We're All Being Pulled Together

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1:24Hello and welcome to The Rest is Science. I'm Michael Stevens. And I'm Hannah Fry. And today I'm asking you, Hannah, when was the last time you fell over? I mean, I fell over quite a lot, I've got to be honest with you. I think my limbs are longer than my brain expects them to be. So I get myself in all kinds of trouble. I think probably the most notable time was I was walking home quite late at night and there was a big queue for a nightclub. And at that moment, a really interesting car drove past. I was so busy looking at the car that I walked into a lamppost and then fell over backwards. And everyone in the queue saw me do it.

1:58that was not a good day. You're a lucky duck because that means you got to experience weightlessness. On the way down. Is that how you should think about it every time you fall over? That's how you should think about it. You lucky duck. That's how you should think about gravity, which is what I want to talk about. Gravity is one of those things that we all experience constantly, can't get away from it. And yet, what the heck is it? What the heck is it indeed? And we've got some answers, but not all the answers, as you will discover. Yeah. So we're going to talk about the nature of that mystery.

2:28And I want to start with a challenge that I gave you a bit earlier, which I've been working on too, which is how would you describe gravity to an alien from another universe that had never experienced gravity? Okay. Let me ask some questions about this alien. Does the alien have a physical body? Yes. And I think, by the way, these questions are very important. They kind of tease out what gravity is. What do you even need to know to experience and understand it? So yeah, let's say that the alien has a physical body. It has a concept of time, very similar to ours. It lives in a world where seconds are pretty fast and years are pretty long.

3:06It understands motion. Okay. It understands position and acceleration. Does it have three dimensions? Yes. It lives in three spatial dimensions. It also lives in one temporal dimension. But for some reason, its universe has no gravity. Okay. So it needs you to explain what it is. Okay, well, then I think that the simplest way to think of it is that in our universe, objects are attracted to each other. And if you, without any interfering from outside, if you just have two objects near each other, they will come together. That's it. I mean, that's it, really. And at this point, the alien goes, what?

3:46That is so odd. Right. And what do you mean by an object? Anything with mass. Anything with mass. Because I think that we sort of imagine gravity as though it's like the Earth is pulling us down. But the thing is, is that we're also pulling the Earth up, right? And if you get much smaller objects than planets and you put them in space, they're pulling each other and will come together. That's right. Yeah. I once calculated the two baseballs placed in intergalactic space, a meter apart, would very slowly collapse in towards each other until they touched. It would take three days for that to happen, but it would be because of their gravitational attraction to each other.

4:26We are gravitationally attracted to each other right now. It cannot overcome the air it would have to push out of the way, the friction between our butts and the seats. But yet we are attracted. In fact, when you're born, right, you've got some zodiac constellation that's like, I don't know. How does astrology work? Something, something, something, Pisces. Right. Okay. So, okay. You're a Pisces if you're born in a particular time of the year, but yet the gravitational influence of Pisces on you is less than the gravitational influence of the doctor who delivered you on you. Because otherwise birth ain't working.

5:07That's why, yeah. People are like, oh, so you're an Aquarius. And I'm like, no, I'm a schnitt cookie because Dr. schnitt cookie was there influencing me at a physical level. Yeah. Not just the catchy, not just the physical touch, but the gravitational attraction to his mass. Right. It's been with me my whole life. Where is he now? Now I want to be this alien again. And I want to say, all right, so objects with mass enjoy each other's company. They come together. They do. What's mass? Oh, no. Now we're getting into trouble. Because, I mean, this very quickly gets into nasty quantum physics territories.

5:44It does. And so let me just posit some possible answers. One is that, look, when I say mass in this context, I just mean the amount of matter something is made of. Yeah. Which in our universe does a couple of things. It affects how attractive and attracting it is to other objects, but it also affects how hard it is to change the motion of the object. and these might be different things. Gravitational mass and what we call inertial mass. Yeah. An object that's really hard to speed up and push around has a lot of inertia. Like resistance to motion. Resistance to motion change. Yes, absolutely. And gravitational attraction, they're both related to mass.

6:35They correlate with it. But is there still debate around whether they're the same thing? One thing I would say that those ideas of inertial mass, that objects resist changes in motion, I like to think that's sort of how I live my life. You know, like when you're in bed in the morning, it's like I'm happy staying in bed. And then I don't like changing. So I don't like getting up. Once I'm working, right, I don't like stopping working. Once I'm awake, I don't like going to bed. It's like I don't like the change. That's right. And to change what you're doing, something kind of forceful is required.

7:09that's the vocabulary that physics uses. If something's motion changes, a force has acted on it. Look, Newton was just a lazy guy. So I want to take a step back and say we've been talking a lot about very like fundamental things in this really abstract way to just explain that things fall down because here on earth, they're attracted to the earth. And you were talking about how it's not just the earth pulling things in. Things pull the earth as well, but the earth is so much bigger than everything else we work with, that equal attraction they have affects other stuff, like a pen, a lot more than it does the earth.

7:49But I once calculated that if you dropped a pen from six feet up, it actually pulls the earth up towards it, nine trillionths the width of a proton. Oh, which is, by my calculation, small. It's very small. So the pen falls the remainder of that distance, which is still pretty much six feet. But they are coming to meet each other. But they're coming to meet each other somewhere in between. Yeah. It just happens to be a much longer trip for the pen. And there you've got both of those senses of mass happening together. The gravitational attraction, but then also that force moves each object with very different accelerations.

8:30I mean, that pen though is particularly light. If you take an object that is heavier, denser, I mean, heavier, actually, there's sort of an implication of gravity in that statement itself, right? But if you take something that has more matter, the amount that the earth would move would change too. That's right. That's right. And so when people say a feather and a hammer dropped in a vacuum, so there's no air to move out of the way, they will fall at the same rate. They'll hit the ground at the same time. I tell you what, why don't we just clear up the question of what is gravity according to what different people thought at different times?

9:09Because everything you're describing so far is essentially like a Newtonian view of gravity. So Newton has this idea that actually gravity is all about objects accelerating towards each other, right? You know, like forces, mass times acceleration was one of his laws. And he was saying that we are accelerating towards the earth, which is the reason why when you chuck an apple or any object, your baseball, if you like, when you chuck it, it accelerates towards the earth and follows this curved path. And everyone for, you know, many hundreds of years was like, that guy Newton, he's got it made. He's done it for us.

9:43That's perfect. But there were still some lingering questions, some little things that didn't quite make sense. So for instance, where is this, how is this force sort of acting, like let's say you took the sun and you had like a magic wand that made the sun disappear instantaneously, it would take eight, nine minutes for the light to hit us. But according to Newton's version of gravity, we would immediately stop accelerating towards the sun, which means that the earth should immediately spin off into the blackness of space. But that sort of doesn't really make any sense, right? Because isn't it that nothing can travel faster than the speed of light?

10:21So how can it be that we would feel the loss of the gravitational pull of the sun before the light switched out? Right. Yeah. And so we know for a fact today that gravity travels how fast? Speed of light. Speed of light. No faster. Well, it's the universal speed limit. Yeah. Certainly it's not instantaneous. Absolutely. Which means that if the sun suddenly vanished, we wouldn't know about it at all. But was that a problem for Newton? Newton? Newton, no. But as the time went on, people were like, there's something a bit fishy going on. There's something a bit weird. I'm not sure I like this. The other one that was a bit weird that people just couldn't quite work out is Mercury's orbit.

11:01The thing about Mercury, closest planet to the sun, it has this elliptical orbit. But that elliptical orbit is itself spinning around. It's affected by the other planets. So it doesn't trace out the same ellipse every single time it orbits the sun. That ellipse is moving around. It's called the perihillion of Mercury's orbit, which sort of makes sense, right? Helion meaning sun. And everyone was cool with that. Everyone was absolutely fine with that, that they knew that, you know, the orbit was going to change because of where different planets were. But when they ran the calculations, according to Newton's version of gravity, that it's essentially just objects accelerating towards each other, something was off, right?

11:41It was like the number of arc seconds of Mercury's orbit just didn't totally make sense. and for a long time, you know, the telescopes weren't that accurate. People were like, maybe we've just made a miscalculation. It's sort of a bit, I don't know. And this was for a long time. Long time. Hundreds of years. And then when Einstein came along and he was like, I think there's something else going on here. Einstein has this great intuition that it's not just that objects are magically accelerating towards each other, but that space-time itself has this curvature to it. So the sun, for instance, this giant gravitational force is literally bending and warping space-time between us and it.

12:22And so if you got a magical wand and you made the sun disappear immediately, there would be this ripple that was sent out from the absence of that sun. Imagine taking a bowling ball on a rubber sheet and then removing it. That rubber sheet is going to kind of bounce up and down and ripple as you remove the weight. And that that ripple would reach us at the speed of light. He had this great intuition, worked out all the calculations for it. And one of the very first things that he turned his equations to was the perhillion of Mercury's orbit to see if his new theory came up with a more accurate prediction than Newton's.

12:58And he absolutely nailed it. Nailed it. Level of precision. I mean, he said that he was happy for days after he looked at those calculations and was like, I've absolutely got it. I found the missing piece to the puzzle. So two things. First, that leap from there's a force acting on things, maybe it's mediated by some particle or whatever, to leap from there to actually maybe gravity is just a change in the shape of space-time is really gigantic. Gigantic. Because space-time is such a bizarrely abstract thing. It's the canvas that we are on. if we were two-dimensional this would be easier we could say you know two-dimensional creature could be painted onto this curtain and if I crumple the curtain up they're still stuck on it and they're going over all of these crinkles but they don't even know it I can bring them together and push them apart if it gets crumpled up or curved you're just going to follow along that curve you cannot leave it and so yeah Einstein is like but what if it's the shape of the canvas that we are on.

14:06Exactly. Even the shape of time and how quickly time runs for you. If we allow that to change, then Mercury's orbit makes sense. Exactly right. They're crumpling the curtain. That's a really nice way to do it. Yeah, I think you need analogies because we're just talking about things that are so outside of our normal day-to-day activities. Totally. We understand forces. We understand pushes and pulls. But to say that space and time themselves push and pull. It's kind of more like you're just in them. But here's the thing, right? The implications of this idea that space time is like a crumpled curtain.

14:45It means that across the surface of the earth, even the gravitational effects are slightly different. So I did some calculations, um, Boulder in Colorado, right? Um, which of course is like a very high altitude compared to Greenwich in London, where I am, the gravitational effect in Boulder is 9.796 meters per second. And what is it in Greenwich? 9.812. Wow. I've got higher gravitational effect than you. Yeah. So you are more attracted to the center of Earth than I am in Boulder. Yep. Because I'm further away. Yep. And the inverse square law says... Exactly. Further away, drops away. That gravitational effect diminishes.

15:27Except that what that means, given Einstein's version of gravity, is that the way that time changes in Boulder is different to the way that time changes in Greenwich. Because what gravity is doing is it's bending and warping space time. So what this means is that time travels slower in Greenwich than it does in Boulder. And the difference is about 5.6 microseconds a year. so uh what i will say is that you are aging faster than me i i am but relative to who is my proper time is going to feel very normal to me sure meaning the time that i'm i'm looking at i'm feeling i'm looking at my watch it all feels normal but if i could somehow look at your watch yeah i would notice that it was running behind uh-huh that's because you're closer to earth's center of mass.

16:21Gravity is stronger where you are. So time runs more slowly. I mean, this stuff is so mind-warping. It's mind-blowing. It's mind-warping. It's space-time warping, but it's also experimentally confirmed. Absolutely. And that's, I think, something that we should pay more attention to when talking about because it makes it seem more real. Like they've done these experiments. They've taken really accurate clocks and put them up at high altitudes and down at low altitudes and seen that this exactly happens, that time runs more slowly on the lower clock. But we should also be said, we're not talking about like stopwatches here, right?

16:59We're talking about like atomic clocks, which are, you know, like based on the most precise measurements that it's possible for humans to take with the technology that we have at the moment. And the calculations, they're not just like close, they are like dead on the nose. If Einstein had had these ideas in the 1200s, confirmation would have eluded them for a long time. I mean, the idea of 5.6 microseconds a year is like, you just don't really notice it. You wouldn't notice it. No. Yeah, he would have been seen as just a guy writing some cool fairy tales. Yeah. One thing that I do think is interesting, though, is that, as you said, right, this is the canvas that we're living on.

17:40It's not something that we're noticing. and actually they've done these different experiments where they've taken earthbound creatures, right, and taken them up into space, you know, not just humans, but to see how they behave and how they react. And fish in particular, something interesting that happens. If you take earth-born fish up into microgravity in space, you know, the ISS or whatever, they're completely disoriented. They sort of don't know which way is up. They're like really struggling to swim. They're just, you know, they don't have a good time. but they can have babies in space right and they have they have managed to breed them when the babies are born right these kind of little fish eggs hatch those babies absolutely fine totally chill about it interesting swim back normally do fish come into the world with as a blank slate ready to learn it and if they learn gravity then they can't forget it well i think that's the hypothesis right but they also did this with octopuses octopi um and the uh the octopuses that were born in space seemed like they were fine.

18:42And then when they came back down, they really struggled. They struggled on Earth. Yeah. I feel bad for those. I know. And now here's like where you're supposed to be living and there's gravity and it just feels like an oppressive force squishing you all the time. Exactly. You're welcome. It would be so much worse that way around, wouldn't it? Right? If you go, I think I would rather be born in high gravity and move to low gravity than the other way around. These poor like water animals that we're subjecting this to, It's harder for them. For us, we need that gravitational force for our bones to develop properly.

19:15A human baby born in space in the so-called zero-G weightless environment, their bones would not grow properly. They would look like they had rickets. I did a video about this, and it's very gruesome what would happen if you tried to grow a human with no gravitational tension on their bones. I think we tried this with rats as well, birthing rats in space. And yeah, they didn't do too well in space. They really need that landlubber gravity. In order to form properly. Yeah. Yeah, I mean, some science experiments aren't that nice, are they? No, they were all okay, by the way. This was just like a minor inconvenience.

19:56And now they've got a good story to tell their friends. It's how I choose to look at it. It was worth it for the anecdote. Yeah, that's right. Yeah. These visualizations of warping of space-time, though, I mean, should we talk about those a little bit? Because I think there's some that are quite common. You see them around a lot, right? Like of, I mean, I sort of even use for myself there, the stretched rubber sheet with Earth as a bowling ball sitting in the middle of it. I mean, they're kind of useful in a way, but they're also quite limited because we ourselves are not capable of comprehending these extra dimensions.

20:29Yeah. I've never really liked the rubber sheet analogy because it doesn't seem to do anything but just show what you already feel, which is that gravity makes things fall. They rely on gravity to demonstrate gravity. And also they're shrunk down to just the two-dimensional plane of a sheet of rubber. We're talking, of course, of the demonstration where you've got a rubber sheet, you put something really heavy in the middle, and it sags. And then when you put like a little marble on it, it rolls down towards the big, heavy, massive object. And you say it's because of gravity. Of course it's because of gravity.

21:02That's why everything rolls down. Well, try and get a gravity-free situation on Earth in which to demonstrate. Yeah. And then also you can take a marble and you can throw it out tangentially towards the slope, and it'll orbit the heavy object, go around and around. But it loses energy so quickly. It always spirals into the thing. And I just feel like, especially for students, they go, well, but Earth isn't doing that. We aren't spiraling into the sun. So clearly I'm not learning anything about Earth and the sun. Yeah, that's where the analogies break down. The one that I do like, and I think this was Richard Feynman, was he was saying the reason why it's difficult for us to imagine additional dimensions.

21:42If you take a piece of paper and you imagine that you have an ant living on this piece of paper, And the ant is so small that it's effectively two-dimensional, right? It cannot conceive of up and down because even when it climbs up a wall, the curve between or the corner between, you know, the ground and the wall, it's so tiny that it's like it feels like it's just this continuous surface. But if you take an ant on a piece of paper and you say, this is point A, this is point B, and you ask the ant what's the quickest way between these two points, the ant's going to say, oh, it's a straight line between the two, right?

22:11Which is like, yeah, great, well done, ant. But you could take that piece of paper because we have an additional dimension, we have three dimensions, whereas the ant only has two, you can fold the piece of paper and make A and B combine in the same point, right? You can effectively create a wormhole that the ant didn't see coming. And this is sort of a way to explain why this stuff is so difficult for us to comprehend. Because once you bend that canvas, once you warp that curtain, I mean, everything sort of changes. The rules slightly go out the window. Yeah, and that analogy is great. But you can also see how hard it is to jump up to where we live.

22:48Yeah. Because I can imagine a curtain folding and things that live on the curtain not understanding. But how do I bend three-dimensional space? Not a flat curtain, but like a whole universe. Where am I bending it into? Some other dimension. Yeah. A fourth spatial dimension. A fourth spatial dimension. Have you ever tried to visualize a fourth spatial dimension? a fourth line that could be perpendicular to three others? I mean, you can't, right? You can only imagine slices of it. You can only imagine three-dimensional slices. You don't think it's possible. I mean, you can describe it mathematically.

23:25That's like very easy. I mean, for instance, you know, you could take like a square, right? Or a cube. I could tell you the corners of a cube are there's one at zero, zero, zero. There's one at one, one, one. There's one at one, zero, zero, and so on and so on. And then if I use the language of mathematics, then it's really easy, right? Because I can just say, well, a four-dimensional one, it has a corner at zero, zero, zero, zero. Yeah, you got four coordinates. Fantastic. I'm still not feeling it. You're never going to feel it. You're never going to see it. Why do you think I can't? I just don't think that your brain is capable.

23:56And that's not just you, Michael. This is how a lot of people count sheep when they go to bed. I try to imagine a way to look inside a hollow sphere by looking from a fourth spatial dimension. And I haven't succeeded yet. You surprise me. All right, well, I hope we've suitably warped your minds. Should we go to a break? Yeah, let's do it.

24:31All right, welcome back from the break. We've talked about Newton and Einstein's theories. Where are we now? Okay, so here's the thing. is that everyone was like, Einstein's such a genius. Everything's stitched up. It's all great. No, there's still so much more. There's still all of these big questions that it's like, why, it doesn't quite work. It doesn't quite work. So one of them, there was a cosmologist called Fritz Zwicky, one of my favourite cosmologists of all time. It's a good name. It's a great name. And also he was a proper curmudgeonly old git. He wasn't a very nice guy, right? And he had a saying where he would say of his colleagues that they were spherical bastards because they were bastards from whichever angle you looked at them.

25:15Very clever. I mean, he is a super smart guy. Yeah. Anyway, he was studying this galaxy, the Coma Cluster, and he was like, well, you know what? Something's going on with gravity here. Something's not quite right. Because these galaxies are spinning around so fast that if the only gravity that's present is the gravity that's coming from the stuff you can see, then these galaxies should be spewing stuff out right they should be ripped apart by the speed that they're spinning and so him and a number of other cosmologists were like okay something is not adding up in these equations there's something that's not that's not right here and so um rather than kind of going back and undoing einstein's versions of gravity or undoing newton's they were like well they look like they're right so maybe what's going on instead is that there's all of this other stuff that we just that we just can't see.

26:09We can't see. So it's like a matter that's like not reflecting light. Maybe we should just let's just call it dark matter. Add an extra term in our equations. Everything good. Everyone can go home. And that's the birth of dark matter. That's the birth of dark matter. And frankly, how dark matter has long remained. Wow. I didn't know that Zwicky was also part of the dark matter invention slash discovery. We know a lot more about dark matter today. We do. We do. We don't. Still haven't found it. what does it mean to find it? Like, what would it, I was going to ask what it looks like. It doesn't look like anything.