Latest / Daniel and Kelly’s Extraordinary Universe / How do we measure the speed of light?

Transcript

How do we measure the speed of light? · Daniel and Kelly’s Extraordinary Universe

• 0:01
  Speaker 1: Hey, it Jorhan Daniel here, and we want to tell
• 0:03
  Speaker 1: you about our new book. It's called Frequently Asked Questions
• 0:06
  Speaker 1: about the Universe because you have questions about the universe,
• 0:09
  Speaker 1: and 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 have a question
• 1:11
  Speaker 1: about the physics of the Internet. Oh, that sounds like fun.
• 1:15
  Speaker 1: Like what is the speed of email? You know, how
• 1:18
  Speaker 1: fast does it travel? If I write you an email?
• 1:20
  Speaker 1: When does it get to you? You know, I think
• 1:22
  Speaker 1: email might actually violate the laws of physics. What do
• 1:25
  Speaker 1: you mean they go faster than the speed of light?
• 1:28
  Speaker 1: Or do you mean like if it falls into a
• 1:30
  Speaker 1: black hole it's actually my inbox? Yeah, exactly like that.
• 1:33
  Speaker 1: I can tell you a story about one time I
• 1:36
  Speaker 1: had email violate causality No way. What happened? Well in college,
• 1:40
  Speaker 1: one time I sent a draft of an essay to
• 1:43
  Speaker 1: my TA for comments. She wrote back, Hey, looks great,
• 1:46
  Speaker 1: no comments. Then I realized I'd never attached it to
• 1:49
  Speaker 1: the email. It sounds actually violated the laws of her
• 1:53
  Speaker 1: responsibilities as a t A, not the laws of physics.
• 1:56
  Speaker 1: That's one interpretation. Is there a non physical interpretation? The
• 2:01
  Speaker 1: grad student union won't allow me to talk about that.
• 2:03
  Speaker 1: It breaks their laws. Hi. I'm Orge, my cartoonist and
• 2:21
  Speaker 1: the creator of PhD comics. Hi, I'm Daniel. I'm a
• 2:24
  Speaker 1: particle physicist and a professor UC Irvine, and I'm recording
• 2:28
  Speaker 1: this podcast on the same microphone Michael Jackson used to
• 2:31
  Speaker 1: record thriller. What what do you mean, like the same
• 2:36
  Speaker 1: microphone or the same brand, the same brand. I just
• 2:38
  Speaker 1: discovered yesterday that apparently I'm using a very well known,
• 2:42
  Speaker 1: famous microphone, which in the industry is known as the thriller.
• 2:45
  Speaker 1: Mike no way, Wow, is it extra? Kind of like
• 2:49
  Speaker 1: it has ex extra pops and extras to go a
• 2:51
  Speaker 1: little bit higher in your falsetto voice. It does make
• 2:54
  Speaker 1: me do a little dance every time you make a
• 2:56
  Speaker 1: really funny joke. So, yeah, you do the moonwalk when
• 3:00
  Speaker 1: we talk about the moon. I don't want you to
• 3:02
  Speaker 1: think about that too much in your head, but yeah,
• 3:04
  Speaker 1: let's say that's what happens. Yeah, But welcome to our podcast.
• 3:07
  Speaker 1: Daniel and Jorge explain the Universe, a production of I
• 3:09
  Speaker 1: Heart Radio, which is sort of like Thriller, and that
• 3:12
  Speaker 1: we take you on a thrill ride around the universe.
• 3:16
  Speaker 1: We don't raise the dead and dance them around, but
• 3:18
  Speaker 1: we do talk about everything in this universe that happens,
• 3:21
  Speaker 1: everything that gets extinguished, everything that flies around and amazes
• 3:25
  Speaker 1: us with everything that can do, and all the laws
• 3:28
  Speaker 1: of physics that seem to work together in harmony to
• 3:31
  Speaker 1: make this universe so crazy, so bonkers, so amazing and
• 3:35
  Speaker 1: yet so discoverable. Yeah, because it is a pretty thrilling universe.
• 3:39
  Speaker 1: And we like to take you on the moonwalk every
• 3:42
  Speaker 1: episode so we can think about the universe and possibly
• 3:45
  Speaker 1: even beat it. What is the moonwalk analogy? There? It
• 3:48
  Speaker 1: looks like you're moving forward, you're actually sliding backwards. Does
• 3:51
  Speaker 1: that mean like we're doing physics? We think we're understanding,
• 3:54
  Speaker 1: but really we understand less and less every year. You know,
• 3:57
  Speaker 1: basically we're faking the whole thing. We're actually moving the
• 4:01
  Speaker 1: progress of science backwards. We're walking backwards on evolution here,
• 4:05
  Speaker 1: but it looks like we're moving forward. I like to
• 4:07
  Speaker 1: think that our podcast helps science move forward in a
• 4:10
  Speaker 1: real way because it excites people and engages them in
• 4:13
  Speaker 1: this Species Why project, they're trying to uncover the mysteries
• 4:16
  Speaker 1: of the universe. Yeah. And also we're wearing red leather
• 4:19
  Speaker 1: jackets right now with a lot of zippers in it.
• 4:22
  Speaker 1: That's right. I only have one sparkly glove on. Does
• 4:24
  Speaker 1: that make me Michael Jackson? I have the other one.
• 4:26
  Speaker 1: It's like we're handshaking glittering gloves across the internet. That's right.
• 4:31
  Speaker 1: And we're trying to merge all the Jackson siblings into
• 4:34
  Speaker 1: one theory of Jackson, the unified theory of entertainment. Yeah,
• 4:40
  Speaker 1: of soul and R and P. But it is a
• 4:43
  Speaker 1: pretty thrilling universe, as we said. And this is an
• 4:46
  Speaker 1: interesting question about how fast do emails travel? Like, if
• 4:49
  Speaker 1: I read you an email and I hit send, does
• 4:51
  Speaker 1: it go to you at the speed of light? Right?
• 4:53
  Speaker 1: Because sort of, right, because electricity and signals over telephone lines,
• 4:58
  Speaker 1: they sort of go basically as fast this light. Right,
• 5:00
  Speaker 1: that's right. That information does travel at the speed of light.
• 5:03
  Speaker 1: You take like a wire, you send a pulse down it,
• 5:06
  Speaker 1: It does travel very very fast, essentially at the speed
• 5:09
  Speaker 1: of light. But we all know, of course, that email
• 5:10
  Speaker 1: doesn't arrive that quickly when you send it, because it's
• 5:13
  Speaker 1: got to go through all sorts of like computers who
• 5:15
  Speaker 1: do algorithms and weight on it and analyze it. And
• 5:18
  Speaker 1: so for example, my email here at you see I
• 5:20
  Speaker 1: sometimes it takes ten minutes to get one. Yeah, and
• 5:22
  Speaker 1: that's not even the emails you send me, which takes
• 5:25
  Speaker 1: me hours to even read. But yeah, the speed of
• 5:27
  Speaker 1: light is pretty fast. It seems to be basically the
• 5:30
  Speaker 1: speed limit of the universe. Right, Nothing in the universe
• 5:33
  Speaker 1: can go faster than the speed of light. That's right,
• 5:35
  Speaker 1: it seems to be this hard and fast limit. It's
• 5:37
  Speaker 1: just like a feature of our universe that there's a
• 5:40
  Speaker 1: maximum speed of information, which is really super cool philosophically
• 5:44
  Speaker 1: to think about, like why is that and how could
• 5:47
  Speaker 1: the universe have been different? And what does it mean?
• 5:50
  Speaker 1: But it's also really fun to think about, like how
• 5:52
  Speaker 1: did we figure that out? You know, something we know now,
• 5:55
  Speaker 1: it's something we definitely understand, but like obviously early humans
• 6:00
  Speaker 1: know that, and so I think it's always fun to
• 6:02
  Speaker 1: return to the moments that we cracked at these problems
• 6:05
  Speaker 1: that we like gained a new understanding of how the
• 6:08
  Speaker 1: universe worked. Yeah, because it's crazy to think that at
• 6:10
  Speaker 1: some point we didn't know what the speed of light was,
• 6:13
  Speaker 1: or even that it sort of had a speed I imagine, right,
• 6:15
  Speaker 1: Like I imagine early man probably thought light was instantaneous,
• 6:19
  Speaker 1: like you light a stick on fire and immediately the
• 6:23
  Speaker 1: light hits your eyeballs. Yeah. The Greeks had a lot
• 6:25
  Speaker 1: of totally uninformed debates on the topic, you know, speculating
• 6:29
  Speaker 1: endlessly about what light was, did it emanate from objects,
• 6:32
  Speaker 1: did it reflect from things? Did it travel instantaneously? Was
• 6:35
  Speaker 1: it a thing or not a thing? What does it
• 6:37
  Speaker 1: mean to be a thing? Man? Like the Greeks went
• 6:40
  Speaker 1: on and on and on with no information. It's amazing
• 6:43
  Speaker 1: to me, for thousands of years you could have uninformed
• 6:45
  Speaker 1: debates because of our discussions these days are informed. Well,
• 6:49
  Speaker 1: we have data, we can do experiments, we can learn things,
• 6:52
  Speaker 1: we can make progress, you know, without just like smoking
• 6:55
  Speaker 1: more of Bannan appeals and thinking about how the universe works. Man, Well,
• 6:59
  Speaker 1: it is pretty interesting thing to think about the speed
• 7:00
  Speaker 1: of light. And I think what's also interesting is that
• 7:02
  Speaker 1: it's not infinite, like it's a number, you know, like
• 7:05
  Speaker 1: the craziest you can go into the universe is a
• 7:08
  Speaker 1: specific number and nothing can go faster than that number.
• 7:11
  Speaker 1: That's right, Yeah, And it's interesting, right every time you
• 7:14
  Speaker 1: see a number in the theory of physics, you wonder
• 7:17
  Speaker 1: why that number? Did it have to be that? Could
• 7:21
  Speaker 1: it have been something else? If it could have been anything,
• 7:24
  Speaker 1: then why is it this value? Or if it could
• 7:26
  Speaker 1: only be one thing, then what are the rules that
• 7:28
  Speaker 1: make it have to be that one thing? And what
• 7:30
  Speaker 1: does that mean? So it's like a huge screaming clue
• 7:33
  Speaker 1: to me. Every time we see a number in physics,
• 7:36
  Speaker 1: what is it? Screaming and screaming, there's a secret here,
• 7:38
  Speaker 1: there's something else to be understood that in a hundred
• 7:41
  Speaker 1: years somebody else is going to win a Nobel Prize
• 7:43
  Speaker 1: for an explanation. And you have all the information you
• 7:46
  Speaker 1: need to arrive at that idea. You just don't see
• 7:48
  Speaker 1: it right. Yeah, And isn't it weird to think about that?
• 7:51
  Speaker 1: All other speeds that are greater than the speed of
• 7:53
  Speaker 1: light are basically impossible, like one per second faster than
• 7:59
  Speaker 1: the speed of light, impact any number from that number
• 8:03
  Speaker 1: to infinity is basically impossible in the universe. Yeah, the
• 8:07
  Speaker 1: universe says no, and it doesn't negotiate. There's no whidle
• 8:10
  Speaker 1: room there. You can't charm the universe into letting you
• 8:13
  Speaker 1: do something a little bit past the speed of light.
• 8:15
  Speaker 1: It's a firm no, it's a hard pass from the universe.
• 8:18
  Speaker 1: And what exactly is the speed of light? Daniel? How
• 8:20
  Speaker 1: how many digits do you know it? Too? Well? It's
• 8:22
  Speaker 1: funny you should ask, because in particle physics we say
• 8:25
  Speaker 1: the speed of light is one, like we use units
• 8:27
  Speaker 1: where the speed of light is just one, because we
• 8:30
  Speaker 1: can't be bothered to write it down all the time
• 8:32
  Speaker 1: because it's everywhere. So we have equations with like speed
• 8:35
  Speaker 1: of light square speed of light to the four speed
• 8:37
  Speaker 1: of light to the eighth, and so it just sort
• 8:38
  Speaker 1: of gets annoying. So we just say, let's just set
• 8:41
  Speaker 1: see equal to one, and then we can ignore it. Mostly,
• 8:43
  Speaker 1: let's ignore reality for our convenience. Let's have another lens
• 8:48
  Speaker 1: in which we look at reality in which it makes
• 8:50
  Speaker 1: more sense than we can boil it down to its
• 8:52
  Speaker 1: true fundamental essence and not get tangled up in little numbers. Right,
• 8:56
  Speaker 1: so you can take more naps, right, you can take
• 8:58
  Speaker 1: more naps. So a particle physicist is the wrong person
• 9:01
  Speaker 1: to ask about the actual value of the speed of
• 9:03
  Speaker 1: light insensible units. But we do define it as two
• 9:07
  Speaker 1: hundred nine million, seven hundred thousand, four hundred and fifty
• 9:11
  Speaker 1: eight meters per second. So that's the exact value of
• 9:15
  Speaker 1: the speed of light. The most people when they do
• 9:16
  Speaker 1: calculations just say three times tended them per second, right,
• 9:20
  Speaker 1: or like three hundred million meters per second. But it
• 9:24
  Speaker 1: is a very specific number, right, it's like eight and
• 9:28
  Speaker 1: like four or five. Nine is too fast. You can't
• 9:30
  Speaker 1: go that fast. That's right. It's a no on four
• 9:33
  Speaker 1: or five, nine and a half, no, five point one. No,
• 9:37
  Speaker 1: there's no flexibility, there's no negotiation. This is in Hollywood.
• 9:40
  Speaker 1: We're like, hey, we can find a deal. Right. It's
• 9:42
  Speaker 1: kind of weird, right that the universe would just pick
• 9:44
  Speaker 1: the number and nothing can go faster. Yeah, it's a
• 9:46
  Speaker 1: huge clue. That's telling you something really deep about the
• 9:49
  Speaker 1: nature of space and time itself, right, Like if the
• 9:53
  Speaker 1: loop quantum gravity is true and space really is a
• 9:55
  Speaker 1: big quantized foam bubble, then maybe this tells us about
• 9:58
  Speaker 1: how those foam bubbles talk to e other and you
• 10:00
  Speaker 1: can't get information from one phone bubble to the other
• 10:03
  Speaker 1: faster than that because they just aren't closely enough connected
• 10:06
  Speaker 1: or something. So I think it really does tell you
• 10:08
  Speaker 1: something deep about the nature of the universe. Yeah, and
• 10:11
  Speaker 1: it's a very specific number, and so I guess the
• 10:13
  Speaker 1: big question is like, how do you know it's that number?
• 10:15
  Speaker 1: To what accuracy do we know that's the right number,
• 10:18
  Speaker 1: that it's a maximum speed limit of the universe, and
• 10:20
  Speaker 1: how did we figure it out? So to be on
• 10:22
  Speaker 1: the podcast, we'll be asking the question, how do we
• 10:29
  Speaker 1: actually know what the speed of light is? That we
• 10:33
  Speaker 1: actually measured it, or are we guessing? You're right, you
• 10:35
  Speaker 1: figured out we always been guessing this whole time, and
• 10:39
  Speaker 1: you have revealed this like a Scooby Doo episode. You've
• 10:41
  Speaker 1: pulled off the mask. Well, a few minutes ago you
• 10:43
  Speaker 1: just said one, right, So I don't know what's true
• 10:46
  Speaker 1: anymore than you. They're all true. It just depends on
• 10:49
  Speaker 1: the units. Well, I guess I'm wondering, like, has anyone
• 10:51
  Speaker 1: actually tested, right, because nobody has actually tried to go
• 10:54
  Speaker 1: faster than the speed of light technically right, Like you haven't,
• 10:57
  Speaker 1: I haven't. I have totally tried apps. I've tried so
• 11:00
  Speaker 1: many times as a kid. I mean I didn't get
• 11:02
  Speaker 1: anywhere near the speed of light. But that doesn't mean
• 11:05
  Speaker 1: I didn't try to try. I mean like a credible try,
• 11:10
  Speaker 1: not like a far off by by ten decimal places.
• 11:14
  Speaker 1: That's true. But you know I did grow up to
• 11:16
  Speaker 1: work at a particle accelerator, which makes a pretty credible
• 11:19
  Speaker 1: attempt to get particles to go faster than the speed
• 11:22
  Speaker 1: of light. We take protons and we accelerate them. We
• 11:25
  Speaker 1: give them so much energy, and they go faster and
• 11:27
  Speaker 1: faster and faster, And what we see is that as
• 11:30
  Speaker 1: you add energy, the particles just don't get going much faster.
• 11:33
  Speaker 1: It's sort of a mind bend there, Like you can
• 11:35
  Speaker 1: add energy, there's more kinetic energy in these particles, but
• 11:39
  Speaker 1: they're not moving much faster. Right, it approaches a speed
• 11:41
  Speaker 1: of light. But I guess a big question is how
• 11:43
  Speaker 1: do you know what the actual speed of light is?
• 11:45
  Speaker 1: Like the actual number, Yes, so that you can measure
• 11:48
  Speaker 1: by actually looking at light and measuring it, but you
• 11:50
  Speaker 1: can also see the protons approach it. Right. We have
• 11:52
  Speaker 1: these limits in physics all the time where you can
• 11:54
  Speaker 1: see something is approaching a limit and as em topically
• 11:57
  Speaker 1: gets closer and closer and closer, and so you can
• 12:00
  Speaker 1: alculate what that limit would be if you went to
• 12:03
  Speaker 1: infinite energy. You can extrapolate mathematically to figure out like
• 12:06
  Speaker 1: what is the limiting case, either from looking at protons
• 12:09
  Speaker 1: to see what they are approaching, or just by directly
• 12:12
  Speaker 1: measuring the speed of actual beams of light itself. Well,
• 12:15
  Speaker 1: that's kind of what we're going to get into today
• 12:16
  Speaker 1: is kind of the history of how we've gone about
• 12:19
  Speaker 1: measuring light and also what are some of our best
• 12:22
  Speaker 1: current measurements of it and some really surprising twists about
• 12:25
  Speaker 1: what we do and don't know about the speed of light,
• 12:29
  Speaker 1: some light twists or some dark twists a little both. So,
• 12:34
  Speaker 1: as usually, we were wondering how many people out there
• 12:36
  Speaker 1: knew the answer to this question, how we measure the
• 12:39
  Speaker 1: speed of light? So Daniel went out there and ask
• 12:41
  Speaker 1: people on the internet if they knew how we measured
• 12:44
  Speaker 1: the speed of light. That's right, So thank you to
• 12:46
  Speaker 1: everybody who, during these strange pandemic times, have stepped up
• 12:50
  Speaker 1: to fill in the gap left by you see Irvine
• 12:52
  Speaker 1: students and answered questions online. If you'd like to participate
• 12:56
  Speaker 1: for a future episode of the podcast, please don't be
• 12:58
  Speaker 1: shy and right to us two questions at Daniel and
• 13:01
  Speaker 1: Jorge dot com. So think about it for a second.
• 13:04
  Speaker 1: What would you answer? Here's what people had to say. Well,
• 13:07
  Speaker 1: if we already had space travel by the time we
• 13:09
  Speaker 1: were trying to measure that, we could have sent a
• 13:15
  Speaker 1: radio transmission from the moon with time stamp and see
• 13:19
  Speaker 1: how long it took to get there. So I don't
• 13:22
  Speaker 1: know exactly how we measured the speed of light, but
• 13:26
  Speaker 1: I would guess by measuring the time required for light
• 13:29
  Speaker 1: to travel a certain distance, maybe by using the mirror experiment,
• 13:34
  Speaker 1: where the time required for light to travel to the
• 13:38
  Speaker 1: mirror and back from the mirror is recorded, and since
• 13:43
  Speaker 1: we know the distance and time, we can find the
• 13:45
  Speaker 1: speed of light. Oh, no idea. I mean it was
• 13:53
  Speaker 1: theoretical at first, and then they tested it. So then
• 13:57
  Speaker 1: a particle accelerator I think Galla Lao tried to measure
• 14:01
  Speaker 1: the speed of light using lanterns at set distances across fields.
• 14:06
  Speaker 1: I assumed that wasn't successful, though, and I assumed that
• 14:10
  Speaker 1: we've also tried to measure it by bouncing signals off
• 14:12
  Speaker 1: the moon in more recent times. I don't know, though,
• 14:16
  Speaker 1: what the first successful measurement of the speed of light was.
• 14:20
  Speaker 1: I guess maybe they worked out like the distance between
• 14:22
  Speaker 1: two objects in space, and then worked out how quickly
• 14:27
  Speaker 1: light traveled between them and did the maths. I don't know.
• 14:30
  Speaker 1: That's a really good question. So I've been racking my
• 14:34
  Speaker 1: brain trying to think of this, because I swear I've
• 14:36
  Speaker 1: learned this in high school or college physics. But I
• 14:39
  Speaker 1: couldn't tell you know how we measured the speed of light.
• 14:43
  Speaker 1: A guy climbed a mountain and shone a light at
• 14:47
  Speaker 1: a rotating mirror on another mountain and counted how long
• 14:52
  Speaker 1: in between for the late come back. So what do
• 14:56
  Speaker 1: you think of these engineering ideas for how to measure
• 14:58
  Speaker 1: this incredible speed. I think they're a little light on substance,
• 15:03
  Speaker 1: but they're pretty good attempts. A lot of people didn't
• 15:05
  Speaker 1: seem to have sort of an idea of how we've
• 15:07
  Speaker 1: done it. Maybe, you know, they could think about ways
• 15:10
  Speaker 1: that they could do it, but I guess not a
• 15:12
  Speaker 1: lot of people. You know what the latest and greatest
• 15:14
  Speaker 1: measurement is. Yeah, and the challenge, of course is that
• 15:17
  Speaker 1: it's super duper duper fast. Like we talked about the
• 15:20
  Speaker 1: speed of light being three million meters per second, that's
• 15:24
  Speaker 1: sort of hard to understand, you know, what does that
• 15:26
  Speaker 1: really mean? I think it's easier sometimes to think about
• 15:28
  Speaker 1: it in terms of how far light goes in a
• 15:31
  Speaker 1: very short amount of time, rather than in a full second. Actually,
• 15:35
  Speaker 1: think about light is traveling about one foot every nanosecond,
• 15:39
  Speaker 1: so like a light nanosecond. You know the concept of
• 15:41
  Speaker 1: a light year, how far light travels in a year.
• 15:43
  Speaker 1: A light nanosecond is about one ft or thirty centimeters.
• 15:47
  Speaker 1: For those of you on the metric system, I totally
• 15:49
  Speaker 1: reject that that way of looking at the first of all,
• 15:53
  Speaker 1: it's English units using feet instead of meters, and also
• 15:58
  Speaker 1: a nanosecond that doesn't have a lot of meaning to me.
• 16:00
  Speaker 1: I guess, well, I guess you don't get a lot
• 16:02
  Speaker 1: of things done in a nanosecond. But I'm pretty efficient,
• 16:04
  Speaker 1: you know, I can answer ten emails in a nanoseconds.
• 16:06
  Speaker 1: So you live in another reality, it seems no, no,
• 16:11
  Speaker 1: obviously not. But you know, when you're talking about like signals,
• 16:14
  Speaker 1: like how long is it going to take my information
• 16:16
  Speaker 1: to go down this cable? I have one piece of
• 16:18
  Speaker 1: equipment over here, another piece of equipment over there ten
• 16:20
  Speaker 1: feet away, so you know it's gonna take ten nanoseconds
• 16:23
  Speaker 1: to get from here or there. And sometimes if you're
• 16:25
  Speaker 1: building electronics, right, you need to know are these signals
• 16:27
  Speaker 1: gonna be coordinated with the gap going to be between them?
• 16:30
  Speaker 1: And so it's helpful sometimes to think about light in
• 16:32
  Speaker 1: terms of how long it takes to move across a
• 16:34
  Speaker 1: reasonable distance, because I don't really know what three hundred
• 16:37
  Speaker 1: million meters is, but how can we measure it in
• 16:40
  Speaker 1: terms of like in the blink of an eye, Like
• 16:42
  Speaker 1: let's say a blink of an eye, is I don't know,
• 16:45
  Speaker 1: a hundred milliseconds, how long can that light travel in
• 16:48
  Speaker 1: those hundred milliseconds. Well, you know, a hundred milliseconds is
• 16:52
  Speaker 1: only a tenth of a second, right, and so a
• 16:55
  Speaker 1: tenth of a second would be thirty million meters, So
• 16:59
  Speaker 1: it's still pretty are that light can go in the
• 17:01
  Speaker 1: tense of a second, right, thirty million meters, it's about
• 17:03
  Speaker 1: thirty tho kilometers, right, which is about yeah, that's right,
• 17:08
  Speaker 1: thirty millions is just under twenty thousand miles. So like,
• 17:12
  Speaker 1: if I blink, then light can travel basically once around
• 17:15
  Speaker 1: the world kind of almost maybe if you're at the
• 17:18
  Speaker 1: latitude of like North America, like could go around the
• 17:21
  Speaker 1: Earth in the blink of an eye. And that's really
• 17:23
  Speaker 1: the challenge of measuring the speed of light that it's
• 17:25
  Speaker 1: so darn fast that for all extents and purposes, for
• 17:29
  Speaker 1: the things we do, it's essentially infinite. And that makes
• 17:32
  Speaker 1: it really really challenging because you either need incredibly vast
• 17:36
  Speaker 1: distances so you can accumulate some like reasonable amount of
• 17:39
  Speaker 1: time between when you send a message and when it arrives,
• 17:42
  Speaker 1: or you need to be able to measure really really
• 17:44
  Speaker 1: short times. And so imagine you're like Aristotle five thousand
• 17:48
  Speaker 1: years ago, How could you possibly set up an experiment
• 17:51
  Speaker 1: to measure something over vast distances or very short times? Yeah,
• 17:56
  Speaker 1: because I guess you know, it's hard to measure something
• 17:58
  Speaker 1: that happens in the blink of an eye, right, especially
• 18:00
  Speaker 1: if you're in the thousand years BC or something that's right,
• 18:03
  Speaker 1: if all you have is like you know, a clay tablet,
• 18:06
  Speaker 1: a stick and a robe, then like what are you
• 18:08
  Speaker 1: gonna do? And these days, even for us, it's not
• 18:10
  Speaker 1: easy to see the impact of the speed of light.
• 18:13
  Speaker 1: You know, it impacts things like spaceflight communication. If you're
• 18:17
  Speaker 1: on Mars driving a rover, then sure, and there's an
• 18:19
  Speaker 1: impact of the speed of light not being infinite. But
• 18:22
  Speaker 1: here on Earth, you know, it doesn't really make a
• 18:24
  Speaker 1: difference in your life very much. If you're like the
• 18:26
  Speaker 1: designer of a computer, then you think about this, like
• 18:29
  Speaker 1: how long did it take the information and go across
• 18:31
  Speaker 1: my CPU? And can I optimize the design of it
• 18:35
  Speaker 1: to bring things closer to speed up my computer? But
• 18:37
  Speaker 1: unless you're driving rovers on Mars or designing CPUs, you
• 18:41
  Speaker 1: probably don't think about the speed of light very much. Yeah,
• 18:43
  Speaker 1: it's probably pretty instantaneous to you in an intuitive sense.
• 18:46
  Speaker 1: But I think if a big question is like, how
• 18:48
  Speaker 1: do we know it's this particular number, right or not? Yeah,
• 18:51
  Speaker 1: well that comes from the things we measure, and we
• 18:53
  Speaker 1: have no like theoretical preference for that number. It's not
• 18:56
  Speaker 1: like the kind of thing we could have derived where
• 18:58
  Speaker 1: we're like, well, it has to be this number, it
• 19:00
  Speaker 1: can only be this number. It's just a measurement, right.
• 19:03
  Speaker 1: Often in physics there are things where you know, there's
• 19:06
  Speaker 1: something we know happens in the universe, but we don't
• 19:08
  Speaker 1: know why it's this way and not the other way,
• 19:10
  Speaker 1: and so we just have to measure it. Things like
• 19:13
  Speaker 1: the mass of the Higgs boson, or the mass of
• 19:15
  Speaker 1: all the particles, or you know, the strength of gravity.
• 19:17
  Speaker 1: These are things we don't know why they are that
• 19:19
  Speaker 1: number and not some other number. And the speed of
• 19:22
  Speaker 1: light is like that. It's just something we have to
• 19:24
  Speaker 1: go out and measure, like it could have been another number, right,
• 19:26
  Speaker 1: it could have been three, or we don't know, right,
• 19:31
  Speaker 1: it could be that it has to be this number
• 19:33
  Speaker 1: because there's some deeper theory of physics that constrains it
• 19:36
  Speaker 1: and makes it only work for this value. But we
• 19:39
  Speaker 1: don't have that theory of physics. According to our theory,
• 19:41
  Speaker 1: it could have been any other number. But of course
• 19:43
  Speaker 1: we don't expect that our theory of physics is the
• 19:45
  Speaker 1: final answer. And it's exactly the kind of place where
• 19:48
  Speaker 1: I see there are opportunities right where we say, well,
• 19:50
  Speaker 1: we don't have an explanation for this, so let's keep
• 19:53
  Speaker 1: looking for an explanation. To me, it's unsatisfying. People say, well,
• 19:56
  Speaker 1: it could have been anything. It was just random or
• 19:59
  Speaker 1: one element in the multi verse. So this is just
• 20:01
  Speaker 1: what it is. There is no explanation to me. It's
• 20:03
  Speaker 1: a clue that says that probably is an explanation. Keep digging, right, like,
• 20:07
  Speaker 1: maybe it tells you something specifically about why the universe
• 20:11
  Speaker 1: was the way it had to be. We just don't
• 20:13
  Speaker 1: see it yet. And it's also interesting to think that
• 20:16
  Speaker 1: it's not just the speed of light. It's like the
• 20:18
  Speaker 1: maximum speed that information can travel in the universe, like anything.
• 20:23
  Speaker 1: It's not just light that travels at the speed of light.
• 20:25
  Speaker 1: It's any particle without mass. Yeah, and it's sort of
• 20:28
  Speaker 1: a misnomer, right. We discovered light moves at this speed first,
• 20:31
  Speaker 1: and it is the speed of light in a vacuum.
• 20:34
  Speaker 1: But really it should be like the speed of space
• 20:36
  Speaker 1: time or the speed of information, because, as you say,
• 20:39
  Speaker 1: anything that doesn't have mass, and that means like a
• 20:42
  Speaker 1: graviton if they exist, or a gluon for example, anything
• 20:46
  Speaker 1: that doesn't have mass has to travel at the speed
• 20:48
  Speaker 1: of light, and only the speed of light, and nothing
• 20:51
  Speaker 1: else that does have mass can travel at that speed.
• 20:55
  Speaker 1: So it really is a special speed in the universe,
• 20:57
  Speaker 1: more than just the speed of photons. Right, maybe you
• 21:00
  Speaker 1: should have been called the speed of nothing. You like
• 21:02
  Speaker 1: that because light is technically nothing. It has no mass,
• 21:06
  Speaker 1: and nothing can go faster than it. No, because it
• 21:08
  Speaker 1: takes no time to do nothing, Right, like you didn't
• 21:10
  Speaker 1: do anything? How long? Did that take? No time at all?
• 21:13
  Speaker 1: So nothing is instantaneous. It's zero divided by zero. So,
• 21:19
  Speaker 1: if anything, I think that would have been more confusing.
• 21:21
  Speaker 1: All right, well, let's get into what we've done throughout
• 21:24
  Speaker 1: the thousands of years of human history to measure the
• 21:27
  Speaker 1: speed of light, and then let's get into our latest
• 21:29
  Speaker 1: measurements of that number. But first let's take a quick break.
• 21:45
  Speaker 1: All right, Daniel, we are measuring the speed of light
• 21:47
  Speaker 1: today on the episode Are you ready? I'm gonna clap
• 21:50
  Speaker 1: and then you tell me how long it took. All right,
• 21:55
  Speaker 1: that was exactly one clap per clap. There you go.
• 22:00
  Speaker 1: That's the joy of particle physics. Units. The answer is
• 22:03
  Speaker 1: always one or E or pie in particle physics. That's
• 22:06
  Speaker 1: why it's always one of the three. But yeah, like
• 22:09
  Speaker 1: you were saying earlier, it's pretty hard to measure the
• 22:11
  Speaker 1: speed of light because it's so fast, right, Like, you
• 22:13
  Speaker 1: can't just like turn on a flashlight, run over there
• 22:15
  Speaker 1: and see when the light arise. Like it moves faster
• 22:18
  Speaker 1: than anything can move. So it's hard to like you know,
• 22:21
  Speaker 1: beat it or coordinate the measurement of it. So, like,
• 22:24
  Speaker 1: how did the people in ancient times even approach this question?
• 22:28
  Speaker 1: I think in ancient times they had a different attitude, right,
• 22:30
  Speaker 1: They were not empirical. They were not of the mind
• 22:33
  Speaker 1: to go out and discover things in the universe by
• 22:36
  Speaker 1: doing experiments. They were much more internal. They thought that
• 22:39
  Speaker 1: they could understand the universe just by thinking about it.
• 22:42
  Speaker 1: You know. They had lots of crazy theories about the
• 22:44
  Speaker 1: way things work, theories that could be easily disproved in
• 22:47
  Speaker 1: like an afternoon of experimentation, you know. Aristatilian physics really
• 22:51
  Speaker 1: doesn't make any sense if you do any experiments. So
• 22:54
  Speaker 1: they really didn't even try to measure the speed of life.
• 22:56
  Speaker 1: They mostly just like talked about it and thought about it.
• 22:59
  Speaker 1: It wasn't all about five hundred years ago when this
• 23:02
  Speaker 1: concept of like, oh, let's go out and measure things
• 23:04
  Speaker 1: in the universe, let's try to see if our theories
• 23:07
  Speaker 1: actually work. This concept of empirical science came about that
• 23:10
  Speaker 1: people really started to actually try to measure things. And
• 23:13
  Speaker 1: the earliest recorded measurement I could find was from Galileo
• 23:17
  Speaker 1: about five years ago. Well do you think maybe people
• 23:19
  Speaker 1: didn't try before because they just had no means to
• 23:22
  Speaker 1: do it. You know, they didn't have accurate clocks or
• 23:25
  Speaker 1: ways to measure things. They certainly didn't have the means
• 23:28
  Speaker 1: to do it. If they had tried, they definitely would
• 23:30
  Speaker 1: have failed. But you know, I don't think they even
• 23:32
  Speaker 1: thought to try. Like I think, it's hard to take
• 23:35
  Speaker 1: your mind out of the current modern concept of science,
• 23:38
  Speaker 1: where we learned from the universe by doing experiments. That's
• 23:41
  Speaker 1: a fairly new idea. Aristotle, for example, had this idea
• 23:45
  Speaker 1: of how things fall, and he thought, for example, if
• 23:47
  Speaker 1: you're on a boat, that was moving and you dropped
• 23:50
  Speaker 1: a ball while you were on the boat, that the
• 23:52
  Speaker 1: ball would somehow get left behind, that it wouldn't just
• 23:54
  Speaker 1: like move with the boat. And like if you just
• 23:56
  Speaker 1: went out with a boat and a ball, you could
• 23:58
  Speaker 1: disprove this idea in an afternoon, right unless it's windy. Yeah,
• 24:04
  Speaker 1: and Galileo, like thousands of years later, proved that this
• 24:07
  Speaker 1: was false and literally overturned all of physics with a
• 24:10
  Speaker 1: boat and a ball in about ten minutes. So these
• 24:14
  Speaker 1: guys weren't limited by their physical capabilities, they just were
• 24:17
  Speaker 1: limited by the idea that you should go out and
• 24:19
  Speaker 1: actually measure stuff. What can you do with the boat
• 24:21
  Speaker 1: and a ball? Galileo used it all up right, like
• 24:24
  Speaker 1: early Nobel prizes. That's all the equipment you needed. I see,
• 24:28
  Speaker 1: the fruit was hanging lower before. Yes, exactly. Nobody even
• 24:31
  Speaker 1: tried to measure stuff back then, so he said, Galileo
• 24:34
  Speaker 1: tried to measure the speed of light. And then how
• 24:36
  Speaker 1: did he do it with lanterns? Yeah, he had lanterns
• 24:38
  Speaker 1: and he put them about a mile apart, and he
• 24:40
  Speaker 1: tried to time it the most accurate clocks you could
• 24:42
  Speaker 1: back then, and he tried to like you know, shine
• 24:45
  Speaker 1: a lantern and then measure how long it took to
• 24:47
  Speaker 1: get from one spot to the other with two coordinated clocks.
• 24:51
  Speaker 1: And you know, he failed to notice any difference. He
• 24:54
  Speaker 1: couldn't measure any time between when the lantern was revealed
• 24:58
  Speaker 1: and when the information arrived at the other side. Interesting
• 25:01
  Speaker 1: like he synchronized two clocks or two watches, and then
• 25:04
  Speaker 1: he had one watch, like go a mile away, and
• 25:07
  Speaker 1: then he said, Okay, I'm gonna turn on this lantern,
• 25:09
  Speaker 1: and when you see it turn on, you record the time.
• 25:12
  Speaker 1: And they came back and said it was the same
• 25:13
  Speaker 1: time exactly. They couldn't measure any difference, right, They couldn't
• 25:16
  Speaker 1: tell the difference between light being super duper fast but
• 25:19
  Speaker 1: finite speed and light actually being infinite speed. And the
• 25:23
  Speaker 1: reason is that the delay, like how long it takes
• 25:25
  Speaker 1: light to go a mile, is just eleven microseconds, and
• 25:28
  Speaker 1: so to measure that unique clocks that are more accurate
• 25:31
  Speaker 1: than eleven microseconds, and five years ago he definitely did
• 25:34
  Speaker 1: not have that. So did he conclude that speed was
• 25:37
  Speaker 1: infinitely fast or that he just didn't He couldn't measure it.
• 25:40
  Speaker 1: It was too fast to measure. It was too fast
• 25:42
  Speaker 1: to measure and these days, what you would conclude from
• 25:44
  Speaker 1: an experiment like that is you would measure a minimum speed.
• 25:47
  Speaker 1: If you knew how accurate your clocks were, you could say, well,
• 25:50
  Speaker 1: light is at least as fast as some number. He
• 25:53
  Speaker 1: just said, well, I don't know. It's either infinite or
• 25:56
  Speaker 1: it's very very fast. Right. He didn't have the right clocks.
• 25:59
  Speaker 1: But then later the people had better clocks. People had
• 26:01
  Speaker 1: better clocks. But actually the first measurement of the speed
• 26:05
  Speaker 1: of light being not infinite didn't come from using a
• 26:08
  Speaker 1: very fast clock. It came from using really really long distances, right,
• 26:12
  Speaker 1: Because that's that's two ways to kind of slow speed down, right,
• 26:15
  Speaker 1: Either give it a long distance to go over, or
• 26:18
  Speaker 1: use a really accurate clock. Yeah. So a Danish astronomer
• 26:22
  Speaker 1: about a hundred years later he realized that light bouncing
• 26:26
  Speaker 1: off of Jupiter's moon Io could be used to measure
• 26:30
  Speaker 1: the speed of light. What. Yeah, because Io orbits Jupiter. Right,
• 26:35
  Speaker 1: it's a moon of Jupiter and it goes around It
• 26:37
  Speaker 1: takes like forty two and a half hours to go
• 26:39
  Speaker 1: around Jupiter. And when it comes around the back of Jupiter,
• 26:43
  Speaker 1: it emerges from the back of Jupiter, and you can
• 26:45
  Speaker 1: see it. So if you're watching Io from Earth, then
• 26:48
  Speaker 1: you see it emerged from behind Jupiter every forty two
• 26:51
  Speaker 1: and a half hours. And so that's sort of like
• 26:53
  Speaker 1: a clock for the universe, right, it should happen every
• 26:56
  Speaker 1: forty two and a half hours, because Io's orbit is
• 26:58
  Speaker 1: very regular. It's a little bit more complicated because either
• 27:01
  Speaker 1: it can be in Jupiter's shadow or can be physically
• 27:03
  Speaker 1: behind Jupiter. But let's put that aside for now. Oh,
• 27:06
  Speaker 1: I see, because you can actually see the moves of
• 27:08
  Speaker 1: Jupiter if you have a telescope from the sixteen hundreds, right, Like,
• 27:11
  Speaker 1: you can see the little dots and you can see
• 27:13
  Speaker 1: the little little dots kind of floating around it. Yeah,
• 27:16
  Speaker 1: and Galileo is the first person to see these, and
• 27:18
  Speaker 1: so with a pretty basic telescope five years ago, you
• 27:21
  Speaker 1: can see these dots and you can plot the trajectory
• 27:23
  Speaker 1: of them, and you can see, like, okay, Io is
• 27:25
  Speaker 1: coming out from behind Jupiter. And people watch these things
• 27:28
  Speaker 1: and look for patterns, and they noticed something really interesting.
• 27:31
  Speaker 1: They noticed that it is true that Io comes out
• 27:33
  Speaker 1: from behind Jupiter every forty two and a half hours,
• 27:36
  Speaker 1: but that during some parts of the year that time
• 27:38
  Speaker 1: is a little bit shorter, and other times of the
• 27:41
  Speaker 1: year that time is a little bit longer. So like
• 27:43
  Speaker 1: the time between Io emerging from behind Jupiter gets longer
• 27:48
  Speaker 1: during one season and shorter during other seasons. And somehow
• 27:51
  Speaker 1: that tells you the speed of light, and that tells
• 27:53
  Speaker 1: you the speed of light because the reason those times
• 27:55
  Speaker 1: gets shorter is because the Earth has now gotten closer
• 27:58
  Speaker 1: to Jupiter and Io than it was last time, and
• 28:01
  Speaker 1: so the light doesn't have is far to go to
• 28:03
  Speaker 1: get to Earth. And the reason the times between the
• 28:06
  Speaker 1: reappearances get longer is when the Earth is moving away
• 28:09
  Speaker 1: from Jupiter. So now the light has further to go
• 28:12
  Speaker 1: when it has to reach Earth to tell you that
• 28:14
  Speaker 1: Io has emerged. If speed of light was infinite, then
• 28:17
  Speaker 1: I would always appear every forty two and a half hours.
• 28:19
  Speaker 1: This wouldn't matter at all. But because the distance between
• 28:22
  Speaker 1: Earth and Io is growing or shrinking, then this period
• 28:26
  Speaker 1: grows or shrinks. And so this Danish astronomer realized, oh
• 28:29
  Speaker 1: my gosh, I can use this information to calculate the
• 28:32
  Speaker 1: speed of light. Whoa interesting right, because sometimes we're in
• 28:37
  Speaker 1: between measurements of when you see the moon the Earth
• 28:40
  Speaker 1: will have moved. Is that what you mean, like sometimes
• 28:42
  Speaker 1: it moves a lot in that time, and sometimes it
• 28:44
  Speaker 1: doesn't move a lot exactly. And so when we are
• 28:47
  Speaker 1: moving further away from Io during that part of the
• 28:49
  Speaker 1: year that we're like zooming away from it, than those
• 28:52
  Speaker 1: times between Io's appearances will get longer. And when we've
• 28:55
  Speaker 1: come around the other side of the Sun and we're
• 28:57
  Speaker 1: zooming towards Io, then we're shortening that instance that light
• 29:00
  Speaker 1: has to go. Like, if you were making these measurements
• 29:03
  Speaker 1: from the Sun where the distance between you and I
• 29:05
  Speaker 1: wasn't changing, then they would be perfectly regular. Or if
• 29:09
  Speaker 1: the speed of light was infinite, so that every time
• 29:11
  Speaker 1: Io came around the back of Jupiter you instantly saw it,
• 29:14
  Speaker 1: then the measurements would be regular. But since the distance
• 29:17
  Speaker 1: is changing and it takes a finite time for light
• 29:20
  Speaker 1: to cross that distance, then you can measure how fast
• 29:23
  Speaker 1: light moves across these incredible distances. I guess the distance
• 29:27
  Speaker 1: is changing, and it's changing in a kind of predictable way,
• 29:30
  Speaker 1: so you can measure the speed of light. And so
• 29:32
  Speaker 1: what did they find. They get pretty close to what
• 29:34
  Speaker 1: the actual speed of light is. He got to within
• 29:36
  Speaker 1: about twenty of the real speed of light. Which is
• 29:39
  Speaker 1: pretty incredible. That's like a B B minus. Yeah, well
• 29:45
  Speaker 1: it's much better than Galileo. Did you know Galileo got
• 29:47
  Speaker 1: an F So at least this guy is passing well technically,
• 29:51
  Speaker 1: I don't know, because Galileo thought it was pretty fast. Yeah.
• 29:54
  Speaker 1: But I love these stories where you like tricked the
• 29:56
  Speaker 1: universe or corner the universe into revealing some piece of formation.
• 30:00
  Speaker 1: This guy didn't build this experiment. He discovered this experiment.
• 30:04
  Speaker 1: He's like, wait a second, this random configuration of stuff
• 30:08
  Speaker 1: reveals this piece of information everybody wants to know, And
• 30:11
  Speaker 1: all I have to do is use my telescope and
• 30:13
  Speaker 1: calculate a few numbers and boom, now I have this number. Interesting,
• 30:18
  Speaker 1: but did he know like the relative positions of the
• 30:20
  Speaker 1: planet in order for him to know exactly like how
• 30:23
  Speaker 1: much more the Earth at moved? Like do we know
• 30:26
  Speaker 1: the orbits that will back then? We didn't know the
• 30:28
  Speaker 1: orbits that well back then. But actually, all you have
• 30:30
  Speaker 1: to know is the orbital distance of the Earth. You
• 30:33
  Speaker 1: just have to know the radius of the Earth's orbit,
• 30:35
  Speaker 1: because that's the difference between the path of light when
• 30:38
  Speaker 1: the Earth is furthest away and when it is closest away,
• 30:40
  Speaker 1: So you can look at the calculations online, but you
• 30:42
  Speaker 1: can do with some pretty basic information about the orbits. Wow,
• 30:46
  Speaker 1: I imagine you could do it today, right Like if
• 30:48
  Speaker 1: you just had a nice back yard telescope, you could
• 30:50
  Speaker 1: measure the speed of light to you could get a
• 30:53
  Speaker 1: b in your back yard. Absolutely you can. And I
• 30:57
  Speaker 1: was chatting with one of our listeners, Brian Fields, if
• 30:59
  Speaker 1: the theoretical particle physicists, and he said he did this
• 31:02
  Speaker 1: lab in college and he actually sent me his rite up,
• 31:05
  Speaker 1: And so it's the kind of thing you can now
• 31:07
  Speaker 1: assign to undergraduates in physics and they can totally extract
• 31:11
  Speaker 1: this basic constant of the universe using simple tools and
• 31:14
  Speaker 1: everyone gets a D in the class. Then what was it?
• 31:16
  Speaker 1: The next step in measuring the speed of light? So
• 31:18
  Speaker 1: the next step was improving on Galileo strategy rather than
• 31:22
  Speaker 1: doing astronomical measurements. There was a guy in the eighteen
• 31:25
  Speaker 1: hundreds named Fizzo who sent a beam of light further away,
• 31:29
  Speaker 1: so instead of one mile, he sent it five miles.
• 31:31
  Speaker 1: So he was trying to measure a longer time distance.
• 31:34
  Speaker 1: He had this really clever trick for measuring really short
• 31:37
  Speaker 1: time periods. He put a beam of light that passed
• 31:40
  Speaker 1: very close to a gear that was rotating. So imagine
• 31:43
  Speaker 1: like gear like the one you have on your bicycle.
• 31:45
  Speaker 1: It's got a little teeth on it, and as it spins,
• 31:48
  Speaker 1: light can go through sometimes when it's not blocked, and
• 31:51
  Speaker 1: then when it hits the gear, when it hits the
• 31:53
  Speaker 1: tooth of the gear, then it's blocked. And if you
• 31:56
  Speaker 1: arrange things just right, then light flies through between the teeth,
• 32:01
  Speaker 1: HiT's a mirror, comes back, and then flies through the
• 32:04
  Speaker 1: next tooth. And so if you arrange things just right,
• 32:07
  Speaker 1: then the light can make it there and come back
• 32:09
  Speaker 1: and not be blocked. And if you're going at the
• 32:11
  Speaker 1: wrong speed, then it's going to hit one of the teeth,
• 32:13
  Speaker 1: either on the way out or the way there. And
• 32:16
  Speaker 1: so you can arrange things just right to get the
• 32:18
  Speaker 1: right speed in the right distance, so you can get
• 32:20
  Speaker 1: light to go there and back and not miss a tooth.
• 32:23
  Speaker 1: And this is the way to measure how long it
• 32:25
  Speaker 1: takes light to go there and back if you know
• 32:27
  Speaker 1: like the rotation speed of your gear. Well, this sounds
• 32:31
  Speaker 1: pretty tricky in advance, I guess the big question is
• 32:34
  Speaker 1: how did they get the light to go five miles
• 32:36
  Speaker 1: bounce off a mirror and come back and still be
• 32:38
  Speaker 1: sort of like you know, legible. Like they didn't have
• 32:41
  Speaker 1: lasers back then today. They only did not have lasers
• 32:45
  Speaker 1: back then. Five a lot, right, Like any beam of light,
• 32:48
  Speaker 1: if it's a little foggy or something, it will make
• 32:50
  Speaker 1: it five miles and back. That's true. But you know,
• 32:52
  Speaker 1: they did have optics, and they had powerful lenses. Even
• 32:55
  Speaker 1: Newton was studying lenses, and so they had ways to
• 32:58
  Speaker 1: concentrate beams of light. But yeah, that was definitely a
• 33:00
  Speaker 1: challenge back then making a powerful enough beam of light.
• 33:03
  Speaker 1: But you know, light can go pretty far, so you
• 33:05
  Speaker 1: need like a clear night, right, This would be an
• 33:07
  Speaker 1: experiment would be better to do in space because you say,
• 33:10
  Speaker 1: light will scatter off of the atmosphere, but you only
• 33:13
  Speaker 1: need a few photons. Also, I see, so they had
• 33:15
  Speaker 1: some sort of like focused beam of light, I guess,
• 33:17
  Speaker 1: but they didn't have electricity, so they must have used
• 33:19
  Speaker 1: like candles or fire. Yeah, that's a really good question.
• 33:22
  Speaker 1: You know. I read a few descriptions of this experiment
• 33:24
  Speaker 1: and they all just say the light source, So I
• 33:26
  Speaker 1: wasn't able to figure out what the actual source of
• 33:29
  Speaker 1: light is. So either some time traveling physicists lend them
• 33:32
  Speaker 1: a laser, or they like really focused beams of light
• 33:34
  Speaker 1: from the sun, or maybe like early electricity allowed them
• 33:38
  Speaker 1: to generate really bright bulbs or aliens. Possibility, the simplest
• 33:43
  Speaker 1: explanation first, that's also on an experiment that like people
• 33:46
  Speaker 1: can do in their backyards, right kind of Yeah, if
• 33:48
  Speaker 1: you have a rotating gear that's very precise and a
• 33:50
  Speaker 1: five mile long backyard, then yeah, go for it. That's right.
• 33:54
  Speaker 1: If you're a billionaire and live in a state, anything
• 33:57
  Speaker 1: is possible for that's right right to us. We'll send
• 34:00
  Speaker 1: a kid for one billion dollars from measuring the speed
• 34:02
  Speaker 1: of light. And then I imagine that we've gotten much
• 34:05
  Speaker 1: better at these kinds of measurements, and so let's get
• 34:07
  Speaker 1: into those and our current understanding of what the speed
• 34:10
  Speaker 1: of light is. But first let's take another quick break.
• 34:26
  Speaker 1: All right, we are measuring the speed of light, and
• 34:29
  Speaker 1: you can do it in your back yard if you're
• 34:30
  Speaker 1: a billionaire. Yeah, you can actually also do it in
• 34:33
  Speaker 1: your kitchen these days. Oh no kidding, Wow, I have
• 34:36
  Speaker 1: a five mile long kitchen. You don't need a five
• 34:39
  Speaker 1: mile long kitchen. Actually, all you need is say microwave
• 34:42
  Speaker 1: and a chocolate bar, and you can measure the speed
• 34:44
  Speaker 1: of light at home in about twenty seconds, no kidding,
• 34:47
  Speaker 1: How does that work? Well, Light, of course is a wave,
• 34:50
  Speaker 1: and so if you know the frequency of the wave
• 34:52
  Speaker 1: and you know the wavelength, you can combine those two
• 34:55
  Speaker 1: pieces of information to get the speed of the waves.
• 34:59
  Speaker 1: And so people do of these now in very high
• 35:01
  Speaker 1: end experiments using cavity residences. The most precise measurements of
• 35:06
  Speaker 1: the speed of light we have sort of inexperiments, come
• 35:08
  Speaker 1: from these cavity residence experiments where you measure the wavelength
• 35:12
  Speaker 1: of the light and you measure it's resident frequency. But
• 35:15
  Speaker 1: you can also do a simpler version of that at home.
• 35:18
  Speaker 1: You just take a chocolate bar and you put it
• 35:20
  Speaker 1: in the microwave. You microwave for about twenty seconds, not
• 35:24
  Speaker 1: enough so it's totally melted, but enough that it's just
• 35:26
  Speaker 1: started to melt. And take it out and you'll notice something.
• 35:29
  Speaker 1: You'll notice that it's more melted in some places than
• 35:32
  Speaker 1: in others. There's like hot spots m interesting, and that
• 35:35
  Speaker 1: is basically the shape of the wave of light of
• 35:38
  Speaker 1: the microwave light that's right. The distance between those hot
• 35:40
  Speaker 1: spots is one half of the wavelength of the microwaves,
• 35:45
  Speaker 1: because that's where they have like added up concretely to
• 35:47
  Speaker 1: give you like the most energy. And so what you're
• 35:50
  Speaker 1: seeing there is like the actual physical wavelength of the
• 35:53
  Speaker 1: photons passing through your chocolate bar, and it's like a
• 35:56
  Speaker 1: few centimeters. So it's something you can reasonably measure using
• 35:59
  Speaker 1: a chalk the bar in your microwave. And then all
• 36:02
  Speaker 1: you have to do is look up the frequency that
• 36:04
  Speaker 1: your microwave uses. Usually it's like two and a half
• 36:07
  Speaker 1: giga hurts or something. Combine those two numbers together and boom,
• 36:11
  Speaker 1: that's the speed of light. Interesting, but I guess you
• 36:13
  Speaker 1: know inside the microwave, isn't it bombarded by microwaves from
• 36:17
  Speaker 1: all directions? Is that the wave inside of my microwave
• 36:19
  Speaker 1: that like coherent, that untouched, that sort of neat? Yeah, unfortunately,
• 36:24
  Speaker 1: it is right, and that's why you have hot spots
• 36:26
  Speaker 1: and cold spots. We have a whole episode about how
• 36:28
  Speaker 1: microwave ovens work, and usually they have like one source
• 36:31
  Speaker 1: of the radiation and so it puts the stuff out
• 36:33
  Speaker 1: in this kind of pattern where you get this constructive
• 36:36
  Speaker 1: and destructive modes. It would be much better if it
• 36:38
  Speaker 1: was like incoherent and evenly distributing the energy, which is
• 36:42
  Speaker 1: why you usually have like a spinner to move your
• 36:44
  Speaker 1: food through this field of microwaves. So they use a
• 36:47
  Speaker 1: sort of a simpler radiator and it has these features
• 36:49
  Speaker 1: to it. I guess the tricky part though, is measuring
• 36:52
  Speaker 1: the frequency of the LightWave, right, because I mean that's
• 36:54
  Speaker 1: like gigaherts. You don't really have a clock that can
• 36:58
  Speaker 1: measure that. You'd have to trust the greave manufacturer. Yeah,
• 37:01
  Speaker 1: it's sort of cheating because they've done the hard part
• 37:03
  Speaker 1: for you of measuring the frequency. But it's also a
• 37:06
  Speaker 1: cool thing to like physically see the impact of light
• 37:09
  Speaker 1: being a wave, to like to see the distance between
• 37:12
  Speaker 1: the crests of the light wave in a physical thing
• 37:14
  Speaker 1: that you can do in your kitchen. That's sort of cool.
• 37:17
  Speaker 1: But you're right, when we make the actual measurements, like
• 37:19
  Speaker 1: when we actually want to figure this out ourselves, then
• 37:23
  Speaker 1: we use very precise cavities and we measure the residents
• 37:25
  Speaker 1: frequency and the wavelength of the most simultaneously, because you
• 37:29
  Speaker 1: can't just look that stuff up. And is it required
• 37:31
  Speaker 1: that you have to eat the chocolate afterwards, because then
• 37:34
  Speaker 1: you're cheating not just the universe, but you're diet a
• 37:36
  Speaker 1: little bit. No, that's the bonus of doing physics. Man,
• 37:39
  Speaker 1: Sometimes you make a delicious experiment that gets a little messy,
• 37:45
  Speaker 1: all right, So nowadays we use much more like constraint environments.
• 37:48
  Speaker 1: I guess a cavity and you have a wave of
• 37:50
  Speaker 1: light and you know exactly what the frequency is, and
• 37:53
  Speaker 1: you can sort of see the wavelength and that gives
• 37:55
  Speaker 1: you one measurement of the speed. Like nowadays, we don't
• 37:57
  Speaker 1: really like do these experiments meants where we send it
• 38:01
  Speaker 1: off to one place and then measure how light it
• 38:03
  Speaker 1: takes to come back. We use something like this. Yeah,
• 38:05
  Speaker 1: it's much more precise to use interference effects or residence
• 38:08
  Speaker 1: effects because they're very very sensitive to very small shifts
• 38:12
  Speaker 1: in one wave to the other. And so the way
• 38:14
  Speaker 1: the cavity residence experiment works is you measure both the
• 38:18
  Speaker 1: wavelength and the resident frequency. Are you build some precise
• 38:21
  Speaker 1: cavity and that determines the wavelengths of like standing modes
• 38:26
  Speaker 1: inside the cavity, or do you have like two mirrors essentially,
• 38:29
  Speaker 1: and you want light to go back and forth between
• 38:31
  Speaker 1: those mirrors in a way that it doesn't cancel itself out.
• 38:35
  Speaker 1: You need light to have a wavelength so that an
• 38:37
  Speaker 1: integer number of those wavelengths adds up to exactly the
• 38:40
  Speaker 1: width of the cavity. So there's only like certain modes
• 38:43
  Speaker 1: of the cavity where you can get this sort of effect.
• 38:45
  Speaker 1: And then you just measure the resident frequency, like at
• 38:48
  Speaker 1: what frequency of light? What color of light do you
• 38:51
  Speaker 1: get these residences? So you can measure the width of
• 38:53
  Speaker 1: your cavity and measure the frequency the color of light
• 38:56
  Speaker 1: that goes in there that achieves residents, and together you
• 38:59
  Speaker 1: can get a very accurate measurement of the speed of light.
• 39:01
  Speaker 1: And that was like nine seventy five. The people really
• 39:04
  Speaker 1: perfected this and got like a super duper precise measurements
• 39:07
  Speaker 1: of the speed of light. But I guess, doesn't that
• 39:09
  Speaker 1: depend on how accurate your clock is to measure the
• 39:12
  Speaker 1: frequency of light and also how good you're you know,
• 39:16
  Speaker 1: ruler is to measure the size of your cavity, right, Like,
• 39:19
  Speaker 1: there are still I guess imagine limitations to how well
• 39:22
  Speaker 1: we know the speed of light. There were still limitations
• 39:25
  Speaker 1: for just those reasons, like people used crazy techniques to
• 39:29
  Speaker 1: measure the size of these cavities. Very very accurately, and
• 39:32
  Speaker 1: it's like a real tourtive force of experimental physics, the
• 39:35
  Speaker 1: clever strategies people came up with to measure these precisely.
• 39:38
  Speaker 1: These days, however, we have actually zero uncertainty on the
• 39:42
  Speaker 1: speed of light, zero uncertainty, like we know it to
• 39:45
  Speaker 1: an infinite number of digits. Yeah, so it's a bit
• 39:47
  Speaker 1: of a cob out answer, right, we don't know the
• 39:49
  Speaker 1: speed of light to an infinite precision if you set
• 39:52
  Speaker 1: an arbitrary length for the meter and an arbitrary length
• 39:55
  Speaker 1: for the second. Instead, we've decided we're going to use
• 39:58
  Speaker 1: the speed of light to define the length. We're going
• 40:01
  Speaker 1: to say we know this better than anything else, so
• 40:03
  Speaker 1: let's define everything else in terms of the speed of light.
• 40:07
  Speaker 1: So now the official definition of a meter is no
• 40:09
  Speaker 1: longer like here's a platinum rod in Paris. Instead, it's
• 40:13
  Speaker 1: how far light travels in a certain amount of time. Right,
• 40:17
  Speaker 1: Because I guess you're saying that if we picked a
• 40:20
  Speaker 1: valley for the meter in a valley for the second,
• 40:22
  Speaker 1: then that makes the speed of light that we measure
• 40:24
  Speaker 1: kind of dependent on what we pick for the meter
• 40:27
  Speaker 1: in the second. So instead it makes more sense maybe
• 40:30
  Speaker 1: from a global point of view. To define the meter
• 40:33
  Speaker 1: and the second by the speed of light. Yes, so
• 40:36
  Speaker 1: we define the meter by how far light travels in
• 40:39
  Speaker 1: a second, and we define the second by the oscillations
• 40:42
  Speaker 1: of some caesium atom. So now the meter is something
• 40:46
  Speaker 1: which depends on the speed of light and the oscillations
• 40:48
  Speaker 1: of the caesium atom. So now instead of asking like,
• 40:51
  Speaker 1: how well do we know the speed of light, it's like, well,
• 40:54
  Speaker 1: how well do you know the length of this platinum
• 40:56
  Speaker 1: rod in Paris? Oh? I see, So you're kind of
• 40:58
  Speaker 1: saying almost like we're coming up with the speed of light,
• 41:01
  Speaker 1: like we're inventing the speed of light, right, because we
• 41:03
  Speaker 1: just picked some numbers and then we call that the meter.
• 41:07
  Speaker 1: So therefore the speed of light is so and so
• 41:08
  Speaker 1: meters per second. Yeah, And it's just like in particle physics,
• 41:11
  Speaker 1: we could define the speed of light to be one
• 41:13
  Speaker 1: and everything is set relative to that. And so here
• 41:16
  Speaker 1: we're defining the meter so that the speed of light
• 41:19
  Speaker 1: is exactly two eight with no decimal places, like it's
• 41:25
  Speaker 1: exactly that number. And you know, I said, we know
• 41:29
  Speaker 1: it accurately. Really, we just define it to be that,
• 41:31
  Speaker 1: and everything is now relative to that. Number just kind
• 41:34
  Speaker 1: of blew my mind. It means we don't know what
• 41:35
  Speaker 1: the speed of light is, right, Like, technically, philosophically, you're
• 41:40
  Speaker 1: trying to say that we don't know what the speed
• 41:41
  Speaker 1: of light is. We just picked the number and said
• 41:43
  Speaker 1: that's it. We picked the number. We said, this is
• 41:45
  Speaker 1: what we call the speed of light. The speed of
• 41:47
  Speaker 1: light is a number, right, and we just assigned to
• 41:49
  Speaker 1: say it's this number, this length. And now the question
• 41:52
  Speaker 1: is what is length to mean? Length is relative to
• 41:54
  Speaker 1: the speed of light. It's just as good as saying
• 41:57
  Speaker 1: length is relative to this rot in Paris. But this
• 41:59
  Speaker 1: row in Paris has no real meeting or physical significance,
• 42:03
  Speaker 1: so it's sort of silly, whereas the speed of light
• 42:05
  Speaker 1: obviously does. And so it makes a lot more sense
• 42:07
  Speaker 1: to define things relative to the speed of light rather
• 42:09
  Speaker 1: than relative to an arbitrary chunk of metal. But yeah,
• 42:12
  Speaker 1: I think by using a number that doesn't have any
• 42:14
  Speaker 1: decimal places, right, you get that to be the meter,
• 42:17
  Speaker 1: and then you use that meter to measure the speed
• 42:19
  Speaker 1: of light. But then the number gives you was it's
• 42:22
  Speaker 1: a consequence of you picking that random number. Yeah, you
• 42:24
  Speaker 1: can't measure the speed of light anymore. You're exactly right.
• 42:27
  Speaker 1: It doesn't make any sense to define the meter in
• 42:30
  Speaker 1: terms of the speed of light and then trying to
• 42:31
  Speaker 1: measure the speed of light. Like, you can't measure the
• 42:33
  Speaker 1: speed of light in terms of the meter because the
• 42:35
  Speaker 1: meter is defined in terms of the speed of light.
• 42:37
  Speaker 1: It's circular. Instead, when you can do is define the
• 42:39
  Speaker 1: speed of light and then measure the length of a
• 42:41
  Speaker 1: rod in Paris in terms of that. Why anybody would
• 42:44
  Speaker 1: care the length of rod in Paris, I don't know,
• 42:46
  Speaker 1: But philosophically that's what you can do now. But I
• 42:48
  Speaker 1: feel like that's kind of like you're avoiding the question.
• 42:51
  Speaker 1: Like there is a speed of light, like there is
• 42:53
  Speaker 1: a certain amount of distance that light covers in in
• 42:55
  Speaker 1: one second in the universe, but it doesn't seem like
• 42:58
  Speaker 1: we know what that is to any sort of decimal place. Well,
• 43:01
  Speaker 1: we don't know what that is relative to that stick
• 43:04
  Speaker 1: in Paris. You're right, and we could spend a lot
• 43:07
  Speaker 1: of time and money measuring how fast light goes relative
• 43:10
  Speaker 1: to this arbitrary unit of distance we defined according to
• 43:13
  Speaker 1: this stick in Paris. But I think people decided that
• 43:15
  Speaker 1: doesn't mean anything anyway, Like what does it matter? How
• 43:19
  Speaker 1: many decimal places you get when your unit is arbitrary.
• 43:22
  Speaker 1: We prefer to make a reasonable unit when that makes sense.
• 43:25
  Speaker 1: And the speed of light is the most important physical
• 43:28
  Speaker 1: constant in the universe, and so let's just define everything
• 43:31
  Speaker 1: relative to that, right, But then you're picking an arbitrary
• 43:33
  Speaker 1: number for that speed, Yes, absolutely, I don't know. I
• 43:38
  Speaker 1: guess it makes it more sense to me as a
• 43:40
  Speaker 1: lay person to pick an arbitrary length and then measure
• 43:44
  Speaker 1: the speed of light than to pick an arbitrary speed
• 43:46
  Speaker 1: of light and then define lights from that. Well, it's
• 43:48
  Speaker 1: philosophically their equivalent, you know. Check out our episode on
• 43:50
  Speaker 1: the basic constants of the universe, and you'll realize that
• 43:53
  Speaker 1: no number that has units on it ever has any meaning,
• 43:56
  Speaker 1: because it just depends on your definition of the units.
• 43:59
  Speaker 1: The old numbers that really have meeting are the ones
• 44:02
  Speaker 1: without any units, the ones that are pure numbers of
• 44:05
  Speaker 1: the universe. So the speed of light in that sense
• 44:07
  Speaker 1: is not actually that fundamental. It folds into the fine
• 44:10
  Speaker 1: structure constant, which is a unitless number and which does
• 44:13
  Speaker 1: determine sort of the structure and the nature of the
• 44:16
  Speaker 1: universe and electromagnetism. But I guess you know, like a
• 44:19
  Speaker 1: rod in Paris is something we can all go and
• 44:22
  Speaker 1: touch and see and like, hold right, and then we
• 44:25
  Speaker 1: can all agree that the speed of light goes so
• 44:27
  Speaker 1: and so fast. But I feel like this way of
• 44:29
  Speaker 1: doing things, like nobody can agree what the speed of
• 44:31
  Speaker 1: light is. Everybody can agree, we just choose a number.
• 44:34
  Speaker 1: Whereas a rod in Paris, it like grows and shrinks
• 44:36
  Speaker 1: when it gets hot in Paris. Does that change the
• 44:38
  Speaker 1: speed of light? Like, it's ridiculous to have the speed
• 44:41
  Speaker 1: of light depend on something so arbitrary as how big
• 44:45
  Speaker 1: this rod in the museum in Paris is like the
• 44:47
  Speaker 1: air conditioning breaks in Paris, and now we're all moving faster,
• 44:50
  Speaker 1: Like it doesn't make any sense. Yeah, why not? I
• 44:53
  Speaker 1: mean that's better than like making up a number for
• 44:56
  Speaker 1: the speed of light. Daniel, I can't handle this all right, Well,
• 45:00
  Speaker 1: come to Dawn and Rhee discussed philosophy. I'll give you that.
• 45:04
  Speaker 1: That's the way you're doing it, even though I don't
• 45:06
  Speaker 1: agree with them, all right, objection noted, Yeah, thank you.
• 45:10
  Speaker 1: I'm sure will cause waves in the physics community. So
• 45:13
  Speaker 1: that's kind of the way we're doing it. That means
• 45:15
  Speaker 1: that we kind of can measure the speed of light. Right,
• 45:17
  Speaker 1: once we've defined it, we can no longer measure things
• 45:19
  Speaker 1: in terms of it. Yes, So to answer the questions,
• 45:22
  Speaker 1: how do we measure the speed of light, we don't anymore.
• 45:24
  Speaker 1: We just picked the number. We just picked the number. Yeah,
• 45:27
  Speaker 1: and that number was based on almost nothing, right. Well,
• 45:31
  Speaker 1: that number, you know, defines a meter to be something
• 45:34
  Speaker 1: close to what it used to be, and so that's
• 45:35
  Speaker 1: pretty nice, but it could have been something else. We're
• 45:38
  Speaker 1: just to pick the number out of historical reasons, kind
• 45:40
  Speaker 1: of to approximate historical history. Yeah, we wanted the new
• 45:43
  Speaker 1: meter to be pretty close to the old meter so
• 45:46
  Speaker 1: that we didn't have to like change everything all that,
• 45:49
  Speaker 1: you know, make all new highway signs like oh, this
• 45:51
  Speaker 1: tunnel is now one meter high whereas it used to
• 45:53
  Speaker 1: be twenty meters high. That would be ridiculous. So once
• 45:56
  Speaker 1: again laziness. Yeah, consistency, manists knap consistency. It is very
• 46:03
  Speaker 1: important to physicists. And I'm getting the sense. But yes,
• 46:06
  Speaker 1: it seems like the answer is you can't measure the
• 46:08
  Speaker 1: speed of light anymore, right, because now we've defined the
• 46:10
  Speaker 1: meter as based on this number of the speed of lights,
• 46:12
  Speaker 1: it makes no sense to leasure the speed of light.
• 46:14
  Speaker 1: It's just what it is. Yeah, that's true. We can
• 46:16
  Speaker 1: no longer measure the speed of light relative to other
• 46:19
  Speaker 1: arbitrary units because it is now the arbitrary unit. Can
• 46:23
  Speaker 1: we measure it relative to some of these you know,
• 46:25
  Speaker 1: fundamental unitless constants that you measured, Like, you know, the
• 46:28
  Speaker 1: universe has these numbers that are immovable and fundamental to
• 46:33
  Speaker 1: the fabric of the universe. Can we use those to
• 46:34
  Speaker 1: get a real measurement of the speed of light? Now?
• 46:37
  Speaker 1: Because those numbers don't have units, and so they can't
• 46:39
  Speaker 1: determine numbers that do have units because those depend on
• 46:43
  Speaker 1: your choice of units, right, that's the problem with numbers
• 46:45
  Speaker 1: that have units. Anything that's like meters per second or
• 46:49
  Speaker 1: pounds per square inch or whatever is going to depend
• 46:52
  Speaker 1: on the units, which is why physics refers to talk
• 46:54
  Speaker 1: about numbers without units. But you know, I guess we
• 46:57
  Speaker 1: base time on some sort of fundamental physical thing, right,
• 47:00
  Speaker 1: like the oscillations of a crystal or whatever. Why can't
• 47:04
  Speaker 1: we do that with distance as well? We do, we do,
• 47:07
  Speaker 1: That's exactly what we do. And that's how a rod
• 47:10
  Speaker 1: in France. But like I don't know the width of
• 47:12
  Speaker 1: a proton or something like that. We'll think about it
• 47:14
  Speaker 1: as a certain number of light wavelengths at a certain frequency.
• 47:18
  Speaker 1: Because we defined the meter in terms of the speed
• 47:20
  Speaker 1: of light. Now light is our ruler. But then that
• 47:23
  Speaker 1: means we can't measure the speed of light. That's true. Yeah,
• 47:27
  Speaker 1: we've given that up because now it's our ruler, because
• 47:29
  Speaker 1: we've decided that that's exactly the most basic unit. Just
• 47:32
  Speaker 1: like you can no longer define how long it takes
• 47:36
  Speaker 1: caesium to do one oscillation because it's to find to
• 47:39
  Speaker 1: be one second, or is to find to be one
• 47:41
  Speaker 1: you know, six billions of a second or whatever. Because
• 47:44
  Speaker 1: we now define time in terms of that basic physical operation,
• 47:47
  Speaker 1: you can no longer measure how long that operation takes.
• 47:51
  Speaker 1: What can you say, like, let's measure the speed of
• 47:53
  Speaker 1: light by the frequency of caesium and also the width
• 47:56
  Speaker 1: of caeson Yeah, you could define distance using something else,
• 47:59
  Speaker 1: but you know it's not as fundamental is the speed
• 48:01
  Speaker 1: of light. The speed of light is really basic and
• 48:03
  Speaker 1: interesting to the universe. So I think that's why they
• 48:05
  Speaker 1: chose it. But you're right, these things are arbitrary, and
• 48:07
  Speaker 1: you could have said, you know, the meter is now
• 48:09
  Speaker 1: defined to be one third of the height of Jorge's room. Like,
• 48:12
  Speaker 1: you could have chosen anything. Some choices are better than others,
• 48:15
  Speaker 1: you know, and I think this is a pretty good one.
• 48:17
  Speaker 1: Let's just pick a number of It seems like a
• 48:19
  Speaker 1: crazy way to run thenation of the universe, Daniel, We're
• 48:25
  Speaker 1: doing our best. Do our best. I know it's not
• 48:28
  Speaker 1: your fault. You have limited power in the physics community.
• 48:32
  Speaker 1: All right, Well, I've mounted the chocolate bar in my mind.
• 48:35
  Speaker 1: I feel like I don't know what to trust anymore
• 48:37
  Speaker 1: in physics, Daniel, things are arbitrarily fast. Now. There is
• 48:40
  Speaker 1: no speed limit. There's just the speed limit that you're
• 48:42
  Speaker 1: telling me is the speed limit. That's right, Go out
• 48:44
  Speaker 1: there and break whatever speedimits you want. Jorhey, you're right, absolutely,
• 48:48
  Speaker 1: there are no rules when it comes to you. Well,
• 48:49
  Speaker 1: I think it's another kind of reminder. You know that
• 48:52
  Speaker 1: this is a tricky universe. You know, it's kind of
• 48:54
  Speaker 1: hard to measure things because everything is relative. Everything can change,
• 48:58
  Speaker 1: everything depends on kind of you know, how fast you're moving,
• 49:01
  Speaker 1: or how hot it is, or what you're measuring relative to.
• 49:05
  Speaker 1: So it's kind of hard to find footing in this universe.
• 49:08
  Speaker 1: It is but it also gives us a sense for
• 49:10
  Speaker 1: how the universe works. And I think it's awesome how
• 49:13
  Speaker 1: we as humans have figured out how to extract this
• 49:16
  Speaker 1: kind of information from the universe. I mean, until we
• 49:18
  Speaker 1: define it away is not interesting anymore. I think it's
• 49:22
  Speaker 1: fascinating to see, sort of like the historical sweep, how
• 49:24
  Speaker 1: long it takes, how the thousands and hundreds of years
• 49:27
  Speaker 1: it takes to figure out this one very basic thing
• 49:30
  Speaker 1: about something we literally see every day. And as the reminder,
• 49:33
  Speaker 1: please obey speed limits in your driving practice, because going
• 49:39
  Speaker 1: at the speed of light might get you a few
• 49:41
  Speaker 1: light tickets. But if you do manage to go at
• 49:44
  Speaker 1: the speed of light, please let us know. We'd like
• 49:46
  Speaker 1: to hear about it. But what if they interrupt your nap, Daniel,
• 49:50
  Speaker 1: It'll be worth it. They'll sit in your inbox for
• 49:52
  Speaker 1: ten seconds before you check all right. Well, we hope
• 49:54
  Speaker 1: you enjoyed that. Thanks for joining us, see you next time.
• 50:05
  Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
• 50:08
  Speaker 1: the Universe is a production of I Heart Radio or
• 50:11
  Speaker 1: more podcast from my Heart Radio. Visit the I Heart
• 50:14
  Speaker 1: Radio Apple Apple Podcasts, or wherever you listen to your
• 50:18
  Speaker 1: favorite shows. Ye