Latest / Daniel and Kelly’s Extraordinary Universe / Daniel answers Listener Questions about a donut Universe, mirrors and black holes!

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Daniel answers Listener Questions about a donut Universe, mirrors and black holes! · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: I like to think sometimes about what it was like
0:11
Speaker 1: to be a caveman or cave one. You know, they
0:14
Speaker 1: probably didn't travel nearly as much as we do, and
0:17
Speaker 1: so they just didn't have a global sense for where
0:20
Speaker 1: they lived in the world. They just saw a little
0:22
Speaker 1: patch and all they could do was wonder about what
0:25
Speaker 1: lay beyond. Now, of course, we know so much more
0:28
Speaker 1: about the world, and so we can look at the
0:31
Speaker 1: bones of Stone Age people and feel like we might
0:34
Speaker 1: know more about their world than they did. There might
0:37
Speaker 1: have been people who lived near the ocean but never
0:40
Speaker 1: saw it, things that were right around the corner they
0:42
Speaker 1: didn't even know about. But aren't we still cave men
0:46
Speaker 1: and cave women in that way? We still see our
0:49
Speaker 1: little patch of the universe, and we don't know what's
0:52
Speaker 1: around the corner. What if there's some crazy awesome thing,
0:56
Speaker 1: but it's just a little bit further than we can see.
1:00
Speaker 1: And so that makes me wonder will future humans look
1:03
Speaker 1: back at us in the same way, chuckling to themselves
1:06
Speaker 1: about how clueless we are? I sure hope so, because
1:10
Speaker 1: it means physicists have been doing their jobs. Hi. I'm Daniel,
1:29
Speaker 1: I'm a particle physicist and Welcome to the podcast Daniel
1:33
Speaker 1: and Jorge Explain the Universe our production of My Heart
1:37
Speaker 1: Radio today and the podcast is just me. Jorge can't
1:40
Speaker 1: be here today to join us, and so I'm gonna
1:43
Speaker 1: be taking you on a tour of all the amazing
1:45
Speaker 1: things in our universe, all the incredible things that we
1:48
Speaker 1: want to understand, all the distant places that we can
1:51
Speaker 1: just barely prob with our telescopes, all of the tiny
1:54
Speaker 1: particles that we seek to understand. This podcast is about
1:58
Speaker 1: all of those things and everything. We want to explain
2:01
Speaker 1: the entire universe in a way that makes you come
2:04
Speaker 1: away thinking I get it, and maybe once or twice
2:08
Speaker 1: even makes you chuckle. And one of the things that
2:10
Speaker 1: we cherish on this podcast is not just the things
2:14
Speaker 1: that science has been working on, but the things that
2:17
Speaker 1: you have been wondering about. Because physics is not something
2:21
Speaker 1: which belongs to a tiny group of people who happen
2:23
Speaker 1: to get to be professors of physics, but it's something
2:26
Speaker 1: we can all do. Every time you wonder about the universe,
2:29
Speaker 1: every time you think to yourself, how does that work?
2:32
Speaker 1: Every time you hear two things you don't quite know
2:35
Speaker 1: how to fit them together in your mind. You are
2:38
Speaker 1: doing physics, you are being a physicist, and we love
2:42
Speaker 1: that here on the podcast, and we want to encourage that.
2:45
Speaker 1: And one way we do that is by answering your questions.
2:48
Speaker 1: So if you have questions about the way the universe works,
2:52
Speaker 1: you have lots of ways to get in touch with us.
2:54
Speaker 1: You can send us an email to questions at Daniel
2:57
Speaker 1: and Jorge dot com. You can engage with us and
3:00
Speaker 1: Twitter at Daniel and Jorge, or you can come and
3:03
Speaker 1: ask me in person. Come to one of my public
3:05
Speaker 1: office hours where I hang out on zoom and answer
3:08
Speaker 1: questions from anybody and everybody about physics. For information on that,
3:13
Speaker 1: go to my website to sites dot you see I
3:16
Speaker 1: dot edu, slash Daniel and you'll find all the relevant
3:20
Speaker 1: connection information. Sometimes I'll get emailed the question from a
3:24
Speaker 1: listener and it's such a good question, such a fun topic,
3:28
Speaker 1: that I think I'm not just gonna write back and
3:30
Speaker 1: said them the answer. I want to talk about this
3:32
Speaker 1: on the podcast, So we'll ask them to send me
3:34
Speaker 1: an audio clip of themselves asking the question, because I
3:37
Speaker 1: think all of you might be interested in the answer.
3:41
Speaker 1: And that's exactly what we're gonna do on today's episode. Today,
3:44
Speaker 1: I'll be catching up with our backlog of listener questions
3:48
Speaker 1: while Jorge is on a break and answering questions from
3:51
Speaker 1: real people. Questions about physics, questions about which corner of
3:55
Speaker 1: the universe we're in, questions about how mirrors work, Questions
3:59
Speaker 1: about the potns and gravity, all the kind of things
4:02
Speaker 1: that thinking people wonder about when they try to understand
4:06
Speaker 1: the world around them. All right, so let's get to it. First.
4:10
Speaker 1: Up is a question about the universe? Are Daniel and Hord.
4:13
Speaker 1: My name is Elia, and I'm a big fan of
4:15
Speaker 1: your podcast. I've always wondered whether or not it is
4:19
Speaker 1: possible to know which corner of the universe we're in,
4:22
Speaker 1: depending upon what we believe the shape of the universe is,
4:26
Speaker 1: and if it's shaped like a doughnut, for example, is
4:29
Speaker 1: it possible to cross through the donut hole and re
4:32
Speaker 1: enter the universe provided inflation suddenly stopped. Thank you so
4:36
Speaker 1: much for teaching and inspiring us. All right, thanks very much,
4:39
Speaker 1: Ilia for that super fun question. I like this question
4:43
Speaker 1: a lot, not just because it talks about doughnuts and
4:46
Speaker 1: that makes me hungry and it makes me wonder was
4:48
Speaker 1: Ilia eating a doughnut when he asked me this question,
4:51
Speaker 1: why a donut? Why not like a pastry shaped universe,
4:55
Speaker 1: or you know, a pie shaped universe. But we'll dig
4:57
Speaker 1: into that and take a bite out of that part
4:59
Speaker 1: of the question. But I also love this question because
5:01
Speaker 1: it touches on something really deep and fascinating, not just
5:05
Speaker 1: about the shape of the universe, but about where we
5:08
Speaker 1: are in the universe. Are we in an interesting part
5:12
Speaker 1: of the universe? Are we near the center of the universe?
5:15
Speaker 1: Are we near some weird edge or corner of the universe?
5:18
Speaker 1: I love when you take a deep question about the universe,
5:21
Speaker 1: how big is it? What shape is it? And you
5:24
Speaker 1: make it a personal question, like where are we in
5:27
Speaker 1: the universe? So let's dig into it. Where are we
5:31
Speaker 1: in the universe? What corner of the universe are we in?
5:35
Speaker 1: And you're totally right ilia that the answer to this
5:37
Speaker 1: question depends a lot on what we think about the
5:40
Speaker 1: shape and the size of the universe. So let's answer
5:44
Speaker 1: it in a few different ways. Let's start with the
5:46
Speaker 1: most likely configuration for the universe and imagine that the
5:50
Speaker 1: universe is infinite. That means that it goes on forever
5:53
Speaker 1: in every direction, there's no edge, there's no point at
5:57
Speaker 1: which the universe stops. It just goes on forever. And
6:00
Speaker 1: it's sort of euclidian in a sense that you can
6:02
Speaker 1: go on forever without looping around on yourself, and the
6:05
Speaker 1: two parallel lines will go on forever without crossing. Why
6:09
Speaker 1: do I say this is the most likely scenario, the
6:11
Speaker 1: most likely set up for a universe. I just think
6:14
Speaker 1: it's the most natural because it has no edges, because
6:18
Speaker 1: it's sort of simple. If you have an edge, then
6:20
Speaker 1: you have to explain why that edge. If you have
6:23
Speaker 1: an edge, you have to answer why is this edge
6:26
Speaker 1: over here and not somewhere else? You need a special case.
6:30
Speaker 1: And I like explainations that don't have special cases, that
6:33
Speaker 1: are simple and that our universal. And so this one
6:36
Speaker 1: that the universe just sort of goes on forever in
6:39
Speaker 1: every direction is nice because it makes no place in
6:43
Speaker 1: the universe special. Every point in the universe has an
6:46
Speaker 1: infinite amount of universe in every direction, and so it's
6:50
Speaker 1: very similar everywhere. And that's the key thing. That's a
6:53
Speaker 1: crucial idea in cosmology recently. This cosmological principle, the one
6:59
Speaker 1: that says that every location in the universe is equivalent.
7:03
Speaker 1: There's no center, there's no edge, there's no corners. Every
7:07
Speaker 1: place in the universe is basically like every other place
7:10
Speaker 1: in the universe. Now, let's unpack that a timely a
7:13
Speaker 1: little bit. What do we really mean when we say
7:16
Speaker 1: every place in the universe is like every other place
7:19
Speaker 1: in the universe. Because my house is not the same
7:21
Speaker 1: as your house, and the Earth is not the same
7:24
Speaker 1: as Jupiter, and there are galaxies, and there are places
7:27
Speaker 1: where there are no galaxies. So like, at the very
7:30
Speaker 1: microscopic level, it's not true that every place in the
7:33
Speaker 1: universe is the same as every other place in the universe.
7:36
Speaker 1: What we mean when we say that is that the
7:37
Speaker 1: laws of physics are the same, that they always apply
7:41
Speaker 1: in the same way. That you don't have different laws
7:43
Speaker 1: and different constants over there than you have over here. Now,
7:48
Speaker 1: if you have the same laws of physics, how is
7:50
Speaker 1: it you can even have any difference? How is it
7:52
Speaker 1: that you can have a planet here and not a
7:54
Speaker 1: planet there, or you have all these galaxies over here
7:57
Speaker 1: and no galaxies over there. Well, the current understanding is
8:01
Speaker 1: that this comes from quantum randomness in the very early
8:04
Speaker 1: universe when it was hot and dense, but still infinite, right,
8:08
Speaker 1: still goes on in every direction, but hotter and denser
8:11
Speaker 1: and younger. That there were quantum fluctuations that made some
8:15
Speaker 1: spots in the universe more dense, with stuff in some
8:18
Speaker 1: spots less dense. Now does that mean that those spots
8:21
Speaker 1: are different from the other spots, Well, they got different
8:24
Speaker 1: random numbers, but they had the same opportunity. They're playing
8:27
Speaker 1: the same game and they just had a different outcome.
8:30
Speaker 1: And quantum mechanics is amazing because the rules are the same.
8:33
Speaker 1: It gives the same probability distribution for every scenario that's identical,
8:38
Speaker 1: but individual outcomes can be different. You know. Quantum mechanics
8:41
Speaker 1: tells us that if you poke a particle the same
8:44
Speaker 1: way a hundred times, you might get a hundred different outcomes.
8:48
Speaker 1: Or you might have a scenario where half the time
8:50
Speaker 1: it goes left and half the time it goes right,
8:52
Speaker 1: even if the initial conditions are exactly the same. So
8:56
Speaker 1: that's what's happening in the very early universe. The initial
8:59
Speaker 1: conditions great energy density is the same everywhere, but different
9:03
Speaker 1: things can happen in different places. They are following the
9:05
Speaker 1: same rules, so there's no special place, but they have
9:08
Speaker 1: a different outcome, and then those little pockets of over
9:12
Speaker 1: density get expanded into massive macroscopic effects, which seed the
9:17
Speaker 1: structure of the universe when the universe expanded very rapidly
9:21
Speaker 1: in the early Big Bang, So you have hot, dense
9:23
Speaker 1: stuff in the very early universe. Random quantum fluctuations make
9:28
Speaker 1: some of it a little bit more dense. Inflation expands
9:31
Speaker 1: those tiny little microscopic things into big macroscopic things, and
9:35
Speaker 1: then gravity takes over. So that's how you can have
9:38
Speaker 1: a universe where the rules of physics are the same everywhere,
9:41
Speaker 1: but you don't actually have the same outcome everywhere. You
9:44
Speaker 1: have galaxies here and no galaxies over there. So in
9:48
Speaker 1: that scenario, the universe is the same everywhere. There's no corners,
9:52
Speaker 1: there's no center, and everywhere in the universe follows the
9:56
Speaker 1: same rules. And so where are we in the universe? Well,
9:59
Speaker 1: where he but here is sort of the same as
10:02
Speaker 1: there so here, or there there or here. It sounds
10:05
Speaker 1: sort of like a Doctor Seuss book, right, but it's
10:07
Speaker 1: actually real. And this I think is the most prevalent
10:10
Speaker 1: idea for where we are in the universe because it
10:12
Speaker 1: doesn't require any special cases. Now, for those of you
10:16
Speaker 1: who are thinking back about how I described the Big
10:18
Speaker 1: Bang and thinking, hold on a second, wasn't the Big
10:21
Speaker 1: Bang at a certain point? I described it as a
10:24
Speaker 1: moment when the universe was hot and dense and young. Right,
10:29
Speaker 1: That doesn't mean that it was hot and dense and localized.
10:33
Speaker 1: The way I think about the Big Bang is not
10:35
Speaker 1: as a tiny dot which then exploded out to fill
10:39
Speaker 1: up the universe with stuff. The more modern view of
10:42
Speaker 1: the Big Bang is that it was a change from
10:44
Speaker 1: high density to low density. I'm using different words there
10:49
Speaker 1: because I mean something different. I mean that the universe
10:51
Speaker 1: was high density but still infinite, So not a tiny
10:55
Speaker 1: little dot of stuff like many people used to describe
10:58
Speaker 1: the Big Bang, but an universe filled with hot, dense
11:02
Speaker 1: stuff which then expanded. So now that hot dense stuff
11:06
Speaker 1: becomes cold and dilute, right, it spreads out. Space itself
11:10
Speaker 1: is expanded. You create new space between stuff in order
11:14
Speaker 1: to go from a hot, dense universe to a cold,
11:17
Speaker 1: dilute universe. So the Big Bang was not an explosion
11:21
Speaker 1: as much as an expansion of space. It's stretched space everywhere,
11:26
Speaker 1: So the Big Bang happened everywhere, all at once. So
11:30
Speaker 1: that's the answer. In the scenario that the universe is infinite,
11:33
Speaker 1: that it goes on forever in every direction, and in
11:36
Speaker 1: that scenario, doesn't really make sense to talk about where
11:39
Speaker 1: we are in the universe because everywhere is the same place.
11:43
Speaker 1: But as we've talked about on the podcast a few times,
11:46
Speaker 1: we don't know that the universe is infinite. We can't
11:50
Speaker 1: know that the universe is infinite. We could only see
11:53
Speaker 1: a finite patch of the universe, just like ancient people's
11:57
Speaker 1: who thought about the universe but never traveled very far
12:00
Speaker 1: from their home. They only saw a tiny little patch,
12:02
Speaker 1: and they could only imagine what was beyond the edge
12:05
Speaker 1: of their knowledge. In the same way, there's a patch
12:08
Speaker 1: of the universe about ninety billion light years across that
12:12
Speaker 1: we can see. We call this the observable universe, and
12:15
Speaker 1: beyond it, we don't know what's there. Why can't we
12:18
Speaker 1: see it, Well, it's a physical limitation. It's because of
12:21
Speaker 1: the speed of light. Light just hasn't had enough time
12:25
Speaker 1: to get to us from there since the beginning of
12:28
Speaker 1: the universe. It's on his way. It's been flying our
12:31
Speaker 1: direction the whole time. The universe has been doing its
12:34
Speaker 1: universe thing. But it started out so far away that
12:38
Speaker 1: even though it's traveling at light speed, it hasn't reached
12:41
Speaker 1: us yet, and it will. And every year that goes
12:43
Speaker 1: by the size of the observable universe, the portion of
12:47
Speaker 1: the universe that we can see gets bigger and bigger,
12:52
Speaker 1: and we can see more and more stars and more
12:53
Speaker 1: planets around them, and all sorts of amazing interesting stuff.
12:57
Speaker 1: And of course, first we see the stuff that's happened
13:00
Speaker 1: were teen billion years ago, because that's when the light
13:03
Speaker 1: left it to get to us. So as we look
13:05
Speaker 1: further out into the universe, we actually see the past
13:08
Speaker 1: more than we see the present. So if there was
13:11
Speaker 1: something crazy happening just past the edge of the observable universe,
13:15
Speaker 1: we wouldn't see it for a while. If it's just
13:17
Speaker 1: happening now, if there's some crazy dancing banana party happening
13:21
Speaker 1: on a planet just past the edge, it's going to
13:23
Speaker 1: take billions of years before those awesome images get to us,
13:27
Speaker 1: so we can embarrass the aliens by posting them on
13:30
Speaker 1: social media. So what lies beyond the edge of the
13:32
Speaker 1: observable universe we don't know, and that's one reason why
13:35
Speaker 1: we don't know the shape of the universe. There are
13:38
Speaker 1: a few scenarios for how the universe could be shaped.
13:41
Speaker 1: One of course, is that it's infinite. Another is that
13:44
Speaker 1: it's not infinite, but it doesn't have an edge, something
13:47
Speaker 1: like the surface of a sphere. As you walk along
13:50
Speaker 1: the Earth, you're not aware of there being any edges.
13:53
Speaker 1: I mean, there are oceans and all sorts of stuff
13:55
Speaker 1: and walls and fences, but sort of geo magically only
13:58
Speaker 1: imagine yourself walking just on the surface of a perfect sphere.
14:01
Speaker 1: You wouldn't notice any edges. You could go on forever
14:04
Speaker 1: without bumping into anything, and eventually you could even come
14:08
Speaker 1: back to where you started. So that's a universe that's
14:10
Speaker 1: not infinite, has a finite amount of area the surface
14:14
Speaker 1: of a sphere to make sort of the leap from
14:16
Speaker 1: the two dimensional surface of a sphere where you're walking
14:19
Speaker 1: around on it now into three dimensional space. How does
14:23
Speaker 1: that work? Are we saying that three dimensional space is
14:26
Speaker 1: sort of like plastered onto the surface of a four
14:28
Speaker 1: dimensional sphere. No, we're just saying that the universe could
14:31
Speaker 1: be connected differently. We don't know if our three dimensional
14:35
Speaker 1: space is like embedded in some higher dimensional space four
14:39
Speaker 1: or eleven or twenty six. Though, if you're interested in
14:42
Speaker 1: that kind of stuff. We have a whole podcast episode
14:44
Speaker 1: about how many dimensions there are in space. But even
14:46
Speaker 1: if we just assume that space has three dimensions, it's
14:49
Speaker 1: still possible for it to curve, because space can be
14:53
Speaker 1: bent and connected in weird ways. You can loop around itself,
14:56
Speaker 1: you can have all sorts of distortions. And we'll talk
14:59
Speaker 1: about this actually later today in the same podcast. But
15:02
Speaker 1: you can imagine space being complex in such a way
15:05
Speaker 1: that bits of it are connected to each other, so
15:08
Speaker 1: that if you did embed them in a four dimensional space,
15:10
Speaker 1: it would look like a weird shape. It would look
15:12
Speaker 1: like the surface of a sphere, or it would look
15:15
Speaker 1: like a donut or a banana or a pastry or
15:18
Speaker 1: something else. And now I'm getting too hungry to continue
15:20
Speaker 1: answering this question. But we just don't know the shape
15:23
Speaker 1: that all of these things are possible, because we just
15:26
Speaker 1: can't see enough of the universe to rule some of
15:28
Speaker 1: this out. Now, it's also possible that the universe isn't
15:32
Speaker 1: infinite and that it does have some weird edge. Remember,
15:35
Speaker 1: space is not something that we understand very well. Only
15:39
Speaker 1: for the last hundred years or so have we even
15:41
Speaker 1: understood that space is a thing that it can bend
15:44
Speaker 1: and distored and expand and grow and do all sorts
15:47
Speaker 1: of weird stuff. So we're at the really the very
15:49
Speaker 1: beginning of our understanding of the nature of space. And
15:53
Speaker 1: it might be the space has some weird edge beyond
15:56
Speaker 1: which there is no more space. What would be there
16:00
Speaker 1: beyond space? Is that nothing? Is that a thing without space?
16:03
Speaker 1: We just don't know. But it's possible for space to
16:06
Speaker 1: have weird connections so that if you get to the
16:08
Speaker 1: edge of it, you just can't go that way anymore.
16:11
Speaker 1: And you might think, well, what does that mean? How
16:13
Speaker 1: is that possible? Well, remember that in black holes, for example,
16:17
Speaker 1: space can be distorted so that you can only move
16:19
Speaker 1: in one direction. Inside a black hole, space is so
16:22
Speaker 1: distorted that every motion takes you towards the center. It's
16:25
Speaker 1: an example of a non simple arrangement of space. So
16:29
Speaker 1: now imagine the edge of the universe where there just
16:32
Speaker 1: is no direction out. There's no direction you can go
16:35
Speaker 1: that would take you beyond the edge of the universe.
16:38
Speaker 1: There's only directions in towards the universe, sort of like
16:41
Speaker 1: when you're in a black hole. There is no path
16:44
Speaker 1: out of the universe. But there is an edge to
16:47
Speaker 1: the black hole, there is an event horizon, so it
16:50
Speaker 1: is possible to have these weird edges, these discontinuities in space,
16:55
Speaker 1: places where you just cannot go. Is the universe like that?
16:58
Speaker 1: We just don't know. Maybe future societies, as they eat
17:02
Speaker 1: their donuts will laugh at us for thinking that this
17:05
Speaker 1: was real. Or maybe future societies, as they eat their
17:08
Speaker 1: healthy snack will be thinking, boy, they were really onto something.
17:11
Speaker 1: They should have dug even deeper. All right, so let's
17:14
Speaker 1: get to the last part of Ilia's question, which is
17:16
Speaker 1: about the donut hole. What if the universe is not
17:18
Speaker 1: infinite and it's not like the surface of a sphere
17:22
Speaker 1: where you can sort of move around in every direction.
17:24
Speaker 1: What if it's connected like a donut. You know, the
17:27
Speaker 1: surface of a donut is connected differently from the surface
17:30
Speaker 1: of a sphere. Right on a sphere, if you go north,
17:33
Speaker 1: you come back to where you started, and if you
17:36
Speaker 1: go east, you come back to where you started. On
17:38
Speaker 1: the surface of a donut, it's a little bit different
17:41
Speaker 1: because on the surface of a donut, if you go north,
17:43
Speaker 1: for example, you come back to where you started, but
17:46
Speaker 1: you don't get to the other side of the donut.
17:48
Speaker 1: To do that, you have to go east right, So
17:50
Speaker 1: there's no trivial mapping from a surface of a donut
17:54
Speaker 1: to the surface of a sphere. A donut has a
17:57
Speaker 1: hole in the middle, and a sphere just doesn't. So really,
18:01
Speaker 1: this awesome question is is it possible to go through
18:03
Speaker 1: the donut hole? Is it possible to get to the
18:06
Speaker 1: other side of the universe without going the long way around? Well,
18:10
Speaker 1: I think to answer your second question, first, you could
18:13
Speaker 1: get to the other side of the universe using a wormhole. Right.
18:16
Speaker 1: If the universe is topologically complex, then you can allow
18:20
Speaker 1: other wrinkles. Instead of just being sort of hooked together
18:23
Speaker 1: like a bunch of chain links. But in the shape
18:25
Speaker 1: of a donut, you could have a connection directly between
18:28
Speaker 1: one side of the donut and the other. That's what
18:31
Speaker 1: a wormhole would be. That's exactly what a wormhole is.
18:33
Speaker 1: It's a connection between two points in space that are
18:36
Speaker 1: otherwise very distant. And so you can imagine using a
18:39
Speaker 1: wormhole to get from one side of the donut to
18:42
Speaker 1: the other. Could you actually be inside the donut? Could
18:45
Speaker 1: you like leave the universe and fly through the donut
18:48
Speaker 1: hole to get to the other side. I think Unfortunately,
18:51
Speaker 1: the answer to this question is no. Even just imagining
18:54
Speaker 1: the universe as having a hole in the middle, that
18:57
Speaker 1: that donut hole is a real thing means that you're
19:00
Speaker 1: taking this analogy one step too far. You're imagining the
19:04
Speaker 1: universe as embedded in some high dimensional space where that
19:07
Speaker 1: whole exists. Remember that this donut is an analogy. It's
19:12
Speaker 1: an analogy of a two D universe that's sort of
19:14
Speaker 1: painted onto the surface of a three D donut. But
19:18
Speaker 1: our universe is three dimensions. And again we don't think
19:21
Speaker 1: that it's painted onto the surface of a four D universe.
19:25
Speaker 1: We just think that the connections between the points in
19:28
Speaker 1: the three D universe might resemble the relationships of the
19:32
Speaker 1: surface of a three D donut. Right, And so if
19:36
Speaker 1: you want to visualize it, you can imagine putting it
19:40
Speaker 1: into four D space to sort of arrange it in
19:42
Speaker 1: your head and think, oh, and in that four D
19:45
Speaker 1: space you get a three D donut. But that doesn't
19:48
Speaker 1: mean that four D space exists, And so that doesn't
19:50
Speaker 1: mean that that place, the whole inside the donut, is
19:54
Speaker 1: a real thing. Right. Said another way, if the universe
19:58
Speaker 1: is a donut, then the whole in the inside doesn't
20:00
Speaker 1: exist as a place you can go anymore than the
20:03
Speaker 1: part of the outside of the donut exists. There's no
20:06
Speaker 1: space there, there's no way to be there, no way
20:09
Speaker 1: you can go there, and so it just doesn't exist.
20:12
Speaker 1: So sorry, Iliot, you can't take a short cut across
20:15
Speaker 1: the donut hole, no matter how many donuts you eat.
20:18
Speaker 1: All right, that was a super fun question, Thanks very much, Ilia.
20:21
Speaker 1: I want to answer some more questions, but first let's
20:24
Speaker 1: take a quick break. All right, we're back and this
20:37
Speaker 1: is Daniel and I'm answering listener questions. We talked about
20:41
Speaker 1: the universe as a donut, and I hope that during
20:43
Speaker 1: that break you all went off and got a snack,
20:45
Speaker 1: healthy or unhealthy. Up to you. And now we're back
20:49
Speaker 1: and we're talking more physics. And here's an awesome question
20:52
Speaker 1: from another listener. Hey, Daniel, and explained the universe amazing
20:57
Speaker 1: programs you have there all the way is always inspurring. Um.
21:03
Speaker 1: I was wondering the other day how mirrors work, and
21:07
Speaker 1: then I remember that everything at the smallest scale, it's
21:11
Speaker 1: even weirder. So how do mirrors actually work at the
21:16
Speaker 1: quantum level? Are photos bouncing off a mirror? Or are
21:22
Speaker 1: they being observed and remit it? How does that work?
21:27
Speaker 1: Thank you love your podcast. All right, this is a
21:31
Speaker 1: really fun question because it digs really deep into understanding
21:36
Speaker 1: what light actually is. And I love the whole spirit
21:39
Speaker 1: of this kind of exercise of looking at things around
21:42
Speaker 1: us in the world and wondering what's really going on
21:45
Speaker 1: at the microscopic level. I mean, that's why I'm a
21:48
Speaker 1: particle physicist, because I have this feeling like everything around
21:52
Speaker 1: us in the world is not the true world, sort
21:55
Speaker 1: of build up of tiny little things which, acting together
21:58
Speaker 1: in vast amounts in aggregate, make these weird effects, these
22:02
Speaker 1: effects like people and weather and bananas. But that's not
22:06
Speaker 1: the true stuff of the universe, right, Bananas are not
22:09
Speaker 1: fundamental to the universe, no matter how much horror Hey
22:12
Speaker 1: likes to snack on them. The true universe is made
22:14
Speaker 1: out of the smallest, tiniest things. And it's my sense
22:18
Speaker 1: that if you understood the universe the smallest, the tiniest bits,
22:22
Speaker 1: then you're understanding the deep truth and you could always
22:24
Speaker 1: work your way up from that deep truth to anything
22:27
Speaker 1: else like supernovas and bananas. And that's why every time
22:31
Speaker 1: I see something weird in the world. I want to
22:33
Speaker 1: understand it in terms of the microscopic effects, like what's
22:37
Speaker 1: going on at the particle level that makes this happen.
22:41
Speaker 1: And so I think that's the motivation for this question,
22:44
Speaker 1: this question about how a mirror works, because you could
22:46
Speaker 1: imagine it like, well, what if light is just sort
22:50
Speaker 1: of like photons anything of photons as particles, And what
22:53
Speaker 1: happens when a photon hits a mirror bounces off? And
22:56
Speaker 1: you have an image in your head of like a
22:58
Speaker 1: tennis ball bounce off of a wall and it bounces
23:01
Speaker 1: off and it sort of reflects off the wall. No
23:04
Speaker 1: big deal, right, But what it really happens microscopically because
23:08
Speaker 1: if you zoom in, you realize, well, the wall is
23:11
Speaker 1: not smooth, right, The wall is man out of a
23:14
Speaker 1: lattice of atoms, and the photon is also small, and
23:18
Speaker 1: so it's sensitive to these little defects. The wall is
23:21
Speaker 1: not perfectly smooth, So how can it bounce off of it?
23:24
Speaker 1: And what is bouncing really mean for a photon? Because
23:28
Speaker 1: what happens when a photon appurchase the wall is that
23:31
Speaker 1: it like interacts with the electrons and the atoms in
23:35
Speaker 1: that lattice of the wall. Right. And photon don't like
23:39
Speaker 1: touch the wall. They don't bounce off the wall the
23:42
Speaker 1: way a tennis ball bounces off the wall. The only
23:45
Speaker 1: way a photon can bounce or touch anything is through
23:49
Speaker 1: its interactions. That means that it like gets absorbed by
23:53
Speaker 1: the atom and then gets re emitted. And if it's
23:56
Speaker 1: getting absorbed and re emitted, how can it bounce off
24:00
Speaker 1: in the right direction? Right? And so I think that's
24:02
Speaker 1: the underlying question here. How do we understand reflection? How
24:06
Speaker 1: do we understand how mirrors bounce photons off in the
24:10
Speaker 1: right direction? If mirrors are just collections of particles and
24:14
Speaker 1: photons are just streams of particles. So I think to
24:17
Speaker 1: understand this question and to keep your handle around it
24:19
Speaker 1: at the microscopic level, there's two important things to understand,
24:24
Speaker 1: two effects that we have to have in our minds
24:27
Speaker 1: to make a clear picture of what's going on. And
24:29
Speaker 1: the first is what's going on between the photon and
24:33
Speaker 1: this particle in the mirror, this bit of the mirror
24:35
Speaker 1: that it's interacting with. So photon approaches the mirror and
24:39
Speaker 1: you can think of the photon as a particle, or
24:41
Speaker 1: you can think about it as a wave. Doesn't really matter.
24:44
Speaker 1: But remember, if you're thinking about it as a particle,
24:46
Speaker 1: its motion is determined by its quantum waves. So it's
24:50
Speaker 1: really the wave like effects that dominate here. Now I'm
24:53
Speaker 1: not saying a photon is a wave. I'm not saying
24:55
Speaker 1: it's a particle. If you've been listening to the podcast,
24:58
Speaker 1: you know that. I think it's neither a particle nera wave.
25:00
Speaker 1: It's some weird other quantum mechanical thing which can't be
25:04
Speaker 1: perfectly described by anything we have intuition for. But sometimes
25:09
Speaker 1: some pictures are more useful than others. So what happens
25:12
Speaker 1: when the photon approaches the bit of the mirror, Well,
25:15
Speaker 1: photons can only really do one thing, which is that
25:18
Speaker 1: they can get absorbed by charged particles and then they
25:21
Speaker 1: can get re emitted. And in this case it's mostly
25:24
Speaker 1: the electrons. Like there are charges also in the atomic nucleus,
25:28
Speaker 1: but the nucleus is surrounded by electrons, and so when
25:30
Speaker 1: the photon approaches the service of the mirror really just
25:34
Speaker 1: sees the electrons. It sees this like wall of electrons.
25:37
Speaker 1: And that's okay. Photons can talk to electrons. They're happy
25:40
Speaker 1: to do that. But the only way for that to
25:42
Speaker 1: happen is for the electron to absorb the photon, then
25:45
Speaker 1: it has more energy and then it can give off
25:47
Speaker 1: that photon. It emits that photon, and then the question
25:51
Speaker 1: is how does it know which direction to emit that photon?
25:55
Speaker 1: When an electron which is spinning around an atom gives
25:57
Speaker 1: off a photon, doesn't it just emit it sort of randomly?
26:01
Speaker 1: And you can imagine all of these atoms is just
26:03
Speaker 1: like tiny little sources of light because they've absorbed the photon.
26:07
Speaker 1: Now it's there's why don't they just shine them all
26:10
Speaker 1: in different directions? Well, that is the right way to
26:12
Speaker 1: think about it. You can think about each of these
26:15
Speaker 1: atoms which receives a photon as a little sort of
26:17
Speaker 1: point source is giving off a little bit of light.
26:20
Speaker 1: And this happens in lots of other examples too. For example,
26:24
Speaker 1: when light goes through a little slit, as it famously
26:26
Speaker 1: does in the double slit experiment, when it comes out
26:29
Speaker 1: of that slit, it's not going necessarily in the same
26:32
Speaker 1: direction as when it went into the slit. Now it's
26:35
Speaker 1: like a little point source at the slit, and that's
26:37
Speaker 1: what you need to have, Like interference between two slits.
26:41
Speaker 1: You need a light to be coming out in lots
26:44
Speaker 1: of different directions, so we can add up constructively or destructively.
26:48
Speaker 1: And that same picture also works for what's happening at
26:50
Speaker 1: the surface of a mirror. The photon is absorbed by
26:53
Speaker 1: an atom and then the atom emits, and it emits
26:56
Speaker 1: in all directions. So why does the photon end bouncing
27:00
Speaker 1: at the correct angle? Right? Because if the photon comes
27:03
Speaker 1: in at a very steep angle, comes out at a
27:05
Speaker 1: steep angle. If photons come in a very shallow angle
27:08
Speaker 1: to come out at a shallow angle, well, the way
27:11
Speaker 1: it works is that the photon comes out not just
27:13
Speaker 1: with the direction, but also with a phase. So the
27:16
Speaker 1: quantum wave function of the photon that determines where it's
27:20
Speaker 1: going has a number in it called the phase, sort
27:24
Speaker 1: of like how much it's spun. And the atom actually
27:26
Speaker 1: emits photons in all directions, but with different phases, and
27:31
Speaker 1: those phases determine whether or not the photons add up
27:34
Speaker 1: constructively or destructively. And the phase that comes out depends
27:39
Speaker 1: on the angle that the photon came in. And so
27:42
Speaker 1: what happens is photon comes in, gets absorbed by the atom,
27:46
Speaker 1: and the atom emits photons sort of in every direction
27:49
Speaker 1: belt with different phases, and only in the right direction,
27:52
Speaker 1: in the direction that sort of corresponds to the angle
27:55
Speaker 1: of photon came in. Do those photons not cancel each
27:58
Speaker 1: other out, So you get destructive interference for all the
28:02
Speaker 1: other directions, and constructive interference in the direction that corresponds
28:07
Speaker 1: to the same angle that came in. And that's what
28:09
Speaker 1: mirrors do. They bounce photons off, so the angle they
28:13
Speaker 1: leave at is the same as the angle they came in.
28:15
Speaker 1: Becoming at ninety degrees, you leave at ninety degrees, if
28:18
Speaker 1: you come in at two degrees, you leave at two
28:20
Speaker 1: degrees in the other direction. So the atom is absorbing
28:24
Speaker 1: that photon and then it is sort of emitting in
28:27
Speaker 1: every direction, but there is one direction where the phase
28:30
Speaker 1: is right for the photons to not get canceled out
28:32
Speaker 1: on top of each other. So that's why one direction dominates.
28:36
Speaker 1: And you can think about that either in terms of
28:38
Speaker 1: like lots of photons coming in in parallel, hitting the
28:42
Speaker 1: surface and then sprang out in all different directions, and
28:45
Speaker 1: the ones that are sort of in the wrong directions
28:47
Speaker 1: end up canceling each other out, and that's why you
28:49
Speaker 1: only get photons coming out in parallel, or you can
28:53
Speaker 1: actually also think about it in terms of one photon.
28:56
Speaker 1: This is sort of mind blowing the same with the
28:58
Speaker 1: double slit experiment. Is the single photon hitting a mirror
29:02
Speaker 1: doesn't have like neighboring photons on either side to do
29:06
Speaker 1: the destructive and constructive interference for it. So how does
29:09
Speaker 1: that happen? Well, when a photon is absorbed by the
29:11
Speaker 1: atom and then emitted, there's a probability distribution there for
29:15
Speaker 1: where it's going to go, and it's those probabilities that
29:18
Speaker 1: can interfere with each other. Just like a single particle
29:21
Speaker 1: going through the double slit experiment has probabilities to go
29:24
Speaker 1: through either slit, a single photon hitting a mirror has
29:28
Speaker 1: probabilities to go in lots of different directions, and those
29:31
Speaker 1: probabilities can interfere with themselves. So single particles probabilities interferes
29:37
Speaker 1: with itself, and that was determines the direction that the
29:40
Speaker 1: particle can go in. Alright, So I said that there
29:43
Speaker 1: were two things you have to understand to figure out
29:46
Speaker 1: how mirrors work. When we just talked about how adams
29:48
Speaker 1: absorbed and re emit the light and how it ends
29:50
Speaker 1: up going in the correct direction, because the quantum mechanical
29:54
Speaker 1: phase of the wave function ensures that that direction is
29:57
Speaker 1: the only one that doesn't get destructive inference. The second
30:02
Speaker 1: is that conductors make better mirrors than non conductors. Things
30:06
Speaker 1: that do conduct electricity make good mirrors. That's why metals,
30:10
Speaker 1: for example, make good mirrors a good sheet of aluminum,
30:14
Speaker 1: and the reason for that is that conductors don't let
30:17
Speaker 1: photons penetrate deep into their surface, and instead they mostly
30:20
Speaker 1: reflected just from the very very thin sheet of the surface,
30:24
Speaker 1: which gives you a coherent image. If a particle went
30:27
Speaker 1: deep into a material and was absorbed before being re emitted,
30:31
Speaker 1: like a centimeter inside the material, it might never emerge,
30:34
Speaker 1: and if it did, it might not be as coherent
30:36
Speaker 1: because they will have interactive with lots of different atoms.
30:39
Speaker 1: To get a nice, really crisp picture, you want to
30:42
Speaker 1: reflect just off the very surface of the material, so
30:46
Speaker 1: you're reflecting from a very flat plane, not getting the
30:49
Speaker 1: distortions you would get if you deflected off of a
30:51
Speaker 1: surface that was not smooth, and so if particles can
30:54
Speaker 1: penetrate different levels of the material, you're basically reflecting off
30:58
Speaker 1: a not smooth mirror and you get models mess So
31:01
Speaker 1: metals are very good mirrors because they don't let photons
31:05
Speaker 1: penetrate very far into them. And that's because they are conductors,
31:09
Speaker 1: because they can rearrange their electrons to basically cancel out
31:13
Speaker 1: any electric field. If you have an electric field and
31:16
Speaker 1: you put a conductor in it, it will move its
31:18
Speaker 1: electrons around to balance that electric field, because electric fields
31:22
Speaker 1: act on those electrons, and if you have a positive
31:25
Speaker 1: voltage in one direction, it will pull electrons in that
31:28
Speaker 1: direction to cancel it out. And so conductors, because they
31:32
Speaker 1: have these electrons to sort of slosh around inside them
31:35
Speaker 1: and reconfigure pretty easily, can cancel any electric field inside
31:40
Speaker 1: of them. That's why, for example, it's very difficult to
31:43
Speaker 1: get a phone call inside an elevator. Right in an elevator,
31:47
Speaker 1: you're surrounded by a metal box, and a phone call
31:50
Speaker 1: is just radio waves. It's a fluctuation in the electromagnetic spectrum,
31:55
Speaker 1: and that will just induce electrons inside the elevator walls
31:59
Speaker 1: to move round to cancel it. So a Faraday cage
32:03
Speaker 1: is nothing but a metal box, and it can pretty
32:05
Speaker 1: effectively cancel almost any electromagnetic signal. The other side of
32:10
Speaker 1: that is that mirrors don't allow photons to go very
32:13
Speaker 1: deep into the material because they can rearrange the electrons
32:17
Speaker 1: to avoid those e M fields going into the material.
32:20
Speaker 1: And that makes them good reflectors because the photons reflect
32:23
Speaker 1: just from the surface rather than deep in the material
32:26
Speaker 1: where the image might get muddled. All right, So next
32:29
Speaker 1: time you're looking in the mirror and you're wondering, wow,
32:31
Speaker 1: why am I so good looking, it's because of quantum
32:34
Speaker 1: mechanics and because of electrons zooming around to give you
32:38
Speaker 1: a nice, crisp, clear picture of your smiling face. All right,
32:42
Speaker 1: I hope that answered your question. I have one more
32:45
Speaker 1: question I really want to get to, but first let's
32:48
Speaker 1: take another break. All right, we are back today. It's
33:04
Speaker 1: just me, Daniel, the particle of hysicist, answering your questions
33:07
Speaker 1: about the nature of the universe, whether the universe is
33:10
Speaker 1: as shapely and as hasty as a donut, whether photons
33:15
Speaker 1: bounce off a mirror in the same way that a
33:17
Speaker 1: tennis ball bounces off a wall, and how does that
33:20
Speaker 1: all work? And now we're gonna get to the last
33:22
Speaker 1: question to day, which is a classic a question I
33:25
Speaker 1: love talking about. So here it is. Hello, Daniel, and JAYI.
33:29
Speaker 1: My name is David Somerville. I live in Fort Worth, Texas,
33:34
Speaker 1: but I'm originally from Yorkshire in England. I love the
33:38
Speaker 1: way you guys bound stuff off each other on the podcast.
33:40
Speaker 1: It really works well, so please keep up the good work.
33:44
Speaker 1: I got a question for you. Black holes arenknown to
33:48
Speaker 1: have such huge gravitational pull that nothing can escape, not
33:52
Speaker 1: even light. However, photons are massless, so how can the
33:57
Speaker 1: gravitational force of a black hole have any effect online
34:01
Speaker 1: at all? No matter whether you use eth equals m
34:04
Speaker 1: A or eth equals g mm over on squad, if
34:08
Speaker 1: the mass of the photon is zero, then it means
34:11
Speaker 1: that that force on the light is zero, meaning it
34:14
Speaker 1: will carry on on its merry way unaffected. Obviously this
34:19
Speaker 1: is wrong, but I don't understand why. If you could
34:21
Speaker 1: explain that on the podcast, I would really appreciate it.
34:24
Speaker 1: Thank you all right, thank you very much for this
34:27
Speaker 1: wonderful question. This is a great question, not just because
34:30
Speaker 1: it's fun to talk about black holes and photons and
34:33
Speaker 1: all that stuff, but because it shows somebody being a
34:36
Speaker 1: physicist in action. You're taking one idea black hole with
34:41
Speaker 1: crazy power from gravity, and then applying your understanding of
34:44
Speaker 1: gravity to it and saying, hold on a second, this
34:47
Speaker 1: doesn't work, This doesn't connect in my head. These two
34:50
Speaker 1: ideas clash, and so help me understand it. And that's
34:54
Speaker 1: what we're here for. We are here to help you
34:57
Speaker 1: understand the universe and resolve all of these ideas is
35:00
Speaker 1: in your head. So this short I answer you to
35:02
Speaker 1: your question is that you're right, it doesn't work because
35:06
Speaker 1: you're using Newton's physics, Newton's law of gravity, which cannot
35:11
Speaker 1: describe what happens inside a black hole, and you've got
35:13
Speaker 1: to upgrade to Einstein's theory of gravity, which can describe it.
35:18
Speaker 1: All right, but let's talk a little bit more about
35:20
Speaker 1: what that means. You were walking through your explanation and
35:23
Speaker 1: your question and you were thinking, well, gravity is a force,
35:26
Speaker 1: and it's a force between things that have mass. Right,
35:30
Speaker 1: that's what we typically think about as gravity. You are
35:33
Speaker 1: held onto the Earth because the Earth has a lot
35:35
Speaker 1: of mass and you have mass, and gravity is a
35:38
Speaker 1: force between objects with mass. That was Newton's picture of gravity.
35:43
Speaker 1: That was Newton's description, and he formulated it mathematically. Force
35:47
Speaker 1: of gravity is big G, which is just a number
35:50
Speaker 1: times the two masses involved. Divided by the distance between
35:55
Speaker 1: them squared. So what does that tell us. That's analyze
35:57
Speaker 1: that for a moment. It means that the more mass
36:00
Speaker 1: as you have, the more force of gravity F equals
36:02
Speaker 1: g m m over our squared. The bigger the ms,
36:06
Speaker 1: the bigger the F and so the more force you have.
36:08
Speaker 1: And that's why, for example, you feel stronger pulled towards
36:12
Speaker 1: a heavier object like the Earth than you do to
36:14
Speaker 1: a grapefruit that's even closer to you. And the one
36:17
Speaker 1: in the bottom though, the bit on the bottom of
36:19
Speaker 1: the equation is the one over our square, and that
36:21
Speaker 1: tells you that gravity gets weaker as you get further,
36:24
Speaker 1: and if you're twice as far away, it gets four
36:27
Speaker 1: times weaker because it's an R squared. So you're right
36:30
Speaker 1: that if you use Newton's idea of gravity, and you
36:33
Speaker 1: plug in the mass of a photon being zero, then
36:37
Speaker 1: you get zero force no matter what the other masses,
36:40
Speaker 1: the other masses ten billion sons. It doesn't matter according
36:44
Speaker 1: to Newton, because Newton thought that gravity was just a
36:48
Speaker 1: force between two objects. But Newton was wrong in a
36:51
Speaker 1: couple of really important ways. One is that he thought
36:55
Speaker 1: that gravity was instantaneous he thought that information about gravity
36:59
Speaker 1: try I will through the universe with no delay. He thought,
37:02
Speaker 1: for example, if the Sun disappeared, you would stop feeling
37:06
Speaker 1: its gravity just like that, there would be no delay. Now,
37:09
Speaker 1: of course, we know that it takes time for all
37:12
Speaker 1: information to propagate through the universe, and that if the
37:14
Speaker 1: Sun disappeared, you wouldn't feel it for eight minutes, because
37:18
Speaker 1: that's the maximum speed of information through the universe. So
37:22
Speaker 1: that's one thing about gravity that Newton got wrong. He
37:25
Speaker 1: thought it was instantaneous, and Einstein showed us that it
37:28
Speaker 1: can't be. But Einstein also showed us something much much deeper.
37:32
Speaker 1: He showed us that the right way to think about
37:34
Speaker 1: gravity is not that it's a force at all. Instead
37:37
Speaker 1: that it's a bending of space, and it's this bending
37:40
Speaker 1: of space that affects the way things move in such
37:43
Speaker 1: a way that it looks as if there was a
37:46
Speaker 1: force we call gravity. Now, you may have heard that
37:49
Speaker 1: a few times, so I don't want to really sink
37:50
Speaker 1: that into your brain. I want to make sure you
37:52
Speaker 1: guys really understand what that means. So let's walk through
37:55
Speaker 1: a little analogy. Say, for example, you're on the surface
37:58
Speaker 1: of the Earth and you're on the equator and your
38:00
Speaker 1: friend is somewhere else on the equator, and you both
38:03
Speaker 1: have perfect compasses, and you say, all right, we are
38:05
Speaker 1: going to head north. Everybody walk north. Now, if you're
38:09
Speaker 1: walking north and your friend is also walking north, then
38:12
Speaker 1: you're walking in parallel directions. Right, If you both walk
38:16
Speaker 1: north on the surface of the Earth, you're walking in
38:18
Speaker 1: parallel directions. And you imagine that two people walking in
38:22
Speaker 1: parallel should never meet. Right. If you take two beams
38:25
Speaker 1: of light and you make them perfectly parallel, they will
38:28
Speaker 1: never touch each other. That's true unless space is curved,
38:33
Speaker 1: just as it is on the surface of a sphere
38:35
Speaker 1: and on the surface of the Earth. What happens if
38:38
Speaker 1: you start on the equator and you walk north. Doesn't
38:41
Speaker 1: matter how far apart you are, everybody will end up
38:44
Speaker 1: at the north pole. Those lines will cross. So parallel
38:49
Speaker 1: lines on a curved surface do in fact cross. Parallel
38:53
Speaker 1: lines on a flat surface will not cross. Right, So
38:57
Speaker 1: what is it like for somebody on that surface If
39:00
Speaker 1: you aren't walking north and your friends started out, you know,
39:03
Speaker 1: maybe ten tho kilometers or even just ten ms away
39:06
Speaker 1: from you, you notice them gradually getting closer and closer
39:10
Speaker 1: to you, what is that like. It's like something is
39:13
Speaker 1: pulling you together. It's as if there was a force there,
39:17
Speaker 1: pulling everybody together, so that by the time you got
39:20
Speaker 1: to the north pole boom, you're clumped together into a
39:23
Speaker 1: little dot. So you can see there how a curved space.
39:27
Speaker 1: Moving in a curved space can make us feel as
39:31
Speaker 1: if there's an apparent force there. And if you didn't
39:34
Speaker 1: understand that you were on the surface of a larger object.
39:37
Speaker 1: If you thought, hey, this is my universe, I'm a
39:39
Speaker 1: two D person, you might come up with some force
39:42
Speaker 1: of gravity to explain why things are moving towards each
39:46
Speaker 1: other even if they take off in parallel directions. That's
39:50
Speaker 1: a helpful analogy, but there's also an important element of
39:53
Speaker 1: it missing as we try to take our minds from
39:56
Speaker 1: moving on the surface of a sphere like the Earth,
39:58
Speaker 1: to moving in our or universe. Right, the service of
40:02
Speaker 1: a sphere is too D and we wrap it around
40:04
Speaker 1: a three dimensional sphere in order to understand it. But
40:07
Speaker 1: our universe is three D. Does that mean that we're
40:10
Speaker 1: wrapping it around some four dimensional sphere? No, Because the
40:14
Speaker 1: curvature that we talk about when we say that mass
40:17
Speaker 1: bends space, when we say that general relativity describes gravity
40:22
Speaker 1: as a curvature of space. The curvature we're talking about
40:25
Speaker 1: there is not extrinsic curvature. It's not curvature relative to
40:30
Speaker 1: some four dimension not wrapped around the surface of some
40:33
Speaker 1: higher dimensional sphere. It's an intrinsic curvature. What does that mean,
40:38
Speaker 1: intrinsic curvature. How do you have curvature if it's not
40:41
Speaker 1: relative to like some outside ruler. Well, an intrinsic curvature
40:46
Speaker 1: is just a change in the relationship between points in space.
40:50
Speaker 1: It says, if you have a flat space and you
40:53
Speaker 1: arrange a grid, then all the points are equidistant. But
40:56
Speaker 1: then if you distort it, if you add mass to it,
41:00
Speaker 1: curve that space, then you change the relationship between those points.
41:04
Speaker 1: Some of those points are now closer to each other
41:07
Speaker 1: than they were a moment ago. Some of the points
41:09
Speaker 1: are now further from each other than they were a
41:11
Speaker 1: moment ago. So what happens, for example, to light traveling
41:15
Speaker 1: through space. Well, light always follows the shortest path between
41:19
Speaker 1: two points, and if space is flat, then light just
41:22
Speaker 1: follows a straight line right and goes from point A
41:24
Speaker 1: to point B, and the shortest path is a straight line. Now,
41:28
Speaker 1: imagine a sheet of graph paper, right, you right point
41:30
Speaker 1: A in one spot and point B in the other spot,
41:33
Speaker 1: and you measure the shortest path between point A and
41:37
Speaker 1: point B. It's going to be a straight line between
41:39
Speaker 1: those points. And the grid is square right space is flat.
41:43
Speaker 1: Now what if I took that graph paper and I
41:46
Speaker 1: distorted it. I made some of those points closer together
41:49
Speaker 1: and other points not as close together, And then I
41:52
Speaker 1: asked you, all right, now find the shortest path from
41:55
Speaker 1: A to B. Some of those points don't cost as
41:59
Speaker 1: much to go between as other points, and so now
42:01
Speaker 1: the shortest path between A T V might not be
42:04
Speaker 1: the same as it was before. It might require some
42:07
Speaker 1: sort of curve or a wiggle or two. That's the
42:10
Speaker 1: kind of intrinsic curvature we're talking about. What happens when
42:14
Speaker 1: you have a blob of mass inside some space. Well,
42:18
Speaker 1: it bends space, and it bends space intrinsically so that
42:22
Speaker 1: the relative distances between points change. And that's why, for example,
42:26
Speaker 1: the Earth goes around the Sun, not because there's a
42:29
Speaker 1: force of gravity on the Earth pulling it, but because
42:32
Speaker 1: the shape of space around the Sun is curb so
42:36
Speaker 1: that an object moving inertially with no forces on it,
42:40
Speaker 1: We'll move in a circle around the Sun because that
42:44
Speaker 1: is the shortest path, that is the natural path for
42:47
Speaker 1: an object in that curved space. All right, so we
42:51
Speaker 1: understand that gravity is not just a force g mm
42:55
Speaker 1: over our squared is a curvature of space. Now, it
42:58
Speaker 1: happens to me that you can't in take Newton's picture
43:01
Speaker 1: and it mostly works. That is, you can assume that
43:03
Speaker 1: space is flat, that there's no curvature, and you can
43:06
Speaker 1: treat gravity as if it was a force, and you
43:08
Speaker 1: can write this mathematical equation and mostly it does work,
43:12
Speaker 1: and that's cool, And that just shows you how a
43:14
Speaker 1: lot of times in physics of the universe you can
43:17
Speaker 1: have different ways to see the same effects. But it
43:20
Speaker 1: only mostly works. It breaks down in some cases. It
43:24
Speaker 1: breaks down when the masses get really really large, for example,
43:27
Speaker 1: in the vicinity of a black hole, because what's going
43:30
Speaker 1: on in a black hole. A black hole is a
43:33
Speaker 1: huge amount of mass, but it's not that it has
43:35
Speaker 1: an incredibly high value for the force of gravity. Just
43:39
Speaker 1: because the m value is so large, it's a distortion
43:42
Speaker 1: of space that's so great that space becomes one directional
43:47
Speaker 1: inside the event horizon. Past this point there are only
43:51
Speaker 1: paths that move towards the center of the black hole.
43:55
Speaker 1: It's not just that light is being pulled by a
43:58
Speaker 1: very very strong force, because if you use Newton's law
44:01
Speaker 1: to calculate the force, you would get zero. It doesn't
44:04
Speaker 1: matter how much mass you have, because gravity bends space
44:09
Speaker 1: so much that even photons cannot escape. And the reason
44:13
Speaker 1: is because there is no path out. There's no direction
44:17
Speaker 1: you can go if you're inside the event horizon, which
44:20
Speaker 1: will lead you out. It doesn't matter how fast you move,
44:23
Speaker 1: doesn't matter what the forces are on you. The direction
44:26
Speaker 1: of space requires you to move towards the center of
44:29
Speaker 1: the black hole. Right, And so that's a key understanding.
44:32
Speaker 1: If you think about gravity not as a force but
44:35
Speaker 1: as a distortion of space, then black holes can make sense.
44:39
Speaker 1: And it's not even just in black holes that photons
44:42
Speaker 1: are affected by gravity, right. Photons are also affected just
44:46
Speaker 1: by the changing of shape and space in other non
44:49
Speaker 1: extreme scenarios. For example, we see things like gravitational lensing.
44:54
Speaker 1: If you have a galaxy in between you, and that
44:57
Speaker 1: galaxy is a huge blob of dark matter, for example,
45:01
Speaker 1: then that dark matter can act like a lens because
45:03
Speaker 1: it's mass changes the shape of space, and its light
45:07
Speaker 1: passes through it, it gets distorted in just the same
45:09
Speaker 1: way as if there was a huge lens in space.
45:13
Speaker 1: Even though light has no mass, it's the shape of
45:15
Speaker 1: space itself that has changed, so that now the shortest
45:19
Speaker 1: path for light to follow from that galaxy to your
45:21
Speaker 1: eyeball is a sort of a curved path. So this
45:24
Speaker 1: example gravitational lensing shows us how light can be affected
45:28
Speaker 1: by gravity. And that's how we know that Einstein is
45:31
Speaker 1: more right than Newton, because we found these scenarios where
45:34
Speaker 1: their predictions are different, and lots of situations they give
45:38
Speaker 1: exactly the same predictions. You can think about the shape
45:41
Speaker 1: of space being curved, or a force between two objects
45:44
Speaker 1: and flat space, and you get exactly the same answer.
45:47
Speaker 1: But in some scenarios, and this is what Einstein cooked
45:50
Speaker 1: up to test his theory, you can see differences. And
45:53
Speaker 1: one example scenario is when light gets bent around a
45:56
Speaker 1: heavy object. Because Newton would say it should not be
45:59
Speaker 1: bent all in Einstein would say space is curved, and
46:03
Speaker 1: even massless objects will be influenced by the curvature of space.
46:07
Speaker 1: All right, I hope that answers your questions and explains
46:11
Speaker 1: why photons feel gravity even if they have no mass.
46:14
Speaker 1: It's because gravity is the distortion of space itself, and
46:18
Speaker 1: photons have to fly through that space, and so they
46:21
Speaker 1: are affected by gravity. And thanks to everybody who's been
46:25
Speaker 1: sending in questions, and mostly thank you to everybody who's
46:28
Speaker 1: been thinking deeply about physics and wondering about the universe
46:32
Speaker 1: on this program. We love to wonder, we love to think,
46:34
Speaker 1: We love that there are questions, and sometimes we just
46:37
Speaker 1: love to luxuriate in our ignorance to know that there
46:41
Speaker 1: are answers out there to these questions. That for these
46:44
Speaker 1: really big science questions, there are factual, objective answers that
46:48
Speaker 1: one day humanity will uncover. If we keep doing science
46:52
Speaker 1: and we keep pushing forward on the forefront of knowledge,
46:56
Speaker 1: we will get answers to some of these really big,
46:58
Speaker 1: deep questions about the universe, things that we cannot imagine
47:02
Speaker 1: what it's like to know. People in the future will
47:05
Speaker 1: know those things, and they will look back at us
47:07
Speaker 1: and they will wonder what was it like to be
47:09
Speaker 1: a caveman and not know that the ocean lay just
47:11
Speaker 1: twenty miles away or fifty miles away. What is it
47:14
Speaker 1: like to be a modern human today and not understand
47:17
Speaker 1: the size of the universe, whether shape or the universe,
47:21
Speaker 1: or how the universe was created. These answers are out there,
47:24
Speaker 1: and the way to getting those answers is to keep
47:26
Speaker 1: doing science, to keep asking questions, to keep wondering how
47:30
Speaker 1: the universe works. So thanks everybody for lending us your
47:34
Speaker 1: curiosity and taking this ride with us. Please send us
47:37
Speaker 1: your questions, Thanks for listening, and remember that Daniel and
47:48
Speaker 1: Jorge explained the universe. Is a production of I Heart Radio.
47:52
Speaker 1: For more podcast from my Heart Radio, visit the I
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Speaker 1: Heart Radio Apple Apple Podcasts, or wherever you listen to
47:58
Speaker 1: your favorite shows.