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