Daniel and Kelly’s Extraordinary Universe - Can you outrun a flash of light?

Episode Date: July 27, 2021

Daniel and Jorge talk about whether its possible to outrun the fastest thing in the Universe Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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Hey, Daniel, what's the fastest a person has ever? run. Well, Usain Bolt, the fastest person in history, actually only runs about 25 miles per hour. But what's the fastest anything has ever gone on Earth? That's about 763 miles per hour in a crazy rocket-powered car. That sounds like a good idea. But what about in space? What's the fastest a spaceship has ever gone? There's the Parker Solar Probe, which is whizzed around the sun, and it reached about 5% of the speed of light. All right, so then technically nothing humans have ever made has ever approached the speed of light. Except for actual light from flashlights. Oh, well, but technically we made the flashlight, but then the flashlight made the light.

We want our names on the plaque as well. Yeah, can we label each photon? Made by Jorge. Hi, I'm Jorge. I'm a cartoonist and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and the creator of many photons. Oh, really? You have a certain glow about you. Is that what you're saying? Do all physicists glow? I think usually that's not a good idea if you're working with radioactive things. Especially if you come from Los Alamos, then you do tend to glow in the dark.

But no, every single one of us gives up a unique pattern of photons that we personally have crafted. My photons are artisanal. Oh, yeah. Hmm. Like you individually craft each one. Yeah, they're not very good. Isn't that what artisanal means? Yeah, I guess what you mean is that you glow like in the infrared, like from our body heat. Yeah, I definitely do give off photons. Like every object in the universe, if you have a temperature, then you glow. Sometimes it's not in the visible light, but you definitely are pumping out photons into the universe, even if you are a black hole. Well, Danny, you are a shining example, I think, for all of us.

And welcome to our podcast, Daniel and Jorge Explain the Universe, a production of IHeart Radio. in which we try to shine the light of knowledge into your mind, pushing back on the forefront of ignorance and talking about all the biggest questions in the universe, questions about the universe, questions around the universe questions in the universe and questions from the universe. And we talk about all of it and try to make jokes to entertain you. Yeah, because there is a lot in the universe out there that is bright and interesting and powerful and curious and exciting.

And there are also dark things out there that just escaped our sight. That's right. we can only see a little fraction of the universe, an unknown fraction of the universe, because information comes to us usually via light, and that light travels at a very, very fast, but not infinite speed. And we have learned all sorts of crazy things about how information travels through the universe. Yeah, because the universe is not just filled with mysterious things, it's also sort of filled with mysterious rules.

The rules of the universe are not quite what we experience in an everyday basis, and some of them kind of go against. our intuition about how things actually work. And thank God for that, or my job would be boring. I think about science as like a grand mystery novel. We're trying to figure out how things work and deduce what the rules are from the little clues left here and there. And you're absolutely right that along the way we have figured out that the universe is not

the one we thought we were living in, that the rules are weird, that the rules are strange, that they require real creativity to unearth how things actually work. Yeah. Are you saying the universe is not cliche, thankfully? It's got, it's a twist and turn. I've seen this universe so many times before. Oh, my gosh. Bang, crunch, bang, crunch, bang, crunch.

Hey, do you think every time there's a Big Bang and a Big Crunch and we come back with the same universe, that would be boring. Yeah, obviously it was made by committee, the Big Bang. Let's just reboot the last universe. That's sold pretty well. Yeah, right? Audiences don't want anything new. People don't want to exist in a new universe. This is the new JJ Abrams Big Bang theory.

Oh, a dig on JJ Abrams. I feel like you're always looking for an excuse there. Hey, man, he created the whole universe. You know, I got notes, but it's pretty impressive. Wait, JJ Abrams created the whole universe? In this theory, yes, there is a JJ Abrams equivalent, you know, writing the simulation or guiding the direction of the universe. That would explain all the lens flares and that I see all the time when I look at the universe.

But yeah, it is a weird universe with weird rules. And there is none weirder than special relativity, which is kind of one of our main theories about the universe. That's right. It turns out that when you start going really, really quickly, the rules that work at slow speeds down here on Earth no longer apply. The rules are actually quite different. And for hundreds and thousands of years, we have been learning rules that only work under special conditions, very, very slow movement. If you actually push the boundaries and start going fast, the universe reveals that things we thought were true are not actually true. These are all sorts of weird consequences that really violate our intuition. They break our notion of like simultaneous. and the idea of time being universal. Right. Yeah.

Do you think like if we were all moving faster, we would be used to these strange special rules and like our current rules would seem weird? We would have to be moving really, really fast. But I think in that scenario, our current rules would just be a natural extension of what we already understood. But it's hard to imagine growing up in a universe where special relativity feels intuitive, where it makes sense to you, where our conclusions about the universe would seem strange.

But I'm sure somebody's written that science fiction story. Oh, or maybe not. It's all there for you, Daniel. Somebody out there write it. But it's called special relativity because it sort of applies to special situations. Why does it have that special need? Special relativity is called special because it's not general relativity. It applies to scenarios where space is flat.

You don't have to think about space actually being curved and like changing the path of photons or changing the motion of Earth around the star. It has to do only with a sort of simpler scenario where space. is mostly empty and you're like shooting laser pulses back and forth or you have light bulbs on trains. It has to do with relative velocities of things and how information moves through the universe. It avoids all complications from curved space. Oh, really? Wow. I never knew that. I always thought that it was called special relativity because it was special. But actually, you're sort of saying the opposite. It's special because it only applies to a boring universe.

Yeah, it's a specialized condition, right? You came up with this first to understand this and he thought, well, you know, it's much trickier if space is actually curved. And then he'd just, generalized it. He made it much more broad. Special relativity is a subset of general relativity under the conditions that basically the universe is mostly empty. He hadn't figured that part out yet. So it's not like special because people think it's like it. It's special. It's just special because actually it's like specially boring relativity. That would be technically more accurate, right? Yeah. It's a special case. It's not a better case. It's a simple case. But we often do this in physics. We think about simple scenarios to help us like distill what's going on and get like a

clearest picture so we can separate these ideas because even in the scenario where you have no big masses distorting the shape of space like black holes or even just the sun there are lots of weird things that happen in special relativity you know clocks that don't agree because you're moving at high speed it's pretty weird and hard to get your mind around even if there is no curvature of space so i think it was sort of a good intellectual exercise but even though it applies to boring situations it's still true right like it still applies to the whole universe it still applies to the whole universe. The rules of special relativity do assume that there are no heavy masses. And so if you have big masses around curving space, you can't use calculations from special relativity. You've got to use

general relativity. But I guess then the irony is that special relativity is actually especially boring relativity, but it gives rise to these really strange situations about the universe that don't seem to make sense. And a lot of that has to do with light, experiments involving light. You know, There was this famous experiment by Michelson and Morley that showed that the speed of light doesn't change no matter how fast the Earth is moving through space, for example. That no matter who measures a photon, they always measure it moving at the speed of light, no matter how fast that person is actually moving relative to anything else. It's pretty weird stuff. Yeah. And so this weirdness is sort of maybe especially illustrated by asking a very simple question that you can ask about the speed of light.

So today on the podcast, we'll be asking the question. Is it possible to outrun a flashlight? First of all, do flashlights run? Do they have little legs that I don't know about? That reminds me the old joke about refrigerators. Is your refrigerator running? No, you better check the power. Your flashlights don't actually run, of course.

But the photons from them come out at a blistering speed, right, of three times 10 to the 8 meters per second. So it's pretty hard to imagine outpacing a flashlight. So it's more like, is it possible to outrun a flash of light from a flashlight? Yeah. I think the scenario I'm imagining is you take off at your highest speed and I'm standing behind you with a flashlight. Is it possible that once I send off a flash of light that you could outpace it,

that that flash of light would not catch you? Like it would not shine on me. Like the photons would never hit me. Yes, exactly. Or from your point of view, is it possible you could run fast enough that you could look backwards and you could not see me, right? That I would be past some sort of horizon. beyond which you could not see.

I see. Like, I would never know that you'd turn on the flashlight because those photons would never reach me. Like, I'd be running and be like, ah, Daniel still hasn't flipped on. But you did, but the light would never reach me. And why are you running away from me so quickly? What did I do?

I don't know. Why are you shining a light on me? What are you trying to do? Blind me? I'm just playing flashlight tag, man. Chill out. So this applies to that 90's toy laser tag. Yes, exactly.

Exactly, exactly. This helps you strategize for laser tag. This is special relativity laser tag. Yeah, physics is useful for all kinds of situations, even ones that require you to travel to the 90s and play laser tag. I knew this degree would come in handy someday. Well, first you have to solve time travel. I'm working on it. I'm working on it. All right. So then that's the scenario we're asking, is if I take off running and you shine a flashlight at me, is it possible for me to outrun those photons or will they inevitably hit me at some point in the future? So that's a pretty interesting question. And so we thought we'd post it to people on the internet and see what happens.

So as usual, Daniel went out there and asked listeners if they thought that one could outrun a flash of light. And so as usual, I'm immensely grateful to all of you who wanted to participate in the podcast and answer these weird and random questions. If you're out there and you're listening to the podcast and you've been itching to participate but you haven't quite yet, please send me a message to questions at Danielanhorpe.com. Think about it for a second. Do you think it is possible to outrun a flashlight? Here's what people had to say. I don't think that I can. I'm not faster than light, so I don't think that I could do that. I'd like to believe that you can outrun a flashlight,

but for that you have to be faster, traveling faster than the speed of light. And as far as I know nothing in the universe can travel faster than the speed of light. Yes, definitely. Just don't throw it too hard. so you can, you know, run faster. I think light from a flashlight travels. It speeds much less than the speed of light in a vacuum. It's slowed down by the materials in its way as it moves from inside the flashlight to outside of it.

And then there are many different materials in the way. So because of each material's index of refraction, the speed will be reduced by a number of factors. As for whether I can outrun it, I don't think so. I think it will still be, faster than my running speed. I guess it depends on how fast it's thrown. No, if you actually mean the light itself, then nothing can travel faster than light, so you can't outrun it, no.

Sure, yeah, the flashlight itself is just like the box that sends out the light, right? So you could just set the flashlight on a table and then run past it. That would outrun a flashlight. No, because it would really piss Einstein and also violate relativity. I think maybe the only way to kind of outrun a flash, the light coming out of a flashlight would be maybe if you could go through a wormhole. So if the light from a flashlight, let's say, is headed towards Jupiter and there's a wormhole between Earth and Jupiter and you took that shortcut in your spaceship, you could

probably, hopefully get to Jupiter faster than the light from your flashlight would reach Jupiter. No, you cannot outrun a flashlight because the light traveling from the torch you are holding will always be traveling at the speed of light relative to you. So we'll always be traveling away from you at the speed of light. All right. Some pretty good answers here. Somebody said you can't outrun the flashlight itself. I feel like that's a different philosophical question. Like, can I throw something add to you that will always hit you? Yeah. Or is the light part of the flashlight? After all, will it always be? Does a flashlight have infinite extent because of the photons that come out of it? It's like the photon of Thesius, kind of maybe.

The flashlight of the flashlight. Yeah, like, is the photon from a flashlight part of the flashlight? Oh. Bum, bum, bum. That's if somebody's PhD thesis right there in philosophy of science. Done. All right. I'll take that degree. But yeah, lots of interesting answers here. Most people say that maybe not because you can't go faster than the speed of light. So if light always goes as fast as anything can go into universe, it will eventually catch you, right? Yeah, and it's a very reasonable answer given most people's understanding of special relativity. So yeah. I see some clever answers here too, like what if there's a wormhole? Did you think about that one? Yeah, but why can't the light go through the wormhole also, right? If you run through a wormhole, the photons right behind

you can't go through the wormhole also. I guess if you open and close that Stargate and quickly. Right. Yeah. Wow. A lot of clever answers here. But let's jump into answering this question. Can you outrun a flash of light? And so I guess maybe we should talk about a little bit about this idea of the speed of light and special relativity and why exactly it is kind of weird or why weird things might happen if you try to this experiment in space. Yeah. And the basic thing that we need to understand is how different people measure velocity. Like the way to think about it intuitively is imagine like somebody throwing a natural object like a ball. If they are in a car and they throw a ball forward at 10 miles an hour, then it's moving a 10 miles an hour. Relative to them, no big deal. But if the

car is also driving a 10 miles an hour relative to the ground, then you might ask, well, how fast is the ball moving relative to the ground? Well, it's 10 miles an hour relative to the car and the car is moving 10 miles an hour relative to the ground. So obviously 20 miles per hour, right? And you think that's obvious and it's intuitive. And what you're doing there is you're applying a rule which you sort of intuitively, we have figured out and applied. And it's a Galilean transformation. It says the speed of the ball relative to the ground is the speed of the ball relative to the car plus the speed of the car relative to the ground. And that mostly works. But what we discovered is that it's not true at very, very high speeds. And most specifically, it's not true for light. So if I'm standing in a car and I shine a

flashlight, how fast is the light leaving my flashlight? Well, the speed of light, right? Now, if the car is moving in 10 miles per hour, how fast is the light traveling relative to the ground? Well, your old rule would say, well, it's the speed of light plus 10 miles per hour, right? Like faster than the speed of light, but that can't happen. And so the light always travels at the speed of light no matter who's measuring it and how fast they're going relative to the thing that made the light. So the person in the car measures the photon is going at the speed of light and the person on the ground measures the photon going at the speed of light.

Right, yeah. You had me in Galilean transformation. I think what you're saying is that we're used to in our everyday lives of this idea that velocities like add right, like they add with simple arithmetic like one plus one equals two. But that things get weird with the speed of light because nothing

can go faster than the speed of light. So like you can't keep adding velocities because that would eventually make them go faster than the speed of light. Yeah, they add non-linearly, right? So they get closer and closer to the speed of light, but they don't just stack up on top of each other in a simple way. And so as you say, things with mass can get faster and faster and approach the speed of light, but nothing can go faster than the speed of light. And so you have to have a new addition rule. It doesn't just like A plus B, it's some weird combination

of A and B that helps you approach the speed of light, but never gets you past it. And for light itself, it's always at the speed of light. Never slower, never faster. Right. It's kind of this weird thing. if you're a photon that's shooting out of a flashlight and so you're going at the speed of light and then somehow you as that photon shoot off another photon in front of you, that photon is not going to go like at twice the speed of light. It's going to go still at the speed of light. That's exactly right. Although a photon itself can't have a frame of reference because a photon can't be at rest. So you can't measure the speed of one photon relative to another one, which is another for this tricky little wrinkle there. But exactly, if somebody's flying in a spaceship near

the speed of light relative to Earth and they turn out a flashlight, that photon is not going at like 1.999 times the speed of light relative to Earth. It's only moving at the speed of light. And that's pretty weird, right? Because these things no longer add up, it's like the people on the spaceship tell a different story about what happened than the people on Earth. Because the people on Earth see that photon as moving at near the speed of light. Its relative speed to the ship is actually quite small. Whereas on the ship, the people see the photon is moving away from them at the speed of light. And so you get like a different story about what happened. And that's the mind bending thing about special relativity is that different observers tell different stories about what

happened. And they're both accurate. They're both like honest observers telling conflicting stories and both being correct. But I guess it's not about different things happening. It's more about our perception of these things happening maybe because it's all related to how it affects time, right? Like time is sort of flexible. Time is not universal. Exactly. We don't have a consistent. cyst and clock through the whole universe that says like what happened at every moment and then what happened at the next moment what happens depends on where you are and how fast you are going relative to the events and sometimes there's flexibility there if there's no like causal connection where one thing has to happen before another then different people can give different orders of

events for what happened and both be correct it's not just an issue of perception although you can you know use special relativity to say well i understand why you are seeing the opposite thing that i'm seeing that's because of special relativity but neither of you can say like a happened before b or b happened before a it depends on who you are and how fast you're going right it's like nobody's clock is official everyone has a different clock and so that you can't sort of say like who's right or who's wrong yeah because in the end everything is relative right velocity itself is relative you can't have a velocity just on its own you can't say my spaceship is going at 90% of the speed of light you have to say well relative to what right as we talked about one

recently, like velocity doesn't even have a meaning if you are in a universe all by yourself. Like you can't have a speed if there's nothing else in the universe. So it's a whole new way to think about the nature of the universe and all these weird consequences. A lot of which come from this idea that light always travels at the speed of light. Yeah. So while it may seem intuitive that if you shine a flashlight at me, no matter how fast I'm going, that light is going to catch up to me, that may not actually be true depending on some of these weird consequences of special relativity. That's right. In the simple case where you're like in a normal universe and space is flat and it's obeying these rules, then if somebody shines a flashlight in your direction,

it doesn't matter how far away they are. Eventually that light will reach you. There's no like effective horizon. Given infinite time, that photon will catch you. There's nothing you can do. Right. And that's because it's moving at the speed of light relative to you because light always moves at the speed of light. So in the sort of simple universe where space is not curved, nothing weird is going on, that flash of light will always catch you. Right. But in our universe, the answer might be different. But first, let's take a quick break.

December 29th, 1975, LaGuardia Airport. The holiday rush. Parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently the explosion actually impelled metal glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene.

In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and order, criminal justice system is back. In season two, we're turning our focus to a threat a threat that hides in plain sight that's harder to predict and even harder to stop listen to the new season of law and order criminal justice system on the iHeart radio app apple podcasts or wherever you get your podcasts my boyfriends professor is way too friendly and now i'm

seriously suspicious oh wait a minute sam maybe her boyfriend's just looking for extra credit well dakota it's back to school week on the okay story time podcast so we'll find out soon This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate.

Well, according to this person, this is her boyfriend's former professor and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend? really cheated with his professor or not. To hear the explosive finale, listen to the OK Storytime podcast

on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. I had this, like, overwhelming sensation that I had to call it right then. And I just hit call. I said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation, and I just want to call on and let her know. There's a lot of people battling some of the very same things you're battling. And there is help out there. The Good Stuff Podcast Season 2 takes a deep look into One Tribe Foundation,

a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission. I was married to a combat army veteran, and he actually took his own life to suicide. One Tribe saved my life twice. There's a lot of love that flows through this place, and it's sincere.

Now it's a personal mission. Don't want to have to go to any more funerals, you know. I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Hey, sis, what if I could promise you you never had to listen to a condescending finance, bro, tell you how to manage your money again.

Welcome to Brown Ambition. This is the hard part when you pay down those credit cards. If you haven't gotten to the bottom of why you were racking up credit or turning to credit cards, you may just recreate the same problem a year from now. you do feel like you are bleeding from these high interest rates, I would start shopping for a debt consolidation loan, starting with your local credit union, shopping around online, looking for some online lenders because they tend to have fewer fees and be more affordable. Listen, I am not here to judge. It is so expensive in these streets. I 100% can see how in just a few months you can have this much credit card debt when it weighs on you. It's really easy to just like stick your head

in the sand. It's nice and dark in the sand. Even if it's scary, it's not going to to go away just because you're avoiding it and in fact it may get even worse for more judgment free money advice listen to brown ambition on the iHeart radio app apple podcast or wherever you get your podcast right daniel you're trying to outrun a flashlight i don't know why i guess you don't want people to see you or you want to remain sort of in the dark and mysterious but you're trying to outrun a flashlight and somebody's trying to flashlight on you. And so it seems like it would inevitably the light would reach me because it's going faster than anything can go in the universe. But you're saying there might be some

instances where I can actually outrun light. And you know, this is not just a silly physics thought experiment. This is really important. If you were playing laser tag and you're cornered and they shoot that thing right at you, you got to find a way to escape. And so that's where we come in. Right. This is totally serious and totally relevant to everyone's everyday life. No, there are some scenarios where somebody points a flashlight at you and it doesn't ever hit you, even in infinite time, even if it's pointed right at you. And you don't actually even have to be moving to avoid it in some scenarios. So you might have heard my sort of legalistic maneuver there to avoid saying that a flashlight will always catch you. The condition is that if you are

operating in flat space, in space that's just where general relativity applies and nothing is growing or shrinking. Right. So that gives us one, maybe the first instance in which you could maybe outrun a flashlight, right? It has to do with this idea that space is not actually flat or constant or kind of boring. Yeah, it turns out that our universe is a lot more exciting than the special relativity universe, right? Most importantly, our universe is expanding. And a lot of people think that that means that galaxies are flying out from some sort of central dot at a high speed that they are moving through space. But it's actually much weirder and more amazing than that. It's the expansion of space itself, right? It's not the motion of things through space. I mean, that's also happening.

But dark energy, this accelerating expansion of the universe, this is creating new space between our galaxy and other galaxies. It's not just moving the galaxies through the existing space. It's like stretching the fabric of the universe itself. Right. I think you're saying sort of like maybe I could outrun Usain Bold if maybe I play around with the track or like how the track is moving. Yeah, exactly. Put Usain Bolt. on a treadmill and stand in front of the treadmill, it doesn't matter how fast he runs. That's what I mean, yeah.

Yeah, and in the same way, the universe is laying new track, new space between us and some really, really distant photons. Photons which left their galaxy or their star billions of years ago and have been straining to reach us, some of them will never get to us,

even though they're pointed right at us, because the space between us and them is expanding faster than light can go through it, faster than the speed of light. Right. Well, it's not that the space itself at any given point is expanding faster than the speed of light, but it's more like there's so much space in between and it's growing a little bit at each spot so much that it overall is expanding

faster than the speed of light. Yes, it's a very gentle effect on a local scale, right? It's not like the distance between you and your partner or between the earth and the sun is growing very, very rapidly, right? It's a very gentle effect over small distances. But as you say, larger distances, it adds up, right? And so between our star and the next star, the acceleration is a larger number. And between our galaxy and the next galaxy is an even bigger number. Between really, really distant objects, then that speed is faster than the speed of light. The space is being created faster than the light can go through it. Right. So I guess maybe paint this scenario out for us. Like if I wanted to outrun a flash of light from a flashlight,

how would I do this? Like, we can't start in the same spot. I would have to go. away from you for a while, right? Or a certain amount of distance. That's right. If you start from the same spot as the photon, it's going to catch you instantly. Like, you know, by definition, if I'm holding the flashlight to your back and I press the button as soon as you start running, I've caught you before you've even gone anywhere, right, at t equals zero.

Right. If we're both like standing next to each other. Yeah. But if I give you a head start and you start running, right, and you get a little distance, you get 10 meters or so or 10 seconds before I shine the flashlight, then if space is expanding between the flashlight and how far you got before I turned the flashlight on, then it might be that that photon never makes it through that space to catch you. Right.

Well, let's take it one step at a time. Let's say I fly to Jupiter or you let me run to Jupiter before you turn on the flashlight. So now I'm in Jupiter and I'm running somehow in space and then you shine a flashlight on me. It's going to take a while, but it will catch me at that point, right? Eventually. It will catch you at that point, yes, because the expansion of space between here and Jupiter is not that impressive. It's not enough to overcome the speed of the photon. Right. It's like the space is growing a little bit, maybe like a, what, like a millimeter or something for a year or something?

Yeah, and a lot of people ask this question. They say, why can't we see the expansion of space and our solar system? Why isn't it tearing things apart? Well, the reason is that gravity is pretty strong on a local scale, right? Remember, it goes like one over distance squared. And so the distances are pretty small, like Earth to Jupiter, then gravity is more powerful than dark energy. So the solar system holds itself together. So the distance between the sun and Jupiter is not actually growing at all due to the expansion of the universe because they're holding tight onto each other. Same way the Earth and the moon are or the same way the atoms in your body are holding on to each other. So that's why we only really see this thing between galaxies or even between galaxy clusters because smaller than that, gravity sort of wins.

It's like tying everybody together. Imagine like a gentle breeze is blowing out everywhere. But people are holding onto each other and so they're able to resist it. But over large distances, this breeze adds up and it becomes a really powerful force. It's a breezy universe. All right, well, let's say then that you give me a really big head start and I start running at the nearest galaxy, which I think is... Andromeda. Andromeda.

Yeah, I was about to say that. So let's say you let me get as far as Andromeda and then I start running and then you shine a flashlight on me. It's still going to catch me. It's still going to catch you because the expansion of space between here and Andromeda is not that impressive. Right. So like I'll start running and how far is Andromeda? Like eight millions of light years. So it'll take millions of light years for the light to sort of get to where I'm running. And in those millions of years, I'll also have run a good bid. But eventually those two things will catch up, right? Like the light will eventually, it might take millions of years, but the light will eventually hit me in the back. That's right. If you are running a constant speed, like let's say you're running at half the speed of light because you're super impressive relative to the earth. And I shine that flashlight at the light. the speed of light, then it will eventually catch you. And you'll look back at the flashlight and you'll say, oh, that light is moving at the speed of light relative to me. And it'll catch up to you you, you know, in several million years if you are several million light years ahead when it begins.

But that's if you're moving at constant speed and there's not that much expansion of the space between us. And between us and Andromeda, it's not enough for you to outrun the flashlight. Right. Like the space between here and Andromat is expanding, but it's maybe expanding at, I don't know 10 meters per second or something that's right and you know there's a little wrinkle there because endromeda actually happens to be moving towards us even though the space is expanding between us gravity there is winning and it's pulling indromeda towards us and you know they're local deviations like space itself is expanding but things are still moving through that space as we talked about like driven by gravity and other forces so indromeda is getting closer to us

even though the space is expanding so it's sort of like swimming upstream against that expansion right I guess I'm just saying like Andromeda is where I start running Not that I stay with Andromeda, right? Right. Even if you start running from Andromeda and go past it, then that photon will still catch you.

The expansion of the universe, not enough to overcome that. Right. Like it's expanding, but only a little bit so light can still rip through it. So then kind of like at what point does the expansion of the universe

start to approach the speed of light so that light can't rip through it? So it's something like 60 billion light years away. If I gave you a head start of 60 billion light years away, light ears and shined a flashlight at you, that light would never catch you. Oh, 60 billion light years. Is there enough universe for me to get that much of a head start?

We just don't know, right? We have no idea what's out beyond the edge of the observable universe. And that's actually the threshold of the observable universe, like photons that were created at T equal zero, the beginning of the universe, 62 billion light years away, will never reach us. Those photons, even if they're pointed right at us, we'll never get here because the expansion of the universe will create new space faster than they can move it. So nothing that's beyond that we will ever, ever see. Right.

It's kind of like the reverse problem, right? Like there might be somebody 60 billion lighters away that shines a flashlight at us to try to tag us, but that light will never catch up to us because the space is expanding too fast. Exactly. And in the same way, a flashlight we send from here to there, If you start running there, then that photon is never going to catch those people. No matter how fast they're going or slow, they could just sit on their butts and they will never be hit by that photon. Right.

And that's what you call the Hubble's Law, right? Like the velocity of how space is growing is getting bigger with distance. That's right. Hubble's Law tells us about the recession velocity, how fast something is moving away from us and how that's getting faster and faster as you go further and further. So as you get further away from us, things are moving away from us faster and faster. At some point, that speed exceeds the speed of light. And that's called the Hubble volume. And the Hubble volume is this sphere that surrounds us, right?

And because the universe is expanding and that expansion is accelerating, then that volume is actually shrinking. Right. We can see a smaller and smaller fraction of the universe as time goes on. As time goes on, you can be closer and closer to us, shoot a flashlight at us, and it will never get to us. Because this expansion is accelerating. Right.

Yeah, that whole part of the universe, maybe the rest of the entire universe is basically dark to us, right? It's invisible. Like, we can never see it. Yeah. And things that we used to be able to see are disappearing, right? It might be that if you shine a flashlight at us, an early part of the universe, it would get here.

But then later on, if you waited too long, if you waited 10 billion years and then shown your flashlight at us, it wouldn't ever get here because the expansion has increased and accelerated beyond that threshold. Well, if it wants to move away that fast away from us, you know, maybe we don't want to see it. What do they got to hide anyway? Yeah, what's wrong with us? What do you mean? Why are they running away from us that fast?

So then what does that mean? That means that the furthest anything we'll ever see is about 62 billion light years away? Yes, 62 billion light years away. If the expansion of the universe continues accelerating the way that it has, then we will never see anything further away than 62 billion light years. Oh, there's a caveat. There's always a caveat. So he's a fine print.

Meaning like maybe space will stop expanding, right? Like, we don't know. Well, we don't understand why that expansion is accelerating and what's doing it. All we see is that it's happening. And since we don't understand why, we have no idea what the mechanism is, we can't predict its future. We don't know what's driving it. We know that it turned on a few billion years ago. So we don't understand any of that.

And so it could stop and it could turn around. It could shrink the universe so that things that were always invisible to us now become visible. The simplest thing to do is to extrapolate that nothing's going to change. But, you know, we've been wrong before. Right. And like you said, like you have been sort of wrong before, like this expansion of the universe wasn't always there. Like it seemed to have turned on at some point. Yeah, exactly.

It's not something we understand very well. We talked about this on the podcast actually once about the history of dark energy. We think that maybe the amount of dark energy is constant, but dark energy doesn't get diluted as the universe grows. Like as more space has created, every unit of space also has more dark energy. And so over time, it comes to sort of dominate what's happening in the universe. And that's why we think maybe it sort of took over about five billion years ago and became the thing that drove the whole universe. All right.

So that's one way to answer the question. Can you outrun a flashlight? And the answer is yes, you know, because of expanding space. If the space between you and the person shooting the flashlight at you is big enough and that space is expanding fast enough, then you can outrun light. Amazingly. I guess you can avoid it. What if I just move sideways, Daniel?

Save us all a lot of trouble. Wow. Podcast simplified. So right now the way to outrun a flashlight is just travel 62 billion light years away and then nobody will ever be able to laser tag you. Or invent a dark energy machine that can stretch space between you and that photon arbitrarily quickly. And then, hey, you can just do it yourself. Wow. That would be like dark energy tag. That would be a totally different product.

All right. Well, it turns out that that's not the only way that you can outrun a flashlight. There is another way that you can do it within the room. rules of the universe and it doesn't require you to go out that far. So let's get into it. But first, let's take another quick break. There's been a bombing at the TWA terminal. Apparently the explosion actually impelled metal glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene.

In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and order, criminal justice system is back. In season two, we're turning our focus to a threat that high in plain sight that's harder to predict and even harder to stop listen to the new season of law and order criminal justice system on the iHeart radio app apple podcasts or wherever you get your podcasts my boyfriend's professor is way too friendly and now i'm seriously suspicious

oh wait a minute sam maybe her boyfriend's just looking for extra credit well dakota it's back to school week on the okay story time podcast so we'll find out soon this person writes My boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate.

Well, according to this person, this is her boyfriend's former professor, and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his person? professor or not. To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast.

Hola, it's Honey German, and my podcast, Grasasas Come Again, is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't audition in like over 25 years. Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters sharing their real stories of failure and success. I feel like this is my destiny.

You were destined to be a start. We talk all about what's viral and trending with a little bit of chisement, a lot of laughs, and those amazing vivas you've come to expect. And of course, we'll explore deeper topics dealing with identity, struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash

because you have to do the code switching? I won't say whitewash, because, At the end of the day, you know, I'm me. Yeah. But the whole pretending and cold, you know, it takes a toll on you. Listen to the new season of Grasas Come Again as part of My Cultura Podcast Network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. A foot washed up a shoe with some bones in it. They had no idea who it was.

Most everything was burned up pretty good from the fire that not a whole lot was salvageable. These are the coldest of cold cases. But everything is about to change. Every case that is a cold case that has DNA right now in a backlog will be identified in our lifetime. A small lab in Texas is cracking the code on DNA. Using new scientific tools, they're finding clues in evidence so tiny you might just miss it. He never thought he was going to get caught. And I just looked at my computer screen.

I was just like, ah, gotcha. On America's Crime Lab, we'll learn about victims and survivors. And you'll meet the team behind the scenes at Othrom. the Houston Lab that takes on the most hopeless cases to finally solve the unsolvable. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

All right, I know, can you out run a flashlight? And we figured out one way to do that, that if you go out far enough, the space is expanding fast, enough that light will never catch up to you, but apparently there is another way that you can outrun light without having to depend on the, you know, the diet of space. That's right. And this is a real mindbender. This one took me a while to figure out it feels like it really contradicts everything you know about special relativity. If you spent a lot of

time thinking about special relativity and adjusting to the idea that light always moves at the speed of light relative to everybody else, this one's going to feel like it breaks that rule. But it actually is a natural consequence of special relativity. And it breaks that rule because it breaks one of the assumptions of special relativity, which is about acceleration. So the idea is that you can outrun a flash of light. If you're moving in a rocket chip with constant acceleration, if you're always, always speeding up, then that photon will never catch you. And the way that it avoids the rules of special relativity is that one word, acceleration. Most of the rules of special relativity require you to not be accelerating to have constant relative velocity. Interesting. I see. So like if I'm trying to outrun a flashlight and I start running, it's not just about never stopping or always running or always, you know, moving. It's about like each second you have to go a little bit faster than you were before. That's what acceleration means. That's right. When Usain Bolt comes out of the blocks, he goes from zero meters per second

to 10 meters per second pretty quickly. He's accelerating. He's changing his space. But then at some point he stays at 10 meters per second, right? Like even Usain Bolt can't accelerate forever on the ground, right? Like somehow there's something about his body and air resistance and his muscles that just prevent them from going faster and faster and faster and faster. That's right. So you need constant acceleration to make this work. And you don't need to be able to go faster than the speed of light. This is not some trick where you're like always adding the same amount of speed per second.

you can be constantly accelerating and asymptotically approaching the speed of light, never even actually reaching the speed of light. But if you're getting faster and faster every second, then you can avoid that photon ever hitting your back. Right, because even if you are always accelerating, always gaining speed, you're still never going to go faster than the speed of light. Like somehow the way special relativity works, it's like you're just going to be trying and trying,

but you'll never actually go faster than the speed of light. You can never reach the speed of light because you have mass, things with mass can never reach the speed of light. What you can do is add more energy, right? You can get more energy. Your energy can increase to arbitrary infinite amounts, but your velocity doesn't track. It approaches the speed of light never actually gets there.

Right. And this is where it gets kind of counterintuitive because, you know, I'm accelerating, I'm going faster and faster, but I'll never actually reach the speed of light. And so you're saying that even if somebody shoots this light at me that is going at the speed of light, even though it will always be going faster than me,

you're saying there's a possibility that it might not catch me. Yeah, it will not catch you. You know, the scenario I'm imagining is that you start out ahead of me. Maybe I'll give you a 10 meter head start and you start accelerating and then I turn on this flash of light. Then, you know, you are going to be going a certain speed, you're going faster and faster and faster, approaching the speed of light. I'm shooting this flashlight that's moving at the speed of light. And so you might think, well, I'm pretty good at special relativity.

If Jorge looks back and asks how fast, is that photon traveling relative to me, he should give the answer of the speed of light because as we said before, photons travel at the speed of light no matter who's measuring them, right? And so then it feels pretty simple. You're like, well, if a photon is moving relative to me

at the speed of light, it will eventually catch me. And that seems pretty solid, but all those calculations you just did in your head assume no acceleration. Those calculations are only true in inertial frames where there's no acceleration, there's only relative constant velocity. It's a little bit different when you add acceleration.

You have to go to general relativity because acceleration, it turns out, is equivalent to gravity, right? That's one of Einstein's big discoveries is that you can't tell the difference, for example, between gravity and acceleration. If you're like in an elevator in space, you can't tell, is that elevator accelerating or am I near some planet that's creating gravity? It's the same thing. And so you have to account for the effect.

effective curvature of space that you're creating for yourself when you're accelerating. Right. But I guess maybe let's be clear. Like there are situations where it will catch you, right? Like if I'm standing next to Usain Bolt and he starts running and he's accelerating, he might be accelerating getting up to his 10 meters per second. But if I shine a flashlight on him right away, it's going to catch him, right? If you shine a flashlight on him soon enough, absolutely. But if his acceleration is high enough, right? And he started out with enough of a head start, then it will never catch him if he has constant acceleration. So those are two special things, right?

Like he needs a head start. Yes. And I also have to wait a certain amount of time before I turn on my flashlight. You can turn on your flashlight at the same moment as long as he has a physical head start. He either needs a head start in time so he can get some distance gap or he needs to start like 10 meters away from you. But he can start running at the same moment as you turn on that flashlight and the flashlight will never catch him if he continues to accelerate forever till the end of the universe. which is pretty tough. Yeah, I don't know if he signed up for that.

He might have other things he wants to do. I mean, somebody give that guy a power bar or something. An infinite number of power bars. Right, because you can look at it the other way and say, well, the photon will catch you at time equals infinity because your speed is approaching the speed of life but never catching it. So at time equals infinity, the photon will get you. But time equals infinity isn't a real time, right?

The universe could go on forever. We'll never get there. Oh, I see. But there are two ingredients to this, right? Like, you need a head start and you need to be always accelerating at a certain rate, right? And I imagine that if I'm only accelerating a little bit at a time, then I need a really big head start. But if I accelerate really fast, if I have like a constant rocket pushing me, then I don't need that big of a head start. Exactly.

And you can calculate it by looking at it from the other point of view. Like from Hussein Bolt's point of view, what is this like? If you're riding on his shoulder, for example, and you're looking back as he's accelerating, then you might think, well, then what this means is that there's a distance beyond which you cannot see, like somebody who shines a flashlight at you from there tries to send you a message, that information will never reach you. So there's like this horizon, this wall beyond which it's just black. And the distance to that wall depends on his acceleration. So if he's accelerating really, really fast, then that horizon is closer. If he's accelerating really, really slowly, the horizon needs to be further, further away. So his acceleration goes to zero,

that horizon is infinitely far away. But the distance to that horizon is the speed of light square, divided by your acceleration. But I guess it still feels counterintuitive, right? Because the light is going at the speed of light, but Usain Bolt will never reach the speed of light. So how is it the light will never reach him, right? Like, wouldn't it eventually make up ground? Yeah, you would think so.

And that would be true if there was no acceleration. But remember, acceleration means all these rules get bent a little bit, the same way like space gets bent. And so the way to sort of incorporate it into your brain is to think about how acceleration is like an effective bending of space. And when space gets bent, all of your intuition about who catches what go out the window. You know, for example, like if somebody's inside a black hole and they shine a flashlight at you, that photon is never getting to you.

It doesn't matter how much time it takes. Why? Because space is bent, right? And those photons follow that bent space. So if you are doing constant acceleration, then it's sort of equivalent to gravity. It's sort of like bending space. And so that's what creates this horizon. It's not an event horizon.

It's not like a real physical horizon, but for you, it creates like an information horizon. Accelerating objects have an information horizon, which is pretty weird. Interesting. So like a content acceleration, it's almost like the space between us is expanding. That's what you're saying. It's like somehow if I accelerate fast enough and I'm further enough away, then that expansion of space, which is really just my acceleration is going to prevent the light from reaching me. Yeah, exactly.

But it's not like a real physical horizon, right? It's only it's from your point of view. And again, different people have different points of view and those things can conflict. If I'm watching this whole series of events from a spaceship floating nearby, I might see the photon catching Usain Bolt, right? And so I can see a different series of events. It's just like with a black hole, right? If I'm watching you fall into a black hole from the outside, I never see you fall into a black hole. You never get in.

You've smeared across the event horizon forever. Oh, wait a minute. For you, you fall right through that event horizon. You're inside the black hole. In the same way, these different accounts can conflict. Now, I feel a little cheat at Daniel. I think what you're saying is that if I shine at Alain Bolt, who's always accelerating,

Usain Bold will think that he outran the flash of light, but we are going to see him lose. The person sending the light won't see it hit him because he's accelerating relative to them. He won't see it hitting them, but another observer moving in a different direction, it's possible for them to see the light hitting him. So the story depends on exactly the location,

and velocity of everybody involved. It gets pretty hairy with general relativity, man. Right. I guess you're saying that to Usain's bold kind of frame of reference, his experience of things, that light will reach him, but only in an infinity, like when time ends for him. But for someone who's moving in another frame or speed,

there is a time at which the light will hit him. Yes, exactly. There's almost always another velocity or location you can get to to tell a different story about the same series of events. That's the lesson of the universe is that there is no universal history. There's no true single account

of what happens in this universe. Well, there is. I mean, the light does catch up to him. It's just that for him it happens at infinity and for us it happens not infinity. Yeah, if happening in infinity counts as happening, then, you know, I'll pay you that 20 bucks I owe you at time equals infinity.

Yeah, let's keep talking here until infinity and see if that's the same thing for everyone. But that's kind of what you mean is like, it does happen, but at infinity, but that's only from his point of view. That's right. Obviously, because relativity is weird in that way. It's pretty weird stuff.

All right. Well, then that's the answer to the question is, can you outrun a flashlight? The answer is yes, if you got far enough and space is expanding fast enough. And also, yes, if you are Usain Bolt, I guess. And you can't wait until infinity or you only consider infinity as never. Or if you have an infinite number of power bars and you can accelerate forever into the future. Right. To you, you will always win.

But maybe to somebody else, the answer will be different. But you'll never have to talk to them because you were accelerating away from them forever. That's the true benefit here. It's never having to talk to or see anyone ever again. All right, that's a pretty mind-bending question. And again, just a reminder of how weird this universe is and how weird the rules of it are, right?

It's not just that there are weird things in it. It's just that it is a weird universe in itself. It certainly is, but we love it. We love the weirdness. Stay weird, universe. All right, well, we hope you enjoyed that. Thanks for joining us. See you next time.

Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order, Criminal Justice.

system on the iHeart radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Hold up. Isn't that against school policy? That seems inappropriate.

Maybe find out how it ends by listening to the OK Storytime Podcasts and the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Every case that is a cold case that has DNA. Right now in a backlog will be identified in our lifetime. On the new podcast, America's Crime Lab, every case has a story to tell. And the DNA holds the truth. He never thought he was going to get caught. And I just looked at my computer screen. I was just like, Got you. This technology is already solving so many cases. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast.