Daniel and Jorge Explain the UniverseDaniel and Jorge talk about the history of how we measured this fastest of all velocities!
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What's that Mango?
I've been trying to write.
A promo for our podcast, Part Time Genius, but even though we've done over two hundred and fifty episodes.
We don't really talk about murderers or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then of course can you game a dog show? So what you're saying is everyone should be listening. Listen to Part Time Genius on the iHeartRadio app or wherever you get your podcasts.
Hey, it's Horehand Daniel here, and we want to tell you about our new book.
It's called Frequently Ask Questions about the Universe.
Because you have questions about the universe, and so we decided to write a book all about them.
We talk about your questions, we give some answers, we make a bunch of silly jokes.
As usual, and we tackle all kinds of questions, including what happens if I fall into a black hole? Or is there another version of you out there that's right?
Like usual, we tackle the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the book and get a copy of not just for yourself, but you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters.
And for the aliens. So get your copy of Frequently Asked Questions about the Universe. It's available for pre order now, coming out November two. You can find more details at the book's website universe faq dot com. Thanks for your support, and.
If you have a hamster that can read, please let us know. We'd love to have them on the podcast. Hey, Daniel, I have a question about the physics of the Internet.
Oh that sounds like fun.
Like what is the speed of email? You know? How fast does it travel? If I write you an email, when does it get to you?
You know, I think email might actually violate the laws of physics.
What do 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 actually 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 am Jorge, my cartoonist and the creator of PhD comics.
Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm 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 mic.
No way, Wow, is it extra? Kind of like it has extra pops and extras to go a little bit higher in your falsetto 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 at 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 iHeart Radio.
Which is sort of like Thriller, and 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 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 to 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, actually moving the progress of science backwards. We're walking backwards on the 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 we're handshakingclittery 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 and B. 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, 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 U SEEI sometimes takes ten 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 be 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.
You speak us 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 crisis 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 one 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 whittle 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 its speed of light to the forest 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 particle physicists is the wrong person to ask about the actual value of the speed of light in sensible units. But we do define it as two hundred and ninety nine million, seven hundred and ninety two thousand, four hundred and fifty eight meters per second. So that's the exact value of the speed of light. Then most people when they do calculations just say three times ten to the eight meters per second.
Right, or like three hundred million meters per second. But it is a very specific number, right, It's like two nine nine seven ninety two four five eight, and like four or five nine is too fast. You can't go that fast.
That's right. It's a no on four five nine, four five eight and a half No, four five eight point one. No, there's no flexibility there, there's no negotiation. This isn't Hollywood where 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 loop quantum gravity is true and space really is a big quantized foam bubble, then maybe this tells us about how those foam 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 a maximum speed limit of the.
Universe and how did we figure it out?
So to the end the podcast, we'll be asking the question, how do we actually know what the speed of light? Is that? 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 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, Thaniel.
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 fried 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 faster.
You try, I mean like a credible try, not like a far off by by ten.
That's some 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 band there, Like you can add energy, there's more kinetic energy in these particles, but they're not moving much 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 the protons approach it. Right. We have these limits in physics all the time where you can see something as approaching a limit and it asymptonically 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 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 for some dark twists, a little of both. So, as usually, 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 Danielanjorge dot com.
So think about it for a second. 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 timestamp.
And see 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 had they tested it, so then a particle accelerator.
I think, galileayar, you 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 mats. 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 like 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 shone 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 fast. Like we talk about the speed of light being three hundred 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 how far light goes in a very short amount of time, rather than in a full second. I actually think about light as 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 thirty centimeters. For those of you on the metric system.
I totally reject that that way of looking at that. First of all, it's English units using feet instead of meters, and also a nanosecond 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 ten emails in a nanosecond.
So oh wow, you live in another reality, it seems.
No, No, obviously not. But you know, when you're talking about like 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 ten feet away, so you know it's going to take ten nanoseconds to get from here or 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 three hundred million meters is about.
How 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, one hundred milliseconds. How long can the light travel in those one hundred milliseconds.
Well, you know, one hundred milliseconds is only a tenth of a second, right, and so a tenth of a second would be thirty million meters, So it's still pretty far that like can go in a tenth of a second.
Right, thirty million meters is about thirty thousand kilometers, right, which is about twenty thousand miles.
Yeah, that's right, thirty million meters is just under twenty thousand 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, like 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 extents and 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 five thousand 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 leave 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 in 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 if 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 a big question is like, how do we know it's this particular number, right or not?
Yeah, Well, that's 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. It's just something we have to go out and measure.
Like it could have been another number, right, It could have been three or a bie.
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. It 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. It 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 does have 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 if you like that, because light is technically nothing. It has no mass, and nothing can go faster than it is.
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.
So, 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.
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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.
Well, there you go.
That's the joy of particle physics units. The answer is always one or E or pi in particle physics. That's why, always one of the three. But yeah, like we 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 that 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, Aristatilician physics really doesn't make any sense if you do any experiments. So they really didn't even try to measure the speed of life. They mostly just like talked about it and thought about it. It wasn't until about five hundred 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 five hundred 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 learned 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 yeah 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 ten 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 and a ball?
Tame Galileo used it all up right, like early Nobel prizes. That's all the equipment you needed.
I see the fruit was hanging lower before.
Yes, exactly. He even tried to measure stuff back then.
So he said, Galileo, try to measure the speed of light? Then 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 in 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, we 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 eleven microseconds, and so to measure that unique clocks that are more accurate then eleven microseconds and five hundred 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 the people had better clocks.
People have better clocks. But actually, the first measurement of the speed of light being not infinite. 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 one hundred 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 orbit's Jupiter, right, it's a moon of Jupiter, and it goes around It takes like forty two 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 forty two and a half hours, and so that's sort of like a clock for the universe, right, it should happen every forty two 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 telescope from the sixteen hundreds, right, you can see the little dot and you can see the little little dots kind of floating around it.
Yeah, and Galileo is the first person to see these, and so with a pretty basic telescope five hundred years ago, you can see these dots. You can plot to trajectory them. 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. I notice that it's true that Io comes out from behind Jupiter every forty two 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 gets shorter is because the Earth has now gotten closer to Jupiter and Io 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 Io would always appear every forty two and a half hours. This wouldn't matter at all. But because the distance between Earth and Io is growing or shrinking, than 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.
Whoa interesting right, because sometimes we're in between measurements of when you see the moon, the Earth will have moved. Is that what it mean? 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, where the distance between you and Io wasn't changing, then they would be perfectly regular. Or if the speed of light was infinite so that every time Io 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 you're 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 twenty percent of the real speed of light, which is pretty incredible.
Twenty percent. Yeah, that's like a B B minus.
Yeah, well it's much better than Galileo. Did you know Galleo got an F So at least this guy's 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, did 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 twenty percent. You could get a bee. I saw a bee in your backyard.
Absolutely you can. And I was chatting with one of our listeners, Brian Field, who is 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 eighteen hundreds named Fizzou who sent a beam of light further away, so instead of one mile, he sent it five miles ailes. So he was trying to measure a longer time distance. He is 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 like gear like the one you have on your bicycle. It's got a 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.
WHOA, this sounds pretty tricky in advance, I guess the 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 only did not have lasers back then.
That's just 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 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 it like a clear night, right, This would be an experiment It 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 does this 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. 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 an estate, anything's possible for.
That's right right to us. We'll send you a kit for one billion dollars 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 in our current understanding of what this speed of light is. But first, let's take another quick break.
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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 twenty 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 inexperiments 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 for about twenty 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, hmmm.
Interesting, and that is basically the shape of the wave of light of the microwave light.
That's right. The distance between those hotspots 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 see 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 you know 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 hotspots and 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 the 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 gigaherds. 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 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 actually want to figure this out ourselves, then we use very precise cavities and we measure the resonance frequency and the wavelengths of the mode 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 your 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 then 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 experiencements 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. Are you build some precise cavity, and that determines the wavelengths of like standing modes inside the cavity. Right, do 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 the 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 nineteen seventy five 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 size of your cavity, right, Like, there are still I guess I imagine limitations to how well we know the speed of light.
There were still limitations for just those reals, Like people used crazy techniques to measure the size of these cavities very very accurately, and it's like a real tour de 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 cob 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 in 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 in 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.
Yes, so we define the meter by how far light travels in a second, and we define the second by the oscillations of some caesium atom. So now the meter is something which depends on the speed of light and the oscillations of the caesium 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 picked 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 two nine nine seven nine two four five eight 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.
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 assigned to say it's this number this length. And now the question is what is length? To 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 meeting 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 use that meter to measure the speed of light. But then the number gives you was a 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 carry 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. There is a speed of light, like there is a certain amount of distance that light covers in one second in the universe, 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 this 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 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 more sense to me as a layperson to pick an arbitrary length and then measure the speed of light than to pick an arbitrary speed of light. Then define lengths from that.
Well, it's philosophically their 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 what 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 is 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 Jorge discussed philosophy.
I'll give you that. That's the way you're doing it, even though I don't.
Agree with the dam 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 kind of can measure the speed of light.
Right, once we've defined it, we can no longer measure things in terms of it.
Yes, So need to ask you this question is how do we measure the speed of light? We don't anymore. We just picked the number.
We just pick 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 twenty meters high. That would be ridiculous.
So once again laziness.
Yes, consistency, man consistency, NAP.
Consistency is very important to physicists and 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 you know, fundamental to a 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 you know, I guess we base time on some sort of fundamental physical thing, right, like the oscillations of a crystal or whatever. Why can't we do that with distance as well?
We do. We do, That's exactly what we do, and that's 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 defined the meter in terms of the speed of light. Now light is our ruler.
But then that means basically, 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 a caesium to do one oscillation because it's defined to be one second, or is to find to be one, you know, six billions 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.
What can you say, like, let's measure the speed of light by the frequency of caesium and also the width of caesium.
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 munication of the universe datum.
We're doing our best to do their 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 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, or hey, 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 hundreds of years it takes to figure out this one very basic thing about something we literally see every day.
And as the 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 ten seconds before you check. 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. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digesters to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.
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