Daniel answers Listener Questions about how to read science headlines, gravitational slingshots, lorentz symmetry and the speed of light!

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Speaker 1: When I checked the news and I see some story trending about a crazy news science discovery, like NASA discovers a parallel universe or Chinese scientists teleport matter into space. These are real headlines. But when I see these, I feel a couple of things at the same time. First, of course, excitement. I love science because it has the potential to teach us crazy new stuff about the universe, to blow our minds and show us that reality is different from what we imaginement, or the things we thought were impossible can now be done. But the second thing I feel is skepticism. There's a lot of clickbait out there where journalists have taken an interesting study and given it a bonker's headline just to get eyeballs. And some days the headlines are real. You know that scientists actually have taken pictures of black holes and discovered new particles and landed robots on the surface of distant moons. So there's no shortage of ways to get your mind blown for real. Hi, I'm Daniel, I'm a particle physicist and I'm always looking for ways to get my mind blown. And Welcome to the podcast. Daniel and Jorge explain the Universe a production of I Heart Radio, our podcast in which we explore all the amazing and crazy things about the universe. We love the bonkers new ideas about how the universe might work, and we unpack what we do know already about how the universe do us actually work And this podcast, curiosity is the driving force, and we let it run wild. We ask crazy questions about the nature of the universe, and then we bring it home and try to explain all of it to you. Now you'll notice that today on the podcast it's again just me Daniel, my friend and co host Jorge can't be here today, so I'm taking the opportunity, as I do occasionally, to catch up on listener questions. We mean it when we say we want to answer every question from listeners because we think everybody's curiosity is valuable. If you are wondering something about the universe, we want to help you figure it out. Everybody out there that's thinking deep thoughts about the way the universe works and the way tiny particles fit together is doing physics, and physics is fun and physics is awesome, and we want to encourage it and we want to enable it. So on these podcasts. When Jorge isn't here, I dig into our backlog of questions from listeners, real people like you, thinking about the universe and sending in their questions. A lot of times these questions are not just hey, man, tell me about black holes. They're more like, I've been thinking about this thing and I don't quite understand it. Can you help me figure it out? Or I've been googling and reading articles about this topic and none of it makes any sense. And that's our job to make these things explainable to you. So if you have questions that you'd like answered, please send them to us two questions at Daniel and Jorge dot com. We answer every email, we respond to every tweet, and we will get to your question, we promise. And if you don't like writing tweets or emails, you could also come to my public office hours. I'm a professor at a public university, and i think it's important to be available sometimes to the public. So I'm hanging out on zoom answering questions from anybody and everybody. Check it out. You can go to sites dot u, c I, dot du slash Daniel and you find information there about my next upcoming public office hours where you can come and ask a physicist any question you like about the universe. I won't offer relationship advice. On today's episode of the podcast, we're doing even more listener questions. That's right, and today's questions are super fun. It's deep fundamental physics, it's particle physics, it's how to get around the solar system. But first, it's about science headlines. So here's a great question from a listener, dan An wal Hey, I know that science headlines are often sensationalized. So when I see a headline, what are some things that I can look out for when evaluating the paper behind or the article alongside the headline. Thanks. This is a great question because we should all be informed and educated critical readers of science journalism. Remember that the goal of science journalism is to educate. They want to take work that scientists have done and explain it to the general public. But also they have an interest in entertainment, in splashy stories to get to to clip on their headline, to get you to read the article, to get a little bit of attention. So it's a great idea to develop some tricks, some tools, and I'm gonna give you some tips into how to read science journalism and know whether it makes any sense. Now, first of all, if it seems like a crazy, big deal, then you'll read about it in lots of places, and you'll read about it in places with a good reputation. So, for example, if you hear that NASA has discovered a parallel universe, whoa, you should see that as a huge headline in the New York Times and in other places. Otherwise you might start to suspect this is not really something which is a scientific consensus or has really penetrated deep into the community. And the idea of a scientific consensus is really important. Any scientists can make some claim and even maybe even write a paper and maybe even get it published, but for a big idea to really be accepted has to be accepted by a broad segment of the science community, people who don't necessarily have an interest in getting that paper published, who just want to dig into the truth. And so the most valuable thing you can look for when you're reading coverage of a new science result is whether there are discussions or quotes from other scientists not involved in the study, but experts in the field reacting to it. You'll see this in the best science journalism, and they'll say things like we asked professor Michelle Blah blah blah, who is not involved in this study but as an expert on the topic, what she thought, and if she says, Wow, this is groundbreak and this is revolutionary, this is a huge leap forward, then you know this is really something to get excited about. But if it's mostly just parroting the claims from the people who did the science and wrote the paper, then that doesn't necessarily mean it's wrong. It means that it hasn't received the same level of review of other experts in the community who don't have the same interests. So that's my number one thing, is to look for quotes from other scientists in the field who are not involved in the study. And it really it comes down to trust, because often you can't digest the science in these articles. I read science journalism very broadly, and there's lots of topics I don't know more than the science journalist about neuroscience are all sorts of crazy stuff, and I'd like to believe them. So what I've done is trying to develop a set of trusted sources, meaning people or magazines whose articles seem credible. For example, there's a magazine called Quantum Magazine, which I really like, and every time I read an article in that magazine that's about my field, I find it well written and fair and accurate. So that allows me to evaluate it. I think, well, they do a good job. They hire good science journalists who actually dig into it and try to represent these results fairly and not in a sensationalist way. And so I trust the articles in that magazine, even when they're not in my field. It's earned my trust. And you might find this about particular journalists, people who's writing you like and who develop a credibility with you, and you can look to them and say, well, if this really is such a big deal, what is my favorite journalist? Can changing in the New York Times, for example, say about this? Or maybe you'll discover that another journalist always blows things out of purports, and so when you see an article by that person, you disregarded. So you have to develop sort of a network of trusted sources, locations science journalists you trust, and also look to see that they have asked other people for their opinion. And the last thing is you know if you see a really big headline, ask yourself why you never heard of this before if it's such a big deal. Sometimes it's not the answer that they're blowing out of proportion, but the question, like, yeah, maybe the scientists have accomplished something. It's just not that significant or not as interesting as everybody said. So not that the experiment didn't work, but that they didn't achieve what they were set out to do. But maybe what they set out to do isn't actually that important. I mean, it's not like you've heard necessarily of people trying to do that for years in the past. In contrast, you know there are other things that you might be aware of, so that when you hear about progress in them, you understand to be impressed. Like the moment that a computer first beat a human world champion at chess. That was a big deal for the world because people have been working up to it for decades of sort of a long standing challenge. Or when people really did walk on the moon that was something everybody acknowledged was important and hard, and so when it was achieved whow we could all be impressed. Or when we discovered the Higgs boson it had been a decades long search and been sort of in the cultural zeitgeist already. People knew it was something to look for, saying with pictures of black holes. So when you see a result that makes a big claim about something you never heard of before, you have to wonder if maybe the question itself is getting blown out of proportion. All right, I hope that was helpful, but I think it's great. Read science journalism, get some trusted sources, and look to see if those sources are asking other scientists not involved in the study. All right, but let's get back to our bread and butter, which is answering science questions. So here's a really fine question about navigating the Solar System and maybe protecting the Earth. Hello, Daniel and Hall, Hey, I have a question. What's the sling shots affect and how does it work? Do we use it to our advantage with space probes? And could we ever use it to deflect asteroids or even planets? Thank you? All right, what a fun question. I love this topic. This is about gravitational slingshots or gravitational assists, and the basic idea is using the gravity of a planet or of the Sun to help navigate the Solar System. Without spending as much fuel. Remember that fuel is expensive, not just because it costs money to make the fuel, but it costs fuel to bring fuel. Every pound of fuel that you want to bring on your spacecraft, if you're on a mission out to Neptune, for example, requires you to bring more fuel in order to push that fuel along with you. So fuel needs fuel to bring it, and the mast fuel needs more fuel, and pretty quickly it gets crazy. So what you really want to do is minimize the amount of fuel you need to bring. It's expensive and it blows up very quickly, requiring more and more fuel just to bring that fuel along. So the idea is if you could somehow get your spaceship to change directions or to pick up speed or even slow down somehow to navigate the Solar System without using fuel, then you can save cost and it's also a lot simpler. And that's the idea of a gravitational slingshot or gravitational assist. You're using the gravity of a planet or the gravity of the Sun, either to change the direction of your spacecraft or to speed it up or to slow it down. So you might wonder, well, how does that work? Right, Well, let's think about it for a minute. You know that if something is falling towards the Sun, it's going to get sped up. Imagine a comet, for example. It's falling in from the outer Solar system, speeding up. As it gets towards the Sun. It does a quick whip around the Sun, and that's the moment when it's at its top speed. It's started in the very outer Solar system, moving very slowly, but it's fall and in towards the Sun and it's picked up speed along the way. So when it whips around the Sun is going at very very high speed, very small distance from the Sun. These are very elliptical orbits, not like the Earth's orbit, which is mostly a circle and the Earth mostly goes in the same speed all the way around. A comet is a very elliptical orbit, so it's very slow when it's far from the Sun, and it speeds up a lot, and then it whips around super fast around the back of the Sun. But here's the thing. After it whips around the Sun, then it starts to slow down because now it's climbing away from the Sun. Right now, it's slowing down, and so that when it gets really far away again it's now slow, so it's sort of stable. It speeds up and it slows down, and it speeds up and it slows down. So the question is, how do you use that to change the direction of your spacecraft, Because you don't just want to swing by a planet, speed up while you're by the planet, and then slow down again. Then there's no point. What you want to do is accomplish some overall speed up. How can you do that? It turns out it is possible, and if you do it in just the right direction, then you can actually steal some of the energy from that planet. Say, for example, you're going to the Outer Solar System, which is really far away, so you want to get there before all of your human scientists have perished waiting for you to reach it. So you need a little bit of a speed up, for example, and Jupiter is on your way to going to study Neptune, so you can use Jupiter to help you speed up. Well, how do you do that? So there's two ways to look at it, from the point of view of the planet and from the point of view of the Sun. Now, from the point of view of the planet, it's just like we talked about before. The satellite is approaching, you're pulling on it. It speeds up and whips around, and then it gets shot off in another direction, but at the same speed as it approached. Right, it speeds up and then it slows down. So from the point of view of the planet, there's no change in the velocity and no speed up. You sped it up as it approached, but then you slowed it down as it left, so that doesn't seem like a wind. But that's if you look at it from the point of view of the planet. If you look at it from the point of view of the sun, you notice something interesting. Not only has it whipped around the planet, but it's changed direction. Right, it comes in one direction and it comes out and another direction. Now if it's new direction is also in the same direction the planet was moving, then now it's velocity relative the Sun is actually bigger than it was before because now it's velocity relative to the Sun gets added to the planet's velocity. So maybe it was perpendicular to the planet's velocity before, Now it gets added to the planet's velocity, so it's actually going faster relative to the Sun. And it's done this by stealing a little bit of the energy from Jupiter. Yeah, that's right. So for example, if you have a one ton spacecraft and it whips around Jupiter and it gets sped up by a kilometer per second, which is a pretty big speed up for a spacecraft, then that slows down Jupiter, but not by a lot because Jupiter is so massive. Jupiter is so huge it hardly notices it slows down by ten to the minus twenty five kilometers per second. See a swing around Jupiter. You get a little bit of speed up from the point of view of the Sun, and Jupiter slows down a tiny bit from the point of view of the Sun. This isn't a big deal until we get the big numbers, like if we wanted to send it ten to the twenty five satellites to use Jupiter for a gravitational assist, then we might actually have some impact on the orbit of Jupiter, but hey, who cares anyway. Another way to get this clear in your head is to imagine what would happen if you were standing on the platform at a train station bouncing a tennis ball up and down right and a train comes by, moving really really fast. If you decided to throw that tennis ball at the train. It's gonna bounce off the front of the train and come back the other direction, and it's gonna be going faster because now it's added to the train's velocity. If you threw it at ten meters per second against the train, it's gonna bounce off at ten per second against the train from the point of view of the train, but on the train platform, that ten per second gets added to the speed of the train, and so now it's going even faster and it's slowed down the train a tiny little bit. If you throw a tennis ball against the front of a train, you are by very tiny little bit slowing down that train. So this is very helpful for speeding up without using any fuel or just changing directions, you can also slow down. Like if you swing around Jupiter and you end up going the opposite direction of Jupiter's motion, you could end up with a smaller velocity relative to the Sun. Imagine, for example, you were able to change your direction so you were moving the opposite of the way that Jupiter was moving and with the same velocity. Then with respect to the Sun, you would have no velocity. You would be stationary. Right, So a change in direction relative to the planet can be a change in velocity relative to the Sun. And this is pretty awesome, right, but it also has limits. You can't just say, hey, Jupiter, I need you to be over there at exactly this moment so I can sling shot around you. You have to use the planets where they are and when they are, so when they make these plans, you might have to spend like a whole year orbiting the Solar System waiting for a planet to be just in the right place. So these gravitational systs can be cool, but they can also add years and years to missions because it takes a long time for the plans to just be in the right place. There was this awesome event in the seventies when all the planets were in perfect alignment to use one slingshot to the other and slingshot to the other and slingshot to the other to get all the way out to the outer Solar System. This is the Grand Tour of the Solar System, and it's not going to happen again for at least another two hundred years. So that's why they sent the voyager probes out in the seventies because there was this perfect alignment so the voyager pros basically slowed down every planet between here and Neptune. But hey, it was worth it. They got some beauty full pictures. Now, this is a great history. It was first used in ninety nine when the Soviet probe Luna three took pictures of the far side of the Moon and used the Moon as a slingshot. And then we've done it a lot of times since Cassini. It passed by Venus twice, and then Earth and then Jupiter. Even before reaching Saturn, the messenger probe did a fly by Earth and then twice past Venus and then three times past Mercury, so that it could arrive at Mercury with just the right velocity to enter the atmosphere without having to do a lot of burns. And you can even use the Sun itself as a gravitational syst. Now, you can't change your velocity relative to the Sun, but it can change the velocity relative to the center of the Milky Way. So if you wanted to go from our solar system to another, that's what you'd have to do. So you could use it and take advantage of the Sun's pretty quick motion around the center of the Milky Way to change your velocity with respect to the center of the galaxy and maybe find your way to other stars. And as the listener asked, you could also use the same sort of technique to help deflect an asteroid. This is called a gravity tractor when you use it in a way to try to change the direction of the object itself. So remember we talked about how doing a slingshot past Jupiter would change the direction of Jupiter, but it wasn't really a very big effect. Well, that can be a big effect if the object is smaller. So we're talking about like a five kilometer rock that's heading towards the Earth. That's big enough to kill all of humanity if it strikes the Earth directly, but it's small enough that if you did a gravitational assist around and you could change its trajectory. And the key thing to saving humanity from incoming asteroids is spotting them early so they only need a tiny little nudge. If you knew that an asteroid was heading towards the Earth but it was still really far away, it would only take a tiny little nudge for it to miss the Earth. It's sort of like hitting a arget where a high powered rifle really really far away. The difference between hitting it and missing it is a tiny change in the direction you point the gun. So if all we need to do is change the direction is after it a tiny little bit and it's not that hard. What you need to do is send up some heavy probe and send it around the asteroids so it deflects it gravitationally, right, Or you could even just have it hang out near the probe and have it constantly tugging on it with its gravity. Gravity super duper weak. But again, you only need a really small deflection. So gravitational assists are a great way to explore the Solar System, to steal energy from planets, to change directions, to speed up to slow down, to help navigate the Solar System without having to pack a lot of extra fuel. Thanks for that great question. I want to answer some more listener questions, but first let's take a quick break. All right, we're back, and this is Daniel hosting today and answering listener questions from the backlog. I've promised to get to every single listener question and i will honor that promise, and I'm using these episodes when Jorge isn't around to catch up on our backlog. So far, we've been talking about reading signs, headlines, and gravitational assists. But here's another question about some recent experimental work. Hey, Daniel and Jorge, I was wondering if you could explain what Lawrence symmetry is, what happens if it can be broken in the search for potential Lawrence symmetry violations. Thanks, all right, that's a great question, and I happen to know that came from a listener whose roommate was working on this question and she wanted to understand it more deeply. So let's get into it. What is Lawrence symmetry. Well, Lawrence is a famous Dutch physicist and when the Nobel Prize in nine two, and he was around during the pivotal time when relativity was being developed. And that's what Lorenz symmetry is really all about. It has to do with how we see the universe and how the universe might look different to different people, people who are moving at various speeds or people who are sitting in different locations. So what Lorentz symmetry actually says is that the same laws of physics, the ones that we know gravity and motion and electromagnetism and all those things. The laws of physics all apply to all observers at every location, moving at constant speed. So no matter where you are in the universe and what speed you're moving at, you should be able to look around you and see that everything seems to be following the laws of physics. And you and I should agree. If I'm here and you're at Alpha Centauri, we should be able to look around us, and everything and we see happen should follow the same laws of physics. We shouldn't have to change the laws of physics because of where we are in the universe. And that's also true if I'm moving towards you. If I'm in the spaceship and I'm moving at half the speed of light towards you and your vacation home in Alpha Centauri, I should still be able to look out my window and use the same laws of physics to observe the universe and describe what I see. Even if I'm moving relative to you, you and I should be able to use the same laws of physics. But that's only true if I'm moving relative to you at constant speed. If I'm accelerating. If I'm speeding up or if I'm slowing down, then things get a little wonkier. So we'll dig into that in a moment. But Lorentz symmetry essentially is that it says that the same laws of physics apply for all observers moving at any constant speed relative to each other. It doesn't mean we all see the same thing. It means we could all use the same laws of physics to describe what we do see. All right, So let's dig into that a little bit more. I mean, this seems pretty reasonable. We think there should be only one set of laws of physics that describe the universe. That's sort of the whole goal of physics, right, is to find one set of laws that describes everything. You wouldn't want a set of laws which were dependent on location. So why is it the Lorentz symmetry only holds if you're moving at constant speed? Right? This requirement that you have inertial observers. And the reason is that if you're not moving a constant speed, if you're accelerating, then you do see different forces at play. For example, if you are in an elevator and that elevator is in space, but it's accelerating, right, it's speeding up. Then what are you going to feel. You're going to feel a force from the floor of the elevator, right, You're gonna feel the force of the floor of the elevator pushing up on you. It's almost as if there's a force of gravity. Right. Somebody in an elevator that's moving a constant acceleration sees the same physics as somebody who's standing on the surface of a planet and feels the force of gravity. And that's different physics than somebody who's just floating in space. We're moving a constant velocity. If you were in a spaceship moving a constant velocity, you wouldn't feel any gravity, you wouldn't feel the floor pushing up on you. You would just be floating, waitless in the middle of your spaceship. So those people have to use different physics to account for what they see. The person who's accelerating feels a new force when they can't otherwise describe. It's as if they were standing on the surface of a giant planet pulling down on them. So when they do their calculations, they have to add this new force to describe what they see and somebody else, and a spaceship that's not speeding up, that's moving at constant speed doesn't have to add that force. So that's like a different set of laws of the universe. That's why we only talk about lorn symmetry being relevant to people moving at constant speed, because if you do have some acceleration, that creates a fictitious force, an apparent force. The same thing is true if you're moving around in a circle. Right, say you're on a merry go around for example, somebody spins. Spinning moving in a circle is also acceleration because it's changing the direction of your velocity. You're going around the merrygor round, you're pointing in one direction. Later you're pointing in another direction. So if your direction of your motion is changing, your velocity is changing, that's acceleration. And what do you feel when you're on a merrygor round, Well, you feel this weird fictitious force that's trying to throw you off the merry go round. Is that a real force? There's no particle, there's no field that's creating. It's not a fundamental force of the universe. It's a fictitious force because you are in a non inertial reference frame. Because you are rotating, you are accelerating. So that's why Lorenz symmetry talks only about inertial observers, people who are at rest or moving relative to each other with constant velocity. So what do we mean when we say you use the same laws of physics? Because we know we've seen enough special relativity examples to know that people moving at very high speeds see things differently from each other. Like if you get on a spaceship and go really really fast, close to the speed of light, but it constant velocity, and I had a telescope and I can look at a clock on your ship. I'll see your clock moving slowly because moving clocks run slow. But if you're on the ship and you look at your clock, you see it running normally. Right, So you and I see different things when we look at the universe, even if we don't have any relative acceleration. So how can we say that observers are all using the same laws of physics? Because there's an important distinction between using the same laws of physics and seeing the same thing. We can see different things happening, but still have them be described by the same laws of physics. Here's an example that I think helped Einstein clarify what was going on in his mind as he developed relativity. Think about a single electron floating in space. What does it do? Well, electrons have a charge, so they have an electric field, right, an the electric field doesn't change. It's floating in space no velocity relative to you. Now, say your friend comes by and she's in a hurry, so she's moving really fast past you. What does she see, Well, she looks at this electron, and according to her, the electron is moving. Right. If she's moving relative to you and the electron is floating in front of you, then she's also moving relative to the electron, which means the electron is moving relative to her. Now, what does she see? She sees a charge in motion. That's a current, right. Electric currents are just charges in motion, and electric currents can create magnetic fields. So what does she see She sees a magnetic field. You see an electric field. She sees a magnetic field. Right. So you see different things, but both of you agree, yes, the laws of electromagnetism are working. You see different things, but you use the same laws to describe what you do see. And that's the beautiful thing about Lorenz symmetry is it says you might not have the same observations, but you can use the same rules to describe what you are seeing. So Lorenz symmetry is really, really deeply woven into the very foundations of physics. Is something we assume is the basis of special relativity. It's basically the same thing as special relativity. If Laurent symmetry was violated, then special relativity would be wrong somehow, and we don't think that it is. It's been tested out the wazoo and into the wazoo and around the wazoo, and nearly the speed of light. We're pretty sure special relativity is correct. People are looking for violations of this. One way they do this is they look to see if the speed of light changes as you move. There's the famous Michaelson Morley experiment that showed that the speed of light is the same in two different orthogonal directions, even though the Earth is in motion around the Sun and the Sun is the motion around the center of the galaxy. And that tells us that the speed of light is uniform no matter who is measuring it and what their speed is. But that's an experimental result, and so people have been trying to improve that. They've got this precision down really, really, really fine, so there's no variation in the measured speed of light down to like one part and ten to the seventeen, which is an incredible virtuoso experiment. People also do crazy stuff like bounce lasers off the Moon. You know, when astronauts went to the Moon, they left a mirror on the surface of the Moon, so we could do cool experiments like shoot laser beams at the Moon, not to blow it up, of course, but just to measure the flight time. And this helps us understand the speed of light and also actually the distance to the moon. The speed of light is so well known that we can use the time that a laser takes to get to the moon, bounce off that mirror and come back as a measurement of the distance to the Moon, which happens to be increasing every year by about a centimeter. So people are looking for violations of this in the way that light moves, basically looking for violations of special relativity. But there are also other deeper ways that we study. Lorenz symmetry. Lorenz symmetry is very closely connected to a symmetry in particle physics called C P T. Each letter there's stands for one symmetry. C is for charge, PS for parody, T is for time, and the combination CPT means all three symmetries. And what CPT symmetry says is that if you take a particle physics experiment and you flip the charge of the particles involved, so like from positive to negative, and you flip the parody, you like take it in a mirror and do the mirror inverse experiment, and you flip the direction of time. So instead of doing it forwards, you try to do it backwards. That the experiment should look exactly the same, C, P and T together should all be conserved. Now we already know that parody is violated. We have a whole fun podcast episode about how parody is violated in the weak force and how that was discovered. Then later people discover that CP is violated the combination of charge and parody. So if you flip in the mirror and switch particles to antiparticles, you still get some violations. But people think that CPT is preserved and the reason they think it's preserved is that it's required by Lorenz symmetry. So if you see a violation of CPT somehow, that would undermine all of modern physics because it would imply that Lorenz symmetry is also violated. Now, there was an experiment recently that claimed to violent Lorenz symmetry. The opera experiment at certain claimed to have sent neutrinos from Certain to Italy at faster than the speed of light, which would be a violation of special relativity and a violation of Lorenz symmetry. Now, those headlines were pretty impressive, and when I read those, I thought, WHOA, this is kind of a bonker's result. And it was pretty skeptical when I read that, because I didn't see a detailed analysis from anybody else who was not involved in the study. And so pretty quickly when other folks were not part of the opera experiment, dug into the details and started asking questions. The opera folks discovered Oops, they made a mistake and a cable hadn't been plugged incorrectly, which led to a wrong calibration constant, which led to a mismeasurement of the speed of their neutrinos, and turns out their neutrinos were just ordinary neutrinos traveling at just under the speed of light, not just over the speed of light. So so far, nobody's ever seen a violation of learn symmetry or CPT, and as far as we know, it's a symmetry about the universe. Then, no matter where you are in the universe and how fast you are moving, you can use the same laws of physics to describe everything that you see, which is pretty cool. Thanks very much for that awesome question. I hope that helped you understand it. I have one more question I want to get to, but first let's take another break. Alright, we're back. This is Daniel and I am answering questions from listeners. Folks who had a question about the universe and roote me an email and then sent me some audio with their questions so you could all hear are their questions. And I've chosen these questions because I suspect that other folks out there like you, might have the same questions and might be interested in hearing the answers. So here's our last question of the day. Hi guys, my name is George Ray. I'm from southern Ontario and I have a question for you. It's regarding the speed of light. Is it possible that in the past the speed of light was faster or slower? Uh? And if so, how do we know that? Thanks? By the way, I love the podcast. All right, what a wonderful question. This question touches on so many cool things about the universe. One thing that we've seen in physics is that we have these equations that describe the universe. They say how things relate to each other. But those equations have numbers in them, Like the equations for electromagnetism have some numbers in them that tell us how fast electromagnetic information moves. That's the speed of light is determined by these numbers in those equations. And every time we see numbers in the equations, we wonder why this number, why not another number? Could it have been a different number? Is that this number for a reason? Is it random? Could have been any possible number? Or is there some deeper theory of physics that explains these numbers, connects these numbers to other numbers we see in other equations. So the question you're asking, is this number the speed of light always been this number, or has it, for example, changed with time is a really deep and fundamental question in physics, and it's the kind of thing we really drill into. It's also really fun and important because the speed of light affects a lot of things in the universe. Right, the universe seems really really big, and one reason is not just that stuff is far away, but that it takes a long time to get from here to there. Like, it doesn't really matter how many billions of kilometers you are away from other stars. If you could go super duper fast, then you could get there in a day or in a half a day, it wouldn't matter. But there's this speed limit on information of the universe, which of course also applies to starships and your travel, which means things are effectively really far away because of this limit of the speed of light. So it makes us wonder is it possible for it to change? Could it change in the future. Now, The first thing to understand is that the speed of light is not actually one of the fundamental numbers we talk about when we talk about the parameters of the universe. You know, the things in the sort of universe control panety might dial up or down or change. And the clue to knowing that the speed of light is not fundamental is that it's the number with units on it. Right, it's three times tend the meters per second, which means it's relative to other things like the definition of the meter and the definition of the second. In particle physics, for example, we use different units. We use units where the speed of light is just one, so that we can erase it from all of our equations, because as otherwise, we're writing the speed of light all the time and calculating big numbers. Imagine doing a little thought experiment to see if you would notice if the speed of light changed. Right, Say, for example, you change a meter to be a tenth of a meter, so to change the whole scale of the universe, and then also change the speed of light to match, and you change the gravitational constant, which sort of affects how far apart things are in space where they balance away from each other. So you could change all of those numbers and you wouldn't notice anything. The universe would seem the same to you because those numbers are all relative to those units and to each other. So what we've done in physics is isolated the numbers that don't have any units. We take all the numbers that we can find, the ones that are connected to each other, speed of light, gravitational constant, all these other numbers that do have units, and we divide them against each other and multiply until we get numbers that have no units. These are numbers that we can't change just by changing our units, or by scaling the universe up or shrinking it down by changing the length of a meter, right, and so these are the ones that really would control the nature of physics. And for example, one of them is called the fine structure constant. It's a weird name. It comes out of the early days of quantum mechanics when we were understanding how atomic orbitals work and where electrons were and how much energy they had. But basically it's a relationship between the speed of light and planks constant and the electric charge and this number alpha. The fine structure constant really does determine sort of the way the universe looks. You can't change the fine structure constant without changing the physics of the universe. It's inescapable if you change the speed of light and planks constant and the electric charge in such a way to keep the fine structure constant constant. Then you wouldn't notice any different Maybe universe would really be bigger or really be smaller, but then so would we, and so we wouldn't notice any difference. So that's the key. You have to find the parameter that actually does make a difference, the one that would change physics as we see it. And it's not the speed of light. It's the fine structure constant one of the other several parameters. We actually have a whole podcast episode about what are these fundamental parameters and which ones are really important in the universe. So there are these fundamental parameters to the universe. There's this whole list of them, and if you change one of them, you would change the way the universe worked. Speed of light not technically one of them, because you can tweak other parameters to accommodate for a change in the speed of light and not change anything else. But imagine for a moment if you just change the speed of light or the speed of light had been changing on its own, could you tell any difference. Well, there is one way that we can tell how the universe worked in the past, and that's because we can see the past. It's like out there in space. The finiteness of the speed of light keeps us from exploring the universe, but it also means we can look back into the history of the universe, because light that was created a long time ago with just now arriving at Earth, if it came from really, really far away. So as we look deeper out into space, we see further into the past. And we can't conduct experiments in the past, but we can see experiments in the past. We can find them. We can watch things happening in the past, and we can ask are these described by the same laws of physics that we know now. Can we understand galaxy formation and star formation and all the stuff we see happening in the past in terms of the same laws of physics, or do we need to change something like the speed of light? And so far, all the things we see in the past are very well described by the speed of light as it is now. There's no evidence that the speed of light has been speeding up or going down in the past, and we would see that happening because it would change the way things work, It would change how quickly things move, It would change how rapidly gravitational information was propagated. All sorts of things would be changed if the speed of light changed, and so far it seems like it hasn't, but there are some nuances to that it might be possible, for example, to reinterpret what we see not in the way we imagine it now, but as a change in the speed of light. For example, some people really really don't like the ideas of cosmic inflation. The ideas of the universe grew very, very rapidly in the early universe. In the first few moments, is stretched by ten to the minus thirty seconds. This crazy stretching of space, a stretching of space that actually happened faster than the speed of light. It doesn't violate special relativity because it's a stretching of space, not a motion through space, and that's an important technicality. But some people still don't like this con spt and they wonder if instead maybe it was just that the speed of light was much much faster. There are these theories of physics, these alternative theories that are kind of fringe theories. They're variable speed of light theories that try to explain what we see in the past, not in terms of cosmic inflation or expansion, but instead in terms of a change of the speed of light. And you know, you can always take the same data and fit another theory to it, but then you have to ask, how does that theory look? Is that theory really work And the problem with these theories, these variable speed of light theories is, frankly, that they violate special relativity. They violate Lorenz invariants, and so that makes us not like them very much. We really do believe in Lawrence invariants. We've tested it out the wazoo. Now it is potentially possible that in the early universe things were really different and Lawrence invariance wasn't as respected. But we have reasons to believe in Lawrence in variances. It's not just like an article of faith. It comes out very simply from the mathematics and from looking at the structure of space itself. We talked once about Nurther's theorem, which is this idea that all symmetries are connected to conservation laws, and in this case we think that Lorenz symmetry is connected to translation symmetry and rotation symmetry. The fact that everywhere in the universe seems to be similar. There's no special location in space. It's not like there's an origin at the center of the Sun or the center of the Milky Way that's different from any other place. No matter where you put your zero zero on your axes, physics should work the same. So that's a pretty basic assumption about the way the universe works. To get rid of that, to toss that out the window would mean tossing out the window a lot about what we understand about the universe. That doesn't make it impossible, but it makes it a big pill to swallows. You need very clear evidence. It's much simpler to say, well, we think everywhere in the universe is the same, and Lorenz symmetry makes a lot sense to us, and the speed of light is a fixed number. Everything sort of clicks together and works very very well. You can describe the universe using other theories, but they don't click together as well. They're not as nice, they don't have the same beautiful symmetries. They're more complicated, and so we tend to favor the one that we have now because it works so well. We don't need variation of the speed of light to explain what we see, so to summarize, and we don't think the speed of light has changed. Actually, you could change it without noticing anything in the universe if you conspire to change a few other fundamental constants. The ones you really should be thinking about are these constants without units, the dimensionless numbers, the pure numbers that control the universe. But because we don't know why the speed of light is what it is, there's nothing necessarily determining its number. It is potentially possible that it could have changed in the early universe, but we don't see that. We look out into the data and we see that the universe is described by the same laws of physics with the same speed of light. So it all fits together with us. But thank you for asking such a deep and fun and wonderful question about the nature of the universe and whether it is what it is, and whether that's what it always was. Thank you to everybody out there who's been thinking deeply about the universe and wondering how things work, and asking questions when they don't make sense. So please keep thinking, keep being curious, keep asking questions, and write to us two questions at Daniel and Jorge dot com because we will answer all of your listener questions. Thanks everyone, tune in next time. Thanks for listening, and remember that Daniel and Jorge explained The Universe is a production of I Heart Radio or more podcast from my heart Radio. Visit the I Heart Radio Apple Apple Podcasts, or wherever you listen to your favorite shows. No