Speaker 1: Hey, Daniel, what's the fastest a person has ever run? Well,
Speaker 1: Hussain Bolt, the fastest person in history, actually only runs
Speaker 1: about twenty miles per hour. But what's the fastest anything
Speaker 1: has ever gone on Earth? That's about seven hundred and
Speaker 1: sixty three miles per hour in a crazy rocket powered car.
Speaker 1: That sounds like a good idea. But what about in space?
Speaker 1: What's the fastest a spaceship has ever gone? There's the
Speaker 1: Parker Solar Probe, which is whizzed around the Sun, and
Speaker 1: it reached about five percent of the speed of light.
Speaker 1: All right, so then technically nothing humans have ever made
Speaker 1: has ever approached the speed of flight except for actual
Speaker 1: light from flashlights. Oh well, but technically we made the flashlight,
Speaker 1: but then the flashlight made the light. We want our
Speaker 1: names on the pack as well? Yeah, can we label
Speaker 1: each photon made by Jorge hi am or? Hey, I'm
Speaker 1: a cartoonist and the creator of PhD comics. Hi, I'm Daniel.
Speaker 1: I'm a particle physicist and the creator of many photons.
Speaker 1: Oh really, you have a certain glow about you. Is
Speaker 1: that what you're saying to all physicists? Glow? I think
Speaker 1: usually it's not a good idea. If you're working with
Speaker 1: radioactive things, especially if you come from Los Almos, then
Speaker 1: you do tend to glow in the dark. But now
Speaker 1: every single one of us gives up a unique pattern
Speaker 1: of photons that we personally have crafted. My photons are artisanal.
Speaker 1: Oh yeah, like you indivigitally craft each one. Yeah, they're
Speaker 1: not very good. Isn't that what artisanal means? Yeah, I
Speaker 1: guess what you mean is that you glow like in
Speaker 1: the infrared, like from our body heat. Yeah, I definitely
Speaker 1: do give off photons like every object in the universe,
Speaker 1: if you have a temperature and then you glow. Sometimes
Speaker 1: it's not in the visible light, but you definitely are
Speaker 1: pumping out photons into the universe. Even if you are
Speaker 1: a black hole, well then you are a shining example
Speaker 1: I think for all of us. Welcome for our podcast,
Speaker 1: Daniel and Jorge Explained the Universe, a production of I
Speaker 1: Heart Radio in which we try to shine the light
Speaker 1: of knowledge into your mind, pushing back on the forefront
Speaker 1: of ignorance and talking about all of the biggest questions
Speaker 1: in the universe, questions about the universe, questions around the universe,
Speaker 1: questions in the universe, and questions from the universe, and
Speaker 1: we talk about all of it and try to make
Speaker 1: jokes to entertain you. Yeah, because there is a lot
Speaker 1: in the universe out there that is bright and interesting
Speaker 1: and powerful and curious and exciting, and there are also
Speaker 1: dark things out there that are just escaped our site.
Speaker 1: That's right. We can only see a little fraction of
Speaker 1: the universe, an unknown fraction of the universe, because information
Speaker 1: comes to us usually via light, and that light travels
Speaker 1: at a very very fast but not infinite speed, and
Speaker 1: we have learned all sorts of crazy things about how
Speaker 1: information travels through the universe. Yeah, because the universe is
Speaker 1: not just filled with mysterious things, it's also sort of
Speaker 1: filled with mysterious rules. The rules of the universe are
Speaker 1: not quite what we experience in an everyday basis, and
Speaker 1: some of them kind of go against our intuition about
Speaker 1: how things actually work. And thank God for that, or
Speaker 1: my job would be boring. I think about science is
Speaker 1: like a grand mystery novel. We're trying to figure out
Speaker 1: how things work and deduce what the rules are from
Speaker 1: the little clues left here and there. And you're absolutely
Speaker 1: right that along the way we have figured out that
Speaker 1: the universe is not the one we thought we were
Speaker 1: living in, that the rules are weird, that the rules
Speaker 1: are strange, that they require real creativity to on earth
Speaker 1: how things actually work. Are you saying the universe is
Speaker 1: not cliche? Thankfully, It's got it's twists and turns. I've
Speaker 1: seen this universe so many times before. Oh my gosh,
Speaker 1: bang crunch, bang, crunch, bang crunch. Hey do you think
Speaker 1: every time there's a big bang and a big crunch
Speaker 1: and we come back with the same universe that would
Speaker 1: be boring? Yeah, obviously it was made by committee the
Speaker 1: big bang. Let's just reboot the last universe that's sold
Speaker 1: pretty well. Yeah right. Audiences don't want anything new. People
Speaker 1: don't want to exist in the in a new universe.
Speaker 1: This is the new J. J Abrams Big Bang theory.
Speaker 1: Oh a dig on J. J Abrams. I feel like
Speaker 1: you're always looking for an excuse there. Hey man, he
Speaker 1: created the whole universe. You know, I got notes, but
Speaker 1: it's pretty impressive. Wait. JJ Abrams created the whole universe
Speaker 1: this theory. Yes, there's a J. J Abrams equivalent, you know,
Speaker 1: writing the simulation or guiding the direction of the universe
Speaker 1: that would explain all the lens flares, and I see
Speaker 1: all the time when I look at the universe. But yeah,
Speaker 1: it is a weird universe with weird rules, and there
Speaker 1: is none weirder than special relativity, which is kind of
Speaker 1: one of our main theories about the universe. That's right.
Speaker 1: It turns out that when you start going really really quickly,
Speaker 1: the rules that work at slow speeds down here on
Speaker 1: Earth no longer apply. The rules are actually quite different,
Speaker 1: and for hundreds and thousands of years we have been
Speaker 1: learning rules that only work under special conditions, very very
Speaker 1: slow movement. If you actually push the boundaries and start
Speaker 1: going fast, the universe reveals that things we thought were
Speaker 1: true are not actually true, leadings all sorts of weird
Speaker 1: consequences that really violate our intuition. They break our notion
Speaker 1: of like simultaneity and the idea of time being universal. Right. Yeah,
Speaker 1: do you think like if we were all moving faster,
Speaker 1: we would be used to these strange special rules, and
Speaker 1: like our current rules would team we would have to
Speaker 1: be moving really really fast. But I think in that
Speaker 1: scenario our current rules would just be a natural extension
Speaker 1: of what we already understood. But it's hard to imagine
Speaker 1: growing up in a universe where special relativity feels intuitive,
Speaker 1: where it makes sense to you, where our conclusions about
Speaker 1: the universe would seem strange. But I'm sure somebody's written
Speaker 1: that science fiction story, or maybe not. It's all there
Speaker 1: for you, Daniel, somebody out there right it. But it's
Speaker 1: called special relativity because it sort of applies to special situations.
Speaker 1: Why does it have that special name? Special relativity is
Speaker 1: called special because it's not general relativity. It applies to
Speaker 1: scenarios where space is flat. We don't have to think
Speaker 1: about space actually being curved and like changing the path
Speaker 1: of photons or changing the motion of Earth around the star.
Speaker 1: It has to do only with a sort of simpler
Speaker 1: scenario where space is mostly empty and you're like shooting
Speaker 1: laser pulses back and forth, or you have light bulbs
Speaker 1: on trains. It has to do with relative velocities of
Speaker 1: things and how information moves through the universe. It avoids
Speaker 1: all complications from curved space. Oh really, wow, I never
Speaker 1: knew that. I always thought that it was called special
Speaker 1: relativity because it was special, But actually you're sort of
Speaker 1: sitting the opposite. It's special because it only applies to
Speaker 1: a boring universe. Yeah, it's a specialized condition. Right. He
Speaker 1: came up with this first to understand this, and he thought, well,
Speaker 1: you know, it's much trickier if space is actually curved,
Speaker 1: and then he generalized it. He made it much more broad.
Speaker 1: Special relativity is a subset of general relativity under the
Speaker 1: conditions that basically the universe is mostly empty. You hadn't
Speaker 1: figured that part out yet. So it's not like special
Speaker 1: because people think it's like it ess special. It's just
Speaker 1: special because actually it's like specially boring relativity that would
Speaker 1: be technically more accurate. Right, Yeah, it's a special case.
Speaker 1: It's not a better case. It's a simple case. But
Speaker 1: we often do this in physics. We think about simple
Speaker 1: scenarios to help us like distill what's going on and
Speaker 1: get the clearest picture, so we can separate these ideas
Speaker 1: because even in the scenario where you have no big
Speaker 1: masses destroying the shape of space like black holes or
Speaker 1: even just the sun, there are lots of weird things
Speaker 1: that happened in special relativity. You know, clocks that don't
Speaker 1: agree because you're moving at high speed. It's pretty weird
Speaker 1: and hard to get your mind around. Even if there
Speaker 1: is no curvature of space. So I think it was
Speaker 1: sort of a good intellectual exercise. But even though it
Speaker 1: applies to boring situations, it's still true, right, Like it's
Speaker 1: still applies to the whole universe. It still applies to
Speaker 1: the whole universe. The rules of special relativity do assume
Speaker 1: that there are no heavy masses, and so if you
Speaker 1: have big masses around curving space, you can't use calculations
Speaker 1: from special relativity. You've got to use general relativity. But
Speaker 1: I guess then the irony is that special relativity is
Speaker 1: actually especially boring relativity. But it gives rise to these
Speaker 1: really strange situations about the universe that don't seem to
Speaker 1: make sense. And a lot of that has to do
Speaker 1: with light experiments involving light. You know, there was this
Speaker 1: famous experiment by Michaelson and Morley that showed that the
Speaker 1: speed of light doesn't change no matter how fast the
Speaker 1: Earth is moving through space. For example, that no matter
Speaker 1: who measures a photon, they always measure it moving at
Speaker 1: the speed of light, no matter how fast that person
Speaker 1: is actually moving relative to anything else. It's pretty weird stuff. Yeah,
Speaker 1: And so this weirdness is sort of a maybe especially
Speaker 1: illustrated by asking a very simple question that you can
Speaker 1: ask about the speed of light. So to be on
Speaker 1: the podcast, we'll be asking the question is it possible
Speaker 1: to outrun a flashlight? First of all, the flashlights run?
Speaker 1: Do they have little legs that I don't know about?
Speaker 1: Or that reminds me the old joke about refrigerators. Is
Speaker 1: your refrigerator running? No, you better check the power. Your
Speaker 1: flashlights don't actually run, of course, but the photons from
Speaker 1: them come out at a blistering speed right of three
Speaker 1: times tend the meters per second, So it's pretty hard
Speaker 1: to imagine our pacing a flashlight. So it's more like,
Speaker 1: is it possible to outrun a flash of light from
Speaker 1: a flashlight? Yeah? I think the scenere I'm imagining is
Speaker 1: you take off as your highest speed and I'm standing
Speaker 1: behind you with a flashlight. Is it possible that once
Speaker 1: I send off a flash of light, that you could
Speaker 1: outpace it, that that flash of light would not catch you,
Speaker 1: like it would not shine on me, like the photons
Speaker 1: would never hit me. Yes, exactly. Or from your point
Speaker 1: of view, is it possible you could run fast enough
Speaker 1: that you could look backwards and you could not see
Speaker 1: me right, that I would be passed some sort of
Speaker 1: horizon beyond which you could not see. I see like
Speaker 1: I would never know that you turned on the flashlight
Speaker 1: because those photons would never reach me. Like I'll be
Speaker 1: running and be like Daniel still hasn't but you did.
Speaker 1: But it's the light would never reach me. And why
Speaker 1: are you running away from me so quickly? What did
Speaker 1: I don't know? Why are you shining a light on me?
Speaker 1: What are you trying to do blind me? I'm just
Speaker 1: playing flashlight tag? Man, chill out. So this applies to
Speaker 1: that nineties toy laser tag, actually exactly. This helps you
Speaker 1: strategize for laser tag. This is special relativity laser tag. Yeah,
Speaker 1: physics is useful for all kinds of situations, even ones
Speaker 1: that required to travel to the nineties and play laser tag.
Speaker 1: I knew this degree would come in handy some day. Well,
Speaker 1: at first, you have to solve time travel. I'm working
Speaker 1: on it. I'm working on it, all right. So then
Speaker 1: that's the scenario we're asking is if I take off
Speaker 1: running and you shine a flashlight at me, is it
Speaker 1: possible for me to outrun those photons or will they
Speaker 1: inevitably hit me at some point in the future. So
Speaker 1: that's a pretty interesting question, and so we thought we'd
Speaker 1: post it to people on the internet and see what happens.
Speaker 1: So usually Daniel went out there and ask listeners if
Speaker 1: they thought that one could outrun a flash of light.
Speaker 1: And so, as usual, I'm immensely grateful to all of
Speaker 1: you who wanted to participate in the podcast and answer
Speaker 1: these weird and random questions. If you're out there and
Speaker 1: you're a listener to the podcast and you've been itching
Speaker 1: to participate but you haven't quite yet, please send me
Speaker 1: a message to questions at Daniel and Jorge dot com.
Speaker 1: Think about it for a second. Do you think it
Speaker 1: is possible to outrun a flashlight? Here's what people had
Speaker 1: to say. Um, I don't think that I can. I'm
Speaker 1: not faster than lights, so I don't think that I
Speaker 1: could do that. I'd like to believe that you can
Speaker 1: outrund a flashlight, but for that you have to be faster,
Speaker 1: traveling faster than the speed of light, and as far
Speaker 1: as I know, nothing in the universe can travel faster
Speaker 1: than the speed of light. Yes, definitely, just don't throw
Speaker 1: it too hard so you can, you know, run faster.
Speaker 1: I think light from a flashlight travels its speeds much
Speaker 1: less than the speed of lights and a vacuum that's
Speaker 1: slowed down by the materials in its way as it
Speaker 1: moves from inside the flashlight outside of it. And then
Speaker 1: there are many different materials in the way, so because
Speaker 1: of each material's index of refraction, the speed will be
Speaker 1: reduced by a number of factors. As for whether I
Speaker 1: can outrun it, I don't think so. I think it
Speaker 1: will still be faster than my running speed. I guess
Speaker 1: it depends on how fast it's the out No, if
Speaker 1: if you actually mean the light itself, then nothing can
Speaker 1: trovel a fast light, so you cannot let it. Sure, Yeah,
Speaker 1: the flashlight itself is just like the box that that
Speaker 1: sends out the light, right, So you could just set
Speaker 1: the flashlight on the table and then run past it.
Speaker 1: Would would outrun a flashlight? No, because it would really
Speaker 1: piss s Einstein and also added additivity, I think maybe
Speaker 1: the only way to kind of out run a flash
Speaker 1: the light coming out of a flashlight would be maybe
Speaker 1: if you could go through a worm hole. So if
Speaker 1: the light from a flashlight, let's say, is headed towards
Speaker 1: Jupiter and there's a wormhole between Earth and Jupiter, and
Speaker 1: you took that shortcut in your spaceship m you could
Speaker 1: probably hopefully get to Jupiter faster than the light from
Speaker 1: your flashlight quid Rich Jupiter. No, you cannot outrun a
Speaker 1: flashlight because the light traveling from the torch you're holding
Speaker 1: will always be traveling at the speed of light relative
Speaker 1: to you, so will always be traveling away from you
Speaker 1: speed a light. All right, some pretty good answers here.
Speaker 1: Somebody said you can't outrun the flashlight itself. I feel
Speaker 1: like that's a different philosophical question, like can I throw
Speaker 1: something at you that will always hit you? Yeah? Or
Speaker 1: is the light part of the flashlight? After all? Will
Speaker 1: always be the flashlight of infinite extent because of the
Speaker 1: photons that come out of it. It's like the photon
Speaker 1: of theseus kind of maybe the flashlight of theseus. Yeah,
Speaker 1: Like is the photon from a flashlight part of the flashlight?
Speaker 1: Bump bump bump. That's if somebody's thesis right there in
Speaker 1: philosophy of science done? All right, I'll take that degree.
Speaker 1: But yeah, lots of interesting answers here. Most people that
Speaker 1: say that maybe not because you can't go faster than
Speaker 1: the speed of light. So if light always goes as
Speaker 1: fast as anything can go into universe, it will eventually
Speaker 1: catch you, right, Yeah, and it's a very reasonable answer
Speaker 1: given most people's understanding of special relativity. So yeah, I
Speaker 1: see some clever answers here too, like what if there's
Speaker 1: a wormhole? Did you think about that one? Yeah? But
Speaker 1: why can't the light go through the wormhole? Also? Right,
Speaker 1: if you run through photons right behind, you can't go
Speaker 1: through the wormhole. So I guess if you open and
Speaker 1: close that stargate really quickly, right, Yeah, Yeah, a lot
Speaker 1: of clever answers here, but let's jump into answering this question,
Speaker 1: can you out run a flash of light? And so
Speaker 1: I guess maybe we should talk about a little bit
Speaker 1: about this idea of the speed of light and special
Speaker 1: relativity and why exactly it is kind of weird or
Speaker 1: why weird things might happen if you try to this
Speaker 1: experiment in space. Yeah, And the basic thing that we
Speaker 1: need to understand is how different people measure velocity. Like
Speaker 1: the way to think about it intuitively is imagine like
Speaker 1: somebody throwing a natural object like a ball, if they
Speaker 1: are in a car and they throw a ball forward
Speaker 1: at ten miles an hour and then it's being a
Speaker 1: ten miles an hour relative to them. No big deal.
Speaker 1: But if the car is also driving at ten miles
Speaker 1: an hour relative to the ground, then you might ask, well,
Speaker 1: how fast is the ball moving relative to the ground. Well,
Speaker 1: it's ten miles an hour relative to the car, and
Speaker 1: the car is moving ten miles an hour relative to
Speaker 1: the ground, So obviously twenty miles per hour, right, And
Speaker 1: you think that's obvious and it's intuitive. And what you're
Speaker 1: doing there is you're applying a rule which you sort
Speaker 1: of intuitely we have figured out and applied, and it's
Speaker 1: a Galilean transformation. It says, the speed of the ball
Speaker 1: relative to the ground is the speed of the ball
Speaker 1: relative to the car plus the speed of the car
Speaker 1: relative to the ground. And that mostly works. But what
Speaker 1: we discovered is that it's not true at very very
Speaker 1: high speeds, and most specifically, it's not true for light.
Speaker 1: So if I'm standing in a car and I shine
Speaker 1: a flashlight, how fast is the light leaving my flashlight? Well,
Speaker 1: the speed of light right now, the car is moving
Speaker 1: in ten miles per hour. How fast is the light
Speaker 1: traveling relative to the ground. Well, you're old rule would say, well,
Speaker 1: if the speed of light plus ten miles per hour, right,
Speaker 1: like faster than the speed of light. But that can't happen.
Speaker 1: And so the light always travels at the speed of light,
Speaker 1: no matter who's measuring it and how fast they are
Speaker 1: going relative to the thing that made the light. So
Speaker 1: the person in the car measures the photon is going
Speaker 1: at the speed of light, and the person on the
Speaker 1: ground measures the photon and going at the speed of light. Right, Yeah,
Speaker 1: you had me on galilee and transformation. I think what
Speaker 1: you're saying is that, you know, we're used in our
Speaker 1: everyday lives this idea that velocities like add like they
Speaker 1: add with simple like arithmetic, like you know one plus
Speaker 1: one equals too, but that things get weird with the
Speaker 1: speed of light because nothing can go faster than the
Speaker 1: speed of light. So like you can't keep adding velocities
Speaker 1: because that would eventually make them go faster than the
Speaker 1: speed of light. Yeah, they add nonlinearly, right, so they
Speaker 1: get closer and closer to the speed of light, but
Speaker 1: they don't just stack up on top of each other
Speaker 1: in a simple way. And so as you say things
Speaker 1: with masking get faster and faster and approached the speed
Speaker 1: of light. But nothing can go faster than the speed
Speaker 1: of light, and so you have to have an new
Speaker 1: addition rule. It doesn't just like A plus B. It's
Speaker 1: some weird combination of A and B that helps you
Speaker 1: approach the speed of light but never gets you past it.
Speaker 1: And for light itself, it's always at the speed of light,
Speaker 1: never slower, never faster, right, It's kind of this weird thing.
Speaker 1: Like if you're a photon that's shooting out of a flashlight,
Speaker 1: and so you're going at the speed of light, and
Speaker 1: then somehow you as that photon shoot off another photon
Speaker 1: in front of you, that photon is not gonna go
Speaker 1: like a twice the speed of light. It's going to
Speaker 1: go still at the speed of light. That's exactly right.
Speaker 1: Although a photon itself can't have a frame of reference
Speaker 1: because photon can't be at rest, so you can't measure
Speaker 1: the speed of one photon relative to another one, which
Speaker 1: is another for the tricky little wrinkle there. But exactly
Speaker 1: if somebody's flying in a spaceship near the speed of
Speaker 1: light relative to Earth and they turn on a flashlight,
Speaker 1: that photon is not going at like one times the
Speaker 1: speed of light relative to Earth. It's only moving at
Speaker 1: the speed of light. And that's pretty weird, right, because
Speaker 1: these things no longer add up. It's like the people
Speaker 1: in the spaceship tell a different story about what happened
Speaker 1: than the people on Earth. Because the people on Earth
Speaker 1: see that floton as moving at near the speed of light,
Speaker 1: it's relative speed to the ship is actually quite small,
Speaker 1: whereas on the ship that people see the photon is
Speaker 1: moving away from them at the speed of light. And
Speaker 1: so you get like a different story about what happened.
Speaker 1: And that's the mind bending thing about special relativities, that
Speaker 1: different observers tell different stories about what happened, and they're
Speaker 1: both accurate. They're both like honest observers telling conflicting stories
Speaker 1: and both being correct. But I guess it's not about
Speaker 1: different things happening. It's more about our perception of these
Speaker 1: things happening, maybe because it's all related to how it
Speaker 1: affects time, right, like time is sort of flexible, time
Speaker 1: is not universal, and exactly we don't have a consistent
Speaker 1: clock through the whole universe that says like what happened
Speaker 1: at every moment, and then what happened at the next moment.
Speaker 1: What happens depends on where you are and how fast
Speaker 1: you are going relative to the events, and sometimes this
Speaker 1: flexibility there. If there's no like causal connection where one
Speaker 1: thing has to happen before another, then different people can
Speaker 1: give different orders of events for what happened and both
Speaker 1: be correct. It's not just an issue of perception. Although
Speaker 1: you can, you know, use special relativity to say, well,
Speaker 1: I understand why you are seeing the opposite thing that
Speaker 1: I'm seeing. That's because of special relativity, but neither of
Speaker 1: you can say like A happen before b or be
Speaker 1: happen for a It depends on who you are and
Speaker 1: how fast you're going. Right, It's like nobody's clock is official.
Speaker 1: Everyone has a different clock, and so that you can't
Speaker 1: sort of say, like who's right or who's wrong, because
Speaker 1: in the end, everything is relative. Right, velocity itself is relative.
Speaker 1: You can't have a velocity just on its own. You
Speaker 1: can't say my spaceship is going at the speed of
Speaker 1: light and say well relative to what right, As we
Speaker 1: talked about once recently, like velocity doesn't even have a
Speaker 1: meaning if you are in a universe all by yourself, like,
Speaker 1: you can't have a speed if there's nothing else in
Speaker 1: the universe. So it's a whole new way to think
Speaker 1: about the nature of the universe and all these weird consequences,
Speaker 1: a lot of which come from this idea that light
Speaker 1: always travels at the speed of light. Yeah, So while
Speaker 1: it may seem intuitive that if you shine a flashlight
Speaker 1: at me, no matter how fast I'm going, that light
Speaker 1: is going to catch up to me, that may not
Speaker 1: actually be true depending on some of these weird consequences
Speaker 1: of special relativity. That's right in a simple case where
Speaker 1: you're like in a normal universe and space is flat
Speaker 1: and it's obeying these rules, then if somebody shines a
Speaker 1: flashlight in your direction, it doesn't matter how far away
Speaker 1: they are. Eventually that light will reach you. There's no
Speaker 1: like effective horizon, give an infinite time, that photon will
Speaker 1: catch you. There's nothing you can do, right, and that's
Speaker 1: because it's moving at the speed of light relative to you.
Speaker 1: Because light always moves at the speed of light. So
Speaker 1: in this sort of simple universe where space is not curved,
Speaker 1: nothing weird is going on. That flash of light will
Speaker 1: always catch you, right. But in our universe, the answer
Speaker 1: might be different. But first, let's take a quick break.
Speaker 1: All right, Daniel, you're trying to outrun a flashlight. I
Speaker 1: don't know why. I guess you don't want people to
Speaker 1: see you, or you want to remain sort of in
Speaker 1: the dark and mysterious. But you're trying to outrun a
Speaker 1: flashlight and somebody's shying a flashlight on you, and so
Speaker 1: it seems like it would inevitably the light would reach
Speaker 1: me because it's going faster than anything can go in
Speaker 1: the universe. But you're saying there might be some instances
Speaker 1: where I can actually outrun light. Mm hmmmm. And you
Speaker 1: know this is not just a silly physics thought experiment.
Speaker 1: This is really important. If you were playing laser tag
Speaker 1: and you're cornered and they shoot that thing right at you,
Speaker 1: you gotta find a way to escape. And so that's
Speaker 1: where we come in, right. This is totally serious and
Speaker 1: totally relevant to everyone's everyday lighte No, there are some
Speaker 1: scenarios where somebody points a flashlight at you and it
Speaker 1: doesn't ever hit you even an infinite time, even if
Speaker 1: it's pointed right at you, and you don't actually even
Speaker 1: have to be moving to avoid it. In some scenarios.
Speaker 1: So you might have heard my sort of legalistic maneuver
Speaker 1: there to avoid saying that the flashlight will always catch you.
Speaker 1: The condition is that if you are operating in flat space,
Speaker 1: in space, that's just where general relativity applies, and nothing
Speaker 1: is growing or shrinking. Right, So that gives us one
Speaker 1: that maybe the first instance in which you could maybe
Speaker 1: outrun a flashlight. Right. It has to do with this
Speaker 1: idea that space is not actually flat or constant or
Speaker 1: kind of boring. Yeah, it turns out that our universe
Speaker 1: is a lot more exciting than the special relativity universe, right.
Speaker 1: Most importantly, our universe is expanding. Now, A lot of
Speaker 1: people think that that means that galaxies are flying out
Speaker 1: from some sort of central dot at a high speed,
Speaker 1: that they're moving through space. But it's actually much weirder
Speaker 1: and more amazing than that. It's the expansion of space itself. Right.
Speaker 1: It's not the motion of things through space. I mean
Speaker 1: that's also happening, but dark energy. There's accelerating expansion of
Speaker 1: the universe. This is creating new space between our galaxy
Speaker 1: and other galaxies. It's not just moving the galaxies through
Speaker 1: the existing space. It's like stre watching the fabric of
Speaker 1: the universe itself. Right, I think that you're saying sort
Speaker 1: of like maybe I could outrun Hussein Bold if maybe
Speaker 1: I play around with the track, or like how the
Speaker 1: track is moving. Yeah, exactly. Put Usain Bolt on a
Speaker 1: treadmill and stand in front of the treadmill. It doesn't
Speaker 1: matter how fast he runs, That's what I mean. Yeah. Yeah,
Speaker 1: And in the same way, the universe is laying new track,
Speaker 1: new space between us and some really really distant photons,
Speaker 1: photons which left their galaxy or their star billions of
Speaker 1: years ago and they've been straining to reach us. Some
Speaker 1: of them will never get to us, even though they're
Speaker 1: pointed right at us, because the space between us and
Speaker 1: them is expanding faster than light can go through it,
Speaker 1: faster than the speed of light. Right. Well, it's it's
Speaker 1: not that the space itself at any given point is
Speaker 1: expanding faster than the speed of light. But it's more
Speaker 1: like there's so much space in between, and it's growing
Speaker 1: a little bit at each spot, so much that it
Speaker 1: overall is expanding faster than the speed of light. Yes,
Speaker 1: it's a very gentle effect on a local scale. Right,
Speaker 1: It's not like the distance between you and your partner,
Speaker 1: or between the Earth and the Sun is growing very
Speaker 1: very rapidly. Right, It's a very gentle effect over small distances.
Speaker 1: But as you say, larger distances, it adds up, right,
Speaker 1: And so between our star and the next star, the
Speaker 1: acceleration is a larger number. Between our galaxy and the
Speaker 1: next galaxy is an even bigger number between really really
Speaker 1: distant objects. Then that speed is faster than the speed
Speaker 1: of light. The space is being created faster than the
Speaker 1: light can go through it. Right, So I guess maybe
Speaker 1: paint the scenario out for us, Like if I wanted
Speaker 1: to outrun a flash of light from a flashlight, how
Speaker 1: would I do this? Like, we can't start in the
Speaker 1: same spot. I would have to go away from you
Speaker 1: for a while, right, or a certain amount of distance.
Speaker 1: That's right, If you start from the same spot as
Speaker 1: the photon, it's going to catch you instantly, Like you
Speaker 1: know by definition. If I'm holding the flashlight to your
Speaker 1: back and I press the button as soon as you
Speaker 1: start running, I caught you before you've even gone anywhere, Right,
Speaker 1: A T equal zero? Right if we're both like standing
Speaker 1: next to each other. Yeah, But if I give you
Speaker 1: a head start, and you start running, right, and you
Speaker 1: get a little distance, you get ten ms or so,
Speaker 1: or ten seconds before I shine the flashlight. Then if
Speaker 1: space is expanding between the flashlight and how far you
Speaker 1: got before I turned the flashlight on, then it might
Speaker 1: be that that photon never makes it through that space
Speaker 1: to catch you. Right. Well, let's take it one step
Speaker 1: at a time. Let's say I fly to Jupiter, or
Speaker 1: you let me run to Jupiter before you turn on
Speaker 1: the flashlight. So now I mean, but in Jupiter and
Speaker 1: I'm running somehow in space, and then you shine a
Speaker 1: flashlight on me. It's going to take a while, but
Speaker 1: it will catch me at that point, right, Eventually it
Speaker 1: will catch you at that point, Yes, because the expansion
Speaker 1: of space between here and Jupiter is not that impressive,
Speaker 1: not enough to overcome the speed of the photon. Right,
Speaker 1: It's like the space is growing a little bit, maybe
Speaker 1: like what like a millimeter or something per year or something. Yeah,
Speaker 1: And a lot of people ask this question, They say,
Speaker 1: why can't we see the expansion of space and our
Speaker 1: solar system? Why isn't it tearing things apart? Well, the
Speaker 1: reason is that gravity is pretty strong on a local scale, right,
Speaker 1: Remember it goes like one who were distant squared, and
Speaker 1: so the distances are pretty small, like Earth to Jupiter.
Speaker 1: Then gravity is more powerful than dark energy, so the
Speaker 1: Solar system holds itself together. So the distance between the
Speaker 1: Sun and Jupiter is not actually growing at all due
Speaker 1: to the expansion of the universe, because they're holding tight
Speaker 1: onto each other, the same way the Earth and the
Speaker 1: Moon are, or the same way the atoms in your
Speaker 1: body are holding on to each other. So that's why
Speaker 1: we only really see this thing between galaxies or even
Speaker 1: between galaxy clusters, because smaller than that, gravity sort of wins.
Speaker 1: It's like tying everybody together. Imagine like a gentle breeze
Speaker 1: is blowing out everywhere, but people are holding on to
Speaker 1: each other and so they're able to resist it. But
Speaker 1: over large distances, this breeze adds up and it becomes
Speaker 1: a really powerful force. It's a breezy universe. Alright. Well,
Speaker 1: let's say then that you give me a really big
Speaker 1: head start and I start running at the nearest galaxy,
Speaker 1: which I think is Andromeda. Andromeda. Yeah, I was about
Speaker 1: to say that. So let's say you let me get
Speaker 1: as far as in Drameda and then I start running,
Speaker 1: and then you shine a flash light on me, It's
Speaker 1: still gonna catch me. It's still gonna catch you because
Speaker 1: the expansion of space between here and Andromeda is not
Speaker 1: that impressive, right, So, like, I'll start running, and how
Speaker 1: far is Andromeda? Like eight millions of light years? Will
Speaker 1: take millions of light years for the light to sort
Speaker 1: of get to where I'm running, And in those millions
Speaker 1: of years, I will also have run a good bit.
Speaker 1: But eventually those two things will catch up, right, Like
Speaker 1: the light will eventually. It might take millions of years,
Speaker 1: but it'll light will eventually hit me in the back.
Speaker 1: That's right. If you are running a constant speed, like
Speaker 1: let's say you were running at half the speed of
Speaker 1: light because you're super impressive relative to the Earth, and
Speaker 1: I shine that flashlight at the speed of light, then
Speaker 1: it will eventually catch you. And you'll look back at
Speaker 1: the flashlight and you'll say, oh, that light is moving
Speaker 1: at the speed of light relative to me, and it
Speaker 1: will catch up to you, you know, in several million years,
Speaker 1: if you are several million light years ahead when it begins.
Speaker 1: But I see, if you're moving at constant speed and
Speaker 1: there's not that much expansion of the space between us
Speaker 1: and between us and Drama. H it's not enough for
Speaker 1: you to outrun the flashlight, right, Like the space between
Speaker 1: here and Drama is is expanding, but it's maybe expanding
Speaker 1: at I don't know, ten meters per second or something.
Speaker 1: That's right, And you know there's a little wrinkled there
Speaker 1: because in Drama actually happens to be moving towards us
Speaker 1: even though the space is expanding between us, gravity there
Speaker 1: is winning and it's pulling and Drameda towards us, and
Speaker 1: you know their local deviations, Like space itself is expanding,
Speaker 1: but things are still moving through that space as we
Speaker 1: talked about, like driven by gravity and other forces. So
Speaker 1: Andrameda is getting closer to us, even though the space
Speaker 1: is expanding, sort of sort of like swimming upstream against
Speaker 1: that expansion, right, I guess I'm just saying, like Andrama
Speaker 1: is where I start running, not that I stay with
Speaker 1: and Drameda, right, Right, even if you start running from
Speaker 1: Andrameda and go past it, then that photon will still
Speaker 1: catch you. The expansion of the universe not enough to
Speaker 1: overcome that, right, Like it's expanding but only a little bit,
Speaker 1: so light can still rip through it. So then kind
Speaker 1: of like, at what point does the expansion of the
Speaker 1: universe start to approach the speed of light so that
Speaker 1: like can't rip through it. So it's something like sixty
Speaker 1: billion light years away. If I gave you a head
Speaker 1: start of sixty billion light years then and shine a
Speaker 1: flashlight at you, that light would never catch you sixty
Speaker 1: billion light years. Is there enough universe for me to
Speaker 1: get that much of a head start. We just don't know, right,
Speaker 1: We have no idea what's out beyond the edge of
Speaker 1: the observable universe, And that's actually the threshold of the
Speaker 1: observable universe. Like photons that were created at T equal zero,
Speaker 1: the beginning of the universe sixty two billion light years
Speaker 1: away will never reach us. Those photons, even if they're
Speaker 1: pointed right at us, we'll never get here because the
Speaker 1: expansion of the universe will create new space faster than
Speaker 1: they can move it. So nothing that's beyond that we
Speaker 1: will ever ever see. Right. It's kind of like the
Speaker 1: resverse problem, right, Like there might be somebody sixty billion
Speaker 1: light years away that shines a flashlight at us to
Speaker 1: try to tag us, but that light will never catch
Speaker 1: up to us because the space is expanding too fast, exactly,
Speaker 1: and in the same way a flashlight we send from
Speaker 1: here to there. If you start running there, then that
Speaker 1: photon is never going to catch those people. No matter
Speaker 1: how fast they're going or slow. They could just sit
Speaker 1: on their butts and they will never be hit by
Speaker 1: that photon. Right, And that's what you call the Hubble's law. Right,
Speaker 1: Like the velocity of how space is growing is getting
Speaker 1: bigger with distance, That's right. Hubble's law tells us about
Speaker 1: the recession velocity, how fast something is moving away from us,
Speaker 1: and how that's getting faster and faster as you go
Speaker 1: further and further. So as you get further away from us,
Speaker 1: things are moving away from us faster and faster. At
Speaker 1: some point that speed exceeds the speed of light. And
Speaker 1: that's called the Hubble volume. And the Hubble volume is
Speaker 1: this spear that surrounds us. Right, And because the universe
Speaker 1: is expanding and that expansion is accelerating, then that volume
Speaker 1: is actually shrinking. Right. We can see a smaller and
Speaker 1: smaller fraction of the universe as time goes on. As
Speaker 1: time goes on, you can be closer and closer to us.
Speaker 1: Shoot a flashlight at us, and it will never get
Speaker 1: to us because this expansion is accelerating. Right, Yeah, that
Speaker 1: whole part of the universe, maybe the rest of the
Speaker 1: entire universe is basically dark to us, right, it's invisible,
Speaker 1: like we can never see it. Yeah, and things that
Speaker 1: we used to be able to see are disappearing. Right.
Speaker 1: It might be that if you shine a flashlight at
Speaker 1: us an early part of the universe, it would get here.
Speaker 1: But then later on if you waited too long, if
Speaker 1: you waited ten billion years and then showing your flashlight
Speaker 1: at us, it wouldn't ever get here because the expansion
Speaker 1: has increased and accelerated beyond that threshold. Well, if it
Speaker 1: wants to move away that fast, away from us, you know,
Speaker 1: maybe we don't want to see it. What have they
Speaker 1: got to hide anyway? Yeah, what's wrong with us? What
Speaker 1: do you mean? Why are they running away from us
Speaker 1: that fast? So then what does that mean? What that
Speaker 1: means that the furthest anything we'll ever see is about
Speaker 1: sixty two billion years away, Yes, sixty two billion light
Speaker 1: years away. If the expansion of the universe continues accelerating
Speaker 1: the way that it has, then we will never see
Speaker 1: anything further away. Than sixty two billion light years. There's
Speaker 1: a caveat. There's always a caveat, a fine print, meaning
Speaker 1: like of the space will stop expanding, right, Like, we
Speaker 1: don't know, well, we don't understand why that expansion is
Speaker 1: accelerating and what's doing it. All we see is that
Speaker 1: it's happening. And since we don't understand why, we have
Speaker 1: no idea what the mechanism is, we can't predict its future.
Speaker 1: We don't know what's driving it. We know that it
Speaker 1: turned on a few billion years ago, so we don't
Speaker 1: understand any of that. And so we could stop, and
Speaker 1: it could turn around. It could shrink the universe so
Speaker 1: that the things that we're always invisible to us now
Speaker 1: become visible. The simplest thing to do is to extrapolate
Speaker 1: that nothing's going to change. But you know, we've been
Speaker 1: wrong before, right And like you said, like you have
Speaker 1: been sort of wrong before, Like this expansion of the
Speaker 1: universe wasn't always there, like it seems to have turned
Speaker 1: on at some point. Yeah, exactly. It's not something we
Speaker 1: understand very well. We talked about this on the podcast
Speaker 1: actually once about the history of dark energy. We think
Speaker 1: that maybe the amount of dark energy is constant, but
Speaker 1: dark energy doesn't get deluded as the universe grows, like
Speaker 1: as more spaces created, every unit of space also has
Speaker 1: more dark energy, and so over time it comes to
Speaker 1: sort of dominate what's happening in the universe. And that's
Speaker 1: why we think maybe it sort of took over about
Speaker 1: five billion years ago and became the thing that drove
Speaker 1: the whole universe. All right, So that's one way to
Speaker 1: answer the question can you outrun a flashlight? And the
Speaker 1: answer is yes, you know, because of expanding space. If
Speaker 1: the space between you and the person shooting the flashlight
Speaker 1: at you is big enough and that space is expanding
Speaker 1: fast enough, then you can't outrun light. Amazingly, I guess
Speaker 1: you can avoid it. What if I just moved sideways,
Speaker 1: Daniel would save us all a lot of trouble. Wow
Speaker 1: podcast simplified. So right now, the way to outrun a
Speaker 1: flashlight is just travel sixty two billion light years away
Speaker 1: and then nobody will ever be able to laser tag
Speaker 1: you or invent a dark energy machine that can stretch
Speaker 1: space between you and that photon arbitrarily quickly, and then hey,
Speaker 1: you can just do it yourself. Wow. That would be
Speaker 1: like dark energy tag that would be a totally different product.
Speaker 1: All right, Well, it turns out that that's not the
Speaker 1: only way that you can outrun a flashlight. There is
Speaker 1: another way that you can do it within the rule
Speaker 1: of the universe, and it doesn't require you to go
Speaker 1: out that far. So let's get into it. But first,
Speaker 1: let's take another quick break, right, Daniel, Can you outrun
Speaker 1: a flashlight? And we figured out one way to do
Speaker 1: that that if you go out far enough, the space
Speaker 1: is expanding fast enough that light will never catch up
Speaker 1: to you. But apparently there is another way that you
Speaker 1: can outrun light without having to depend on the you know,
Speaker 1: the diet of space. That's right, And this is a
Speaker 1: real mind bender. This one took me a while to
Speaker 1: figure out. It feels like it really contradicts everything you
Speaker 1: know about special relativity. If you spent a lot of
Speaker 1: time thinking about special relativity and adjusting to the idea
Speaker 1: that light always moves at the speed of light relative
Speaker 1: to everybody else, this one's gonna feel like it breaks
Speaker 1: that rule. But it actually is a natural consequence of
Speaker 1: special relativity, and it breaks that rule because it breaks
Speaker 1: one of the assumptions of special relativity, which is about acceleration.
Speaker 1: So the idea is that you can out run a
Speaker 1: flash of light if you're moving in a rocket ship
Speaker 1: with constant acceleration, if you're always always speeding up, then
Speaker 1: that photon will never catch you. And the way that
Speaker 1: it avoids the rules of special relativity is that one
Speaker 1: word acceleration. Most of the rules of special relativity require
Speaker 1: you to not be accelerating to have constant relative velocity. Interesting,
Speaker 1: I see, so like, if I'm trying to outrun a
Speaker 1: flashlight and I start running, it's not just about never
Speaker 1: stopping or always running or always you know, moving, It's
Speaker 1: about like each second you have to go a little
Speaker 1: bit faster than you were before. That's what acceleration means.
Speaker 1: That's right, when Usain Bolt comes out of the blocks,
Speaker 1: he goes from zero meters per second to ten per
Speaker 1: second pretty quickly. He's accelerating, he's changing his speed, but
Speaker 1: then at some point he stays at ten ms or second,
Speaker 1: right Like, even losing Bold can't accelerate forever on the ground,
Speaker 1: right Like, somehow there's something about his body and air
Speaker 1: resistance and his muscles that just prevent them from going
Speaker 1: faster and faster and faster and faster. That's right. So
Speaker 1: you need constant acceleration to make this work, and you
Speaker 1: don't need to be able to go faster than the
Speaker 1: speed of light. Is not some trick where you're like
Speaker 1: always adding the same amount of speed per second. You
Speaker 1: can be constantly accelerating and asthemptotically approaching the speed of light,
Speaker 1: never even actually reaching the speed of light. But if
Speaker 1: you're getting faster and faster every second, then you can
Speaker 1: avoid that photon ever hitting your back. Right, Because even
Speaker 1: you are always accelerating, always gaining speed, you're still never
Speaker 1: going to go faster than the speed of light. Like somehow,
Speaker 1: the way special relativity works, it's like you're just gonna
Speaker 1: be trying and trying, but you'll never actually go faster
Speaker 1: than the speed of light. You can never reach the
Speaker 1: speed of light because you have mass things with mask
Speaker 1: and never reach the speed of light. What you can
Speaker 1: do is add more energy, right, You can get more energy.
Speaker 1: You're energy can increase the arbitrary infinite amounts, but your
Speaker 1: velocity doesn't track it approaches the speed of light, never
Speaker 1: actually gets there, right, And this is where it gets
Speaker 1: kind of counterintuitive, because you know, I mean, I'm accelerating,
Speaker 1: going faster and faster, but I'll never actually reach the
Speaker 1: speed of light. And so you're saying that even if
Speaker 1: somebody shoots his light at me that is going at
Speaker 1: the speed of light, even though it will always be
Speaker 1: going faster than me, you're saying, there's a possibility that
Speaker 1: it might not catch me. Yeah, it will not catch you.
Speaker 1: You know. The scenario I'm imagining is that you're like,
Speaker 1: you start out ahead of me, maybe I'll give you
Speaker 1: a ten meter head start, and you start accelerating, and
Speaker 1: then I turned on this flash of light. Then you
Speaker 1: know you are going to be going a certain speed,
Speaker 1: going faster and faster and faster, approaching the speed of light.
Speaker 1: I'm shooting this flashlight that's moving at the speed of light,
Speaker 1: and so you might think, well, I'm pretty good special relativity.
Speaker 1: If Jorge looks back and asks how fast is that
Speaker 1: photon traveling relative to me, he should give the answer
Speaker 1: of the speed of light, because, as we said before,
Speaker 1: photons travel at the speed of light no matter who's
Speaker 1: measuring them, right, And so then it feels pretty simple.
Speaker 1: You're like, well, if the photon is moving relative to
Speaker 1: me at the speed of light, it will eventually catch me.
Speaker 1: And that seems pretty solid. But all those calculations you
Speaker 1: just did in your head assume no acceleration. Those calculations
Speaker 1: are only true in inertial frames, where there's no acceleration,
Speaker 1: there's only relative constant velocity. It's a little bit different.
Speaker 1: When you add acceleration. You have to go to general
Speaker 1: relativity because acceleration, it turns out, is equivalent to gravity
Speaker 1: a right. That's one of Einstein's big discoveries is that
Speaker 1: you can't tell the difference, for example, between gravity and acceleration.
Speaker 1: If you're like in an elevator in space, you can't
Speaker 1: tell is that elevator accelerating or am I near some
Speaker 1: planet that's creating gravity. It's the same thing, and so
Speaker 1: you have to account for the effective curvature of space
Speaker 1: that you're creating for yourself when you're accelerating. Right. But
Speaker 1: I guess, maybe let's be clear, like, there are situations
Speaker 1: where it will catch you. Right. If I'm standing next
Speaker 1: to using bold and he starts running and he's accelerating,
Speaker 1: he might be accelerating getting up to his ten per second,
Speaker 1: But if I shine a flashlight on him right away,
Speaker 1: it's going to catch him, right, if you shine a
Speaker 1: flashlight on him soon enough. Absolutely, But if his acceleration
Speaker 1: is high enough, right, and he started out with enough
Speaker 1: of a head start, then it will never catch him.
Speaker 1: If he has constant acceleration. So those are two special things, right,
Speaker 1: Like he needs a head start, yes, and I also
Speaker 1: have to wait a certain amount of time before I
Speaker 1: turn on my flashlight. You can turn on your flashlight
Speaker 1: at the same moment as long as he has a
Speaker 1: physical head start. He either needs a head start in
Speaker 1: time so he can get some distance gap, or he
Speaker 1: needs to start like ten ms away from you. But
Speaker 1: he can start running at the same moment as you
Speaker 1: turn on that flashlight, and the flashlight will never catch
Speaker 1: him if he continues to accelerate forever till the end
Speaker 1: of which is pretty tough. Yeah, I don't know if
Speaker 1: he signed up for that. He might have other things
Speaker 1: he wants to do. I mean, somebody give that guy
Speaker 1: a power bar or something. I mean in a number
Speaker 1: of power bars, right, Because you can look at it
Speaker 1: the other way and say, well, the photon will catch
Speaker 1: you at time equals infinity because your speed is approaching
Speaker 1: the speed of life but never catching it. So at
Speaker 1: time equals infinity, the photon will get to you. But
Speaker 1: time equals infinity isn't a real time, right, the universe
Speaker 1: could go on forever, will never get there. I see.
Speaker 1: But there are two ingredients of this, right, Like you
Speaker 1: need a head start, and you need to be always
Speaker 1: accelerating at a certain rate, right, And I imagine that
Speaker 1: if I'm only accelerating a little bit at a time,
Speaker 1: that I need a really big head start. But if
Speaker 1: I accelerate really fast, if I have like a constant
Speaker 1: rocket pushing me, then I don't need that big of
Speaker 1: a head start exactly. And you can calculate it by
Speaker 1: looking at it from the other point of view, like
Speaker 1: from us sain Bolt point of view, What is this
Speaker 1: Like if you're riding on his shoulder, for example, and
Speaker 1: you're looking back as he's accelerating, then you might think, well,
Speaker 1: then what this means is that there's a distance beyond
Speaker 1: which you cannot see, Like somebody who shines a flashlight
Speaker 1: at you from there tries to send you a message
Speaker 1: that information will never reach you. So there's like this horizon,
Speaker 1: this wall beyond which it's just lack and the distance
Speaker 1: to that wall depends on his acceleration. So if he's
Speaker 1: accelerating really really fast, then that horizon is closer. If
Speaker 1: he's accelerating really really slowly, the horizon needs to be
Speaker 1: further further away, So as the acceleration goes to zero,
Speaker 1: that horizon is infinitely far away. But the distance to
Speaker 1: that horizon is the speed of light squared divided by
Speaker 1: your acceleration. But I guess it still feels counterintuitive, right
Speaker 1: because the light is going at the speed of light,
Speaker 1: but Hussein Bulb will never reach the speed of light.
Speaker 1: So how is it the light will never reach him? Right, Like,
Speaker 1: wouldn't it eventually make up ground? Yeah, you would think so,
Speaker 1: and that would be true if there was no acceleration.
Speaker 1: But remember, acceleration means all these rules get bent a
Speaker 1: little bit, the same way like space gets bent. And
Speaker 1: so the way that sort of incorporated into your brain
Speaker 1: is to think about how acceleration is like an effective
Speaker 1: bending of space, and when space gets bent, all of
Speaker 1: your intuition about who catches what go out the window.
Speaker 1: You know, for example, like if somebody's inside a black
Speaker 1: hole and they shine a flashlight at you, that photon
Speaker 1: is never getting to you. It doesn't matter how much
Speaker 1: time it takes. Why because space is bent, right, and
Speaker 1: those photons follow that bent space. So if you are
Speaker 1: doing constant acceleration, then it's sort of equivalent to gravity.
Speaker 1: It's sort of like bending space. And so that's what
Speaker 1: creates this horizon. It's not an event horizon. It's not
Speaker 1: like a real physical horizon, but for you, it creates
Speaker 1: like an information horizon. Accelerating objects have an information horizon,
Speaker 1: which is pretty weird interesting, sort of like a content acceleration.
Speaker 1: It's almost like the space between us is expanding, that's
Speaker 1: what you're saying. It's like somehow if I accelerate fast
Speaker 1: enough and I'm further enough away, then that expansion of space,
Speaker 1: which is really just my acceleration, is going to prevent
Speaker 1: the light from reaching me. Yeah, exactly. But it's not
Speaker 1: like a real physical horizon, right, It's only it's from
Speaker 1: your point of view. And again, different people have different
Speaker 1: points of view, and those things can conflict. If I'm
Speaker 1: watching this whole series of events from a spaceship floating nearby,
Speaker 1: I might see the photon catching Usain bolt, right, and
Speaker 1: so I can see a different series of events. It's
Speaker 1: just like with a black hole, right, if I'm watching
Speaker 1: you fall into a black hole from the outside, I
Speaker 1: never see you following a black hole. You never get in.
Speaker 1: You smeared across the event horizon forever. Oh wait a minute,
Speaker 1: for you, you fall right through that event horizon. You're
Speaker 1: inside the black hole. In the same way. These different
Speaker 1: accounts can conflict. Uh Now I feel a little cheated, Daniel.
Speaker 1: I think what you're saying is that if I try
Speaker 1: to Lusain Bold, who's always accelerating, Hussein Bold will think
Speaker 1: that he ut ran the flash of light, but we
Speaker 1: are going to see him lose. The person sending the
Speaker 1: light won't see it hit him, because he's accelerating relative
Speaker 1: to them, he won't see it hitting them. But another
Speaker 1: observer moving in a different direction it's possible for them
Speaker 1: to see the light hitting him. The story depends on
Speaker 1: exactly the location and velocity of everybody involved. It gets
Speaker 1: pretty hairy with general relativity. Man, right, I guess that
Speaker 1: you're saying that to us Saint's Bold kind of frame
Speaker 1: of reference, his experience of things, that light will reach
Speaker 1: him but only in an infinity, like when time ends
Speaker 1: for him. But for someone who's moving in another frame
Speaker 1: or speed, there is a time at which the light
Speaker 1: will hit him. Yes, exactly. There's almost always another velocity
Speaker 1: or location you can get to to tell a different
Speaker 1: story about the same series of events. That's the lesson
Speaker 1: of the universe is that there is no universal history.
Speaker 1: There's no true single account of what happens in this universe.
Speaker 1: Well there is. I mean, the light does catch up
Speaker 1: to him, it's just that for him it happens at infinity,
Speaker 1: and for us it happens non infinity. Yeah, if happening
Speaker 1: in infinity accounts is happening, then you know, I'll pay
Speaker 1: you that twenty infinity. Yeah, let's keep talking here until
Speaker 1: infinity and see if that's the same thing for everyone.
Speaker 1: But that's kind of what you mean, is like, it
Speaker 1: does happen, but at infinity. But that's only from his
Speaker 1: point of view. That's right, because relativity is weird in
Speaker 1: that way. It's pretty weird stuff, all right. Well, then
Speaker 1: that's the answer to the question is can you running flashlight?
Speaker 1: The answer is yes if you got far enough and
Speaker 1: space is expanding fast enough. And also yes if you
Speaker 1: are bold, I guess and you you can't wait till
Speaker 1: an infinity or you only consider it infinity as never,
Speaker 1: or if you have an infinite number of power bars
Speaker 1: and you can accelerate forever into the future. Right to you,
Speaker 1: you will always win. But maybe to somebody else the
Speaker 1: answer will be different. But you'll never have to talk
Speaker 1: to them because you're accelerating away from them forever. That's
Speaker 1: the true benefit here is never haven't you talked to
Speaker 1: or see anyone? Ever? Again? All right, that's a pretty
Speaker 1: mind bending question, and again just a reminder of how
Speaker 1: weird this universe is and how weird the rules of
Speaker 1: it are. Right, it's not just that there are weird
Speaker 1: things in it. It's just that it is a weird
Speaker 1: universe in itself. It certainly is. But we love it.
Speaker 1: We love the weirdness. Stay weird universe. All right, Well,
Speaker 1: we hope you enjoyed that. Thanks for joining us, See
Speaker 1: you next time. Thanks for listening, and remember that Daniel
Speaker 1: and Jorge explain the Universe is a production of I
Speaker 1: Heart Radio or more podcast from my Heart Radio. Visit
Speaker 1: the I Heart Radio, Apple Apple Podcasts, or wherever you
Speaker 1: listen to your favorite shows. H