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Is the speed of light the same in all directions? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, Daniel, how do your kids get to school these days? Oh?
0:11
Speaker 1: They're super independent and awesome about it. They actually bike
0:14
Speaker 1: themselves there and back nice And how long does it
0:17
Speaker 1: take them? Oh? That depends on what the weather, what
0:20
Speaker 1: they had for breakfast, those but mostly on the direction.
0:24
Speaker 1: It's about twice as long on the way to school
0:26
Speaker 1: as it is on the way home. Is that because
0:28
Speaker 1: it's downhill one way and uphill the other way? Nope?
0:32
Speaker 1: So what's going on? Is it some kind of quantum effect?
0:35
Speaker 1: I asked my daughter about it. Actually, she said, well,
0:37
Speaker 1: you know, I'm excited to get home. That's my time.
0:41
Speaker 1: She didn't say because she was excited to see you.
0:44
Speaker 1: I'm sure that's what she meant. That's the quantum effect
0:47
Speaker 1: right there. Hi am poor handmade cartoonists and the co
1:04
Speaker 1: author of Frequently Asked Questions about the Universe. Hi. I'm Daniel.
1:08
Speaker 1: I'm a particle physicist and a professor at U C Irvine,
1:11
Speaker 1: and I am a quantum mechanics. Yeah. Do you fix
1:14
Speaker 1: quantum cars? I do when I don't, and then they
1:16
Speaker 1: work and not work at the same time. And do
1:19
Speaker 1: people pay you and not pay you? Also? Yeah, I
1:21
Speaker 1: build them and I don't. That's not like a cool
1:23
Speaker 1: Marvel movie plot device, like a quantum engine. Yeah exactly.
1:27
Speaker 1: It generates any plot you need to fill any hole
1:29
Speaker 1: in the Marvel universe. It's like magic, you know. I
1:32
Speaker 1: mean you're a quantum mechanical engineer, because I'm a mechanical engineer,
1:35
Speaker 1: but you know, I would love to add quantum in
1:38
Speaker 1: front of it. You just need to use super tiny
1:40
Speaker 1: little tools. That's the trick. I just need to pretend
1:43
Speaker 1: to know what I'm talking about. Is that the secret?
1:45
Speaker 1: Just wave your hands a bunch. That's especially effective on
1:47
Speaker 1: a podcast. I see. Yeah, just say you know Shoredinger, Shoredinger, Heisenberg, Heisenberg,
1:52
Speaker 1: her mission matrices, way function, way function. Suddenly I get
1:58
Speaker 1: a promotion. Yeah exactly. Mechanics is just handway functioning. But anyways,
2:02
Speaker 1: welcome to our podcast, Daniel and Jorge Explain the Universe,
2:05
Speaker 1: a production of My Heart Radio, in which we talk
2:07
Speaker 1: about the craziness of our universe, everything that can be
2:11
Speaker 1: understood and everything that can't be understood, and everything that
2:14
Speaker 1: you think you might understand but you simultaneously are confused
2:17
Speaker 1: about all at the same time. Your quantum mechanical understanding
2:21
Speaker 1: of our crazy universe. We embrace it, we explore it,
2:25
Speaker 1: we explode it, and mostly we explain all of it
2:28
Speaker 1: to you. That's right, because humanity is cruising through this
2:31
Speaker 1: universe driven by an engine of curiosity, and we love
2:34
Speaker 1: to look out the window and wonder about all the
2:36
Speaker 1: things that are out there and how they work. And
2:38
Speaker 1: that curiosity has led us to some very strange conclusions
2:42
Speaker 1: about the nature of that universe out there beyond our skulls.
2:47
Speaker 1: It seems to follow rules, but those rules are very
2:50
Speaker 1: different from the ones that we imagined they were many
2:53
Speaker 1: many years ago. Yeah, it is a very perplexing universe,
2:56
Speaker 1: and our physicists and scientists asking questions about it all
2:58
Speaker 1: the time, and we like to talk talked about what
3:00
Speaker 1: they talk about, what questions they're asking, and break it
3:03
Speaker 1: down for you. And most importantly, we like to think
3:06
Speaker 1: about what science has figured out and what science still
3:09
Speaker 1: does not yet know. The boundaries of our knowledge, the
3:12
Speaker 1: realms where future discoveries lie, where crazy revelations for the
3:17
Speaker 1: future will upend our entire understanding of how the universe works. Yeah, because,
3:21
Speaker 1: as you said, I think the universe has sort of
3:23
Speaker 1: surprises quite a few times in the history of science
3:26
Speaker 1: for humans right, Oh, so many times in the past,
3:29
Speaker 1: and I hope many many times in the future, moments
3:31
Speaker 1: when the whole field collectively went, what are you additic
3:35
Speaker 1: to plot? Twist? Is that what it is? You just
3:38
Speaker 1: go to the movies for the shocking moment? I think
3:41
Speaker 1: fundamentally it comes from the fact that the universe seems
3:44
Speaker 1: to work very differently on different scales. You know, the
3:48
Speaker 1: rules of big quantum mechanics, things that apply to like
3:51
Speaker 1: ten to twenty five particles like me and you and
3:54
Speaker 1: baseballs seem to be different than the rules that apply
3:57
Speaker 1: the very very small scale. The rules are very slow
4:00
Speaker 1: things seem to be different from the rules for very
4:02
Speaker 1: very fast things. So because we can't have a single
4:05
Speaker 1: effective description that works for every situation, we keep getting
4:08
Speaker 1: surprised when we discover a new situation that requires a
4:11
Speaker 1: new understanding. Yeah, because the universe seems to work differently
4:15
Speaker 1: a different scale scales of size and skills of speed.
4:17
Speaker 1: And you know, we're as humans used to one scale
4:21
Speaker 1: of and moving in one kind of speed. But somehow
4:23
Speaker 1: the universe kind of likes to change things up. It
4:25
Speaker 1: certainly does, And we've spent hundreds or thousands of years
4:28
Speaker 1: trying to figure out how the universe works. But as
4:31
Speaker 1: you said, we live at a certain speed, and as
4:33
Speaker 1: we explore the universe more broadly, we discover that the
4:36
Speaker 1: rules we devised that work for throwing rocks at each
4:40
Speaker 1: other or hunting antelope or climbing up to get that
4:43
Speaker 1: fresh piece of fruit don't always work when it comes
4:46
Speaker 1: to planets whizzing very close to their stars or approaching
4:49
Speaker 1: black holes. Yeah, you know, I kind of like the
4:51
Speaker 1: speed we're moving out right now, you know, I feel
4:53
Speaker 1: like I don't I'm not sure I want to go faster,
4:56
Speaker 1: or maybe I'm just getting gold. Well, then, you know,
4:58
Speaker 1: just don't volunteer to go up in space sort visit
5:00
Speaker 1: that black hole. Send them of our podcast listeners instead,
5:03
Speaker 1: send the physicist, no, thank you. They're the ones who
5:07
Speaker 1: want to go anyways, Right, why would you send anyone else?
5:10
Speaker 1: I want to know where they're I don't want to
5:12
Speaker 1: be there myself, Right, you don't. But what if nobody
5:15
Speaker 1: else wants to go? Would you go? How? Situation depends
5:19
Speaker 1: on the snacks they're offering along the way. I suppose
5:21
Speaker 1: What if it's just bananas all the way down? Scratch it?
5:25
Speaker 1: Forget about it. You can slide your way into the
5:29
Speaker 1: black hole. It's a slippery slope. But science has blown
5:32
Speaker 1: our minds many times over the last few centuries, and
5:35
Speaker 1: maybe no instance more so than when Einsang discovered special relativity.
5:39
Speaker 1: Einstein's new view of how space and time are linked together,
5:43
Speaker 1: how light moves through space, how time is not universal,
5:47
Speaker 1: really blew up a lot of basic ideas we had
5:50
Speaker 1: about the way the universe worked, ideas that date all
5:53
Speaker 1: the way back to Isaac Newton. Yeah, he blew everyone's mind,
5:56
Speaker 1: and he sort of did it, like from the comfort
5:57
Speaker 1: of his desk, right, he did, and would with a
6:00
Speaker 1: pen on a piece of paper. It all sort of
6:02
Speaker 1: started with a little thought experiment. There were a lot
6:04
Speaker 1: of thought experiments, but you know, he was motivated by
6:06
Speaker 1: the work and the experiments done by a lot of
6:09
Speaker 1: other folks. In the hundred years before Einstein, people have
6:12
Speaker 1: been studying light and electromagnetism, which raised some questions about
6:16
Speaker 1: how fast things moved and whether they were moving relative
6:19
Speaker 1: to something else, whether life propagated through an either or
6:22
Speaker 1: whether it just moved through empty space. And so there
6:24
Speaker 1: were a lot of really interesting and puzzling experimental results
6:27
Speaker 1: in the last hundred years that needed somebody to sort
6:30
Speaker 1: of sit down and think clearly and bring it all together.
6:33
Speaker 1: I think that's really motivating in the history of physics
6:36
Speaker 1: times when all of the ideas were out there, all
6:39
Speaker 1: the information you needed to make that discovery, it was
6:42
Speaker 1: already present. It was public information, and it just needed
6:46
Speaker 1: somebody to sit down and think carefully and make those connections.
6:49
Speaker 1: That's inspiring to me because we're in a similar moment
6:51
Speaker 1: right now when there are a lot of results we
6:53
Speaker 1: just don't understand. Maybe somebody just needs to think about
6:56
Speaker 1: it the right way and have that moment of clarity. Yeah.
7:00
Speaker 1: Pretty cool, And I guess I'm just saying that. You know,
7:02
Speaker 1: he revolutionized physics, but he did it without a fifty
7:05
Speaker 1: billion particle collider. You know, is that a challenge? Are
7:09
Speaker 1: you saying I should be able to do the same thing.
7:10
Speaker 1: You're not a real physicist if you can do it
7:12
Speaker 1: for the price of pen and paper. I didn't say that,
7:15
Speaker 1: You just said that. Well, the point I'm making is that, yeah,
7:18
Speaker 1: he only spent money and pen and paper, but he
7:20
Speaker 1: was relying on the experiments of people who spent a
7:23
Speaker 1: lot more time and money and blood and sweat to
7:26
Speaker 1: extract that information from the universe. Something that really distinguishes
7:30
Speaker 1: modern science from like what the Greeks were doing is
7:33
Speaker 1: that it's empirical. We actually demand that it describes the
7:35
Speaker 1: universe and that it survives experimental tests, not just ideas
7:39
Speaker 1: in our minds. Yeah, I guess he collided ideas sort of. Right.
7:43
Speaker 1: It's a two part harmony experiment in theory, right, you
7:45
Speaker 1: need both voices to really make the song sing. Yeah,
7:49
Speaker 1: but he upturned our notion of the universe, and he
7:51
Speaker 1: did it from his desk with a piece of paper,
7:53
Speaker 1: and he did it sort of by asking a very
7:55
Speaker 1: kind of simple question, right about the universe and the
7:58
Speaker 1: speed of light. So it's day on the podcast we'll
8:01
Speaker 1: be tackling the question is the speed of light the
8:08
Speaker 1: same in all directions? I'm not even sure why we're
8:13
Speaker 1: asking this question, Like is it possible for light to
8:15
Speaker 1: go a different speeds in the like up or down
8:19
Speaker 1: rather than side to side. Yeah, it's a really fascinating question,
8:22
Speaker 1: and there's sort of two steps in getting here. One
8:25
Speaker 1: is first, just like accepting the idea that the speed
8:29
Speaker 1: of light should be the same for all observers. This
8:32
Speaker 1: is sort of the big revelation of special relativity from
8:35
Speaker 1: Einstein that the speed of light is the speed of light,
8:38
Speaker 1: no matter who's measuring it, no matter what the sources.
8:41
Speaker 1: If you're holding a flashlight and you flick the switch,
8:43
Speaker 1: light leaves the flashlight at the speed of light. If
8:45
Speaker 1: you're sitting in a car going sixty miles per hour
8:49
Speaker 1: and turn on a flashlight, then for you, light still
8:51
Speaker 1: leaves your flashlight at the speed of light. But somebody
8:54
Speaker 1: on the ground seeing you move at sixty miles per
8:56
Speaker 1: hour relative to the ground and seeing you flick on
8:59
Speaker 1: that fly light, they don't measure that light going at
9:02
Speaker 1: the speed of light plus sixty miles per hour. They
9:05
Speaker 1: still see it going at the speed of light. This
9:08
Speaker 1: is one of the core ideas of special relativity and
9:10
Speaker 1: one of the hardest to get your minds around, that
9:12
Speaker 1: the speed of light is invariant. Yeah, it's kind of
9:15
Speaker 1: a fundamental feature of the universe. But I guess it
9:18
Speaker 1: gets a little tricky, right because we also know that
9:21
Speaker 1: kind of like gravity and heavy objects bend space, so
9:25
Speaker 1: you can also sort of bend the speed of light
9:27
Speaker 1: in a way. Absolutely everything we're talking about today is
9:29
Speaker 1: assuming that space is flat. There's no curvature, there's no
9:33
Speaker 1: heavy masses, there's no black holes. In general relativity, where
9:36
Speaker 1: space is curved, things get even wank here, and light
9:40
Speaker 1: can have all sorts of weird speeds. And you can
9:42
Speaker 1: observe light going at crazy higher speeds or even crawling
9:45
Speaker 1: to zero as it tries to escape the gravity well
9:48
Speaker 1: of a black hole. But that's a completely separate rabbit hole,
9:51
Speaker 1: which I think we should close off for today. Wait,
9:54
Speaker 1: so you're saying speed of light can change kind of
9:57
Speaker 1: depending on what's around it. It's an even weirder because
10:00
Speaker 1: in general relativity we can't even talk about the definition
10:04
Speaker 1: of the speed of light. If the light is far away.
10:06
Speaker 1: You can only measure the speed of light confidently in
10:09
Speaker 1: your local frame because we don't know how to define
10:12
Speaker 1: the velocity of distant objects in general relativity in an
10:16
Speaker 1: invariant way. If you're talking about the speed of light
10:19
Speaker 1: for a photon that's very very far away from you
10:21
Speaker 1: in general relativity and there's curvature between you and there,
10:24
Speaker 1: then you don't even really know how to define velocity.
10:27
Speaker 1: So wait, so then the speed of light doesn't move
10:29
Speaker 1: the same in all directions, or is it that we
10:31
Speaker 1: just don't know how to measure it. We don't even
10:32
Speaker 1: know how to define what we mean by velocity of
10:35
Speaker 1: a photon for very different objects in curved space. Right.
10:40
Speaker 1: We talked about this once on the podcast, about how
10:42
Speaker 1: to compare things that are moving that are very very
10:45
Speaker 1: far apart from each other if the universe is curved
10:48
Speaker 1: between those objects. Like if you have runners near each
10:52
Speaker 1: other running in a race in Chicago, you can talk
10:55
Speaker 1: about who's faster because they're all in the same location
10:58
Speaker 1: in space is pretty much flat between. But if the
11:00
Speaker 1: Earth is curved and one of your runners is in
11:02
Speaker 1: Chicago and the other one is in South America, then
11:05
Speaker 1: you have to take that curvature into account when you're
11:07
Speaker 1: comparing their velocities, and there's different ways to do that,
11:11
Speaker 1: and so their relative velocity is no longer like a
11:13
Speaker 1: crisply defined thing. You can get different answers based on
11:16
Speaker 1: exactly how you compare them. The same is true in
11:19
Speaker 1: general relativity. If you fire a photon on the other
11:21
Speaker 1: side of a black hole, for example, then people could
11:23
Speaker 1: disagree about how you define the velocity of that photon
11:27
Speaker 1: because space is curved between us and them. I guess
11:30
Speaker 1: that feels like maybe next level podcast um topic. But
11:34
Speaker 1: I think when you talk about Einstein and what he
11:36
Speaker 1: was thinking about back then, he was maybe mostly thinking
11:39
Speaker 1: about the local velocity of light, right, Like, if I'm
11:41
Speaker 1: sitting here and I point a flash light up, down, left, right,
11:44
Speaker 1: bowards or back, is that flash of light going to
11:47
Speaker 1: move at the same speed? That's right? So Einstein first says, look,
11:50
Speaker 1: the speed of light is invariant. Everybody measures it to
11:52
Speaker 1: be the same thing. But there's a wrinkle there. It
11:55
Speaker 1: turns out that Einstein's equations that all of our experiments
11:58
Speaker 1: could also be consisting it with light moving at different
12:01
Speaker 1: speeds in different directions. That maybe light moves faster in
12:05
Speaker 1: one direction than in the other direction. All right, well,
12:08
Speaker 1: I guess that question is more about whether the speed
12:10
Speaker 1: of light is the same whether you're moving or not.
12:12
Speaker 1: But I think today we're asking a different question, which
12:15
Speaker 1: is like, does the universe have kind of a preferred
12:17
Speaker 1: direction for the speed of light or does the speed
12:19
Speaker 1: of light somehow move faster or slower in a particular direction.
12:23
Speaker 1: I guess relative to the rest of the stuff in
12:26
Speaker 1: the universe exactly. And remember there are famous experiments the
12:29
Speaker 1: Michaelson Morley experiment that tried to ask the question about
12:32
Speaker 1: whether light is moving relative to some ether or whether
12:35
Speaker 1: it's just propagetting through empty space. And people were trying
12:38
Speaker 1: to measure that, and so they did these experiments where
12:40
Speaker 1: they shot beams of light against a mirror and back
12:42
Speaker 1: to measure it. And they discovered that the speed of
12:44
Speaker 1: light seems to be the same in all directions, even
12:47
Speaker 1: as the Earth turns, etcetera. And so there probably wasn't
12:49
Speaker 1: any ether. But you know, in special relativity, there are
12:52
Speaker 1: always loopholes and you have to ask questions about those loopholes.
12:56
Speaker 1: And more of the loopholes and those experiments is that
12:58
Speaker 1: they're measuring the speed of light there and back, sort
13:02
Speaker 1: of like you throw a baseball to somebody and they
13:04
Speaker 1: throw it back, and you're measuring the average velocity in
13:06
Speaker 1: both directions. And so there's a question there about whether
13:10
Speaker 1: the speed of light really is the same there and back,
13:13
Speaker 1: or whether it could be different on the way there
13:15
Speaker 1: and on the way back. So it's not about up
13:17
Speaker 1: and down, lift and right. It's more about their on back.
13:19
Speaker 1: It's more about their and back. There have to be
13:21
Speaker 1: some direction in which is preferred for it to be
13:24
Speaker 1: the same or not. Okay, So then does this scenario
13:27
Speaker 1: then assume that we're both sort of stationary and not
13:30
Speaker 1: moving with the rest with the rest of the universe,
13:32
Speaker 1: or can we be moving super fast? This question isn't
13:35
Speaker 1: about how fast you're moving a relative to the rest
13:37
Speaker 1: of the universe or whether there's an ether. The question
13:40
Speaker 1: asks does light actually move the same speed in both directions?
13:45
Speaker 1: Eyeshine and beam of light at you and you hold
13:47
Speaker 1: a mirror and reflective back to me. How do we
13:49
Speaker 1: know that it's gone at the same speed on the
13:51
Speaker 1: way to you and on the way back. And this
13:55
Speaker 1: is no matter where you're standing, Like if I you're
13:58
Speaker 1: standing in front of me and I shine a light
13:59
Speaker 1: to you and back, or to the side of me
14:01
Speaker 1: and I shoud a light to you and back, doesn't
14:03
Speaker 1: matter where you are relative to me? Or are we
14:05
Speaker 1: always just thinking about there and back? It might matter. Yeah,
14:08
Speaker 1: The question is sort of like, does the universe have
14:10
Speaker 1: a preferred direction where the speed of light is faster
14:13
Speaker 1: in one direction and slower in the opposite direction. And
14:16
Speaker 1: that would be really weird, you know, because we expect
14:18
Speaker 1: the universe to not prefer any directions, So that would
14:20
Speaker 1: be very, very strange. That would be super strange. Should
14:23
Speaker 1: be like if there was some kind of like wind
14:26
Speaker 1: overall winds to the universe that that always pushes light
14:29
Speaker 1: faster in one way and not the other exactly. So
14:31
Speaker 1: might surprise people how little we know about the relative
14:34
Speaker 1: speed of light on the way there and on the
14:36
Speaker 1: way back. So, as usually, we were wondering how many
14:38
Speaker 1: people have thought about this question, whether the speed of
14:41
Speaker 1: light can need the same in all directions, And so
14:43
Speaker 1: Daniel went out there into the internet to find out
14:46
Speaker 1: what people thought. So thanks very much to everybody who
14:49
Speaker 1: answered this question and our entire team of question answers.
14:52
Speaker 1: If you'd like to join that team, please don't be shy.
14:55
Speaker 1: Everybody's welcome right to us. Two questions at Daniel and
14:58
Speaker 1: Jorge dot com. Think about it for a second. Do
15:00
Speaker 1: you think the speed of light is the same in
15:02
Speaker 1: all directions? There's what people have to say. I don't
15:05
Speaker 1: think so. I think it is it should be the
15:12
Speaker 1: same in all directions unless there's a something blocking it
15:18
Speaker 1: or bending of space time. I believe that the speed
15:21
Speaker 1: of lights is the same in all directions. However, I'm
15:26
Speaker 1: not sure because when we measure the speed of lights
15:28
Speaker 1: we send lights in one direction and then it comes
15:32
Speaker 1: back to us, and we measure the time and then
15:35
Speaker 1: divided that by true getting through um the speed of light.
15:40
Speaker 1: But we don't know if the speed is the same
15:42
Speaker 1: when it's going towards the mirror as it is when
15:46
Speaker 1: it's coming back. We just know that the total speeds there.
15:50
Speaker 1: Space has no preferred direction, and the speed of light
15:53
Speaker 1: is the constant vacuum. So I'm gonna say, yes, the
15:56
Speaker 1: speed of light is the same in all directions, should
15:58
Speaker 1: be the same in all directions. And you see, it
16:01
Speaker 1: depends on what direction do you do you go, for instance,
16:07
Speaker 1: when you go on a vocation. Yes, um, the speed
16:11
Speaker 1: of light is faster than you come back from your vacation.
16:17
Speaker 1: Coming back from a vacation might affect even the speed
16:21
Speaker 1: of light. I'm pretty sure the speed of light is
16:23
Speaker 1: the same in all directions because it is a constant.
16:27
Speaker 1: So you can't really have light going above the speed
16:31
Speaker 1: of light, or I guess slower than the speed of light.
16:35
Speaker 1: I'm not sure. I thought the speed of light was continuous,
16:39
Speaker 1: so it was that six thousand miles per second, so
16:44
Speaker 1: that would be surely the same in all directions. Is
16:47
Speaker 1: it's not. It doesn't kind of follow the same rules
16:50
Speaker 1: that we would apply to normal speed limits. Saying that,
16:54
Speaker 1: oh god, yeah, no, I'm going to say now, so immediately,
16:57
Speaker 1: I'm kind of thinking of like a star and lights
17:00
Speaker 1: shining from every angle because it's uh spherical. Um, I
17:04
Speaker 1: think it's shines more brightly in the direction where there's
17:07
Speaker 1: like solar flares, because those are essentially like the matter
17:11
Speaker 1: shooting out and then the light leaving those bits of matter.
17:14
Speaker 1: So no, I think it kind of depends where the
17:17
Speaker 1: star is most active in the vacuum of space. The
17:20
Speaker 1: speed of light is the same in every direction. I
17:23
Speaker 1: learned that by listening to your podcast. Yes, the speed
17:27
Speaker 1: of light is the same in all directions. Relative directionality
17:30
Speaker 1: can affect wavelength, and I believe there's experiments where they've
17:33
Speaker 1: captured or slowed down photons, But the speed of light
17:37
Speaker 1: is the same in all directions. I want to say, yes,
17:39
Speaker 1: it's a constant. However, things may change with specitification occurs,
17:47
Speaker 1: or lensing around the black hole. Perhaps, all right, most
17:51
Speaker 1: people think that it should be the same. A few
17:53
Speaker 1: people thought it shouldn't be the same. Yeah, Overwhelmingly people think, look,
17:57
Speaker 1: it's a constant, and so it's the speed of light
17:59
Speaker 1: is the speed of light, and that's been really drummed
18:01
Speaker 1: into people in physics and in popular science for a
18:04
Speaker 1: long time. So it makes sense that people believe that. Yeah, because,
18:07
Speaker 1: as you said, I think everyone has heard of, you know,
18:08
Speaker 1: that famous experiment where you should light in one way
18:11
Speaker 1: and the other way and you measure the speed of
18:13
Speaker 1: light to be the same. So I guess people just
18:15
Speaker 1: assume that it's always the same exactly. But in physics
18:17
Speaker 1: we always have to ask, what did we really learn
18:20
Speaker 1: from this experiment? What other ideas might also explain this experiment,
18:26
Speaker 1: and is there any way we can distinguish between these
18:28
Speaker 1: like alternative hypotheses and the one that we favor. I see,
18:32
Speaker 1: you can't just leave well enough alone. Never we will
18:35
Speaker 1: always have more questions. Keep picking. That's how we got
18:39
Speaker 1: to where we are, right. The whole way that we
18:41
Speaker 1: discovered crazy new ideas about the universe is by picking
18:44
Speaker 1: at things, by tugging on those little threads that didn't
18:47
Speaker 1: quite sit well with somebody. Yes, and are we better off? Danny,
18:52
Speaker 1: I have a job, so yeah, I guess, so one
18:55
Speaker 1: person is better off? All right, Well, let's get down
18:57
Speaker 1: to the nitty gritty of it, and let's start with
18:59
Speaker 1: the basics, Daniel, what is the speed of light? How
19:02
Speaker 1: do you define it? What does it mean in the universe?
19:04
Speaker 1: Usually when we talk about the speed of light, we
19:06
Speaker 1: mean the speed of light of a photon moving in
19:08
Speaker 1: a vacuum. And it's really not just the speed of light,
19:11
Speaker 1: it's the speed of information. Any massless particle, a gluon graviton,
19:16
Speaker 1: if it exists, would move at the speed of light
19:19
Speaker 1: and has to move at the speed of light because
19:21
Speaker 1: things that are massless have nothing to them. They are
19:24
Speaker 1: just motion. So these objects move at this ridiculously high
19:28
Speaker 1: speed three million meters per second, and so it seems
19:32
Speaker 1: to be sort of like a fundamental speed to the universe,
19:35
Speaker 1: not just to light. Right, It's kind of like the
19:38
Speaker 1: basic speed of information. But I guess maybe a basic
19:41
Speaker 1: question is how do you define it? How do you
19:43
Speaker 1: how do you measure it? Like I shoot a photon
19:45
Speaker 1: here and then I measure how long it takes for
19:47
Speaker 1: it to get to a meter in front of me.
19:50
Speaker 1: Is that how you would define the speed of light?
19:52
Speaker 1: But then how does a person a meter in front
19:54
Speaker 1: of me know when I shot the photon? Yeah, exactly.
19:57
Speaker 1: And this is precisely the kind of question you need
19:59
Speaker 1: to think very carefully about in special relativity, because measuring
20:03
Speaker 1: velocity and special relativity is a bit subtle. What you
20:06
Speaker 1: need to do is define a distance. Actually, have a
20:09
Speaker 1: point A and the point B. You measure the distance
20:12
Speaker 1: between them. You shoot something from A to B, and
20:15
Speaker 1: then you need to measure the time it took to
20:16
Speaker 1: go from A to B, right, And that's where the
20:18
Speaker 1: subtlety comes in, knowing how long it took to go
20:21
Speaker 1: from A to B because you have a clock at
20:23
Speaker 1: A and you have a clock could be But to
20:25
Speaker 1: measure the time from A to B, you can't just
20:27
Speaker 1: necessarily subtract the time on the clock at B and
20:30
Speaker 1: the time on the clock at A, because how do
20:32
Speaker 1: you know they were synchronized, right, So you have to
20:34
Speaker 1: come up with some way to synchronize those clocks. And
20:37
Speaker 1: so this is where a lot of the complication comes
20:39
Speaker 1: in is in how to synchronize clocks at different locations.
20:43
Speaker 1: And all special relativity is about simultaneity across distances and
20:48
Speaker 1: how you define these kinds of things. Right, It's really
20:50
Speaker 1: hard to synchronize clocks because I guess like you could
20:53
Speaker 1: come here to South Pasadena and we could make sure
20:56
Speaker 1: our clocks start at the same time. But by the
20:58
Speaker 1: time you get down there to Orange Tinny, where you live,
21:01
Speaker 1: your clock might be out of sink. In fact, it
21:02
Speaker 1: certainly will be, because what we've learned is that moving
21:06
Speaker 1: clocks runs slow. So if I get in my car
21:09
Speaker 1: and I drive at sixty or you know, in l
21:12
Speaker 1: a more like five relative to your house, than by
21:15
Speaker 1: the time my clock arrives in Irvine, it will no
21:18
Speaker 1: longer be synchronized with yours, right, And so it's very
21:22
Speaker 1: complicated to synchronize separated clocks. What if I drive the
21:26
Speaker 1: opposite way the same amount, wouldn't they still be in sync?
21:29
Speaker 1: Then you're just gonna give me a big calculational headache.
21:34
Speaker 1: I mean, I'll end up in the mountains here, sanka
21:37
Speaker 1: Ro mountains. But wouldn't that work. Wouldn't the clocks still
21:39
Speaker 1: be synchronized. No, fundamentally, that doesn't change anything, because it's
21:42
Speaker 1: still just relative. Velocity just increases my relative velocity to
21:46
Speaker 1: you or your relative velocity to me. But because velocity
21:48
Speaker 1: is all relative, it doesn't matter what your velocity is
21:51
Speaker 1: relative to the ground. Doesn't change the fact that our
21:54
Speaker 1: relative velocities is what determines the time dilation. I see.
21:58
Speaker 1: I think what you're saying is it's complicated. I'm saying
22:01
Speaker 1: stay home or hey, yeah good, I hate I hate
22:04
Speaker 1: driving anywhere? Can I take an uber? What if I
22:06
Speaker 1: take an uber or lift? Does it still dilate time? Oh?
22:09
Speaker 1: You have to pay extra to not dilate time in
22:11
Speaker 1: your uber. It's the uber quantum. All right. Well, so
22:13
Speaker 1: that's the speed of light, and let's get into whether
22:15
Speaker 1: or not it moves the same in all directions and
22:17
Speaker 1: whether or not we've actually measured that. But first, let's
22:20
Speaker 1: take a quick break. All right, we're talking about the
22:34
Speaker 1: speed of light, which is the I guess, the opposite
22:38
Speaker 1: of how fast my kids move whenever they don't want
22:41
Speaker 1: to go somewhere, the speed of parenting exactly. Yeah. The
22:45
Speaker 1: speed of light is something that really comes up a
22:47
Speaker 1: lot in questions about the universe because it seems to
22:50
Speaker 1: be something deep and true, something like that reflects how
22:53
Speaker 1: the universe itself works. You know that if you have
22:56
Speaker 1: empty space with nothing in it, that photons ripple through
22:59
Speaker 1: it at certain speed, and people are often asking, you know, why,
23:02
Speaker 1: why is the speed of light? This number and not
23:05
Speaker 1: some other number, right, like is it a fundamental part
23:08
Speaker 1: of the universe. But you just raise an interesting point,
23:10
Speaker 1: which is that the speed of light is really just
23:12
Speaker 1: kind of like how fast of photon or ripple in
23:15
Speaker 1: the electromagnetic field moves. So maybe the real question we're
23:19
Speaker 1: asking here is whether those fields themselves can move or
23:24
Speaker 1: maybe they have a preferred direction for ripples to move it. Yeah,
23:26
Speaker 1: we're talking about the rippling of these quantum fields and
23:29
Speaker 1: how well we understand it, you know. And I think
23:31
Speaker 1: the thing to remember is that our description of special
23:34
Speaker 1: relativity doesn't come from some deep underlying understanding of space
23:39
Speaker 1: and time. We haven't like derived the speed of light
23:42
Speaker 1: from saying, oh, space is a bunch of quantum foam
23:46
Speaker 1: and they're linked together in this way, and the speed
23:48
Speaker 1: of light comes from how those things are linked or
23:50
Speaker 1: anything like that. It's just our most successful and most
23:54
Speaker 1: compact description of all the experiments that we've seen. So
23:58
Speaker 1: you can look at it and you can ask, like, well,
24:00
Speaker 1: under why it's this way and not some other way,
24:01
Speaker 1: And we don't have an answer to that. We just
24:03
Speaker 1: have like a very effective description that works really really well,
24:08
Speaker 1: and we can ask questions about it and we can
24:09
Speaker 1: use it. We don't always understand why this is the
24:12
Speaker 1: theory of the universe or why the number is this number.
24:15
Speaker 1: It might be that is determined by deeper underlying truths
24:19
Speaker 1: about the universe, or it might be that it's not
24:21
Speaker 1: you know, that we live in a multiverse, and in
24:23
Speaker 1: different universes there are different speeds of light, and this
24:25
Speaker 1: is just the one we got right, or even just
24:27
Speaker 1: the idea that there is a speed limit to the universe,
24:30
Speaker 1: you know, like, you know, we assume that nothing can
24:32
Speaker 1: move faster than the speed of light, and physicists always
24:34
Speaker 1: saying and I think people have internalized that, but really
24:37
Speaker 1: we don't know why, Like we only know that because
24:39
Speaker 1: we've measured that. Right, that's right, that is our best
24:42
Speaker 1: description of the universe. It really isn't a great why.
24:45
Speaker 1: The answer to why is like, well, if you assume
24:48
Speaker 1: that that's true, then you develop a theory which is
24:51
Speaker 1: very effective and seems very accurate and describes our universe.
24:55
Speaker 1: It's not really an answer to the why. It's a
24:58
Speaker 1: whether you should think this is probably accurate, not why
25:01
Speaker 1: is it this way and not some other way? Right?
25:03
Speaker 1: And it's all based on what we've observed exactly. And
25:07
Speaker 1: that's why it's so important to remember what the experiments
25:10
Speaker 1: actually tell us and what they don't tell us, because
25:13
Speaker 1: it can be tempting to draw like overly broad conclusions
25:16
Speaker 1: about what's going on in the universe and what we understand.
25:19
Speaker 1: But in the end it's always grounded in those experiments,
25:21
Speaker 1: not just the pen and paper folks. Right, So let's
25:24
Speaker 1: get into that maybe in more detail. And when you
25:27
Speaker 1: talk about the speed of light and measuring the speed
25:29
Speaker 1: of light, like what does that actually mean? Like I
25:31
Speaker 1: asked earlier, like does it mean that somebody shot a
25:33
Speaker 1: flash light here and somebody measured how long it took
25:35
Speaker 1: to get to the other person? But then how did
25:36
Speaker 1: they do it? Where the clock sinks? And all that
25:38
Speaker 1: gets kind of tricky. It gets pretty tricky. If you
25:40
Speaker 1: want to measure the speed of light like from me
25:42
Speaker 1: to you, then either you have to know exactly how
25:46
Speaker 1: our clocks go out of sync when we move them apart,
25:50
Speaker 1: but that require special relativity which depends on the speed
25:53
Speaker 1: of light. Or you can try to use like light
25:55
Speaker 1: pulses to synchronize our clocks which are separated, but again
25:59
Speaker 1: that requires under standing how long it takes light to
26:01
Speaker 1: go from me to you, So there's no way to
26:04
Speaker 1: independently think two distant clocks in order to measure the
26:09
Speaker 1: speed of light just in one direction. And so what
26:11
Speaker 1: people have done, the actual experiments we have done, is
26:14
Speaker 1: to measure the round trip speed of light. So you
26:16
Speaker 1: don't have a clock in Pasadena, you just have a mirror.
26:19
Speaker 1: I have a clock, one single clock. I shoot my
26:22
Speaker 1: laser beam up to Pasadena, bounces off of your mirror,
26:24
Speaker 1: it comes back to me, and I can measure that
26:27
Speaker 1: they're in back time on my clock, and we know
26:29
Speaker 1: what the distance is, so then we can derive the
26:32
Speaker 1: speed of light. But what I'm measuring there is the
26:34
Speaker 1: round trip speed of light, not the one way speed
26:37
Speaker 1: of light. Interesting, then you don't need to clocks right exactly, Okay,
26:42
Speaker 1: So then that is that kind of the basis of
26:44
Speaker 1: most experiments that have measured the speed of light is
26:46
Speaker 1: that there and back kind of timing. That's the basis
26:49
Speaker 1: of every measurement of the speed of light. We had
26:52
Speaker 1: a whole podcast episode about how this was measured, and
26:54
Speaker 1: you know, the first ones were pretty cool. There were
26:56
Speaker 1: tricks about like Io disappearing behind Jupiter and how along
27:00
Speaker 1: that takes when I was moving towards US versus away
27:02
Speaker 1: from us. But more recent measurements are all about like
27:05
Speaker 1: shooting beams of light against distant mirrors and having them
27:08
Speaker 1: come back. Phizoue did this cool experiment with rotating gears
27:12
Speaker 1: that moved really fast and blocking the light, etcetera. But
27:14
Speaker 1: all of them are there and back measurements for that reason,
27:17
Speaker 1: because there's no way to synchronize two distant clocks without
27:20
Speaker 1: knowing what the speed of light is already, right, right,
27:23
Speaker 1: So that seems like a pretty straightforward way to measure
27:26
Speaker 1: the speed of light. Is you shoot it to get
27:27
Speaker 1: up to a mirror, You know the distance from you
27:29
Speaker 1: to the mirror, so you measure how long light takes
27:31
Speaker 1: to go to the mirror and back. And why isn't
27:33
Speaker 1: that just isn't that straightforward? Like that should tell you
27:37
Speaker 1: how fast light moves? Right, that should tell you how
27:39
Speaker 1: fast light moves. But if you go back and read
27:41
Speaker 1: Einstein's original paper from five, he makes a point that
27:45
Speaker 1: we don't actually know what the speed of light is
27:48
Speaker 1: in all directions like his special relativity, and all of
27:50
Speaker 1: these experiments are also consistent with a very different, very
27:55
Speaker 1: strange idea of the universe that light could move faster
27:58
Speaker 1: between Irvine and Passive NA than it does between Pasadena
28:01
Speaker 1: and Irvine. Let's say, for the sake of argument, that
28:03
Speaker 1: it should take two seconds for light to get from
28:06
Speaker 1: Irvine to Pasadena and back. One possibility is that it
28:09
Speaker 1: takes one second to get from Irvine to Pasadena and
28:11
Speaker 1: one second to get back. Another possibility is that it
28:14
Speaker 1: takes half a second to get from Irvine to Pasadena
28:16
Speaker 1: and a second and a half to get back, And
28:19
Speaker 1: both of those would be consistent with what we measure,
28:21
Speaker 1: because we only measure the round trip time. And Einstend
28:24
Speaker 1: and his paper pointed out, like, there's an ambiguity here.
28:27
Speaker 1: We don't actually know what the one way time is.
28:29
Speaker 1: So let's just assume that it's the same in every
28:32
Speaker 1: direction because that's simplest. So that's called the Einstein convention,
28:37
Speaker 1: and that's the one on which special relativity is built.
28:39
Speaker 1: But we've never actually checked that that's true. WHOA, That
28:42
Speaker 1: is pretty land blowing. I guess that the idea is
28:44
Speaker 1: that you wouldn't be able to tell the difference between
28:47
Speaker 1: the two scenarios, right, like you wouldn't be able to
28:49
Speaker 1: tell if light took one second each way, or whether
28:51
Speaker 1: it took one and a half seconds one way and
28:53
Speaker 1: half a second the other way. Like, there's no real
28:56
Speaker 1: way for you to know, right, there's no experiment you
28:58
Speaker 1: can do to tell the friends. But what if I,
29:00
Speaker 1: like do Irvine to Pasadena and Pasadian back, Right, that's
29:04
Speaker 1: one sort of direction. What if I go, let's say
29:07
Speaker 1: that's like north south. What if I go east west
29:10
Speaker 1: and I measure it to be the same. Wouldn't that
29:11
Speaker 1: sort of tell me that there is some sort of
29:13
Speaker 1: invariants in the universe. That's a Michael said Morley experiment. Right,
29:16
Speaker 1: it doesn't matter what direction your apparatus has pointed relative
29:19
Speaker 1: to the Earth or the cosmos or anything. You always
29:22
Speaker 1: get the same answer. But it could be that the
29:24
Speaker 1: east west version is different from the north south version.
29:26
Speaker 1: But again you can't tell because you don't measure the
29:28
Speaker 1: intermediate time. You only measure the round trip time. And
29:32
Speaker 1: it's even possible for it to be instantaneous in one
29:35
Speaker 1: direction and then take the full round trip time on
29:38
Speaker 1: the way back. Zero seconds from Ermine to Pasadena, two
29:42
Speaker 1: seconds on the way back. I see, So even if
29:45
Speaker 1: you measure in north south, east west, up and down
29:49
Speaker 1: because you're measuring a round trip thing. If there was
29:51
Speaker 1: sort of a bias, let's say the universe out of
29:53
Speaker 1: bias towards northwest to southeast, you still wouldn't be able
29:57
Speaker 1: to tell the difference. I think that's what you're saying.
29:58
Speaker 1: That's right. All round trip measurements of the speed of
30:01
Speaker 1: light are not sensitive to the one way speed of light.
30:04
Speaker 1: The speed of light just from a to be what
30:08
Speaker 1: if I don't know, should shoot a triangle like a
30:10
Speaker 1: form of triangle with like tumors, Do you know what
30:14
Speaker 1: I mean? Like, wouldn't that give me a little bit
30:16
Speaker 1: more of a sensitivity to the direction of the universe. Yeah,
30:18
Speaker 1: But if you're not measuring the time and the intermediate steps,
30:21
Speaker 1: that doesn't matter how many intermediate steps you have. You're
30:24
Speaker 1: still just measuring the round trip time. As long as
30:26
Speaker 1: you have a single clock in one location, you can't
30:29
Speaker 1: tell how long it's taken to get part of the
30:31
Speaker 1: way through the trip, right, all right? So that sounds
30:34
Speaker 1: like a quite a pickle there that we can't tell
30:37
Speaker 1: the universe has a preferred direction for the speed of light.
30:40
Speaker 1: So what does that mean? It means two things, right,
30:42
Speaker 1: It means on one hand that we should think deeply
30:45
Speaker 1: about what we actually know about the universe, what's really
30:48
Speaker 1: happening out there, And on the other hand, it also
30:50
Speaker 1: means we should think carefully about the questions we're asking,
30:53
Speaker 1: like does this question have any meaning? It sounds really
30:56
Speaker 1: deep and meaningful, but is it just really the same
30:58
Speaker 1: as like saying that zones or time zones and it
31:01
Speaker 1: just depends on how you define time. So it raises
31:04
Speaker 1: some interesting questions about like what we mean by these
31:07
Speaker 1: theories and how we understand the universe. Wait, wait, are
31:10
Speaker 1: you saying that you're giving up like we we can't
31:12
Speaker 1: tell from an experiment where you bounce light off of
31:15
Speaker 1: a mirror, So does that mean we can never tell
31:17
Speaker 1: if light has a preferred direction in the universe. If
31:20
Speaker 1: special relativity is correct, then it's consistent with all of
31:23
Speaker 1: these different ideas, and there should be no way to
31:26
Speaker 1: tell the difference between these various ideas that light is
31:30
Speaker 1: instantaneous in one direction and half the speed of light
31:33
Speaker 1: in the other, or the speed of light in every direction.
31:36
Speaker 1: So according to special relativity, Einstein's ideas are consistent with
31:40
Speaker 1: all of those right, And every experiment we've ever done
31:43
Speaker 1: is also consistent with all of those ideas. There is
31:46
Speaker 1: no way that we are aware of to tell the difference. Wow,
31:49
Speaker 1: does that mean we'll never know or does it mean
31:52
Speaker 1: we just, um, you know, need a bit of better theory.
31:55
Speaker 1: If this theory is correct, then we'll never know. But
31:58
Speaker 1: it might be possible that special relativity is not correct.
32:01
Speaker 1: We know that it's part of general relativity and general relativity.
32:04
Speaker 1: It's probably not the fundamental theory of the universe that
32:07
Speaker 1: needs to be modified to include quantum mechanics, which has
32:10
Speaker 1: its own weird ideas about time. So you know, in
32:13
Speaker 1: some future theory of quantum gravity, perhaps we'll figure this out.
32:16
Speaker 1: Or if we develop a deep understanding of space and
32:19
Speaker 1: time and we understand, like where the speed of light
32:21
Speaker 1: actually comes from, then maybe that'll put some constraint on it.
32:24
Speaker 1: But if our current theories are true, then there's no
32:27
Speaker 1: way to tell the difference. I mean, there isn't anything
32:29
Speaker 1: that would like, I don't know, give you a theoretical
32:32
Speaker 1: like inconsistency if the universe wasn't if it had a
32:36
Speaker 1: preferred direction, you know what I mean, like if it
32:38
Speaker 1: had a preferred like Let's assume it hasn't preferred direction.
32:40
Speaker 1: Wouldn't that make the equations break or wouldn't that make things,
32:44
Speaker 1: you know, all these symmetries break somehow. No, it makes
32:46
Speaker 1: the equations much more complicated because, now, like time dilation
32:50
Speaker 1: is asymmetric, clocks run slower differently depending on the direction
32:54
Speaker 1: there are going. But it always makes the same predictions.
32:57
Speaker 1: It always cancels itself out. It always comes out to
32:59
Speaker 1: predict the same conclusions for every experiment. The twin paradox
33:03
Speaker 1: and spaceships and general relativity and all that stuff, time
33:07
Speaker 1: dilation all comes out to them the same answers. So
33:10
Speaker 1: the math is more complicated, and it's also not as appealing, right,
33:13
Speaker 1: it's weird, it's strange, it doesn't sit well with us.
33:16
Speaker 1: It's not the simplest possible explanation, which is why Einstein
33:20
Speaker 1: made his choice. He's like, this seems more reasonable, This
33:22
Speaker 1: seems like the right way to go. But theoretically it
33:25
Speaker 1: all does hang together. Well, well, I guess let's think
33:28
Speaker 1: about the scenario where the the universe does have a
33:32
Speaker 1: preferred direction, like let's say the universe northeast to southwest
33:35
Speaker 1: or something like that. What could be the cause of that?
33:39
Speaker 1: Does that mean that all of the quantum fields are
33:41
Speaker 1: somehow moving in that direction, and why that direction. Well,
33:45
Speaker 1: I think there's two schools have thought about this. One
33:47
Speaker 1: is like, this could be real. It could be true
33:50
Speaker 1: that things actually move instantaneously in one direction and you know,
33:54
Speaker 1: our half the speed of light in the other. And
33:56
Speaker 1: you ask, like what could cause that? And yeah, it
33:59
Speaker 1: could be you know, of resulting from the way space
34:01
Speaker 1: and time is linked together. You know, remember that we
34:04
Speaker 1: just don't really understand what space is. Space is not
34:07
Speaker 1: just this backdrop in which the universe happens. It might
34:11
Speaker 1: emerge somehow from like woven together quantum pieces, and the
34:14
Speaker 1: way that information travels through that space could be determined
34:17
Speaker 1: by the rules for how that space works at the
34:20
Speaker 1: lowest level. So because we don't know why the speed
34:22
Speaker 1: of light is what it is, we have no reason
34:25
Speaker 1: to expect that it is some value in this direction
34:27
Speaker 1: and some value in another direction. There's interesting consequences there also,
34:30
Speaker 1: for like momentum conservation and angular momentum conservation, because remember
34:34
Speaker 1: that momentum conservation comes from assuming that the universe is
34:37
Speaker 1: the same everywhere. An angular momentum conservation comes from assuming
34:41
Speaker 1: that it's same in every direction. That if you spin
34:44
Speaker 1: your experiment, you don't get any different answers. That's one
34:47
Speaker 1: school of thought. Another school of thought is that this
34:50
Speaker 1: is all overblown and none of this really means anything.
34:53
Speaker 1: It just has to do with like how you define
34:55
Speaker 1: time and simultaneity. But really, nothing mysterious is going on. Well,
34:59
Speaker 1: did us to go back to what you said a
35:01
Speaker 1: moment earlier? I did have a question about that light,
35:03
Speaker 1: Like if the light is moving faster in one direction
35:05
Speaker 1: than another direction, and I shoot a photon to a
35:08
Speaker 1: mirror in that direction, I mean it's going faster one
35:11
Speaker 1: way and slow the other way. I guess what's causing
35:15
Speaker 1: that photon to slow down? Does it need something to
35:18
Speaker 1: like push it, or like, wouldn't it take some energy
35:21
Speaker 1: to change its speed? And that's an interesting question. Remember
35:23
Speaker 1: that photons don't just bounce off of mirrors, right, So
35:26
Speaker 1: it's not really the same photon that goes there and
35:29
Speaker 1: comes back. A photon is like absorbed and re emitted
35:32
Speaker 1: in a really complicated process. The microphysics of like how
35:35
Speaker 1: photons bounce off of objects and are reflected at the
35:38
Speaker 1: correct angle. It is actually really complicated. We should dig
35:41
Speaker 1: into it in some future episode of the podcast. But
35:43
Speaker 1: you can sort of think of it like as a
35:44
Speaker 1: new photon and what could cause it? You know, like
35:47
Speaker 1: we don't know why the speed of light is what
35:49
Speaker 1: it is, and so this is just saying that there's
35:51
Speaker 1: more freedom to our theories of the universe than we expected.
35:54
Speaker 1: That there's a knob that can be set to different
35:56
Speaker 1: values in different directions and everything is still sort of
36:00
Speaker 1: works out. Because we don't know why that knobby is
36:02
Speaker 1: what it is at all, then it's a little bit
36:05
Speaker 1: suspect to assume that it's the same knob in every direction.
36:10
Speaker 1: I see. What about things moving in a circle, like
36:12
Speaker 1: you have particles and sometimes light, you know, light instead
36:15
Speaker 1: of go around a black hole in a circle, would
36:18
Speaker 1: in a preferred direction cossum walking? Isn't that or some
36:21
Speaker 1: weird kind of angular momentum differences? You can think of
36:26
Speaker 1: motion in a circle is decomposed into just two different
36:29
Speaker 1: one directional motions, right, It's just like put an access
36:32
Speaker 1: on an X and Y. Motion is circle is motion
36:35
Speaker 1: in X and motion in WHY and so you can
36:37
Speaker 1: just break it down into two pieces of linear motion.
36:40
Speaker 1: And if the speed of light is different in one direction,
36:43
Speaker 1: then you know it would be faster around the back
36:45
Speaker 1: of the black hole, for example, and then slow down
36:47
Speaker 1: on the way back. But if you're not measuring it
36:49
Speaker 1: halfway around the black hole, then you have no idea
36:52
Speaker 1: how long it's taken to do the orbit, and whether
36:54
Speaker 1: it's gone smoothly around the black hole, like zipped faster
36:58
Speaker 1: around one bit and taking its time around another bit. Right,
37:01
Speaker 1: But it's weird to think that the photon would slow
37:03
Speaker 1: down in the middle, Right, wouldn't that cost some different
37:06
Speaker 1: forces on on it or the black hole? Yeah, and
37:09
Speaker 1: it's complicated because you have to account for all the
37:11
Speaker 1: forces here. But we're talking about photons moving through curved space,
37:15
Speaker 1: then there's already effective forces on them. You need to
37:18
Speaker 1: think about space as curved and photons moving along geodesics,
37:21
Speaker 1: or you could think about space as flat, sort of
37:23
Speaker 1: in a Newtonian way, thinking about the force on the
37:25
Speaker 1: photon to make it move in a circle. So in
37:28
Speaker 1: any case, there's already like strange things happening to this
37:31
Speaker 1: photon to change its direction and therefore its energy. Remember
37:35
Speaker 1: that energy itself is not an invariant in the universe.
37:39
Speaker 1: Different people will see the same photons having different energies anyway, Right,
37:43
Speaker 1: based on their velocity relative to the object that emitted it.
37:47
Speaker 1: I could see a photon is being redder than you
37:49
Speaker 1: see that same photon based on our velocities relative to
37:52
Speaker 1: what sent that photon out because of relativistic Doppler effects.
37:56
Speaker 1: So energy already is pretty wonky, even if you think
38:00
Speaker 1: that the speed of light is the same in all directions.
38:02
Speaker 1: I see, all right, I guess you can. I'll just
38:04
Speaker 1: blame it on the universe being wonky. Al Right, Well,
38:08
Speaker 1: it seems like we may never know if light can
38:11
Speaker 1: go faster in any particular speed. It could it could
38:13
Speaker 1: be pretty wild and crazy out there, or it could
38:15
Speaker 1: be pretty boring and the same everywhere. But we we
38:18
Speaker 1: might never know. And so let's get into what it
38:20
Speaker 1: could all mean about our understanding of the universe and
38:23
Speaker 1: our future experiments. But first, let's take another quick break.
38:39
Speaker 1: We're talking about the speed of light and whether it's
38:41
Speaker 1: the same in all directions. Daniel, seems like you're telling
38:43
Speaker 1: us that it's not possible to know ever, maybe whether
38:48
Speaker 1: it's going faster in any particular direction. It's not possible
38:51
Speaker 1: to know whether the universe prefers our current description where
38:55
Speaker 1: the speed of light is the same in every direction
38:57
Speaker 1: or a different description. We change the speed of light
39:00
Speaker 1: to be different in different directions. You know, they all
39:03
Speaker 1: describe the universe in the same way. And so until
39:06
Speaker 1: you like get into the heart of black holes and
39:08
Speaker 1: understand quantum gravity and have a reason to predict why
39:11
Speaker 1: the speed of light should be a certain number, than
39:14
Speaker 1: all these theories work the same way. And remember, the
39:16
Speaker 1: reason we like special relativity, that we accept it, not
39:19
Speaker 1: that we believe that it's fundamentally true, is because it works.
39:23
Speaker 1: And so if you apply that same premise right like,
39:26
Speaker 1: you should apply that to all theories that work. The
39:28
Speaker 1: reason that we prefer the one where the speed of
39:30
Speaker 1: light is the same in all directions is just that
39:32
Speaker 1: it's simpler. It makes the math easier, and we like
39:35
Speaker 1: simpler explanations of the universe. Some people think, like, all right,
39:38
Speaker 1: this is kind of like overblown clickbait. Really doesn't mean
39:41
Speaker 1: anything different about the physical universe. It's just about like
39:44
Speaker 1: how we write things down on paper. I see, like
39:47
Speaker 1: it's maybe gets to a more philosophical area of like
39:50
Speaker 1: what is even time or what is even distance or
39:52
Speaker 1: what is even speed? Exactly? It has a lot to
39:55
Speaker 1: do with how you define time all these measurements of
39:57
Speaker 1: the speed of light are really ratios of distances and times.
40:01
Speaker 1: As you're talking about time intervals, and it's very easy
40:04
Speaker 1: to get confused about what time intervals mean. In special relativity,
40:08
Speaker 1: it's simplest if you just have one clock never moves
40:10
Speaker 1: relative to you. But now we're trying to do something else.
40:13
Speaker 1: We're trying to think about, like what is the reading
40:15
Speaker 1: of a clock that's far away? And you know this
40:17
Speaker 1: is hard to do even with time zones on Earth.
40:19
Speaker 1: You know, for example, like France is an hour ahead
40:22
Speaker 1: of England in time zones, right, or like Arizona is
40:25
Speaker 1: an hour ahead of California in time zones. So you
40:29
Speaker 1: could like set off on a journey from Arizona to California,
40:33
Speaker 1: leaving at noon in Arizona and arriving after an hour
40:37
Speaker 1: at noon in California. Does that mean that you've traveled instantaneously? Right?
40:42
Speaker 1: Like you left at noon, you arrived at noon. According
40:45
Speaker 1: to some definition velocity, you've gone a distance and no
40:48
Speaker 1: time is elapsed on the clocks, and so you've moved
40:51
Speaker 1: instantaneously in some senses. This is sort of making that
40:54
Speaker 1: same argument that you can r raturely define the meaning
40:58
Speaker 1: of clocks at distant location, so that the speed of
41:01
Speaker 1: light becomes really weird and wonky. Right, And I was thinking, like,
41:04
Speaker 1: not even our notions of time or how we measure
41:07
Speaker 1: it is that pure in themselves, right, Like we measure
41:10
Speaker 1: time by how some crystal oscillates, right usually, or some
41:14
Speaker 1: atom like spins or something like that. But even those
41:17
Speaker 1: things have to do with motion of these particles through space,
41:20
Speaker 1: and then so those are also dependent on the speed
41:23
Speaker 1: of light, right, Yeah, absolutely everything is linked in that way.
41:26
Speaker 1: As you say, time is linked to motion, right, Time
41:28
Speaker 1: is a measurement of change. You can't have a clock
41:32
Speaker 1: that doesn't change. And everything there is linked to the
41:34
Speaker 1: fundamental processes which are connected to the speed of light.
41:37
Speaker 1: So more than thinking like wow, light might move instantaneously
41:41
Speaker 1: between here and Mars and take twice as long on
41:43
Speaker 1: the way back, instead of thinking that that might be
41:46
Speaker 1: the way our universe is. The point is to realize
41:48
Speaker 1: that there's a deep connection between the speed of light
41:50
Speaker 1: and the whole idea of time, and to understand how
41:53
Speaker 1: those ideas flow from one to the other, and how
41:55
Speaker 1: little we really know about what's happening far away. And
41:58
Speaker 1: I think there's a temptation to say we have special relativity.
42:01
Speaker 1: Therefore we can think about how long it takes, like
42:03
Speaker 1: to get from here to there, But really we get
42:05
Speaker 1: confused when we think about what's happening somewhere far away.
42:08
Speaker 1: We don't really know what's happening to clocks that are
42:11
Speaker 1: far away from us. Right. It kind of goes back
42:13
Speaker 1: to what we talked about earlier, how we're used to
42:16
Speaker 1: the world working one way because that's what it seems
42:18
Speaker 1: to work on as we grow up, and maybe there
42:21
Speaker 1: the real universe works in a totally different way, you know,
42:23
Speaker 1: Like we're used to this idea of you know, my
42:26
Speaker 1: time being the same as your time, or if I
42:28
Speaker 1: synchronize clocks and then we walk away from each other,
42:30
Speaker 1: the clocks will still be synchronized. But really that's not
42:33
Speaker 1: maybe how the universe really work in these extreme or
42:36
Speaker 1: nitty gritty situations. Yeah, and we have to be careful
42:39
Speaker 1: about relying too much on conventions that sound reasonable and
42:43
Speaker 1: seem like good assumptions, but in the end are just conventions,
42:47
Speaker 1: you know, Like, for example, we set the electrons charge
42:49
Speaker 1: to be minus one, we could have chosen to be
42:52
Speaker 1: something else. We could have chosen to be plus sixty two,
42:54
Speaker 1: and then a lot of things would be different, but
42:56
Speaker 1: fundamentally the universe wouldn't be different. It's just like how
42:59
Speaker 1: we're writing things down on paper, how we are thinking
43:01
Speaker 1: about things that we shouldn't describe too much universality to
43:04
Speaker 1: these arbitrary choices that we make. And in this case,
43:07
Speaker 1: the lesson is relating back to what you were talking
43:10
Speaker 1: about earlier about local measurements. Right, Really, what this tells
43:14
Speaker 1: us is that we can only measure things that are
43:16
Speaker 1: very very close to us and know what that means
43:18
Speaker 1: in any intuitive sense. Once we start talking about things
43:21
Speaker 1: that are really any distance away from us with respect
43:23
Speaker 1: to the speed of light, then things start to get fuzzy,
43:26
Speaker 1: and the ideas you have about what's happening far away
43:28
Speaker 1: relying more on convention than actual experiment. Right, It's all
43:32
Speaker 1: sort of depending on how you measure things or what
43:34
Speaker 1: things in. But there has to be something true about
43:37
Speaker 1: the universe that is invariant, right, like it Maybe our
43:40
Speaker 1: note of what time is is not sort of immovable,
43:43
Speaker 1: but there has to be some nugget of truth in
43:45
Speaker 1: the universe, right, some consistency in the law. Maybe we
43:48
Speaker 1: just need to sort of like realign our conceptions and
43:51
Speaker 1: to get at that to sort of understand the truth. Yeah,
43:53
Speaker 1: it's possible, but it's also possible that we have an
43:56
Speaker 1: overblown sense of truth and the universe sality of our
44:00
Speaker 1: picture of the universe based on what we've learned here.
44:03
Speaker 1: You know, Newton's biggest leap was to unify the heavens
44:07
Speaker 1: and the Earth and say, oh, look, the law of
44:09
Speaker 1: gravity that works down here on Earth also works on
44:11
Speaker 1: the planets in the sky. And that must have been
44:14
Speaker 1: a great moment. Unfortunately he was kind of wrong, right, Like,
44:16
Speaker 1: the same rules don't apply everywhere, and so the universality
44:21
Speaker 1: of rules that you deduced by looking at things in
44:23
Speaker 1: your experience don't necessarily tell you anything about the rest
44:27
Speaker 1: of the universe. And so it might be that there's
44:29
Speaker 1: truth to the universe, but that truth might also just
44:32
Speaker 1: be local. It might just be like, the universe is crazy,
44:35
Speaker 1: chaotic mess, and we can summarize parts of it in
44:38
Speaker 1: some circumstances approximately and describe them, but there might not
44:42
Speaker 1: be any like deep, simple truth about the universe. Well,
44:45
Speaker 1: but but it's not random, right, It's not like anything
44:48
Speaker 1: goes like if something works at a certain scale, it
44:51
Speaker 1: seems to work all the time at that scale, or
44:53
Speaker 1: something works at a different scale, seems to work all
44:54
Speaker 1: the time and that scale. Yeah, it does seem to
44:56
Speaker 1: follow some laws. Exactly what those laws are universal, and
45:00
Speaker 1: whether you can extend your understanding out from the cases
45:03
Speaker 1: that you've studied is another question, right, Like, the experiments
45:06
Speaker 1: we've done are the experiments we've done, and if you
45:08
Speaker 1: repeat them, you get the same answers, But you should
45:10
Speaker 1: be careful about, like drawing overly broad conclusions from those
45:14
Speaker 1: experiments about the meaning of what's happening elsewhere and on
45:17
Speaker 1: different scales, right, right, Well, I thought of an interesting
45:20
Speaker 1: scenario and maybe you can run this through with us here,
45:23
Speaker 1: like a situation where light would be different in different directions.
45:26
Speaker 1: We've talked about how the universe is expanding, right, and
45:29
Speaker 1: space is always expanding everywhere all the time. That would
45:32
Speaker 1: make light sort of take a different amount of time
45:35
Speaker 1: to go outwards than it would to come back. Right,
45:39
Speaker 1: Like you and I, the space between you and me
45:41
Speaker 1: is constantly expanding, It's getting bigger. If you shoot a
45:44
Speaker 1: photon at me, it's going to travel a certain speed
45:46
Speaker 1: or take a certain amount of time. But once it
45:48
Speaker 1: bounces off my mirror and goes back to you. It's
45:50
Speaker 1: sort of like going upstream off of the expansion of
45:53
Speaker 1: the universe. That's a cool idea. But then on the
45:56
Speaker 1: way back, it's also going upstream against the expansion of
45:58
Speaker 1: the universe, right, because the niverse is always expanding, And
46:01
Speaker 1: if it's expanding the same way in every direction as
46:04
Speaker 1: we think it is, then it's sort of always expanding
46:06
Speaker 1: in front of that photon no matter what direction it's going.
46:09
Speaker 1: But what if the expansion is also not Oh yeah,
46:14
Speaker 1: let's add epicycles to epicycles. Now, this is a really
46:17
Speaker 1: cool idea. And actually, yesterday I was reading a really
46:19
Speaker 1: fascinating paper exploring something similar about whether we could see
46:22
Speaker 1: the impact of a non universal speed of light on
46:25
Speaker 1: the distant universe, Like, should we be able to see
46:28
Speaker 1: galaxies in one direction less red shifted than in another
46:31
Speaker 1: direction because the speed of light is different in that direction, right?
46:35
Speaker 1: Or if the universe is expanding differently in one direction
46:37
Speaker 1: then in another direction, and the speed of light is
46:39
Speaker 1: different in that direction, could that also explain things? And
46:42
Speaker 1: it turns out that you can't tell the difference, right,
46:45
Speaker 1: that it's possible for the universe to be expanding at
46:47
Speaker 1: different speeds in different directions, and the speed of light
46:50
Speaker 1: to be different in different directions, and it all cancels
46:53
Speaker 1: out in a very nice way so that it looks uniform,
46:56
Speaker 1: which is what we see today. We see the red
46:58
Speaker 1: shift being the same in every direct action, for example.
47:00
Speaker 1: And then it has to do with the connection between
47:02
Speaker 1: time dilation and the speed of light. The speed of
47:05
Speaker 1: light is different in different directions than time dilation is different,
47:08
Speaker 1: and so like a red shifting is different in different directions.
47:11
Speaker 1: So even if weird things are happening out there, it
47:13
Speaker 1: all looks uniform to us, right, right, But if I,
47:16
Speaker 1: like shoot a photon to the edge of the observable universe,
47:19
Speaker 1: it's going to take a certain amount of time to
47:21
Speaker 1: get there, right, because there's a certain amount of space
47:24
Speaker 1: between here and there. But on the way back it
47:27
Speaker 1: should take longer because there's more space on the way back. Right. Yeah,
47:31
Speaker 1: it will take longer on the way back, exactly. But
47:33
Speaker 1: because you don't measure it halfway there, you don't measure
47:35
Speaker 1: when it gets to the end of the observable universe.
47:38
Speaker 1: You don't know how long it's taken to get there
47:39
Speaker 1: versus how long it's taken to get back if you're
47:41
Speaker 1: only measuring the round trip time. But it should have
47:45
Speaker 1: taken more time, right, because there was more space on
47:47
Speaker 1: the way back. Yes, it should take more time on
47:49
Speaker 1: the way back. But even in the case where the
47:51
Speaker 1: speed of light is the same in every direction, that's true, right,
47:54
Speaker 1: it's just this more space on the way back, all right. Well,
47:56
Speaker 1: it's pretty mind blowing to think that we may not
47:59
Speaker 1: know how the universe and it works. Even today, it
48:02
Speaker 1: seems like maybe the speed of light could be moving
48:04
Speaker 1: differently in different directions. Yeah, that's one way to describe
48:07
Speaker 1: the universe. It sort of requires a weirder definition of
48:10
Speaker 1: time than the one we have. And so I like
48:13
Speaker 1: the idea that the speed of light on one way
48:15
Speaker 1: trips is the same as the way it is on
48:17
Speaker 1: the way back because it's just simpler and it describes
48:20
Speaker 1: all of the experiments, and it also doesn't break my brain.
48:23
Speaker 1: But the truth is that we don't know, and we
48:25
Speaker 1: have to keep our minds open too crazier ideas about
48:28
Speaker 1: the universe than we even imagine. Right, it's because it's
48:31
Speaker 1: weird to think that it could be a totally different
48:33
Speaker 1: way and still the laws of the universe that we
48:35
Speaker 1: have now is still work. Yeah, and we have lots
48:38
Speaker 1: of examples of that in our history of generalizing from
48:41
Speaker 1: the ideas that are time tested and believed to be
48:43
Speaker 1: true to a larger set of ideas which reveals some
48:47
Speaker 1: deeper truth about the universe. Well, there is something that
48:49
Speaker 1: does have a preferred direction, and that is this podcast
48:53
Speaker 1: because we've reached the end of our time here today.
48:57
Speaker 1: But hopefully it made you think a little bit about
48:59
Speaker 1: what time really means in the universe, and it's maybe
49:01
Speaker 1: something that we may never really know or know the
49:03
Speaker 1: real truth of it out there. So are we doing
49:06
Speaker 1: this podcast one way? We're not going to go all
49:07
Speaker 1: the way to the end and then go back. That's
49:09
Speaker 1: why we're not going to turn around and say all
49:11
Speaker 1: the same things we just said. But backwards. Yeah, that's
49:13
Speaker 1: your job, folks. He's replayed button backwards and listen to
49:16
Speaker 1: it at two X speed, that's right, or listen to
49:19
Speaker 1: it um backwards. There might be a hidden message in
49:22
Speaker 1: the audio, you know, like in those old records. Send bananas,
49:27
Speaker 1: Send two billion dollars in bananas now, please please send
49:34
Speaker 1: mine in real dollars please. But anyways, thanks for joining us.
49:38
Speaker 1: We hope you enjoyed that. See you next time. Thanks
49:48
Speaker 1: for listening, and remember that Daniel and Jorge Explain the
49:50
Speaker 1: Universe is a production of I heart Radio. For more
49:53
Speaker 1: podcast for my heart Radio, visit the i heart Radio app,
49:57
Speaker 1: Apple Podcasts, or wherever you listen to your favorite Chills