What is space?

00:00:06
Speaker 1: Some of the deepest mysteries of the universe are so big, so imposing, so important that even asking questions about what they are and how they work it can leave us confused. That's because we have to spend some time thinking about what exactly we're asking and what kind of answer we want. But we shouldn't shy away from these kinds of questions because they are the most important questions. Understanding the nature of our reality means understanding the context of our lives, where we live, how we live, maybe even why we live. And what the universe has taught us is that it always requires persistent, careful effort to unpack the deepest mysteries. We have been chiseling away at the rock face of physics for thousands of years, mostly cluelessly, but occasionally a great hunk of understanding will open up and will see everything in a new light. So it's worth asking the biggest questions, the ones where the question itself can be confusing, because looking for answers might eventually help us figure it all out, or help us realize we're asking the wrong question. So today on the podcast, we're going to dig deep into what we know about maybe the most basic question about the universe. We'll be asking what is space.

00:01:37
Speaker 2: Welcome to Daniel and Kelly's Extraordinary Universe. Today we are talking about what is space? And I am Kelly Wienersmith, and I take up space. How about you?

00:01:49
Speaker 1: I'm Daniel Whitson. I'm a professor of physics, which means I probably should understand something about space.

00:01:56
Speaker 2: I'm going to start with a confession. So, like when I was in high school and college, I did really well in physics. I was in the honors classes, I got a's. I really liked the stuff about circuitry. But whenever we would talk about what is space or what is time? I would feel very frustrated because like you were taking this thing that I felt like I could work with in my day to day life, and it felt like it was being made unnecessarily complicated. And I think I also felt a little insecure because it's like I can't even understand what is space, and I think it just made me frustrated and I would kind of shut down. And as an adult, I feel different about it. I think it's interesting. I think thinking about it creates like testable predictions that teach us about the universe and then we can do practical things with that knowledge. But like, what was your journey along questions like this? And am I the only physics student in classes who isn't like, oh, this is fun and is like, no, I knew this, Why are you doing this to me?

00:02:49
Speaker 1: I think that's a whole spectrum of people in physics, and I think it's a big tent, and I'm glad there are people who are like, hey, look, this gives us tools so that we can calculate how our cannonballs fly over castle walls. That's really all we care about, So let's just do that and move on. And then there's the folks on the sort of philosophical side of things that really want to understand why balls fly over castle walls because they have deeper questions about like why is there anything and how does it all work? And the amazing thing about physics is that it not only lets you do practical stuff bill transistors and iPhones, but then you get to turn around and ask like, well, why does this work at all? And what does that tell us about the actual universe we live in? So yeah, touches on like technological, fascinating, useful stuff all the way to philosophical stuff, and yeah, I always found myself sort of on the philosophical side of things, and I was the kind of kid who was like, whoa, what is space anyway? Man? And this is before I smoked any banana peels. I was wondering about what would it be like to live in space that was four dimensions or two dimensions? And why is it that we can only think in three dimensions? So for me, these questions were cat in it, even though I didn't understand anything about them. And it's only now as a professional physicist that I understand why we don't understand anything about them.

00:04:03
Speaker 2: I hope that we can get to that in the show today so that the people like me who are just like, well now you've just made me confused, have a better sense of why we're asking these questions and why they're fundamentally important and not just physicists making things complicated. Like every once in a while, I'll be like, well, we haven't married relativity and quantum mechanics, and so what if when people say like space bends and time is blow, Like, what if none of that is true because we haven't married these things, And so then why do I have to do this, and then I get super frustrated, and I don't know, maybe I shouldn't be so negative. At the beginning of our show. I'm not actually negative, like I'm excited about it, but I'm remembering Kelly from the past feeling very frustrated about this stuff.

00:04:46
Speaker 1: Well, I think we're going to be aiming today's episode at Kelly of the past, somebody who is curious about how the world works but doesn't want like a wall of confusing language where words suddenly mean things that they didn't mean before. And let's remember that that's the whole project of physical We take the world that we kind of understand we've been living in any way, and we try to systematize it. We try to say, well, you know, at what angle will your cannon ball fly over the council walls? And do you have to factor in the wind resistance? And then we get to turn around and be like, well, why does this work? What does that tell us it? Lets us do something which I think is really awesome. We just peel back that layer of intuitive reality and say, hey, we thought the universe was like this, Actually it turns out it works like that. The universe is different from the way we thought it was, and that's wonderful. That's the incredible experience we're going for in physics. We want to pull the veil from our eyes and figure out how the universe actually is. I think what we'll discover today is that, wow, we really just don't know that the universe works. But it's important that everybody understand what we do know and what we don't know about these really basic questions about the nature.

00:05:48
Speaker 2: Of the universe, right And if you stop asking the questions, then you never get to the answer. So it's important to keep.

00:05:53
Speaker 1: Going exactly and that thing. It might be helpful for people when we're talking about something as fuzzy and difficult to grapple with a space to think about like what kind of question we're asking and what kind of answers are satisfactory? You know, because when we ask a vague question like what is space? What are we really asking when we say that? Even so often in philosophy, I replace a confusing word with something familiar, like replace the confusing word with an elephant, and think, what does the question mean if I'm talking about an elephant? You know, So if somebody asks you, hey, what is an elephant. You know, what kind of question is that? What kind of answers do we expect to that kind of question?

00:06:26
Speaker 2: When I was reading through our outline today, one of the things that got me excited was thinking back to grad school and when we would talk about behavior and we'd be like, well, what is the behavior? And there's like at least four different ways to think about it, you know, like how did it evolve? What neurotransmitters make this behavior happen? What things that happened beforehands initiate the behavior? And so in the case of behavior, there's a lot of different answers you could have for that question. Is it the same for space? Are we looking for like an equation? So yeah, like what would make a satisfying answer for something like this? Or are there lots?

00:06:55
Speaker 1: No, You're right, what we're looking for our answers to questions like well what can space do?

00:07:00
Speaker 3: Right?

00:07:00
Speaker 1: Like what is an elephant? Well, an elephant has big ears, it does this thing? Right, you can describe it, You can describe what it does. You can also ask like, well, why is there space? You know the same way you can be like why do we have elephants? You know, there's a story there that tells us something about elephants the relationship to other living things. Right, we can also ask, well, what is space made out of? Is it itself the fundamental thing in the universe where like you just got to have it, you don't have universe without it? Or somehow like elephants and ice cream, does it emerge from the workings of other little bits deeper inside it that somehow weave themselves together to make this experience we call space. That's what I was referring to earlier, where in that scenario, you're like pulling back a layer of reality understanding what's going on underneath. So you discover that your experience is not like fundamental, it's just sort of like one thing that the universe can do. It can make elephants, it can make ice cream, it can also not. So those are the kind of questions I want to know the answers to about space, like what can it do? Why do we have it? Is it itself made us something smaller? Or is it fundamental? Is it a requirement for the universe.

00:08:03
Speaker 2: So one of the things that I was worried about when we started talking about this question was the word space has been used by so many other fields that now it's not confusing from a physics perspective, it's just confusing because humans use that word for lots of different things. But I think I was definitely proved wrong there because when you asked the audience to tell us what they thought space was, nobody was like, well, space is the literature on space, and like everyone got it. I guess they know us well enough to know what we were probably talking about. So let's hear what the listeners had to say.

00:08:35
Speaker 1: That's right, And if you would like to participate in the audience participation segments of this podcast, please write to us to questions at Danielankelly dot org. We will set you up and you can hear your voice speculating basistly on the podcast. Here's a bunch of people answering the question what is space.

00:08:53
Speaker 4: I don't think that space is a real thing.

00:08:54
Speaker 1: I think that space is just an abstraction.

00:08:56
Speaker 5: Perhaps space is a medium in which fields can exist, that has shape and perhaps has density and can change in its form. Space is like an invisible lattice, geometric framework.

00:09:17
Speaker 1: Or other diffuse concept that is just a name we give our experience.

00:09:22
Speaker 2: The composite of all energy fields, gravitational fields, and dimensions.

00:09:28
Speaker 4: The medium that we are traveling through. It's like combining the trajectory of the Earth of some galaxy.

00:09:35
Speaker 1: Universe. Space is that which matter can move within.

00:09:41
Speaker 3: Space is the background fabric of the entire universe.

00:09:45
Speaker 4: I think to think of space as a huge collection of spots that can have multiple states of excitement, and the excitement of a spot interferes with its neighbors.

00:09:55
Speaker 1: The replacement or nothing.

00:09:58
Speaker 4: A physical meta that we can experience and move around and in which events occur that we can observe and try to understand.

00:10:08
Speaker 3: Space is this kind of thing in which everything happens, but we can't see or feel it. We can only see the evidence of it.

00:10:20
Speaker 1: I think this is outdated now, but I still just think of spice as a volume that we can put stuff into.

00:10:25
Speaker 4: I think space used to be seen as like a substrate where everything happened on or within, But I believe now space is kind of understood to be the thing that is happening.

00:10:36
Speaker 2: So none of the people who answered the question were like Daniel, I hate you, or like Daniel go away, So it seems like everybody was, you know, enjoying thinking it through and giving you an answer, and we got some pretty good answers. So maybe the world is not filled with angry young versions of Kelly, which is great, would be better. I was also like a goth chick covered in black all the time. I've cheered up a bit too.

00:10:59
Speaker 1: I want to see when those pictures at some point.

00:11:01
Speaker 2: All right, I'll share one with you, but not with everyone else. So, yeah, what did you think of these answers?

00:11:06
Speaker 4: Yeah?

00:11:06
Speaker 1: I thought it was good. And I also love hearing people grapple with a hard question.

00:11:10
Speaker 3: You know.

00:11:10
Speaker 1: It's the kind of thing, as you say, everybody's got some intuition about what space is because we live in it, right, and yet it's difficult to say, like exactly what is it and where it comes from a lot of people described what it can do, right, or it can hold, you can have things in space. So I think those are all fine ways to approach this problem. But I hope by the end of the podcast we give people a really comprehensive view of like what physics says about space and all the different, confusing, contradictory things the physics says about what space might be.

00:11:39
Speaker 2: All right, So then what do you think is the best place to start. So to me, I'm just like I don't know space. It's like there's like stuff in front of me. But if my table is there, it still counts as space because my table's just like in the space. And to be honest, that I think is most everything I've thought about this question. So where do we go from here?

00:12:00
Speaker 1: Maybe the best place to start is to try to answer, like what is we're asking about? You know, when we say space, what do we even mean by it before we talk about like where it comes from and how it works and what the rules are and what physics has learned, Like what is the thing we're asking about? You know, let's at least pinpoint the elephant here. And you know, in my mind, space is not about the stuff in the universe, It's about what's underneath it. It's about the underlying fabric. So like, take a chunk of universe wherever it is, and remove everything you can remove, so it maybe there's a peanut in the universe, toss that out, Maybe there's a planet there, toss that out, Maybe there's a galaxy whatever, pushed out all to the side. Empty it as much as possible, right, because the thing we're not talking about is like particles and matter and photons and stuff. Let's talk about what's underneath it. That's really what's exciting to me about this question is that we're like digging under the carpet of the universe, right, And so to me, space is what's left when you remove everything that you could remove from a portion of the universe.

00:13:00
Speaker 2: So, like in my office right now, and if we were to try to figure out what space is in terms of my office, we'd turn it into a vacuum. So I should leave my office if we're going to do this experiment, like we take everything out, and now we're asking what are we left with?

00:13:15
Speaker 1: Yeah, well we'd have to take you out because you're not space right your stuff? Yeah, so yeah, remove all the stuff what is left? And that opens up immediately like, well is there anything left? Does it mean anything to have space there without stuff in it? Is space just defined to be the place between stuff? Or is it a thing itself? Right? Is space a kind of stuff? Right? I Mean, I know we're getting like really banana pels behind the gym over here, but these are the questions we're grappling with, and to me, this is what's exciting about physics is that these questions are really fuzzy, and yeah, you could you know, smoke banana peels and talk about them all afternoon and really make no progress, or for thousands of years and make no progress. But physics gives you a way forward. Physics gives you this method to like understand your intuitive experience and make it make sense by asking, like, can we build a model that describes what he can do? And then can we look at that model and say, like, what does that mean about what it is? So to me, the reason I'm a physicist and not a philosopher is that we can't actually make some progress if we think about it like mathematically and systematically.

00:14:17
Speaker 2: Okay, so we've now gotten to like you've removed everything, so you've got a vacuum, and now I feel like there's a vacuum in my brain and I'm like, well, where do you go from there? How does physics tackle this question?

00:14:27
Speaker 3: Then?

00:14:27
Speaker 1: Physics thinks about space in terms of location and motion, because what do you have left once you've emptied your office? There's just space there, which means the possibility to put something in it. Right, you can put a proton in it, But The interesting thing is you can choose where to put the proton. You can put the proton where Kelly's desk used to be, or you could put the proton where Kelly's head used to be. Those in principle are different, right, And so space offers us these choices. You can be here, you can be there. Space seems to have inherent in it this like location, right, and and those locations can change. So, like, very very early on, before we were doing science, the way we think about it science, you know, the Greeks, they were thinking about space in terms of change, like motion and change. So the way physics begins attacking this problem is like why are there locations? And what are the rules about locations? Like how do things go from here to there? And why is here different from there? And can you tell the difference between here and there?

00:15:24
Speaker 2: In my head? Right now, I've got like a three dimensional graph, and you identify space as like a point on that graph, and does it stay there forever? Or does space move? Or have you just jumped to a different point on the graph?

00:15:40
Speaker 1: Yeah? Right, great questions right, like are we moving relative to space? Can you measure our motion relative to space? Or can we only measure our motion relative to like other things in space. Right, that's an early basic question, and this is the kind of thing Aristotle was thinking about. Aristotle was like, well, you know, why do things move move at all? Wasn't everything just like stay the same place you put a proton? Wasn't it just stay there forever? And of course he was working on the surface of the Earth, and so he noticed like, hey, things fall down, right, Why do things fall down? Why do things seem to move through this space? And so you see that, like very early on, the questions of space and motion were tied together. And you know, Aristotle didn't have like a mathematical picture of how the universe worked or how anything happened. He was sort of like words based, you know, it was like vibes based signs. And he just basically said, look, things fall down because things move according to their nature. Matter tends to fall down. That's just it's sort of like a descriptive. It's not really explanatory. He's just like, stuff falls down because it's in the nature of things to fall down. Such a circular answer. I don't even know why it was ever satisfactory.

00:16:46
Speaker 2: So we were talking about the absence of stuff, and now we're talking about the movement of stuff, and so the connection to space is that space is standing still while the stuff is moving, or just that this is the first time people have thought about the relationship between space and stuff.

00:17:03
Speaker 1: Yeah, I think all of that early on people trying to figure out what space is by understanding how things move through space, like what does it mean to go from here to there? What does it mean to fall down? And why do things fall down? Anyway, and it gives you a handle like what does speed mean? But you know, Aristotle's view of what this meant was basically the way people thought about it for thousands of years until around Galileo. And Galileo was the first person to think like, well, what do you really mean? Aristotle like, what are you talking about things fall down? Because down seems to kind of depend on who you are. And he had this famous thought experiment way before Einstein was thinking about stuff. He was like, say you're on a boat and you're inside the boat, so you can't see the outside. You're like below decks, and you drop a ball. What's going to happen? Well, Aristotle says, the ball's going to fall down. Anybody who dropped. Anything on a boat knows the ball falls down. Cool, But now what happens if there's somebody on the ground and they're watching your experiment somehow does the ball fall down according to the person on the boat, which means it's then moving with the boat, or does the ball fall down according to the person on the ground, in which case it would be left behind. Right, there's actually different predictions there. If the ball falls down according to the person on the dock, it falls like sort of straight down, then as the boat keeps moving, the ball gets left behind and the person on the boat should see the ball like weirdly fall backwards. Whereas if the ball falls down according to the person on the boat right, then it falls down for them, but the person on the dock sees it moving forward with the boat, So you can't have it fall straight down for both people. This is what Galleo realized.

00:18:41
Speaker 2: Okay, so my brain is now again trying to so it feels like we're talking about stuff, but we're not talking about space. And so the connection is the stuff is moving through space, and by understanding the movement of the stuff, we can understand the space better.

00:18:58
Speaker 1: Exactly because galle is experiment helps us think about what speed means. Right, what is velocity? Are you moving relative to space? Are you only moving relative to other things? And Galileo's experiment and what we call Galleyan relativity is the velocity is just relative. You're not moving relative to space. Space is not something like grid that fills the universe and you could just move through space relative to that grid. You can only measure your velocity relative to other stuff because, like you're in the boat, there's no experiment that you can do to measure your velocity relative to the ground. Right, you can drop the ball, but the motion of the ball doesn't depend on your speed relative to the ground, So it doesn't tell you how fast you're going. You could be standing still, you could be going super fast. You can't tell the difference. And you're asking like, well, okay, but aren't we supposed to be talking about space? And what this tells us is about motion through space, and it tells us something really deep and important that velocity is only relative to other stuff. Space is not a thing you can have a speed relative to. So Aristotles think, you know, the universe is filled with space, and stuff moves through space. According to it's like natural tendency. Galleo's like, nu uh uh, space isn't a thing. You actually can just move relative to other stuff in space. So that's already like a big clue about what space might be.

00:20:16
Speaker 2: Okay, So is it fair to summarize by being like Galileo would say, there is no space. There's no grid in the world, so no space. I'm guessing other people have come to other conclusions. Otherwise that would be a short episode. And so let's take a break and when we come back, we'll find out if space exists. All right, So Galileo telling us space doesn't exist, who are we going to talk about next.

00:21:00
Speaker 1: I love the way you say that because I imagine like you saying that to Galleo, and he would probably not agree with your phrasing of it, though I agree, you know, I think that what Galileo tells us is that maybe space exists, but it's just the distance between objects, right, So, like you can have space, it's just not absolute, like it can't exist without stuff in it. The thought experiment we did, we were like, take everything out of Kelly's office. It would be like, well, there's nothing in there. Space is not a thing. If there's nothing in there. If you evacuated the whole universe, right, got all rid of all the particles and all the energy and everything, then there would be no space. I think that's what Galileo would say, not the space doesn't exist, but that it only exists between stuff, not on its own. It's not like a thing on its own.

00:21:40
Speaker 2: So like if you had a vacuum and you cleared out everything inside the vacuum, nothing exists in there, or like you now have a space of non existence.

00:21:48
Speaker 1: Yeah, I think that's Galileo's view. But this was very confusing to people, and folks like Newton who spent a lot of time thinking about motion. You know, he developed physics basically and calculus, and he thought carefully about the mathematical flow of things, and he unified our understanding of gravity on the ground and in the sky. He agreed with Galileo about how velocity works, right, obviously, he wrote down the equations. You know, he was the first guy to really be able to predict these things. So he agreed the velocity is relative, but he fundamentally disagreed about the nature of space. He was like, no, space is absolute, it exists in and of itself and you can have a velocity relative to it. So Galileo's like, velocity is purely relative, and Newton is like, I agree with you about the equations and that you can only measure your velocity relative to other stuff. But I still believe that space is a thing underlying everything.

00:22:39
Speaker 2: So does that suggest that the things that they were measuring are not important for understanding space because they could get the same information and come to different conclusions.

00:22:49
Speaker 1: I think it suggests that Newton was a little crazy, okay, because his conclusion is not supported. Like, the problem with Newton's idea that maybe space is absolute is that you still can't ever measure your velocity relative to it. So he believed in this thing that existed that you couldn't ever measure, you know, Galleo says, velocity is purely relative. There's no like velocity relative to space itself, because space itself on its own doesn't exist, right, only exists relative to stuff. Newton is like, yeah, I agree with you about velocity. You can only measure with between things, but still unobservable, unknowable to us, there is this absolute space. Even though he could think of no way to measure speed relative to it. So Newton believed that you could be, for example, at rest with respect to space, and it was some like special frame of reference there. Gallet was like, no, no, that doesn't exist, and plus you can't measure it. So Newton is just sort of going out in a limb thinking that it exists even if you can't measure it.

00:23:45
Speaker 2: All right, Well, I think just about every scientist you talk to has blind spots where they're like, no, it's true, and I can't tell you why, but I'm sure that it's true. We're all human. So we've been saying the word relative a lot, which makes me feel like we're gonna have to get to Einstein eventually. Is he the next person on your list?

00:24:02
Speaker 1: He's definitely the next person on the list. And Einstein gets a lot of credit for relativity. You think of relativity, you think of Einstein. But the truth is that Einstein's relativity, his concepts of space really just go back to Galileo. It's Galileyan relativity that Einstein took and just sort of like said, hey, let's go back to this. This made a lot of sense to me. Let's just like build this in at the foundations because Einstein was thinking about light. Only a few decades earlier, Maxwell had figured out that light is a wiggle in the electromagnetic field. And the confusing thing about it was that Maxwell's equation said that light should travel at a specific speed, the speed of light, and that it shouldn't depend on your velocity at all. Like, no matter whether you're on the Earth that's going around the Sun, or you're in a spaceship or you're just floating in deep space, everybody should see light travel at the same speed, said Maxwell. And Einstein was like, hey, well that's cool. That's actually what Galileo was saying, right. Gallea was saying that you shouldn't be able to measure your velocity, that there's no absolute for life, the velocity is only relative. And so what Einstein did was apply Galleyan relativity to Maxwell's equations and say that means that everybody measures the speed of light to be the speed of light, regardless of what else you're doing. That's the foundation of Einstein's idea. But really it's taking Galileo's relativity and just being like, hey, let's take.

00:25:19
Speaker 2: This seriously, okay, And so all of that. Whenever you're talking about the history of something. I feel like I get it in my head. I'm like, oh, that makes sense that it's like, oh no, wait, but then that got overturned and you're like, oh, but I just understood it, okay, And so that so all of that we still believe in.

00:25:36
Speaker 1: Right, We wait until you understand something, Klay, and then we overturn it. That's that's the whole plan there.

00:25:41
Speaker 2: You know that I've suspected that for a long time. But the whole world is about me and my understanding of things. So light is always moving at the same speed, no matter who was viewing it. That is still something we believe.

00:25:53
Speaker 1: That is still something we believe. And that is the earthquake of Einstein's relativity. The concepts came from Galileo, but because he applied it to light and had all sorts of consequences. Also because Einstein connected space and time and we're going to talk about what time means in another episode and what it even is. But the fact that light always moves at the same speed for all observers connects space and time in these really unique ways. And the short version of the story is that it means that different people have different clocks. So like clocks tike at different speeds at different parts of the universe, and that's a direct consequence of how light moves through space, because everybody sees light move at the same speed no matter what. Then you can't have clocks that all agree all the way through the universe. And if you want to know more about how that works, check out the companion episode about what time is. We'll explain all of that.

00:26:41
Speaker 2: Now we've talked about the movement of light through space. What does that tell us about how we can define space?

00:26:48
Speaker 1: Yeah, so Einstein's relativity means something really important about what space is because it connects space and time, which tells us that time is relative also not just space. Time is relative and the same for everybody, and it has an important meaning for what distances are between things. It means that like I can measure the length of a ruler, and you can measure the length of a ruler, which is like the distance between two points in space, and we can get different answers and we can both be correct. This is something in special loyaltuty that goes by the name length traction, and basically it just means, hey, you assume that things have a length and have a length, and that length is their length, and it doesn't matter how fast you're going or where you are. Turns out that's not true. It turns out the distances between points what we even mean by space, depends on where you're looking at them from and how fast you're going relative to them.

00:27:39
Speaker 2: Okay, So if we want to try to understand space, does that mean we need to try to find ways to hold constant how fast you're moving and the distance between things, and then we can start to get a handle on space.

00:27:53
Speaker 1: Yeah, exactly, we want to talk about space. Space is about the distance between things. So now we have to think about, well, how do you measure the distance between things? And you know, you can be pretty pedantic about it. You can be like, well, I'm gonna hold up a ruler between two things. I'm going to measure where thing one is and thing two are at the same time, and I'm gonna say, well, the difference between the marks and the ruler, that's how far apart they are. You know, I have my left hand and my right hand. I put them on a ruler. There's ten centimeters long ticks between them, So I say they're ten centimeters apart right. And the crucial thing that I've done there is I've done it at the same time. I said, I'm gonna look at where my left hand is right now and where my right hand is right now, and measure their distances at the same time, and then I'm going to call that the distance between them. The problem is that Einstein's relativity and this whole speed of light business changes what we mean by at the same time because time is not universal anymore. So I might say I'm measuring where my left and my right hand are at the same time. But you might think, actually, Daniel, you messed up. You measured where your left hand was and then a second later where your right hand was, and if you're moving, then now the whole measurement is messed up. So connecting space and time changes how we think our clocks work, which also upends how we measure distances, and in the end, that's what space is about. Right We're talked about where a proton is and where another proton is and how far apart they are. Now it turns out, according to Einstein, we don't even agree about the distances. That's not even a fundamental thing about space. That everybody looks at a distance and agrees about what the distance is, like how many centimeters are there between the protons?

00:29:21
Speaker 2: Okay, so it sounds like we've decided that a definition of space has to include distance. Yeah, but people measure distances differently depending on their conditions. Is there a way to get around that or no, there's no way to get two people? I need ice cream?

00:29:40
Speaker 1: Yeah, exactly, there's no way around that. That turns out to just be a feature of space that we never noticed before because mostly we had basically no velocity relative to each other, mostly slow speeds on the surface of the Earth, and we didn't look at very long distances. So we have this intuition that things have a size, and that size is just what they are, and you should measure that size no matter who you are and how fast you're going. That just turns out to be wrong. Like sometimes people ask me, you know, why do things get shorter if you see them at high speeds. You know, you have a ruler stick flying by you at nine tenths of the speed of light, Why do you measure it to be less than a meter stick? If it was a meter when you're holding it and the answer is not that it's shrunk there. You're imposing your like intuition, you're prejudice that things have a size and that they have to shrink to get shorter. The answer is lengths depends on velocity. Like, that's just the way space works. You can't escape it, Kelly, is no way around it. Space just is different from the way our intuition work. And this is why it's so important to explore it like systematically and mathematically, because it contradicts our intuition and it leads us, we hope, at least, to some true insights about the nature of reality.

00:30:45
Speaker 2: So why can't we just say that depending on conditions, you get different distances, but the only thing you're differing in is how much space you're talking about. But you can still talk about space, like why do we have to be able to measure it to have a definition of it?

00:31:01
Speaker 1: Why do we have to measure it to have a definition of it? Wow, awesome question. You know. I think that's probably because the way we do science is we measure stuff, right, Like, you have to be able to take measurements to have data so you can talk about what that data means. Right, Otherwise, what are you doing. You just smoking banana peels and having conversations, which is fine.

00:31:22
Speaker 2: But so distance isn't the only way to measure things. Why is distance the measurement that we have to have in order to understand space?

00:31:31
Speaker 1: Oh yeah, I see a great question. Well, I think because we imagine that space is about locations, right, Even if you think about space as some three D grid, those are all locations, and so distances are differences between locations, right, And either those are relative, like the only thing that exists are distances between two points, two protons or two ends of the ruler, or there's some absolute grid, and you can measure your distance relative to space itself. But in the end, distance is the thing that space is describing.

00:32:02
Speaker 5: Right.

00:32:03
Speaker 1: If you don't have space, you can't have distance, right, And so that's sort of like the way we get handle on space.

00:32:09
Speaker 2: This whole podcast is about Kelly understanding herself better. It's like part Kelly Psychology, part what is space? And I think part of why I enjoy this conversation more as an adult is I'm way more comfortable now being like, this is probably a really not smart question, but I don't care. I'm asking it, and I think in its classes. I'd just be like, no, I can't say that, because that would be anyway. Maybe our listeners are going to write it and be like, you shouldn't ask those questions. They're not good questions, but anyway.

00:32:34
Speaker 1: Okay, they're perfect questions. They're perfect questions.

00:32:37
Speaker 2: Yes, okay, you have to have distance, but you cannot measure distance. Where do we go from here?

00:32:44
Speaker 1: You can measure distance, it's just that the distance is not the same for everybody. Right, that's two people measuring the same quantities can get different answers. So distance is not universal, but it can still exist, and then things get crazier. Everything we're talking about so far is just Einstein's view of how things move in sort of Newton's idea of space. But Einstein then introduced another concept. So now we go from special relativity to general relativity. He said, space can do even more. We can change the relative distances between things without those things moving. This is the idea of space itself curving. And there's a lot of descriptions out there in popular science about what it means for space to curve, and many of them are very misleading. You know, there's the famous one about the rubber sheet that most pop side folks go to. And I really discourage you from thinking about space in terms of a rubber sheet, because if you think about it carefully, it leads to all sorts of misunderstandings, you know. And so we're going to cover the rubber sheet analogy just to talk about why it's misleading. The general picture of the rubber sheet is like you stretch out this rubber sheet and then you put a bowling ball in it. The bowling ball bends the rubber sheet, and that's supposed to represent like how space is curving. The problem with that analogy is that it's showing you to D space the rubber sheet in some third dimension, like it's bending outside the universe itself. The universe is supposed to be two D, and the rubber sheet is bending into some other dimension, whereas in our space. The way Einstein thinks about space bending is it's intrinsic. There's no additional dimension. We're not bending our three D space into some fourth dimension like a rubber sheet bending into some new dimension we can't see. That's not what space curvature is. It's just changing the relative distances between things. So, like Kelly and I have a certain number of thousands of miles between us right now, what if while we both sit in our chair, you could just change that distance, so now it's a thousand miles, or now it's ten thousand miles. We just change the amount of space between us.

00:34:43
Speaker 2: When I think about the curvature of space, I feel like, but isn't there now space on either side of whatever just curved? But I guess that area is supposed to have no space, So there's places that have no space? Is that right?

00:34:56
Speaker 1: What do you mean on either side? You mean like are we getting pushed out in to other space or something?

00:35:01
Speaker 2: So I guess maybe my brain is still stuck on the rubber sheet. So you've got the sheet, it like goes in where the bowling ball is. But in my head, there's space above where the bend is, Like you know, the sheet was there and now it's down, but there's still something where the sheet used to be, because there's still space there. But is that not how to think about it? Like when it bends, there's space and there's nothing, there's like the absence.

00:35:21
Speaker 1: Yeah, exactly. That's why the rubber sheet is so confusing because as soon as you dig into it. It leads to questions that don't have answers because the herb sheet is just not the way it works right. The two D examples are useful because they are easier to think about instead. Imagine like a map, right, so you have your favorite country, US, Argentina or whatever, and think about a bunch of cities on that map, and they all have distances between them, right, and old maps you could like look up how far it was between New York and La or between Seattle and Miami, right, and those are distances all right. Cool, And if you took a ruler to the map, you could like measure those things and the map would lay flat on a table and you could measure those distances with rule. Cool. Now, what if I came in and I had magic fingers, and I'm like, I'm just going to change the distance between LA and Seattle and I'm not going to change anything else, or I'm gonna make it longer. I'm gonna make it shorter that distance. Now, imagine like, does that map lie flat on the table anymore? If I play with enough cities, then a order accommodate having more or less space, I'm gonna end up with wrinkles in that map. There's gonna be no way to lay that map flat on the table. And that's essentially what's happening in space. You put a mass in space, and space shrinks, right. It changes the relative distances between stuff in a way that it's no longer conceptually flat in your mind, the way that a sheet of paper would no longer be flat if you just like magically change the distances between two points on that sheet of paper.

00:36:46
Speaker 2: All right, So I'm thinking about the map, and you shortened the distance between Seattle and LA. And maybe I'm taking everything too literally, Like has San Francisco disappeared? Are all the people in San Francisco shorter because everything has trunk? Or are you tunneling through the map to shorten the distance, or do we not know what space is doing? It could be like any of those things.

00:37:10
Speaker 1: We're not tunneling through San Francisco, and we're not killing anybody in San Francisco. I hope you all are safe out there. We're just changing the distance between Seattle and LA. We're just saying, hey, if you turn on a flashlight in La, how long would it take for that light to get to Seattle? Because the speed of light is constant. This is a good way to measure how much space there is. So we're saying, hey, these things are now closer. That's what it means to shrink space. And the idea of curvature is that it's local, right, Like you could make everything further apart and everything closer together and keep the map flat. But if you only squeeze two different cities to make them closer together or further apart, then now space has weird bends in it, right, in order to make light take a certain amount of time to go here and a certain amount of time to go there, and to make all the light times work out, space has to have weird curves and it can no longer be flat in the same way that like, if you magically made the time from La to Seattle five times as long, then there would have to be more space in there. You couldn't have a flat sheet of paper. You need to like add more space, more land, more road for you to drive on.

00:38:11
Speaker 2: We can move forward, but I have to admit I'm having a little trouble, Like, does that mean we have stretched out what already exists or made more? Are there new cities in between now or did we just take what existed and it's like taffy and wh'ere just it's like like all the people in between are like twice as wide.

00:38:29
Speaker 1: Yeah, okay, great question, And this is something we actually know the answer to because we've done it. Like when gravitational waves hit the Earth, they literally do this. They stretch space and they shrink space. Right, So this actually happens. And what happens when a gravitational wave passes through the Earth, this is a wave in space itself again, which just means you're changing their relative distances. What happens in reality is the stuff that's holding us together, the land, the electrochemical bond, keeps us at the same distance. And so, for example, simplify things and say you and I were out floating in space and we were holding a rod that was one kilometer long, each holding one end, and then a gravitational wave comes by and it stretches space. Well, you and I are holding the rod, and the rod has bonds, and so it's going to keep us at exactly one kilometer apart, even though space is stretching around us. So if we show in a flashlight, we would still measure one kilometer. But if we weren't holding onto the rod, right, there was nothing keeping us at one kilometer. Then when space wiggles around us, we would use our flashlights and we would measure more than one kilometer distance because it would take light longer. So your question is, like, is it making more space or is it stretching the space that's already there. Nobody knows the answer to that question because we don't really know what space is. Like, what is happening you're asking, like the underlying mechanisms of all this, Yeah, we have no idea what's really going on there. We just have this mathematical theory that tells us how to do these calculations. We don't know what's underlying going on.

00:39:59
Speaker 2: But the car the way we understand these things, can it be used to like predict and understand things we've actually seen. This isn't just smoking banana peels. We tested these things. Our understanding is.

00:40:13
Speaker 1: Useful, absolutely, And we can measure this curvature, like if you shine light through this space, you can tell is space curved or is it not? Like if you shine two parallel lines through space that's not curved. They should never touch two photon beams through flat space should never touch if they're in parallel, but in curved space they will either cross or they'll split apart. Right, And we have measured this, Like the famous proof of relativity from Einstein was seeing light bend around the moon during the eclipse, right, that was seeing light move through curved space and bending, And so this is definitely something we've seen. We have very accurate models, and general relativity is amazingly precise. It predicts all sorts of things that nobody else could predict. So it tells us what space does because not only is space curved and light moves through it in curved ways, but like other stuff moves through it in curved ways. You have the Earth moving around the Sun because the Sun has curved space around it. So it's not just smoking banana peels. It works really really really well. But you know, then we can ask like, well, all right, so general relativity tells us that, like if you have mass in space, it curves it, and that not only our distance is relative, but like distances between stuff can change even without that stuff moving. So like what does that mean about what space is? Right?

00:41:32
Speaker 2: And let's dig into that question after we go all grab some more banana peels. Okay, so you just finished saying that the distance between two objects can change even if both of the objects like perceive that they have stayed still. But what does that tell us about space?

00:42:04
Speaker 1: So we can measure our velocity relative to other stuff, and we can tell that space can sometimes expand and shrink between it, and we have these amazing calculations from general relativity, what does that tell us about what space actually is? You know? Well, back to the conversation with Galileo and with Newton. Einstein agrees that velocity is relative, right, you can only measure your velocity relative to other stuff. And it seems like he's saying that space is a thing because space can do stuff like it can wiggle, it can bend, it can expand. So we went from like Galleo saying, now space is just the distance between stuff, to Newton saying, no, space is a thing, even though you can't measure your velocity to it, to Einstein being like, well, space has interesting properties, so it's pretty hard to say it's not a thing. Right. The weirdest part about Einstein's space, though, is that you still can't measure your velocity relative to it, like, even if you think it's out there and it has curvature and has these properties, you can't measure your speed relative to just space. It's something Matt Strassler calls emotional, like there's no way to measure your speed relative to space. So in one hand, Einstein agrees with Galleo like you have velocities relative, but he also agrees with Newton like space is a thing. But then back to Galleo, He's like, actually, but you can't measure your speed relative to it, So like what is it?

00:43:22
Speaker 4: Man?

00:43:22
Speaker 2: Yeah, so is this something that like when we understand dark matter and dark energy, it could help us understand space. Are those like just completely different problems.

00:43:35
Speaker 1: They could be completely different problems or they could be connected. You never know what thread of investigation is it really going to help you, like figure out what's going on and where the next breakthrough is going to come from. What we do know is that general relativity is a great description of space and motion, but we don't know what's going on underneath it. Like people often ask me, you put mass in space and space bends. Why does it bend? What is the mechanism for bending it? What is doing the bending? And we don't know the answer to those questions. Remember, general relativity is a description of what we've seen, and we can look at it and be like, well, what does that mean about space? And it's not a final answer. And part of the reason we know it's not a final answer is that we have this completely separate idea about how the universe works and how space is that comes from the other branch of physics that we've been developing over the last one hundred years, which is quantum mechanics, and quantum mechanics tells us a completely different story about space, what it is and how it works.

00:44:28
Speaker 2: So quantum mechanics would not agree that space bends in the way that we've been talking about.

00:44:33
Speaker 1: Quantum mechanics has no answer to the question of what is bending light around the moon. Quantum mechanics can't explain space bending. Quantum mechanics can explain gravity at all. Quantum mechanics can explain electromagnetism, it can explain the weak force, it can explain particles that it can explain all the strange experiments we saw right one hundred years ago and the experiments we do at the particle collider, and it's an extraordinarily accurate description of everything basically particle related. But it's built on a different assumption about space. It thinks of space the way Newton did, just like the backdrop on which things play out in the universe. You know, general relativity is what we call background free. It's like space itself is just the distances between stuff, and quantum mechanics is like, no, there's a background there, Just lay space out, roll it out like AstroTurf, and then particles do their dance in that space. And so it starts from a very different place, and it can't explain how space bends. In fact, the space bends too much, quantum mechanics breaks down. We don't know how to do calculations for quantum mechanics if space is super bendy. And it also tells us something very different about empty space. Like the exercise we started out with where we said, take your office, remove Kelly, remove all of her books, remove all of her weird samples of parasites and other gross stuff I can see in the background, and all the air. What's left. Well, Einstein says there's nothing there, but quantum mechanics says that's not possible because quantum mechanics says, space in itself is filled with fields, Like what is it that light is moving through? Anyway? It's moving through the electromagnetic field. Well, you can't take that out of space. Quantum mechanics says, you can't like rip the field itself out. You can say, I'm going to take all the photons out, but the field itself is like the capacity for light to move through it. It's like a parking lot with no cars in it, right, the field itself is always there. And quantum mechanics says that we have the electromagnetic field and the electron field and the muon fil in all sorts of fields that they're a part of space itself. And not only that, but these fields can never be totally zero. You can never pull all the energy out of them. They have a minimum quantum fuzziness, which means that there's always energy in space. So quantum mechanics view of space is really different. It's like you have this absolute background on which you put these fields, and these fields are always buzzing even if you do your best job of pulling everything out of that space.

00:46:56
Speaker 2: So are these the two main theories for space? So there's no other like theories that physicists take seriously. I'm sure there's plenty of people who have additional theories, but there's no theories people take seriously.

00:47:08
Speaker 1: Yeah, we have narrated down to at least two ideas about space, both of which we're pretty sure are wrong. Hey, no, that's progress.

00:47:15
Speaker 2: That is No, that is progress. So does anybody have any promising experiments designed to follow up on this next or what comes next? We're at a stalemate.

00:47:25
Speaker 1: It seems we are at a stalemate. And one issue is that we don't know whether the quantum mechanics version of space or the general relativity version of space is correct because almost every experiment we can think of only involves one of them. Like we can do experiments to test general relativity, like photons bending around moons, but then quantum effects are irrelevant because quantum effects get averaged out when you have something as big as a moon, Or we can think of quantum mechanical experiments we have like one particle bouncing off another particle, but then gravity is irrelevant because the gravity of a particle is basically zero because gravity is super duper weak. So the only place you could do a test to say, like, well, whose idea of space is correct? Quantum mechanics or gravity are experiments that are particle sized but have the masses of moons. And so now we're talking about black holes, and so the answer to like what space really is and how does it all work? Is hiding behind the event horizons of black holes, like what's in there? General relativity says it's a singularity. Quantum mechanics says that's nonsense. They can't both be right. They could both be wrong. So, yeah, the answer to your question is build a spaceship, fly into a black hole, get the answer, but never be able to tell anybody about it because you're chopped forever.

00:48:35
Speaker 2: Is there any reason to hope that we'll be able to get the answer? Someday? Will our children be having the same conversation?

00:48:43
Speaker 1: I think we probably will figure it out. There are other ways to explore this, like the hearts of neutron stars are not quite dense enough to become black holes, but they are dense enough, or gravity and some quantum mechanical effects are both important, So by studying the insides of neutron stars we might be able to get a clue. But there's also just a lot of like thinking that we need to do. You know, it's not like we have a great theory that predicts what's going to happen that we need to go test. So we have some more thinking to do about like how to bring these things together, and there are definitely people working on it, you know. String theory is one effort to try to describe things that incorporates gravity and quantum mechanics. This other approaches loop quantum gravity. One of my favorite ideas is that space itself is made of chunks, right, like little pixels of space, and that what's happening when space increases is that you're like adding more pixels, and that when space shrinks is that you're decreasing these pixels. The cool thing about this is that it gives you a way to think about a universe without space. Like imagine a whole bunch of pixels in the beginning of the universe, and these pixels are not tied together in any way. It's like a pile of beads before you do your project. Then somebody comes along and they weave all these beads together into a sheet, right, or like a three D grid. That's what space is there's a bunch of these pixels woven together with maybe quantum forces or something into this three D grid that we live in and experience. But you could also imagine that there was a time before that happened, when space was like disorganized, where you couldn't let go from one to the other because they weren't connected the way they are now. So thinking about the nature of space and thinking about how to bring these ideas together is maybe a fruitful way to make progress because it forces you to think, like, well, what does this mean, and what else could it be? And could you have a universe without space? All this kind of stuff.

00:50:29
Speaker 2: So for the pixel theory, like when you get more pixels or lose pixels, where do they come from? And where do they go?

00:50:35
Speaker 1: Yeah? Great question, And that question assumes that they have to come from somewhere, right you imagining like things like energy in the universe are concerned. Right, you can't just like pop new pixels out of nothing. But you know, we don't actually know that. We don't know that energy has to be conserved in the universe. We know that, for example, when the universe expands, photons get red shifted, they get stretched to longer wavelengths. That means they lose energy. Where's that energy go? Nowhere? It doesn't have to go anywhere, because maybe energy itself is not conserved in the universe. So these are great questions because it might be that they are the wrong questions, and the contradictions that come up when we ask them lead us to asking the right questions, which we don't know what those questions are yet. But it's sort of like, you know, knowing the answer is forty two and then going back and realizing, hmm, maybe we asked the wrong question, or maybe we're thinking about this whole thing wrong, you know. I think the takeaway message for listeners is like, what is space? Well, we don't know. We have two really nice descriptions of what space might be, both of which work in different scenarios, but both of which raise a lot of questions and they don't agree with each other, and so we really just don't know what space is. Even though we've made a lot of progress and we can build iPhones and launch rockets to Mars and all sorts of stuff. We can move through space, we can manipulate space, doesn't mean we yet know what it is, and it might be a century or a thousand years before we really figure it out.

00:51:57
Speaker 2: Well, that's exciting. I'm going to try to convince my daughter to become a physicist. She's been wearing her Sern outfits in her Stern hard hat since visiting CERN, so maybe she's on the path.

00:52:07
Speaker 1: I think about space the way I think about like a photon. People are often told, like a photon is a particle. No, it's a wave. Or sometimes it's a particle, sometimes it's a wave. The way I think about it is like a photon. It's neither a particle or a wave. Sometimes it's particle like, sometimes it's a wave like. It's something else we haven't yet figured out. And the same is true of space. We can describe space sometimes using general relativity, we can describe it sometimes using quantum mechanics. But space is probably something else. We've never even imagined, something beyond yet our current thinking that maybe one of our listeners is smart enough to.

00:52:38
Speaker 2: Figure out that would be awesome.

00:52:40
Speaker 1: All right, Well, thanks for going on this journey with us into the philosophical underpinnings of physics. I hope I've convinced you that physics is a way to think about these big, deep questions without getting lost in the meaning of the words, because it lets us be mathematical to try to be precise, and then to ask philosophical questions about those mathematical models, be like, what does it mean that I can calculate this, but I can't measure that? What does it tell us about the nature of the universe? Even when those answers are or we really.

00:53:06
Speaker 2: Just have no clue, my brain today hurt in a good way. I really enjoyed thinking about this today. It was nice to have us someone to ask silly questions to well.

00:53:16
Speaker 1: Thank you for all the silly and wonderful questions, and thanks everybody else out there for thinking about the nature of the universe. If you have questions about how things work, don't be shy. Write to us if you have questions at Daniel and Kelly dot org.

00:53:34
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00:53:40
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