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

0:08
Speaker 1: Hey, Daniel, what's the fastest a person has ever run? Well,
0:12
Speaker 1: Hussain Bolt, the fastest person in history, actually only runs
0:16
Speaker 1: about twenty miles per hour. But what's the fastest anything
0:20
Speaker 1: has ever gone on Earth? That's about seven hundred and
0:23
Speaker 1: sixty three miles per hour in a crazy rocket powered car.
0:28
Speaker 1: That sounds like a good idea. But what about in space?
0:31
Speaker 1: What's the fastest a spaceship has ever gone? There's the
0:34
Speaker 1: Parker Solar Probe, which is whizzed around the Sun, and
0:37
Speaker 1: it reached about five percent of the speed of light.
0:40
Speaker 1: All right, so then technically nothing humans have ever made
0:44
Speaker 1: has ever approached the speed of flight except for actual
0:47
Speaker 1: light from flashlights. Oh well, but technically we made the flashlight,
0:50
Speaker 1: but then the flashlight made the light. We want our
0:52
Speaker 1: names on the pack as well? Yeah, can we label
0:54
Speaker 1: each photon made by Jorge hi am or? Hey, I'm
1:13
Speaker 1: a cartoonist and the creator of PhD comics. Hi, I'm Daniel.
1:17
Speaker 1: I'm a particle physicist and the creator of many photons.
1:20
Speaker 1: Oh really, you have a certain glow about you. Is
1:22
Speaker 1: that what you're saying to all physicists? Glow? I think
1:24
Speaker 1: usually it's not a good idea. If you're working with
1:26
Speaker 1: radioactive things, especially if you come from Los Almos, then
1:29
Speaker 1: you do tend to glow in the dark. But now
1:32
Speaker 1: every single one of us gives up a unique pattern
1:34
Speaker 1: of photons that we personally have crafted. My photons are artisanal.
1:39
Speaker 1: Oh yeah, like you indivigitally craft each one. Yeah, they're
1:43
Speaker 1: not very good. Isn't that what artisanal means? Yeah, I
1:46
Speaker 1: guess what you mean is that you glow like in
1:47
Speaker 1: the infrared, like from our body heat. Yeah, I definitely
1:50
Speaker 1: do give off photons like every object in the universe,
1:53
Speaker 1: if you have a temperature and then you glow. Sometimes
1:55
Speaker 1: it's not in the visible light, but you definitely are
1:57
Speaker 1: pumping out photons into the universe. Even if you are
2:00
Speaker 1: a black hole, well then you are a shining example
2:03
Speaker 1: I think for all of us. Welcome for our podcast,
2:05
Speaker 1: Daniel and Jorge Explained the Universe, a production of I
2:08
Speaker 1: Heart Radio in which we try to shine the light
2:10
Speaker 1: of knowledge into your mind, pushing back on the forefront
2:13
Speaker 1: of ignorance and talking about all of the biggest questions
2:16
Speaker 1: in the universe, questions about the universe, questions around the universe,
2:20
Speaker 1: questions in the universe, and questions from the universe, and
2:24
Speaker 1: we talk about all of it and try to make
2:25
Speaker 1: jokes to entertain you. Yeah, because there is a lot
2:28
Speaker 1: in the universe out there that is bright and interesting
2:30
Speaker 1: and powerful and curious and exciting, and there are also
2:34
Speaker 1: dark things out there that are just escaped our site.
2:37
Speaker 1: That's right. We can only see a little fraction of
2:39
Speaker 1: the universe, an unknown fraction of the universe, because information
2:43
Speaker 1: comes to us usually via light, and that light travels
2:46
Speaker 1: at a very very fast but not infinite speed, and
2:49
Speaker 1: we have learned all sorts of crazy things about how
2:52
Speaker 1: information travels through the universe. Yeah, because the universe is
2:55
Speaker 1: not just filled with mysterious things, it's also sort of
2:58
Speaker 1: filled with mysterious rules. The rules of the universe are
3:02
Speaker 1: not quite what we experience in an everyday basis, and
3:05
Speaker 1: some of them kind of go against our intuition about
3:08
Speaker 1: how things actually work. And thank God for that, or
3:11
Speaker 1: my job would be boring. I think about science is
3:14
Speaker 1: like a grand mystery novel. We're trying to figure out
3:17
Speaker 1: how things work and deduce what the rules are from
3:19
Speaker 1: the little clues left here and there. And you're absolutely
3:22
Speaker 1: right that along the way we have figured out that
3:24
Speaker 1: the universe is not the one we thought we were
3:26
Speaker 1: living in, that the rules are weird, that the rules
3:28
Speaker 1: are strange, that they require real creativity to on earth
3:32
Speaker 1: how things actually work. Are you saying the universe is
3:34
Speaker 1: not cliche? Thankfully, It's got it's twists and turns. I've
3:38
Speaker 1: seen this universe so many times before. Oh my gosh,
3:42
Speaker 1: bang crunch, bang, crunch, bang crunch. Hey do you think
3:44
Speaker 1: every time there's a big bang and a big crunch
3:46
Speaker 1: and we come back with the same universe that would
3:48
Speaker 1: be boring? Yeah, obviously it was made by committee the
3:50
Speaker 1: big bang. Let's just reboot the last universe that's sold
3:55
Speaker 1: pretty well. Yeah right. Audiences don't want anything new. People
3:59
Speaker 1: don't want to exist in the in a new universe.
4:01
Speaker 1: This is the new J. J Abrams Big Bang theory.
4:04
Speaker 1: Oh a dig on J. J Abrams. I feel like
4:06
Speaker 1: you're always looking for an excuse there. Hey man, he
4:09
Speaker 1: created the whole universe. You know, I got notes, but
4:12
Speaker 1: it's pretty impressive. Wait. JJ Abrams created the whole universe
4:15
Speaker 1: this theory. Yes, there's a J. J Abrams equivalent, you know,
4:18
Speaker 1: writing the simulation or guiding the direction of the universe
4:22
Speaker 1: that would explain all the lens flares, and I see
4:24
Speaker 1: all the time when I look at the universe. But yeah,
4:26
Speaker 1: it is a weird universe with weird rules, and there
4:29
Speaker 1: is none weirder than special relativity, which is kind of
4:33
Speaker 1: one of our main theories about the universe. That's right.
4:36
Speaker 1: It turns out that when you start going really really quickly,
4:39
Speaker 1: the rules that work at slow speeds down here on
4:42
Speaker 1: Earth no longer apply. The rules are actually quite different,
4:45
Speaker 1: and for hundreds and thousands of years we have been
4:48
Speaker 1: learning rules that only work under special conditions, very very
4:52
Speaker 1: slow movement. If you actually push the boundaries and start
4:55
Speaker 1: going fast, the universe reveals that things we thought were
4:58
Speaker 1: true are not actually true, leadings all sorts of weird
5:01
Speaker 1: consequences that really violate our intuition. They break our notion
5:05
Speaker 1: of like simultaneity and the idea of time being universal. Right. Yeah,
5:09
Speaker 1: do you think like if we were all moving faster,
5:11
Speaker 1: we would be used to these strange special rules, and
5:14
Speaker 1: like our current rules would team we would have to
5:17
Speaker 1: be moving really really fast. But I think in that
5:19
Speaker 1: scenario our current rules would just be a natural extension
5:23
Speaker 1: of what we already understood. But it's hard to imagine
5:26
Speaker 1: growing up in a universe where special relativity feels intuitive,
5:30
Speaker 1: where it makes sense to you, where our conclusions about
5:32
Speaker 1: the universe would seem strange. But I'm sure somebody's written
5:35
Speaker 1: that science fiction story, or maybe not. It's all there
5:38
Speaker 1: for you, Daniel, somebody out there right it. But it's
5:41
Speaker 1: called special relativity because it sort of applies to special situations.
5:45
Speaker 1: Why does it have that special name? Special relativity is
5:47
Speaker 1: called special because it's not general relativity. It applies to
5:51
Speaker 1: scenarios where space is flat. We don't have to think
5:54
Speaker 1: about space actually being curved and like changing the path
5:57
Speaker 1: of photons or changing the motion of Earth around the star.
6:01
Speaker 1: It has to do only with a sort of simpler
6:03
Speaker 1: scenario where space is mostly empty and you're like shooting
6:06
Speaker 1: laser pulses back and forth, or you have light bulbs
6:08
Speaker 1: on trains. It has to do with relative velocities of
6:11
Speaker 1: things and how information moves through the universe. It avoids
6:14
Speaker 1: all complications from curved space. Oh really, wow, I never
6:18
Speaker 1: knew that. I always thought that it was called special
6:20
Speaker 1: relativity because it was special, But actually you're sort of
6:22
Speaker 1: sitting the opposite. It's special because it only applies to
6:25
Speaker 1: a boring universe. Yeah, it's a specialized condition. Right. He
6:29
Speaker 1: came up with this first to understand this, and he thought, well,
6:31
Speaker 1: you know, it's much trickier if space is actually curved,
6:33
Speaker 1: and then he generalized it. He made it much more broad.
6:36
Speaker 1: Special relativity is a subset of general relativity under the
6:40
Speaker 1: conditions that basically the universe is mostly empty. You hadn't
6:43
Speaker 1: figured that part out yet. So it's not like special
6:45
Speaker 1: because people think it's like it ess special. It's just
6:48
Speaker 1: special because actually it's like specially boring relativity that would
6:52
Speaker 1: be technically more accurate. Right, Yeah, it's a special case.
6:56
Speaker 1: It's not a better case. It's a simple case. But
6:58
Speaker 1: we often do this in physics. We think about simple
7:01
Speaker 1: scenarios to help us like distill what's going on and
7:04
Speaker 1: get the clearest picture, so we can separate these ideas
7:07
Speaker 1: because even in the scenario where you have no big
7:09
Speaker 1: masses destroying the shape of space like black holes or
7:12
Speaker 1: even just the sun, there are lots of weird things
7:14
Speaker 1: that happened in special relativity. You know, clocks that don't
7:17
Speaker 1: agree because you're moving at high speed. It's pretty weird
7:20
Speaker 1: and hard to get your mind around. Even if there
7:22
Speaker 1: is no curvature of space. So I think it was
7:24
Speaker 1: sort of a good intellectual exercise. But even though it
7:26
Speaker 1: applies to boring situations, it's still true, right, Like it's
7:29
Speaker 1: still applies to the whole universe. It still applies to
7:33
Speaker 1: the whole universe. The rules of special relativity do assume
7:37
Speaker 1: that there are no heavy masses, and so if you
7:39
Speaker 1: have big masses around curving space, you can't use calculations
7:43
Speaker 1: from special relativity. You've got to use general relativity. But
7:46
Speaker 1: I guess then the irony is that special relativity is
7:49
Speaker 1: actually especially boring relativity. But it gives rise to these
7:53
Speaker 1: really strange situations about the universe that don't seem to
7:57
Speaker 1: make sense. And a lot of that has to do
7:59
Speaker 1: with light experiments involving light. You know, there was this
8:02
Speaker 1: famous experiment by Michaelson and Morley that showed that the
8:05
Speaker 1: speed of light doesn't change no matter how fast the
8:08
Speaker 1: Earth is moving through space. For example, that no matter
8:10
Speaker 1: who measures a photon, they always measure it moving at
8:14
Speaker 1: the speed of light, no matter how fast that person
8:16
Speaker 1: is actually moving relative to anything else. It's pretty weird stuff. Yeah,
8:20
Speaker 1: And so this weirdness is sort of a maybe especially
8:24
Speaker 1: illustrated by asking a very simple question that you can
8:27
Speaker 1: ask about the speed of light. So to be on
8:29
Speaker 1: the podcast, we'll be asking the question is it possible
8:37
Speaker 1: to outrun a flashlight? First of all, the flashlights run?
8:41
Speaker 1: Do they have little legs that I don't know about?
8:43
Speaker 1: Or that reminds me the old joke about refrigerators. Is
8:46
Speaker 1: your refrigerator running? No, you better check the power. Your
8:49
Speaker 1: flashlights don't actually run, of course, but the photons from
8:52
Speaker 1: them come out at a blistering speed right of three
8:56
Speaker 1: times tend the meters per second, So it's pretty hard
8:58
Speaker 1: to imagine our pacing a flashlight. So it's more like,
9:01
Speaker 1: is it possible to outrun a flash of light from
9:04
Speaker 1: a flashlight? Yeah? I think the scenere I'm imagining is
9:07
Speaker 1: you take off as your highest speed and I'm standing
9:09
Speaker 1: behind you with a flashlight. Is it possible that once
9:13
Speaker 1: I send off a flash of light, that you could
9:14
Speaker 1: outpace it, that that flash of light would not catch you,
9:17
Speaker 1: like it would not shine on me, like the photons
9:19
Speaker 1: would never hit me. Yes, exactly. Or from your point
9:23
Speaker 1: of view, is it possible you could run fast enough
9:25
Speaker 1: that you could look backwards and you could not see
9:27
Speaker 1: me right, that I would be passed some sort of
9:30
Speaker 1: horizon beyond which you could not see. I see like
9:33
Speaker 1: I would never know that you turned on the flashlight
9:36
Speaker 1: because those photons would never reach me. Like I'll be
9:38
Speaker 1: running and be like Daniel still hasn't but you did.
9:44
Speaker 1: But it's the light would never reach me. And why
9:46
Speaker 1: are you running away from me so quickly? What did
9:48
Speaker 1: I don't know? Why are you shining a light on me?
9:51
Speaker 1: What are you trying to do blind me? I'm just
9:54
Speaker 1: playing flashlight tag? Man, chill out. So this applies to
9:58
Speaker 1: that nineties toy laser tag, actually exactly. This helps you
10:01
Speaker 1: strategize for laser tag. This is special relativity laser tag. Yeah,
10:05
Speaker 1: physics is useful for all kinds of situations, even ones
10:09
Speaker 1: that required to travel to the nineties and play laser tag.
10:11
Speaker 1: I knew this degree would come in handy some day. Well,
10:14
Speaker 1: at first, you have to solve time travel. I'm working
10:16
Speaker 1: on it. I'm working on it, all right. So then
10:18
Speaker 1: that's the scenario we're asking is if I take off
10:21
Speaker 1: running and you shine a flashlight at me, is it
10:23
Speaker 1: possible for me to outrun those photons or will they
10:26
Speaker 1: inevitably hit me at some point in the future. So
10:29
Speaker 1: that's a pretty interesting question, and so we thought we'd
10:31
Speaker 1: post it to people on the internet and see what happens.
10:34
Speaker 1: So usually Daniel went out there and ask listeners if
10:37
Speaker 1: they thought that one could outrun a flash of light.
10:41
Speaker 1: And so, as usual, I'm immensely grateful to all of
10:44
Speaker 1: you who wanted to participate in the podcast and answer
10:47
Speaker 1: these weird and random questions. If you're out there and
10:49
Speaker 1: you're a listener to the podcast and you've been itching
10:52
Speaker 1: to participate but you haven't quite yet, please send me
10:54
Speaker 1: a message to questions at Daniel and Jorge dot com.
10:57
Speaker 1: Think about it for a second. Do you think it
10:59
Speaker 1: is possible to outrun a flashlight? Here's what people had
11:03
Speaker 1: to say. Um, I don't think that I can. I'm
11:08
Speaker 1: not faster than lights, so I don't think that I
11:10
Speaker 1: could do that. I'd like to believe that you can
11:13
Speaker 1: outrund a flashlight, but for that you have to be faster,
11:17
Speaker 1: traveling faster than the speed of light, and as far
11:19
Speaker 1: as I know, nothing in the universe can travel faster
11:22
Speaker 1: than the speed of light. Yes, definitely, just don't throw
11:26
Speaker 1: it too hard so you can, you know, run faster.
11:29
Speaker 1: I think light from a flashlight travels its speeds much
11:32
Speaker 1: less than the speed of lights and a vacuum that's
11:35
Speaker 1: slowed down by the materials in its way as it
11:38
Speaker 1: moves from inside the flashlight outside of it. And then
11:42
Speaker 1: there are many different materials in the way, so because
11:46
Speaker 1: of each material's index of refraction, the speed will be
11:49
Speaker 1: reduced by a number of factors. As for whether I
11:52
Speaker 1: can outrun it, I don't think so. I think it
11:55
Speaker 1: will still be faster than my running speed. I guess
11:58
Speaker 1: it depends on how fast it's the out No, if
12:02
Speaker 1: if you actually mean the light itself, then nothing can
12:04
Speaker 1: trovel a fast light, so you cannot let it. Sure, Yeah,
12:09
Speaker 1: the flashlight itself is just like the box that that
12:14
Speaker 1: sends out the light, right, So you could just set
12:16
Speaker 1: the flashlight on the table and then run past it.
12:19
Speaker 1: Would would outrun a flashlight? No, because it would really
12:22
Speaker 1: piss s Einstein and also added additivity, I think maybe
12:28
Speaker 1: the only way to kind of out run a flash
12:32
Speaker 1: the light coming out of a flashlight would be maybe
12:36
Speaker 1: if you could go through a worm hole. So if
12:40
Speaker 1: the light from a flashlight, let's say, is headed towards
12:44
Speaker 1: Jupiter and there's a wormhole between Earth and Jupiter, and
12:49
Speaker 1: you took that shortcut in your spaceship m you could
12:53
Speaker 1: probably hopefully get to Jupiter faster than the light from
12:59
Speaker 1: your flashlight quid Rich Jupiter. No, you cannot outrun a
13:03
Speaker 1: flashlight because the light traveling from the torch you're holding
13:08
Speaker 1: will always be traveling at the speed of light relative
13:11
Speaker 1: to you, so will always be traveling away from you
13:14
Speaker 1: speed a light. All right, some pretty good answers here.
13:18
Speaker 1: Somebody said you can't outrun the flashlight itself. I feel
13:21
Speaker 1: like that's a different philosophical question, like can I throw
13:24
Speaker 1: something at you that will always hit you? Yeah? Or
13:28
Speaker 1: is the light part of the flashlight? After all? Will
13:31
Speaker 1: always be the flashlight of infinite extent because of the
13:34
Speaker 1: photons that come out of it. It's like the photon
13:36
Speaker 1: of theseus kind of maybe the flashlight of theseus. Yeah,
13:40
Speaker 1: Like is the photon from a flashlight part of the flashlight?
13:44
Speaker 1: Bump bump bump. That's if somebody's thesis right there in
13:47
Speaker 1: philosophy of science done? All right, I'll take that degree.
13:51
Speaker 1: But yeah, lots of interesting answers here. Most people that
13:53
Speaker 1: say that maybe not because you can't go faster than
13:55
Speaker 1: the speed of light. So if light always goes as
13:58
Speaker 1: fast as anything can go into universe, it will eventually
14:01
Speaker 1: catch you, right, Yeah, and it's a very reasonable answer
14:03
Speaker 1: given most people's understanding of special relativity. So yeah, I
14:07
Speaker 1: see some clever answers here too, like what if there's
14:09
Speaker 1: a wormhole? Did you think about that one? Yeah? But
14:13
Speaker 1: why can't the light go through the wormhole? Also? Right,
14:16
Speaker 1: if you run through photons right behind, you can't go
14:19
Speaker 1: through the wormhole. So I guess if you open and
14:21
Speaker 1: close that stargate really quickly, right, Yeah, Yeah, a lot
14:25
Speaker 1: of clever answers here, but let's jump into answering this question,
14:28
Speaker 1: can you out run a flash of light? And so
14:31
Speaker 1: I guess maybe we should talk about a little bit
14:33
Speaker 1: about this idea of the speed of light and special
14:36
Speaker 1: relativity and why exactly it is kind of weird or
14:40
Speaker 1: why weird things might happen if you try to this
14:42
Speaker 1: experiment in space. Yeah, And the basic thing that we
14:46
Speaker 1: need to understand is how different people measure velocity. Like
14:50
Speaker 1: the way to think about it intuitively is imagine like
14:52
Speaker 1: somebody throwing a natural object like a ball, if they
14:56
Speaker 1: are in a car and they throw a ball forward
14:58
Speaker 1: at ten miles an hour and then it's being a
15:00
Speaker 1: ten miles an hour relative to them. No big deal.
15:03
Speaker 1: But if the car is also driving at ten miles
15:05
Speaker 1: an hour relative to the ground, then you might ask, well,
15:08
Speaker 1: how fast is the ball moving relative to the ground. Well,
15:11
Speaker 1: it's ten miles an hour relative to the car, and
15:13
Speaker 1: the car is moving ten miles an hour relative to
15:15
Speaker 1: the ground, So obviously twenty miles per hour, right, And
15:18
Speaker 1: you think that's obvious and it's intuitive. And what you're
15:21
Speaker 1: doing there is you're applying a rule which you sort
15:24
Speaker 1: of intuitely we have figured out and applied, and it's
15:27
Speaker 1: a Galilean transformation. It says, the speed of the ball
15:30
Speaker 1: relative to the ground is the speed of the ball
15:33
Speaker 1: relative to the car plus the speed of the car
15:35
Speaker 1: relative to the ground. And that mostly works. But what
15:38
Speaker 1: we discovered is that it's not true at very very
15:41
Speaker 1: high speeds, and most specifically, it's not true for light.
15:45
Speaker 1: So if I'm standing in a car and I shine
15:47
Speaker 1: a flashlight, how fast is the light leaving my flashlight? Well,
15:51
Speaker 1: the speed of light right now, the car is moving
15:54
Speaker 1: in ten miles per hour. How fast is the light
15:56
Speaker 1: traveling relative to the ground. Well, you're old rule would say, well,
16:00
Speaker 1: if the speed of light plus ten miles per hour, right,
16:03
Speaker 1: like faster than the speed of light. But that can't happen.
16:06
Speaker 1: And so the light always travels at the speed of light,
16:09
Speaker 1: no matter who's measuring it and how fast they are
16:12
Speaker 1: going relative to the thing that made the light. So
16:14
Speaker 1: the person in the car measures the photon is going
16:16
Speaker 1: at the speed of light, and the person on the
16:18
Speaker 1: ground measures the photon and going at the speed of light. Right, Yeah,
16:21
Speaker 1: you had me on galilee and transformation. I think what
16:25
Speaker 1: you're saying is that, you know, we're used in our
16:26
Speaker 1: everyday lives this idea that velocities like add like they
16:30
Speaker 1: add with simple like arithmetic, like you know one plus
16:33
Speaker 1: one equals too, but that things get weird with the
16:36
Speaker 1: speed of light because nothing can go faster than the
16:38
Speaker 1: speed of light. So like you can't keep adding velocities
16:41
Speaker 1: because that would eventually make them go faster than the
16:44
Speaker 1: speed of light. Yeah, they add nonlinearly, right, so they
16:47
Speaker 1: get closer and closer to the speed of light, but
16:49
Speaker 1: they don't just stack up on top of each other
16:51
Speaker 1: in a simple way. And so as you say things
16:53
Speaker 1: with masking get faster and faster and approached the speed
16:56
Speaker 1: of light. But nothing can go faster than the speed
16:59
Speaker 1: of light, and so you have to have an new
17:00
Speaker 1: addition rule. It doesn't just like A plus B. It's
17:03
Speaker 1: some weird combination of A and B that helps you
17:05
Speaker 1: approach the speed of light but never gets you past it.
17:08
Speaker 1: And for light itself, it's always at the speed of light,
17:11
Speaker 1: never slower, never faster, right, It's kind of this weird thing.
17:15
Speaker 1: Like if you're a photon that's shooting out of a flashlight,
17:18
Speaker 1: and so you're going at the speed of light, and
17:20
Speaker 1: then somehow you as that photon shoot off another photon
17:23
Speaker 1: in front of you, that photon is not gonna go
17:26
Speaker 1: like a twice the speed of light. It's going to
17:28
Speaker 1: go still at the speed of light. That's exactly right.
17:31
Speaker 1: Although a photon itself can't have a frame of reference
17:34
Speaker 1: because photon can't be at rest, so you can't measure
17:37
Speaker 1: the speed of one photon relative to another one, which
17:40
Speaker 1: is another for the tricky little wrinkle there. But exactly
17:43
Speaker 1: if somebody's flying in a spaceship near the speed of
17:45
Speaker 1: light relative to Earth and they turn on a flashlight,
17:48
Speaker 1: that photon is not going at like one times the
17:51
Speaker 1: speed of light relative to Earth. It's only moving at
17:54
Speaker 1: the speed of light. And that's pretty weird, right, because
17:57
Speaker 1: these things no longer add up. It's like the people
18:00
Speaker 1: in the spaceship tell a different story about what happened
18:02
Speaker 1: than the people on Earth. Because the people on Earth
18:05
Speaker 1: see that floton as moving at near the speed of light,
18:08
Speaker 1: it's relative speed to the ship is actually quite small,
18:11
Speaker 1: whereas on the ship that people see the photon is
18:13
Speaker 1: moving away from them at the speed of light. And
18:16
Speaker 1: so you get like a different story about what happened.
18:18
Speaker 1: And that's the mind bending thing about special relativities, that
18:21
Speaker 1: different observers tell different stories about what happened, and they're
18:25
Speaker 1: both accurate. They're both like honest observers telling conflicting stories
18:29
Speaker 1: and both being correct. But I guess it's not about
18:32
Speaker 1: different things happening. It's more about our perception of these
18:35
Speaker 1: things happening, maybe because it's all related to how it
18:37
Speaker 1: affects time, right, like time is sort of flexible, time
18:42
Speaker 1: is not universal, and exactly we don't have a consistent
18:45
Speaker 1: clock through the whole universe that says like what happened
18:48
Speaker 1: at every moment, and then what happened at the next moment.
18:51
Speaker 1: What happens depends on where you are and how fast
18:54
Speaker 1: you are going relative to the events, and sometimes this
18:58
Speaker 1: flexibility there. If there's no like causal connection where one
19:01
Speaker 1: thing has to happen before another, then different people can
19:03
Speaker 1: give different orders of events for what happened and both
19:06
Speaker 1: be correct. It's not just an issue of perception. Although
19:09
Speaker 1: you can, you know, use special relativity to say, well,
19:12
Speaker 1: I understand why you are seeing the opposite thing that
19:15
Speaker 1: I'm seeing. That's because of special relativity, but neither of
19:18
Speaker 1: you can say like A happen before b or be
19:20
Speaker 1: happen for a It depends on who you are and
19:23
Speaker 1: how fast you're going. Right, It's like nobody's clock is official.
19:27
Speaker 1: Everyone has a different clock, and so that you can't
19:29
Speaker 1: sort of say, like who's right or who's wrong, because
19:31
Speaker 1: in the end, everything is relative. Right, velocity itself is relative.
19:36
Speaker 1: You can't have a velocity just on its own. You
19:38
Speaker 1: can't say my spaceship is going at the speed of
19:40
Speaker 1: light and say well relative to what right, As we
19:44
Speaker 1: talked about once recently, like velocity doesn't even have a
19:47
Speaker 1: meaning if you are in a universe all by yourself, like,
19:50
Speaker 1: you can't have a speed if there's nothing else in
19:53
Speaker 1: the universe. So it's a whole new way to think
19:55
Speaker 1: about the nature of the universe and all these weird consequences,
19:58
Speaker 1: a lot of which come from this idea that light
20:00
Speaker 1: always travels at the speed of light. Yeah, So while
20:03
Speaker 1: it may seem intuitive that if you shine a flashlight
20:05
Speaker 1: at me, no matter how fast I'm going, that light
20:08
Speaker 1: is going to catch up to me, that may not
20:10
Speaker 1: actually be true depending on some of these weird consequences
20:14
Speaker 1: of special relativity. That's right in a simple case where
20:17
Speaker 1: you're like in a normal universe and space is flat
20:20
Speaker 1: and it's obeying these rules, then if somebody shines a
20:22
Speaker 1: flashlight in your direction, it doesn't matter how far away
20:26
Speaker 1: they are. Eventually that light will reach you. There's no
20:29
Speaker 1: like effective horizon, give an infinite time, that photon will
20:32
Speaker 1: catch you. There's nothing you can do, right, and that's
20:35
Speaker 1: because it's moving at the speed of light relative to you.
20:38
Speaker 1: Because light always moves at the speed of light. So
20:40
Speaker 1: in this sort of simple universe where space is not curved,
20:43
Speaker 1: nothing weird is going on. That flash of light will
20:45
Speaker 1: always catch you, right. But in our universe, the answer
20:49
Speaker 1: might be different. But first, let's take a quick break.
21:04
Speaker 1: All right, Daniel, you're trying to outrun a flashlight. I
21:07
Speaker 1: don't know why. I guess you don't want people to
21:08
Speaker 1: see you, or you want to remain sort of in
21:11
Speaker 1: the dark and mysterious. But you're trying to outrun a
21:13
Speaker 1: flashlight and somebody's shying a flashlight on you, and so
21:16
Speaker 1: it seems like it would inevitably the light would reach
21:19
Speaker 1: me because it's going faster than anything can go in
21:21
Speaker 1: the universe. But you're saying there might be some instances
21:24
Speaker 1: where I can actually outrun light. Mm hmmmm. And you
21:28
Speaker 1: know this is not just a silly physics thought experiment.
21:30
Speaker 1: This is really important. If you were playing laser tag
21:33
Speaker 1: and you're cornered and they shoot that thing right at you,
21:35
Speaker 1: you gotta find a way to escape. And so that's
21:37
Speaker 1: where we come in, right. This is totally serious and
21:40
Speaker 1: totally relevant to everyone's everyday lighte No, there are some
21:44
Speaker 1: scenarios where somebody points a flashlight at you and it
21:47
Speaker 1: doesn't ever hit you even an infinite time, even if
21:51
Speaker 1: it's pointed right at you, and you don't actually even
21:53
Speaker 1: have to be moving to avoid it. In some scenarios.
21:56
Speaker 1: So you might have heard my sort of legalistic maneuver
21:59
Speaker 1: there to avoid saying that the flashlight will always catch you.
22:02
Speaker 1: The condition is that if you are operating in flat space,
22:05
Speaker 1: in space, that's just where general relativity applies, and nothing
22:08
Speaker 1: is growing or shrinking. Right, So that gives us one
22:11
Speaker 1: that maybe the first instance in which you could maybe
22:14
Speaker 1: outrun a flashlight. Right. It has to do with this
22:16
Speaker 1: idea that space is not actually flat or constant or
22:20
Speaker 1: kind of boring. Yeah, it turns out that our universe
22:23
Speaker 1: is a lot more exciting than the special relativity universe, right.
22:26
Speaker 1: Most importantly, our universe is expanding. Now, A lot of
22:30
Speaker 1: people think that that means that galaxies are flying out
22:33
Speaker 1: from some sort of central dot at a high speed,
22:36
Speaker 1: that they're moving through space. But it's actually much weirder
22:39
Speaker 1: and more amazing than that. It's the expansion of space itself. Right.
22:44
Speaker 1: It's not the motion of things through space. I mean
22:46
Speaker 1: that's also happening, but dark energy. There's accelerating expansion of
22:50
Speaker 1: the universe. This is creating new space between our galaxy
22:54
Speaker 1: and other galaxies. It's not just moving the galaxies through
22:58
Speaker 1: the existing space. It's like stre watching the fabric of
23:01
Speaker 1: the universe itself. Right, I think that you're saying sort
23:03
Speaker 1: of like maybe I could outrun Hussein Bold if maybe
23:06
Speaker 1: I play around with the track, or like how the
23:08
Speaker 1: track is moving. Yeah, exactly. Put Usain Bolt on a
23:12
Speaker 1: treadmill and stand in front of the treadmill. It doesn't
23:15
Speaker 1: matter how fast he runs, That's what I mean. Yeah. Yeah,
23:18
Speaker 1: And in the same way, the universe is laying new track,
23:22
Speaker 1: new space between us and some really really distant photons,
23:26
Speaker 1: photons which left their galaxy or their star billions of
23:30
Speaker 1: years ago and they've been straining to reach us. Some
23:33
Speaker 1: of them will never get to us, even though they're
23:35
Speaker 1: pointed right at us, because the space between us and
23:38
Speaker 1: them is expanding faster than light can go through it,
23:41
Speaker 1: faster than the speed of light. Right. Well, it's it's
23:44
Speaker 1: not that the space itself at any given point is
23:47
Speaker 1: expanding faster than the speed of light. But it's more
23:49
Speaker 1: like there's so much space in between, and it's growing
23:52
Speaker 1: a little bit at each spot, so much that it
23:56
Speaker 1: overall is expanding faster than the speed of light. Yes,
24:00
Speaker 1: it's a very gentle effect on a local scale. Right,
24:02
Speaker 1: It's not like the distance between you and your partner,
24:05
Speaker 1: or between the Earth and the Sun is growing very
24:08
Speaker 1: very rapidly. Right, It's a very gentle effect over small distances.
24:12
Speaker 1: But as you say, larger distances, it adds up, right,
24:15
Speaker 1: And so between our star and the next star, the
24:18
Speaker 1: acceleration is a larger number. Between our galaxy and the
24:21
Speaker 1: next galaxy is an even bigger number between really really
24:25
Speaker 1: distant objects. Then that speed is faster than the speed
24:28
Speaker 1: of light. The space is being created faster than the
24:31
Speaker 1: light can go through it. Right, So I guess maybe
24:33
Speaker 1: paint the scenario out for us, Like if I wanted
24:35
Speaker 1: to outrun a flash of light from a flashlight, how
24:38
Speaker 1: would I do this? Like, we can't start in the
24:40
Speaker 1: same spot. I would have to go away from you
24:42
Speaker 1: for a while, right, or a certain amount of distance.
24:44
Speaker 1: That's right, If you start from the same spot as
24:46
Speaker 1: the photon, it's going to catch you instantly, Like you
24:48
Speaker 1: know by definition. If I'm holding the flashlight to your
24:51
Speaker 1: back and I press the button as soon as you
24:53
Speaker 1: start running, I caught you before you've even gone anywhere, Right,
24:56
Speaker 1: A T equal zero? Right if we're both like standing
24:59
Speaker 1: next to each other. Yeah, But if I give you
25:01
Speaker 1: a head start, and you start running, right, and you
25:03
Speaker 1: get a little distance, you get ten ms or so,
25:06
Speaker 1: or ten seconds before I shine the flashlight. Then if
25:10
Speaker 1: space is expanding between the flashlight and how far you
25:14
Speaker 1: got before I turned the flashlight on, then it might
25:17
Speaker 1: be that that photon never makes it through that space
25:20
Speaker 1: to catch you. Right. Well, let's take it one step
25:22
Speaker 1: at a time. Let's say I fly to Jupiter, or
25:24
Speaker 1: you let me run to Jupiter before you turn on
25:26
Speaker 1: the flashlight. So now I mean, but in Jupiter and
25:29
Speaker 1: I'm running somehow in space, and then you shine a
25:31
Speaker 1: flashlight on me. It's going to take a while, but
25:33
Speaker 1: it will catch me at that point, right, Eventually it
25:36
Speaker 1: will catch you at that point, Yes, because the expansion
25:38
Speaker 1: of space between here and Jupiter is not that impressive,
25:41
Speaker 1: not enough to overcome the speed of the photon. Right,
25:45
Speaker 1: It's like the space is growing a little bit, maybe
25:48
Speaker 1: like what like a millimeter or something per year or something. Yeah,
25:52
Speaker 1: And a lot of people ask this question, They say,
25:53
Speaker 1: why can't we see the expansion of space and our
25:56
Speaker 1: solar system? Why isn't it tearing things apart? Well, the
25:58
Speaker 1: reason is that gravity is pretty strong on a local scale, right,
26:02
Speaker 1: Remember it goes like one who were distant squared, and
26:05
Speaker 1: so the distances are pretty small, like Earth to Jupiter.
26:08
Speaker 1: Then gravity is more powerful than dark energy, so the
26:11
Speaker 1: Solar system holds itself together. So the distance between the
26:14
Speaker 1: Sun and Jupiter is not actually growing at all due
26:16
Speaker 1: to the expansion of the universe, because they're holding tight
26:19
Speaker 1: onto each other, the same way the Earth and the
26:21
Speaker 1: Moon are, or the same way the atoms in your
26:23
Speaker 1: body are holding on to each other. So that's why
26:25
Speaker 1: we only really see this thing between galaxies or even
26:28
Speaker 1: between galaxy clusters, because smaller than that, gravity sort of wins.
26:32
Speaker 1: It's like tying everybody together. Imagine like a gentle breeze
26:36
Speaker 1: is blowing out everywhere, but people are holding on to
26:38
Speaker 1: each other and so they're able to resist it. But
26:41
Speaker 1: over large distances, this breeze adds up and it becomes
26:44
Speaker 1: a really powerful force. It's a breezy universe. Alright. Well,
26:48
Speaker 1: let's say then that you give me a really big
26:50
Speaker 1: head start and I start running at the nearest galaxy,
26:54
Speaker 1: which I think is Andromeda. Andromeda. Yeah, I was about
26:58
Speaker 1: to say that. So let's say you let me get
27:00
Speaker 1: as far as in Drameda and then I start running,
27:02
Speaker 1: and then you shine a flash light on me, It's
27:05
Speaker 1: still gonna catch me. It's still gonna catch you because
27:07
Speaker 1: the expansion of space between here and Andromeda is not
27:09
Speaker 1: that impressive, right, So, like, I'll start running, and how
27:13
Speaker 1: far is Andromeda? Like eight millions of light years? Will
27:16
Speaker 1: take millions of light years for the light to sort
27:18
Speaker 1: of get to where I'm running, And in those millions
27:21
Speaker 1: of years, I will also have run a good bit.
27:23
Speaker 1: But eventually those two things will catch up, right, Like
27:26
Speaker 1: the light will eventually. It might take millions of years,
27:29
Speaker 1: but it'll light will eventually hit me in the back.
27:31
Speaker 1: That's right. If you are running a constant speed, like
27:33
Speaker 1: let's say you were running at half the speed of
27:35
Speaker 1: light because you're super impressive relative to the Earth, and
27:38
Speaker 1: I shine that flashlight at the speed of light, then
27:41
Speaker 1: it will eventually catch you. And you'll look back at
27:43
Speaker 1: the flashlight and you'll say, oh, that light is moving
27:45
Speaker 1: at the speed of light relative to me, and it
27:47
Speaker 1: will catch up to you, you know, in several million years,
27:50
Speaker 1: if you are several million light years ahead when it begins.
27:53
Speaker 1: But I see, if you're moving at constant speed and
27:56
Speaker 1: there's not that much expansion of the space between us
27:58
Speaker 1: and between us and Drama. H it's not enough for
28:01
Speaker 1: you to outrun the flashlight, right, Like the space between
28:04
Speaker 1: here and Drama is is expanding, but it's maybe expanding
28:07
Speaker 1: at I don't know, ten meters per second or something.
28:10
Speaker 1: That's right, And you know there's a little wrinkled there
28:12
Speaker 1: because in Drama actually happens to be moving towards us
28:15
Speaker 1: even though the space is expanding between us, gravity there
28:18
Speaker 1: is winning and it's pulling and Drameda towards us, and
28:21
Speaker 1: you know their local deviations, Like space itself is expanding,
28:24
Speaker 1: but things are still moving through that space as we
28:27
Speaker 1: talked about, like driven by gravity and other forces. So
28:30
Speaker 1: Andrameda is getting closer to us, even though the space
28:33
Speaker 1: is expanding, sort of sort of like swimming upstream against
28:36
Speaker 1: that expansion, right, I guess I'm just saying, like Andrama
28:39
Speaker 1: is where I start running, not that I stay with
28:41
Speaker 1: and Drameda, right, Right, even if you start running from
28:43
Speaker 1: Andrameda and go past it, then that photon will still
28:46
Speaker 1: catch you. The expansion of the universe not enough to
28:48
Speaker 1: overcome that, right, Like it's expanding but only a little bit,
28:51
Speaker 1: so light can still rip through it. So then kind
28:53
Speaker 1: of like, at what point does the expansion of the
28:57
Speaker 1: universe start to approach the speed of light so that
28:59
Speaker 1: like can't rip through it. So it's something like sixty
29:02
Speaker 1: billion light years away. If I gave you a head
29:06
Speaker 1: start of sixty billion light years then and shine a
29:09
Speaker 1: flashlight at you, that light would never catch you sixty
29:13
Speaker 1: billion light years. Is there enough universe for me to
29:16
Speaker 1: get that much of a head start. We just don't know, right,
29:19
Speaker 1: We have no idea what's out beyond the edge of
29:21
Speaker 1: the observable universe, And that's actually the threshold of the
29:25
Speaker 1: observable universe. Like photons that were created at T equal zero,
29:29
Speaker 1: the beginning of the universe sixty two billion light years
29:32
Speaker 1: away will never reach us. Those photons, even if they're
29:36
Speaker 1: pointed right at us, we'll never get here because the
29:39
Speaker 1: expansion of the universe will create new space faster than
29:42
Speaker 1: they can move it. So nothing that's beyond that we
29:45
Speaker 1: will ever ever see. Right. It's kind of like the
29:48
Speaker 1: resverse problem, right, Like there might be somebody sixty billion
29:52
Speaker 1: light years away that shines a flashlight at us to
29:55
Speaker 1: try to tag us, but that light will never catch
29:57
Speaker 1: up to us because the space is expanding too fast, exactly,
30:01
Speaker 1: and in the same way a flashlight we send from
30:04
Speaker 1: here to there. If you start running there, then that
30:06
Speaker 1: photon is never going to catch those people. No matter
30:08
Speaker 1: how fast they're going or slow. They could just sit
30:10
Speaker 1: on their butts and they will never be hit by
30:12
Speaker 1: that photon. Right, And that's what you call the Hubble's law. Right,
30:16
Speaker 1: Like the velocity of how space is growing is getting
30:19
Speaker 1: bigger with distance, That's right. Hubble's law tells us about
30:22
Speaker 1: the recession velocity, how fast something is moving away from us,
30:25
Speaker 1: and how that's getting faster and faster as you go
30:27
Speaker 1: further and further. So as you get further away from us,
30:30
Speaker 1: things are moving away from us faster and faster. At
30:32
Speaker 1: some point that speed exceeds the speed of light. And
30:35
Speaker 1: that's called the Hubble volume. And the Hubble volume is
30:37
Speaker 1: this spear that surrounds us. Right, And because the universe
30:41
Speaker 1: is expanding and that expansion is accelerating, then that volume
30:45
Speaker 1: is actually shrinking. Right. We can see a smaller and
30:48
Speaker 1: smaller fraction of the universe as time goes on. As
30:52
Speaker 1: time goes on, you can be closer and closer to us.
30:55
Speaker 1: Shoot a flashlight at us, and it will never get
30:57
Speaker 1: to us because this expansion is accelerating. Right, Yeah, that
31:02
Speaker 1: whole part of the universe, maybe the rest of the
31:04
Speaker 1: entire universe is basically dark to us, right, it's invisible,
31:08
Speaker 1: like we can never see it. Yeah, and things that
31:10
Speaker 1: we used to be able to see are disappearing. Right.
31:13
Speaker 1: It might be that if you shine a flashlight at
31:15
Speaker 1: us an early part of the universe, it would get here.
31:18
Speaker 1: But then later on if you waited too long, if
31:20
Speaker 1: you waited ten billion years and then showing your flashlight
31:22
Speaker 1: at us, it wouldn't ever get here because the expansion
31:25
Speaker 1: has increased and accelerated beyond that threshold. Well, if it
31:28
Speaker 1: wants to move away that fast, away from us, you know,
31:31
Speaker 1: maybe we don't want to see it. What have they
31:33
Speaker 1: got to hide anyway? Yeah, what's wrong with us? What
31:36
Speaker 1: do you mean? Why are they running away from us
31:38
Speaker 1: that fast? So then what does that mean? What that
31:40
Speaker 1: means that the furthest anything we'll ever see is about
31:42
Speaker 1: sixty two billion years away, Yes, sixty two billion light
31:46
Speaker 1: years away. If the expansion of the universe continues accelerating
31:49
Speaker 1: the way that it has, then we will never see
31:51
Speaker 1: anything further away. Than sixty two billion light years. There's
31:55
Speaker 1: a caveat. There's always a caveat, a fine print, meaning
31:59
Speaker 1: like of the space will stop expanding, right, Like, we
32:02
Speaker 1: don't know, well, we don't understand why that expansion is
32:05
Speaker 1: accelerating and what's doing it. All we see is that
32:07
Speaker 1: it's happening. And since we don't understand why, we have
32:09
Speaker 1: no idea what the mechanism is, we can't predict its future.
32:12
Speaker 1: We don't know what's driving it. We know that it
32:14
Speaker 1: turned on a few billion years ago, so we don't
32:16
Speaker 1: understand any of that. And so we could stop, and
32:18
Speaker 1: it could turn around. It could shrink the universe so
32:21
Speaker 1: that the things that we're always invisible to us now
32:23
Speaker 1: become visible. The simplest thing to do is to extrapolate
32:26
Speaker 1: that nothing's going to change. But you know, we've been
32:28
Speaker 1: wrong before, right And like you said, like you have
32:31
Speaker 1: been sort of wrong before, Like this expansion of the
32:33
Speaker 1: universe wasn't always there, like it seems to have turned
32:36
Speaker 1: on at some point. Yeah, exactly. It's not something we
32:39
Speaker 1: understand very well. We talked about this on the podcast
32:42
Speaker 1: actually once about the history of dark energy. We think
32:44
Speaker 1: that maybe the amount of dark energy is constant, but
32:47
Speaker 1: dark energy doesn't get deluded as the universe grows, like
32:50
Speaker 1: as more spaces created, every unit of space also has
32:54
Speaker 1: more dark energy, and so over time it comes to
32:57
Speaker 1: sort of dominate what's happening in the universe. And that's
33:00
Speaker 1: why we think maybe it sort of took over about
33:02
Speaker 1: five billion years ago and became the thing that drove
33:05
Speaker 1: the whole universe. All right, So that's one way to
33:07
Speaker 1: answer the question can you outrun a flashlight? And the
33:10
Speaker 1: answer is yes, you know, because of expanding space. If
33:15
Speaker 1: the space between you and the person shooting the flashlight
33:18
Speaker 1: at you is big enough and that space is expanding
33:21
Speaker 1: fast enough, then you can't outrun light. Amazingly, I guess
33:25
Speaker 1: you can avoid it. What if I just moved sideways,
33:28
Speaker 1: Daniel would save us all a lot of trouble. Wow
33:32
Speaker 1: podcast simplified. So right now, the way to outrun a
33:35
Speaker 1: flashlight is just travel sixty two billion light years away
33:38
Speaker 1: and then nobody will ever be able to laser tag
33:40
Speaker 1: you or invent a dark energy machine that can stretch
33:43
Speaker 1: space between you and that photon arbitrarily quickly, and then hey,
33:47
Speaker 1: you can just do it yourself. Wow. That would be
33:49
Speaker 1: like dark energy tag that would be a totally different product.
33:52
Speaker 1: All right, Well, it turns out that that's not the
33:54
Speaker 1: only way that you can outrun a flashlight. There is
33:57
Speaker 1: another way that you can do it within the rule
34:00
Speaker 1: of the universe, and it doesn't require you to go
34:03
Speaker 1: out that far. So let's get into it. But first,
34:06
Speaker 1: let's take another quick break, right, Daniel, Can you outrun
34:21
Speaker 1: a flashlight? And we figured out one way to do
34:24
Speaker 1: that that if you go out far enough, the space
34:26
Speaker 1: is expanding fast enough that light will never catch up
34:29
Speaker 1: to you. But apparently there is another way that you
34:32
Speaker 1: can outrun light without having to depend on the you know,
34:37
Speaker 1: the diet of space. That's right, And this is a
34:40
Speaker 1: real mind bender. This one took me a while to
34:42
Speaker 1: figure out. It feels like it really contradicts everything you
34:45
Speaker 1: know about special relativity. If you spent a lot of
34:48
Speaker 1: time thinking about special relativity and adjusting to the idea
34:52
Speaker 1: that light always moves at the speed of light relative
34:55
Speaker 1: to everybody else, this one's gonna feel like it breaks
34:58
Speaker 1: that rule. But it actually is a natural consequence of
35:01
Speaker 1: special relativity, and it breaks that rule because it breaks
35:04
Speaker 1: one of the assumptions of special relativity, which is about acceleration.
35:08
Speaker 1: So the idea is that you can out run a
35:11
Speaker 1: flash of light if you're moving in a rocket ship
35:14
Speaker 1: with constant acceleration, if you're always always speeding up, then
35:19
Speaker 1: that photon will never catch you. And the way that
35:22
Speaker 1: it avoids the rules of special relativity is that one
35:24
Speaker 1: word acceleration. Most of the rules of special relativity require
35:29
Speaker 1: you to not be accelerating to have constant relative velocity. Interesting,
35:34
Speaker 1: I see, so like, if I'm trying to outrun a
35:36
Speaker 1: flashlight and I start running, it's not just about never
35:39
Speaker 1: stopping or always running or always you know, moving, It's
35:42
Speaker 1: about like each second you have to go a little
35:45
Speaker 1: bit faster than you were before. That's what acceleration means.
35:48
Speaker 1: That's right, when Usain Bolt comes out of the blocks,
35:50
Speaker 1: he goes from zero meters per second to ten per
35:54
Speaker 1: second pretty quickly. He's accelerating, he's changing his speed, but
35:58
Speaker 1: then at some point he stays at ten ms or second,
36:00
Speaker 1: right Like, even losing Bold can't accelerate forever on the ground,
36:05
Speaker 1: right Like, somehow there's something about his body and air
36:08
Speaker 1: resistance and his muscles that just prevent them from going
36:11
Speaker 1: faster and faster and faster and faster. That's right. So
36:14
Speaker 1: you need constant acceleration to make this work, and you
36:16
Speaker 1: don't need to be able to go faster than the
36:19
Speaker 1: speed of light. Is not some trick where you're like
36:22
Speaker 1: always adding the same amount of speed per second. You
36:25
Speaker 1: can be constantly accelerating and asthemptotically approaching the speed of light,
36:29
Speaker 1: never even actually reaching the speed of light. But if
36:32
Speaker 1: you're getting faster and faster every second, then you can
36:35
Speaker 1: avoid that photon ever hitting your back. Right, Because even
36:39
Speaker 1: you are always accelerating, always gaining speed, you're still never
36:42
Speaker 1: going to go faster than the speed of light. Like somehow,
36:45
Speaker 1: the way special relativity works, it's like you're just gonna
36:47
Speaker 1: be trying and trying, but you'll never actually go faster
36:50
Speaker 1: than the speed of light. You can never reach the
36:52
Speaker 1: speed of light because you have mass things with mask
36:54
Speaker 1: and never reach the speed of light. What you can
36:57
Speaker 1: do is add more energy, right, You can get more energy.
36:59
Speaker 1: You're energy can increase the arbitrary infinite amounts, but your
37:03
Speaker 1: velocity doesn't track it approaches the speed of light, never
37:07
Speaker 1: actually gets there, right, And this is where it gets
37:09
Speaker 1: kind of counterintuitive, because you know, I mean, I'm accelerating,
37:13
Speaker 1: going faster and faster, but I'll never actually reach the
37:15
Speaker 1: speed of light. And so you're saying that even if
37:19
Speaker 1: somebody shoots his light at me that is going at
37:22
Speaker 1: the speed of light, even though it will always be
37:24
Speaker 1: going faster than me, you're saying, there's a possibility that
37:27
Speaker 1: it might not catch me. Yeah, it will not catch you.
37:30
Speaker 1: You know. The scenario I'm imagining is that you're like,
37:32
Speaker 1: you start out ahead of me, maybe I'll give you
37:34
Speaker 1: a ten meter head start, and you start accelerating, and
37:37
Speaker 1: then I turned on this flash of light. Then you
37:38
Speaker 1: know you are going to be going a certain speed,
37:40
Speaker 1: going faster and faster and faster, approaching the speed of light.
37:43
Speaker 1: I'm shooting this flashlight that's moving at the speed of light,
37:47
Speaker 1: and so you might think, well, I'm pretty good special relativity.
37:49
Speaker 1: If Jorge looks back and asks how fast is that
37:53
Speaker 1: photon traveling relative to me, he should give the answer
37:56
Speaker 1: of the speed of light, because, as we said before,
37:59
Speaker 1: photons travel at the speed of light no matter who's
38:01
Speaker 1: measuring them, right, And so then it feels pretty simple.
38:03
Speaker 1: You're like, well, if the photon is moving relative to
38:05
Speaker 1: me at the speed of light, it will eventually catch me.
38:09
Speaker 1: And that seems pretty solid. But all those calculations you
38:13
Speaker 1: just did in your head assume no acceleration. Those calculations
38:17
Speaker 1: are only true in inertial frames, where there's no acceleration,
38:21
Speaker 1: there's only relative constant velocity. It's a little bit different.
38:24
Speaker 1: When you add acceleration. You have to go to general
38:27
Speaker 1: relativity because acceleration, it turns out, is equivalent to gravity
38:32
Speaker 1: a right. That's one of Einstein's big discoveries is that
38:35
Speaker 1: you can't tell the difference, for example, between gravity and acceleration.
38:39
Speaker 1: If you're like in an elevator in space, you can't
38:42
Speaker 1: tell is that elevator accelerating or am I near some
38:45
Speaker 1: planet that's creating gravity. It's the same thing, and so
38:49
Speaker 1: you have to account for the effective curvature of space
38:52
Speaker 1: that you're creating for yourself when you're accelerating. Right. But
38:56
Speaker 1: I guess, maybe let's be clear, like, there are situations
38:58
Speaker 1: where it will catch you. Right. If I'm standing next
39:01
Speaker 1: to using bold and he starts running and he's accelerating,
39:04
Speaker 1: he might be accelerating getting up to his ten per second,
39:07
Speaker 1: But if I shine a flashlight on him right away,
39:09
Speaker 1: it's going to catch him, right, if you shine a
39:11
Speaker 1: flashlight on him soon enough. Absolutely, But if his acceleration
39:15
Speaker 1: is high enough, right, and he started out with enough
39:17
Speaker 1: of a head start, then it will never catch him.
39:20
Speaker 1: If he has constant acceleration. So those are two special things, right,
39:24
Speaker 1: Like he needs a head start, yes, and I also
39:26
Speaker 1: have to wait a certain amount of time before I
39:28
Speaker 1: turn on my flashlight. You can turn on your flashlight
39:30
Speaker 1: at the same moment as long as he has a
39:32
Speaker 1: physical head start. He either needs a head start in
39:34
Speaker 1: time so he can get some distance gap, or he
39:37
Speaker 1: needs to start like ten ms away from you. But
39:39
Speaker 1: he can start running at the same moment as you
39:41
Speaker 1: turn on that flashlight, and the flashlight will never catch
39:44
Speaker 1: him if he continues to accelerate forever till the end
39:47
Speaker 1: of which is pretty tough. Yeah, I don't know if
39:50
Speaker 1: he signed up for that. He might have other things
39:53
Speaker 1: he wants to do. I mean, somebody give that guy
39:55
Speaker 1: a power bar or something. I mean in a number
39:57
Speaker 1: of power bars, right, Because you can look at it
39:59
Speaker 1: the other way and say, well, the photon will catch
40:01
Speaker 1: you at time equals infinity because your speed is approaching
40:05
Speaker 1: the speed of life but never catching it. So at
40:07
Speaker 1: time equals infinity, the photon will get to you. But
40:09
Speaker 1: time equals infinity isn't a real time, right, the universe
40:13
Speaker 1: could go on forever, will never get there. I see.
40:16
Speaker 1: But there are two ingredients of this, right, Like you
40:18
Speaker 1: need a head start, and you need to be always
40:21
Speaker 1: accelerating at a certain rate, right, And I imagine that
40:24
Speaker 1: if I'm only accelerating a little bit at a time,
40:27
Speaker 1: that I need a really big head start. But if
40:29
Speaker 1: I accelerate really fast, if I have like a constant
40:32
Speaker 1: rocket pushing me, then I don't need that big of
40:35
Speaker 1: a head start exactly. And you can calculate it by
40:37
Speaker 1: looking at it from the other point of view, like
40:39
Speaker 1: from us sain Bolt point of view, What is this
40:41
Speaker 1: Like if you're riding on his shoulder, for example, and
40:43
Speaker 1: you're looking back as he's accelerating, then you might think, well,
40:46
Speaker 1: then what this means is that there's a distance beyond
40:49
Speaker 1: which you cannot see, Like somebody who shines a flashlight
40:52
Speaker 1: at you from there tries to send you a message
40:54
Speaker 1: that information will never reach you. So there's like this horizon,
40:57
Speaker 1: this wall beyond which it's just lack and the distance
41:01
Speaker 1: to that wall depends on his acceleration. So if he's
41:04
Speaker 1: accelerating really really fast, then that horizon is closer. If
41:07
Speaker 1: he's accelerating really really slowly, the horizon needs to be
41:10
Speaker 1: further further away, So as the acceleration goes to zero,
41:13
Speaker 1: that horizon is infinitely far away. But the distance to
41:16
Speaker 1: that horizon is the speed of light squared divided by
41:18
Speaker 1: your acceleration. But I guess it still feels counterintuitive, right
41:22
Speaker 1: because the light is going at the speed of light,
41:24
Speaker 1: but Hussein Bulb will never reach the speed of light.
41:27
Speaker 1: So how is it the light will never reach him? Right, Like,
41:30
Speaker 1: wouldn't it eventually make up ground? Yeah, you would think so,
41:33
Speaker 1: and that would be true if there was no acceleration.
41:36
Speaker 1: But remember, acceleration means all these rules get bent a
41:38
Speaker 1: little bit, the same way like space gets bent. And
41:41
Speaker 1: so the way that sort of incorporated into your brain
41:43
Speaker 1: is to think about how acceleration is like an effective
41:46
Speaker 1: bending of space, and when space gets bent, all of
41:49
Speaker 1: your intuition about who catches what go out the window.
41:53
Speaker 1: You know, for example, like if somebody's inside a black
41:56
Speaker 1: hole and they shine a flashlight at you, that photon
41:59
Speaker 1: is never getting to you. It doesn't matter how much
42:01
Speaker 1: time it takes. Why because space is bent, right, and
42:04
Speaker 1: those photons follow that bent space. So if you are
42:07
Speaker 1: doing constant acceleration, then it's sort of equivalent to gravity.
42:11
Speaker 1: It's sort of like bending space. And so that's what
42:14
Speaker 1: creates this horizon. It's not an event horizon. It's not
42:16
Speaker 1: like a real physical horizon, but for you, it creates
42:19
Speaker 1: like an information horizon. Accelerating objects have an information horizon,
42:24
Speaker 1: which is pretty weird interesting, sort of like a content acceleration.
42:27
Speaker 1: It's almost like the space between us is expanding, that's
42:30
Speaker 1: what you're saying. It's like somehow if I accelerate fast
42:33
Speaker 1: enough and I'm further enough away, then that expansion of space,
42:37
Speaker 1: which is really just my acceleration, is going to prevent
42:39
Speaker 1: the light from reaching me. Yeah, exactly. But it's not
42:42
Speaker 1: like a real physical horizon, right, It's only it's from
42:45
Speaker 1: your point of view. And again, different people have different
42:48
Speaker 1: points of view, and those things can conflict. If I'm
42:50
Speaker 1: watching this whole series of events from a spaceship floating nearby,
42:54
Speaker 1: I might see the photon catching Usain bolt, right, and
42:57
Speaker 1: so I can see a different series of events. It's
43:00
Speaker 1: just like with a black hole, right, if I'm watching
43:03
Speaker 1: you fall into a black hole from the outside, I
43:06
Speaker 1: never see you following a black hole. You never get in.
43:08
Speaker 1: You smeared across the event horizon forever. Oh wait a minute,
43:12
Speaker 1: for you, you fall right through that event horizon. You're
43:14
Speaker 1: inside the black hole. In the same way. These different
43:18
Speaker 1: accounts can conflict. Uh Now I feel a little cheated, Daniel.
43:21
Speaker 1: I think what you're saying is that if I try
43:23
Speaker 1: to Lusain Bold, who's always accelerating, Hussein Bold will think
43:27
Speaker 1: that he ut ran the flash of light, but we
43:31
Speaker 1: are going to see him lose. The person sending the
43:33
Speaker 1: light won't see it hit him, because he's accelerating relative
43:36
Speaker 1: to them, he won't see it hitting them. But another
43:38
Speaker 1: observer moving in a different direction it's possible for them
43:41
Speaker 1: to see the light hitting him. The story depends on
43:44
Speaker 1: exactly the location and velocity of everybody involved. It gets
43:48
Speaker 1: pretty hairy with general relativity. Man, right, I guess that
43:51
Speaker 1: you're saying that to us Saint's Bold kind of frame
43:53
Speaker 1: of reference, his experience of things, that light will reach
43:56
Speaker 1: him but only in an infinity, like when time ends
44:00
Speaker 1: for him. But for someone who's moving in another frame
44:03
Speaker 1: or speed, there is a time at which the light
44:05
Speaker 1: will hit him. Yes, exactly. There's almost always another velocity
44:09
Speaker 1: or location you can get to to tell a different
44:11
Speaker 1: story about the same series of events. That's the lesson
44:14
Speaker 1: of the universe is that there is no universal history.
44:16
Speaker 1: There's no true single account of what happens in this universe.
44:20
Speaker 1: Well there is. I mean, the light does catch up
44:22
Speaker 1: to him, it's just that for him it happens at infinity,
44:24
Speaker 1: and for us it happens non infinity. Yeah, if happening
44:28
Speaker 1: in infinity accounts is happening, then you know, I'll pay
44:30
Speaker 1: you that twenty infinity. Yeah, let's keep talking here until
44:35
Speaker 1: infinity and see if that's the same thing for everyone.
44:38
Speaker 1: But that's kind of what you mean, is like, it
44:39
Speaker 1: does happen, but at infinity. But that's only from his
44:42
Speaker 1: point of view. That's right, because relativity is weird in
44:45
Speaker 1: that way. It's pretty weird stuff, all right. Well, then
44:48
Speaker 1: that's the answer to the question is can you running flashlight?
44:51
Speaker 1: The answer is yes if you got far enough and
44:53
Speaker 1: space is expanding fast enough. And also yes if you
44:56
Speaker 1: are bold, I guess and you you can't wait till
44:59
Speaker 1: an infinity or you only consider it infinity as never,
45:04
Speaker 1: or if you have an infinite number of power bars
45:06
Speaker 1: and you can accelerate forever into the future. Right to you,
45:10
Speaker 1: you will always win. But maybe to somebody else the
45:12
Speaker 1: answer will be different. But you'll never have to talk
45:14
Speaker 1: to them because you're accelerating away from them forever. That's
45:18
Speaker 1: the true benefit here is never haven't you talked to
45:21
Speaker 1: or see anyone? Ever? Again? All right, that's a pretty
45:24
Speaker 1: mind bending question, and again just a reminder of how
45:27
Speaker 1: weird this universe is and how weird the rules of
45:29
Speaker 1: it are. Right, it's not just that there are weird
45:31
Speaker 1: things in it. It's just that it is a weird
45:33
Speaker 1: universe in itself. It certainly is. But we love it.
45:36
Speaker 1: We love the weirdness. Stay weird universe. All right, Well,
45:39
Speaker 1: we hope you enjoyed that. Thanks for joining us, See
45:42
Speaker 1: you next time. Thanks for listening, and remember that Daniel
45:52
Speaker 1: and Jorge explain the Universe is a production of I
45:55
Speaker 1: Heart Radio or more podcast from my Heart Radio. Visit
45:59
Speaker 1: the I Heart Radio, Apple Apple Podcasts, or wherever you
46:02
Speaker 1: listen to your favorite shows. H