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Can fast-moving potatoes become black holes? · Daniel and Kelly’s Extraordinary Universe

0:05
Speaker 1: The world out there is a wonderful, intriguing puzzle. Sometimes
0:09
Speaker 1: it feels like it makes little sense, constantly refusing to
0:12
Speaker 1: obey our intuition or behave the way that we expect
0:15
Speaker 1: it to. Quantum mechanics tells us electrons don't follow smooth paths,
0:19
Speaker 1: relativity tells us time doesn't flow the same way for everyone,
0:23
Speaker 1: And yet we can make some sense of it. We
0:26
Speaker 1: have math that describes it and predicts it. It's incredible
0:30
Speaker 1: that the universe is so finely balanced. It's complicated enough
0:34
Speaker 1: that it's taking us thousands of years to unravel its truths,
0:38
Speaker 1: but it's simple enough that our tiny brains can make
0:41
Speaker 1: progress year after year. So let's keep hoping that truant continues.
0:46
Speaker 1: But even when we have wrangled a bit of math
0:48
Speaker 1: that seems to work, it doesn't mean our intuition goes
0:51
Speaker 1: along for the ride. Sometimes when we translate the math
0:54
Speaker 1: into popular science intuition and dramatic clickbait, the reality is lost.
1:00
Speaker 1: You've heard that a potato approaching the speed of light
1:02
Speaker 1: can turn into a black hole, or that photons don't
1:05
Speaker 1: experience time. Unfortunately, neither of those things are accurate. Today,
1:09
Speaker 1: we're going to try to untangle some of these widespread
1:12
Speaker 1: misconceptions about special relativity and show you that the truth
1:15
Speaker 1: of the universe is plenty weird and wonderful without clickbait.
1:19
Speaker 1: Do Photon's experience time. Does a potato become a black
1:23
Speaker 1: hole if it goes fast enough? Welcome to Daniel and
1:26
Speaker 1: Kelly's extraordinary but amazingly comprehensible universe.
1:44
Speaker 2: Hello, I'm Kelly Widerspin. I'm a biologist, and I am
1:46
Speaker 2: super excited to let Daniel be the wet blanket today.
1:52
Speaker 1: Hi, I'm Daniel. I'm a particle physicist, and I'm not
1:55
Speaker 1: going to throw a wet blanket on the universe. I'm
1:57
Speaker 1: going to show you how it's even weirder than what
2:00
Speaker 1: you've been told.
2:01
Speaker 2: Yeah, that's what I do all the time. Also, Daniel,
2:04
Speaker 2: it's all about packaging the idea.
2:06
Speaker 1: That's right, exactly, make it sound positive, that's right.
2:10
Speaker 2: So my question for you today is when you were
2:12
Speaker 2: a kid and you watched like physics y kinds of movies,
2:16
Speaker 2: were you concerned with whether or not they were accurate
2:19
Speaker 2: or did kid Daniel just enjoy whatever he was watching.
2:23
Speaker 1: Hey, there's an implication there that adult Daniel can't enjoy movies.
2:27
Speaker 1: I don't appreciate that. That's not true.
2:30
Speaker 2: Go ahead and tell us about the trajectory of your
2:33
Speaker 2: life and how it has evolved with movie watching.
2:35
Speaker 1: You know, when I watched movies like that as a kid,
2:37
Speaker 1: I was just amazed and odd. I thought it was
2:39
Speaker 1: fantastic to think about these things and to push against
2:42
Speaker 1: the edge of our knowledge. Could we live in four
2:44
Speaker 1: dimensional space? Could aliens be six dimensional beings? Is it
2:48
Speaker 1: possible to travel through time? I thought those movies were
2:51
Speaker 1: wonderful because they pushed us past the edge of our
2:53
Speaker 1: current knowledge and imagined how the universe could be different.
2:57
Speaker 1: And now, of course, as a grown up scientist, I
2:59
Speaker 1: know something about what we do and don't know, and
3:01
Speaker 1: which of those ideas are possible and which of those
3:03
Speaker 1: ideas are not really possible. But I appreciate the creativity
3:07
Speaker 1: in that. I think it's important that we push past
3:09
Speaker 1: that and then we try to break outside of the
3:11
Speaker 1: box of our current ideas.
3:12
Speaker 2: So you could watch a movie that clearly had an
3:15
Speaker 2: incorrect physics concept and straight up enjoy it. Is that right?
3:21
Speaker 1: You know, if they're going to embrace life in a
3:23
Speaker 1: different universe with different laws, that's fine. Go for it.
3:25
Speaker 1: But if they're going to pretend to be in this
3:27
Speaker 1: universe and then they drop a bunch of like pop
3:29
Speaker 1: sign nonsense to make it sound like they talk to
3:32
Speaker 1: a physicist when they didn't, and they're all higgs bos
3:35
Speaker 1: on this and quantum fluctuation that, then yeah, that's annoying
3:38
Speaker 1: because like, hey, reach out to a scientist. It's not
3:40
Speaker 1: so hard to get like actual realistic scientific babble. Instead
3:44
Speaker 1: appear nonsense from your chat GPT, like come on email me,
3:48
Speaker 1: I'll answer. I'll help you get actual science into your movie.
3:51
Speaker 2: I think the one time where I was like, maybe
3:54
Speaker 2: I'm going to ask for my money back? What was?
3:56
Speaker 2: The movie was with John Cusack and I loved John Cusack,
3:59
Speaker 2: but it was like the new Trinos have mutated and everything.
4:03
Speaker 2: I was like, oh, can I get past this? I
4:05
Speaker 2: don't think I can. It was one of those like
4:07
Speaker 2: Ends of the World disaster movies.
4:10
Speaker 1: The new Trenos are going to kill us, all that's right.
4:13
Speaker 2: I was like, they're neutral, guys, It's all right anyway.
4:17
Speaker 1: Yeah. I mean, if people want to embrace something awesome
4:20
Speaker 1: and new, that's fantastic. And I think that being created
4:22
Speaker 1: that way is cool but if you want to use
4:24
Speaker 1: real physics, like reach out to a real scientist. There's
4:27
Speaker 1: so many folks who would happily help you make the
4:29
Speaker 1: science in your show real. And you know, then it's
4:32
Speaker 1: even better because the people who are listening and know
4:34
Speaker 1: something are then like not jerked out of their experience
4:38
Speaker 1: by the bizarre nonsense you just injected into their brains.
4:42
Speaker 2: We would all love to be involved in TV shows
4:44
Speaker 2: and movies that makes us like the cool kid in
4:46
Speaker 2: the room, So like, give us the chance to be
4:48
Speaker 2: the cool kid man.
4:49
Speaker 1: Yeah, Christopher Nolan write to me, come.
4:51
Speaker 2: On, that's right, that's right, And there's got to be
4:54
Speaker 2: a bunch of parasite movies that people could be, you know,
4:56
Speaker 2: talking to me about. But all right, well, today we're
4:59
Speaker 2: focusing on two very specific ideas that are often wrong
5:03
Speaker 2: in popsid.
5:04
Speaker 1: Yeah, that's right, because there's a lot of popular science
5:06
Speaker 1: descriptions of special relativity what happens as you approach the
5:10
Speaker 1: speed of light and what's it like to be a
5:12
Speaker 1: photon and all this kind of stuff, and some of
5:15
Speaker 1: it sounds really cool and it's really fun to read about,
5:17
Speaker 1: but it's actually kind of nonsense, and the worst part
5:20
Speaker 1: is that it's obscuring the reality, the awesomeness of our universe,
5:25
Speaker 1: and the truth is always so much weirder than the nonsense,
5:29
Speaker 1: which I love. And so because people write in quite
5:31
Speaker 1: often asking these kinds of questions, I thought it'd be
5:34
Speaker 1: fun to try to disentangle some of the common misconceptions
5:37
Speaker 1: about special relativity. But before we dig into those particular topics,
5:41
Speaker 1: I was curious what people thought were the most popular
5:44
Speaker 1: misconceptions about special relativity, the things that people didn't understand
5:49
Speaker 1: about it. So I went out and asked our listeners
5:51
Speaker 1: if you would like to join our group of volunteers,
5:54
Speaker 1: please don't be shy or right to us two questions
5:56
Speaker 1: at Danielankelly dot org.
5:58
Speaker 2: We were going through and Daniel and I really to survey,
6:00
Speaker 2: which you can get on our website Daniel and Kelly
6:02
Speaker 2: dot com. And I was going through some of the answers.
6:05
Speaker 1: Daniel and Kelly dot org unless you're a guest at
6:07
Speaker 1: Daniel and Kelly's wedding, in which case go to Daniel
6:09
Speaker 1: Kelly dot com. Congratulations you too, love he thank you
6:13
Speaker 1: for the correction.
6:14
Speaker 2: Yeah, I was looking through the answers and somebody wrote
6:17
Speaker 2: to say that they would like to hear a lot
6:18
Speaker 2: more female voices for this question and answer session, and
6:22
Speaker 2: so if you are a woman who is on that list,
6:26
Speaker 2: we would really love to hear from you. Or if
6:27
Speaker 2: you're a woman who would like to answer these questions,
6:29
Speaker 2: we would really love to hear from you. I had
6:31
Speaker 2: a journalist reach out to me the other day and
6:33
Speaker 2: I told them, oh, sorry, I'm not an expert in
6:35
Speaker 2: that area, so I'm gonna pass. And they wrote back
6:38
Speaker 2: and they said, men never pass. Women are the ones
6:43
Speaker 2: who are like, oh, I'm not quite sure I'm an
6:44
Speaker 2: expert in this. And so what I'm saying is, ladies,
6:47
Speaker 2: let's go all in on the confidence and just answer it.
6:49
Speaker 2: And if the answer is I don't know, that's fine too.
6:51
Speaker 2: Let's get your voice out there.
6:53
Speaker 1: Absolutely, ladies, please chime in. All right, without further ado,
6:58
Speaker 1: here's what our listeners had to say.
7:00
Speaker 3: Whether I'm thinking of special general relativity a time dilation.
7:05
Speaker 2: If you put a gun to my head, I couldn't
7:07
Speaker 2: tell you.
7:08
Speaker 1: The difference between special and general relativity time dilation.
7:12
Speaker 3: That if you somehow get into a ship and approach
7:15
Speaker 3: the speed of light or close to the speed of light,
7:18
Speaker 3: that your clock or un slower.
7:19
Speaker 1: The speed of light is constant for all observers, regardless
7:24
Speaker 1: of their motion or location or whatever. Why does the
7:27
Speaker 1: speed of light always appear to be at the speed
7:30
Speaker 1: of light regardless of my speed? Well, I'm ashamed to
7:34
Speaker 1: say I can't remember the difference between special and general relativity.
7:39
Speaker 1: How to become special enough to be recognized by the
7:44
Speaker 1: Special relativity crew. The visualizations of invisible fabrics, as well
7:48
Speaker 1: as two dimensional sheets used to explain four dimensional curvature,
7:51
Speaker 1: makes wascetime more confusing. Why is it special?
7:56
Speaker 3: There's more photons than anything else, so when that kind
8:00
Speaker 3: to be a general thing?
8:01
Speaker 1: How observers can see events in different orders.
8:05
Speaker 2: That if you exceed light speed the time will run backwards.
8:08
Speaker 3: I've never understood why we say the universe is about
8:11
Speaker 3: fourteen billion years old, but a photon traveling at the
8:15
Speaker 3: speed of light since the Big Bang has experienced no
8:19
Speaker 3: change in.
8:20
Speaker 1: Time god fact of the speed of light. Why is
8:24
Speaker 1: the speed of light three times twenty eighth meters per
8:27
Speaker 1: second and not some other number.
8:29
Speaker 4: I don't understand how, despite the variables, light can never
8:35
Speaker 4: exceed three hundred thousand kilometers per second, that.
8:40
Speaker 3: The speed of light is constant, no matter where you're
8:43
Speaker 3: observing it from or how fast you're going.
8:46
Speaker 1: It's the fact that the speed of light is a constant.
8:49
Speaker 1: Misunderstood is E equals mc squared because most people think
8:52
Speaker 1: it's got something to do with relativity.
8:55
Speaker 3: That there is no god beside the universe with an
8:57
Speaker 3: absolute clock who judges who was right.
9:01
Speaker 1: Fixed speed in a vacuum, no matter what your speed is, All.
9:05
Speaker 2: Right, some great answers. And I got to say, every
9:07
Speaker 2: time we have a conversation, I get closer to understanding relativity,
9:11
Speaker 2: but not all of it sticks. And so I'm looking
9:14
Speaker 2: forward to being reminded of the difference between special and
9:17
Speaker 2: general relativity today.
9:19
Speaker 1: Right, So today we're only digging into special relativity. General
9:22
Speaker 1: relativity is a whole other hairy beast we're not even
9:25
Speaker 1: going to try to tackle today because special relativity, even
9:28
Speaker 1: though it's simpler than general relativity, is plenty confusing. You know,
9:32
Speaker 1: it already requires you to distort how your brain works
9:35
Speaker 1: and accept the time flows differently for people in different
9:38
Speaker 1: parts of the universe. Briefly, special relativity is physics in
9:41
Speaker 1: flat space, where there's no curvature, there's no mass, there's
9:44
Speaker 1: no gravity, it's just like beams of light and clocks
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Speaker 1: and things moving fast, and it tells us how time
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Speaker 1: flows and how things are relative, even outside of like
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Speaker 1: black holes and all that weirdness.
9:56
Speaker 2: So this is not dealing with any bending of space
9:59
Speaker 2: or time.
10:00
Speaker 1: That's right, Yeah, exactly. Imagine space is purely flat and
10:04
Speaker 1: we got a bunch of physics nerds doing experiments with
10:06
Speaker 1: lasers and clocks and cats and all sorts of stuff.
10:09
Speaker 1: But we've got no black holes, no gravity, nothing like that.
10:12
Speaker 2: All right, great, I think I can wrap my head
10:14
Speaker 2: around that. Where do we start.
10:15
Speaker 1: Then, Well, my favorite thing about special relativity is that
10:20
Speaker 1: it lets you dig deep into your understanding of like
10:22
Speaker 1: the very fabric of reality. And I know that sounds
10:25
Speaker 1: like pomps and grandiose, but it's true because that's what
10:28
Speaker 1: it's dealing with. It's describing space and time and helping
10:32
Speaker 1: us grapple with like what that really means. And you know, philosophically,
10:35
Speaker 1: it's awesome that you can say, hey, here's a mathematical
10:38
Speaker 1: model that describes what's happening out there in the universe.
10:41
Speaker 1: And if it works, if it describes the universe faithfully,
10:44
Speaker 1: then I can look at that model and say, hmm,
10:47
Speaker 1: maybe I can learn something about the universe by studying
10:49
Speaker 1: this model. If I have a mathematical description of the universe,
10:52
Speaker 1: does that description reveal something about reality to me? That's
10:56
Speaker 1: the juice in all of this physics. And you know,
10:58
Speaker 1: one of the most important things to understand and in
11:00
Speaker 1: special relativity are very basic things like location, like velocity,
11:05
Speaker 1: like acceleration. Like what do these things mean in special relativity?
11:09
Speaker 1: And how are they different from how Newton or Archimedes
11:12
Speaker 1: or Aristotle thought about these very basic concepts.
11:16
Speaker 2: A while back, we had two topics that I think
11:17
Speaker 2: would be helpful intros to this, if anybody wants to
11:20
Speaker 2: go back and get some more background. We had an
11:22
Speaker 2: episode on what is time and what is space? And
11:24
Speaker 2: as I was looking at your outline, I thought, oh,
11:26
Speaker 2: that's helpful background information for this discussion.
11:28
Speaker 1: Yeah, And so the headlines for special relativity are that
11:31
Speaker 1: location and velocity are purely relative. There's no absolute measure
11:36
Speaker 1: of location velocity, but acceleration is not. Acceleration is an
11:40
Speaker 1: absolute quantity. So let's start with location. What do we
11:44
Speaker 1: mean when we say location is relative? It just means
11:48
Speaker 1: that your location can only be described relative to other stuff.
11:52
Speaker 1: There's no like set of markers in the universe, no
11:55
Speaker 1: like axis glowing in space where you can say my
11:58
Speaker 1: location is in this quadrant. It's always like I am
12:01
Speaker 1: five kilometers from that, or I am two meters from this.
12:06
Speaker 1: That's the only way to define your location in space.
12:10
Speaker 1: There's no reference points. Another way to think about this
12:12
Speaker 1: is that location is a property of a pair of points,
12:16
Speaker 1: not a single object. Like a single object doesn't have
12:18
Speaker 1: a location. Two objects have a distance between them, But
12:23
Speaker 1: there's no meaning to say my coffee is at this location.
12:27
Speaker 1: You can say my coffee is a meter from the ground,
12:29
Speaker 1: or my coffee is six thousand kilometers from the center
12:32
Speaker 1: of the Earth. But my coffee has no location on
12:35
Speaker 1: its own, and.
12:36
Speaker 2: There's no greater question than where is my coffee? So
12:42
Speaker 2: thank you physicists.
12:45
Speaker 1: Exactly. I mean, I'm trying to be practical here. You know,
12:47
Speaker 1: it's not all space ships and cats flying through space.
12:50
Speaker 1: And this is sort of hard for a lot of
12:51
Speaker 1: people to wrap their minds around because they think of
12:54
Speaker 1: locations as absolute the way Newton did. Right. Newton thought
12:57
Speaker 1: of space as absolutely thought. Even if there's nothing in
13:00
Speaker 1: the universe, there is still space. But you know, we
13:02
Speaker 1: think of space as just the distance between things, and
13:05
Speaker 1: that's really important framework for understanding general relativity if you
13:09
Speaker 1: get there, because the bending of space can be understood
13:12
Speaker 1: not as like the curature of space relative to some
13:16
Speaker 1: other external metric. There is no other external metric, but
13:19
Speaker 1: just the changing distances between things, because that's all we have.
13:24
Speaker 1: So it's important to remember that distances are just relative. Right,
13:27
Speaker 1: your location is just relative, And that's also true about velocity.
13:33
Speaker 1: Velocity also purely a relative quantity. This one, I think
13:37
Speaker 1: is even harder for people to grapple with.
13:39
Speaker 2: So you said, there's no like grid, and this is
13:41
Speaker 2: a stupid question, but like, would our understanding of location
13:46
Speaker 2: change if we had infinite money and we put a
13:48
Speaker 2: grid in space and like you know, every one light
13:52
Speaker 2: year we put like a blinking buoy like in the ocean.
13:55
Speaker 2: How would we still be in the same position even
13:57
Speaker 2: if we had that grid.
13:58
Speaker 1: Yeah, if we had infinite money, wouldn't coommend spending it
14:01
Speaker 1: on that grid because it wouldn't change our relationship with space,
14:03
Speaker 1: Like you could still define your location relative to a
14:07
Speaker 1: point on that grid. That's fine, But now you've created
14:10
Speaker 1: an object. You've put it there, and you said I'm
14:12
Speaker 1: going to create axis at this location, and therefore I
14:15
Speaker 1: can define my distance from this buoy or from that
14:17
Speaker 1: booey or from the other thing. That's no different from
14:20
Speaker 1: saying here I am relative to my coffee or a
14:22
Speaker 1: relative to the center of the earth. But you haven't
14:24
Speaker 1: anchored those booies to like space itself. Right. You could
14:27
Speaker 1: shift all of space over and nothing would change in
14:30
Speaker 1: your measurements because space itself doesn't have a location. There's
14:33
Speaker 1: no frame to space itself. Right. You can't say my
14:37
Speaker 1: booys are here relative to space. That means nothing, right,
14:41
Speaker 1: space has no frame.
14:43
Speaker 2: You just saved us all a lot of money.
14:47
Speaker 1: I'd like to take five percent of that infinite dollars
14:49
Speaker 1: divert it to my own research.
14:51
Speaker 2: You might need it, all right, So let's move on
14:53
Speaker 2: to velocity and acceleration. And I'm going to be honest,
14:56
Speaker 2: I often forget the definition of velocity and the definition
15:00
Speaker 2: of acceleration. So let's just start there.
15:03
Speaker 1: Okay, cool, Well, velocity and principle is very simple. It's
15:06
Speaker 1: how is your location changing? Okap. Location is in units
15:10
Speaker 1: of like distance, meters, kilometers, whatever. Velocity is distance per time, right,
15:15
Speaker 1: So meters per second? How is your location changing? So
15:19
Speaker 1: if you're in a car on the ground, your location
15:22
Speaker 1: is where are you relative to your house, for example,
15:25
Speaker 1: and your velocity is how quickly is that location changing? Right,
15:30
Speaker 1: So that's why you measure it in like meters per
15:32
Speaker 1: second or kilometers per hour something.
15:34
Speaker 2: Okay, So the way you just said it, it's also
15:36
Speaker 2: relative to something.
15:38
Speaker 1: You don't have an absolute velocity exactly. You always measure
15:42
Speaker 1: relative to something because there are no absolute references, right,
15:45
Speaker 1: How could you measure it in an absolute sense? If
15:47
Speaker 1: space itself is not absolute, if there's nothing to grab
15:50
Speaker 1: onto on space, nothing everybody can agree on. So you
15:53
Speaker 1: and I can measure the speed of a passing baseball
15:56
Speaker 1: and we can disagree because maybe you're in a car
15:58
Speaker 1: and I'm on the ground, and so you measure the
16:00
Speaker 1: baseball traveling at one speed and I'm measured traveling at
16:02
Speaker 1: another speed. The baseball has no speed inherently. It has
16:06
Speaker 1: a speed relative to you and a speed relative to me.
16:10
Speaker 1: I think that makes sense to people, but it tells
16:12
Speaker 1: us that speeding in is not a property of the object.
16:15
Speaker 1: It's a property of a pair of objects. And I
16:18
Speaker 1: think most people are totally cool with that, and I
16:20
Speaker 1: see you nodding. But then you get a lot of
16:22
Speaker 1: people talking about what happens when you're moving near the
16:26
Speaker 1: speed of light. My question is always near the speed
16:29
Speaker 1: of light relative to what There is no I'm moving
16:32
Speaker 1: near the speed of light or I'm not moving near
16:34
Speaker 1: the speed of light. You're already moving near the speed
16:37
Speaker 1: of light relative to particles that are shooting at the Earth.
16:40
Speaker 1: If those particles are moving towards the Earth at near
16:43
Speaker 1: the speed of light, the Earth is moving towards them
16:45
Speaker 1: at near the speed of light. So you're already moving
16:48
Speaker 1: near the speed of light. You're also moving at zero
16:50
Speaker 1: velocity relative to your shoes right, which are probably I
16:53
Speaker 1: hope attached to you, or to your head, let's say
16:56
Speaker 1: your head, for example. So you have multiple speeds. You
16:59
Speaker 1: have speed relative to any potential observer. So it makes
17:02
Speaker 1: no sense to say I'm moving near the speed of
17:05
Speaker 1: light or I'm not moving near the speed of light.
17:07
Speaker 1: You're moving at any speed relative to any observer that
17:10
Speaker 1: could be observing you.
17:12
Speaker 2: I didn't realize I could claim that I was going
17:13
Speaker 2: so fast. I'm gonna go ahead and across the exercise
17:16
Speaker 2: box off of my new year's resolution list. I am
17:19
Speaker 2: speeding around this universe.
17:22
Speaker 1: I always tell people you're very quick.
17:23
Speaker 2: Yes, you're very quick too, We're all very quick. It
17:26
Speaker 2: depends on what you're comparing to.
17:28
Speaker 1: Yeah, exactly. And this is an amazing and confusing thing
17:31
Speaker 1: about special relativity because time depends on this speed. Right.
17:36
Speaker 1: What we say in special relativity is that moving clocks
17:38
Speaker 1: run slow. So if there's a spaceship zooming past the
17:42
Speaker 1: Earth and it has high speed relative to the Earth, right,
17:46
Speaker 1: people writing with relativity questions and they say a spaceship
17:49
Speaker 1: is moving near the speed of light, and I say
17:51
Speaker 1: near the speed of light relative to who? Right, relative
17:54
Speaker 1: to what? In this case, the spaceship is moving near
17:56
Speaker 1: the speed of light relative to the Earth. The Earth
17:59
Speaker 1: sees a space ship's clocks as running slowly. That might
18:02
Speaker 1: make sense. People are cool with that. But then because
18:06
Speaker 1: it's symmetric, because the spaceship also sees the Earth as
18:10
Speaker 1: moving near the speed of light. Because hey, we just
18:11
Speaker 1: told you velocity is relative, right, That means that the
18:15
Speaker 1: spaceship sees the Earth's clocks as running slow. And that's
18:19
Speaker 1: the amazing thing about special relativity. If you really grasp
18:22
Speaker 1: the relativity of velocity. It means that you can't have
18:26
Speaker 1: the same clock everywhere, that everybody doesn't have to agree
18:29
Speaker 1: about how time flows. I'm right that the spaceship's clocks
18:32
Speaker 1: are running slow. The spaceship is right that my clocks
18:35
Speaker 1: are running slow. That's both true because we don't have
18:38
Speaker 1: to have the same story about what's happening in the universe.
18:41
Speaker 1: Special relativity tells us there's no absolute space, there's no
18:44
Speaker 1: absolute time, there is no absolute history of the universe.
18:48
Speaker 1: Boom boom.
18:50
Speaker 2: Now that does seem like a good place to start
18:52
Speaker 2: a movie or a sci fi novel. So I can
18:54
Speaker 2: see why this comes up so often.
18:55
Speaker 1: Right, So keep that in mind when we're later talking
18:57
Speaker 1: about whether potatoes can into black holes or what it's
19:01
Speaker 1: like to be a photon. But there is one thing
19:03
Speaker 1: in the universe which really amazingly, fascinatingly is absolute about
19:08
Speaker 1: the universe, and that's acceleration. So we started with location.
19:12
Speaker 1: That's just like, where are you relative to some arbitrary
19:15
Speaker 1: grid where you anchorated some object. You know, distance from
19:19
Speaker 1: my coffee cup, or distance from my toes, or distance
19:21
Speaker 1: from the center of the sun. That's location. Velocity is
19:24
Speaker 1: how is your location changing acceleration is how is your
19:28
Speaker 1: velocity changing. So for the math nerds out there, we're
19:31
Speaker 1: now two derivatives. In right, velocity is the slope or
19:35
Speaker 1: derivative of your location. If you're plotting where is your
19:39
Speaker 1: location versus time, then velocity is the slope of that plot.
19:43
Speaker 1: If you then plot your velocity, acceleration is the slope
19:46
Speaker 1: of the velocity plot. So it's just like, how much
19:48
Speaker 1: is velocity changing? Velocity is how much is location changing?
19:51
Speaker 2: Got it? I kind of liked calculus.
19:56
Speaker 1: I love the connection between calculus and physics, right, Like
19:59
Speaker 1: knowing this makes physics so simple and straightforward. You're like, oh,
20:03
Speaker 1: acceleration is just the derivative of velocity, which is just
20:06
Speaker 1: the derivative of location. So I can just like derive
20:09
Speaker 1: my equations of emotions from one fact like a constant acceleration. Boom,
20:14
Speaker 1: I know the accelerations emotion, Just integrate twice to get
20:16
Speaker 1: the position. Boom, You're done.
20:18
Speaker 2: Elementary maybe not quite that easy.
20:23
Speaker 1: But I had this great moment because my son is
20:25
Speaker 1: taking physics right now, and he's also taking calculus, and
20:28
Speaker 1: so he knows these tools and he was learning about
20:31
Speaker 1: equations of motion and I was like dude, you can
20:33
Speaker 1: just integrate this twice and get that answer. And he
20:36
Speaker 1: was like, oh, and that's why it's one half at squared.
20:38
Speaker 1: I get it. Cool. And he had this like moment,
20:40
Speaker 1: We're all clicked in his mind, and I was like, dude,
20:43
Speaker 1: math and physics dancing together to explain the universe.
20:46
Speaker 2: Aw. That must have been a proud moment as a
20:48
Speaker 2: father to be able to observe that.
20:50
Speaker 1: It was twenty years in the making, but it was cool.
20:53
Speaker 2: Sometimes you got to wait a long time for the payoffs,
20:55
Speaker 2: but you got there.
20:56
Speaker 1: But the amazing thing about acceleration is that it is absolute.
21:00
Speaker 1: It's the one thing which does belong to you. Right.
21:03
Speaker 1: Your location is relative, it's a property of a pair
21:05
Speaker 1: of objects. Your velocity is relative. It only has meaning
21:09
Speaker 1: in relation to something else you're measuring with respect to
21:12
Speaker 1: acceleration is something you own. Your acceleration is just your own.
21:16
Speaker 1: You don't need to measure it relative to anything else.
21:19
Speaker 1: And this is Einstein's famous thought experiment. Right in a box,
21:23
Speaker 1: you can't tell where you are or how fast you're
21:25
Speaker 1: going because those things only have meaning relative to stuff
21:28
Speaker 1: outside the box. But inside the box, you can measure
21:30
Speaker 1: your acceleration.
21:31
Speaker 2: My first question is, so is it because the reference
21:34
Speaker 2: point is now what you were doing like a second ago,
21:36
Speaker 2: and because that's like an internal comparison, that's why it's
21:40
Speaker 2: not relative silences. I always think, was this a great
21:45
Speaker 2: question or a really dumb question? Dane was trying to
21:47
Speaker 2: figure out, how do I not insult Kelly's intelligence?
21:49
Speaker 1: No, not at all. The fun moments for me and
21:52
Speaker 1: teaching are hearing somebody's question and then trying to work
21:55
Speaker 1: backwards to what is going on in your mind that
21:57
Speaker 1: made you ask that question, so that my an or
22:00
Speaker 1: is the most helpful, and for me, like that's the
22:02
Speaker 1: fun part about teaching. That's always the puzzle.
22:05
Speaker 2: I enjoy giving you lots of those opportunities.
22:07
Speaker 1: Daniel, You know, it's a really amazing property of the universe,
22:14
Speaker 1: and it's not something I think we understand philosophically. It's
22:18
Speaker 1: just something we observe, like this is our description of
22:22
Speaker 1: the universe as we experience it. We know that you
22:24
Speaker 1: cannot measure your velocity or your location, but you can
22:27
Speaker 1: measure your acceleration, So fundamentally, this is a description of
22:31
Speaker 1: something we see in the universe, and this is one
22:33
Speaker 1: of those moments when you can say, Okay, this is
22:35
Speaker 1: the description of the universe. What does it mean about
22:38
Speaker 1: the universe that velocity is relative and acceleration is not.
22:41
Speaker 1: And it's actually one of the connections to general relativity
22:44
Speaker 1: because acceleration can be seen as equivalent to curvature. Acceleration
22:49
Speaker 1: has all the same effects on the motion of an
22:51
Speaker 1: object as curvature. In fact, we describe curvature sometimes as
22:55
Speaker 1: a pseudo force, right, that's what gravity is. Curvature is gravity,
22:59
Speaker 1: and gravity is a pseudoforce that generates apparent acceleration. So
23:04
Speaker 1: that's actually a much more complex topic and connect special
23:07
Speaker 1: in general relativity. So I think for the purposes of
23:09
Speaker 1: today's conversation, let's just say this is something we observe
23:12
Speaker 1: in the universe, and we do our best to describe it.
23:15
Speaker 1: And I think the best way to get your handle
23:17
Speaker 1: on the intuition for acceleration is to imagine, like, well,
23:20
Speaker 1: how would you measure it? I told you you can't
23:22
Speaker 1: measure your velocity accept relative to other stuff, And you
23:25
Speaker 1: can imagine, like being in a box flying through space
23:28
Speaker 1: where you don't have access to anything outside, how would
23:30
Speaker 1: you measure your velocity? There's no experiment you can come
23:33
Speaker 1: up with that can measure your velocity because you need
23:35
Speaker 1: access to stuff outside because that's the only way to
23:37
Speaker 1: define your velocity. But you can measure your acceleration inside
23:41
Speaker 1: that box.
23:42
Speaker 2: And how would I do that? Like say I was
23:44
Speaker 2: standing in there with the ball, would that help me?
23:46
Speaker 1: Yes? Absolutely? Just drop the ball. See what happens right now.
23:50
Speaker 1: If you're not accelerating, the ball will just hang there
23:53
Speaker 1: with you. If you are accelerating, then the ball will
23:56
Speaker 1: move because there'll be a pseudo force generated by your acceleration.
24:00
Speaker 1: And we do this all the time. You feel this
24:02
Speaker 1: every time you drive somewhere. Right, If you're driving the
24:05
Speaker 1: car and somebody hits the accelerator right, then what happens.
24:08
Speaker 1: You feel pressed back into your seat. If you dropped
24:11
Speaker 1: a ball right, then the ball would fly backwards. If
24:14
Speaker 1: somebody hits the brakes, which is also an acceleration, then
24:17
Speaker 1: you feel pushed forwards.
24:18
Speaker 3: Right.
24:18
Speaker 1: There's a phudo force there as you're pushed forward. If
24:20
Speaker 1: you dropped the ball, it would fly forwards. Right. The
24:23
Speaker 1: reason you have a seat belt is because of this effect.
24:26
Speaker 1: So it's very easy to tell whether you're accelerating. Just
24:29
Speaker 1: bring a ball or bring a scale. Right, a scale,
24:32
Speaker 1: it literally is measuring acceleration. That's why, for example, when
24:35
Speaker 1: you stand on a scale in space, you measure nothing
24:38
Speaker 1: because you're in free fall, there is no acceleration, whereas
24:41
Speaker 1: if you stand on the surface of the Earth, you
24:43
Speaker 1: do measure acceleration. You're measuring the Earth accelerating up and
24:47
Speaker 1: out to keep you from falling towards the center of
24:50
Speaker 1: the Earth.
24:51
Speaker 2: We are recording this pretty close to the end of
24:53
Speaker 2: the holiday, so I'm going to stick with the ball
24:55
Speaker 2: instead of the scale. Let's take a break, and when
24:58
Speaker 2: we get back from the break, going to talk about
25:00
Speaker 2: whether or not potatoes turn into black holes if they're
25:03
Speaker 2: going fast enough. Okay, Daniel, So here's my question for you.
25:23
Speaker 2: Do things change their mass as you approach the speed
25:27
Speaker 2: of lie?
25:29
Speaker 1: Yeah? Right? And this question was inspired by a listener
25:32
Speaker 1: who wrote in and heard on another podcast that potatoes,
25:35
Speaker 1: if they approach the speed of light, were turned into
25:38
Speaker 1: black holes. And like, I won't comment on whether you
25:41
Speaker 1: might turn into a black hole if you eat too
25:42
Speaker 1: many potatoes after the holidays, but I do want to
25:45
Speaker 1: dig into this question of what happens to a potato
25:48
Speaker 1: immediately your eyebras scrunch up when you hear this question,
25:51
Speaker 1: because even the question like what happens to a potato
25:54
Speaker 1: when approaches the speed of light, you might think like, well,
25:56
Speaker 1: the speed of light relative to what? Right? Because velocity
25:59
Speaker 1: is not the property of a potato. So it doesn't
26:02
Speaker 1: make sense to even talk about a potato having a speed.
26:05
Speaker 1: Is it moving at that speed relative to the Earth,
26:07
Speaker 1: relative to a spaceship, relative to some particle? The question
26:11
Speaker 1: itself already doesn't make sense, and that tells you that
26:14
Speaker 1: the answer can't.
26:15
Speaker 2: Be yes and we're done.
26:18
Speaker 1: I mean, for example, like take a potato. Maybe have
26:21
Speaker 1: a potato in your kitchen. What is the velocity of
26:23
Speaker 1: that potato? Well, it's moving at zero relative to your kitchen.
26:27
Speaker 1: Probably it's already moving near the speed of light relative
26:31
Speaker 1: to anything that's moving towards the Earth at near the
26:33
Speaker 1: speed of light. And there's lots of stuff moving towards
26:36
Speaker 1: the Earth at near the speed of light. There's particles
26:38
Speaker 1: shooting from space at super high energy. Is at ninety
26:41
Speaker 1: nine point nine percent of the speed of light. Your
26:45
Speaker 1: potato is moving near the speed of light relative to
26:48
Speaker 1: that particle. Is your potato a black hole?
26:50
Speaker 3: No?
26:51
Speaker 1: So the answer. Everybody who has a potato is doing
26:54
Speaker 1: this experiment right now, so we know your potato is
26:57
Speaker 1: not turning into a black hole. And the amazing thing
27:00
Speaker 1: about black holes is that they are observer independent. Some
27:04
Speaker 1: of the things we talked about earlier are observer dependent,
27:07
Speaker 1: like I see your clock as slowing down if I
27:09
Speaker 1: see you moving quickly, and that's observer dependent because it
27:13
Speaker 1: depends on my velocity relative to you. But black holes
27:17
Speaker 1: are not observer dependent. They exist in every frame, if
27:21
Speaker 1: they exist at all. So it's not like I can
27:24
Speaker 1: see the potato as a black hole because it's moving
27:26
Speaker 1: fast relative to me, but you don't see the potato
27:29
Speaker 1: as a black hole because it's sitting next to you.
27:31
Speaker 1: Everybody has to agree whether it's a black hole or not.
27:34
Speaker 2: So if we assumed that when they were talking about
27:36
Speaker 2: the speed of the potato it was relative to Earth,
27:39
Speaker 2: does that solve because we all have the same frame
27:41
Speaker 2: of reference.
27:42
Speaker 1: No, because you can always possibly some observer are moving
27:44
Speaker 1: with the potato somewhere else. Yeah, exactly, and there's always
27:47
Speaker 1: some particle there to do that observing. And the root
27:50
Speaker 1: of this comes from a historically sort of fascinating idea
27:53
Speaker 1: about mass. You often hear that mass increases as your
27:57
Speaker 1: approach to the speed of light. And again, I hope
27:59
Speaker 1: your ears to turn up at that and go like, hmm,
28:02
Speaker 1: who's measuring the speed in that case? And though this
28:04
Speaker 1: is often quoted, it makes little sense, right, because it
28:07
Speaker 1: doesn't make sense for mass to be observer dependent. If
28:12
Speaker 1: you're moving past me near the speed of light, does
28:15
Speaker 1: it make sense for me to measure mass as larger
28:18
Speaker 1: than somebody else to measure your mass? Right? Mass can't
28:21
Speaker 1: be observed dependent if it has consequences like if you
28:24
Speaker 1: have enough of it you turn into a black hole, right,
28:26
Speaker 1: And we know that's not observer dependent. So what's going
28:29
Speaker 1: on here is an old concept in relativity which are
28:34
Speaker 1: sort of picked up on and propagated and been repeated
28:36
Speaker 1: over and over and over again, even though it doesn't
28:38
Speaker 1: really make much sense.
28:39
Speaker 2: What is the old idea that's being repeated.
28:42
Speaker 1: So this is basically all Einstein's fault, because when Einstein
28:45
Speaker 1: was developing relativity, he had to think about how some
28:49
Speaker 1: of these basic concepts change in this new notion, in
28:52
Speaker 1: this new perspective of the universe, right, And so he
28:55
Speaker 1: was thinking about speed and momentum and energy and mass,
29:00
Speaker 1: And you know, some of these things are the same
29:02
Speaker 1: for Newton, and some of these things are different. Right,
29:04
Speaker 1: velocity is similar, but it has a maximum value now,
29:08
Speaker 1: and so that changes. And what does that mean about
29:10
Speaker 1: changing energy and changing momentum, Because like energy doesn't have
29:13
Speaker 1: a maximum value. You can have an infinite amount of
29:15
Speaker 1: energy even if your speed only approaches a certain value.
29:18
Speaker 1: So like the relationship between these quantities have to change.
29:22
Speaker 1: And so Einstein had to reimagine what these quantities were,
29:25
Speaker 1: and for a moment, he came up with this idea
29:28
Speaker 1: of relativistic mass, saying like, well, let's treat an object
29:32
Speaker 1: as if it had more mass if its velocity is greater.
29:35
Speaker 1: And so in his early writing he came up with
29:38
Speaker 1: this concept, and he wrote the equations for mass increasing
29:41
Speaker 1: with velocity.
29:42
Speaker 2: So anyone who was confused a moment ago about whether
29:45
Speaker 2: or not mass changes with velocity can feel good knowing
29:47
Speaker 2: that Einstein was making the same mistake.
29:49
Speaker 1: Yes, exactly, And you know it's fair like you're exploring
29:52
Speaker 1: these new concepts, you're wondering, like, how do we generalize
29:56
Speaker 1: We went from one idea to another. What gets change,
29:59
Speaker 1: what doesn't get change? What's the most sensible way for
30:01
Speaker 1: things to change, and it's fine for your first idea
30:04
Speaker 1: to not be the best idea. The problem with relativistic
30:07
Speaker 1: mass is that it doesn't really make sense and it's
30:09
Speaker 1: not really necessary. Doesn't really make sense because it means
30:13
Speaker 1: that your mass now depends on your velocity. So like
30:16
Speaker 1: that potato would have more mass or less mass based
30:19
Speaker 1: on who's measuring it, and also it would have different
30:22
Speaker 1: masses in different directions, right, Like, what does happen to
30:27
Speaker 1: the potato as we see it approach to the speed
30:29
Speaker 1: of light relative to us is that it gets harder
30:32
Speaker 1: to accelerate in one direction and not in others. Like,
30:35
Speaker 1: if the potato is already going at ninety nine percent
30:37
Speaker 1: of the speed of light as it whizzes by us,
30:40
Speaker 1: then it's harder for us to increase its velocity in
30:42
Speaker 1: the direction it's already moving, right, because it's already going
30:46
Speaker 1: near the speed of light in that direction. It's easier
30:49
Speaker 1: for me to push it perpendicular to its motion because
30:51
Speaker 1: it's not already moving at a very high velocity.
30:54
Speaker 2: It's easier for you to push it perpendicular because it's
30:57
Speaker 2: not already moving at a high velocity.
30:58
Speaker 1: Yeah, relativity velocity has this weird maximum. Right, nothing can
31:03
Speaker 1: go faster than the speed of light relative to anything else.
31:07
Speaker 1: And as you approach the speed of light relative to something,
31:10
Speaker 1: you can pour additional energy into that object. You can
31:13
Speaker 1: give it a push without increasing its velocity as much.
31:17
Speaker 1: Like if I apply the same force to a potato
31:20
Speaker 1: that said zero meters per second relative to me, it's
31:23
Speaker 1: going to speed up. And if I apply that same
31:25
Speaker 1: force to a potato that's already ninety nine point nine
31:27
Speaker 1: to nine percent of the speed of light relative to me,
31:29
Speaker 1: it's not going to speed up as much. It's just
31:31
Speaker 1: like not room for it to speed up as much.
31:33
Speaker 1: So I can pour energy into it without getting the
31:36
Speaker 1: same velocity return.
31:37
Speaker 2: Sure, but what was the perpendicular part?
31:39
Speaker 1: So if you try to think of that as, oh,
31:42
Speaker 1: the potato has additional mass, it's harder to accelerate because
31:46
Speaker 1: it's more massive, then you might think initially, okay, that
31:50
Speaker 1: makes sense. I can describe this as additional mass. It's
31:53
Speaker 1: harder to accelerate the potato if it's always already going
31:56
Speaker 1: really fast, so the same force doesn't give the same
31:59
Speaker 1: increase in velocity. That kind of makes sense, right, except
32:02
Speaker 1: that only makes sense in the direction the potato is going,
32:05
Speaker 1: because the potato can have no velocity in other directions. Right,
32:09
Speaker 1: so I'm free to apply a force to the potato
32:11
Speaker 1: in another direction and I get the same boost as
32:13
Speaker 1: I always did. And so now the potato has to
32:16
Speaker 1: have like a mass in this direction because it's hard
32:19
Speaker 1: to speed it up in that direction, and a mass
32:21
Speaker 1: in other directions where it's easier to speed it up.
32:24
Speaker 1: And so now mass has to have like directionality to it, right,
32:28
Speaker 1: instead of just being like a property of the object.
32:31
Speaker 2: I feel like I just felt the pieces click like
32:33
Speaker 2: in my hand. There. Okay, let's keep going.
32:35
Speaker 1: And so there is a way that could make sense
32:38
Speaker 1: if you're willing to have mass be this weird directional thing.
32:41
Speaker 1: But Einstein was like, Okay, actually, this doesn't make any sense,
32:44
Speaker 1: and you don't need it because you already have a
32:47
Speaker 1: concept of energy, the total energy. The object already captures
32:50
Speaker 1: this behavior, So you don't need this new weird directional
32:54
Speaker 1: relativistic mass. It doesn't give you anything, it doesn't help
32:56
Speaker 1: you at all. Let that be part of energy. And
32:59
Speaker 1: then Einstein and others decided, well, let's just keep mass
33:03
Speaker 1: to be a number and it'll be the amount of
33:06
Speaker 1: energy something has when it's at rest. So it's like
33:09
Speaker 1: the rest energy of the object, and that makes it
33:11
Speaker 1: invariant because you defined it to be the amount at rest.
33:15
Speaker 1: And so this is what we call invariant mass, and
33:17
Speaker 1: it means that energy is now nicely broken in two parts.
33:21
Speaker 1: The energy you have a rest which you call the
33:22
Speaker 1: invariant mass. So take for example, an electron. It has
33:25
Speaker 1: a mass even if you're holding it in your hand, right,
33:27
Speaker 1: that's what we call the invariant mass. That's the rest
33:30
Speaker 1: mass of the object. And you can also have energy
33:33
Speaker 1: if it's in motion, so that's its momentum. So energy
33:36
Speaker 1: now has two components, the rest mass, the invariant mass,
33:39
Speaker 1: and the motion part right, the energy of its motion.
33:43
Speaker 1: So those are two separate things. And the invariant mass
33:45
Speaker 1: by definition, doesn't grow with velocity because you measure it
33:49
Speaker 1: when it's at rest. So you might think that's just
33:53
Speaker 1: defining stuff, you're just defining it to be invariant trick. Yeah, yes,
33:57
Speaker 1: that's true. We're defining it to be at rest. That's
34:00
Speaker 1: really what mass is. And we're free to invent these
34:02
Speaker 1: quantities to be useful and to make sense to us,
34:04
Speaker 1: because hey, we're the audience of.
34:05
Speaker 2: It, right, right, we're creating tools that help us with stuff,
34:08
Speaker 2: so why not?
34:09
Speaker 1: Yeah, And if you're following along, remember our original question
34:13
Speaker 1: is does a potato turn into a black hole near
34:16
Speaker 1: the speed of light? And our answer so far is,
34:19
Speaker 1: let's be careful when we talk about energy of mass
34:22
Speaker 1: and energy of momentum. Energy momentum is relative, energy of
34:26
Speaker 1: mass is not. So listeners following along might be like, Okay,
34:29
Speaker 1: Daniels told us about the definition of energy and how
34:33
Speaker 1: it might be mass and how it might be menum.
34:35
Speaker 1: But in general relativity, we know that curvature space depends
34:39
Speaker 1: just on energy, right, It doesn't depend only on mass.
34:42
Speaker 1: It depends on more complex notions of energy. Because like
34:44
Speaker 1: photons can help bend space and they have no mass.
34:48
Speaker 1: So why don't potatoes create enough curvature if they have
34:52
Speaker 1: enough velocity, they have enough momentum, why can't that energy
34:56
Speaker 1: density then create black holes? And I have two answers
34:59
Speaker 1: to that. One is this is a really really complicated
35:02
Speaker 1: calculation to do because in general relativity, it's not just
35:06
Speaker 1: like a number. It's not like Newton's gravity where you
35:08
Speaker 1: have mass and more mass means more gravity. Einstein's equations
35:12
Speaker 1: are tensor equations, which means they're matrices. Is all sorts
35:15
Speaker 1: of complicated stuff, and different kinds of energy enter in
35:18
Speaker 1: different ways, So energy of mass enters differently from energy
35:21
Speaker 1: of velocity, and so it's a really complicated calculation. And
35:26
Speaker 1: we know that the answer has to be the same
35:28
Speaker 1: for a potato at rest and a potato in motion
35:31
Speaker 1: because black holes are not observer dependent. That's just like
35:34
Speaker 1: a bedrock fact in general relativity. So instead of doing
35:37
Speaker 1: a really complicated calculation where the potato is in motion,
35:40
Speaker 1: we know we'll get the same answer when the potato
35:42
Speaker 1: is at rest. And if the potato at rest doesn't
35:45
Speaker 1: give you a black hole, then the potato at motion
35:47
Speaker 1: can't give you a black hole, and it can't go
35:49
Speaker 1: through all the complicated math here on the podcast. But
35:52
Speaker 1: that's the sort of the end run around having to
35:55
Speaker 1: do all that math. We know a potato turning into
35:58
Speaker 1: a black hole if it's not already a black hole in.
36:01
Speaker 2: Your kitchen, So I don't have to live in fear
36:02
Speaker 2: of my potatoes. That is a relief.
36:07
Speaker 1: Well, potatoes going to hurt you in lots of ways,
36:09
Speaker 1: but they're not going to turn into black.
36:10
Speaker 2: Holes, that's right, all right. So we have struck down
36:13
Speaker 2: one misconception about potatoes and black holes, which I'm sure
36:18
Speaker 2: everybody woke up this morning thinking that they'd hear about
36:20
Speaker 2: potatoes and black holes. And when we get back, we're
36:22
Speaker 2: going to ask do photons experience time? All right? Daniel?
36:43
Speaker 2: You know I've often heard it said that light doesn't
36:46
Speaker 2: experience time? Is that right?
36:53
Speaker 1: Have you heard it said that photons experience our podcast
36:55
Speaker 1: and enjoy it? Are they faithful listeners?
36:58
Speaker 2: I mean, Daniel, how could they not everything from super
37:03
Speaker 2: organisms down to the smallest particles enjoy dk EU. How
37:06
Speaker 2: could they not?
37:07
Speaker 1: Exactly? And we're so good looking, right, and what do
37:09
Speaker 1: photons do if not appreciate our looks?
37:11
Speaker 2: Right?
37:12
Speaker 1: Yeah?
37:13
Speaker 2: Sure, I think we're getting on something ic here. But
37:17
Speaker 2: let's move forward, all.
37:18
Speaker 1: Right before we undermine our credibility too far. Yes, this
37:22
Speaker 1: is something you see in popular science all the time.
37:24
Speaker 1: Photons fly through the universe not experiencing time. So let's
37:28
Speaker 1: try and understand where this comes from, and then let's
37:31
Speaker 1: talk about what we actually know about it.
37:33
Speaker 3: All right?
37:33
Speaker 2: Where does this come from?
37:35
Speaker 1: So it's a not unreasonable extrapolation of what we know
37:38
Speaker 1: about special relativity, we say that moving clocks run slow.
37:43
Speaker 1: So I put Kelly and her potato on a spaceship
37:45
Speaker 1: with a clock, and I tell them to accelerate. They're
37:49
Speaker 1: going now near the speed of light relative to me.
37:52
Speaker 1: I look at their clock through a telescope. I see
37:54
Speaker 1: that it ticks more slowly than a clock sitting next
37:57
Speaker 1: to me. So two clocks, one that has no volo
38:00
Speaker 1: city relative to me, taking one second per second, and
38:03
Speaker 1: Kelly's clock near the speed of light relative to me,
38:06
Speaker 1: ticking at one second per year or something. And that's cool,
38:09
Speaker 1: that's fascinating, that's amazing, right, whoop.
38:12
Speaker 2: So I'll note if you have Kelly and a potato
38:14
Speaker 2: pretty soon, and you're only going to have Kelly, but
38:16
Speaker 2: are fascinating and amazing.
38:18
Speaker 1: Yeah, And special relativity tells me how to calculate that.
38:21
Speaker 1: It says, okay, Kelly's moving fast. I can calculate how
38:24
Speaker 1: quickly her clock is ticking. And I can also go
38:27
Speaker 1: to Kelly's reference frame. Because Kelly has a reference frame,
38:30
Speaker 1: she has a potatoes, she's a clock. She is sitting
38:32
Speaker 1: in her spaceship sipping her coffee. I can go from
38:35
Speaker 1: her reference frame to my reference frame. And that's really
38:37
Speaker 1: the core of special relativities. It tells you how to
38:40
Speaker 1: translate from one reference frame to another. Right, I create mine.
38:43
Speaker 1: We said there is no absolute space. But I can
38:46
Speaker 1: create a reference frame and say, here's my origin. Here's
38:49
Speaker 1: location equals zero, here's location equals one. I can measure
38:52
Speaker 1: location and velocity relative to my reference frame. Like if
38:55
Speaker 1: we spent a zillion dollars building your grid, that would
38:58
Speaker 1: be Kelly's reference frame. Wouldn't be special or absolute in anyway,
39:01
Speaker 1: but it'd be yours, and it'd be wonderful. I'm sure
39:07
Speaker 1: it'd be special to me and special to you, but
39:09
Speaker 1: not special to the universe, all right. They wouldn't change
39:11
Speaker 1: the laws of physics in any way. And so I
39:14
Speaker 1: have a reference frame, You have a reference frame, and
39:16
Speaker 1: special relativity tells us how to transform between these different
39:19
Speaker 1: reference frames. And special relativity tells us that as the
39:23
Speaker 1: velocity of two reference frames grows to near the speed
39:26
Speaker 1: of light, which is the maximum, the time that they
39:29
Speaker 1: see each other's clocks ticking goes to zero. If you're
39:32
Speaker 1: at ninety nine percent of the speed of light, I
39:34
Speaker 1: see your clock ticking like one second per day. If
39:37
Speaker 1: you go to ninety nine point ninety nine percent of it,
39:39
Speaker 1: I see your clock ticking. It's one second per year.
39:42
Speaker 1: If you go at ninety nine point nine and whatever
39:45
Speaker 1: percent of the speed of light, maybe I see your
39:47
Speaker 1: clock ticking one second every thousand years. So it's tempting
39:51
Speaker 1: to extrapolate this and say, well, what happens if you
39:53
Speaker 1: go at the speed of light? Do I see your
39:55
Speaker 1: time as stopping. That's where this comes from because photons
39:59
Speaker 1: we move at the speed of light, and so people imagine, oh, okay,
40:03
Speaker 1: put a photon in a spaceship, take that spaceship to
40:06
Speaker 1: the speed of light. If the photon has a little
40:08
Speaker 1: clock next to it, what does that clock read? Well,
40:11
Speaker 1: it's very tempting to say, at the speed of light,
40:14
Speaker 1: time stops.
40:15
Speaker 2: All right, so is this going to be a speed
40:17
Speaker 2: of light relative to something? I'm not going to try
40:20
Speaker 2: to jump the gun? What do we talk about next, Daniel.
40:23
Speaker 1: So it's very fun to say that time stops with
40:25
Speaker 1: that photon, but it's not really true because that photon
40:28
Speaker 1: doesn't have a reference frame.
40:30
Speaker 2: Well, that's what I was trying to get at. Y
40:31
Speaker 2: I'm brilliant, all right, Sorry, go ahead, PhD In physics.
40:35
Speaker 1: Yeah, because photons don't have a reference frame. Photons move
40:38
Speaker 1: at the speed of light relative to every observer. So
40:40
Speaker 1: I see the move at the speed of light. You
40:41
Speaker 1: see the move at the speed of light. You have
40:43
Speaker 1: this confusing scenario where you're in your spaceship moving really fast,
40:47
Speaker 1: you turn on a flashlight. You see the photons moving
40:49
Speaker 1: relative to you at the speed of light. I see
40:52
Speaker 1: the photons moving relative to you at the speed of light,
40:55
Speaker 1: even though I also see you moving at nine of
40:58
Speaker 1: the speed of light. It's very confused. But the thing
41:00
Speaker 1: about photons, the reason they move at the speed of
41:02
Speaker 1: light relative to everybody, is that you can never catch them.
41:05
Speaker 1: You can't like zoom up next to a photon and
41:07
Speaker 1: say like, hey, look there's a photon the way you
41:09
Speaker 1: can relative to a potato, for example, or relative to
41:12
Speaker 1: Kelly's ship. This weird fact that two observers always see
41:16
Speaker 1: light moving at the speed of light, even if they
41:18
Speaker 1: have a high velocity relative to each other. This is
41:21
Speaker 1: not like a little detail. It's the whole foundation of
41:24
Speaker 1: special relativity. From this one fact and the assumption that
41:28
Speaker 1: the laws of physics are the same everywhere, you can
41:30
Speaker 1: derive all of special relativity, time dilation, length contraction, Lorentz
41:35
Speaker 1: transformations all over. That depends on this Why is that
41:39
Speaker 1: the case? Why is there a universe this way? Well,
41:41
Speaker 1: we aren't actually sure, but it is an observed fact
41:45
Speaker 1: we have verified with experiments, and everything flows from it,
41:50
Speaker 1: and it has all of these weird consequences, including that
41:53
Speaker 1: you can't catch up to a photon. You can't ever
41:55
Speaker 1: join a photon in its reference frame and say, hey,
41:59
Speaker 1: what's it like to be a photon? And that's kind
42:01
Speaker 1: of the short answer is that photons have no reference frame,
42:04
Speaker 1: and so it makes no sense to say, what does
42:06
Speaker 1: a photon experience? Does a photon experience time? Photons don't
42:10
Speaker 1: experience anything the way like lumps of coal don't have
42:13
Speaker 1: political views. Right, It's sort of like a category error
42:16
Speaker 1: to even ask the question.
42:18
Speaker 2: Unfortunately, Okay, I'm taking a second to wrap my head
42:22
Speaker 2: around the fact that photons have no reference frame because essentially,
42:26
Speaker 2: for any reference frame you pick, they're always moving at
42:29
Speaker 2: the speed a light.
42:30
Speaker 1: Yes, exactly, And special relativity, this calculation on which this
42:34
Speaker 1: whole idea is based, can only translate between reference frames.
42:37
Speaker 1: It says, my potatoes reference frame, Kelly's reference frame, my
42:40
Speaker 1: coffee is reference rame, my cats reference frame. I can
42:42
Speaker 1: tell you how clocks in any of those reference frame
42:44
Speaker 1: appear in my reference frame, or I can tell you
42:48
Speaker 1: how my clocks appear in those reference frames. But photons
42:51
Speaker 1: don't have a reference frame. There is no axis moving
42:55
Speaker 1: with the photon where the photon is at rest. There's
42:58
Speaker 1: no special set of buize for that photon that move
43:01
Speaker 1: along with it where it has no velocity. Right, no
43:04
Speaker 1: matter what set of booies you build, photons will always
43:07
Speaker 1: be moving at the speed of light relative to those booies.
43:10
Speaker 1: So there is no reference frame, so you can't use
43:12
Speaker 1: special relativity to calculate what's it like to be a photon.
43:15
Speaker 2: I think my brain feels like physics should be intuitive
43:19
Speaker 2: because I exist in this universe and feel like I
43:22
Speaker 2: understand it. But the more I talk to you, the
43:24
Speaker 2: more I feel like it's not necessarily intuitive, which makes
43:27
Speaker 2: it all the more amazing that we have figured it out.
43:29
Speaker 2: And so I totally get why people jumped to photons
43:32
Speaker 2: don't experience time, because intuitively, that does feel right to me.
43:36
Speaker 2: I'll be sleeping tonight thinking about photons not having a
43:39
Speaker 2: reference frame. Yeah, is this intuitive to you? Or is
43:42
Speaker 2: this it makes sense now because you've been thinking about
43:44
Speaker 2: it for so long.
43:45
Speaker 1: It makes sense mathematically. Intuitively, it's always a struggle it
43:49
Speaker 1: makes sense of the universe. The way I think about
43:51
Speaker 1: it is that there's a very tempting intuitive path which
43:53
Speaker 1: is wrong, which is to think about photons as the
43:57
Speaker 1: extrapolation of what happens when you go really really fast,
44:00
Speaker 1: because they're moving at the speed of light, and we
44:02
Speaker 1: can go almost to the speed of light, and so
44:04
Speaker 1: it seems like they're right there, like at the end
44:07
Speaker 1: of that curve, right, But they're not. They're really in
44:10
Speaker 1: a different category. So even though you might want to
44:13
Speaker 1: organize them like at the end of that curve they're
44:16
Speaker 1: similar to a spaceship moving really really fast, they're not.
44:18
Speaker 1: They're really in a completely different regime because they have
44:22
Speaker 1: no mass. Right, So you can't like build a clock
44:24
Speaker 1: in this photons reference frame. You can't build a clock
44:27
Speaker 1: out of pure photons. There's nothing it's like to be
44:31
Speaker 1: a photon. It's really a completely different kind of object
44:33
Speaker 1: than anything that does have mass. So think about two
44:36
Speaker 1: different categories. Has mass, doesn't have mass. Special relativity can
44:40
Speaker 1: tell you about the experience and how time clicks for
44:42
Speaker 1: anything that has mass, anything that doesn't have mass. There's
44:46
Speaker 1: no reference frame there. So special relativity has no handle
44:49
Speaker 1: on it. It can't tell you at all what it's
44:51
Speaker 1: like to be a photon. Does that mean photons don't
44:53
Speaker 1: have an experience? You know, maybe alien philosophers when they
44:57
Speaker 1: come and it tell us about how the universe works,
44:59
Speaker 1: will have some way thinking about what it's like to
45:01
Speaker 1: be a photon, you know, the way Thomas Nagel struggled
45:04
Speaker 1: with what it's like to be a bat. So I
45:06
Speaker 1: don't want to totally rule it out. And you know,
45:08
Speaker 1: maybe there are crazy aliens out there whose minds are
45:11
Speaker 1: just made out of photons somehow, or ripples and electromagnetism,
45:15
Speaker 1: and so I don't want to piss them off either.
45:18
Speaker 2: You're really hedging your bets here, Dan.
45:21
Speaker 4: Hey.
45:21
Speaker 1: You know, like at the beginning of the podcast, we've
45:23
Speaker 1: got to be open minded, and we don't want to
45:25
Speaker 1: be full of hubris and declare that we know everything
45:27
Speaker 1: about the universe. So I think the crispest thing we
45:30
Speaker 1: can say is that special relativity can't say anything about
45:33
Speaker 1: what it's like to be a photon. It certainly doesn't
45:36
Speaker 1: tell us that photons experience zero time. And you know
45:40
Speaker 1: that already, because if you take that idea to its
45:43
Speaker 1: logical conclusion, then there's all sorts of confusing contradictions. Like a
45:47
Speaker 1: photon that's omitted instantly is across the universe, right like,
45:50
Speaker 1: it doesn't that feel like it violates all sorts of
45:52
Speaker 1: principles and transformation of information, and you get quickly into
45:56
Speaker 1: paradoxes and confusions about causality. It really doesn't make sense
46:00
Speaker 1: at all. And for those of you who are students
46:02
Speaker 1: of special relativity, photons do take time to fly through
46:06
Speaker 1: the universe. They have this thing called a space time
46:08
Speaker 1: interval which is zero, which just means that they follow
46:12
Speaker 1: the shortest path through space time, but they still take
46:15
Speaker 1: time to move through space. And so, you know, special
46:19
Speaker 1: relativity is our best description of how space works, and
46:22
Speaker 1: it tells us a lot about the nature of space
46:23
Speaker 1: and the nature of velocity and the nature of acceleration
46:26
Speaker 1: even but it doesn't tell us what it's like to
46:28
Speaker 1: be a photon.
46:29
Speaker 2: Got it all right? Just like we'll never know what
46:31
Speaker 2: the bees see with the colors that they can see
46:33
Speaker 2: and we stand, we'll never know what it's like to
46:36
Speaker 2: be a photon. I gotta be honest, I'm a little
46:37
Speaker 2: sad that I'll never know what it's like to be
46:39
Speaker 2: a bee, but I can live without knowing what it's
46:41
Speaker 2: like to be a photon. But maybe this is a
46:43
Speaker 2: fundamental difference between biologists and physicists.
46:45
Speaker 1: But hey, if you're a photon and you've been listening
46:48
Speaker 1: to this podcast and we offended you right in, tell
46:50
Speaker 1: us what is it like to be a photon and
46:53
Speaker 1: be pissed off at the podcast? I want to know. Also,
46:56
Speaker 1: if you're a bee right in, because Kelly wants to hear.
46:58
Speaker 2: From you, that's right, it will have you on the show.
47:01
Speaker 1: And if you're not a photon and you're not a bee,
47:03
Speaker 1: and you're somehow a human listening to this podcast, we
47:06
Speaker 1: still want to hear from you. Did this make sense
47:08
Speaker 1: to you? Did this help you understand why potatoes are
47:10
Speaker 1: not black holes? And what it's like to be a photon?
47:13
Speaker 1: Did we just confuse you? Send us some feedback, Send
47:15
Speaker 1: us some love, send us some grumpy emails. Whatever. We
47:18
Speaker 1: love to hear from you. Write to us to Danielankelly
47:21
Speaker 1: dot org. Don't send hard physics questions to Daniel and
47:24
Speaker 1: Kelly dot com. I don't think they know the answers.
47:27
Speaker 2: They might just might not appreciate it. You know, a
47:28
Speaker 2: lot of work was at the planning a wedding. They're
47:30
Speaker 2: very busy people.
47:31
Speaker 1: That's right, Yeah, exactly. They got a full spreadsheet. They
47:34
Speaker 1: don't need your question on top of the to do list.
47:37
Speaker 2: Good luck, Daniel and Kelly.
47:39
Speaker 1: Congratulations.
47:47
Speaker 2: Daniel and Kelly's Extraordinary Universe is produced by Iheartreading. We
47:51
Speaker 2: would love to hear from you, We really would.
47:53
Speaker 1: We want to know what questions you have about this
47:56
Speaker 1: Extraordinary Universe.
47:58
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48:01
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48:04
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48:05
Speaker 1: We really mean it. We answer every message. Email us
48:08
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48:11
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48:13
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48:17
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48:21
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