
34:24
Speaker 1: They see your time moving slowly and they see you
34:27
Speaker 1: sort of smeared across the event horizon. But for you,
34:30
Speaker 1: you actually fall into the black hole, you don't notice
34:33
Speaker 1: anything different changing, right, you notice the rest of the
34:36
Speaker 1: universe's clock speeding up. But from your perspective, you fall
34:39
Speaker 1: into the black hole, you pass the event horizon, you
34:41
Speaker 1: get sucked into the singularity, so your time is definitely
34:45
Speaker 1: finite from your perspective. And this is not something that
34:48
Speaker 1: we understand very well. There's all sorts of weird paradoxes
34:50
Speaker 1: and contradictions here about how one person sees you not
34:53
Speaker 1: falling into the black hole to the end of time,
34:55
Speaker 1: and you see yourself actually falling in. It's not something
34:58
Speaker 1: that we know how to reckon. SI. I think what
35:00
Speaker 1: you're saying is that the person falling in, you're saying
35:03
Speaker 1: they'll see themselves falling through, But we don't actually know
35:05
Speaker 1: if that's true, right, Like they might just actually freeze
35:08
Speaker 1: at the edge, which just don't know. Yeah, we don't
35:10
Speaker 1: know it is true because nobody's done it and come
35:12
Speaker 1: to report back. It's possible they actually just freeze the
35:15
Speaker 1: edge and they think that they're inside, but it's actually
35:17
Speaker 1: that the inside of a black hole is a hologram
35:20
Speaker 1: projected from the surface of the black hole. We just
35:23
Speaker 1: don't really know what's going on inside a black hole.
35:26
Speaker 1: And so this effect of gravity on time doesn't just
35:29
Speaker 1: happen in black holes. I mean, black hole is sort
35:31
Speaker 1: of the extreme example, but it actually happens like every
35:34
Speaker 1: day and everywhere, like here on Earth, the Earth is
35:37
Speaker 1: slowing down time and even like I'm slowing down time
35:41
Speaker 1: for the things around me, right, yeah, absolutely, everywhere there
35:44
Speaker 1: is curvature, time is slowed down and the Earth curved space, right,
35:48
Speaker 1: because the Earth has a lot of mass. That's why,
35:50
Speaker 1: for example, you don't fall off the Earth. You're feeling
35:53
Speaker 1: it's gravity. So anywhere you're in a situation where you're
35:55
Speaker 1: feeling gravity, you're also having your time affected. And because
35:59
Speaker 1: gravity is stronger as you get closer to the Earth
36:02
Speaker 1: and weaker as you move away from it, that means
36:04
Speaker 1: that the clocks are variable. Time flows in a variable
36:08
Speaker 1: way as a function of the distance from the center
36:10
Speaker 1: of the Earth. And this is something you can measure,
36:13
Speaker 1: like over your life, your feet will age one second
36:17
Speaker 1: more than your head, only if you spent a lot
36:20
Speaker 1: of time standing up Daniel, which cartoon is don't do
36:23
Speaker 1: a lot of So I guess our feet are still young.
36:26
Speaker 1: We are still light on our feet exactly. That's why
36:28
Speaker 1: you lay down all the time, just to keep your
36:30
Speaker 1: body like in sync, just to keep my feet young.
36:33
Speaker 1: All right, well, let's get into what this all means.
36:36
Speaker 1: Does that mean that we're all moving slower through time
36:39
Speaker 1: than we should? And whether does that mean that also
36:42
Speaker 1: there's no universal clock actually to measure time in the universe,
36:46
Speaker 1: So let's get into that. But first let's take another
36:49
Speaker 1: quick break. All right, Daniel, my favorite question and all
37:04
Speaker 1: these topics, what does it all mean? Man? So anything
37:08
Speaker 1: with gravity bends space around it, which causes acceleration, and
37:12
Speaker 1: accelerating things slowed down in time. So things are always
37:16
Speaker 1: slowing down in time everywhere all the time. Yeah, everywhere
37:19
Speaker 1: there's a gravitational field, clocks are being slowed. So if
37:23
Speaker 1: you're in a gravitational field, then your sense of now
37:26
Speaker 1: is moving forward differently than other people who are like
37:30
Speaker 1: out in deep space. And so if you spend a
37:32
Speaker 1: lot of time near and gravitational objects, then you are
37:36
Speaker 1: younger than you otherwise would be. Right, But it's not
37:38
Speaker 1: just gravity too, right, It's like if I get in
37:40
Speaker 1: my car and I accelerate up to the freeway, I
37:43
Speaker 1: somehow slowed down time for myself. Yeah, gravity and acceleration.
37:47
Speaker 1: They're equivalent, and so both of them will slow down time.
37:51
Speaker 1: Every time you accelerate, the universe sort of leaps forward
37:54
Speaker 1: a little bit relative to you. If you accelerated a
37:57
Speaker 1: lot for a long time, you would notice clocks around
38:00
Speaker 1: you seeming to move forward faster than one second per
38:03
Speaker 1: second on your clock. And so I guess that means
38:05
Speaker 1: first of all, that there's no real age to the universe.
38:09
Speaker 1: Is that really true? That Does that mean that you
38:11
Speaker 1: know there's no like absolute time? Yeah, there's no absolute time,
38:15
Speaker 1: which is really frustrating if you'd like to have a
38:17
Speaker 1: sense that you know, there's truth, that there's something really
38:20
Speaker 1: going on out there outside of our skulls. It's frustrating
38:23
Speaker 1: to imagine that. Like, different people can tell different stories
38:26
Speaker 1: and they can both be correct. But there's an even
38:29
Speaker 1: deeper problem if you try to extrapolate back to time
38:32
Speaker 1: equal zero. You know, we say this thing the universe
38:35
Speaker 1: is thirteen point eight billion years old. Well, according to
38:38
Speaker 1: what clock, right, is that clock been moving on a spaceship?
38:41
Speaker 1: Has that clock spent a lot of time near a
38:42
Speaker 1: black hole? Because if so, it's going to have a
38:44
Speaker 1: different answer. And so, because different parts of the universe
38:47
Speaker 1: have different curvature, right like near black holes or near
38:50
Speaker 1: Sun's or whatever. Then different parts of the universe have
38:53
Speaker 1: aged differently since it's beginning. So the universe does not
38:58
Speaker 1: have one single age. Just like your feet and your
39:00
Speaker 1: head are not the same age, assuming you haven't spent
39:04
Speaker 1: your whole life in bed, the parts of the universe
39:06
Speaker 1: have different ages. I do a lot of handstands, so
39:09
Speaker 1: I'm trying not to go bald, so I'm keeping my head.
39:11
Speaker 1: I think we all need a video of you doing
39:13
Speaker 1: a handstand. Let's see that. That's good. But I guess
39:18
Speaker 1: that confuses me because you told me earlier that acceleration
39:21
Speaker 1: is absolute, so I can measure exceloration absolutely, and time
39:26
Speaker 1: is actually bent by exploration. So couldn't I, I don't know,
39:30
Speaker 1: find a spot in the universe that's never been accelerated
39:33
Speaker 1: and say that, like, that's the absolute age of the universe. Well,
39:37
Speaker 1: that's the age of that part of the universe, and
39:39
Speaker 1: that would be the oldest part of the universe. That
39:42
Speaker 1: part would have experienced the most time. But you know,
39:45
Speaker 1: if you had put a clock somewhere else in the
39:47
Speaker 1: universe and let it run since the beginning, it would
39:51
Speaker 1: have a different number. So like different parts of the
39:53
Speaker 1: universe have different ages, and you might reasonably say, well,
39:57
Speaker 1: the oldest part of the universe, I'm gonna use that
39:59
Speaker 1: as the a age of the universe. Yeah, nobody cares
40:02
Speaker 1: how young my feet are. I care, I care, you care.
40:07
Speaker 1: It sounded a lot like I care though, Right, that
40:08
Speaker 1: sounded sincere didn't it. But yeah, so there's sort of
40:12
Speaker 1: an age limit to the universe, right, Like you're you're
40:14
Speaker 1: saying that there is an absolute age of the universe
40:17
Speaker 1: by which we can measure all other ages. Yeah, there's
40:20
Speaker 1: a maximum age to the universe, right, there is a
40:23
Speaker 1: number beyond which no part of the universe could have
40:26
Speaker 1: experienced more time than that if you wanted to find
40:29
Speaker 1: that as the age of the whole universe. I guess
40:32
Speaker 1: that makes sense, But I think more conceptually it makes
40:34
Speaker 1: sense to imagine, like how many clock ticks have there
40:36
Speaker 1: been in a given part of the universe, And that's
40:39
Speaker 1: not equal all over the universe. It depends on how
40:42
Speaker 1: much gravity there is nearby, right, Because I guess even
40:45
Speaker 1: our solar system is being accelerated around the Milky Way,
40:48
Speaker 1: so therefore our time is sort of being slowed down
40:51
Speaker 1: in that way too. Yeah, the curvature of the center
40:54
Speaker 1: of the Milky Way and that supermassive black hole does
40:56
Speaker 1: affect the motion of the Sun and the curvature nearby,
40:59
Speaker 1: which slows down our time. And I guess you were
41:01
Speaker 1: trying to tell me earlier that there is sort of
41:04
Speaker 1: a philosophical question here, which is like, does acceloration change
41:10
Speaker 1: time or does time change acceleration. Yeah, it's familiar to
41:15
Speaker 1: think about general relativity is saying that mass causes space
41:20
Speaker 1: to bend, and then the curvature of space tells masses
41:24
Speaker 1: how to move. Right, that you get an appearance of acceleration,
41:28
Speaker 1: this effective force of gravity because space itself is bent.
41:31
Speaker 1: There's a missing component there, right, which is that time
41:35
Speaker 1: is also curved by mass. So you have mass somewhere,
41:38
Speaker 1: it doesn't just curve space, It also curves time, which
41:41
Speaker 1: is what we've been talking about today, And the curving
41:43
Speaker 1: of time also contributes to this force of gravity. So
41:47
Speaker 1: gravity is an apparent force that comes not just from
41:50
Speaker 1: the bending of space but also from the bending of time.
41:53
Speaker 1: The two work together because spacetime really is sort of
41:56
Speaker 1: one thing to create this effect of gravity. You're saying time.
42:01
Speaker 1: You can also bend time, just like you can bend space,
42:04
Speaker 1: and somehow that's where gravity comes from. Yeah, exactly. It's
42:08
Speaker 1: familiar to use like a rubber sheet analogy, where parts
42:11
Speaker 1: of space are bent and that changes how an object
42:13
Speaker 1: moves sort of naturally. And you know, that's a little
42:16
Speaker 1: confusing because in what direction is the rubber sheet bending.
42:19
Speaker 1: It's bending in some sort of like external direction. You
42:22
Speaker 1: can measure it in terms of like another dimension in reality,
42:24
Speaker 1: in our space. It's intrinsic curvature. It just changes the
42:28
Speaker 1: relationship between points in space their relative distances. So that's
42:32
Speaker 1: a familiar way to think about how the curvature space
42:34
Speaker 1: affects the motion of an object. You can do something
42:38
Speaker 1: similar for a time. You can imagine like different parts
42:40
Speaker 1: of the universe flowing with different time. Right, It's sort
42:43
Speaker 1: of like you're moving down a river and different parts
42:46
Speaker 1: of the river are moving faster than others, and that
42:49
Speaker 1: will affect the motion of objects. Like you have a
42:51
Speaker 1: really big object in a river and the river is
42:54
Speaker 1: flowing faster on the left side than on the right side.
42:56
Speaker 1: It's going to change the way that object moves it
42:58
Speaker 1: like tug it towards it's the slow moving part of
43:01
Speaker 1: the river, and that's part of how the curvature space
43:05
Speaker 1: and time creates this effect of gravity that objects no
43:09
Speaker 1: longer move in what we perceive to be straight lines.
43:12
Speaker 1: I see you're saying, like you can think of it
43:13
Speaker 1: the other way around, Like me falling to the Earth
43:16
Speaker 1: or me being an orbit around Earth is actually a
43:19
Speaker 1: consequence of the differences in time, and like differences in
43:23
Speaker 1: time cause me to move from one place to another, exactly,
43:26
Speaker 1: it's space and time being heard, both affect your trajectory. Cool,
43:31
Speaker 1: So in a way, asking why does gravity slow down time,
43:34
Speaker 1: you could also mainly ask why does time slowing downtime
43:39
Speaker 1: cause gravity? Yeah, exactly. Another way to think about it
43:41
Speaker 1: is that the effective gravity we observe is coming from
43:45
Speaker 1: the curvature space and the curvature of time, and that
43:47
Speaker 1: gravitational time dilation is just another aspect of the curving
43:52
Speaker 1: of space time in respect to mass. So I guess
43:54
Speaker 1: in the end, you just have to say that it's
43:56
Speaker 1: all sort of kind of the same thing. It's all
43:59
Speaker 1: sort of relate it in and it's all you know.
44:01
Speaker 1: You can look at it from one way, or you
44:03
Speaker 1: can look at it upside down. But at the end
44:05
Speaker 1: of the day, and it all comes down to really
44:07
Speaker 1: I think exceloration, right, Like things that accelerate have to
44:10
Speaker 1: slow down in time because of the speed limit of
44:13
Speaker 1: the universe. Yeah, that's a consistent way to think about it.
44:16
Speaker 1: I prefer the sort of geometrical way to think about it,
44:18
Speaker 1: that we're living in a universe that's courage and work,
44:20
Speaker 1: but we don't perceive it directly, and so the way
44:23
Speaker 1: things move through space and time are affected by these
44:26
Speaker 1: like invisible warping. That sort of geometrically makes the most
44:29
Speaker 1: sense to me. But you can also think about it
44:31
Speaker 1: just in terms of acceleration in flat space. Yeah, all right, Well,
44:34
Speaker 1: I guess that answer is sort of the question why
44:36
Speaker 1: does gravity slow down time? The answer is because of
44:39
Speaker 1: exploration and why does exceleraation slow than time? Well, we
44:43
Speaker 1: probably need a whole new podcast episode about it. Yeah,
44:46
Speaker 1: why does gravity slow down time? Because time is bent
44:48
Speaker 1: in the presence of mass, just like spaces. I think
44:52
Speaker 1: you just went all the way around. Why does the
44:56
Speaker 1: excelation slow in time because slowing downtime causes excelraation. It
45:00
Speaker 1: sounds like a great answer, and it causes podcasts to
45:02
Speaker 1: go in circles. Yeah, and now you can just hit
45:04
Speaker 1: replay and listen to this episode all over again and
45:08
Speaker 1: it should make sense. Right. It's all pretty tricky stuff.
45:10
Speaker 1: In the end, it all comes down to consequences from
45:13
Speaker 1: our observation that the speed of light is the maximum
45:16
Speaker 1: speed of the universe that just happens to be the
45:18
Speaker 1: universe we live in. And when we build in those
45:20
Speaker 1: constraints into our theories, these are all the consequences that
45:23
Speaker 1: come out of it. And maybe that point is what
45:25
Speaker 1: you were saying earlier than you know. We have the
45:27
Speaker 1: speed limit and it causes time to do weird things.
45:30
Speaker 1: So maybe time is not a fundamental thing, right, Like
45:33
Speaker 1: it's not outside of the universe. Maybe time it's something
45:37
Speaker 1: that comes out of how the universe works, yeah, or
45:40
Speaker 1: how we are perceiving it or measuring it. And you know,
45:43
Speaker 1: there's a whole universe of crazy ideas about time, even
45:46
Speaker 1: ideas that like time is not essential part of the
45:49
Speaker 1: universe but comes out of something else, or that there
45:51
Speaker 1: are multiple dimensions of time, the way there are multiple
45:54
Speaker 1: dimensions of space, and we're just moving on like a
45:57
Speaker 1: one D line through threedom mentional time. Man, there's a
46:02
Speaker 1: whole like crazy, crazy set of really fun ideas to
46:05
Speaker 1: dig into. Unfortunately we are out of time for this episode.
46:09
Speaker 1: We'll have to find more time to get into it more.
46:12
Speaker 1: But we hope you enjoyed that and maybe got you
46:14
Speaker 1: to think a little bit more about how young your
46:17
Speaker 1: feet are and how you should maybe do more handstand
46:19
Speaker 1: I want to see that video if you're doing a handstand,
46:22
Speaker 1: I can do what actually, but now it's not the time,
46:24
Speaker 1: all right, Well, thanks for joining us, see you next time.
46:35
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
46:38
Speaker 1: the Universe is a production of I Heart Radio or More.
46:41
Speaker 1: Podcast for my Heart Radio, visit the I Heart Radio
46:44
Speaker 1: Apple Apple Podcasts, or wherever you listen to your favorite shows.