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

Episode Date: November 16, 2021

Daniel and Jorge talk about the history of how we measured this fastest of all velocities! Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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What? You mean they go faster than the speed of light? Or do you mean like if it falls into a black hole, it's actually my inbox? Yeah, exactly like that. I can tell you a story about one time I had email violate causality. No way. What happened? Well, in college one time, I sent a draft of an essay to my TA for comments. She wrote back, hey, looks great, no comments. Then I realized I'd never attached it to the email.

It sounds like she violated the laws of her. responsibilities as a TA, not the laws of physics. That's one interpretation. Is there a non-physical interpretation? The grad student union won't allow me to talk about that. It breaks their laws. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm recording this podcast on the same microphone Michael Jackson used to record Thriller. What? What do you mean? Like the same microphone or the same brand? The same brand. I just discovered yesterday that apparently I'm using a very well-known famous microphone, which in the industry is known as the Thriller Mike. No way. Wow. Is it extra kind of like it has extra pops and extra to go a little bit higher in your false.

voice. It does make me do a little dance every time you make a really funny joke. So yeah. You do the moonwalk whenever we talk about the moon? I don't want you to think about that too much in your head. But yeah, yeah, let's say that's what happens. Yeah, but welcome to our podcast, Daniel and Jorge Explain the Universe, a production of I-Hard Radio. Which is sort of like thriller in that we take you on a thrill ride around the universe. We don't raise the dead and dance them around, but we do talk about everything in this universe that happens, everything that gets extinguished everything that flies around and amazes us with everything that it can do and all the laws of physics that seem to work together in harmony to make this universe so crazy, so bonkers,

so amazing, and yet so discoverable. Yeah, because it is a pretty thrilling universe and we like to take you on a moonwalk every episode so we can think about the universe and possibly even beat it. What is the moonwalk analogy there? It looks like you're moving forward, you're actually sliding backwards. Does that mean like we're doing physics? We think we're understanding, but really we understand less and less every year. You know, basically we're faking the whole thing. We're actually moving the progress of science backwards. We're walking backwards on evolution here. But it looks like we're moving forward. I like to think that our podcast helps science move forward in a real way because it excites people and engages them in this species wide project. They're trying to

uncover the mysteries of the universe. Yeah. And also we're wearing red leather jackets right now. with a lot of zippers in it. That's right. I only have one sparkly glove on. Does that make me Michael Jackson? I have the other one. It's like we're handshaking, glittery gloves across the internet. That's right.

And we're trying to merge all the Jackson siblings into one theory of Jackson. The unified theory of entertainment. Yeah, of soul and R&D. But it is a pretty thrilling universe, as we said. And this is an interesting question about how fast do emails travel? Like if I write you an email and I hit, send, does it go to you at the speed of light, right? Because sort of, right? Because electricity and signals over telephone lines, they sort of go basically as fast as light, right? That's right. That

information does travel at the speed of light. You take like a wire and you send a pulse down it. It does travel very, very fast, essentially at the speed of light. But we all know, of course, that email doesn't arrive that quickly when you send it because it's got to go through all sorts of like computers who do algorithms and wait on it and analyze it. And so, for example, my email here at UCI, which takes 10 minutes to get one. Yeah, and that's not even the emails you send me, which take me hours to even read. But yeah, the speed of light is pretty fast.

It seems to be basically the speed limit of the universe, right? Nothing in the universe can go faster than the speed of light. That's right. It seems to me this hard and fast limit. It's just like a feature of our universe that there's a maximum speed of information, which is really super cool philosophically to think about like, why is that and how could the universe have been different

and what does it mean? But it's also really fun to think about like, how did we figure that out? You know, something we know now. It's something we definitely understand. But like, obviously early humans didn't know that. And so I think it's always fun to return to the moments that we cracked at these problems, that we like gained a new understanding of how the universe worked. Yeah, because it's crazy to think that at some point we didn't know what the speed of light was

or even that it sort of had a speed, I imagine, right? Like, I imagine early man probably thought light was instantaneous. Like, you light a stick on fire and immediately the light hits your eyeballs. Yeah, the Greeks had a lot of totally uninformed debates on the topic. You know, speculating endlessly about what light was. Did it emanate from objects? Did it reflect from things? Did it travel instantaneously?

Was it a thing or not a thing? What does it mean to be a thing, man? Like, the Greeks went on and on and on with no information. It's amazing to me. For thousands of years, you could have uninformed debates. beats. You speak as if our discussions these days are informed. Well, we have data. We can do experiments. We can learn things. We can make progress, you know, without just like smoking more banana peels and thinking about how the universe works, man. Well, it is pretty interesting to think about the speed of light. And I think what's also interesting is that it's not infinite. Like, it's a number. You know, like the craziest you can go in the universe is a specific number. And nothing can go faster than that number. That's right. Yeah. And it's interesting. Right. Every time you see a number, in the theory of physics you wonder why that number did it have to be that could it have been something else if it could have been anything then why is it this value or if it could only be one

thing then what are the rules that make it have to be that one thing and what does that mean so it's like a huge screaming clue to me every time we see a number in physics what is it screaming it's screaming there's a secret here there's something else to be understood that in a hundred years somebody else is going to win a Nobel Prize for an explanation and you have all the information you need to arrive at that idea, you just don't see it. Right, yeah. And isn't it weird to think about that all other speeds that are greater than the speed of light are basically impossible. Like one meter per second faster than the speed of light, impossible. Any number from that number to infinity is basically impossible in the

universe. Yeah, the universe says no and it doesn't negotiate. There's no wiggle room there. You can't charm the universe into letting you do something a little bit past the speed of light. It's a firm no. It's a hard pass from the universe. And what exactly is the speed of light, Daniel? How many digits do you know it too? Well, it's funny you should ask because in particle physics, we say the speed of light is one. Like we use units where the speed of light is just one because we can't be bothered to write it down all the time because it's everywhere.

So we have equations with like speed of light square, speed of light to the fourth, speed of light to the eighth. And so it just sort of gets annoying. So we just say, let's just set C equal to one and then we can ignore it mostly. Let's ignore reality for our convenience. Let's have another lens in which we look at reality in which it makes more sense and we can boil it down to its true fundamental essence and not get tangled up in little numbers. Right. So you can take more naps, right? You can take more naps.

So a particle physicist is the wrong person to ask about the actual value of the speed of light in sensible units. But we do define it as 299,792,458 meters per second. So that's the exact value, the speed of light. Though most people, when they do calculations, just say three times 10 to the 8 meters per second. Right. Or like 300 million meters per second. But it is a very specific number, right? It's like 299-7-9-2-458 and like 459 is too fast.

You can't go that fast. That's right. It's a no on 459. 458 and a half? No. 458.1? No. There's no flexibility there.

There's no negotiation. This isn't in Hollywood. We're like, hey, we can find a deal. Right? It's kind of weird, right? that the universe would just pick the number and nothing can go faster. Yeah, it's a huge clue that's telling you something really deep about the nature of space and time itself, right?

Like, if the loop quantum gravity is true and space really is a big quantized foam bubble, then maybe this tells us about how those phone bubbles talk to each other, and you can't get information from one phone bubble to the other faster than that because they just aren't closely enough connected or something. So I think it really does tell you something deep about the nature of the universe. Yeah, and it's a very specific number. And so I guess the big question is, like, how do you know it's that number? To what accuracy do we know that's the right number that is the maximum speed limit of the universe?

And how did we figure it out? So to the end of the podcast, we'll be asking the question. How do we actually know what the speed of light is? Have we actually measured it or are we guessing? You're right. You figured it out. We've all just been guessing this whole time. And you have revealed this. It's like a Scooby-Doo episode.

you've pulled off the mask. Well, a few minutes ago, you just said one, right? So I don't know what's true anymore, Daniel. They're all true. It just depends on the units. Well, I guess I'm wondering, like, has anyone actually tested, right? Because nobody has actually tried to go faster than the speed of light, technically, right? Like, you haven't.

I haven't. I have totally tried. Absolutely. I've tried so many times as a kid. I mean, I didn't get anywhere near the speed of light, but that doesn't mean I didn't try to go fast. Oh, I see. He tried.

I mean, like, a credible try. Not like a far off by 10 decimal places. That's true. But you know, I did grow up to work at a particle accelerator, which makes a pretty credible attempt to get particles to go faster than the speed of light. We take protons and we accelerate them. We give them so much energy. And they go faster and faster and faster.

And what we see is that as you add energy, the particles just don't get going much faster. It's sort of a mind bend there like you can add energy. There's more kinetic energy in these particles, but they're not moving much faster. faster. Right. It approaches the speed of light, but I guess a big question is, how do you know what the actual speed of light is, like the actual number? Yeah, so that you can measure by actually looking at light and measuring it. But you can also see that protons approach it, right? We have

these limits in physics all the time where you can see something is approaching a limit and asymptotically gets closer and closer and closer. So you can calculate what that limit would be if you went to infinite energy. You can extrapolate mathematically to figure out like what is the limiting case, either from looking at protons to see what they are approaching or just by directly measuring the speed of actual beams of light itself. Well, that's kind of what we're going to get into today is kind of the history of how we've gone about measuring light and also what are some of our best current measurements of it. And some really surprising twists about what we do and don't know about the speed of light. Some light twists or some dark twists. A little above. So as usual,

we were wondering how many people out there knew the answer to this question, how we measure the speed of light. So Daniel went out there and asked people on the Internet if they knew how we measured the speed of light. That's right. So thank you to everybody who during these strange pandemic times have stepped up to fill in the gap left by UC Irvine students and answered questions online. If you'd like to participate for a future episode of the podcast,

please don't be shy and write to us to questions at Danielanhorpe.com. So think about it for a second. And what would you answer? Here's what people had to say. Well, if we already had space travel by the time we were trying to measure that, we could have sent a radio transmission from the moon with a time stamp and seen how long it took to get there. So I don't know exactly how we measured the speed of light,

but I would guess by measuring the time required for light to travel a certain distance, maybe by using the mirror experiment where the time required for light to travel to the mirror and back from the mirror is recorded and since we know the distance and time, we can find the speed of light. Oh, no idea. I mean, it was theoretical at first

and then they tested it, so then a particle accelerator? I think Galileo, tried to measure the speed of light using lanterns at set distances across fields. I assume that wasn't successful though. And I assume that we've also tried to measure it by bouncing signals off the moon in more recent times. I don't know though what the first successful measurement of the speed of light was. I guess maybe they worked out like the distance between two objects in space and then worked out

how quickly light travel between them and did the maths. I don't know. That's a pretty good question. So I've been racking my brain trying to think of this because I swear I've learned this in high school or college physics, but I couldn't tell you now how we measured the speed of light. A guy climbed a mountain and shown a light at a rotating mirror on another mountain and counted how long in between for the light to come back. So what do you think of these engineering ideas for how to measure this incredible speed? I think they're a little light on substance, but they're pretty good attempts. A lot of people didn't seem to have sort of an idea of how we've done it. Maybe, you know, they could think about ways that they could do it.

But I guess not a lot of people knew what the latest and greatest measurement is. Yeah. And the challenge, of course, is that it's super duper, duper fast. Like we talk about the speed of light being 300 million meters per second. That's sort of hard to understand. You know, what does that really mean? I think it's easier sometimes to think about it in terms of. of how far light goes in a very short amount of time,

rather than in a full second. I actually think about light is traveling about one foot every nanosecond. So like a light nanosecond, you know the concept of a light year, how far light travels in a year? A light nanosecond is about one foot or 30 centimeters for those of you on the metric system.

I totally reject that way of looking at that. First of all, it's English units. You're using feet instead of meters. And also a nanosecond. And that doesn't have a lot of meaning to me, I guess. Well, I guess you don't get a lot of things done in a nanosecond, but I'm pretty efficient. You know, I can answer 10 emails in a nanosecond. So, wow.

You live in another reality, it seems. No, no, obviously not. But, you know, when you're talking about, like, your signals, like how long is it going to take my information to go down this cable? I have one piece of equipment over here, another piece of equipment over there, 10 feet away. So you know it's going to take 10 nanoseconds to get from here to there. And sometimes if you're building electronics, right, you need to know, are these signals going to be coordinated, What's the gap going to be between them? And so it's helpful sometimes to think about light

in terms of how long it takes to move across a reasonable distance because I don't really know what 300 million meters is. But can we measure it in terms of like in the blink of an eye? Like let's say a blink of an eye is, I don't know, 100 milliseconds. How long can the light travel in those 100 milliseconds? Well, you know, 100 milliseconds is only a 10th of a second, right? And so a 10th of a second would be 30 million meters. So it's still pretty far that light can go in a tenth of a second.

Right. 30 million meters is about 30,000 kilometers, right? Which is about 20,000 miles. Yeah, that's right. 30 million meters is just under 20,000 miles. So like if I blink, then light can travel basically once around the world, kind of almost. Maybe if you're at the latitude of like North America, light could go around the earth in the blink of an eye. And that's really the challenge of measuring the speed of light that it's so darn fast that for all extent.

purposes for the things we do. It's essentially infinite. And that makes it really, really challenging because you either need incredibly vast distances so you can accumulate some like reasonable amount of time between when you send a message and when it arrives, or you need to be able to measure really, really short times. And so imagine you're like Aristotle 5,000 years ago. How could you possibly set up an experiment to measure something over vast distances or very short times? Yeah, because I guess, you know, it's hard to measure something that happens in the blink of an eye, right? Especially if you're in the thousand years BC or something. That's right. If all you have is like, you know, a clay tablet, a stick and a robe, then like, what are you going to do? And these days,

even for us, it's not easy to see the impact of the speed of light. You know, it impacts things like spaceflight communication. If you're on Mars driving a rover, then sure, and there's an impact of the speed of light not being infinite. But here on Earth, you know, it doesn't really make a difference in your life very much. If you're like the designer of a computer, then you think about this, like how long does it take the information and go across my CPU and can I optimize the design of it to bring things closer to speed up my computer? But unless you're driving rovers on Mars or designing CPUs, you probably don't think about

the speed of light very much. Yeah, it's probably pretty instantaneous to you in an intuitive sense. But I think of a big question is like, how do we know it's this particular number, right? Or not? Yeah, well, that comes from the things we measure. and we have no like theoretical preference for that number. It's not like the kind of thing we could have derived where we're like, well, it has to be this number.

It can only be this number. It's just a measurement, right? Often in physics there are things where, you know, there's something we know happens in the universe, but we don't know why it's this way and not the other way. And so we just have to measure it. Things like the mass of the Higgs boson or the mass of all the particles or, you know, the strength of gravity.

These are things we don't know why they are that number and not some other number. And the speed of light is like that. is just something we have to go out and measure. Like it could have been another number, right? It could have been three or a bazillion. We don't know, right? It could be that it has to be this number

because there's some deeper theory of physics that constrains it and makes it only work for this value. But we don't have that theory of physics. According to our theory, it could have been any other number. But of course, we don't expect that our theory of physics is the final answer. And it's exactly the kind of place where I see there are opportunities, right? Where we say, well, we don't have an explanation for this.

So let's keep looking for an explanation. To me, it's unsatisfying. People say, well, it could have been anything. It was just random. We're one element in the multiverse. So this is just what it is. There is no explanation. To me, it's a clue that says that probably is an explanation.

Keep digging. Right. Like maybe it tells you something specifically about why the universe was the way it had to be. We just don't see it yet. And it's also interesting to think that it's not just the speed of light. It's like the maximum speed that information can travel in the universe. like anything. It's not just light that travels at the speed of light.

It's any particle without mass. Yeah, and it's sort of a misnomer, right? We discovered light moves at this speed first, and it is the speed of light in a vacuum. But really, it should be like the speed of space time or the speed of information. Because as you say, anything that doesn't have mass, and that means like a graviton, if they exist,

or a gluon, for example, anything that doesn't have mass has to travel at the speed of light and only the speed of light and nothing else that doesn't. has mass can travel at that speed. So it really is a special speed in the universe more than just the speed of photons. Right. Maybe you should have been called the speed of nothing. Do you like that? Because light is technically nothing. It has no mass. And nothing can go faster than it. No, because it takes no time to do nothing. Right. Like you didn't do anything. How long did that take?

No time at all. So nothing is instantaneous. It's zero divided by zero. If anything, I think that would have been more confusing. All right, well, let's get into what we've done throughout the thousands of years of human history to measure the speed of light, and then let's get into our latest measurements of that number. But first, let's take a quick break. Imagine that you're on an airplane, and all of a sudden you hear this. Attention passengers, the pilot is having an emergency, and we need someone, anyone, to land this plane. Think you could do it?

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All right Daniel we are measuring the speed of light today on the episode. Are you ready? I'm going to clap and then you tell me how long it took. All right. Go. That was exactly one clap per clap. Oh, there you go. That's the joy of particle physics units. The answer is always one or E or pie in particle physics. That's why. It's always one of the three. But yeah, like you were saying

earlier, it's pretty hard to measure the speed of light because it's so fast, right? Like you can't just like turn on a flashlight, run over there and see when the light arise. Like, it moves faster than anything can move. So it's hard to, like, you know, beat it or coordinate the measurement of it. So, like, how did the people in ancient times even approach this question? I think in ancient times they had a different attitude, right? They were not empirical. They were not of the mind to go out and discover things in the universe by doing experiments.

They were much more internal. They thought they could understand the universe just by thinking about it. You know, they had lots of crazy theories about the way things work. theories that could be easily disproved in like an afternoon of experimentation. You know, Aristotelian physics really doesn't make any sense if you do any experiments. So they really didn't even try to measure the speed of light. They mostly just like talked about it and thought about it. It wasn't until about 500 years ago when this concept of like, oh, let's go out and measure things in the universe.

Let's try to see if our theories actually work. This concept of empirical science came about that people really started to actually try to measure things. And the earliest recorded measurement I could find was from Galileo about 500 years ago. Well, do you think maybe people didn't try before because they just had no means to do it? You know, they didn't have accurate clocks or ways to measure things. They certainly didn't have the means to do it. If they had tried, they definitely would have failed. But, you know, I don't think they even thought to try.

Like, I think it's hard to take your mind out of the current modern concept of science where we learn from the universe by doing experiments. That's a fairly new idea. Aristotle, for example, had this idea of how things fall. And he thought, for example, if you were on a boat that was moving and you dropped a ball while you were on the boat, that the ball would somehow get left behind, that it wouldn't just, like, move with the boat. And like, if you just went out with a boat and a ball, you could disprove this idea in an afternoon, right? Unless it's windy.

Unless it's windy. Then that it misses you up. Yeah. And Galileo, like thousands of years later, proved that this was false and literally overturned all of physics with a boat and a ball in about 10 minutes. So these guys weren't limited by their physical capabilities. They just were limited by the idea that you should go out and actually measure stuff. What can you do with a boat in a baltane? Galileo used it all up, right?

Like early Nobel Prizes, that's all the equipment you needed. Oh, I see. The fruit was hanging lower before. Yes, exactly. Nobody had even tried to measure stuff back then. So you said Galileo tried to measure the speed of light. And how did he do it with lanterns? Yeah, he had lanterns and he put them about a mile apart.

And he tried to time it. He had the most accurate clocks you could back then, and he tried to, like, you know, shine a lantern and then measure how long it took to get from one spot to the other with two coordinated clocks. And, you know, he failed to notice any difference. He couldn't measure any time between when the lantern was revealed and when the information arrived at the other side. Interesting, like he synchronized two clocks or two watches, and then he had one watch, like, go a mile away. And then he said, okay, I'm going to turn on this lantern. and when you see it turn on, you record the time. And they came back and said it was the same time.

Exactly. They couldn't measure any difference, right? They couldn't tell the difference between light being super duper fast, but finite speed and light actually being infinite speed. And the reason is that the delay, like how long it takes light to go a mile, is just 11 microseconds. And so to measure that, you need clocks that are more accurate than 11 microseconds. And 500 years ago, he definitely did not have that.

So did he conclude that speed was infinitely fast? or that he just didn't, he couldn't measure it. It was too fast to measure. It was too fast to measure. And these days what you would conclude from an experiment like that is you would measure a minimum speed. If you knew how accurate your clocks were, you could say, well, light is at least as fast as some number.

He just said, well, I don't know. It's either infinite or it's very, very fast. Right. He didn't have the right clocks. But then later, people had better clocks. People had better clocks, but actually the first measurement of the speed of light being not infinite It didn't come from using a very fast clock. It came from using really, really long distances.

Right, because that's two ways to kind of slow speed down, right? Either give it a long distance to go over or use a really accurate clock. Yeah, so a Danish astronomer about 100 years later, he realized that light bouncing off of Jupiter's moon Io could be used to measure the speed of light. What? Yeah, because Io orbits Jupiter, right? It's a moon of Jupiter. goes around, it takes like 42 and a half hours to go around Jupiter. And when it comes around the back of Jupiter, it emerges from the back of Jupiter and you can see it. So if you're watching

Io from Earth, then you see it emerge from behind Jupiter every 42 and a half hours. And so that's sort of like a clock for the universe, right? It should happen every 42 and a half hours because Io's orbit is very regular. It's a little bit more complicated because either it can be in Jupiter's shadow or it can be physically behind Jupiter. But let's put that aside for now. Oh, I see. Because you can actually see the moons of Jupiter if you have a telescope from the 1600s, right? You can see the little dot and you can see the little dots

kind of floating around it. Yeah, and Galileo was the first person to see these. And so with a pretty basic telescope 500 years ago, you can see these dots and you can plot the trajectory of them and you can see like, okay, IOS coming out from behind Jupiter. And people watch these things and look for patterns.

And they notice something really interesting. They noticed that it's true that IO comes out from behind Jupiter every 42 and a half hours, but that during some parts of the year, that time is a little bit shorter, and other times of the year, that time is a little bit longer. So like the time between Io emerging from behind Jupiter gets longer during one season and shorter during other seasons.

And somehow that tells you the speed of light? And that tells you the speed of light, because the reason those times get shorter is because the Earth has now gotten closer to Jupiter and I.O. than it was last time. And so the light doesn't have as far to go to get to Earth. And the reason the times between the reappearances get longer is when the Earth is moving away from Jupiter. So now light has further to go when it has to reach Earth to tell you that Io has emerged. If speed of light was infinite, then I.O would always appear every 42 and a half hours.

This wouldn't matter at all. But because the distance between Earth and I.O. is growing or shrinking, then this period grows or shrinks. And so this Danish astronomer realized, oh my gosh, I can use this information to calculate the speed of light. Wow. Interesting, right? Because sometimes we're in between measurements of when you see the moon, the Earth will have moved. Is that what you mean? Like sometimes it moves a lot in that time and sometimes it doesn't move a lot. Exactly. And so when we are moving further away from Io during that part of the year that we're like zooming away from it, then those times between Io's appearances will get longer. And when we've come around the other side of the sun and we're zooming towards Io, then we're shortening the distance that light has to go. Like if you were making these measurements from the sun, Sun where the distance between you and I.O. wasn't changing, then they would be perfectly regular. Or if the speed of light was infinite so that every time I.O. came around the back of Jupiter, you instantly saw it, then the measurements would be regular. But since the distance is changing and it takes a finite time for light to cross that distance, then you can measure

how fast light moves across these incredible distances. I guess the distance is changing and it's changing in a kind of predictable way so you can measure the speed of light. And so what did they find? they get pretty close to what the actual speed of light is? He got to within about 20% of the real speed of light, which is pretty incredible. 20%. Yeah. That's like a B. B minus.

Yeah, well, it's much better than Galileo did. Galileo got an F. So at least this guy is passing. Well, technically, I don't know, because Galileo thought it was pretty fast. Yeah. But I love these stories where you've, like, tricked the universe or cornered the universe into revealing some piece of information. This guy didn't build this experiment.

he discovered this experiment. He's like, wait a second, this random configuration of stuff reveals this piece of information everybody wants to know. And all I have to do is use my telescope and calculate a few numbers and boom, now I have this number. Interesting, but did he know like the relative positions of the planet in order for him to know exactly like how much more the Earth had moved? Like, do we know the orbits that well back then?

We didn't know the orbits that well back then, but actually all you have to know is the orbital. distance of the Earth. You just have to know the radius of the Earth's orbit because that's the difference between the path of light when the Earth is furthest away and when it's closest away. So you can look up the calculations online, but you can do it with some pretty basic information about the orbits. Wow. I imagine you could do it today, right? Like if you just had a nice backyard telescope, you could measure the speed of light to a 20%. You could get a B, a solid B, in your backyard. Absolutely you can. And I was chatting with one of our listeners, Brian

Field, who's a theoretical particle physicist, and he said he did this lab in college, and he actually sent me his write-up. And so it's the kind of thing you can now assign to undergraduates in physics, and they can totally extract this basic constant of the universe using simple tools. And everyone gets a B in the class. Then what was it the next step in measuring the speed of light? So the next step was improving on Galileo's strategy. Rather than doing astronomical measurements, there was a guy in the 1800s named Fizzou, who sent a beam of light further away. So Instead of one mile, he sent it five miles, so he was trying to measure a longer time distance. He had this really clever trick for measuring really short time periods.

He put a beam of light that passed very close to a gear that was rotating. So imagine a gear like the one you have on your bicycle, it's got little teeth on it. And as it spins, light can go through sometimes when it's not blocked. And then when it hits the gear, when it hits the tooth of the gear, then it's blocked. And if you arrange things just right, then light flies. through between the teeth, hits a mirror, comes back, and then flies through the next tooth. And so if you arrange things just right, then the light can make it there and come back and not be blocked. And if you're going at the wrong speed, then it's going to hit one of the teeth either

on the way out or the way there. And so you can arrange things just right to get the right speed in the right distance so you can get light to go there and back and not miss a tooth. And this is a way to measure how long it takes light to go there and back. If you know like the rotation speed of your gear. Wow. This sounds pretty tricky in advance. I guess a big question is, how did they get the light to go five miles,

bounce off a mirror, and come back, and still be sort of like, you know, legible? Like, they didn't have lasers back then, did they? They definitely did not have lasers back then. Like five miles is a lot, right? Like any beam of light, if it's a little foggy or something, it won't make it five miles and back.

That's true. But, you know, they did have optics and they had powerful lenses. Even Newton was studying lenses. And so they had ways to concentrate beams of light. But yeah, that was definitely a challenge back then, making a powerful enough beam of light. But, you know, light can go pretty far. So you need like a clear night, right? This would be an experiment would be better to do in space because you say light will scatter

off of the atmosphere. But you only need a few photons also. I see. So they had some sort of like focused beam of light, I guess, but they didn't have electricity. So they must have used like candles or fire. Yeah, that's a really good question. You know, I read a few descriptions of this experiment and they all just say the light source. So I wasn't able to figure out what the actual source of light is. So either some time traveling physicists lent them a laser or they like really focused beams of light from the sun or maybe like early electricity allowed them to generate really bright bulbs or aliens. That's always a possibility. The simplest explanation first. That's also an experiment that like people can do in their backyards, right? Kind of. Yeah. If you have

a rotating gear that's very precise and a five mile long backyard, then yeah, go for it. That's right. If you're a billionaire and live in a estate, anything's possible for you. That's right. Right to us, we'll send you a kit for $1 billion for measuring the speed of light. And then I imagine that we've gotten much better at these kinds of measurements. And so let's get into those and our current understanding of what the speed of light is. But first, let's take another quick break. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers. The pilot is having an emergency and we need someone.

anyone to land this plane. Think you could do it? It turns out that nearly 50% of men think that they could land the plane with the help of air traffic control. And they're saying like, okay, pull this, do this, pull that, turn this. It's just... I can do my eyes close.

I'm Manny. I'm Noah. This is Devin. And on our new show, no such thing. We get to the bottom of questions like these. Join us as we talk to the leading expert on overconfidence. Those who lack expertise lack the expertise they need to recognize that they lack expertise. And then, as we try the whole thing out for real. Wait, what?

Oh, that's the run right. I'm looking at this thing. See? Listen to no such thing on the Iheart radio app, Apple Podcasts, or wherever you get your podcasts. A foot washed up a shoe with some bones in it. They had no idea who it was. Most everything was burned up pretty good from the fire that not a whole lot was salvageable. These are the coldest of cold cases, but everything is about to change.

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to finally solve the unsolvable. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Hey, sis, what if I could promise you you never had to listen to a condescending finance, bro, tell you how to manage your money again. Welcome to Brown Ambition. This is a hard part. when you pay down those credit cards, if you haven't gotten to the bottom of why you were

racking up credit or turning to credit cards, you may just recreate the same problem a year from now. When you do feel like you are bleeding from these high interest rates, I would start shopping for a debt consolidation loan, starting with your local credit union, shopping around online, looking for some online lenders because they tend to have fewer fees and be more affordable. Listen, I am not here to judge. It is so expensive in these streets. I 100% can see how in just a few months you can have this much credit card debt when it weighs on you. It's really easy to just like stick your head in the sand. It's nice and dark in the sand. Even if it's scary, it's not going to go away just because you're avoiding it. And in fact,

it may get even worse. For more judgment-free money advice, listen to Brown Ambition on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy. Hi, I'm Danny Shapiro, and these are just a few of the profound and powerful stories I'll be mining on our 12th season of Family Secrets. With over 37 million downloads, we continue to be moved and inspired by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes with you, stories of tangled up identities, concealed truths,

and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests for this new season of Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we are measuring the speed of light and can do it in your backyard if you're a billionaire. Yeah, you can actually also do it in your kitchen these days. Oh, no kidding. Wow. I have a five mile long kitchen. You don't need a five mile long kitchen. Actually, all you need is a microwave and a chocolate bar, and you can measure the speed of light at home in about 20 seconds.

No kidding. How does that work? Well, light, of course, is a wave. And so if you know the frequency of the wave and you know the wavelength, You can combine those two pieces of information to get the speed of the waves. And so people do these now in very high-end experiments using cavity resonances. The most precise measurements of the speed of light we have sort of in experiments come from these cavity resonance experiments where you measure the wavelength of the light and you measure its resonant frequency. But you can also do a simpler version of that at home.

You just take a chocolate bar and you put it in the microwave. You microwave it for about 20 seconds. Not enough so it's totally melted, but enough that it's just started to melt. And take it out and you'll notice something. You'll notice that it's more melted in some places than in others. There's like hotspots. Interesting. And that is basically the shape of the wave of light, of the microwave light?

That's right. The distance between those hot spots is one half of the wavelength of the microwaves. Because that's where they've like added up concretely to give you like the most energy. And so what you're seeing there is like the actual physical wavelength of the photons, passing through your chocolate bar. And it's like a few centimeters. So it's something you can reasonably measure using a chocolate bar in your microwave.

And then all you have to do is look up the frequency that your microwave uses. Usually it's like two and a half gigahertz or something. Combine those two numbers together and boom, that's the speed of light. Interesting. But I guess inside the microwave, isn't it bombarded by microwaves from all directions?

Is the wave inside of my microwave that, like, coherent, that untouched, that sort of neat? Yeah, unfortunately it is, right? And that's why you have hot spots and cold spots. We have a whole episode about how microwave ovens work. And usually they have like one source of the radiation. And so it puts this stuff out in this kind of pattern where you get this constructive and destructive modes. It'd be much better if it was like incoherent and evenly distributing the energy, which is why you usually have like a spinner to move your food through this field of microwaves. So they use a sort of a simpler radiator and it has these features to it. I guess the tricky part, though, is measuring the frequency of the light wave, right?

Because, I mean, that's like gigahertz. You don't really have a clock that can measure that. You'd have to trust the microwave manufacturer. Yeah, it's sort of cheating because they've done the hard part for you of measuring the frequency. But it's also a cool thing to, like, physically see the impact of light being a wave, to like to see the distance between the crests of the light wave in a physical thing that you can do in your kitchen. That's sort of cool. But you're right.

When we make the actual measurements, like when we make the actual measurements, like when we're we actually want to figure this out ourselves, then we use very precise cavities and we measure the resonance frequency and the wavelengths of the modes simultaneously because you can't just look that stuff up. And is it required that you have to eat the chocolate afterwards? Because then you're cheating not just the universe,

but you're diet a little bit. No, that's the bonus of doing physics, man. Sometimes you make a delicious experiment. That gets a little messy. All right, so nowadays we use much more like constraint environments, I guess, a cavity and you have a wave of light and you know exactly what the frequency is and you can sort of see the wavelength and that gives you one measurement of the speed. Like nowadays, we don't really like do these experiments where we send it off to one place and then measure how long it takes

to come back. We use something like this. Yeah, it's much more precise to use interference effects or resonance effects because they're very, very sensitive to very small shifts in one wave to the other. And so the way the cavity resonance experiment works is you measure both the wavelength, and the resonant frequency. You build some precise cavity and that determines the wavelengths of like standing modes inside the cavity. You have like two mirrors essentially

and you want light to go back and forth between those mirrors in a way that it doesn't cancel itself out. You need light to have a wavelength so that an integer number of those wavelengths adds up to exactly the width of the cavity. So there's only like certain modes of the cavity where you can get this sort of effect. And then you just measure the resonant frequency

like at what frequency of light, what color of light do you get these resonances? So you can measure the width of your cavity and measure the frequency, the color of light that goes in there that achieves resonance and together you can get a very accurate measurement of the speed of light. And that was like 1975 that people really perfected this and got like super duper precise measurements of the speed of light. But I guess doesn't that depend on how accurate your clock is to measure the frequency of light and also how good your ruler is to measure the of your cavity, right? Like, there are still, I guess, imagine, limitations to how well we know the speed of light. There were still limitations for just those reasons. Like people used crazy

techniques to measure the size of these cavities very, very accurately. And it's like a real tour to force of experimental physics, the clever strategies people came up with to measure these precisely. These days, however, we have actually zero uncertainty on the speed of light. Zero uncertainty. Like we know it to an infinite number of digits? Yeah. So it's a bit of a hop-out answer, right? We don't know the speed of light to an infinite precision if you set an arbitrary length for the meter and an arbitrary length for the second. Instead, we've decided we're going to use the speed of light to define length. We're going to say, we know this better than anything else. So let's define everything else in terms of the speed of light. So now the official

definition of a meter is no longer like, here's a platinum rod in Paris. Instead, it's how far light travels in a certain amount of time. Right. Because I guess you're saying that if we picked a valley for the meter and a valley for the second, then that makes the speed of light that we measure kind of dependent on what we pick for the meter and the second. So instead, it makes more sense maybe from a global point of view to define the meter and the second by the speed of light. Yeah, so we define the meter by how far light travels in a second, and we define the second by the oscillations of some cesium atom.

So now the meter is something which depends on the speed of light. and the oscillations of the cesium atom. So now instead of asking, like, how well do we know the speed of light? It's like, well, how well do you know the length of this platinum rod in Paris? Oh, I see. So you're kind of saying almost like we're coming up with the speed of light. Like we're inventing the speed of light, right? Because we just pick some numbers and then we call that the meter.

So therefore the speed of light is so-and-so meters per second. Yeah, and it's just like in particle physics. We could define the speed of light to be one, and everything is set relative to that. And so here we're defining the meter. so that the speed of light is exactly 299792-458 with no decimal places. Like it's exactly that number. And, you know, I said we know it accurately. Really, we just define it to be that.

And everything is now relative to that number. You just kind of blew my mind. It means we don't know what the speed of light is. Right? Like technically, philosophically, you're trying to say that we don't know what the speed of light is. We just picked the number and said that's it. We picked the number. We said this is what we call the speed of light.

the speed of light is a number, right? And we just assign to say it's this number, this length. And now the question is, what does length mean? Length is relative to the speed of light. It's just as good as saying length is relative to this rod in Paris, but this rod in Paris has no real meaning or physical significance. So it's sort of silly, whereas the speed of light obviously does. And so it makes a lot more sense to define things relative to the speed of light

rather than relative to an arbitrary chunk of metal. But I think by using a number that doesn't have any decimal places, right, you get that to be the meter and then you use that meter to measure the speed of light, but then the number it gives you was the consequence of you picking that random number. Yeah, well, you can't measure the speed of light anymore. You're exactly right. It doesn't make any sense to define the meter in terms of the speed of light and then trying to measure the speed of light.

Like you can't measure the speed of light in terms of the meter because the meter is defined in terms of the speed of light. It's circular. Instead, what you can do is define the speed of light and then measure the length of a rod in Paris in terms of that. Why anybody would care the length of a rod in Paris? I don't know, but philosophically, that's what you can do now. But I feel like that's kind of like you're avoiding the question.

Like there is a speed of light. Like there is a certain amount of distance that light covers in one second in the universe. But it doesn't seem like we know what that is to any sort of decimal place. Well, we don't know what that is relative to that stick in Paris. You're right. And we could spend a lot of time and money measuring how fast light goes relative to this arbitrary unit of distance we defined according to the stick in Paris. but I think people decided that doesn't mean anything anyway. Like, what does it matter how many decimal places you get

when your unit is arbitrary? We prefer to make a reasonable unit one that makes sense. And the speed of light is the most important physical constant in the universe. And so let's just define everything relative to that. Right, but then you're picking an arbitrary number for that speed. Yes, absolutely. I don't know. I guess it makes sense to me as a layperson to pick an arbitrary length

and then measure the speed of light, then to pick an arbitrary speed of light, and then define lengths from that. Well, it's philosophically they're equivalent. You know, check out our episode on the basic constants of the universe and you'll realize that no number that has units on it ever has any meaning because it just depends on your definition of the units. The only numbers that really have meaning are the ones without any units,

the ones that are pure numbers of the universe. So the speed of light in that sense is not actually that fundamental. It folds into the fine structure constant, which is a unitless number. and which does determine sort of the structure and the nature of the universe and electromagnetism. But I guess, you know, like a rod in Paris is something we can all go and touch and see and like hold, right?

And then we can all agree that the speed of light goes so and so fast. But I feel like this way of doing things, like nobody can agree with the speed of light is. Everybody can agree. We just choose a number. Whereas a rod in Paris, it like grows and shrinks. When it gets hot in Paris, does that change the speed of light? Like, it's ridiculous to have the speed of light

depend on something so arbitrary as how big this rod in the museum in Paris. It's like the air conditioning breaks in Paris and now we're all moving faster. Like it doesn't make any sense. Yeah, why not? I mean that's better than like making up a number for the speed of light. Daniel, I can't

handle this. All right. Well, welcome to Daniel and Horhead discuss philosophy. I'll give you that. That's the way you're doing it, even though I don't agree with it. All right. Objection noted. Yeah, thank you. I'm sure it'll cause waves in the physics community. So that's kind of the way we're doing it.

That means that we can't measure the speed of light, right? Once we've defined it, we can no longer measure things in terms of it, yes. So to answer this question is how do we measure the speed of light? We don't anymore. We just pick the number. We just picked the number, yeah. And that number was based on almost nothing, right? Well, that number, you know, defines a meter to be something close to what it used to be.

And so that's pretty nice. But it could have been something else. We're just to pick that number out of historical reasons, kind of, to approximate historical history. Yeah, we wanted the new meter to be pretty close to the old meter so that we didn't have to like change everything, all the, you know, make all new highway signs like, oh, this tunnel is now one meter high, whereas it used to be 20 meters high. That would be ridiculous. So once again, laziness, yes. Consistency, man. Consistency. Nap consistency is very important to physicists. I'm getting the sense. But yeah, it seems like the answer is you can't measure the speed of light anymore, right?

because now we've defined the meter as based on this number of the speed of light. So it makes no sense to measure the speed of light. It's just what it is. Yeah, that's true. We can no longer measure the speed of light relative to other arbitrary units because it is now the arbitrary unit. Can we measure it relative to some of these fundamental unitless constants that you measured? Like, you know, the universe has these numbers that are immovable and fundamental to the fabric of the universe. Can we use those to get a real measurement of the speed of light? No, because those numbers don't have units.

And so they can't determine numbers that do have units because those depend on your choice of units, right? That's the problem with numbers that have units. Anything that's like meters per second or pounds per square inch or whatever is going to depend on the units, which is why physics prefers to talk about numbers without units. But I guess we base time on some sort of fundamental physical thing, right?

Like the oscillations of a crystal or whatever. Why can we do that with distance as well? We do. We do. That's exactly what we do. And that's exactly what. Not a rod in France, but like, I don't know, the width of a proton or something like that. Well, think about it as a certain number of light wavelengths at a certain frequency.

Because we've defined the meter in terms of the speed of light. Now light is our ruler. But then that means we can't measure the speed of light. That's true. Yeah, we've given that up because now it's our ruler because we've decided that that's exactly the most basic unit. Just like you can no longer define how long it takes cesium to do one oscillation. because it's defined to be one second or it's defined to be one, you know, six billionths of a second or whatever. Because we now define time in terms of that basic physical operation, you can no longer measure how long that operation takes.

But couldn't you say like let's measure the speed of light by the frequency of cesium and also the width of cesium? Yeah, you could define distance using something else. But, you know, it's not as fundamental as the speed of light. The speed of light is really basic and interesting to the universe. So I think that's why they chose it. But you're right. These things are arbitrary. And you could have said, you know, the meter is now defined to be one-third of the height of Jorge's room.

Like, you could have chosen anything. Some choices are better than others, you know, and I think this is a pretty good one. Let's just pick a number. It seems like a crazy way to run the investigation of the universe, Daniel. We're doing our best, man. We're doing our best. I know it's not your fault. You have limited power in the physics community.

All right. Well, I've melted the chocolate bar in my mind. I feel like I don't know what to try. trust anymore in physics, Daniel. Things are arbitrarily fast now. There is no speed limit. There's just the speed limit that you're telling me is the speed limit. That's right. Go out there and break whatever speed limits you want, Jorge. You're right. Absolutely. There are no rules when it comes to you. Well, I think it's another kind of reminder, you know, that this is a tricky universe. You know, it's kind of hard to measure things because everything is relative. Everything can change. Everything

depends on kind of, you know, how fast you're moving or how hot it is or what you're measuring relative too. So it's kind of hard to find footing in this universe. It is, but it also gives us a sense for how the universe works. And I think it's awesome how we as humans have figured out how to extract this kind of information from the universe. I mean, until we define it away as not interesting anymore, I think it's fascinating to see sort of like the historical sweep, how long it takes, how the thousands and hundreds of years it takes to figure out this one very basic thing about something we literally see every day. And as a reminder, please obey speed limits in your driving practice because going at the speed of light might get you a few light

tickets. But if you do manage to go at the speed of light, please let us know. We'd like to hear about it. But what if they interrupt your nap, Daniel? It'll be worth it. It'll sit in your inbox for 10 seconds before you check it. All right, well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. I'm Manny. I'm Noah. This is Devin.

And we're best friends and journalists with a new podcast called No Such Thing, where we get to the bottom of questions like that. Why are you screaming? I can't expect what to do. Now, if the rule was the same, go off on me. I deserve it. You know, lock him up. Listen to No Such Thing on the IHeart Radio app,

Apple Podcasts, or wherever you get your podcast. No Such Thing. I'm Dr. Joy Harden-Bradford, host of the Therapy for Black Girls podcast. I know how overwhelming it can feel if flying makes you anxious. In session 418 of the Therapy for Black Girls podcast, Dr. Angela Neal-Barnett and I discuss flight anxiety. What is not a norm is to allow it to prevent you from doing the things that you want to do, the things that you were meant to do. Listen to Therapy for Black Girls on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Your entire identity has been fabricated.

Your beloved brother goes missing without a trace. You discover the depths of your mother's illness. I'm Danny Shapiro, and these are just a few of the powerful stories I'll be mining on our upcoming 12th season of Family Secrets. We continue to be moved and inspired by our guests and their courageously told stories. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast. Thank you.