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What is a light cone? · Daniel and Kelly’s Extraordinary Universe

• 0:09
  Speaker 1: Or Hey, do you ever wish that the speed of
• 0:11
  Speaker 1: light was faster? I guess I'm not dissatisfied with the
• 0:15
  Speaker 1: speed of light. I'm just glad it's not slower, because
• 0:17
  Speaker 1: you know, I don't want to wait ten minutes after
• 0:19
  Speaker 1: I flip on the light switch. But you know, if
• 0:21
  Speaker 1: it was faster, and then we could see like further
• 0:24
  Speaker 1: out into the universe. M m. That's true, and then
• 0:27
  Speaker 1: we might be able to travel further to explore the stars.
• 0:30
  Speaker 1: Also true. But there's another side to it, as always, well,
• 0:34
  Speaker 1: how could it be bad to have a faster speed
• 0:36
  Speaker 1: of light? Well, light speed was a million times faster
• 0:39
  Speaker 1: than technically any alien out there in the universe could
• 0:42
  Speaker 1: kill us instantly with a death ray. That's true. But
• 0:45
  Speaker 1: you know, witnessing an alien death ray would be pretty cool.
• 0:49
  Speaker 1: It would be pretty cool for about a nano second,
• 0:51
  Speaker 1: and then you'd be dead. Also, I'm pretty sure a
• 0:53
  Speaker 1: death ray would be hot and not cool. Might still
• 0:56
  Speaker 1: be worth it, just for that nana second, you would
• 0:58
  Speaker 1: know that AIDS existed. As my brain is getting FRIEDA.
• 1:16
  Speaker 1: I am or haemade cartoonists and the creator of PhD comics. Hi,
• 1:20
  Speaker 1: I'm Daniel, I'm a particle physicist, and I don't spend
• 1:23
  Speaker 1: a lot of time worrying about alien death rays. You don't,
• 1:26
  Speaker 1: do you know something I don't. I know that the
• 1:28
  Speaker 1: speed of light protects us from most of the universe
• 1:30
  Speaker 1: shooting us with alien death rays. Really like it forms
• 1:34
  Speaker 1: a force field around us, protecting this like a blanket
• 1:37
  Speaker 1: helping us sleep at night. It just makes it kind
• 1:39
  Speaker 1: of ineffective. If you're going to shoot an alien death
• 1:41
  Speaker 1: ray millions of light years, then you're gonna kill your
• 1:43
  Speaker 1: target in millions of years. And like, who's geopolitical strategy
• 1:47
  Speaker 1: really has that long and arc to it? But what
• 1:49
  Speaker 1: if there are patient aliens? Then we're toast? Then we
• 1:53
  Speaker 1: might literally be toast. Welcome to our podcast, Daniel and
• 1:56
  Speaker 1: Jorge Explain the Universe, a production of I Heart Radio
• 1:59
  Speaker 1: in which we beam our joke rays into your brain,
• 2:02
  Speaker 1: hoping to transmit some of the incredible wonder and knowledge
• 2:06
  Speaker 1: about the universe, all the things that science has learned
• 2:09
  Speaker 1: about this Bonker's wondrous, beautiful, glittering cosmos, and all the
• 2:13
  Speaker 1: things that science is still working on, all the vague
• 2:16
  Speaker 1: questions about how the universe got here, where it's going,
• 2:19
  Speaker 1: and who lives in it, and how to actually build
• 2:21
  Speaker 1: a joke Ray. I mean, that's probably the next Nobel Prize.
• 2:25
  Speaker 1: The question it doesn't make you groan or does it
• 2:27
  Speaker 1: make you laugh? Is that where you're gonna ask when
• 2:29
  Speaker 1: the aliens come, I'm gonna be like, tell us the
• 2:30
  Speaker 1: secrets of spacetime and you're like, have you developed the
• 2:33
  Speaker 1: joke Ray yet? We'll think about what it would mean
• 2:35
  Speaker 1: for the mental health of humanity. You know, we could
• 2:38
  Speaker 1: just have a laugh about everything. Laughter is the best medicine.
• 2:41
  Speaker 1: So anytime you go to the doctor, they're just like, yep,
• 2:43
  Speaker 1: we're prescribing you five shots with the joke Ray. And
• 2:45
  Speaker 1: then as the aliens as they you know, enslave us
• 2:48
  Speaker 1: and possibly eat us, we could be laughing all the way.
• 2:51
  Speaker 1: But it is a bizarre and jokey universe because it
• 2:55
  Speaker 1: seems like a lot of it is beyond our control
• 2:57
  Speaker 1: or even beyond our ability to whoever interact with it.
• 3:01
  Speaker 1: That's true, and it's also sometimes beyond our ability to
• 3:05
  Speaker 1: really understand. We have one way of thinking about the
• 3:08
  Speaker 1: world where things move and slide and the way time works.
• 3:11
  Speaker 1: But about a hundred years ago we discovered that that
• 3:14
  Speaker 1: wasn't really the way it worked, that there were different
• 3:16
  Speaker 1: rules the things change when you move fast or when
• 3:19
  Speaker 1: space is bent, and that we needed new rules and
• 3:22
  Speaker 1: new ideas to understand what actually goes on in this universe. Yeah,
• 3:26
  Speaker 1: because I guess we realized a few hundred years ago
• 3:28
  Speaker 1: that the universe, the world doesn't really behave the way
• 3:31
  Speaker 1: it behaves like on an everyday basis, like when you're
• 3:33
  Speaker 1: juggling baseball or or you know, balancing a basketball. It's
• 3:37
  Speaker 1: like that's one way that we think of the world
• 3:39
  Speaker 1: is working. But really, like if you get down to it,
• 3:41
  Speaker 1: in some situations, it's totally different. You know. In fact,
• 3:44
  Speaker 1: there are really fundamentally different rules that govern how things
• 3:48
  Speaker 1: move and how time flows. It just happens to be
• 3:51
  Speaker 1: that in certain situations, like when you're moving really slow
• 3:55
  Speaker 1: and when you're far from a black hole, things look
• 3:58
  Speaker 1: a lot simpler. So we're living in a sort of
• 4:01
  Speaker 1: a simplified corner of a very weird, very complicated universe. Right,
• 4:05
  Speaker 1: It's almost like we're in a special case, like our
• 4:08
  Speaker 1: perception of the universe. It's like a small example of
• 4:11
  Speaker 1: all the ways that the universe could be. It's like
• 4:12
  Speaker 1: you're playing the warm up levels on some video game
• 4:15
  Speaker 1: before all the bad guys come in, just you can
• 4:17
  Speaker 1: sort of like figure out how the controls work. Right,
• 4:19
  Speaker 1: That's what we've been doing for physics for like thousands
• 4:22
  Speaker 1: of years, and now we finally understand the full scope
• 4:25
  Speaker 1: of the game. And wouldn't be great a good respond
• 4:28
  Speaker 1: in this universe? We can just keep trying, well, we
• 4:31
  Speaker 1: sort of do. I mean, like physicists start from where
• 4:34
  Speaker 1: previous generations of physicists ended, right, We don't have to
• 4:36
  Speaker 1: start from scratch on physics every single generation. So physics
• 4:39
  Speaker 1: sort of responds in our minds, right, And we're going
• 4:42
  Speaker 1: to run out of lives at some point. Do we
• 4:44
  Speaker 1: need to put in more quarters into the arcade of
• 4:46
  Speaker 1: the universe? That depends on the alien death right or
• 4:50
  Speaker 1: the joke ray Neither one might help us or not
• 4:53
  Speaker 1: help us. But yeah, the universe is kind of tricky,
• 4:56
  Speaker 1: and it's tricky not just in like extreme situations like
• 4:59
  Speaker 1: in a that hole or moving on close to the
• 5:01
  Speaker 1: speed of light, because who knows, like we could be
• 5:03
  Speaker 1: moving right now really fast compared to other aliens and
• 5:07
  Speaker 1: other galaxies right we are. We are moving really fast
• 5:10
  Speaker 1: compared to the Sun, and the Sun is moving really
• 5:12
  Speaker 1: fast compared to the center of the galaxy. But all
• 5:14
  Speaker 1: those things are kind of abstract. What really matters is
• 5:17
  Speaker 1: your speed relative to the people who are watching you
• 5:20
  Speaker 1: and relative to your clock. And so that's something that
• 5:24
  Speaker 1: special relativity has taught us, is that the world looks
• 5:27
  Speaker 1: different from different perspectives. Based on who's doing the measuring.
• 5:31
  Speaker 1: You can get kind of a different account of what's
• 5:33
  Speaker 1: going on and what's happened in the world. Yeah, you
• 5:35
  Speaker 1: might not agree with your relatives about relativity, but yeah,
• 5:39
  Speaker 1: things are kind of tricky in this universe, and so
• 5:42
  Speaker 1: even physicists sort of struggle with it, right, I mean,
• 5:45
  Speaker 1: it's sort of hard to rab anyone's head around these
• 5:48
  Speaker 1: topics and how it actually works. And and so physicists
• 5:52
  Speaker 1: have some ways to kind of think about how these
• 5:54
  Speaker 1: things work that might help other people. Yeah, because in
• 5:57
  Speaker 1: science we have our intuition, like give a fling about
• 6:00
  Speaker 1: how things might work and what might happen and how
• 6:03
  Speaker 1: things are put together. And then there's the mathematics. And
• 6:06
  Speaker 1: the mathematics is what actually happens, what really is going on,
• 6:09
  Speaker 1: the rules that really do control the universe. But sometimes
• 6:12
  Speaker 1: you're looking at like a page of equations and you're like,
• 6:15
  Speaker 1: I don't see it. I don't really get it. And
• 6:17
  Speaker 1: for me personally, it's easier to think about these things
• 6:20
  Speaker 1: like geometrically to understand like the relationship in space and
• 6:24
  Speaker 1: time of these things. So sometimes you can get more
• 6:27
  Speaker 1: of an intuition for weird things if you think about
• 6:30
  Speaker 1: their structure and space and time sort of geometrically, like
• 6:33
  Speaker 1: the shapes that are created by all of these ideas, right,
• 6:36
  Speaker 1: and then you draw cartoons about it, which is where
• 6:40
  Speaker 1: right exactly? It helps to draw cartoons, no joke. Cartoons
• 6:43
  Speaker 1: are a great way to visualize this stuff. That's why
• 6:46
  Speaker 1: you and I work together on a lot of these books,
• 6:48
  Speaker 1: and folks out there should check out our book We
• 6:50
  Speaker 1: Have No Idea, which has all sorts of really clever
• 6:53
  Speaker 1: cartoons visualizing really tricky concepts. Yeah, and our upcoming book
• 6:57
  Speaker 1: which is coming out in a few months frequently questions
• 7:00
  Speaker 1: about the universe, which you can pre order right now. Man,
• 7:03
  Speaker 1: we totally snuck in those book plugs, Daniel, totally unplanned.
• 7:07
  Speaker 1: I keep getting emails from listeners saying, you guys should
• 7:09
  Speaker 1: write a book. I'm like, what do we not talk
• 7:11
  Speaker 1: about the book enough on the podcast? I guess I
• 7:13
  Speaker 1: guess we don't. We should talk about it every time.
• 7:16
  Speaker 1: I think we should, honestly. Anyway, back to the podcast. Yeah,
• 7:19
  Speaker 1: back to the podcast. Let's talk about one of these
• 7:21
  Speaker 1: tools that physicists used to kind of wrap your head
• 7:23
  Speaker 1: around some of these tricky concepts about relativity and the
• 7:26
  Speaker 1: speed of light. So today on the podcast, we'll be
• 7:28
  Speaker 1: asking the question, what is a light cone? Daniel, this
• 7:38
  Speaker 1: is a light coming, I think, I guess you mean light,
• 7:40
  Speaker 1: not like not heavy cone is exactly light as in
• 7:44
  Speaker 1: photons is and that thing that speeds along at the
• 7:47
  Speaker 1: maximum speed of the universe. Now, this sounds sort of
• 7:50
  Speaker 1: like maybe a dessert, but maybe we're not a dessert
• 7:53
  Speaker 1: you want to eat or that would taste very good.
• 7:56
  Speaker 1: Sounds like the kind of dessert your mom would order
• 7:57
  Speaker 1: for you when what you really want is scubavice. Where
• 8:00
  Speaker 1: you really want is a heavy cone, the heavy cream cone.
• 8:03
  Speaker 1: You don't want the like fat free light cone. But yeah,
• 8:07
  Speaker 1: So this is a concept that comes up in special
• 8:09
  Speaker 1: relativity and it has some pretty interesting consequences about causality
• 8:14
  Speaker 1: and like what's actually real or like what's actually happening
• 8:19
  Speaker 1: or happened in the universe. Yeah, and it helps you
• 8:21
  Speaker 1: think about what's going on inside a black hole and
• 8:24
  Speaker 1: even possibly maybe allows you to figure out a way
• 8:27
  Speaker 1: to do real life time travel. Sounds like a very
• 8:31
  Speaker 1: powerful dessert here. All right, Well, before we go on,
• 8:34
  Speaker 1: we just want to give a quick shout out to Biorn,
• 8:37
  Speaker 1: right he sort of helped inspire this episode. Daniel. Yeah,
• 8:40
  Speaker 1: thanks Born from Sweden who sent me a link to
• 8:43
  Speaker 1: his Solar System inspired music which I listened to while
• 8:47
  Speaker 1: preparing for this podcast. You can find him on Spotify
• 8:50
  Speaker 1: at ord cloud Services. Check them out if you like
• 8:52
  Speaker 1: the sort of space ambiance music. Thanks a lot, Biorn
• 8:55
  Speaker 1: for sharing music. Yeah, and again that's or Clouds. That's
• 8:57
  Speaker 1: o RT cloud service is on Spotify, which is a
• 9:01
  Speaker 1: pun because it's like the Orc Cloud and also cloud services.
• 9:05
  Speaker 1: And what is the orc cloud done for us recently? Well,
• 9:07
  Speaker 1: I guess it delivers spacey music and you can upload
• 9:10
  Speaker 1: your pictures to it. You might never get them back,
• 9:13
  Speaker 1: but yeah, you can upload them. It probably takes like
• 9:16
  Speaker 1: twenty years just to get there, and then the download
• 9:19
  Speaker 1: speed it's probably even worse. All right, Well, we were curious,
• 9:23
  Speaker 1: as usually how many people out there are familiar with
• 9:25
  Speaker 1: the concept we're talking about today. So Daniel went out
• 9:28
  Speaker 1: there into the internet to ask people what is a
• 9:32
  Speaker 1: light cone? And if you are out there on the
• 9:34
  Speaker 1: internet and always wanted to participate in a podcast, please
• 9:38
  Speaker 1: write to us two questions at Daniel and Jorge dot
• 9:40
  Speaker 1: com and you can be unprepared to answer future questions
• 9:43
  Speaker 1: for the podcast. Right, And if you're not on the Internet,
• 9:46
  Speaker 1: please tell us your secret. I mean I would to
• 9:49
  Speaker 1: sometimes not be on the Internet. I guess we're on
• 9:52
  Speaker 1: the radio too, right, Daniel. I think some stations broadcasts,
• 9:55
  Speaker 1: so they might be listening on the radio. Yeah, So
• 9:58
  Speaker 1: thanks everybody who's out there listening to us on the
• 10:00
  Speaker 1: radio as well. We appreciate all of your support and
• 10:02
  Speaker 1: all of your listening. Did you know that we recently
• 10:04
  Speaker 1: passed fifteen million episode downloads? Wow? Really, it's amazing. That's
• 10:10
  Speaker 1: like fifty million hours that people have been listening to us.
• 10:13
  Speaker 1: That's like fifteen million banana jokes at least at least
• 10:17
  Speaker 1: because you know, we're a multi banana joke death Ray
• 10:20
  Speaker 1: a joke Ray here. We try to in our in
• 10:22
  Speaker 1: our best episodes, like a shotgun rate joke. Really, we
• 10:26
  Speaker 1: just splatter bananappeals all over the place and now I
• 10:29
  Speaker 1: imagining fifteen million tiny bananas coming out of the nozzle
• 10:32
  Speaker 1: of a gun at very high to be coming out
• 10:35
  Speaker 1: of our mouth, that's a little disturbing. But anyways, think
• 10:39
  Speaker 1: about it for a second. If someone asked you what
• 10:41
  Speaker 1: do you think a light cone is? What would you say?
• 10:45
  Speaker 1: Here's what people had to say. I would say, that's
• 10:47
  Speaker 1: what comes out of the shiny end of a flash light. Um.
• 10:50
  Speaker 1: I believe this is a space time diagram where it
• 10:54
  Speaker 1: shows a three dimensional object and as you go up,
• 11:01
  Speaker 1: that's time passing, so it makes a calm like if
• 11:04
  Speaker 1: you look at the diagram of the Big Bang and
• 11:06
  Speaker 1: are we sure a tiny point? Then it slowly goes
• 11:09
  Speaker 1: up into a cone as time progresses. Um. I don't
• 11:13
  Speaker 1: know what a light cone is. I'm thinking about there
• 11:16
  Speaker 1: was some propulsion the idea that was. I think it
• 11:21
  Speaker 1: was largely theoretical, but it was to do with kind
• 11:24
  Speaker 1: of using light to push spacecraft and it would capture
• 11:30
  Speaker 1: the light using light sales. I don't know if that
• 11:32
  Speaker 1: would be to do with it. I remember seeing a
• 11:34
  Speaker 1: model in the Science Museum in London, but there was
• 11:37
  Speaker 1: a cone, but that was probably just because it was
• 11:39
  Speaker 1: made of plastic. Light corne is representation of how from
• 11:47
  Speaker 1: the light travels from a point. For instance, if you
• 11:52
  Speaker 1: have if I have a little flash light in my hand,
• 11:56
  Speaker 1: then I turned it on the lightbulb trouble in space
• 12:02
  Speaker 1: and time. Like cone, you can represent it as a cone.
• 12:09
  Speaker 1: That is something I've heard of before. Um, it's the
• 12:13
  Speaker 1: realm of like what you can see. So it's like
• 12:16
  Speaker 1: ninety three billion light years across as in our observable universe,
• 12:21
  Speaker 1: because that's like how far we could observes or something
• 12:25
  Speaker 1: into space, or like the photons that have reached us
• 12:29
  Speaker 1: from that distance. Although I don't understand how that's bigger
• 12:33
  Speaker 1: than thirteen point eight billion light years since that's the
• 12:37
  Speaker 1: age of the universe. But I'm still quite unclear on that.
• 12:40
  Speaker 1: But I know that the light cone is basically all
• 12:42
  Speaker 1: that we can see within our realm of light, anything
• 12:46
  Speaker 1: that can reach us in any form. I think a
• 12:49
  Speaker 1: light cone is an ice cream cone that's been left
• 12:52
  Speaker 1: out in the sun too long and all the ice
• 12:54
  Speaker 1: cream melted. A light cone is an ice cream cone
• 12:58
  Speaker 1: that has no chocolate ice cream minute whatsoever, which is
• 13:02
  Speaker 1: fundamentally flawed, and I think it violates one of the
• 13:04
  Speaker 1: laws of thermodynamics, because why would you want vanilla ice cream?
• 13:07
  Speaker 1: One chocolate is an option, so light cones make no sense.
• 13:11
  Speaker 1: All right. A lot of people went with my interpretation,
• 13:14
  Speaker 1: which is that it sounds like a dessert possibly and
• 13:18
  Speaker 1: that it's light. Yeah, and we also heard some people
• 13:20
  Speaker 1: throwing shade on vanilla ice cream. Wow, that was rough.
• 13:23
  Speaker 1: I know, how can you be down in vanilla ice cream?
• 13:26
  Speaker 1: It's like the nicest flavor gets along with everybody. That's true.
• 13:30
  Speaker 1: I mean I would always pick chocolate over vanilla. But
• 13:32
  Speaker 1: you know, if all you got is vanilla, I'm happy
• 13:35
  Speaker 1: to get. But if you take vanilla and sprinkle chocolate
• 13:37
  Speaker 1: on it, see that's the versatility of vanilla. Or take
• 13:41
  Speaker 1: chocolate and sprinkled chocolate on it, it's still better. But
• 13:44
  Speaker 1: you can't take chocolate and sprinkle vanilla on it. That's
• 13:46
  Speaker 1: when you do. That's vanilla doesn't add to anything. It's
• 13:49
  Speaker 1: just sort of like the substrate he lean. It makes
• 13:53
  Speaker 1: most cookies taste good. It's just sort of vanilla. That's
• 13:55
  Speaker 1: the reason why they call it vanilla vanilla, you know,
• 13:58
  Speaker 1: because it's so dependent and solid and she just always
• 14:02
  Speaker 1: there for you. Right. If somebody reads you're writing and
• 14:05
  Speaker 1: they describe it as a vanilla. Would you take that
• 14:07
  Speaker 1: as a compliment? Depends am I writing the bag of
• 14:11
  Speaker 1: an ice cream carton? Then it would meet its cool
• 14:14
  Speaker 1: But yeah, so a light cone, Daniel tell me, this
• 14:17
  Speaker 1: is something I was surprised to see in our topic
• 14:19
  Speaker 1: because it is it's like a graphical interpretation of physics, right,
• 14:23
  Speaker 1: And so it's a little tricky to describe on a podcast.
• 14:26
  Speaker 1: It's a little tricky to describe on a podcast. We'll
• 14:28
  Speaker 1: see how we do. But I thought it was useful
• 14:30
  Speaker 1: because I find that every time we talk about special relativity,
• 14:33
  Speaker 1: I have light cones in my mind. I got these
• 14:36
  Speaker 1: things in my brain. They really helped me think about
• 14:39
  Speaker 1: sort of the structure of space and time, and they
• 14:42
  Speaker 1: lead to a lot of interesting consequences. And so I
• 14:44
  Speaker 1: hope if we can somehow download the concept of a
• 14:47
  Speaker 1: light cone into the minds of our listeners, it will
• 14:49
  Speaker 1: help them think about special relativity. We need like a
• 14:52
  Speaker 1: light cone ray to shoot listen to people. Maybe, Yeah,
• 14:55
  Speaker 1: I think it's a podcast starring me and you're talking
• 14:57
  Speaker 1: about throwing out vananative. All right, well, m step us
• 15:01
  Speaker 1: through it. What is technically a light cone? So a
• 15:05
  Speaker 1: light cone defines the part of the universe that you
• 15:08
  Speaker 1: can influence, the part of the future that you can
• 15:12
  Speaker 1: visit geometrically. And you can think about it like if
• 15:16
  Speaker 1: you turn on a flashlight for a moment, so you
• 15:19
  Speaker 1: shine photons in every direction, they leave you in sort
• 15:21
  Speaker 1: of a spear. After one second, they are one light
• 15:24
  Speaker 1: second away from you. After one year, they are one
• 15:28
  Speaker 1: light year away from you. So imagine a sphere and
• 15:30
  Speaker 1: then that sphere sort of growing as time goes on, right,
• 15:34
  Speaker 1: So that set of spears versus time as those spears grow,
• 15:38
  Speaker 1: this is your future light cone. This is the part
• 15:41
  Speaker 1: of the universe that you can influence where your choices,
• 15:45
  Speaker 1: the things that you do as a person could have
• 15:47
  Speaker 1: sort of an effect downstream. It's very easy to understand why.
• 15:52
  Speaker 1: The reason is that everything that's outside of that growing
• 15:55
  Speaker 1: set of spears you can't influence because information would have
• 15:58
  Speaker 1: to travel faster and the speed of light to get there.
• 16:02
  Speaker 1: So it's a direct outcome of the fact that we
• 16:03
  Speaker 1: have a maximum speed limit to the universe. It divides
• 16:06
  Speaker 1: the universe into parts that you can influence and parts
• 16:10
  Speaker 1: you cannot. So the light cone is the part of
• 16:12
  Speaker 1: these universe, including the future. Right, we're talking space and
• 16:15
  Speaker 1: time here, the part of the future universe that you
• 16:18
  Speaker 1: can have any sort of influence over. So I turn
• 16:21
  Speaker 1: on a light like a light bulb, and that light
• 16:24
  Speaker 1: is growing comes out of the light bulb of growth
• 16:27
  Speaker 1: in a sphere, and you're saying, that's the light cone.
• 16:30
  Speaker 1: So it really should be called like a light sphere.
• 16:31
  Speaker 1: Maybe it'd be a better name, like the light sphere
• 16:34
  Speaker 1: is the sphere around you which you can reach with light.
• 16:40
  Speaker 1: The reason it's often called the light cone is that
• 16:42
  Speaker 1: it's hard to draw four dimensions on a sheet of paper.
• 16:45
  Speaker 1: The most we can really draw on a sheet of
• 16:47
  Speaker 1: papers three dimensions, you know, by having like some perspective,
• 16:50
  Speaker 1: and so you need to have time on there. So
• 16:52
  Speaker 1: you can really only have two other dimensions of space
• 16:55
  Speaker 1: on your sheet of paper. And so when you're drawing
• 16:57
  Speaker 1: these diagrams, people typically treats base as if it was
• 17:00
  Speaker 1: a two D surface instead of like our three D world,
• 17:03
  Speaker 1: just to sort of simplify it. So in two dimensions
• 17:06
  Speaker 1: the sphere we were just talking about, it becomes a circle. Right. Imagine,
• 17:10
  Speaker 1: for example, taking a smooth sheet of water and hitting
• 17:13
  Speaker 1: it somewhere. What happens you have a wave that leaves
• 17:16
  Speaker 1: where you hit it and it travels outwards. So now
• 17:19
  Speaker 1: you get a set of circles. Now, if you take
• 17:21
  Speaker 1: those set of circles and you extend them like vertically
• 17:23
  Speaker 1: above the surface in time and make time that vertical direction,
• 17:27
  Speaker 1: then what you get is a cone. Right. It starts
• 17:30
  Speaker 1: from a point when you hit the surface of the water,
• 17:32
  Speaker 1: and then as time goes on, that circle gets larger
• 17:35
  Speaker 1: and larger, and that defines a cone. So it's a
• 17:38
  Speaker 1: cone only in a sort of universe that has two
• 17:41
  Speaker 1: spatial dimensions and one time dimension. Right, It's kind of
• 17:44
  Speaker 1: like if you take a slice of those light spheres
• 17:46
  Speaker 1: we were talking about, Like if you take a slice
• 17:48
  Speaker 1: of it and you see those that circle on that
• 17:51
  Speaker 1: slice growing, and then you imagine that circle growing but
• 17:56
  Speaker 1: growing and also moving up maybe at the same time,
• 18:00
  Speaker 1: it kind of makes like a cone, yeah, exactly, and
• 18:02
  Speaker 1: it moves up because now we're imagining time to be
• 18:05
  Speaker 1: one of our axes. Right, we have two dimensions of
• 18:07
  Speaker 1: space and we have another direction, which is time. And
• 18:10
  Speaker 1: obviously things just flow forward in time at one second
• 18:13
  Speaker 1: per second, and so this circle just sort of grows
• 18:17
  Speaker 1: in time. If you instead had something which wasn't moving. Right,
• 18:20
  Speaker 1: if you just like drew a circle around you, then
• 18:23
  Speaker 1: that circle would slide through time and make a cylinder, right,
• 18:27
  Speaker 1: because it wouldn't be growing. But if it is growing,
• 18:29
  Speaker 1: then it makes a cone. It starts from the point
• 18:31
  Speaker 1: where you've made it, and it makes a cone. And
• 18:34
  Speaker 1: I guess if you're having trail visual license, you can
• 18:36
  Speaker 1: always just pop on Google images or something and look
• 18:38
  Speaker 1: for light cone. I imagine you'll get a picture of this, right.
• 18:41
  Speaker 1: How do people do podcast before Google image search anymore?
• 18:45
  Speaker 1: They probably avoided topics that involved geometry and and multiple dimensions.
• 18:51
  Speaker 1: But it's kind of an interesting concept, and I think
• 18:54
  Speaker 1: you as a physics you like it because it defines
• 18:57
  Speaker 1: kind of like a volume, right. It defines kind of
• 19:00
  Speaker 1: like a set of space time that is either inside
• 19:03
  Speaker 1: the cone or outside the cone. That's kind of the
• 19:05
  Speaker 1: power of this kind of visualization. Yeah, and it's not
• 19:08
  Speaker 1: just arbitrary, like you can make whatever shape you want
• 19:10
  Speaker 1: in spacetime. But this one is special. This one is
• 19:13
  Speaker 1: really important because everything that's inside your future light cone
• 19:17
  Speaker 1: are things that you can influence. Right. You can't send
• 19:21
  Speaker 1: the message to anything outside your future light cone, like
• 19:24
  Speaker 1: anything outside your future. Light cone by definition, is too
• 19:27
  Speaker 1: far away for any message to reach because you would
• 19:30
  Speaker 1: have to travel faster than the speed of light. And
• 19:33
  Speaker 1: if you can send a message, it means you can't
• 19:35
  Speaker 1: send any information. That means you can't influence it at all.
• 19:39
  Speaker 1: And one of the really tricky, complicated things about special
• 19:41
  Speaker 1: relativity is that it plays sometimes with the order of
• 19:44
  Speaker 1: events and it seems to have consequences for causality. So
• 19:47
  Speaker 1: this helps you sort of keep track of that the
• 19:50
  Speaker 1: part of the universe that you can influence, and also
• 19:53
  Speaker 1: looking backwards, the part of the universe that can influence you. Right,
• 19:56
  Speaker 1: all right, Well, it's get into some of these weird
• 19:58
  Speaker 1: consequences for because reality and simultaneity and alien death rays.
• 20:03
  Speaker 1: But first let's take a quick break. Alright. We're talking
• 20:18
  Speaker 1: about the speed of light, causality, the universe, and light cones,
• 20:23
  Speaker 1: which are sort of like a visualization in physics that
• 20:26
  Speaker 1: help you kind of figure out what you can't affect
• 20:29
  Speaker 1: and what you cannot affect because I guess that's something
• 20:33
  Speaker 1: is far enough away you can't affect it right now,
• 20:36
  Speaker 1: right because you have to wait for the speed of
• 20:38
  Speaker 1: light to get from here there or there to hear
• 20:41
  Speaker 1: it doesn't even have to be far away. Something that's
• 20:44
  Speaker 1: one foot away from you, like literally one foot away
• 20:47
  Speaker 1: from you right now. You can have no influence on
• 20:49
  Speaker 1: what's happening right there, right now, because even if you
• 20:53
  Speaker 1: shot your death ray, it would only affect what's happening
• 20:55
  Speaker 1: to foot away from you in the time it takes
• 20:57
  Speaker 1: for light to travel from the tip of your death
• 20:59
  Speaker 1: ray one foot. There's nothing you can do about what's
• 21:03
  Speaker 1: happening one foot away from you right now. Oh wow,
• 21:06
  Speaker 1: that's kind of weird, right to know that there's stuff
• 21:09
  Speaker 1: around me that is unaccessible to me. Yeah, I mean,
• 21:12
  Speaker 1: even if you're seeing something that happens a foot away
• 21:14
  Speaker 1: from you, that means it actually happened in the past,
• 21:16
  Speaker 1: so it's already done anyway. Right, So all you can
• 21:20
  Speaker 1: do is affect the future, right That seems obvious. But
• 21:23
  Speaker 1: you also can't affect the future everywhere, Like if you
• 21:25
  Speaker 1: want to change the future of Andromeda, you know intuitively,
• 21:29
  Speaker 1: you can't change the future of Andromeda tomorrow or the
• 21:31
  Speaker 1: next day, or even the next year. It would take
• 21:34
  Speaker 1: millions of years for any decision you make to affect
• 21:37
  Speaker 1: anything in Andromeda, because nothing you do will get there.
• 21:41
  Speaker 1: No choices you make can propagate that far before a
• 21:44
  Speaker 1: million years because it's a million light years away, right, Yeah,
• 21:47
  Speaker 1: I guess that's the big consequence of having a speed
• 21:50
  Speaker 1: limit in the universe is this idea that you know
• 21:53
  Speaker 1: there are things in the present, but you can't really
• 21:57
  Speaker 1: touch them, more affect them, or even know what they
• 22:00
  Speaker 1: are right now in the same president as you. That's right,
• 22:03
  Speaker 1: there's a limit on the information and how information travels.
• 22:06
  Speaker 1: It's really fascinating. You can only ever see one slice
• 22:09
  Speaker 1: of the universe, and your slice the universe might tell
• 22:12
  Speaker 1: a different story from other slices of the universe. And
• 22:14
  Speaker 1: this is really hard to get into your head because
• 22:16
  Speaker 1: you imagine that what we're doing with physics is trying
• 22:19
  Speaker 1: to build up like an absolute picture of the universe,
• 22:22
  Speaker 1: like what's really out there, what is really going on,
• 22:25
  Speaker 1: what happened in the universe, and what's going to happen,
• 22:28
  Speaker 1: And it's hard to accept that one you can never
• 22:31
  Speaker 1: really know everything about what's going on in the universe
• 22:33
  Speaker 1: because you're limited to the slice of information you're going
• 22:36
  Speaker 1: to get, but also that what you do learn about
• 22:38
  Speaker 1: the universe might not be like the absolute truth. It
• 22:41
  Speaker 1: might just be like what things look like from your perspective, right.
• 22:45
  Speaker 1: I think we talked about this once in the podcast.
• 22:47
  Speaker 1: It's like, imagine like how the human society was before
• 22:50
  Speaker 1: the Internet or before telecommunications, you know, when we had
• 22:54
  Speaker 1: to rely on writing letters and you know, waiting for
• 22:58
  Speaker 1: newspaper to be delivered to your door. It's like, there
• 23:01
  Speaker 1: could be things happening right now in other parts of
• 23:03
  Speaker 1: the Earth, but you know it'll be a long time
• 23:06
  Speaker 1: before you find out about them, or you know that
• 23:08
  Speaker 1: they actually happened at the same time that they happen.
• 23:11
  Speaker 1: That's right, and speeding up our communications has sort of
• 23:14
  Speaker 1: expanded our cone of understanding. Right, It's shrunk the time
• 23:18
  Speaker 1: it gets for information yet here, but not to zero.
• 23:21
  Speaker 1: Even with cell phones and speed of light electronics, you
• 23:23
  Speaker 1: still can't know exactly what's happening across the world right now.
• 23:27
  Speaker 1: There's still a time delay. And that's why it's cool
• 23:29
  Speaker 1: to see in some of these TV shows that they
• 23:31
  Speaker 1: really incorporate that. You know that if you're trying to
• 23:33
  Speaker 1: coordinate your army on Mars from your base on Earth,
• 23:36
  Speaker 1: it takes time for your messages to get there. If
• 23:39
  Speaker 1: there's a battle going on around you know, the atmosphere
• 23:42
  Speaker 1: of Jupiter that by the time you've heard about the beginning,
• 23:45
  Speaker 1: it's probably over already and there's nothing you can do
• 23:47
  Speaker 1: to change the outcome. I guess, like before when you
• 23:49
  Speaker 1: had a battle on you have to like send a
• 23:51
  Speaker 1: messenger or send a scout or something you had, there
• 23:54
  Speaker 1: was this just huge delay, and so now thinks they're faster,
• 23:58
  Speaker 1: but they're still limited, like there's still no simultaneity, Like
• 24:01
  Speaker 1: there's still no ability to know what is going on
• 24:04
  Speaker 1: right now somewhere else. Yeah, exactly. And the same concept
• 24:08
  Speaker 1: is light cone, which tells you, like what you can influence.
• 24:10
  Speaker 1: It also works in the other direction. You have like
• 24:12
  Speaker 1: a backwards light cone. If you think in the other direction,
• 24:16
  Speaker 1: like how close does somebody have to be to me
• 24:18
  Speaker 1: to shoot a death ray and hit me right now?
• 24:21
  Speaker 1: You know, if somebody is in Andromeda and they shoot
• 24:24
  Speaker 1: a death ray at you, you're safe for a few
• 24:26
  Speaker 1: million years, right Nobody in Andromeda can kill you right now.
• 24:30
  Speaker 1: Even if they shot their death ray a hundred thousand
• 24:32
  Speaker 1: years ago and it's been hurtling towards you for a
• 24:34
  Speaker 1: hundred thousand years, you're still safe. It's not going to
• 24:37
  Speaker 1: get here for a while. Because they are not in
• 24:39
  Speaker 1: your backwards light cone yet. But if they shot at
• 24:42
  Speaker 1: a million years ago, it would be hitting us right
• 24:44
  Speaker 1: now potentially. And if they shoot it right now, they
• 24:48
  Speaker 1: were still going to get hit, But it's going to
• 24:49
  Speaker 1: be a million years from now, that's right. If they
• 24:52
  Speaker 1: shoot it right now, it's not in our past light cone,
• 24:54
  Speaker 1: but our past light cone moves up this time access
• 24:57
  Speaker 1: and eventually they will be inside our ask light gone
• 25:00
  Speaker 1: and yes it will get here, right but you're safe
• 25:03
  Speaker 1: right now, right now, there is this past like cone
• 25:06
  Speaker 1: that dictates all the things that could be influencing what's
• 25:08
  Speaker 1: happening at your location, and somebody really far away, even
• 25:12
  Speaker 1: a foot away, can't influence what's happening to you right now.
• 25:16
  Speaker 1: They can only influence your future. Right well, they can't
• 25:19
  Speaker 1: influence you right now. But I mean doesn't mean you're
• 25:21
  Speaker 1: safe from a death ray, because they could have shot
• 25:23
  Speaker 1: it a while ago and you would still die from
• 25:25
  Speaker 1: the death ray arriving now, you're right, And the weirdest thing,
• 25:28
  Speaker 1: the worst thing about death rays is that they travel
• 25:30
  Speaker 1: at the fastest speed possible, so there's literally no way
• 25:33
  Speaker 1: to get any warning. Right the moment you see it
• 25:35
  Speaker 1: is the moment that arrives at the moment it impacts you.
• 25:38
  Speaker 1: But hopefully, you know, a million years ago, you know,
• 25:40
  Speaker 1: we were just cavemen and women, and you know we
• 25:43
  Speaker 1: didn't do anything to offendiate, hopefully, Or maybe it's a
• 25:48
  Speaker 1: natural process. Maybe the gamma rays from a supernova that
• 25:51
  Speaker 1: blew up, you know, a while ago, and those sterilizing
• 25:54
  Speaker 1: death rays are just now arriving here at Earth. Man,
• 25:57
  Speaker 1: this line makes me feel not at all safer. Daniel,
• 26:01
  Speaker 1: See how you can drop comfort from this light cone.
• 26:04
  Speaker 1: It's like a pretty flimsy protection against giant gamma ray bers.
• 26:08
  Speaker 1: It just means that somebody's got to plan ahead. If
• 26:10
  Speaker 1: they really want to fry us now, they're got to
• 26:12
  Speaker 1: either be close by or they have that planned ahead
• 26:16
  Speaker 1: so that athetic exploding start. Are you saying it's not
• 26:20
  Speaker 1: very good at planning so we're saved. Is that it's
• 26:22
  Speaker 1: a procrastinator saves us from procrastinators exactly? But what about
• 26:27
  Speaker 1: type A supernovas? Oh, the kind of plants ahead, there's
• 26:31
  Speaker 1: nothing we can do. Yeah, we're toast, all right. So
• 26:35
  Speaker 1: it sort of protects us, and I think it also
• 26:38
  Speaker 1: kind of affects what we might agree on. Is kind
• 26:42
  Speaker 1: of a consistent history of the universe. Right, Yeah, it's
• 26:45
  Speaker 1: really interesting. The whole universe can be divided into three regions.
• 26:49
  Speaker 1: You have your future like cone, your path like cone,
• 26:52
  Speaker 1: and then there's everything else. Right, and this is this
• 26:55
  Speaker 1: called this the absolute elsewhere. It sounds like an awesome
• 26:58
  Speaker 1: name for a Netflix show or something. So the absolute
• 27:01
  Speaker 1: elsewhere is the spot that you can't affect, you can't influence, right,
• 27:04
  Speaker 1: And that's really interesting because things that happened in the
• 27:07
  Speaker 1: absolute elsewhere, you might disagree with people about the order
• 27:10
  Speaker 1: in which they happen. Like you might look at a
• 27:13
  Speaker 1: series of events and say A happens before B, and
• 27:15
  Speaker 1: somebody else might look at it and say, no, no, no,
• 27:17
  Speaker 1: I think B happens before A. And you could both
• 27:20
  Speaker 1: be right. If these two things are not inside each
• 27:23
  Speaker 1: other's light cones, and these events don't have overlapping light cones,
• 27:27
  Speaker 1: then they can be differently ordered and not affect anything
• 27:30
  Speaker 1: from a causality point of view. WHOA, So I guess
• 27:34
  Speaker 1: I can have a light cone and you can have
• 27:35
  Speaker 1: a light cone. And if those two don't interact, is
• 27:38
  Speaker 1: that what you mean? Like if they don't touch, then
• 27:40
  Speaker 1: we were looking at literally like different universes. Well, they
• 27:43
  Speaker 1: will eventually touch, right, because their light cones will eventually impact,
• 27:47
  Speaker 1: and so there is some overlap in their light cone
• 27:49
  Speaker 1: far in the future where both of them can influence.
• 27:52
  Speaker 1: But it doesn't matter which one happened first. It doesn't
• 27:55
  Speaker 1: matter if A happened in then B, or B happened
• 27:58
  Speaker 1: and then A, because these two events are the events
• 28:01
  Speaker 1: themselves are not inside each other's light cones. Right, B
• 28:05
  Speaker 1: is not in a's light cone, and A is not
• 28:06
  Speaker 1: in b s light cone. Even though the two light
• 28:09
  Speaker 1: cones do overlap, if they are not inside each other's
• 28:12
  Speaker 1: light cones, then you can swap their order just by
• 28:15
  Speaker 1: changing your speed. If you decide to zip by them
• 28:18
  Speaker 1: in a spaceship as close to the speed of light,
• 28:20
  Speaker 1: you will see one happened before the other. If you
• 28:22
  Speaker 1: go in the other direction and nearly the speed of light,
• 28:25
  Speaker 1: you will see the opposite order of events. And it
• 28:28
  Speaker 1: all works out downstream. Because these two things aren't causally linked,
• 28:31
  Speaker 1: they're not inside each other's light cones, so you're free
• 28:34
  Speaker 1: to change their order of events. But then you know
• 28:37
  Speaker 1: the TV who are involved would have a different interpretation, right,
• 28:40
  Speaker 1: like one of them will see one thing happening before
• 28:42
  Speaker 1: the other. Yeah, exactly, But there is no absolute definition
• 28:47
  Speaker 1: of what happened. First, all the observers have their own interpretation,
• 28:50
  Speaker 1: and they're all correct because the answer depends on your position.
• 28:54
  Speaker 1: That's not true for two events where one is inside
• 28:57
  Speaker 1: the other one's light cone. If I shoot a joke
• 29:00
  Speaker 1: ray and I hit you in the head, then you laugh, right,
• 29:03
  Speaker 1: put it on joke mode, please, there's that. It's don
• 29:07
  Speaker 1: and then there's a joke open on banana mode. So
• 29:10
  Speaker 1: I hit the ray and the banana hits you in
• 29:12
  Speaker 1: the head. Right now, you are in my light cone.
• 29:15
  Speaker 1: Because the two events me pressing the button on the
• 29:18
  Speaker 1: gun and you having banana arrived in your mouth, they're
• 29:20
  Speaker 1: not at the same time. What happens before the other,
• 29:23
  Speaker 1: and there's a causal structure for them. I have to
• 29:26
  Speaker 1: press the button before the banana ray arrives at your head.
• 29:30
  Speaker 1: And that's okay, because the banana ray arriving at your
• 29:33
  Speaker 1: head is an event in my future. It's inside my
• 29:35
  Speaker 1: light cone. But for example, you and I both have
• 29:38
  Speaker 1: these guns and we point them in random directions or whatever,
• 29:41
  Speaker 1: and we try to fire them at the same time.
• 29:43
  Speaker 1: Some people might say we fired them at the same time.
• 29:45
  Speaker 1: Other people flying behind the spaceship might disagree about who
• 29:49
  Speaker 1: pressed their trigger first because they would be moving and
• 29:53
  Speaker 1: because of the way that light along the light takes
• 29:56
  Speaker 1: to get to them and the speed limit of the universe,
• 30:00
  Speaker 1: they might reconstruct the events in a different way. Yeah,
• 30:02
  Speaker 1: but it's not just because how long light takes to
• 30:05
  Speaker 1: get to them. Even if you factor that in, right,
• 30:07
  Speaker 1: even if you account for how long it takes the
• 30:09
  Speaker 1: signals and the images to appear, there's still a difference
• 30:13
  Speaker 1: in the order of events. And that's because time flows
• 30:15
  Speaker 1: differently based on where you are and how fast you
• 30:18
  Speaker 1: are going. So your speed affects how you see time flowing,
• 30:22
  Speaker 1: and so you can see one event happening before the other.
• 30:25
  Speaker 1: You can see Jorge presses button first, or somebody else
• 30:27
  Speaker 1: going in the other direction can see Daniel press the
• 30:29
  Speaker 1: button first, even if they account for the transmission delay
• 30:32
  Speaker 1: from light. But they would be wrong. I guess what
• 30:35
  Speaker 1: I mean is they would have their own interpretation, but
• 30:38
  Speaker 1: other people would have their different interpretation. Yeah, but nobody's
• 30:41
  Speaker 1: right or everybody's right. You know, if these two events
• 30:44
  Speaker 1: are not inside each other's light, cones right, their light
• 30:46
  Speaker 1: cones can overlap in the future, but the two events
• 30:48
  Speaker 1: themselves are not inside each other's light cones, then their
• 30:51
  Speaker 1: order is not well defined. And this is all because
• 30:55
  Speaker 1: the universe has a speed limit, right, Like, if it
• 30:57
  Speaker 1: didn't have a speed limit, then you could talk about
• 30:59
  Speaker 1: like absolute history of the universe. That's right exactly, And
• 31:03
  Speaker 1: that speed limit defines the size of this cone. Right. Imagine,
• 31:07
  Speaker 1: for example, if the speed of light was much much slower,
• 31:10
  Speaker 1: then your light cone would be narrower. The part of
• 31:13
  Speaker 1: the universe, the part of the future that you could influence,
• 31:15
  Speaker 1: would be a much smaller part of the universe. If
• 31:17
  Speaker 1: you shot death rays in every direction and the speed
• 31:20
  Speaker 1: of light was much smaller, then you would kill a
• 31:22
  Speaker 1: smaller fraction of the universe. Or if you shot joke raise,
• 31:25
  Speaker 1: you would make a smaller fraction in the universe laugh.
• 31:28
  Speaker 1: Then if the speed of light was ten times faster
• 31:30
  Speaker 1: than it is, then you could influence more of the universe,
• 31:33
  Speaker 1: and aliens out there with their joke raise could make
• 31:36
  Speaker 1: us laugh more effectively. So this number the speed of
• 31:39
  Speaker 1: light really changes the whole structure of causality of space. Right,
• 31:43
  Speaker 1: But what if you make like a light joke like
• 31:45
  Speaker 1: this vanilla is the best. Ha ha ha. That's a joke,
• 31:48
  Speaker 1: like a light vanilla joke. Yeah, that wouldn't harm anyone,
• 31:51
  Speaker 1: and people would just let it go by. But it
• 31:54
  Speaker 1: really is sort of like a cone. I'm thinking, like
• 31:56
  Speaker 1: a doc cone. You know those cones they put on
• 31:58
  Speaker 1: dogs so that data and harm themselves or just to
• 32:01
  Speaker 1: protect them. It really is like a cone that you
• 32:04
  Speaker 1: put on our we put on our heads that kind
• 32:06
  Speaker 1: of limits what we can see and what we can
• 32:08
  Speaker 1: interact with, right it sort of does. Now imagining you
• 32:12
  Speaker 1: with one of these on your head trying to eat
• 32:13
  Speaker 1: a banana, even though you can't, Daniel shoot me with
• 32:17
  Speaker 1: the bananas exactly, then you need me to shoot you
• 32:20
  Speaker 1: with the banana ray or I guess she went up
• 32:22
  Speaker 1: in the air and then catch it and the cone
• 32:25
  Speaker 1: would help me catch this. There you go, there you
• 32:27
  Speaker 1: go pretty soon be surrounded with bananas. I'd have a
• 32:30
  Speaker 1: ConA around my head filmed with bananas and I'll go
• 32:32
  Speaker 1: along with a scoop of vanilla on top, and then
• 32:36
  Speaker 1: for some childhood. But the amazing thing to me is
• 32:38
  Speaker 1: that we still don't know why the speed of light
• 32:40
  Speaker 1: is this number. You know, it has such a dramatic
• 32:43
  Speaker 1: effect on what part of the universe we can see
• 32:45
  Speaker 1: and what part of the universe we can influence, and
• 32:48
  Speaker 1: how the universe is sort of like tied together, you know,
• 32:50
  Speaker 1: what affects what It really changes everything. And it's just
• 32:54
  Speaker 1: like a number. We measure it. We don't know why
• 32:56
  Speaker 1: it is what it is. It could have been better,
• 32:58
  Speaker 1: it could have been smaller, for all we know. Our
• 33:00
  Speaker 1: laws of physics just have like an empty spot there.
• 33:02
  Speaker 1: If you're like right in the number you measure, there's
• 33:04
  Speaker 1: no reason it has to be this or that that
• 33:07
  Speaker 1: we know of. And that's like a real mystery, you
• 33:09
  Speaker 1: know, it it makes you wonder why it's this and not
• 33:11
  Speaker 1: something else. Alright, cool, Well, let's get into now what
• 33:14
  Speaker 1: consequences is this light cone has in our in the
• 33:17
  Speaker 1: universe and possibly in our ability to travel through time.
• 33:22
  Speaker 1: But first let's take another quick break. Are we're talking
• 33:37
  Speaker 1: about light counts and the speed of light and whether
• 33:41
  Speaker 1: or not we can affect things in the universe that
• 33:43
  Speaker 1: are happening right now, which is basically no, right, Like,
• 33:47
  Speaker 1: we can't affect anything else in the universe happening at
• 33:50
  Speaker 1: this moment, that's right, nothing except for what's happening right here,
• 33:54
  Speaker 1: right now, or stuff that's happening in the future. But
• 33:57
  Speaker 1: at this very moment, you can't affect anything except for
• 34:00
  Speaker 1: exactly the spot where you are. Well, technically nothing right
• 34:03
  Speaker 1: because even things that are one millimeter away from my
• 34:06
  Speaker 1: eye in the present moment, I can't even see or effect.
• 34:09
  Speaker 1: That's true. We have to go down to zero millimeters.
• 34:12
  Speaker 1: And that's only even only if you believe in free
• 34:14
  Speaker 1: will and if your brain operates instantaneously, So technically you're right.
• 34:17
  Speaker 1: Any decision you make right now can only influence the future,
• 34:21
  Speaker 1: except that maybe there's a possibility to use this concept
• 34:24
  Speaker 1: to do time travel. Daniel so explained that to me. Yes,
• 34:27
  Speaker 1: so what we talked about so far, this idea of
• 34:29
  Speaker 1: light cones these circles of light expanding out into the
• 34:32
  Speaker 1: universe and into your future. This is the way we
• 34:34
  Speaker 1: think about space. If it's not curved, so we call
• 34:37
  Speaker 1: space flat, if it's just sort of like has no
• 34:40
  Speaker 1: shape to it, if there's no bending of it that
• 34:43
  Speaker 1: affects like the path of photons. And we know that
• 34:46
  Speaker 1: space can be bent. We know that, for example, everything
• 34:50
  Speaker 1: with mass and energy bend space, like the Sun bend space.
• 34:54
  Speaker 1: That's why the Earth is moving around it instead of
• 34:56
  Speaker 1: just flying off into deep space. We know the Eube
• 34:59
  Speaker 1: and in the space because you have mass, and so
• 35:02
  Speaker 1: everything out there changes the shape of space, which means
• 35:05
  Speaker 1: it changes how light goes through it. Remember that gravity
• 35:09
  Speaker 1: is not just like a force that tugs on things,
• 35:12
  Speaker 1: because light, for example, has no mass, but gravity can
• 35:16
  Speaker 1: affect its path because gravity changes the very shape of
• 35:20
  Speaker 1: space and light travels through that space. And so we
• 35:23
  Speaker 1: know that things are more complicated when space gets curved.
• 35:26
  Speaker 1: So what we talked about so far is only in
• 35:28
  Speaker 1: the event that the universe is empty and there's no curvature,
• 35:31
  Speaker 1: there's no mass or anything to change the shape. So
• 35:33
  Speaker 1: in flat space, when there's a lot of mass around,
• 35:35
  Speaker 1: things get curved and things get pretty different and weird.
• 35:38
  Speaker 1: These light cones, which you can imagine sort of like
• 35:41
  Speaker 1: going up and going down now, start to get tilted. Right.
• 35:45
  Speaker 1: Is this something we can actually see? Right? You can
• 35:47
  Speaker 1: see light getting bent out there in space by heavy objects. Yeah,
• 35:52
  Speaker 1: it's called gravitational lensing. If there's like a really big
• 35:55
  Speaker 1: heavy object between us and some source of light, like
• 35:58
  Speaker 1: a background galaxy, then as those photons come towards Earth,
• 36:02
  Speaker 1: they could bent around that massive object. And this is
• 36:05
  Speaker 1: one way we can detect, for example, the existence of
• 36:07
  Speaker 1: dark matter and try to figure out, like roughly where
• 36:09
  Speaker 1: it is based on how it distorts the light that's
• 36:12
  Speaker 1: passing near it or through it. So it's definitely a
• 36:15
  Speaker 1: real thing that's happening. It's something we've observed. We're sure
• 36:18
  Speaker 1: it is true. So if a space can bend light,
• 36:21
  Speaker 1: or land space can sort of affect the trajectory of light,
• 36:24
  Speaker 1: that would make sense that it would bend these light
• 36:27
  Speaker 1: cones that we have to kind of figure out what
• 36:30
  Speaker 1: we can see and cannot see in the universe exactly.
• 36:32
  Speaker 1: Because the shape of the light cone is defined by
• 36:34
  Speaker 1: where light goes right where light can reach, and so
• 36:38
  Speaker 1: space is bent in a way that light can't get somewhere,
• 36:41
  Speaker 1: then the light cones are also bent. So this idea
• 36:44
  Speaker 1: of them being like simple cones is only in flat
• 36:46
  Speaker 1: space near something really massive, they bend towards the mass, right,
• 36:51
  Speaker 1: because light moves towards mass. Right. If you have, for example,
• 36:55
  Speaker 1: the Sun, and you shoot a photon near the Sun,
• 36:58
  Speaker 1: it will get curved sort of in the dire action
• 37:00
  Speaker 1: of the Sun. And if you have a black hole,
• 37:03
  Speaker 1: then it's going to get curved room towards the black hole.
• 37:06
  Speaker 1: So as you get closer and closer something massive, these
• 37:08
  Speaker 1: light cones tilt more and more. Well, they start to
• 37:12
  Speaker 1: look like maybe horns and not so much cones or yeah,
• 37:14
  Speaker 1: or like bow ties or something yeah, or like a bugle,
• 37:17
  Speaker 1: remember that snack, the little like cones, curved cones. Yeah, exactly, Yeah,
• 37:23
  Speaker 1: light bugles. Will call it light bugles exactly. And so
• 37:27
  Speaker 1: inside a black hole, of course, you can think about
• 37:29
  Speaker 1: what happens to a photon. While a photon inside a
• 37:31
  Speaker 1: black hole always goes towards the center, not because gravity
• 37:35
  Speaker 1: is so strong, but because the shape of space requires it,
• 37:37
  Speaker 1: because there's only one direction inside a black hole. So
• 37:41
  Speaker 1: inside a black hole, these things have tilted totally sideways.
• 37:45
  Speaker 1: You shoot a photon and it goes just towards the singularity.
• 37:48
  Speaker 1: All the light cones are now tilted towards the center.
• 37:51
  Speaker 1: You hear people say sometimes this thing that inside a
• 37:53
  Speaker 1: black hole, space and time switch roles. Right, This is
• 37:57
  Speaker 1: like a geometric way to understand that, because now instead
• 38:01
  Speaker 1: of time flowing up, it's sort of flowing sideways. Right,
• 38:05
  Speaker 1: Because the light cone has tilted over by forty five degrees. Well,
• 38:10
  Speaker 1: I guess I don't understand it at all. So the
• 38:15
  Speaker 1: light cone tilts, and somehow time and space which positions?
• 38:19
  Speaker 1: Is that what you just said? Yeah, because your future,
• 38:21
  Speaker 1: all possible futures are the singularity, and so the only
• 38:25
  Speaker 1: way to move forward in time is to move in
• 38:28
  Speaker 1: this one direction of space. And so sort of like
• 38:31
  Speaker 1: moving through space towards the singularity is the same thing
• 38:35
  Speaker 1: as moving forwards in time. And you can graph that
• 38:39
  Speaker 1: geometrically by thinking about the light zone being tilted over
• 38:42
  Speaker 1: from being vertical to being horizontal. It's like there's only
• 38:45
  Speaker 1: space and time become one way and everything goes in
• 38:49
  Speaker 1: the same direction. Yeah, exactly, all right, And so then
• 38:51
  Speaker 1: how does that relate to time travel? You said it
• 38:53
  Speaker 1: might be possible to do time travel. It might be
• 38:56
  Speaker 1: possible to do time travel if you use these cones
• 38:59
  Speaker 1: you and construct this really weird structure. Right, So we
• 39:03
  Speaker 1: talked about how near something really heavy, these cones can
• 39:07
  Speaker 1: tilt like up to forty five degrees right towards the
• 39:10
  Speaker 1: center of a black hole. Now, imagine if you could
• 39:12
  Speaker 1: have a cone that tilted more than forty five degrees,
• 39:16
  Speaker 1: And what that means is that you're tilting so that
• 39:18
  Speaker 1: part of the light cone is now in the past, right,
• 39:22
  Speaker 1: So it's possible if you could do this, it would
• 39:25
  Speaker 1: mean that these particles are moving not just into the future,
• 39:28
  Speaker 1: but like the future is now dipping into the past.
• 39:32
  Speaker 1: What you mean you you bend the light cone so
• 39:35
  Speaker 1: much it wraps around and touches kind of the past
• 39:38
  Speaker 1: light cone, m exactly, and you think, is this possible?
• 39:42
  Speaker 1: Doesn't that contradict itself? Will remember that, like the definition
• 39:45
  Speaker 1: of time and the definition of space depends a little
• 39:49
  Speaker 1: bit on how fast you're going and the environment you're in,
• 39:52
  Speaker 1: and that gets even more complicated when space is bent.
• 39:56
  Speaker 1: And so what you can have is like a particle
• 39:58
  Speaker 1: moving forward into it's future like it's local time still
• 40:02
  Speaker 1: says I'm moving forward in the future one second, then
• 40:05
  Speaker 1: the next second, and the next second. But it could
• 40:07
  Speaker 1: be moving into the past from the point of view
• 40:09
  Speaker 1: of another observer. So like from a distant observer who's
• 40:12
  Speaker 1: watching this from far away, whose space isn't curved, they
• 40:16
  Speaker 1: could see that particle moving into like their past, even
• 40:18
  Speaker 1: though if it's still moving into its own future. Whoa wait,
• 40:22
  Speaker 1: are you saying that space and time actually curve around itself,
• 40:26
  Speaker 1: or it only looks curved around to a different observer,
• 40:29
  Speaker 1: the structure of it is actually occurred, and people think
• 40:33
  Speaker 1: in some weird configurations, if there's a very special arrangement
• 40:36
  Speaker 1: of mass, then you might be able to construct a
• 40:40
  Speaker 1: structure of space that allows particles to have their local future,
• 40:45
  Speaker 1: their personal timeline extend into like the past from the
• 40:49
  Speaker 1: point of view of other observers. From the point of
• 40:51
  Speaker 1: view of other observers, but the particle itsels wouldn't be
• 40:54
  Speaker 1: traveling into the past, or like me, I wouldn't be
• 40:57
  Speaker 1: traveling into the past. Well, it's like into their own future.
• 41:01
  Speaker 1: But that's sort of like what happens on a time
• 41:02
  Speaker 1: travel show. Right, If you are on a time travel
• 41:05
  Speaker 1: show and you want to go into the past, you're
• 41:07
  Speaker 1: still living your life forward on your timeline. Right, you
• 41:10
  Speaker 1: were forty six years old and seven days and two minutes,
• 41:13
  Speaker 1: And then when you go into the past, you're not
• 41:15
  Speaker 1: like younger again, right, You're just like keep going forward.
• 41:18
  Speaker 1: Now you're forty seven years, two days and five minutes
• 41:21
  Speaker 1: and seven minutes and ten minutes. So what you want
• 41:23
  Speaker 1: is your own timeline to continue, but you want the
• 41:26
  Speaker 1: structure of space around you to change that your timeline,
• 41:29
  Speaker 1: your future now goes into like the rest of the
• 41:32
  Speaker 1: universe's past. So this is called in physics, it's called
• 41:35
  Speaker 1: a closed time like curve because if you can tilt
• 41:38
  Speaker 1: a bunch of these cones just the right way, then
• 41:41
  Speaker 1: you can make this path that loops on around itself
• 41:44
  Speaker 1: and comes back to the same point where it originated.
• 41:47
  Speaker 1: So it's like shooting a light beam and having it
• 41:50
  Speaker 1: hit you in the back of the head, almost like
• 41:51
  Speaker 1: a wormhole kind of in time, almost like a wormhole. Yeah,
• 41:55
  Speaker 1: it's not actually a wormhole, because a wormhole is like
• 41:57
  Speaker 1: a shortcut between two distant locations. This is like a
• 42:00
  Speaker 1: structure in space. It's like a loop in space time. Right.
• 42:04
  Speaker 1: It is a complicated reconnection and rearrangement of how space works.
• 42:07
  Speaker 1: But it's technically a little bit different from a wormhole. Right,
• 42:10
  Speaker 1: wouldn't you be stuck in this loop like you'd be
• 42:13
  Speaker 1: be like groundhogs day? For this particle, it's just reliving,
• 42:16
  Speaker 1: going around and around and around. It thinks it's going
• 42:18
  Speaker 1: in the future, but really it's like revisiting the same
• 42:21
  Speaker 1: time over and over and over. Exactly. It's a structure
• 42:23
  Speaker 1: that reinforces itself, and it's only stable if it does
• 42:26
  Speaker 1: revisit itself. And so you're right. You wouldn't like get
• 42:29
  Speaker 1: to go back and choose a different flavor of ice
• 42:31
  Speaker 1: cream or set your death ray to banana instead of
• 42:34
  Speaker 1: death right. You have to make the same choices over
• 42:37
  Speaker 1: and over again. It's like this self supporting structure in
• 42:39
  Speaker 1: space and time so not recommended. Well, unless you want
• 42:43
  Speaker 1: to keep watching the same shows maybe, or if you
• 42:45
  Speaker 1: really like vanilla and you want to eat only vanilla
• 42:49
  Speaker 1: flavor for the rest of your lives for eternity. Yeah, exactly.
• 42:52
  Speaker 1: And this is something which is theoretically conceivable, like people
• 42:56
  Speaker 1: have worked this out on paper and it seems like
• 42:58
  Speaker 1: it might work. But again, general relativity doesn't tell us
• 43:02
  Speaker 1: like how to bend space to make this happen. People like,
• 43:06
  Speaker 1: if you could bend space in this particular way, then
• 43:09
  Speaker 1: we think this could happen. That doesn't mean necessarily we
• 43:12
  Speaker 1: know how to make space bend in that particular way.
• 43:15
  Speaker 1: You know, people have ideas, they have this crazy structure
• 43:19
  Speaker 1: that's like an infinite spinning cylinder of dust, and they
• 43:23
  Speaker 1: do the calculations and they think maybe that would allow
• 43:26
  Speaker 1: for these closed timelike curves to happen, but other people
• 43:29
  Speaker 1: think it's impossible, there's no way it could work, and
• 43:31
  Speaker 1: that we have to sort of throw up our hands
• 43:33
  Speaker 1: and say, let's figure it out when we get the
• 43:35
  Speaker 1: unified theory of everything. Well, you're saying that it's according
• 43:39
  Speaker 1: to the equations, it's possible, or like you know, anything
• 43:42
  Speaker 1: that can bend, if you bend it enough, this will happen,
• 43:45
  Speaker 1: But we don't know if it's physically possible to do it.
• 43:48
  Speaker 1: We don't know if it's physically possible. If you believe
• 43:50
  Speaker 1: general relativity is absolutely true and you can construct an
• 43:54
  Speaker 1: infinite spinning cylinder of dust, then you should believe this
• 43:57
  Speaker 1: is physically possible and can actually happen. Personally, I don't
• 44:01
  Speaker 1: think general relativity is a true theory of nature because
• 44:04
  Speaker 1: it ignores quantum mechanics, and so it can't be correct,
• 44:07
  Speaker 1: and we need some updated version. And it might be
• 44:09
  Speaker 1: that in that updated version and incorporates quantum mechanics, this
• 44:12
  Speaker 1: doesn't work anymore. So this could be like sort of
• 44:15
  Speaker 1: like a singularity of a black hole, like a sign
• 44:17
  Speaker 1: that the theory is wrong because it predicts this sort
• 44:19
  Speaker 1: of nonsense result. Right, Because I guess if it's a
• 44:22
  Speaker 1: closed loop. How would you even get into that loop? Right? Yeah,
• 44:25
  Speaker 1: you have to have somebody else build that infinite spinning
• 44:27
  Speaker 1: cylinder for you in the infinite past, because you'll always
• 44:31
  Speaker 1: be in that loop. So it might exist and there
• 44:33
  Speaker 1: might be a particle stuck in it or from the
• 44:36
  Speaker 1: beginning of time, but you wouldn't be able to get
• 44:38
  Speaker 1: in or out of it. That's right. If it exists,
• 44:40
  Speaker 1: it would be stuck forever. But you know, now we're
• 44:43
  Speaker 1: talking about like construction of a time loop, so we're
• 44:46
  Speaker 1: talking about the variation of the loop as a function
• 44:49
  Speaker 1: of time. So it's like an extra level of meta
• 44:52
  Speaker 1: time there, which gets tricky. You can just use the
• 44:54
  Speaker 1: same vanilla equations, you get to go extra spicy or
• 44:58
  Speaker 1: sprinkle some bugles on for some extra crunching iss. All right,
• 45:03
  Speaker 1: So that's the light on one, I guess, and I
• 45:04
  Speaker 1: guess you're saying it's one way that that it helps
• 45:07
  Speaker 1: you think about all these weird situations of space and
• 45:10
  Speaker 1: black holes and bending time and spacetime looping around itself.
• 45:15
  Speaker 1: You use that in in your everyday you know, theoretical
• 45:18
  Speaker 1: musings I do, and also when people write to me
• 45:21
  Speaker 1: and they ask me some like crazy question about special
• 45:24
  Speaker 1: relativity involving you know, spaceships and snowballs whatever. I usually
• 45:28
  Speaker 1: end up drawing light cones so I can understand, like, well,
• 45:31
  Speaker 1: this happened there, What will this person see or is
• 45:33
  Speaker 1: it possible for them to know this. It's just a
• 45:36
  Speaker 1: nice way to sort of like separate things because in
• 45:38
  Speaker 1: physics problems because much simpler when you can separate and
• 45:41
  Speaker 1: say these things are independent from those things, so I
• 45:43
  Speaker 1: can just deal with it on its own. They're not
• 45:45
  Speaker 1: like entangled. So a light cone shows you how to
• 45:48
  Speaker 1: like disentangle all the causal threads of the universe into
• 45:51
  Speaker 1: a bunch of different packets. Right, Yeah, everything is better
• 45:54
  Speaker 1: with the snow cone. That's our next episode. What is
• 45:58
  Speaker 1: a physics snow cone? It's a commort physic You say
• 46:03
  Speaker 1: no all the time, it's no, can I travel back
• 46:07
  Speaker 1: in times? No? Cyes c Yes? All right, Well, hopefully
• 46:12
  Speaker 1: it helps all of you out there and think about
• 46:15
  Speaker 1: cantality and what we can and cannot affect about the
• 46:19
  Speaker 1: present and maybe makes you appreciate a little bit more
• 46:22
  Speaker 1: where you are and what we're able to see of
• 46:25
  Speaker 1: this big, crazy universe. Well, thanks for joining us, see
• 46:29
  Speaker 1: you next time. Thanks for listening, and remember that Daniel
• 46:39
  Speaker 1: and Jorge explained. The Universe is a production of I
• 46:42
  Speaker 1: heart Radio. Or more podcast from my heart Radio, visit
• 46:46
  Speaker 1: the i heart Radio app, Apple Podcasts, or wherever you
• 46:49
  Speaker 1: listen to your favorite shows.