Daniel and Jorge Explain the UniverseDaniel and Jorge talk about whether its possible to outrun the fastest thing in the Universe
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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 twenty five miles per hour.
But what's the fastest anything has ever gone on Earth?
That's about seven hundred and sixty three 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 five percent 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, like you individually craft each one.
Yeah, they're not very good. Isn't that what archisenal 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 and then you glow. Sometimes it's not the visit 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. Welcome for our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we try to shine the light of knowledge into your mind, pushing back on the forefront of ignorance and talking about all of 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 are just escaped our site.
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 strained, that they require real creativity to undearth how things actually work.
Yeah, are you saying the universe is not cliche? Thankfully, its twists and turns. I've seen this universe so many times before. Oh my gosh, bang, crunch, bang, crunch, bank crunch. Hey do you think every time there's a big bang and a big crunch 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 sold pretty well.
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 thig 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's 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, verse reveals that things we thought were true are not actually true. Lean These all sorts of weird consequences that really violate our intuition. They break our notion of like simultaneity 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.
Team 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 name?
Special relativity is called special because it's not general relativity. It applies to scenarios where space is flat. We 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 sitting 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 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 and 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, 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 the 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 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 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 Michaelson 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 a 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, the flashlights run. Do they have little legs that I don't know about?
Or that reminds me of the old joke about refrigerators. Is your refrigerator running? No, you better check the power. Flashlights don't actually run, of course, but the photons from them come out at a blistering speed right of three times ten to the eight 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? The thing, the scenario I'm imagining is you take off at your highest speed and I'm standing behind you with the 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 pass some sort of horizon beyond which you could not see I.
See, like I would never know that you turn on the flashlight because those photons would never reach me. Like I'd be running and be like, ah, Daniel still hasn't a photon, but you did, but the light would never reach me.
And why are you running away from me so quickly? What did I do?
Why are you shining a light on me? What are you trying to do blind me?
I'm just playing flashlight tag man.
So this applies to that nineties toy laser tag.
Yes, exactly, exactly. This helps you strategize for laser tag. This is special relativity laser tag.
Yeah, physics. It's useful for all kinds of situations, even ones that require to travel to the nineties 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 ask 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 a listener to the podcast and you've been itching to participate but you haven't quite yet, please send me a message to questions at dani Yinhorge dot 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 lights, 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 run faster.
I think from a flash light travels its speeds much less than the speed of lights. In a vacuum, it's slowed down by the materials in its way as it moves from inside the flash light 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 run, 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 the light, so you can'tot run 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 bese Einstein and also 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 will 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 at you that will always hit you?
Yeah? Or is the light part of the flashlight after all, will always be Does a flashlight have infinite extent because of the photons that come out of it?
It's like the photon of theseus kind of maybe.
The flashlight of theseus?
Yeah, Like is the photon from a flashlight part of the flashlight?
A bum that's of somebody's thesis right there in philosophy of science.
Done, all right, I'll take that degree. But yeah, lots of interesting answers here. Most people that 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 the universe, it will eventually catch you, right.
Yeah, And that'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 of photons right behind, you can't go through the wormhole. So I guess if you open and close that stargate really quickly?
Right? Yeah, 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 ten miles an hour, then it's moving at ten miles an hour relative to them, No big deal. But if the car is also driving at ten 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 ten miles an hour relative to the car, and the car is moving ten miles an hour relative to the ground, so obviously twenty 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 galilee in 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 ten miles per hour, how fast is light traveling relative to the ground. Well, your old rule would say, well, it's the speed of light plus ten 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 are 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 Galilee and transformation. I think what you're saying is that, you know, we're used in our everyday lives, this idea that velocities like add right, Like they add with simple like arithmetic, like well, you know one plus one equals too, 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 nonlinearly, 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 masks can get faster and faster and have pro to 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 amb 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. Like 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 a 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 tricky little wrinkle there. But exactly, if somebody's flying in a spaceship near the speed of light relative to Earth and they turn on a flashlight, that photon is not going at like one point nine to ninety nine 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, that people see the photon as 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 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 happen 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 ninety percent of the speed of light and say well relative to what right, As we talked about once 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 that you 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 a 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, give an 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.
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All 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.
Mmmmmm. And you know this is not just a silly physics thought experiment. This is really important. If you are 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 every day 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 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 that 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 a 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.
Now.
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 Ussein 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 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 galaxies, and even bigger number between really really distant objects than that speed is faster than the speed of light if space is being created faster than the light can go through it.
Right, So I guess maybe paint the 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, A 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 ten meters or so, or ten seconds before I shine the flashlight, then if space is expanding between the flashlight and how far you got before I turn 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 it 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 what like a millimeter or something per 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 square, and so if 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 onto 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 on to 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 Hear 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 we'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 will 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 it the fit will eventually hit me in the back.
That's right. If you are running at 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 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 will catch up to you, you know, in several million years, if you are several million light years ahead when it begins. But I see, 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 Andrama is expanding, but it's maybe expanding at I don't know, ten meters per second or something.
That's right. And you know, there's a little wrinkle there because Andrama actually happens to be moving towards us even though the space is expanding between us. Gravity there is winning and it's pulling Andrameda towards us. And you know, there are 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 Andromeda is getting closer to us even though the space is expanding sort of 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.
It 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 sixty billion light years away. If I gave you a head start of sixty billion light years and shining to flashlight at you, that light would never catch you.
Oh sixty billion light years? Is there enough universe for me to get that much of a headstart?
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 photon that were created at T equal zero the beginning of the universe sixty two billion light years away will never reach us. Those photons even if the pointed right at us will 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 sixty billion lightyers 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 can 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 ten billion years and then show in your flashlight at us, it wouldn't ever get here because the expansion has increased and accelerated beyond the 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? What's that means? That the furthest anything we'll ever see is about sixty two billion guy years away.
Yes, sixty two 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 sixty two 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 the things that were always invisible to us now become visible. The simplest thing to do is to extrapoli is 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 is 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 standing save us all a lot of trouble.
Wow podcast simplified.
So right now, the way to outrun a flashlight is just travel sixty two 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 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.
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Right, Daniel, can you outrun 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 diet of space.
That's right, And this is a real mind bender. This one took me a while to figure out. It feels like it really contradicts everything you know about special relativity. If you spend 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 ship with con instant 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 ten meters per second pretty quickly. He's accelerating, he's changing his speed.
But then at some point he stays at ten meters per second, right Like, even loosing Bold 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 as symptotically 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 gonna 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 masks 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. Approach is 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, like you start out ahead of me. Maybe I'll give you a ten 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, 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 a 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 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 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 in bold and he starts running and he's accelerating, he might be accelerating getting up to his ten 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 headstart in time so he can get some distance gap, or he needs to start like ten 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 annis, which is pretty tough.
Yeah, I don't know if he's signed up for that. He might have other things he wants to do.
I mean, somebody give that guy a power bar or something. I mean a 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 light but never catching it. So at time equals infinity, the photon will get to you. But time equals infinity isn't a real time, right, the universe could go on forever, We'll never get there.
I see. But there are two ingredients to this, right, Like, you need a headstart, 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 Usain 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 as his acceleration goes to zero, that horizon is infinitely far away. But the distance to that horizon is the speed of light squared divided by your acceleration.
But I guess it still feels counterintuitive. Right, because the light is going at the speed of light. But Hussein Bulb will never reach the speed of light. So how is this The light will never reach him, right, Like, wouldn't it eventually make up the 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 ways it sort of incorporated 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, 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 a 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 Ussein 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 smeared across the event horizon forever.
Oh wait a minute, for you, you fall.
Right through that event horizon. You're in inside the black hole. In the same way, these different accounts can conflict.
Oh now I feel a little cheat to Daniel, I think what you're saying me is that if I tin to at Hussein Bolt, who's always accelerating. Hussein Bold will think that he upran 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. Well, so the story depends on exactly the location and velocity of everybody involved. It gets pretty hairy with general relativity.
Man, right, I guess so you're saying that to Ussein'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 but 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 as it happens, not infinity.
Yeah, if happening in infinity counts as happening, then you know, I'll pay you that twenty bucks out or as infinity.
Yeah, let's keep talking here to 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, I see, 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 have runny flashlight? The answer is yes if you got far enough and space is expanding fast enough. And also yes if you are Hussein bold I guess and you can't wait till infinity or you only consider it 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 them because you were accelerating away from them forever.
That's the true benefit here, is never having to talk to you 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 iHeart Radio. For more podcasts from iHeart Radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.
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When you pop a piece of cheese into your mouth. You're probably not thinking about the environmental impact, but the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
There are children, friends, and families walking riding on paths and roads every day. Remember they're real people with loved ones who need them to get home safely. Protect our cyclists and pedestrians because they're people too.
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