Speaker 1: Some of the deepest mysteries of the universe are so big,
Speaker 1: so imposing, so important that even asking questions about what
Speaker 1: they are and how they work it can leave us confused.
Speaker 1: That's because we have to spend some time thinking about
Speaker 1: what exactly we're asking and what kind of answer we want.
Speaker 1: But we shouldn't shy away from these kinds of questions
Speaker 1: because they are the most important questions. Understanding the nature
Speaker 1: of our reality means understanding the context of our lives,
Speaker 1: where we live, how we live, maybe even why we live.
Speaker 1: And what the universe has taught us is that it
Speaker 1: always requires persistent, careful effort to unpack the deepest mysteries.
Speaker 1: We have been chiseling away at the rock face of
Speaker 1: physics for thousands of years, mostly cluelessly, but occasionally a
Speaker 1: great hunk of understanding will open up and will see
Speaker 1: everything in a new light. So it's worth asking the
Speaker 1: biggest questions, the ones where the question itself can be confusing,
Speaker 1: because looking for answers might eventually help us figure it
Speaker 1: all out, or help us realize we're asking the wrong question.
Speaker 1: So today on the podcast, we're going to dig deep
Speaker 1: into what we know about maybe the most basic question
Speaker 1: about the universe. We'll be asking what is space.
Speaker 2: Welcome to Daniel and Kelly's Extraordinary Universe. Today we are
Speaker 2: talking about what is space? And I am Kelly Wienersmith,
Speaker 2: and I take up space. How about you?
Speaker 1: I'm Daniel Whitson. I'm a professor of physics, which means
Speaker 1: I probably should understand something about space.
Speaker 2: I'm going to start with a confession. So, like when
Speaker 2: I was in high school and college, I did really
Speaker 2: well in physics. I was in the honors classes, I
Speaker 2: got a's. I really liked the stuff about circuitry. But
Speaker 2: whenever we would talk about what is space or what
Speaker 2: is time? I would feel very frustrated because like you
Speaker 2: were taking this thing that I felt like I could
Speaker 2: work with in my day to day life, and it
Speaker 2: felt like it was being made unnecessarily complicated. And I
Speaker 2: think I also felt a little insecure because it's like
Speaker 2: I can't even understand what is space, and I think
Speaker 2: it just made me frustrated and I would kind of
Speaker 2: shut down. And as an adult, I feel different about it.
Speaker 2: I think it's interesting. I think thinking about it creates
Speaker 2: like testable predictions that teach us about the universe and
Speaker 2: then we can do practical things with that knowledge. But like,
Speaker 2: what was your journey along questions like this? And am
Speaker 2: I the only physics student in classes who isn't like, oh,
Speaker 2: this is fun and is like, no, I knew this,
Speaker 2: Why are you doing this to me?
Speaker 1: I think that's a whole spectrum of people in physics,
Speaker 1: and I think it's a big tent, and I'm glad
Speaker 1: there are people who are like, hey, look, this gives
Speaker 1: us tools so that we can calculate how our cannonballs
Speaker 1: fly over castle walls. That's really all we care about,
Speaker 1: So let's just do that and move on. And then
Speaker 1: there's the folks on the sort of philosophical side of
Speaker 1: things that really want to understand why balls fly over
Speaker 1: castle walls because they have deeper questions about like why
Speaker 1: is there anything and how does it all work? And
Speaker 1: the amazing thing about physics is that it not only
Speaker 1: lets you do practical stuff bill transistors and iPhones, but
Speaker 1: then you get to turn around and ask like, well,
Speaker 1: why does this work at all? And what does that
Speaker 1: tell us about the actual universe we live in? So yeah,
Speaker 1: touches on like technological, fascinating, useful stuff all the way
Speaker 1: to philosophical stuff, and yeah, I always found myself sort
Speaker 1: of on the philosophical side of things, and I was
Speaker 1: the kind of kid who was like, whoa, what is
Speaker 1: space anyway? Man? And this is before I smoked any
Speaker 1: banana peels. I was wondering about what would it be
Speaker 1: like to live in space that was four dimensions or
Speaker 1: two dimensions? And why is it that we can only
Speaker 1: think in three dimensions? So for me, these questions were
Speaker 1: cat in it, even though I didn't understand anything about them.
Speaker 1: And it's only now as a professional physicist that I
Speaker 1: understand why we don't understand anything about them.
Speaker 2: I hope that we can get to that in the
Speaker 2: show today so that the people like me who are
Speaker 2: just like, well now you've just made me confused, have
Speaker 2: a better sense of why we're asking these questions and
Speaker 2: why they're fundamentally important and not just physicists making things complicated.
Speaker 2: Like every once in a while, I'll be like, well,
Speaker 2: we haven't married relativity and quantum mechanics, and so what
Speaker 2: if when people say like space bends and time is blow, Like,
Speaker 2: what if none of that is true because we haven't
Speaker 2: married these things, And so then why do I have
Speaker 2: to do this, and then I get super frustrated, and
Speaker 2: I don't know, maybe I shouldn't be so negative. At
Speaker 2: the beginning of our show. I'm not actually negative, like
Speaker 2: I'm excited about it, but I'm remembering Kelly from the
Speaker 2: past feeling very frustrated about this stuff.
Speaker 1: Well, I think we're going to be aiming today's episode
Speaker 1: at Kelly of the past, somebody who is curious about
Speaker 1: how the world works but doesn't want like a wall
Speaker 1: of confusing language where words suddenly mean things that they
Speaker 1: didn't mean before. And let's remember that that's the whole
Speaker 1: project of physical We take the world that we kind
Speaker 1: of understand we've been living in any way, and we
Speaker 1: try to systematize it. We try to say, well, you know,
Speaker 1: at what angle will your cannon ball fly over the
Speaker 1: council walls? And do you have to factor in the
Speaker 1: wind resistance? And then we get to turn around and
Speaker 1: be like, well, why does this work? What does that
Speaker 1: tell us it? Lets us do something which I think
Speaker 1: is really awesome. We just peel back that layer of
Speaker 1: intuitive reality and say, hey, we thought the universe was
Speaker 1: like this, Actually it turns out it works like that.
Speaker 1: The universe is different from the way we thought it was,
Speaker 1: and that's wonderful. That's the incredible experience we're going for
Speaker 1: in physics. We want to pull the veil from our
Speaker 1: eyes and figure out how the universe actually is. I
Speaker 1: think what we'll discover today is that, wow, we really
Speaker 1: just don't know that the universe works. But it's important
Speaker 1: that everybody understand what we do know and what we
Speaker 1: don't know about these really basic questions about the nature.
Speaker 2: Of the universe, right And if you stop asking the questions,
Speaker 2: then you never get to the answer. So it's important
Speaker 2: to keep.
Speaker 1: Going exactly and that thing. It might be helpful for
Speaker 1: people when we're talking about something as fuzzy and difficult
Speaker 1: to grapple with a space to think about like what
Speaker 1: kind of question we're asking and what kind of answers
Speaker 1: are satisfactory? You know, because when we ask a vague
Speaker 1: question like what is space? What are we really asking
Speaker 1: when we say that? Even so often in philosophy, I
Speaker 1: replace a confusing word with something familiar, like replace the
Speaker 1: confusing word with an elephant, and think, what does the
Speaker 1: question mean if I'm talking about an elephant? You know,
Speaker 1: So if somebody asks you, hey, what is an elephant.
Speaker 1: You know, what kind of question is that? What kind
Speaker 1: of answers do we expect to that kind of question?
Speaker 2: When I was reading through our outline today, one of
Speaker 2: the things that got me excited was thinking back to
Speaker 2: grad school and when we would talk about behavior and
Speaker 2: we'd be like, well, what is the behavior? And there's
Speaker 2: like at least four different ways to think about it,
Speaker 2: you know, like how did it evolve? What neurotransmitters make
Speaker 2: this behavior happen? What things that happened beforehands initiate the behavior?
Speaker 2: And so in the case of behavior, there's a lot
Speaker 2: of different answers you could have for that question. Is
Speaker 2: it the same for space? Are we looking for like
Speaker 2: an equation? So yeah, like what would make a satisfying
Speaker 2: answer for something like this? Or are there lots?
Speaker 1: No, You're right, what we're looking for our answers to
Speaker 1: questions like well what can space do?
Speaker 3: Right?
Speaker 1: Like what is an elephant? Well, an elephant has big ears,
Speaker 1: it does this thing? Right, you can describe it, You
Speaker 1: can describe what it does. You can also ask like, well,
Speaker 1: why is there space? You know the same way you
Speaker 1: can be like why do we have elephants? You know,
Speaker 1: there's a story there that tells us something about elephants
Speaker 1: the relationship to other living things. Right, we can also ask, well,
Speaker 1: what is space made out of? Is it itself the
Speaker 1: fundamental thing in the universe where like you just got
Speaker 1: to have it, you don't have universe without it? Or
Speaker 1: somehow like elephants and ice cream, does it emerge from
Speaker 1: the workings of other little bits deeper inside it that
Speaker 1: somehow weave themselves together to make this experience we call space.
Speaker 1: That's what I was referring to earlier, where in that scenario,
Speaker 1: you're like pulling back a layer of reality understanding what's
Speaker 1: going on underneath. So you discover that your experience is
Speaker 1: not like fundamental, it's just sort of like one thing
Speaker 1: that the universe can do. It can make elephants, it
Speaker 1: can make ice cream, it can also not. So those
Speaker 1: are the kind of questions I want to know the
Speaker 1: answers to about space, like what can it do? Why
Speaker 1: do we have it? Is it itself made us something smaller?
Speaker 1: Or is it fundamental? Is it a requirement for the universe.
Speaker 2: So one of the things that I was worried about
Speaker 2: when we started talking about this question was the word
Speaker 2: space has been used by so many other fields that
Speaker 2: now it's not confusing from a physics perspective, it's just
Speaker 2: confusing because humans use that word for lots of different things.
Speaker 2: But I think I was definitely proved wrong there because
Speaker 2: when you asked the audience to tell us what they
Speaker 2: thought space was, nobody was like, well, space is the
Speaker 2: literature on space, and like everyone got it. I guess
Speaker 2: they know us well enough to know what we were
Speaker 2: probably talking about. So let's hear what the listeners had
Speaker 2: to say.
Speaker 1: That's right, And if you would like to participate in
Speaker 1: the audience participation segments of this podcast, please write to
Speaker 1: us to questions at Danielankelly dot org. We will set
Speaker 1: you up and you can hear your voice speculating basistly
Speaker 1: on the podcast. Here's a bunch of people answering the
Speaker 1: question what is space.
Speaker 4: I don't think that space is a real thing.
Speaker 1: I think that space is just an abstraction.
Speaker 5: Perhaps space is a medium in which fields can exist,
Speaker 5: that has shape and perhaps has density and can change
Speaker 5: in its form. Space is like an invisible lattice, geometric framework.
Speaker 1: Or other diffuse concept that is just a name we
Speaker 1: give our experience.
Speaker 2: The composite of all energy fields, gravitational fields, and dimensions.
Speaker 4: The medium that we are traveling through. It's like combining
Speaker 4: the trajectory of the Earth of some galaxy.
Speaker 1: Universe. Space is that which matter can move within.
Speaker 3: Space is the background fabric of the entire universe.
Speaker 4: I think to think of space as a huge collection
Speaker 4: of spots that can have multiple states of excitement, and
Speaker 4: the excitement of a spot interferes with its neighbors.
Speaker 1: The replacement or nothing.
Speaker 4: A physical meta that we can experience and move around
Speaker 4: and in which events occur that we can observe and
Speaker 4: try to understand.
Speaker 3: Space is this kind of thing in which everything happens,
Speaker 3: but we can't see or feel it. We can only
Speaker 3: see the evidence of it.
Speaker 1: I think this is outdated now, but I still just
Speaker 1: think of spice as a volume that we can put
Speaker 1: stuff into.
Speaker 4: I think space used to be seen as like a
Speaker 4: substrate where everything happened on or within, But I believe
Speaker 4: now space is kind of understood to be the thing
Speaker 4: that is happening.
Speaker 2: So none of the people who answered the question were
Speaker 2: like Daniel, I hate you, or like Daniel go away,
Speaker 2: So it seems like everybody was, you know, enjoying thinking
Speaker 2: it through and giving you an answer, and we got
Speaker 2: some pretty good answers. So maybe the world is not
Speaker 2: filled with angry young versions of Kelly, which is great,
Speaker 2: would be better. I was also like a goth chick
Speaker 2: covered in black all the time. I've cheered up a
Speaker 2: bit too.
Speaker 1: I want to see when those pictures at some point.
Speaker 2: All right, I'll share one with you, but not with
Speaker 2: everyone else. So, yeah, what did you think of these answers?
Speaker 4: Yeah?
Speaker 1: I thought it was good. And I also love hearing
Speaker 1: people grapple with a hard question.
Speaker 3: You know.
Speaker 1: It's the kind of thing, as you say, everybody's got
Speaker 1: some intuition about what space is because we live in it, right,
Speaker 1: and yet it's difficult to say, like exactly what is
Speaker 1: it and where it comes from a lot of people
Speaker 1: described what it can do, right, or it can hold,
Speaker 1: you can have things in space. So I think those
Speaker 1: are all fine ways to approach this problem. But I
Speaker 1: hope by the end of the podcast we give people
Speaker 1: a really comprehensive view of like what physics says about
Speaker 1: space and all the different, confusing, contradictory things the physics
Speaker 1: says about what space might be.
Speaker 2: All right, So then what do you think is the
Speaker 2: best place to start. So to me, I'm just like
Speaker 2: I don't know space. It's like there's like stuff in
Speaker 2: front of me. But if my table is there, it
Speaker 2: still counts as space because my table's just like in
Speaker 2: the space. And to be honest, that I think is
Speaker 2: most everything I've thought about this question. So where do
Speaker 2: we go from here?
Speaker 1: Maybe the best place to start is to try to answer,
Speaker 1: like what is we're asking about? You know, when we
Speaker 1: say space, what do we even mean by it before
Speaker 1: we talk about like where it comes from and how
Speaker 1: it works and what the rules are and what physics
Speaker 1: has learned, Like what is the thing we're asking about?
Speaker 1: You know, let's at least pinpoint the elephant here. And
Speaker 1: you know, in my mind, space is not about the
Speaker 1: stuff in the universe, It's about what's underneath it. It's
Speaker 1: about the underlying fabric. So like, take a chunk of
Speaker 1: universe wherever it is, and remove everything you can remove,
Speaker 1: so it maybe there's a peanut in the universe, toss
Speaker 1: that out, Maybe there's a planet there, toss that out,
Speaker 1: Maybe there's a galaxy whatever, pushed out all to the side.
Speaker 1: Empty it as much as possible, right, because the thing
Speaker 1: we're not talking about is like particles and matter and
Speaker 1: photons and stuff. Let's talk about what's underneath it. That's
Speaker 1: really what's exciting to me about this question is that
Speaker 1: we're like digging under the carpet of the universe, right,
Speaker 1: And so to me, space is what's left when you
Speaker 1: remove everything that you could remove from a portion of
Speaker 1: the universe.
Speaker 2: So, like in my office right now, and if we
Speaker 2: were to try to figure out what space is in
Speaker 2: terms of my office, we'd turn it into a vacuum.
Speaker 2: So I should leave my office if we're going to
Speaker 2: do this experiment, like we take everything out, and now
Speaker 2: we're asking what are we left with?
Speaker 1: Yeah, well we'd have to take you out because you're
Speaker 1: not space right your stuff? Yeah, so yeah, remove all
Speaker 1: the stuff what is left? And that opens up immediately like,
Speaker 1: well is there anything left? Does it mean anything to
Speaker 1: have space there without stuff in it? Is space just
Speaker 1: defined to be the place between stuff? Or is it
Speaker 1: a thing itself? Right? Is space a kind of stuff? Right?
Speaker 1: I Mean, I know we're getting like really banana pels
Speaker 1: behind the gym over here, but these are the questions
Speaker 1: we're grappling with, and to me, this is what's exciting
Speaker 1: about physics is that these questions are really fuzzy, and yeah,
Speaker 1: you could you know, smoke banana peels and talk about
Speaker 1: them all afternoon and really make no progress, or for
Speaker 1: thousands of years and make no progress. But physics gives
Speaker 1: you a way forward. Physics gives you this method to
Speaker 1: like understand your intuitive experience and make it make sense
Speaker 1: by asking, like, can we build a model that describes
Speaker 1: what he can do? And then can we look at
Speaker 1: that model and say, like, what does that mean about
Speaker 1: what it is? So to me, the reason I'm a
Speaker 1: physicist and not a philosopher is that we can't actually
Speaker 1: make some progress if we think about it like mathematically
Speaker 1: and systematically.
Speaker 2: Okay, so we've now gotten to like you've removed everything,
Speaker 2: so you've got a vacuum, and now I feel like
Speaker 2: there's a vacuum in my brain and I'm like, well,
Speaker 2: where do you go from there? How does physics tackle
Speaker 2: this question?
Speaker 3: Then?
Speaker 1: Physics thinks about space in terms of location and motion,
Speaker 1: because what do you have left once you've emptied your office?
Speaker 1: There's just space there, which means the possibility to put
Speaker 1: something in it. Right, you can put a proton in it,
Speaker 1: But The interesting thing is you can choose where to
Speaker 1: put the proton. You can put the proton where Kelly's
Speaker 1: desk used to be, or you could put the proton
Speaker 1: where Kelly's head used to be. Those in principle are different, right,
Speaker 1: And so space offers us these choices. You can be here,
Speaker 1: you can be there. Space seems to have inherent in
Speaker 1: it this like location, right, and and those locations can change. So, like,
Speaker 1: very very early on, before we were doing science, the
Speaker 1: way we think about it science, you know, the Greeks,
Speaker 1: they were thinking about space in terms of change, like
Speaker 1: motion and change. So the way physics begins attacking this
Speaker 1: problem is like why are there locations? And what are
Speaker 1: the rules about locations? Like how do things go from
Speaker 1: here to there? And why is here different from there?
Speaker 1: And can you tell the difference between here and there?
Speaker 2: In my head? Right now, I've got like a three
Speaker 2: dimensional graph, and you identify space as like a point
Speaker 2: on that graph, and does it stay there forever? Or
Speaker 2: does space move? Or have you just jumped to a
Speaker 2: different point on the graph?
Speaker 1: Yeah? Right, great questions right, like are we moving relative
Speaker 1: to space? Can you measure our motion relative to space?
Speaker 1: Or can we only measure our motion relative to like
Speaker 1: other things in space. Right, that's an early basic question,
Speaker 1: and this is the kind of thing Aristotle was thinking about.
Speaker 1: Aristotle was like, well, you know, why do things move
Speaker 1: move at all? Wasn't everything just like stay the same
Speaker 1: place you put a proton? Wasn't it just stay there forever?
Speaker 1: And of course he was working on the surface of
Speaker 1: the Earth, and so he noticed like, hey, things fall down, right,
Speaker 1: Why do things fall down? Why do things seem to
Speaker 1: move through this space? And so you see that, like
Speaker 1: very early on, the questions of space and motion were
Speaker 1: tied together. And you know, Aristotle didn't have like a
Speaker 1: mathematical picture of how the universe worked or how anything happened.
Speaker 1: He was sort of like words based, you know, it
Speaker 1: was like vibes based signs. And he just basically said, look,
Speaker 1: things fall down because things move according to their nature.
Speaker 1: Matter tends to fall down. That's just it's sort of
Speaker 1: like a descriptive. It's not really explanatory. He's just like,
Speaker 1: stuff falls down because it's in the nature of things
Speaker 1: to fall down. Such a circular answer. I don't even
Speaker 1: know why it was ever satisfactory.
Speaker 2: So we were talking about the absence of stuff, and
Speaker 2: now we're talking about the movement of stuff, and so
Speaker 2: the connection to space is that space is standing still
Speaker 2: while the stuff is moving, or just that this is
Speaker 2: the first time people have thought about the relationship between
Speaker 2: space and stuff.
Speaker 1: Yeah, I think all of that early on people trying
Speaker 1: to figure out what space is by understanding how things
Speaker 1: move through space, like what does it mean to go
Speaker 1: from here to there? What does it mean to fall down?
Speaker 1: And why do things fall down? Anyway, and it gives
Speaker 1: you a handle like what does speed mean? But you know,
Speaker 1: Aristotle's view of what this meant was basically the way
Speaker 1: people thought about it for thousands of years until around Galileo.
Speaker 1: And Galileo was the first person to think like, well,
Speaker 1: what do you really mean? Aristotle like, what are you
Speaker 1: talking about things fall down? Because down seems to kind
Speaker 1: of depend on who you are. And he had this
Speaker 1: famous thought experiment way before Einstein was thinking about stuff.
Speaker 1: He was like, say you're on a boat and you're
Speaker 1: inside the boat, so you can't see the outside. You're
Speaker 1: like below decks, and you drop a ball. What's going
Speaker 1: to happen? Well, Aristotle says, the ball's going to fall down.
Speaker 1: Anybody who dropped. Anything on a boat knows the ball
Speaker 1: falls down. Cool, But now what happens if there's somebody
Speaker 1: on the ground and they're watching your experiment somehow does
Speaker 1: the ball fall down according to the person on the boat,
Speaker 1: which means it's then moving with the boat, or does
Speaker 1: the ball fall down according to the person on the ground,
Speaker 1: in which case it would be left behind. Right, there's
Speaker 1: actually different predictions there. If the ball falls down according
Speaker 1: to the person on the dock, it falls like sort
Speaker 1: of straight down, then as the boat keeps moving, the
Speaker 1: ball gets left behind and the person on the boat
Speaker 1: should see the ball like weirdly fall backwards. Whereas if
Speaker 1: the ball falls down according to the person on the
Speaker 1: boat right, then it falls down for them, but the
Speaker 1: person on the dock sees it moving forward with the boat,
Speaker 1: So you can't have it fall straight down for both people.
Speaker 1: This is what Galleo realized.
Speaker 2: Okay, so my brain is now again trying to so
Speaker 2: it feels like we're talking about stuff, but we're not
Speaker 2: talking about space. And so the connection is the stuff
Speaker 2: is moving through space, and by understanding the movement of
Speaker 2: the stuff, we can understand the space better.
Speaker 1: Exactly because galle is experiment helps us think about what
Speaker 1: speed means. Right, what is velocity? Are you moving relative
Speaker 1: to space? Are you only moving relative to other things?
Speaker 1: And Galileo's experiment and what we call Galleyan relativity is
Speaker 1: the velocity is just relative. You're not moving relative to space.
Speaker 1: Space is not something like grid that fills the universe
Speaker 1: and you could just move through space relative to that grid.
Speaker 1: You can only measure your velocity relative to other stuff because,
Speaker 1: like you're in the boat, there's no experiment that you
Speaker 1: can do to measure your velocity relative to the ground. Right,
Speaker 1: you can drop the ball, but the motion of the
Speaker 1: ball doesn't depend on your speed relative to the ground,
Speaker 1: So it doesn't tell you how fast you're going. You
Speaker 1: could be standing still, you could be going super fast.
Speaker 1: You can't tell the difference. And you're asking like, well, okay,
Speaker 1: but aren't we supposed to be talking about space? And
Speaker 1: what this tells us is about motion through space, and
Speaker 1: it tells us something really deep and important that velocity
Speaker 1: is only relative to other stuff. Space is not a
Speaker 1: thing you can have a speed relative to. So Aristotles think,
Speaker 1: you know, the universe is filled with space, and stuff
Speaker 1: moves through space. According to it's like natural tendency. Galleo's like,
Speaker 1: nu uh uh, space isn't a thing. You actually can
Speaker 1: just move relative to other stuff in space. So that's
Speaker 1: already like a big clue about what space might be.
Speaker 2: Okay, So is it fair to summarize by being like
Speaker 2: Galileo would say, there is no space. There's no grid
Speaker 2: in the world, so no space. I'm guessing other people
Speaker 2: have come to other conclusions. Otherwise that would be a
Speaker 2: short episode. And so let's take a break and when
Speaker 2: we come back, we'll find out if space exists. All right,
Speaker 2: So Galileo telling us space doesn't exist, who are we
Speaker 2: going to talk about next.
Speaker 1: I love the way you say that because I imagine
Speaker 1: like you saying that to Galleo, and he would probably
Speaker 1: not agree with your phrasing of it, though I agree,
Speaker 1: you know, I think that what Galileo tells us is
Speaker 1: that maybe space exists, but it's just the distance between objects, right, So,
Speaker 1: like you can have space, it's just not absolute, like
Speaker 1: it can't exist without stuff in it. The thought experiment
Speaker 1: we did, we were like, take everything out of Kelly's office.
Speaker 1: It would be like, well, there's nothing in there. Space
Speaker 1: is not a thing. If there's nothing in there. If
Speaker 1: you evacuated the whole universe, right, got all rid of
Speaker 1: all the particles and all the energy and everything, then
Speaker 1: there would be no space. I think that's what Galileo
Speaker 1: would say, not the space doesn't exist, but that it
Speaker 1: only exists between stuff, not on its own. It's not
Speaker 1: like a thing on its own.
Speaker 2: So like if you had a vacuum and you cleared
Speaker 2: out everything inside the vacuum, nothing exists in there, or
Speaker 2: like you now have a space of non existence.
Speaker 1: Yeah, I think that's Galileo's view. But this was very
Speaker 1: confusing to people, and folks like Newton who spent a
Speaker 1: lot of time thinking about motion. You know, he developed
Speaker 1: physics basically and calculus, and he thought carefully about the
Speaker 1: mathematical flow of things, and he unified our understanding of
Speaker 1: gravity on the ground and in the sky. He agreed
Speaker 1: with Galileo about how velocity works, right, obviously, he wrote
Speaker 1: down the equations. You know, he was the first guy
Speaker 1: to really be able to predict these things. So he
Speaker 1: agreed the velocity is relative, but he fundamentally disagreed about
Speaker 1: the nature of space. He was like, no, space is absolute,
Speaker 1: it exists in and of itself and you can have
Speaker 1: a velocity relative to it. So Galileo's like, velocity is
Speaker 1: purely relative, and Newton is like, I agree with you
Speaker 1: about the equations and that you can only measure your
Speaker 1: velocity relative to other stuff. But I still believe that
Speaker 1: space is a thing underlying everything.
Speaker 2: So does that suggest that the things that they were
Speaker 2: measuring are not important for understanding space because they could
Speaker 2: get the same information and come to different conclusions.
Speaker 1: I think it suggests that Newton was a little crazy, okay,
Speaker 1: because his conclusion is not supported. Like, the problem with
Speaker 1: Newton's idea that maybe space is absolute is that you
Speaker 1: still can't ever measure your velocity relative to it. So
Speaker 1: he believed in this thing that existed that you couldn't
Speaker 1: ever measure, you know, Galleo says, velocity is purely relative.
Speaker 1: There's no like velocity relative to space itself, because space
Speaker 1: itself on its own doesn't exist, right, only exists relative
Speaker 1: to stuff. Newton is like, yeah, I agree with you
Speaker 1: about velocity. You can only measure with between things, but
Speaker 1: still unobservable, unknowable to us, there is this absolute space.
Speaker 1: Even though he could think of no way to measure
Speaker 1: speed relative to it. So Newton believed that you could be,
Speaker 1: for example, at rest with respect to space, and it
Speaker 1: was some like special frame of reference there. Gallet was like, no, no,
Speaker 1: that doesn't exist, and plus you can't measure it. So
Speaker 1: Newton is just sort of going out in a limb
Speaker 1: thinking that it exists even if you can't measure it.
Speaker 2: All right, Well, I think just about every scientist you
Speaker 2: talk to has blind spots where they're like, no, it's true,
Speaker 2: and I can't tell you why, but I'm sure that
Speaker 2: it's true. We're all human. So we've been saying the
Speaker 2: word relative a lot, which makes me feel like we're
Speaker 2: gonna have to get to Einstein eventually. Is he the
Speaker 2: next person on your list?
Speaker 1: He's definitely the next person on the list. And Einstein
Speaker 1: gets a lot of credit for relativity. You think of relativity,
Speaker 1: you think of Einstein. But the truth is that Einstein's relativity,
Speaker 1: his concepts of space really just go back to Galileo.
Speaker 1: It's Galileyan relativity that Einstein took and just sort of
Speaker 1: like said, hey, let's go back to this. This made
Speaker 1: a lot of sense to me. Let's just like build
Speaker 1: this in at the foundations because Einstein was thinking about light.
Speaker 1: Only a few decades earlier, Maxwell had figured out that
Speaker 1: light is a wiggle in the electromagnetic field. And the
Speaker 1: confusing thing about it was that Maxwell's equation said that
Speaker 1: light should travel at a specific speed, the speed of light,
Speaker 1: and that it shouldn't depend on your velocity at all. Like,
Speaker 1: no matter whether you're on the Earth that's going around
Speaker 1: the Sun, or you're in a spaceship or you're just
Speaker 1: floating in deep space, everybody should see light travel at
Speaker 1: the same speed, said Maxwell. And Einstein was like, hey,
Speaker 1: well that's cool. That's actually what Galileo was saying, right.
Speaker 1: Gallea was saying that you shouldn't be able to measure
Speaker 1: your velocity, that there's no absolute for life, the velocity
Speaker 1: is only relative. And so what Einstein did was apply
Speaker 1: Galleyan relativity to Maxwell's equations and say that means that
Speaker 1: everybody measures the speed of light to be the speed
Speaker 1: of light, regardless of what else you're doing. That's the
Speaker 1: foundation of Einstein's idea. But really it's taking Galileo's relativity
Speaker 1: and just being like, hey, let's take.
Speaker 2: This seriously, okay, And so all of that. Whenever you're
Speaker 2: talking about the history of something. I feel like I
Speaker 2: get it in my head. I'm like, oh, that makes
Speaker 2: sense that it's like, oh no, wait, but then that
Speaker 2: got overturned and you're like, oh, but I just understood it, okay,
Speaker 2: And so that so all of that we still believe in.
Speaker 1: Right, We wait until you understand something, Klay, and then
Speaker 1: we overturn it. That's that's the whole plan there.
Speaker 2: You know that I've suspected that for a long time.
Speaker 2: But the whole world is about me and my understanding
Speaker 2: of things. So light is always moving at the same speed,
Speaker 2: no matter who was viewing it. That is still something
Speaker 2: we believe.
Speaker 1: That is still something we believe. And that is the
Speaker 1: earthquake of Einstein's relativity. The concepts came from Galileo, but
Speaker 1: because he applied it to light and had all sorts
Speaker 1: of consequences. Also because Einstein connected space and time and
Speaker 1: we're going to talk about what time means in another
Speaker 1: episode and what it even is. But the fact that
Speaker 1: light always moves at the same speed for all observers
Speaker 1: connects space and time in these really unique ways. And
Speaker 1: the short version of the story is that it means
Speaker 1: that different people have different clocks. So like clocks tike
Speaker 1: at different speeds at different parts of the universe, and
Speaker 1: that's a direct consequence of how light moves through space,
Speaker 1: because everybody sees light move at the same speed no
Speaker 1: matter what. Then you can't have clocks that all agree
Speaker 1: all the way through the universe. And if you want
Speaker 1: to know more about how that works, check out the
Speaker 1: companion episode about what time is. We'll explain all of that.
Speaker 2: Now we've talked about the movement of light through space.
Speaker 2: What does that tell us about how we can define space?
Speaker 1: Yeah, so Einstein's relativity means something really important about what
Speaker 1: space is because it connects space and time, which tells
Speaker 1: us that time is relative also not just space. Time
Speaker 1: is relative and the same for everybody, and it has
Speaker 1: an important meaning for what distances are between things. It
Speaker 1: means that like I can measure the length of a ruler,
Speaker 1: and you can measure the length of a ruler, which
Speaker 1: is like the distance between two points in space, and
Speaker 1: we can get different answers and we can both be correct.
Speaker 1: This is something in special loyaltuty that goes by the
Speaker 1: name length traction, and basically it just means, hey, you
Speaker 1: assume that things have a length and have a length,
Speaker 1: and that length is their length, and it doesn't matter
Speaker 1: how fast you're going or where you are. Turns out
Speaker 1: that's not true. It turns out the distances between points
Speaker 1: what we even mean by space, depends on where you're
Speaker 1: looking at them from and how fast you're going relative
Speaker 1: to them.
Speaker 2: Okay, So if we want to try to understand space,
Speaker 2: does that mean we need to try to find ways
Speaker 2: to hold constant how fast you're moving and the distance
Speaker 2: between things, and then we can start to get a
Speaker 2: handle on space.
Speaker 1: Yeah, exactly, we want to talk about space. Space is
Speaker 1: about the distance between things. So now we have to
Speaker 1: think about, well, how do you measure the distance between things?
Speaker 1: And you know, you can be pretty pedantic about it.
Speaker 1: You can be like, well, I'm gonna hold up a
Speaker 1: ruler between two things. I'm going to measure where thing
Speaker 1: one is and thing two are at the same time,
Speaker 1: and I'm gonna say, well, the difference between the marks
Speaker 1: and the ruler, that's how far apart they are. You know,
Speaker 1: I have my left hand and my right hand. I
Speaker 1: put them on a ruler. There's ten centimeters long ticks
Speaker 1: between them, So I say they're ten centimeters apart right.
Speaker 1: And the crucial thing that I've done there is I've
Speaker 1: done it at the same time. I said, I'm gonna
Speaker 1: look at where my left hand is right now and
Speaker 1: where my right hand is right now, and measure their
Speaker 1: distances at the same time, and then I'm going to
Speaker 1: call that the distance between them. The problem is that
Speaker 1: Einstein's relativity and this whole speed of light business changes
Speaker 1: what we mean by at the same time because time
Speaker 1: is not universal anymore. So I might say I'm measuring
Speaker 1: where my left and my right hand are at the
Speaker 1: same time. But you might think, actually, Daniel, you messed up.
Speaker 1: You measured where your left hand was and then a
Speaker 1: second later where your right hand was, and if you're moving,
Speaker 1: then now the whole measurement is messed up. So connecting
Speaker 1: space and time changes how we think our clocks work,
Speaker 1: which also upends how we measure distances, and in the end,
Speaker 1: that's what space is about. Right We're talked about where
Speaker 1: a proton is and where another proton is and how
Speaker 1: far apart they are. Now it turns out, according to Einstein,
Speaker 1: we don't even agree about the distances. That's not even
Speaker 1: a fundamental thing about space. That everybody looks at a
Speaker 1: distance and agrees about what the distance is, like how
Speaker 1: many centimeters are there between the protons?
Speaker 2: Okay, so it sounds like we've decided that a definition
Speaker 2: of space has to include distance. Yeah, but people measure
Speaker 2: distances differently depending on their conditions. Is there a way
Speaker 2: to get around that or no, there's no way to
Speaker 2: get two people? I need ice cream?
Speaker 1: Yeah, exactly, there's no way around that. That turns out
Speaker 1: to just be a feature of space that we never
Speaker 1: noticed before because mostly we had basically no velocity relative
Speaker 1: to each other, mostly slow speeds on the surface of
Speaker 1: the Earth, and we didn't look at very long distances.
Speaker 1: So we have this intuition that things have a size,
Speaker 1: and that size is just what they are, and you
Speaker 1: should measure that size no matter who you are and
Speaker 1: how fast you're going. That just turns out to be wrong.
Speaker 1: Like sometimes people ask me, you know, why do things
Speaker 1: get shorter if you see them at high speeds. You know,
Speaker 1: you have a ruler stick flying by you at nine
Speaker 1: tenths of the speed of light, Why do you measure
Speaker 1: it to be less than a meter stick? If it
Speaker 1: was a meter when you're holding it and the answer
Speaker 1: is not that it's shrunk there. You're imposing your like intuition,
Speaker 1: you're prejudice that things have a size and that they
Speaker 1: have to shrink to get shorter. The answer is lengths
Speaker 1: depends on velocity. Like, that's just the way space works.
Speaker 1: You can't escape it, Kelly, is no way around it.
Speaker 1: Space just is different from the way our intuition work.
Speaker 1: And this is why it's so important to explore it
Speaker 1: like systematically and mathematically, because it contradicts our intuition and
Speaker 1: it leads us, we hope, at least, to some true
Speaker 1: insights about the nature of reality.
Speaker 2: So why can't we just say that depending on conditions,
Speaker 2: you get different distances, but the only thing you're differing
Speaker 2: in is how much space you're talking about. But you
Speaker 2: can still talk about space, like why do we have
Speaker 2: to be able to measure it to have a definition
Speaker 2: of it?
Speaker 1: Why do we have to measure it to have a
Speaker 1: definition of it? Wow, awesome question. You know. I think
Speaker 1: that's probably because the way we do science is we
Speaker 1: measure stuff, right, Like, you have to be able to
Speaker 1: take measurements to have data so you can talk about
Speaker 1: what that data means. Right, Otherwise, what are you doing.
Speaker 1: You just smoking banana peels and having conversations, which is fine.
Speaker 2: But so distance isn't the only way to measure things.
Speaker 2: Why is distance the measurement that we have to have
Speaker 2: in order to understand space?
Speaker 1: Oh yeah, I see a great question. Well, I think
Speaker 1: because we imagine that space is about locations, right, Even
Speaker 1: if you think about space as some three D grid,
Speaker 1: those are all locations, and so distances are differences between locations, right,
Speaker 1: And either those are relative, like the only thing that
Speaker 1: exists are distances between two points, two protons or two
Speaker 1: ends of the ruler, or there's some absolute grid, and
Speaker 1: you can measure your distance relative to space itself. But
Speaker 1: in the end, distance is the thing that space is describing.
Speaker 5: Right.
Speaker 1: If you don't have space, you can't have distance, right,
Speaker 1: And so that's sort of like the way we get
Speaker 1: handle on space.
Speaker 2: This whole podcast is about Kelly understanding herself better. It's
Speaker 2: like part Kelly Psychology, part what is space? And I
Speaker 2: think part of why I enjoy this conversation more as
Speaker 2: an adult is I'm way more comfortable now being like,
Speaker 2: this is probably a really not smart question, but I
Speaker 2: don't care. I'm asking it, and I think in its classes.
Speaker 2: I'd just be like, no, I can't say that, because
Speaker 2: that would be anyway. Maybe our listeners are going to
Speaker 2: write it and be like, you shouldn't ask those questions.
Speaker 2: They're not good questions, but anyway.
Speaker 1: Okay, they're perfect questions. They're perfect questions.
Speaker 2: Yes, okay, you have to have distance, but you cannot
Speaker 2: measure distance. Where do we go from here?
Speaker 1: You can measure distance, it's just that the distance is
Speaker 1: not the same for everybody. Right, that's two people measuring
Speaker 1: the same quantities can get different answers. So distance is
Speaker 1: not universal, but it can still exist, and then things
Speaker 1: get crazier. Everything we're talking about so far is just
Speaker 1: Einstein's view of how things move in sort of Newton's
Speaker 1: idea of space. But Einstein then introduced another concept. So
Speaker 1: now we go from special relativity to general relativity. He said,
Speaker 1: space can do even more. We can change the relative
Speaker 1: distances between things without those things moving. This is the
Speaker 1: idea of space itself curving. And there's a lot of
Speaker 1: descriptions out there in popular science about what it means
Speaker 1: for space to curve, and many of them are very misleading.
Speaker 1: You know, there's the famous one about the rubber sheet
Speaker 1: that most pop side folks go to. And I really
Speaker 1: discourage you from thinking about space in terms of a
Speaker 1: rubber sheet, because if you think about it carefully, it
Speaker 1: leads to all sorts of misunderstandings, you know. And so
Speaker 1: we're going to cover the rubber sheet analogy just to
Speaker 1: talk about why it's misleading. The general picture of the
Speaker 1: rubber sheet is like you stretch out this rubber sheet
Speaker 1: and then you put a bowling ball in it. The
Speaker 1: bowling ball bends the rubber sheet, and that's supposed to
Speaker 1: represent like how space is curving. The problem with that
Speaker 1: analogy is that it's showing you to D space the
Speaker 1: rubber sheet in some third dimension, like it's bending outside
Speaker 1: the universe itself. The universe is supposed to be two D,
Speaker 1: and the rubber sheet is bending into some other dimension,
Speaker 1: whereas in our space. The way Einstein thinks about space
Speaker 1: bending is it's intrinsic. There's no additional dimension. We're not
Speaker 1: bending our three D space into some fourth dimension like
Speaker 1: a rubber sheet bending into some new dimension we can't see.
Speaker 1: That's not what space curvature is. It's just changing the
Speaker 1: relative distances between things. So, like Kelly and I have
Speaker 1: a certain number of thousands of miles between us right now,
Speaker 1: what if while we both sit in our chair, you
Speaker 1: could just change that distance, so now it's a thousand miles,
Speaker 1: or now it's ten thousand miles. We just change the
Speaker 1: amount of space between us.
Speaker 2: When I think about the curvature of space, I feel like,
Speaker 2: but isn't there now space on either side of whatever
Speaker 2: just curved? But I guess that area is supposed to
Speaker 2: have no space, So there's places that have no space?
Speaker 2: Is that right?
Speaker 1: What do you mean on either side? You mean like
Speaker 1: are we getting pushed out in to other space or something?
Speaker 2: So I guess maybe my brain is still stuck on
Speaker 2: the rubber sheet. So you've got the sheet, it like
Speaker 2: goes in where the bowling ball is. But in my head,
Speaker 2: there's space above where the bend is, Like you know,
Speaker 2: the sheet was there and now it's down, but there's
Speaker 2: still something where the sheet used to be, because there's
Speaker 2: still space there. But is that not how to think
Speaker 2: about it? Like when it bends, there's space and there's nothing,
Speaker 2: there's like the absence.
Speaker 1: Yeah, exactly. That's why the rubber sheet is so confusing
Speaker 1: because as soon as you dig into it. It leads
Speaker 1: to questions that don't have answers because the herb sheet
Speaker 1: is just not the way it works right. The two
Speaker 1: D examples are useful because they are easier to think
Speaker 1: about instead. Imagine like a map, right, so you have
Speaker 1: your favorite country, US, Argentina or whatever, and think about
Speaker 1: a bunch of cities on that map, and they all
Speaker 1: have distances between them, right, and old maps you could
Speaker 1: like look up how far it was between New York
Speaker 1: and La or between Seattle and Miami, right, and those
Speaker 1: are distances all right. Cool, And if you took a
Speaker 1: ruler to the map, you could like measure those things
Speaker 1: and the map would lay flat on a table and
Speaker 1: you could measure those distances with rule. Cool. Now, what
Speaker 1: if I came in and I had magic fingers, and
Speaker 1: I'm like, I'm just going to change the distance between
Speaker 1: LA and Seattle and I'm not going to change anything else,
Speaker 1: or I'm gonna make it longer. I'm gonna make it
Speaker 1: shorter that distance. Now, imagine like, does that map lie
Speaker 1: flat on the table anymore? If I play with enough cities,
Speaker 1: then a order accommodate having more or less space, I'm
Speaker 1: gonna end up with wrinkles in that map. There's gonna
Speaker 1: be no way to lay that map flat on the table.
Speaker 1: And that's essentially what's happening in space. You put a
Speaker 1: mass in space, and space shrinks, right. It changes the
Speaker 1: relative distances between stuff in a way that it's no
Speaker 1: longer conceptually flat in your mind, the way that a
Speaker 1: sheet of paper would no longer be flat if you
Speaker 1: just like magically change the distances between two points on
Speaker 1: that sheet of paper.
Speaker 2: All right, So I'm thinking about the map, and you
Speaker 2: shortened the distance between Seattle and LA. And maybe I'm
Speaker 2: taking everything too literally, Like has San Francisco disappeared? Are
Speaker 2: all the people in San Francisco shorter because everything has trunk?
Speaker 2: Or are you tunneling through the map to shorten the distance,
Speaker 2: or do we not know what space is doing? It
Speaker 2: could be like any of those things.
Speaker 1: We're not tunneling through San Francisco, and we're not killing
Speaker 1: anybody in San Francisco. I hope you all are safe
Speaker 1: out there. We're just changing the distance between Seattle and LA.
Speaker 1: We're just saying, hey, if you turn on a flashlight
Speaker 1: in La, how long would it take for that light
Speaker 1: to get to Seattle? Because the speed of light is constant.
Speaker 1: This is a good way to measure how much space
Speaker 1: there is. So we're saying, hey, these things are now closer.
Speaker 1: That's what it means to shrink space. And the idea
Speaker 1: of curvature is that it's local, right, Like you could
Speaker 1: make everything further apart and everything closer together and keep
Speaker 1: the map flat. But if you only squeeze two different
Speaker 1: cities to make them closer together or further apart, then
Speaker 1: now space has weird bends in it, right, in order
Speaker 1: to make light take a certain amount of time to
Speaker 1: go here and a certain amount of time to go there,
Speaker 1: and to make all the light times work out, space
Speaker 1: has to have weird curves and it can no longer
Speaker 1: be flat in the same way that like, if you
Speaker 1: magically made the time from La to Seattle five times
Speaker 1: as long, then there would have to be more space
Speaker 1: in there. You couldn't have a flat sheet of paper.
Speaker 1: You need to like add more space, more land, more
Speaker 1: road for you to drive on.
Speaker 2: We can move forward, but I have to admit I'm
Speaker 2: having a little trouble, Like, does that mean we have
Speaker 2: stretched out what already exists or made more? Are there
Speaker 2: new cities in between now or did we just take
Speaker 2: what existed and it's like taffy and wh'ere just it's
Speaker 2: like like all the people in between are like twice
Speaker 2: as wide.
Speaker 1: Yeah, okay, great question, And this is something we actually
Speaker 1: know the answer to because we've done it. Like when
Speaker 1: gravitational waves hit the Earth, they literally do this. They
Speaker 1: stretch space and they shrink space. Right, So this actually happens.
Speaker 1: And what happens when a gravitational wave passes through the Earth,
Speaker 1: this is a wave in space itself again, which just
Speaker 1: means you're changing their relative distances. What happens in reality
Speaker 1: is the stuff that's holding us together, the land, the
Speaker 1: electrochemical bond, keeps us at the same distance. And so,
Speaker 1: for example, simplify things and say you and I were
Speaker 1: out floating in space and we were holding a rod
Speaker 1: that was one kilometer long, each holding one end, and
Speaker 1: then a gravitational wave comes by and it stretches space. Well,
Speaker 1: you and I are holding the rod, and the rod
Speaker 1: has bonds, and so it's going to keep us at
Speaker 1: exactly one kilometer apart, even though space is stretching around us.
Speaker 1: So if we show in a flashlight, we would still
Speaker 1: measure one kilometer. But if we weren't holding onto the rod, right,
Speaker 1: there was nothing keeping us at one kilometer. Then when
Speaker 1: space wiggles around us, we would use our flashlights and
Speaker 1: we would measure more than one kilometer distance because it
Speaker 1: would take light longer. So your question is, like, is
Speaker 1: it making more space or is it stretching the space
Speaker 1: that's already there. Nobody knows the answer to that question
Speaker 1: because we don't really know what space is. Like, what
Speaker 1: is happening you're asking, like the underlying mechanisms of all this, Yeah,
Speaker 1: we have no idea what's really going on there. We
Speaker 1: just have this mathematical theory that tells us how to
Speaker 1: do these calculations. We don't know what's underlying going on.
Speaker 2: But the car the way we understand these things, can
Speaker 2: it be used to like predict and understand things we've
Speaker 2: actually seen. This isn't just smoking banana peels. We tested
Speaker 2: these things. Our understanding is.
Speaker 1: Useful, absolutely, And we can measure this curvature, like if
Speaker 1: you shine light through this space, you can tell is
Speaker 1: space curved or is it not? Like if you shine
Speaker 1: two parallel lines through space that's not curved. They should
Speaker 1: never touch two photon beams through flat space should never
Speaker 1: touch if they're in parallel, but in curved space they
Speaker 1: will either cross or they'll split apart. Right, And we
Speaker 1: have measured this, Like the famous proof of relativity from
Speaker 1: Einstein was seeing light bend around the moon during the eclipse, right,
Speaker 1: that was seeing light move through curved space and bending,
Speaker 1: And so this is definitely something we've seen. We have
Speaker 1: very accurate models, and general relativity is amazingly precise. It
Speaker 1: predicts all sorts of things that nobody else could predict.
Speaker 1: So it tells us what space does because not only
Speaker 1: is space curved and light moves through it in curved ways,
Speaker 1: but like other stuff moves through it in curved ways.
Speaker 1: You have the Earth moving around the Sun because the
Speaker 1: Sun has curved space around it. So it's not just
Speaker 1: smoking banana peels. It works really really really well. But
Speaker 1: you know, then we can ask like, well, all right,
Speaker 1: so general relativity tells us that, like if you have
Speaker 1: mass in space, it curves it, and that not only
Speaker 1: our distance is relative, but like distances between stuff can
Speaker 1: change even without that stuff moving. So like what does
Speaker 1: that mean about what space is? Right?
Speaker 2: And let's dig into that question after we go all
Speaker 2: grab some more banana peels. Okay, so you just finished
Speaker 2: saying that the distance between two objects can change even
Speaker 2: if both of the objects like perceive that they have
Speaker 2: stayed still. But what does that tell us about space?
Speaker 1: So we can measure our velocity relative to other stuff,
Speaker 1: and we can tell that space can sometimes expand and
Speaker 1: shrink between it, and we have these amazing calculations from
Speaker 1: general relativity, what does that tell us about what space
Speaker 1: actually is? You know? Well, back to the conversation with
Speaker 1: Galileo and with Newton. Einstein agrees that velocity is relative, right,
Speaker 1: you can only measure your velocity relative to other stuff.
Speaker 1: And it seems like he's saying that space is a
Speaker 1: thing because space can do stuff like it can wiggle,
Speaker 1: it can bend, it can expand. So we went from
Speaker 1: like Galleo saying, now space is just the distance between stuff,
Speaker 1: to Newton saying, no, space is a thing, even though
Speaker 1: you can't measure your velocity to it, to Einstein being like, well,
Speaker 1: space has interesting properties, so it's pretty hard to say
Speaker 1: it's not a thing. Right. The weirdest part about Einstein's space, though,
Speaker 1: is that you still can't measure your velocity relative to it, like,
Speaker 1: even if you think it's out there and it has
Speaker 1: curvature and has these properties, you can't measure your speed
Speaker 1: relative to just space. It's something Matt Strassler calls emotional,
Speaker 1: like there's no way to measure your speed relative to space.
Speaker 1: So in one hand, Einstein agrees with Galleo like you
Speaker 1: have velocities relative, but he also agrees with Newton like
Speaker 1: space is a thing. But then back to Galleo, He's like, actually,
Speaker 1: but you can't measure your speed relative to it, So
Speaker 1: like what is it?
Speaker 4: Man?
Speaker 2: Yeah, so is this something that like when we understand
Speaker 2: dark matter and dark energy, it could help us understand space.
Speaker 2: Are those like just completely different problems.
Speaker 1: They could be completely different problems or they could be connected.
Speaker 1: You never know what thread of investigation is it really
Speaker 1: going to help you, like figure out what's going on
Speaker 1: and where the next breakthrough is going to come from.
Speaker 1: What we do know is that general relativity is a
Speaker 1: great description of space and motion, but we don't know
Speaker 1: what's going on underneath it. Like people often ask me,
Speaker 1: you put mass in space and space bends. Why does
Speaker 1: it bend? What is the mechanism for bending it? What
Speaker 1: is doing the bending? And we don't know the answer
Speaker 1: to those questions. Remember, general relativity is a description of
Speaker 1: what we've seen, and we can look at it and
Speaker 1: be like, well, what does that mean about space? And
Speaker 1: it's not a final answer. And part of the reason
Speaker 1: we know it's not a final answer is that we
Speaker 1: have this completely separate idea about how the universe works
Speaker 1: and how space is that comes from the other branch
Speaker 1: of physics that we've been developing over the last one
Speaker 1: hundred years, which is quantum mechanics, and quantum mechanics tells
Speaker 1: us a completely different story about space, what it is
Speaker 1: and how it works.
Speaker 2: So quantum mechanics would not agree that space bends in
Speaker 2: the way that we've been talking about.
Speaker 1: Quantum mechanics has no answer to the question of what
Speaker 1: is bending light around the moon. Quantum mechanics can't explain
Speaker 1: space bending. Quantum mechanics can explain gravity at all. Quantum
Speaker 1: mechanics can explain electromagnetism, it can explain the weak force,
Speaker 1: it can explain particles that it can explain all the
Speaker 1: strange experiments we saw right one hundred years ago and
Speaker 1: the experiments we do at the particle collider, and it's
Speaker 1: an extraordinarily accurate description of everything basically particle related. But
Speaker 1: it's built on a different assumption about space. It thinks
Speaker 1: of space the way Newton did, just like the backdrop
Speaker 1: on which things play out in the universe. You know,
Speaker 1: general relativity is what we call background free. It's like
Speaker 1: space itself is just the distances between stuff, and quantum
Speaker 1: mechanics is like, no, there's a background there, Just lay
Speaker 1: space out, roll it out like AstroTurf, and then particles
Speaker 1: do their dance in that space. And so it starts
Speaker 1: from a very different place, and it can't explain how
Speaker 1: space bends. In fact, the space bends too much, quantum
Speaker 1: mechanics breaks down. We don't know how to do calculations
Speaker 1: for quantum mechanics if space is super bendy. And it
Speaker 1: also tells us something very different about empty space. Like
Speaker 1: the exercise we started out with where we said, take
Speaker 1: your office, remove Kelly, remove all of her books, remove
Speaker 1: all of her weird samples of parasites and other gross
Speaker 1: stuff I can see in the background, and all the air.
Speaker 1: What's left. Well, Einstein says there's nothing there, but quantum
Speaker 1: mechanics says that's not possible because quantum mechanics says, space
Speaker 1: in itself is filled with fields, Like what is it
Speaker 1: that light is moving through? Anyway? It's moving through the
Speaker 1: electromagnetic field. Well, you can't take that out of space.
Speaker 1: Quantum mechanics says, you can't like rip the field itself out.
Speaker 1: You can say, I'm going to take all the photons out,
Speaker 1: but the field itself is like the capacity for light
Speaker 1: to move through it. It's like a parking lot with
Speaker 1: no cars in it, right, the field itself is always there.
Speaker 1: And quantum mechanics says that we have the electromagnetic field
Speaker 1: and the electron field and the muon fil in all
Speaker 1: sorts of fields that they're a part of space itself.
Speaker 1: And not only that, but these fields can never be
Speaker 1: totally zero. You can never pull all the energy out
Speaker 1: of them. They have a minimum quantum fuzziness, which means
Speaker 1: that there's always energy in space. So quantum mechanics view
Speaker 1: of space is really different. It's like you have this
Speaker 1: absolute background on which you put these fields, and these
Speaker 1: fields are always buzzing even if you do your best
Speaker 1: job of pulling everything out of that space.
Speaker 2: So are these the two main theories for space? So
Speaker 2: there's no other like theories that physicists take seriously. I'm
Speaker 2: sure there's plenty of people who have additional theories, but
Speaker 2: there's no theories people take seriously.
Speaker 1: Yeah, we have narrated down to at least two ideas
Speaker 1: about space, both of which we're pretty sure are wrong. Hey, no,
Speaker 1: that's progress.
Speaker 2: That is No, that is progress. So does anybody have
Speaker 2: any promising experiments designed to follow up on this next
Speaker 2: or what comes next? We're at a stalemate.
Speaker 1: It seems we are at a stalemate. And one issue
Speaker 1: is that we don't know whether the quantum mechanics version
Speaker 1: of space or the general relativity version of space is
Speaker 1: correct because almost every experiment we can think of only
Speaker 1: involves one of them. Like we can do experiments to
Speaker 1: test general relativity, like photons bending around moons, but then
Speaker 1: quantum effects are irrelevant because quantum effects get averaged out
Speaker 1: when you have something as big as a moon, Or
Speaker 1: we can think of quantum mechanical experiments we have like
Speaker 1: one particle bouncing off another particle, but then gravity is
Speaker 1: irrelevant because the gravity of a particle is basically zero
Speaker 1: because gravity is super duper weak. So the only place
Speaker 1: you could do a test to say, like, well, whose
Speaker 1: idea of space is correct? Quantum mechanics or gravity are
Speaker 1: experiments that are particle sized but have the masses of moons.
Speaker 1: And so now we're talking about black holes, and so
Speaker 1: the answer to like what space really is and how
Speaker 1: does it all work? Is hiding behind the event horizons
Speaker 1: of black holes, like what's in there? General relativity says
Speaker 1: it's a singularity. Quantum mechanics says that's nonsense. They can't
Speaker 1: both be right. They could both be wrong. So, yeah,
Speaker 1: the answer to your question is build a spaceship, fly
Speaker 1: into a black hole, get the answer, but never be
Speaker 1: able to tell anybody about it because you're chopped forever.
Speaker 2: Is there any reason to hope that we'll be able
Speaker 2: to get the answer? Someday? Will our children be having
Speaker 2: the same conversation?
Speaker 1: I think we probably will figure it out. There are
Speaker 1: other ways to explore this, like the hearts of neutron
Speaker 1: stars are not quite dense enough to become black holes,
Speaker 1: but they are dense enough, or gravity and some quantum
Speaker 1: mechanical effects are both important, So by studying the insides
Speaker 1: of neutron stars we might be able to get a clue.
Speaker 1: But there's also just a lot of like thinking that
Speaker 1: we need to do. You know, it's not like we
Speaker 1: have a great theory that predicts what's going to happen
Speaker 1: that we need to go test. So we have some
Speaker 1: more thinking to do about like how to bring these
Speaker 1: things together, and there are definitely people working on it,
Speaker 1: you know. String theory is one effort to try to
Speaker 1: describe things that incorporates gravity and quantum mechanics. This other
Speaker 1: approaches loop quantum gravity. One of my favorite ideas is
Speaker 1: that space itself is made of chunks, right, like little
Speaker 1: pixels of space, and that what's happening when space increases
Speaker 1: is that you're like adding more pixels, and that when
Speaker 1: space shrinks is that you're decreasing these pixels. The cool
Speaker 1: thing about this is that it gives you a way
Speaker 1: to think about a universe without space. Like imagine a
Speaker 1: whole bunch of pixels in the beginning of the universe,
Speaker 1: and these pixels are not tied together in any way.
Speaker 1: It's like a pile of beads before you do your project.
Speaker 1: Then somebody comes along and they weave all these beads
Speaker 1: together into a sheet, right, or like a three D grid.
Speaker 1: That's what space is there's a bunch of these pixels
Speaker 1: woven together with maybe quantum forces or something into this
Speaker 1: three D grid that we live in and experience. But
Speaker 1: you could also imagine that there was a time before
Speaker 1: that happened, when space was like disorganized, where you couldn't
Speaker 1: let go from one to the other because they weren't
Speaker 1: connected the way they are now. So thinking about the
Speaker 1: nature of space and thinking about how to bring these
Speaker 1: ideas together is maybe a fruitful way to make progress
Speaker 1: because it forces you to think, like, well, what does
Speaker 1: this mean, and what else could it be? And could
Speaker 1: you have a universe without space? All this kind of stuff.
Speaker 2: So for the pixel theory, like when you get more
Speaker 2: pixels or lose pixels, where do they come from? And
Speaker 2: where do they go?
Speaker 1: Yeah? Great question, And that question assumes that they have
Speaker 1: to come from somewhere, right you imagining like things like
Speaker 1: energy in the universe are concerned. Right, you can't just
Speaker 1: like pop new pixels out of nothing. But you know,
Speaker 1: we don't actually know that. We don't know that energy
Speaker 1: has to be conserved in the universe. We know that,
Speaker 1: for example, when the universe expands, photons get red shifted,
Speaker 1: they get stretched to longer wavelengths. That means they lose energy.
Speaker 1: Where's that energy go? Nowhere? It doesn't have to go anywhere,
Speaker 1: because maybe energy itself is not conserved in the universe.
Speaker 1: So these are great questions because it might be that
Speaker 1: they are the wrong questions, and the contradictions that come
Speaker 1: up when we ask them lead us to asking the
Speaker 1: right questions, which we don't know what those questions are yet.
Speaker 1: But it's sort of like, you know, knowing the answer
Speaker 1: is forty two and then going back and realizing, hmm,
Speaker 1: maybe we asked the wrong question, or maybe we're thinking
Speaker 1: about this whole thing wrong, you know. I think the
Speaker 1: takeaway message for listeners is like, what is space? Well,
Speaker 1: we don't know. We have two really nice descriptions of
Speaker 1: what space might be, both of which work in different scenarios,
Speaker 1: but both of which raise a lot of questions and
Speaker 1: they don't agree with each other, and so we really
Speaker 1: just don't know what space is. Even though we've made
Speaker 1: a lot of progress and we can build iPhones and
Speaker 1: launch rockets to Mars and all sorts of stuff. We
Speaker 1: can move through space, we can manipulate space, doesn't mean
Speaker 1: we yet know what it is, and it might be
Speaker 1: a century or a thousand years before we really figure
Speaker 1: it out.
Speaker 2: Well, that's exciting. I'm going to try to convince my
Speaker 2: daughter to become a physicist. She's been wearing her Sern
Speaker 2: outfits in her Stern hard hat since visiting CERN, so
Speaker 2: maybe she's on the path.
Speaker 1: I think about space the way I think about like
Speaker 1: a photon. People are often told, like a photon is
Speaker 1: a particle. No, it's a wave. Or sometimes it's a particle,
Speaker 1: sometimes it's a wave. The way I think about it
Speaker 1: is like a photon. It's neither a particle or a wave.
Speaker 1: Sometimes it's particle like, sometimes it's a wave like. It's
Speaker 1: something else we haven't yet figured out. And the same
Speaker 1: is true of space. We can describe space sometimes using
Speaker 1: general relativity, we can describe it sometimes using quantum mechanics.
Speaker 1: But space is probably something else. We've never even imagined,
Speaker 1: something beyond yet our current thinking that maybe one of
Speaker 1: our listeners is smart enough to.
Speaker 2: Figure out that would be awesome.
Speaker 1: All right, Well, thanks for going on this journey with
Speaker 1: us into the philosophical underpinnings of physics. I hope I've
Speaker 1: convinced you that physics is a way to think about
Speaker 1: these big, deep questions without getting lost in the meaning
Speaker 1: of the words, because it lets us be mathematical to
Speaker 1: try to be precise, and then to ask philosophical questions
Speaker 1: about those mathematical models, be like, what does it mean
Speaker 1: that I can calculate this, but I can't measure that?
Speaker 1: What does it tell us about the nature of the universe?
Speaker 1: Even when those answers are or we really.
Speaker 2: Just have no clue, my brain today hurt in a
Speaker 2: good way. I really enjoyed thinking about this today. It
Speaker 2: was nice to have us someone to ask silly questions
Speaker 2: to well.
Speaker 1: Thank you for all the silly and wonderful questions, and
Speaker 1: thanks everybody else out there for thinking about the nature
Speaker 1: of the universe. If you have questions about how things work,
Speaker 1: don't be shy. Write to us if you have questions
Speaker 1: at Daniel and Kelly dot org.
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