Latest / Daniel and Kelly’s Extraordinary Universe / How can we measure the curvature of space?

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How can we measure the curvature of space? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, what concept in physics twist your brain more
• 0:12
  Speaker 1: than anything else? I think it would have to be
• 0:14
  Speaker 1: general relativity because it's super beautiful and gorgeous, but it's
• 0:18
  Speaker 1: also really hard to actually wrap your mind around. Isn't
• 0:22
  Speaker 1: that all relative? Though? But I guess what makes it
• 0:24
  Speaker 1: hard all the math. There's a lot of math, but
• 0:27
  Speaker 1: fundamentally it's the concepts. You know. It's just such a
• 0:29
  Speaker 1: different view of how to see the universe. It says
• 0:32
  Speaker 1: that space is like a thing and it's invisibly doing
• 0:35
  Speaker 1: stuff that we're blind to. Yes, pretty well, and now
• 0:39
  Speaker 1: we got like neutrinos, dark energy, dark matter. Seems like
• 0:42
  Speaker 1: most of the universe is invisible to us. We're definitely
• 0:44
  Speaker 1: more clueless than clued in when it comes to the universe.
• 0:48
  Speaker 1: It's all a giant game of clue. You know. It
• 0:51
  Speaker 1: was the dark energy that killed the dark matter in
• 0:53
  Speaker 1: the space library within lutrinos. That might be the first
• 0:56
  Speaker 1: time neutrinos ever killed anybody. Hi am Orhammy cartoonists and
• 1:15
  Speaker 1: the creator of PhD comics. Hi, I'm Daniel. I'm a
• 1:17
  Speaker 1: particle physicist and a professor. You see Irvine, And if
• 1:20
  Speaker 1: I could choose the way I go out, I'd like
• 1:22
  Speaker 1: it to be with neutrinos. Oh yeah, why is that?
• 1:24
  Speaker 1: Because it sounds unique. You know, I'd like to be
• 1:26
  Speaker 1: the first person ever killed by neutrinos. I don't even
• 1:29
  Speaker 1: know if that's possible. Like, imagine the crazy intense neutrino
• 1:33
  Speaker 1: beam necessary to even heat you up a little bit,
• 1:35
  Speaker 1: not to mention kill you. Sounds like you thought about
• 1:37
  Speaker 1: this a lot about how to use neutrinos to kill someone. Yeah,
• 1:42
  Speaker 1: for about ten seconds so far. I guess it is hard,
• 1:45
  Speaker 1: but I guess with enough of them that they could
• 1:47
  Speaker 1: be deadly, right, Yeah, if you have a powerful enough beam,
• 1:49
  Speaker 1: they could actually deposit enough energy and you had to
• 1:52
  Speaker 1: fry you just like a laser. Oh, neutrino lasers. Would
• 1:55
  Speaker 1: it be like a neutral tan? It'd be like a
• 1:58
  Speaker 1: little neutral tan. Right, because this newtrie No, maybe our
• 2:01
  Speaker 1: next product idea should be neutrinos blocking pree. Now there's
• 2:05
  Speaker 1: a science scam. Add the word quantum to it and
• 2:08
  Speaker 1: it'll sell cell cell good of a reading of NPF
• 2:11
  Speaker 1: one thousand lutrino protection factor. But anyways, welcome to our podcast.
• 2:16
  Speaker 1: Daniel and Jorge explain the Universe, a production of iHeartRadio,
• 2:19
  Speaker 1: in which we try to beam into your brain all
• 2:22
  Speaker 1: of the incredible mysteries and knowledge about the universe without
• 2:25
  Speaker 1: toasting it. We try to protect that precious little blob
• 2:28
  Speaker 1: of matter while also injecting ideas and questions and curiosity
• 2:33
  Speaker 1: into it. We hope to stimulate your brain to think
• 2:36
  Speaker 1: about the nature of the entire universe, what it looks like,
• 2:40
  Speaker 1: what it seems like, what it's actually doing behind our backs,
• 2:43
  Speaker 1: without toasting it aggressively, or at least neutraally toasting it.
• 2:47
  Speaker 1: Because it is a fascinating universe full of amazing things
• 2:50
  Speaker 1: out there, and it's an ever changing universe. It's a
• 2:53
  Speaker 1: universe that's expanding and growing and shifting and moving and
• 2:57
  Speaker 1: rotating space doing all kinds of things. Is doing a
• 3:00
  Speaker 1: whole lot of things, and as you mentioned, is doing
• 3:02
  Speaker 1: a lot of things that we can't see. Our senses
• 3:05
  Speaker 1: are like tiny little portals into the vast and complex
• 3:08
  Speaker 1: workings of the universe. Most of what's out there is
• 3:11
  Speaker 1: really invisible to us. Yeah, and that's one of the
• 3:14
  Speaker 1: wonderful things about the universe. So that it doesn't reveal
• 3:17
  Speaker 1: itself right away, we need to probe it. We need
• 3:19
  Speaker 1: to think about it. We need to find clever ways
• 3:22
  Speaker 1: to figure out what's going on out there, and so
• 3:24
  Speaker 1: the history of physics is filled with people noticing something weird,
• 3:28
  Speaker 1: something they can't quite explain, something that doesn't quite fit.
• 3:32
  Speaker 1: Usually that's a thread we can pull on to unravel
• 3:34
  Speaker 1: an entire story about something going on in the universe.
• 3:38
  Speaker 1: We weren't even aware of the discovery of neutrinos being
• 3:42
  Speaker 1: pumped out from the Sun, the discovery of vast quantities
• 3:45
  Speaker 1: of dark matter floating out there in space and changing
• 3:48
  Speaker 1: the way that galaxies spin and the entire universe is formed.
• 3:52
  Speaker 1: And that's not the end of the invisible things that
• 3:54
  Speaker 1: the universe is doing right under our noses. Yeah. Maybe
• 3:58
  Speaker 1: one of the most mind blowing revelations about the universe
• 4:01
  Speaker 1: that humans have discovered in the last hundred or so
• 4:03
  Speaker 1: years is this idea that space is not fixed. It's
• 4:07
  Speaker 1: not this kind of emptiness that we are used to
• 4:10
  Speaker 1: in our everyday lives. As we move around in space,
• 4:13
  Speaker 1: space is actually bending and curved. Yeah, space has a
• 4:17
  Speaker 1: lot more properties then Isaac Newton might have imagined. It
• 4:20
  Speaker 1: can do stuff. It's not just there. It's not just
• 4:24
  Speaker 1: the emptiness, the lack of stuff. It is actually a physical,
• 4:28
  Speaker 1: dynamical thing that has properties and can affect things in
• 4:31
  Speaker 1: all sorts of important ways. Yeah, and so one of
• 4:34
  Speaker 1: the most interesting facts about that is this idea that
• 4:37
  Speaker 1: space can curve, that space is not just straight up
• 4:40
  Speaker 1: emptiness with nothing in it. It can actually kind of bend.
• 4:43
  Speaker 1: This idea that space can be described geometrically is having
• 4:47
  Speaker 1: curvature is of course one of the great insights that
• 4:50
  Speaker 1: underpinn Einstein's theory of general relativity. We've had this idea
• 4:55
  Speaker 1: for about one hundred years and it completely reshapes the
• 4:58
  Speaker 1: way we think about the universe. But still going to
• 5:00
  Speaker 1: be pretty tricky to understand what it means. What are
• 5:03
  Speaker 1: we talking about here? If you have a chunk of
• 5:06
  Speaker 1: space in front of you, what does it mean for
• 5:08
  Speaker 1: it to be curved? And is it possible to actually
• 5:11
  Speaker 1: see it? This is literally a mind bending topic. If
• 5:16
  Speaker 1: your mind is part of space, then yes, bending of
• 5:19
  Speaker 1: space will also bend your mind. I do feel like
• 5:22
  Speaker 1: my head is in outer space a lot of the time,
• 5:25
  Speaker 1: or outer space or outer space both. We want to
• 5:28
  Speaker 1: blow your mind into outer space when neutrino lasers, is
• 5:31
  Speaker 1: that where all this is going neutrino lasers are not
• 5:35
  Speaker 1: really very useful for anything except for joking about how
• 5:38
  Speaker 1: Daniel wants to go out. Well, and technically would they
• 5:41
  Speaker 1: be called lasers or lasers? M yeah, good question. I
• 5:44
  Speaker 1: guess it depends on the frequency, right, But it'd be
• 5:47
  Speaker 1: pretty tricky to build an apparatus that could resonate or
• 5:50
  Speaker 1: focus neutrinos. Neutrino optics would be quite challenging to design.
• 5:55
  Speaker 1: It's even hard to make X ray lasers. So I
• 5:57
  Speaker 1: think neutrino lasers are pretty far from our technology capability.
• 6:00
  Speaker 1: So I can safely joke about using them to fry
• 6:03
  Speaker 1: my brain. Well, they're also pretty far from the topic.
• 6:05
  Speaker 1: I don't know how we just can I need degree attorney. Here.
• 6:09
  Speaker 1: We were almost on track there. We're talking about the
• 6:13
  Speaker 1: curvature space and how space is kind of bendy, and
• 6:17
  Speaker 1: so today on the program, we'll be asking the question
• 6:25
  Speaker 1: can we measure the curvature of space? Maybe we should
• 6:28
  Speaker 1: be measuring the curvature of the podcast, Like can we
• 6:30
  Speaker 1: actually keep a conversation going in a straight line or
• 6:33
  Speaker 1: do we constantly bend off the topic into other areas
• 6:37
  Speaker 1: like neutrino lasers. I think the experimental data says that
• 6:40
  Speaker 1: we do bend a lot, sometimes in ninety degrees, sometimes
• 6:45
  Speaker 1: three sixty degrees. Yeah, and sometimes those are the best
• 6:47
  Speaker 1: moments on the podcast when we talk about something totally
• 6:50
  Speaker 1: unexpected and discover a fun little corner of physics. Yeah,
• 6:54
  Speaker 1: but let's maybe stick to the straight and narrow here
• 6:56
  Speaker 1: and stay on the topic of the curvature space. This
• 7:00
  Speaker 1: is an interesting topic because, first of all, maybe a
• 7:03
  Speaker 1: lot of people out there, at least maybe in the
• 7:04
  Speaker 1: general public, don't know that space can curve. Yes, spatial
• 7:08
  Speaker 1: curature is really the foundation of general relativity. It's the
• 7:11
  Speaker 1: idea that gravity is not actually a force, but that
• 7:14
  Speaker 1: the reason things move as if there was gravity is
• 7:17
  Speaker 1: because space is invisibly doing this thing. It's bending, it's curving,
• 7:21
  Speaker 1: it's changing how things move through it. It really requires
• 7:24
  Speaker 1: complete shift in your understanding of gravity and what the
• 7:27
  Speaker 1: universe really is all about. Now, technically, Daniel, don't we
• 7:31
  Speaker 1: need to say that we're talking about the curvature of
• 7:33
  Speaker 1: space time? Right? Yes, because space by itself doesn't really bend.
• 7:38
  Speaker 1: It's really more like a bending if you include time
• 7:40
  Speaker 1: into it. Well, technically I think space does bend, but
• 7:43
  Speaker 1: you're absolutely right there. The equations and the important like
• 7:46
  Speaker 1: conservation laws are expressed in terms of space time because
• 7:50
  Speaker 1: relativity takes time and treats it as the fourth dimension
• 7:53
  Speaker 1: of space. So really it thinks about a four dimensional object,
• 7:57
  Speaker 1: not just three D space. So yes, space time is
• 8:00
  Speaker 1: more accurate description. Right. Wait, are you saying that three
• 8:02
  Speaker 1: D space bends or it's only that we should really
• 8:06
  Speaker 1: be using the word space time to mean the four
• 8:08
  Speaker 1: D concept is what bends? Well, each dimension does bend, Right,
• 8:12
  Speaker 1: X bends, why bends, Z bends, So space itself as x,
• 8:15
  Speaker 1: y Z does bend, but time also bends, and they
• 8:18
  Speaker 1: all sort of bend together. And that's one thing that
• 8:21
  Speaker 1: Einstein realized is that it makes much more sense to
• 8:24
  Speaker 1: think of these as four components of one larger mathematical object.
• 8:28
  Speaker 1: But each individual one does bend the way. For example,
• 8:31
  Speaker 1: time bends, right, that's time dilation. It certainly does bend,
• 8:34
  Speaker 1: but it also bends in conjunction with the other dimensions.
• 8:37
  Speaker 1: That some of the beautiful mathematics of relativities. Seeing how
• 8:40
  Speaker 1: all four work together cool well as usual, we were
• 8:43
  Speaker 1: wondering how many people out there had thought about the
• 8:45
  Speaker 1: question of whether and how you can measure the curvature
• 8:49
  Speaker 1: of space. So thanks very much to everybody who answers
• 8:51
  Speaker 1: these questions for us. We'd love to hear your voice
• 8:54
  Speaker 1: on the podcast as well. Write to me two questions
• 8:56
  Speaker 1: at Daniel d Jorge dot com and I'll email you
• 9:00
  Speaker 1: a batch of questions for future episodes. To think about
• 9:03
  Speaker 1: it for a second, how would you measure the curvature
• 9:05
  Speaker 1: of space? Here's what people had to say. Yes, we
• 9:09
  Speaker 1: can measure the curvature of space. I think we did
• 9:14
  Speaker 1: that gravitational wave detection recently, and I know at least
• 9:18
  Speaker 1: like the calculations work out, so I'm going to go it. Yeah,
• 9:22
  Speaker 1: I know we can detect distortions due to gravitational lensing
• 9:26
  Speaker 1: from massive objects, but I don't think that was what
• 9:31
  Speaker 1: you mean. I don't know how we would detect the
• 9:35
  Speaker 1: curvature of space well, being within the space time continuum,
• 9:41
  Speaker 1: I would think you'd have to be outside of it
• 9:42
  Speaker 1: to be able to see the curvature. Yes, I don't
• 9:45
  Speaker 1: know exactly how it works, but probably we can measure
• 9:50
  Speaker 1: it with a light, with photos something, but I don't
• 9:54
  Speaker 1: know exactly. Oh this might work. If we can't measure
• 9:58
  Speaker 1: the curvature of space, why is general relativity all about
• 10:03
  Speaker 1: all right. Everyone seemed pretty positive about the fact that
• 10:06
  Speaker 1: you can do it. Yeah, it's definitely something we know
• 10:08
  Speaker 1: is happening out there, right, which is sort of cool
• 10:11
  Speaker 1: philosophically and conceptually to accept that this thing is happening
• 10:14
  Speaker 1: all around you. It's sort of invisible to you, but
• 10:18
  Speaker 1: it's necessary to understand how things work, right, to accept
• 10:22
  Speaker 1: that a big fraction of the nature of the universe
• 10:25
  Speaker 1: itself is invisible to us. Yeah, I guess it's kind
• 10:27
  Speaker 1: of a weird question because like if space bent and
• 10:30
  Speaker 1: right here in front of me, I would probably be
• 10:32
  Speaker 1: able to tell, right, Well, that's the question is how
• 10:34
  Speaker 1: could you tell, Like, imagine it was just space, no matter,
• 10:37
  Speaker 1: no particles, everything was totally empty. If you had a
• 10:40
  Speaker 1: chunk of space in front of you, how would you
• 10:42
  Speaker 1: measure its curac or could you measure it without its
• 10:45
  Speaker 1: influence on other things? Like you can't see it bent
• 10:48
  Speaker 1: the way you can look at a road and say, okay,
• 10:50
  Speaker 1: I can see that the road is bending ahead of me.
• 10:52
  Speaker 1: You can't do the same thing with space because it's
• 10:54
  Speaker 1: bending is not directly visible. What do you mean it's
• 10:57
  Speaker 1: not directly visible? Like if the space in front of
• 10:59
  Speaker 1: me curve. Put an i'd be able to see a curve. Well,
• 11:02
  Speaker 1: imagine an invisible road, right, If you can't see the road,
• 11:05
  Speaker 1: but you can follow the cars moving along it, then
• 11:08
  Speaker 1: that's the way you're seeing that the road curves. So
• 11:10
  Speaker 1: that's a difference between a visible road where you could,
• 11:13
  Speaker 1: for example, see that it's bendy even when there aren't
• 11:15
  Speaker 1: any cars on it, and an invisible road like at night,
• 11:18
  Speaker 1: if you can't tell where the cars are on the mountain,
• 11:21
  Speaker 1: but you can follow their headlights, so you can infer
• 11:23
  Speaker 1: where the road must be. In the same way, space
• 11:26
  Speaker 1: we can't directly see its curvature unless there's matter being
• 11:29
  Speaker 1: influenced by that space, we can't directly tell what the
• 11:32
  Speaker 1: curvature is. Well, I see what you're saying. You're saying space,
• 11:35
  Speaker 1: but itself is invisible. You can't like see space, and
• 11:40
  Speaker 1: so therefore how do you know if it's bending or not? Yeah, exactly,
• 11:43
  Speaker 1: And you might think, oh, that's obvious, right, we're talking
• 11:46
  Speaker 1: about space. Space is invisible because it's space, right, But
• 11:48
  Speaker 1: remember that we now know that space is a thing.
• 11:51
  Speaker 1: It has properties. So at each point in space, it
• 11:54
  Speaker 1: has this property, this amount of curvature that's somehow stored
• 11:58
  Speaker 1: in it, and yet we can directly see it. So
• 12:01
  Speaker 1: even though it's invisible, there is something to it. All right, Well,
• 12:04
  Speaker 1: let's dig into this topic. Daniel steps through the basics
• 12:08
  Speaker 1: of this, like, what do you mean by curvature? What
• 12:09
  Speaker 1: does it mean for space to be curved? So first
• 12:12
  Speaker 1: let's dispense with a sort of common misunderstanding of curvature.
• 12:15
  Speaker 1: A lot of people have seen this example of like
• 12:18
  Speaker 1: a rubber sheet with a bowling ball in it, and
• 12:20
  Speaker 1: have been told that this is an example or an
• 12:22
  Speaker 1: analogy for the curvature of space. And you know, this
• 12:25
  Speaker 1: is helpful in some ways because it makes you think
• 12:28
  Speaker 1: about how space could be bendy, but it's also really
• 12:31
  Speaker 1: misleading in some important ways. First of all, it treats
• 12:34
  Speaker 1: our universe as if it was two dimensional, just like
• 12:36
  Speaker 1: the surface of the rubber sheet, and it suggests the
• 12:39
  Speaker 1: bending is happening in some third dimension up and down.
• 12:43
  Speaker 1: So it suggests the bending is extrinsic, that it's like
• 12:45
  Speaker 1: relative to some fixed external ruler. But in our universe
• 12:49
  Speaker 1: we think that the bending of space is intrinsic. There
• 12:52
  Speaker 1: is no external ruler, no fourth dimension where our space
• 12:57
  Speaker 1: is sort of like bending out towards the bending of
• 13:00
  Speaker 1: space for us, and in general, relativities intrinsic, which means
• 13:03
  Speaker 1: it just changes the relative distances between things, like how
• 13:06
  Speaker 1: far two points are apart. I guess that's weird to
• 13:09
  Speaker 1: me because I think what you're saying, is it curvatual
• 13:12
  Speaker 1: is something that happens within space, not relative to anything
• 13:15
  Speaker 1: outside of it. But if it's not happening to anything
• 13:19
  Speaker 1: relative outside of it and we're all in space, I mean,
• 13:22
  Speaker 1: is that still curvature or is it just that's the
• 13:25
  Speaker 1: way space is? You know what I mean? Like, what's
• 13:27
  Speaker 1: the difference between a curve space and a man curve space? Well,
• 13:31
  Speaker 1: you can measure it. And what does it mean? Right? Well,
• 13:33
  Speaker 1: we know that space can be curved and it can
• 13:35
  Speaker 1: also be not curved, because we have chunks of space
• 13:37
  Speaker 1: in our universe that are not curved that are like
• 13:39
  Speaker 1: far from masses and energy, and chunks of space that
• 13:42
  Speaker 1: are highly curved near large masses or even so curved
• 13:45
  Speaker 1: that they become like one directional inside a black hole.
• 13:49
  Speaker 1: So it's definitely something that space can do, and space
• 13:51
  Speaker 1: can do differently. They have different amounts of curvature. Well,
• 13:55
  Speaker 1: I feel like you're now defining it relative to how
• 13:58
  Speaker 1: it's not curved. Right, You're saying it curves relative to
• 14:03
  Speaker 1: how it's not curve. But isn't that also used like
• 14:05
  Speaker 1: an external measure of it or an extremes definition of it. Well,
• 14:08
  Speaker 1: I think it's still relative, and you can use that definition,
• 14:11
  Speaker 1: as we will talk about in this episode, to construct
• 14:13
  Speaker 1: like ways to measure that curvature by, for example, passing
• 14:17
  Speaker 1: matter through it and seeing the influence on it. Right,
• 14:19
  Speaker 1: in curvespace, things that are not under acceleration don't appear
• 14:23
  Speaker 1: to move in straight lines, whereas in flat space they do.
• 14:26
  Speaker 1: So there's definitely a difference in the behavior of objects
• 14:29
  Speaker 1: in flat space and in curved space. And it all
• 14:32
  Speaker 1: comes down to this definition of relative distances. Right. This metric,
• 14:35
  Speaker 1: which is the solution to Einstein's equations, tells you the
• 14:38
  Speaker 1: amount of curvature at every point, and that tells you
• 14:41
  Speaker 1: how things move, and that basically tells you what the
• 14:43
  Speaker 1: shortest path is between two points in space. And so
• 14:47
  Speaker 1: it's all about the relative differences, not in reference to
• 14:50
  Speaker 1: any external ruler, but yeah, it is relative to some
• 14:53
  Speaker 1: internal ruler, right, which is flat space. That's true. Right,
• 14:56
  Speaker 1: So you sort of comparing it to like a universe
• 14:59
  Speaker 1: without any masses or anything energy in it basically, right,
• 15:02
  Speaker 1: which is yeah, which is kind of like thinking about
• 15:05
  Speaker 1: it as like the exterior measure of space, like relative
• 15:08
  Speaker 1: to not space, which is kind of like an outside
• 15:10
  Speaker 1: point of view. Right. Well, I think it's a nice
• 15:12
  Speaker 1: way to think about it as a benchmark, compare curved
• 15:15
  Speaker 1: space to flat space. That's definitely a nice way to
• 15:18
  Speaker 1: think about it. But that flat space doesn't have to
• 15:19
  Speaker 1: be an external metric. It's not like our curved space
• 15:22
  Speaker 1: is sitting inside some larger flat space that's being used
• 15:26
  Speaker 1: to measure it. We can measure the curvature internally without
• 15:30
  Speaker 1: referencing anything outside of our universe. Maybe that's what you
• 15:33
  Speaker 1: mean by intrinsing, is that you can measure this curvature
• 15:36
  Speaker 1: without knowing what it would be like without any masses
• 15:39
  Speaker 1: in it. Yeah, exactly. And it's amazing that we can
• 15:41
  Speaker 1: that we can detect this in our universe, And in
• 15:44
  Speaker 1: some sense it's kind of obvious to us, like we
• 15:46
  Speaker 1: notice the effect of curvature all the time because we
• 15:50
  Speaker 1: grew up experiencing it. Our experience of gravity turns out
• 15:53
  Speaker 1: not to be due to some mysterious force of gravity,
• 15:57
  Speaker 1: as Newton described, But it's the effect of curvature changing
• 16:01
  Speaker 1: how things move through space. So we experienced the curvature
• 16:04
  Speaker 1: space all the time. It's not subtle, all right. So
• 16:07
  Speaker 1: curvature is kind of a property of space itself. It's
• 16:10
  Speaker 1: not relative to some outside space that space sits in.
• 16:15
  Speaker 1: And so is it related to the force of gravity? Right,
• 16:18
  Speaker 1: And so Newton's idea was, look, gravity is a force.
• 16:21
  Speaker 1: I noticed the Earth pulls on things like this apple
• 16:23
  Speaker 1: or that bowling ball, or my bowl of yogurt or whatever.
• 16:26
  Speaker 1: And so Newton explained this thing. He observed that masses
• 16:30
  Speaker 1: tend to attract each other in terms of some force.
• 16:32
  Speaker 1: And he didn't understand like a mechanism of it. He
• 16:35
  Speaker 1: didn't understand deep down what's happening. He just described it
• 16:38
  Speaker 1: and said, here's a mathematical description for what's happening. I
• 16:41
  Speaker 1: have an equation that describes it. It all seems to work.
• 16:44
  Speaker 1: So that was Newton's description. But it turns out that
• 16:47
  Speaker 1: gravity is not actually a force in our universe the
• 16:49
  Speaker 1: way for example, electromagnetism is, or the strong force or
• 16:53
  Speaker 1: the weak force. It turns out it's an apparent force,
• 16:57
  Speaker 1: something that seems to be a force but is actually
• 16:59
  Speaker 1: caused by something else. But wait, I feel like you're saying,
• 17:02
  Speaker 1: maybe the electromagnetic force is apparent. It's not a real force.
• 17:06
  Speaker 1: It's an apparent force. Now I was saying the opposite,
• 17:08
  Speaker 1: that electromagnetism and the weak force and the strong force,
• 17:11
  Speaker 1: those are real forces. But that gravity is different. Gravity
• 17:14
  Speaker 1: is an apparent force. It's not actually a force in
• 17:16
  Speaker 1: the universe. It's just caused by a curvature. And because
• 17:19
  Speaker 1: we can't see the curvature, we need to invent this
• 17:22
  Speaker 1: force in order to explain what we are seeing. Oh right,
• 17:25
  Speaker 1: I got that backwards. So then the curvature space is
• 17:28
  Speaker 1: Gravity is your curvaturo space. It's not gravity. So gravity
• 17:32
  Speaker 1: is our way to explain the effect of the curvature
• 17:35
  Speaker 1: of space, because we didn't realize that space was curved.
• 17:38
  Speaker 1: We didn't understand it was happening. Let's take a simple
• 17:41
  Speaker 1: example of apparent forces we sort of invent to explain things. Say,
• 17:44
  Speaker 1: for example, you're driving a truck and you've got a
• 17:47
  Speaker 1: tennis ball in the back. Now, when your truck is
• 17:49
  Speaker 1: not going anywhere, or it's driving a constant speed. This
• 17:51
  Speaker 1: tennis ball in your truck is just going to sit
• 17:53
  Speaker 1: in the back. It's not going to roll forwards or backwards.
• 17:56
  Speaker 1: But now if you hit the gas and the truck accelerates,
• 17:58
  Speaker 1: then what happens to the tennis ball. It suddenly rolls
• 18:01
  Speaker 1: to the back of the truck. Right, But in your frame,
• 18:04
  Speaker 1: the frame of the truck, why is the tennis ball
• 18:06
  Speaker 1: rolling backwards? Right? There's no force on the tennis ball.
• 18:10
  Speaker 1: For somebody sitting in the back of the truck, they
• 18:12
  Speaker 1: just see it roll backwards. Well, I guess you know,
• 18:14
  Speaker 1: if you were there in the truck, you would also
• 18:16
  Speaker 1: feel that force. Right, Yes, you're back. You're saying, maybe,
• 18:18
  Speaker 1: like if you had a camera inside the truck filming
• 18:21
  Speaker 1: this ball, someone looking at the footage, would you see
• 18:23
  Speaker 1: the ball suddenly start to move? Yeah, exactly. Somebody would
• 18:26
  Speaker 1: see the ball suddenly start to move. And so they
• 18:28
  Speaker 1: would say, where's this force coming from? Right, there's nothing
• 18:31
  Speaker 1: touching it. What is pushing on the ball. And we
• 18:34
  Speaker 1: know the answer is that the truck is accelerating, right,
• 18:36
  Speaker 1: it's actually how you measure acceleration. But in the frame
• 18:39
  Speaker 1: of the truck, the camera that's sitting in the back
• 18:41
  Speaker 1: you can't explain it without adding some external force and saying, well,
• 18:45
  Speaker 1: there must be some external force on this ball, right,
• 18:49
  Speaker 1: So you add this apparent force in order to explain
• 18:51
  Speaker 1: the motion you see. It's the same thing is happening
• 18:54
  Speaker 1: like on a merry go round. If you're on a
• 18:56
  Speaker 1: merry go round that's spinning, you feel this apparent force outward. Right.
• 19:00
  Speaker 1: There's no real force pushing you off the merrygo round.
• 19:03
  Speaker 1: It's just the fact that it's spinning, which again is
• 19:06
  Speaker 1: a kind of acceleration, creates this apparent force. So if
• 19:10
  Speaker 1: you want to do like F equals M and you
• 19:11
  Speaker 1: want to explain all acceleration in terms of forces, you
• 19:14
  Speaker 1: have to create this apparent force to explain what's going
• 19:18
  Speaker 1: on when you accelerate the truck or when you're on
• 19:20
  Speaker 1: the merrygoround. Those are just examples of other places where
• 19:24
  Speaker 1: we've had to add apparent forces in order to explain
• 19:27
  Speaker 1: the dynamics that we're seeing. Well, I guess maybe I'm
• 19:29
  Speaker 1: a little confused on this subtle point because you know,
• 19:32
  Speaker 1: in the case of the ball on the truck, there
• 19:34
  Speaker 1: is a force going on, right, like something is pushing
• 19:36
  Speaker 1: the truck, but nothing is pushing the ball. Yeah, but
• 19:38
  Speaker 1: the truck is being pushed by the engine and the
• 19:41
  Speaker 1: wheels and the friction with the road. What I'm seeing
• 19:45
  Speaker 1: is the ball not being accelerated with the truck. But
• 19:48
  Speaker 1: there is a force going on, right there is there
• 19:50
  Speaker 1: is a regular electromagnetic force pushing the truck. You're exactly right,
• 19:54
  Speaker 1: and that's the key insight. There is a force on
• 19:56
  Speaker 1: the truck and therefore the full frame of reference there
• 19:59
  Speaker 1: is excel rating. So what you're really seeing there is
• 20:02
  Speaker 1: that the frame of reference is accelerating, which makes it
• 20:04
  Speaker 1: a non inertial frame, and the equation F equals MA
• 20:08
  Speaker 1: only works in inertial frames because there's no force on
• 20:12
  Speaker 1: the ball. If you want to understand the acceleration of
• 20:14
  Speaker 1: the ball from the point of view of your camera
• 20:16
  Speaker 1: in the back of the truck, there is no force
• 20:17
  Speaker 1: in the ball, like nothing is touching it, nothing is
• 20:19
  Speaker 1: pushing on it. Something instead is pushing the camera, which
• 20:23
  Speaker 1: is attached to the truck. It's changing the frame of reference.
• 20:26
  Speaker 1: So if you want to use F equals m A,
• 20:28
  Speaker 1: you have to add in a force to compensate for that.
• 20:30
  Speaker 1: So the acceleration creates this apparent force on the ball,
• 20:34
  Speaker 1: even though again nothing is actually pushing the ball, but
• 20:37
  Speaker 1: you see it moving as if there was a force
• 20:39
  Speaker 1: on it. Right. But I guess that's only because you
• 20:41
  Speaker 1: don't know that the truck is accelerating. But if you did,
• 20:43
  Speaker 1: you could figure it out, you could account for it
• 20:46
  Speaker 1: because of the forces that are there. No exactly right,
• 20:49
  Speaker 1: there's two different ways to think about this. To think, oh,
• 20:51
  Speaker 1: I'm in an accelerating frame, so I shouldn't be using
• 20:53
  Speaker 1: F equals MA, I have to account for the fact
• 20:55
  Speaker 1: that I'm accelerating. But if you didn't know that the
• 20:57
  Speaker 1: camera was accelerating, then you have to add a force
• 21:00
  Speaker 1: to account for it. That would be an apparent force.
• 21:02
  Speaker 1: And that's exactly our situation when it comes to curvature
• 21:05
  Speaker 1: and gravity. We can't see curvature. We don't know that
• 21:08
  Speaker 1: it's out there. In fact, for hundreds or thousands of years,
• 21:11
  Speaker 1: we didn't even know it was happening, and so in
• 21:13
  Speaker 1: order to explain the motion of objects as we saw them,
• 21:16
  Speaker 1: we had to create this apparent force we call gravity
• 21:20
  Speaker 1: to explain the things that otherwise didn't have an explanation.
• 21:23
  Speaker 1: Now we know that space is curved, like knowing that
• 21:25
  Speaker 1: the truck was accelerating, and that can be our explanation
• 21:29
  Speaker 1: instead of creating this apparent fictitious force. Let's dig it
• 21:33
  Speaker 1: more into this idea of curvature. And then finally, how
• 21:36
  Speaker 1: do you even measure something that's invisible out there in space?
• 21:40
  Speaker 1: But first, let's take a quick break. All right, we
• 21:54
  Speaker 1: are bending our minds here with the curvature of space,
• 21:57
  Speaker 1: and I have to say I got a little bit confused.
• 22:00
  Speaker 1: I feel like we've talked about this for hours and
• 22:03
  Speaker 1: hours on this podcast, but it's still kind of hard
• 22:06
  Speaker 1: to process. It's kind of hard to tell the difference
• 22:09
  Speaker 1: between like, hey, gravity is not really a force, it's
• 22:14
  Speaker 1: a space that bends, and the alternative point of view,
• 22:17
  Speaker 1: which is like, hey, maybe if you just look at
• 22:19
  Speaker 1: it differently, you can see it this way, that it's
• 22:22
  Speaker 1: a bending us space, you know what I mean? Like,
• 22:24
  Speaker 1: is it that we can see it as a bending
• 22:26
  Speaker 1: of space or that it is a bending of space? Oh? Yeah,
• 22:29
  Speaker 1: great question? Right? Is it just a philosophical distinction or
• 22:33
  Speaker 1: is it actually a physical distinction? Doesn't matter? The answer
• 22:36
  Speaker 1: is that it does matter. We know that it is
• 22:37
  Speaker 1: the bending of space, but most of the time it
• 22:40
  Speaker 1: doesn't matter. Most of the time, treating it like a
• 22:43
  Speaker 1: force and treating it like the bending of space give
• 22:45
  Speaker 1: exactly the same prediction for the Earth going around the Sun,
• 22:48
  Speaker 1: and for all sorts of things. In some edge cases,
• 22:51
  Speaker 1: some corner cases, some extreme circumstances, they do give slightly
• 22:55
  Speaker 1: different predictions. And that's how we know that Einstein's theory
• 22:58
  Speaker 1: was right and then Newton's is wrong. What are some
• 23:01
  Speaker 1: of these extreme cases. So there are a couple of examples.
• 23:03
  Speaker 1: One is like spinning masses. Another is the effect on light.
• 23:07
  Speaker 1: So Newton's theory predicts that spinning masses have exactly the
• 23:11
  Speaker 1: same gravity as masses that don't spin. Like if you're
• 23:13
  Speaker 1: an outer space and the Earth is spinning under you,
• 23:16
  Speaker 1: the gravity from the Earth is not affected by the
• 23:19
  Speaker 1: fact that the Earth is spinning. It just depends on
• 23:21
  Speaker 1: the amount of mass. But Einstein says, actually there's a
• 23:23
  Speaker 1: little effect there. If the Earth is spinning, it's like
• 23:26
  Speaker 1: dragging the curvature of space with it, and this causes
• 23:30
  Speaker 1: a weird, little twisting effect on things out in space.
• 23:33
  Speaker 1: We talked about a really precise experiment called gravity Probe B,
• 23:37
  Speaker 1: which actually measured this and confirmed that this is happening.
• 23:41
  Speaker 1: The other examples the bending of light. Newton's theory says
• 23:44
  Speaker 1: that masses attract there's a force of gravity between objects
• 23:47
  Speaker 1: with mass, but photons have no mass, and yet we
• 23:51
  Speaker 1: see they are bent by massive objects. Einstine's theory was
• 23:55
  Speaker 1: famously proven when we saw light being bent around the Sun.
• 23:58
  Speaker 1: This is gravitational insane, and that's light moving through curved space.
• 24:03
  Speaker 1: So there are some differences between the view that gravity
• 24:06
  Speaker 1: is just a force like Newton said, and that gravity
• 24:08
  Speaker 1: is just an apparent force due to the bending of space, right.
• 24:12
  Speaker 1: I know we talked about both of those cases before
• 24:14
  Speaker 1: in the podcast, but I guess maybe a question I
• 24:17
  Speaker 1: wonder if other people have, is that whether we're used
• 24:20
  Speaker 1: to defining gravity only as like what happens between things
• 24:23
  Speaker 1: with masses, But if you know mass is also energy,
• 24:27
  Speaker 1: what if you just expand that definition of gravity to
• 24:30
  Speaker 1: be what happens between things with energy and so then
• 24:32
  Speaker 1: you can include things like light and that would also
• 24:35
  Speaker 1: explain the bending of light, wouldn't it. Or like I
• 24:39
  Speaker 1: wonder if if you also include energy, then the spinning
• 24:42
  Speaker 1: of the Earth that could also be sort of like
• 24:45
  Speaker 1: extra rotational energy. I don't know, I'm just making things up.
• 24:48
  Speaker 1: I'm just wondering. No, it's really cool. Could you add
• 24:50
  Speaker 1: these features, these bells and whistles to Newton's theory to
• 24:54
  Speaker 1: make them work? And there's a whole bunch of people
• 24:56
  Speaker 1: trying to think about exactly how to abridge Einstein and
• 24:58
  Speaker 1: Newton's theory to make Newton's theory like a special case
• 25:01
  Speaker 1: of Einstein's theory, to sort of like put it as
• 25:03
  Speaker 1: a point in a larger sort of Einsteinian space. And
• 25:06
  Speaker 1: you can try to do that, but it doesn't quite work.
• 25:08
  Speaker 1: You know, for example, treating photons as if they have
• 25:11
  Speaker 1: energy and therefore gravity doesn't work because photons are the
• 25:14
  Speaker 1: same frequency, bend through space the same amount. It depends
• 25:17
  Speaker 1: on the curvature of space, not on the energy of
• 25:20
  Speaker 1: the photon. So it doesn't quite work because I guess
• 25:23
  Speaker 1: photons can have different energies to them. Yeah, photons of
• 25:26
  Speaker 1: different frequencies have different energies, but how much they bend
• 25:29
  Speaker 1: depends on the curvature of space, not on the energy
• 25:31
  Speaker 1: of the photon. Unless there maybe there's something you're missing. Yeah,
• 25:36
  Speaker 1: you can add more bells and whistles if you like.
• 25:38
  Speaker 1: Potentially it's possible for somebody to come up with a
• 25:40
  Speaker 1: modification to Newtonian gravity to make it work like Einsteins
• 25:44
  Speaker 1: and think about it as a force. You know, in
• 25:46
  Speaker 1: some sense, we can never really know what's real out
• 25:49
  Speaker 1: there and what is just our description of the universe.
• 25:52
  Speaker 1: But we have a very compelling description of all of
• 25:55
  Speaker 1: these effects using the concept of space being curved. It's
• 25:59
  Speaker 1: very susful and so we'd like to think that it's real,
• 26:02
  Speaker 1: but ultimately we never can know what's actually happening out
• 26:05
  Speaker 1: there as compared to our mental image of the universe.
• 26:09
  Speaker 1: All right, well, then we have to sort of accept
• 26:12
  Speaker 1: them that it's a real curvature, like it really does
• 26:14
  Speaker 1: ben It seems to be a very accurate description of
• 26:17
  Speaker 1: what's happening in the universe. So it's very tempting to
• 26:19
  Speaker 1: say it's real. You know, philosophically, what does it mean
• 26:22
  Speaker 1: for it to be real? It means that, like it's
• 26:24
  Speaker 1: happening even if we don't look like, if humans weren't
• 26:27
  Speaker 1: here to measure the curvature, space would still be bent.
• 26:29
  Speaker 1: So that's a philosophical claim, not a scientific one. That's
• 26:32
  Speaker 1: not something we could ever actually test. But yeah, it's
• 26:35
  Speaker 1: pretty convincing when you have a theory that works this well,
• 26:38
  Speaker 1: it feels like you've discovered how the universe is working
• 26:41
  Speaker 1: rather than just describing it. Well. Maybe one thing that's
• 26:44
  Speaker 1: a little bit also confusing is that in terms of
• 26:46
  Speaker 1: the curvature space, it's sort of like you can't tell
• 26:48
  Speaker 1: it's curved if you're in it. You can only tell
• 26:51
  Speaker 1: its curved if somebody's watching you from the outside. Right, Like,
• 26:55
  Speaker 1: if you were writing that light being being bent through space,
• 26:58
  Speaker 1: you wouldn't feel any force, is right, you would think
• 27:01
  Speaker 1: you were going in a straight line. But to someone
• 27:03
  Speaker 1: outside of you, you'd be like, oh, that like rain bent. Yeah,
• 27:06
  Speaker 1: you're exactly right. Anything moving according to curvature feels no acceleration.
• 27:11
  Speaker 1: Like if you built an accelerometer, and basically that tennis
• 27:14
  Speaker 1: ball in the back of a truck is an accelerometer,
• 27:16
  Speaker 1: something that measures whether there is acceleration. Say you have
• 27:19
  Speaker 1: an accelerometer with you and you're in a spaceship and
• 27:21
  Speaker 1: you're flying through totally flat space at constant velocity. You're
• 27:25
  Speaker 1: watching the accelerometer, nothing happens, no surprise there. Now, say
• 27:29
  Speaker 1: you're flying through curved space. As you say, could you
• 27:32
  Speaker 1: tell that you were flying through curved space? Just by
• 27:35
  Speaker 1: looking at stuff inside your ship and your accelerometer. After all,
• 27:39
  Speaker 1: you are bending, right, you're changing your direction. Well, the
• 27:42
  Speaker 1: answer is no, you do not feel any acceleration inside
• 27:45
  Speaker 1: your spaceship. Your accelerometer does not register any acceleration because
• 27:49
  Speaker 1: you're moving along the curvature of space. The accelerometer only
• 27:54
  Speaker 1: measures sort of like forces against the curvature of space,
• 27:57
  Speaker 1: like resisting gravities flow. So, yeah, the only way you
• 28:00
  Speaker 1: can measure that curvature is by fight, for example, comparing
• 28:03
  Speaker 1: your position to other objects out there. You mean, as
• 28:07
  Speaker 1: you were writing the lightbeam. Yeah, as you're writing the lightbeam.
• 28:10
  Speaker 1: This concept of freefall, of moving through space without any
• 28:13
  Speaker 1: other forces, just letting gravity control you, it is really
• 28:16
  Speaker 1: important in general relativity. That's the sort of like concept
• 28:20
  Speaker 1: of an inertial observer. Somebody who's like skydiving, they jump
• 28:23
  Speaker 1: out of an airplane. They're in freefall, ignore air resistance.
• 28:27
  Speaker 1: Newton would say, oh, they're being pulled down by the
• 28:29
  Speaker 1: force of gravity, right, and Einstein would say, no, they're
• 28:32
  Speaker 1: just moving along the curvature of space. There's no force
• 28:35
  Speaker 1: on them. They're just in freefall. And Einstein's right that
• 28:39
  Speaker 1: if you had like an accelerometer with you after you
• 28:41
  Speaker 1: jumped out of the plane, it would not measure any acceleration. Right.
• 28:45
  Speaker 1: Maybe this is where it gets sort of tricky and
• 28:46
  Speaker 1: you kind of have to include the definition of time
• 28:49
  Speaker 1: into it, right, because I guess if you jumped out
• 28:51
  Speaker 1: of an airplane, you would get moved, right, like your
• 28:53
  Speaker 1: pocisition in space would change. Yeah, exactly, even though there's
• 28:57
  Speaker 1: no acceleration, right. That's because in curved space time you
• 29:01
  Speaker 1: have to accelerate just to remain stationary. Right. The airplane,
• 29:04
  Speaker 1: for example, it's applying a force to stay up. It's
• 29:07
  Speaker 1: actually accelerating upwards. When you jump out of the airplane,
• 29:11
  Speaker 1: you are no longer accelerating. You're now in freefall, so
• 29:14
  Speaker 1: there are no forces on you because remember gravity not
• 29:17
  Speaker 1: a force. You're just moving according to the curvature space,
• 29:20
  Speaker 1: just like that space ship out in space moving through
• 29:23
  Speaker 1: bent space. It's not going to notice anything. You jump
• 29:25
  Speaker 1: out of the airplane. You're not going to measure any acceleration.
• 29:28
  Speaker 1: The airplane is staying up hopefully, and so it's accelerating
• 29:32
  Speaker 1: up right, just like somebody who's standing on the surface
• 29:35
  Speaker 1: of the Earth in order to avoid moving down towards
• 29:38
  Speaker 1: the center of the Earth. The Earth is accelerating them upwards.
• 29:41
  Speaker 1: It's providing a force, the ground is pushing them up.
• 29:44
  Speaker 1: It's actually accelerating them all upwards. You are in freefall.
• 29:48
  Speaker 1: You don't measure any acceleration. You measure everybody else accelerating upwards. Right.
• 29:52
  Speaker 1: I think maybe this is where you kind of have
• 29:54
  Speaker 1: to say the word space time, right, because I mean
• 29:57
  Speaker 1: you can't just say like, you're following the curvature of
• 29:59
  Speaker 1: space because that only works if you also include time
• 30:03
  Speaker 1: into the word. Absolutely. Yes, time is crucial here because
• 30:06
  Speaker 1: we're talking about motion. So you do kind of have
• 30:08
  Speaker 1: to say the word really that it's a curvature of
• 30:10
  Speaker 1: space time, right, Yeah, curvature of space time absolutely all right.
• 30:14
  Speaker 1: Like you said, it's kind of hard to know that
• 30:16
  Speaker 1: spacetime is bending around you if you're in it, if
• 30:19
  Speaker 1: you're being moved along its curvature, And so I guess
• 30:23
  Speaker 1: the question is how do you measure then that space
• 30:26
  Speaker 1: is being bent? What are some of the ways that
• 30:28
  Speaker 1: people do that? So the most straightforward way to test
• 30:31
  Speaker 1: weather space is bend is to see its effect on stuff, right,
• 30:34
  Speaker 1: This famous description of general relativity is that matter tells
• 30:38
  Speaker 1: space time. How to bend. Spacetime tells matter how to move.
• 30:42
  Speaker 1: So if you see stuff moving in straight lines, that
• 30:44
  Speaker 1: tells you that spacetime in front of you is flat.
• 30:48
  Speaker 1: If you're an inertial observer and you see things moving
• 30:50
  Speaker 1: in curves, that tells you that space time in front
• 30:53
  Speaker 1: of you is curved somehow. So you can just watch
• 30:56
  Speaker 1: the motion of objects, just like looking at those cars
• 30:59
  Speaker 1: to sending down the mountain at night, look at their headlights,
• 31:02
  Speaker 1: you can tell if the road they're on is bent.
• 31:05
  Speaker 1: So basically you have to be kind of like an
• 31:08
  Speaker 1: outside or observer, or I guess you have to sit
• 31:11
  Speaker 1: at a distance imagine what that space in front of
• 31:15
  Speaker 1: you would be like if there wasn't any bending, and
• 31:19
  Speaker 1: if something moves through there differently than it would through
• 31:22
  Speaker 1: empty space, then you know it's being a bent. Yeah. Say,
• 31:25
  Speaker 1: for example, you didn't know the Sun was there, and
• 31:27
  Speaker 1: you threw a planet into the Solar System and it
• 31:29
  Speaker 1: didn't fly right through. Instead, it bent and it ended
• 31:32
  Speaker 1: up in an orbit. Right, that's definitely not the motion
• 31:35
  Speaker 1: you expect through flat space, And so you can tell
• 31:38
  Speaker 1: that space is curved because the object is not moving
• 31:41
  Speaker 1: in a straight line. It's following the curvature of space.
• 31:44
  Speaker 1: It's on what we call a geodesic them hath a
• 31:46
  Speaker 1: particle follows if there's no acceleration on it, And so
• 31:50
  Speaker 1: you can tell, for example, that the space in our
• 31:53
  Speaker 1: Solar System is curved because the Earth is not moving
• 31:56
  Speaker 1: in a straight line. Right. I think we've talked about
• 31:58
  Speaker 1: this before. Like if you took out the Sun and
• 32:00
  Speaker 1: you replace it with a black hole with the same
• 32:02
  Speaker 1: mass as the Sun, it would be super tiny, right,
• 32:06
  Speaker 1: I think maybe around the size of a bowling ball
• 32:08
  Speaker 1: or something like that, which you would never see from
• 32:10
  Speaker 1: this distance, right, because it's millions of miles away, But
• 32:14
  Speaker 1: the planets would still keep orbiting the same way as
• 32:16
  Speaker 1: they would before. Yeah, I think the Sun would actually
• 32:18
  Speaker 1: compressed about three kilometers. But you're absolutely right on the
• 32:21
  Speaker 1: point of gravity. Our Earth would move around it in
• 32:24
  Speaker 1: the same way, and so space was like invisibly bent
• 32:27
  Speaker 1: by a black hole. Then you could tell, and that's
• 32:29
  Speaker 1: exactly what we do. At the heart of our galaxy.
• 32:32
  Speaker 1: We can tell that there's a black hole there even
• 32:34
  Speaker 1: though it's largely invisible by the motion of the stars nearby.
• 32:38
  Speaker 1: They whizz around as if there was some very massive
• 32:41
  Speaker 1: object there. Curving space. That's what I mean. Like a
• 32:45
  Speaker 1: three kilometer wide bowling ball, I want to see the pins.
• 32:50
  Speaker 1: Yeah yeah, but you still wouldn't see that from here, probably, right,
• 32:53
  Speaker 1: something an object three kilometers, why you probably wouldn't see
• 32:56
  Speaker 1: that from here, would you, Especially if it's a black
• 32:58
  Speaker 1: in a black backdrop. I don't know, if it's one
• 33:00
  Speaker 1: of those like swirly galaxy. Bowling balls might also bend
• 33:03
  Speaker 1: into the background. Oh yeah, they do have those glow
• 33:05
  Speaker 1: in the dark bowling balls. Yeah, exactly. And so that
• 33:09
  Speaker 1: seems sort of obvious, and maybe that sounds like a cheat,
• 33:12
  Speaker 1: like we're just saying, oh, gravity is actually the curvature space.
• 33:15
  Speaker 1: So anywhere you see the effective gravity, you're seeing the
• 33:17
  Speaker 1: curvature space, and therefore you know that space is curved.
• 33:20
  Speaker 1: But remember we also talked about the curvature space doing
• 33:23
  Speaker 1: things that just Newton's gravity can't do, like bending light
• 33:27
  Speaker 1: around the Sun. And this was Einstein's famous test of
• 33:30
  Speaker 1: general relativity. He predicted that in an eclipse, we would
• 33:33
  Speaker 1: be able to see the bending of light from distant
• 33:37
  Speaker 1: stars as it goes around the Sun. Right Like, during
• 33:40
  Speaker 1: an eclipse, right actisse, you can see the light race
• 33:42
  Speaker 1: kind of bend around the eclipsing moon. Actually, the bending
• 33:46
  Speaker 1: is of distant stars well behind the Sun, around the Sun,
• 33:50
  Speaker 1: and the reason we use the eclipse it is not
• 33:52
  Speaker 1: because we're looking for the light being bent around the Moon,
• 33:55
  Speaker 1: but just that then the Moon mostly blocks out the
• 33:57
  Speaker 1: Sun's light, so it's easier to see these stars are
• 34:00
  Speaker 1: very close to the Sun. In principle, you could see
• 34:02
  Speaker 1: this at any time. Stars that are sort of just
• 34:05
  Speaker 1: behind the Sun are having their light bent by it,
• 34:08
  Speaker 1: but it's pretty hard to do when the Sun is on.
• 34:10
  Speaker 1: So basically we use the eclipse to turn the Sun
• 34:12
  Speaker 1: off to block it, and then we can see the
• 34:14
  Speaker 1: stuff around it more easily. But also technically the sun
• 34:18
  Speaker 1: rays are probably being bent by the Moon in front
• 34:21
  Speaker 1: of the Sun. Oh right. Also, yeah, absolutely a little bit,
• 34:24
  Speaker 1: but maybe not noticeably. Yeah, for sure, you put your
• 34:26
  Speaker 1: hand up to block the Sun, and your hand is
• 34:29
  Speaker 1: bending the light rays of the Sun, right because your
• 34:32
  Speaker 1: hand curves space. Everything with mass curve space. It's a
• 34:36
  Speaker 1: pretty subtle effect. I mean, even the Sun bends this
• 34:39
  Speaker 1: light by like a thousands of a degree, so it's
• 34:42
  Speaker 1: pretty hard to see. All right. Well, I think what
• 34:44
  Speaker 1: you're saying is that one way to know if curvature
• 34:48
  Speaker 1: of space caused by a mass is to see if
• 34:52
  Speaker 1: things that fly near it, including light bed Yeah, exactly.
• 34:56
  Speaker 1: And one time on the podcast you made a really
• 34:58
  Speaker 1: cool point that the best way to see this actually
• 35:00
  Speaker 1: use like two beams of light, you like, shine a
• 35:02
  Speaker 1: laser through space and see if they stay parallel, right,
• 35:06
  Speaker 1: because if space is flat, then they will stay parallel forever.
• 35:09
  Speaker 1: But if space is curved, then they will bend and
• 35:11
  Speaker 1: they may even cross. Okay, So that's one way to
• 35:14
  Speaker 1: measure how a space can bend is if you see things,
• 35:18
  Speaker 1: the trajectory of things, even including light bending around something.
• 35:21
  Speaker 1: You know that bending relative to what you're looking at.
• 35:24
  Speaker 1: That means that there's something there and it's bending space.
• 35:27
  Speaker 1: What are some other ways that we can measure the
• 35:29
  Speaker 1: bending of space? Time? Yes, space time exactly, very good point.
• 35:33
  Speaker 1: And time is also bent with space, as you've said, right,
• 35:36
  Speaker 1: and so the curvature is not just in space but
• 35:38
  Speaker 1: in space time, which means that time is also curved.
• 35:41
  Speaker 1: And that's this feature we call gravitational time dilation. The
• 35:45
  Speaker 1: curvature of space makes clocks slow down. And this is
• 35:49
  Speaker 1: really super fascinating and different from the kind of time
• 35:52
  Speaker 1: dilation we're used to thinking about from velocity. Like if
• 35:55
  Speaker 1: you see somebody in a spaceship traveling really really fast,
• 35:59
  Speaker 1: we know that your view of their clocks sees their
• 36:02
  Speaker 1: clocks slow down. Moving clocks run slow. That's one really
• 36:06
  Speaker 1: cool effect, but it's actually totally separate from this kind
• 36:09
  Speaker 1: of time dilation. This is time dilation just caused by
• 36:12
  Speaker 1: the curvature of space. So if you're in a part
• 36:15
  Speaker 1: of space that has a lot of curvature, your clock
• 36:17
  Speaker 1: will run more slowly. And so if you look out
• 36:20
  Speaker 1: into the universe and everybody else has clocks and you
• 36:23
  Speaker 1: see their clocks running faster, that means that you are
• 36:26
  Speaker 1: in curved space. You looked at your clock and it
• 36:29
  Speaker 1: seems to be running normally. Everybody else's clocks are running faster.
• 36:33
  Speaker 1: They see your clock as running more slowly. So that's
• 36:36
  Speaker 1: one way to detect the curvature of space. M I
• 36:39
  Speaker 1: think you mean like the example of like you know,
• 36:42
  Speaker 1: you can measure the curvature of space by seeing how
• 36:44
  Speaker 1: they bend, how their trajectory bends in space. That tells
• 36:47
  Speaker 1: you there's something there, But like there might be a
• 36:49
  Speaker 1: situation where you can't tell that space is bent even
• 36:53
  Speaker 1: though there's something there. Like for example, if I shoot
• 36:54
  Speaker 1: a laser straight at a black hole, I'm not going
• 36:58
  Speaker 1: to see the path of the laser move were changed, right,
• 37:01
  Speaker 1: just gonna keep going in a straight line, I guess,
• 37:03
  Speaker 1: until it hits the black hole. But before that you
• 37:05
  Speaker 1: wouldn't be able to tell that space with bending unless
• 37:08
  Speaker 1: you use something else like time. Yeah, good point. If
• 37:10
  Speaker 1: you shoot a laser being directly at the heart of
• 37:12
  Speaker 1: a black hole, like pointed bang on to the singularity,
• 37:15
  Speaker 1: then its path wouldn't bend right because it's sort of
• 37:19
  Speaker 1: moving along that curvature, but it's time would bend right,
• 37:23
  Speaker 1: and so anything falling towards the black hole, it's time
• 37:26
  Speaker 1: gets dilated. And this is something we've actually measured. But
• 37:29
  Speaker 1: what would that mean for a laser though, Like would
• 37:31
  Speaker 1: you see the lasers slow down? So this gets into
• 37:33
  Speaker 1: very tricky territory and general relativity about measuring velocity of
• 37:37
  Speaker 1: distant things. If you are near those photons as they
• 37:40
  Speaker 1: pass you, you measure them as having the velocity of
• 37:43
  Speaker 1: the speed of light. If you are far away from
• 37:45
  Speaker 1: them and they're moving to curved space, then the rule
• 37:48
  Speaker 1: that light always travels at the speed of light no
• 37:50
  Speaker 1: longer applies. That only applies to flat space near inertial observers,
• 37:56
  Speaker 1: so you could actually see light travel are all sorts
• 37:58
  Speaker 1: of different weird velocities. You would see it's slow down, Yes,
• 38:02
  Speaker 1: it is pretty weird. You sort of have to need
• 38:04
  Speaker 1: to like plant the clock in that light laser beam
• 38:06
  Speaker 1: in order to know that the space was curved. Otherwise
• 38:09
  Speaker 1: would you know? I suppose by measuring its velocity as
• 38:12
  Speaker 1: a distant observer you could measure the curvature of space there.
• 38:15
  Speaker 1: But your right time is a really cool way to
• 38:17
  Speaker 1: measure the curvature as well. And this I think is
• 38:20
  Speaker 1: really cool because these are experiments we have done. We
• 38:23
  Speaker 1: shot a laser into the heart of a black hole.
• 38:26
  Speaker 1: Did I miss that headline? Unfortunately, nothing's so cool because
• 38:30
  Speaker 1: we don't have laser beams orbiting black holes. To do
• 38:32
  Speaker 1: these experiments. We have to make do with measuring the
• 38:35
  Speaker 1: curvature of space around our Earth. And so we've done
• 38:38
  Speaker 1: is built really really precise clocks and see that they
• 38:41
  Speaker 1: run differently at different altitudes for example. And again this
• 38:44
  Speaker 1: is not velocity based time dilation. This has not put
• 38:47
  Speaker 1: a clock up in a spaceship and orbit the Earth.
• 38:49
  Speaker 1: Really really fast. They have, for example, a super precise
• 38:51
  Speaker 1: atomic clock that they can raise and lower by like
• 38:54
  Speaker 1: a foot, and they can see a difference in how
• 38:57
  Speaker 1: fast it runs if they raise and low or just
• 39:00
  Speaker 1: buy one foot, because the curvature is different as you
• 39:03
  Speaker 1: move further away from the surface of the Earth. All right, well,
• 39:05
  Speaker 1: let's get into some of the other ways that you
• 39:07
  Speaker 1: can measure the curvature of space, and then let's talk
• 39:10
  Speaker 1: about what this all means. Man, But first let's take
• 39:13
  Speaker 1: another a quick break. All right, we're talking about the
• 39:27
  Speaker 1: curvature of space. It's bending my mind a bit. And
• 39:31
  Speaker 1: how you might measure this bending of space If you
• 39:33
  Speaker 1: didn't know, I guess if space was being bent, I
• 39:35
  Speaker 1: guess if it's not obvious, the space is invisible technically, yeah,
• 39:38
  Speaker 1: if you can't see the curvature directly, how can you
• 39:41
  Speaker 1: tell that it's there? And I love this philosophical question
• 39:44
  Speaker 1: of even if you measure it, do you really know
• 39:45
  Speaker 1: that it's there? Or if it's just some weird effect,
• 39:48
  Speaker 1: Like could somebody come up with another theory of physics
• 39:51
  Speaker 1: it doesn't require the curvature of space, but requires some
• 39:54
  Speaker 1: other weird change in our understanding of reality. That can
• 39:57
  Speaker 1: also explain everything we see, possibly, and then you might
• 40:01
  Speaker 1: be forced to believe that that's how reality actually is,
• 40:04
  Speaker 1: That space doesn't curve, it's actually this other thing that's happening. Yeah,
• 40:07
  Speaker 1: so we're sort of along from the ride as physics
• 40:10
  Speaker 1: is figuring it out. Is this shifting of the clocks
• 40:12
  Speaker 1: in relativity, like this idea that time slows down maybe
• 40:16
  Speaker 1: also another way to show that gravity is not like
• 40:20
  Speaker 1: Newton imagined it, right, because there's nothing in Newton's laws
• 40:23
  Speaker 1: that account for like time slowing down. Right, Yeah, great,
• 40:26
  Speaker 1: point is there? I don't know, No, there is not.
• 40:29
  Speaker 1: Newton thinks of space and time as absolute and universal, right,
• 40:33
  Speaker 1: So this is another feature of relativities, connecting space and
• 40:36
  Speaker 1: time together, tying it all up into one four D
• 40:39
  Speaker 1: mathematical object, and accepting that they're related to each other
• 40:42
  Speaker 1: and in special relativity, space and time are all twisted
• 40:45
  Speaker 1: up together. How you measure clocks depends on where you
• 40:47
  Speaker 1: are and how fast you're going, so they're definitely tied
• 40:50
  Speaker 1: together in a way that Newton never anticipated. Right. All right, Well,
• 40:53
  Speaker 1: we're talking about how to measure this invisible thing of
• 40:56
  Speaker 1: an invisible thing, and so what are some of the
• 40:58
  Speaker 1: other ways that you might measure the bend of space.
• 41:00
  Speaker 1: One really cool way to measure the bending of space
• 41:03
  Speaker 1: is to look at the geometry of objects. Like you
• 41:06
  Speaker 1: can tell if space is curved by building triangles or
• 41:09
  Speaker 1: by measuring the circumference of circles. And this is easiest
• 41:13
  Speaker 1: to understand if you think about like what happens on
• 41:15
  Speaker 1: the surface of a sphere versus like what happens out
• 41:18
  Speaker 1: in space. If you're like out in space and you
• 41:19
  Speaker 1: build a triangle and you measure its angles, you get
• 41:21
  Speaker 1: one hundred and eighty degrees. Now, if you're on the
• 41:23
  Speaker 1: surface of the Earth and you build like a really
• 41:25
  Speaker 1: big triangle, then you measure all of its angles and
• 41:28
  Speaker 1: add them up, you'll discover that they don't actually add
• 41:31
  Speaker 1: up to one hundred and eighty degrees. To add up
• 41:33
  Speaker 1: to a little bit more, because the angles of a
• 41:35
  Speaker 1: triangle add up to one hundred and eighty degrees only
• 41:37
  Speaker 1: on a flat surface, not on a curved surface. Well,
• 41:40
  Speaker 1: I guess only if you kind of project that shape
• 41:44
  Speaker 1: onto the surface of the Earth, right, you can still
• 41:47
  Speaker 1: have a perfect triangle. It's just sitting on top of
• 41:50
  Speaker 1: the Earth in a weird way, isn't it. Yes, if
• 41:53
  Speaker 1: you follow the curvature of the Earth, then that triangle
• 41:56
  Speaker 1: has an angle greater than one hundred and eighty degrees.
• 41:58
  Speaker 1: If you don't follow the curvature of the Earth in
• 42:00
  Speaker 1: your triangles like sort of awkwardly sitting on top of
• 42:02
  Speaker 1: the Earth, then yeah, it can still be flat, but
• 42:05
  Speaker 1: if it follows the curvature, it won't have angles that
• 42:08
  Speaker 1: add up to one hundred and eighty m. That's the
• 42:10
  Speaker 1: one way that you can tell if that the surface
• 42:12
  Speaker 1: of the Earth is curved, right, Yeah, exactly. You can
• 42:15
  Speaker 1: measure the curvature of the Earth. And that's sort of
• 42:17
  Speaker 1: a famous example, but it applies to other objects too,
• 42:20
  Speaker 1: and I think maybe people haven't heard about this, and
• 42:22
  Speaker 1: I think it's really cool. You can also measure the
• 42:24
  Speaker 1: curvature by measuring pie right like draw circle, measure the
• 42:27
  Speaker 1: diameter and the circumference. The ratio is pie, and on
• 42:31
  Speaker 1: a flat surface, pie is three point one, four, one, five, nine, etc. Etc.
• 42:35
  Speaker 1: But on a curved surface it's not right. On a
• 42:38
  Speaker 1: curved surface, the diameter gets longer. Instead of just being
• 42:42
  Speaker 1: like straight across the circle, it's now like rising above
• 42:45
  Speaker 1: the circle and coming back down. So pie changes as
• 42:49
  Speaker 1: space gets curved. I think what you mean is that
• 42:52
  Speaker 1: if you drew a circle across the equator, or if
• 42:54
  Speaker 1: you thought of the equator as a circle, and then
• 42:57
  Speaker 1: you try to measure its radius from the north pole,
• 43:00
  Speaker 1: you wouldn't get pie. You would get something else because
• 43:02
  Speaker 1: you're measuring the radius along the surface of the Earth. Right,
• 43:07
  Speaker 1: You're measuring this basically the curvature, the longitude basically line
• 43:10
  Speaker 1: which are longer than if you just do a line
• 43:13
  Speaker 1: through the center of the Earth on that circle exactly.
• 43:15
  Speaker 1: And so if you were like not aware that the
• 43:17
  Speaker 1: Earth was curved, you try this huge circle and you
• 43:20
  Speaker 1: walk from one part of the equator across the north
• 43:23
  Speaker 1: bold and then back down to your circle and measure that.
• 43:26
  Speaker 1: Then you would get an answer that's much longer than
• 43:29
  Speaker 1: just drilling a hole through the center of the Earth.
• 43:31
  Speaker 1: And so basically that's a measurement of pie. And so
• 43:33
  Speaker 1: pie is a measurement of curvature. And so if you
• 43:36
  Speaker 1: go out in space and make a really big circle
• 43:38
  Speaker 1: and then measure its diameter using a laser beam, you
• 43:42
  Speaker 1: can measure the curvature of that space by comparing what
• 43:45
  Speaker 1: you get to pie. But what if you make a
• 43:48
  Speaker 1: giant pie the size of the equator, wouldn't you still
• 43:51
  Speaker 1: be measuring pie and fill it with neutrinos? I can't
• 43:54
  Speaker 1: tell if you're joking. Are you talking to giant pie pie? Yeah,
• 44:00
  Speaker 1: I mean a giant apple pie to say, ride with
• 44:02
  Speaker 1: the diameter of the equator. But then you still measure pie. Well,
• 44:07
  Speaker 1: that's a good question. If you build a flat pie
• 44:10
  Speaker 1: out in space that's ignoring the curvature of space, it's
• 44:12
  Speaker 1: being held together by electromagnetic bonds which are really strong,
• 44:16
  Speaker 1: then it could still be flat, right, But if you're
• 44:18
  Speaker 1: following the curvature of space, you're using like a laser
• 44:21
  Speaker 1: beam to follow the curvature of space, then you would
• 44:23
  Speaker 1: not get pie measured across your giant apple pie, which
• 44:27
  Speaker 1: would have to cut I guess, with a neutrino laser
• 44:29
  Speaker 1: beam because it'd be so big. What kind of ice
• 44:33
  Speaker 1: cream do you serve with neutrino apple pie? I don't
• 44:36
  Speaker 1: even know. Obviously a dark ice all right. So that's
• 44:39
  Speaker 1: another way to measure the curvature space is using geometry,
• 44:43
  Speaker 1: which is like a sixth grade subject just measuring angles
• 44:47
  Speaker 1: between things, and they don't come out to be what
• 44:50
  Speaker 1: you would expect in flat space, then you know your
• 44:52
  Speaker 1: space is what are some other ways that we can
• 44:54
  Speaker 1: measure the vending space? So I think one of the
• 44:57
  Speaker 1: coolest ways is this experiment we talked about much earlier,
• 45:00
  Speaker 1: which measures frame dragging. This is an effect that doesn't
• 45:03
  Speaker 1: happen in Newtonian gravity at all. So, as we said before,
• 45:07
  Speaker 1: the Earth is spinning, and as it spins, it's sort
• 45:10
  Speaker 1: of like drags space time with it a little bit.
• 45:13
  Speaker 1: And so if you're an object out orbiting the Earth,
• 45:15
  Speaker 1: you feel a different force because the Earth is spinning
• 45:18
  Speaker 1: than you would if it wasn't spinning. Would you feel
• 45:21
  Speaker 1: is a little torque, like a little twist, not just
• 45:23
  Speaker 1: a force inwards towards the center of the Earth, but
• 45:26
  Speaker 1: like a little twist that spins you a little bit,
• 45:28
  Speaker 1: because how space is sort of flowing over you. So
• 45:31
  Speaker 1: there's these awesome experiments called gravity Probe B that built
• 45:34
  Speaker 1: like the most spherical objects known to man, these super
• 45:37
  Speaker 1: precise gyroscopes out in space that were like mind in
• 45:42
  Speaker 1: Brazil and then polished by like German grandmothers for years
• 45:45
  Speaker 1: and years and years, and these were able to measure
• 45:47
  Speaker 1: this very very small effect. But it's real, you know,
• 45:50
  Speaker 1: we have a whole episode about this. But this sort
• 45:51
  Speaker 1: of related to tidal forces too, right, Like, if you
• 45:54
  Speaker 1: have an object down in space near Earth spinning, some
• 45:57
  Speaker 1: things are sort of closer to it than others, and
• 45:59
  Speaker 1: so there's some delay in how the gravity kind of
• 46:03
  Speaker 1: goes from one end to the other. Yeah, exactly, this
• 46:05
  Speaker 1: only happens on objects that are not points, that objects
• 46:08
  Speaker 1: that have an extent, because as you say, they're experiencing
• 46:10
  Speaker 1: space differently, and so it's that relative effect across the
• 46:14
  Speaker 1: object that ends up causing the torque. So you're right. Conceptually,
• 46:17
  Speaker 1: it is similar to tidal forces in that way. The
• 46:20
  Speaker 1: effect is larger for bigger objects and zero for point
• 46:23
  Speaker 1: like objects. But this frame dragging is a way to
• 46:26
  Speaker 1: confirm that space can curve, but you sort of need
• 46:30
  Speaker 1: as guidance spinning object to cause that kind of effect. Yeah,
• 46:33
  Speaker 1: you don't get frame dragging around objects that are not spinning.
• 46:36
  Speaker 1: So in one way, it's really a test of Einstein's
• 46:39
  Speaker 1: theory to say, is this effect that Einstein predicts but
• 46:42
  Speaker 1: Newton doesn't, is it real in our universe? And the
• 46:44
  Speaker 1: answer is yes, And that's a consequence of all of
• 46:47
  Speaker 1: Einstein's math and his concept that space is curved. This
• 46:51
  Speaker 1: is a direct result of space being curved and how
• 46:55
  Speaker 1: space reacts and how that curvature reacts to mass, especially
• 46:58
  Speaker 1: spinning masses, and so in that sense, it's an indirect
• 47:02
  Speaker 1: confirmation that space is actually curved. The scientists who work
• 47:06
  Speaker 1: on this project, they think of this is like one
• 47:08
  Speaker 1: of the most direct measurements of the curvature of space.
• 47:11
  Speaker 1: Because so many other measurements could be explained by Newton's theory,
• 47:14
  Speaker 1: this one only can be explained by Einstein's theory. So
• 47:17
  Speaker 1: they take it as really proof that Einstein was right
• 47:20
  Speaker 1: and therefore space is curved or space can curve right, Yes,
• 47:25
  Speaker 1: can curve, are more likely space time can curry. Spacetime
• 47:30
  Speaker 1: has the ability to have curvature, which is really still
• 47:33
  Speaker 1: bongles my mind. Yeah, so I guess maybe to wrap
• 47:36
  Speaker 1: it all up, like, let's say I wanted to tell
• 47:39
  Speaker 1: if the space around our Solar System or even the
• 47:42
  Speaker 1: space around our galaxy was curved, maybe not due to
• 47:47
  Speaker 1: the mass of the galaxy, but just like overall, like
• 47:50
  Speaker 1: are we living in a spherical universe or are we
• 47:53
  Speaker 1: living in a cube universe? Or are we living in
• 47:55
  Speaker 1: a donut universe? Like how do you tell that you're
• 47:59
  Speaker 1: space around is curve Would you be able to tell
• 48:02
  Speaker 1: with any of these methods, or do would some of
• 48:04
  Speaker 1: these methods work, and other thoughts. So, so, to measure
• 48:06
  Speaker 1: the curvature is space on like a cosmological scale, you
• 48:10
  Speaker 1: could use this if you could construct a giant triangle
• 48:14
  Speaker 1: bigger than galaxies, or shoot laser beams between galaxies in
• 48:18
  Speaker 1: a big triangle, then you could use these methods to
• 48:21
  Speaker 1: measure them. But that's not really practical, right, But what
• 48:24
  Speaker 1: we can do instead is see the effect of space
• 48:27
  Speaker 1: on things that are already out there. For example, we've
• 48:30
  Speaker 1: looked at the cosmic microwave background radiation, this light left
• 48:34
  Speaker 1: over from just after the Big Bang, and that light
• 48:37
  Speaker 1: has like wiggles in it has like hot spots and
• 48:39
  Speaker 1: cold spots, and we know something about how big those
• 48:42
  Speaker 1: hot spots should be because of how much time things
• 48:46
  Speaker 1: had to like even out and cool off. And then
• 48:48
  Speaker 1: the curvature of space affects the size of those hot
• 48:51
  Speaker 1: spots as we see them. If space is curved in
• 48:54
  Speaker 1: one way, then the spots get bigger, they get like
• 48:56
  Speaker 1: blown up by lensing. If space is curved in another way,
• 48:59
  Speaker 1: they get shrunk. So we can actually measure like the
• 49:02
  Speaker 1: overall curvature of space near us by looking at the
• 49:06
  Speaker 1: size of hot spots in the cosmic microwave background radiation,
• 49:10
  Speaker 1: because I guess that light comes from really really really
• 49:13
  Speaker 1: far away kind of and all around us. Yeah, and
• 49:15
  Speaker 1: all around us. It's been traveling from billions and billions
• 49:18
  Speaker 1: of years, and so it's probing a really really big space.
• 49:20
  Speaker 1: It's like the oldest light that we can see. So
• 49:22
  Speaker 1: it comes from like the very edge of the observable universe.
• 49:27
  Speaker 1: What does that light say? Is the universe bending right
• 49:29
  Speaker 1: or left? That light says that, to within our accuracy
• 49:33
  Speaker 1: to measure it, the universe is flat. Like there are
• 49:36
  Speaker 1: little bendy spots here and there near galaxies, but that
• 49:39
  Speaker 1: overall there is no curvature to the universe. That the
• 49:42
  Speaker 1: universe seems to be mostly flat. That's sort of a
• 49:45
  Speaker 1: weird result, though, I would maybe say maybe you're just wrong,
• 49:50
  Speaker 1: And it's sort of like when you're trying to see
• 49:54
  Speaker 1: if something is right or let and you say, oh,
• 49:56
  Speaker 1: it's neither. I'm like, well, how do you know you
• 49:59
  Speaker 1: got it right? Yeah, it's a good question. And lots
• 50:01
  Speaker 1: of people have done these experiments with lots of different
• 50:03
  Speaker 1: technologies and look at lots of different aspects of it.
• 50:06
  Speaker 1: So we're pretty confident. But you know, there's also statistical
• 50:10
  Speaker 1: uncertainty there. It's like flat to within about one percent,
• 50:14
  Speaker 1: and so there's a possibility that space is a little
• 50:16
  Speaker 1: bit bent one way or the other sort of overall,
• 50:20
  Speaker 1: but we know that space is bent near us, right
• 50:22
  Speaker 1: effect of all, the mass of the galaxy in the
• 50:24
  Speaker 1: Solar System is definitely a curving space in our neighborhood.
• 50:28
  Speaker 1: What if we build a giant pie the size of
• 50:31
  Speaker 1: the galaxy. I'm in, I don't need to hear anymore.
• 50:36
  Speaker 1: I mean, well, technically you could do also do that
• 50:38
  Speaker 1: right that thing. That's what you meant earlier by giant lasers.
• 50:41
  Speaker 1: Like if you build a giant circle the size of
• 50:45
  Speaker 1: the galaxy and measure pie by measuring the conference and radius,
• 50:49
  Speaker 1: then you would be able to tell right that things
• 50:51
  Speaker 1: are bent. Hm. Yes, galaxy sized pie is a good
• 50:55
  Speaker 1: experiment in any case, because even if it turns out
• 50:59
  Speaker 1: to be inconclusive, it might still be delicious fun fun
• 51:02
  Speaker 1: fund all right, Well, it sounds like there are many
• 51:05
  Speaker 1: ways to measure the curvature of space, some of them
• 51:09
  Speaker 1: more delicious and or dangerous than others. But the amazing
• 51:12
  Speaker 1: fact is that is space time does bend. It's been confirmed,
• 51:17
  Speaker 1: and there are experiments you can do to measure it.
• 51:19
  Speaker 1: That's right, and it is possible to get a grasp
• 51:22
  Speaker 1: on what's going on with our universe understanding it's otherwise
• 51:26
  Speaker 1: invisible doings if we are clever enough, and if we
• 51:29
  Speaker 1: follow the threads of all those weird little things we
• 51:32
  Speaker 1: can't otherwise explain. I wonder what people who think that
• 51:34
  Speaker 1: the Earth is flat think about this concept, like it's
• 51:37
  Speaker 1: it's just too much for them, or do you think
• 51:39
  Speaker 1: there are people out there who say, yes, the Earth
• 51:42
  Speaker 1: is flat, but the universe is cur I don't know.
• 51:45
  Speaker 1: I'm a flat universe er. All right. Another exploration of
• 51:48
  Speaker 1: how mind bending and also space time bending the universe is,
• 51:53
  Speaker 1: I guess a good reminder that sometimes we think the
• 51:56
  Speaker 1: universe is one way from our local experience, but if
• 51:58
  Speaker 1: you go out there into the universe or to explore
• 52:01
  Speaker 1: extreme situations, it turns out that the universe is different.
• 52:06
  Speaker 1: Hope you enjoyed that. Thanks for joining us, see you
• 52:08
  Speaker 1: next time. Thanks for listening, and remember that Daniel and
• 52:18
  Speaker 1: Jorge explain the universe is a production of iHeartRadio. For
• 52:22
  Speaker 1: more podcast from my heart Radio, visit the iHeartRadio app,
• 52:26
  Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.