The Physics of Gravity

What is gravity, really? Why do objects pull towards each other at all? And if Einstein 'fixed' Newton’s theory, why does gravity remain one of science’s biggest unsolved mysteries? A clumsy trip into a lamppost leads Michael and Hannah into a whirlwind tour of our changing understanding of gravity, from falling apples and making wormholes for ants, to the puzzles we still can’t crack. Why does time tick faster on a mountaintop than by the sea? Why are galaxies spinning in ways our equations can’t explain? And could an invisible particle finally reveal what gravity is made of? Join Professor Hannah Fry and YouTube educator Michael Stevens as they uncover the strange, unfinished story of the force that holds the universe together. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠cancerresearchuk.org/restisscience⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠clicking here.⁠ ------------------- Video Producer: Adam Thornton Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Producer: Becki Hills Senior Producer: Lauren Armstrong-Carter Head of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 0:02Michael Stevens?

  Imagine this, inside all of us, billions of cells follow millions of instructions written in microscopic code. And when a new cell grows, it copies those instructions, but the smallest error can lead cancer to develop.

• 0:17Hannah Fry?

  Right, and this is the reason why there isn't a single cure for cancer, because, you know, there are more than 200 different types, each of them have got different distinct characteristics, you know, different challenges, different mysteries. And that means that trying to cure cancer isn't like following a single path, it's like trying to map out an entire forest.

• 0:34Michael Stevens?

  That's right. And Cancer Research UK is the world's largest charitable funder of cancer research. I mean, their work spans more than 20 countries, with over 4,000 scientists, doctors and nurses pushing knowledge forward to save and improve lives worldwide.

• 0:50Hannah Fry?

  You know, over the last 50 years, the work that this charity has done has helped to double cancer survival in the UK. And you have to think about, that is, that is more parents at the dinner table, right? That is more friends at their birthday parties, that is more people who are living longer, better lives.

• 1:08Michael Stevens?

  For more information about Cancer Research UK, their research, breakthroughs, and how you can support them, visit cancerresearchuk.org/rastoscience.

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• 2:54Michael Stevens?

  Hello and welcome to The Rest Is Science. I'm Michael Stevens.

• 2:57Hannah Fry?

  And I'm Hannah Fry.

• 2:58Michael Stevens?

  And today, I'm asking you, Hannah, when was the last time you fell over?

• 3:02Hannah Fry?

  I mean, I fall over quite a lot, I've got to be honest with you.

• 3:05Michael Stevens?

  Very good.

• 3:06Hannah Fry?

  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. It was not a good day.

• 3:30Michael Stevens?

  You're a lucky duck. Because that means you got to experience weightlessness.

• 3:34Hannah Fry?

  On the way down? Is that how you should think about it every time you fall over?

• 3:37Michael Stevens?

  That's how you should think about it.

• 3:38Hannah Fry?

  You lucky duck.

• 3:39Michael Stevens?

  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?

• 3:49Hannah Fry?

  What the heck is it, indeed? And we've got some answers, but not all the answers, as you will discover.

• 3:56Michael Stevens?

  Yeah, so we're going to talk about the nature of that mystery. And 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?

• 4:10Hannah Fry?

  Mm-hmm. Okay, let me ask some questions about this alien. Does the alien have a physical body?

• 4:16Michael Stevens?

  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?

• 4:24Hannah Fry?

  Yeah.

• 4:25Michael Stevens?

  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. It understands motion, okay? It understands position and acceleration.

• 4:41Hannah Fry?

  Does it have three dimensions?

• 4:43Michael Stevens?

  Yes, it lives in three spatial dimensions. It also lives in one temporal dimension. But for some reason, its universe has no gravity.

• 4:52Hannah Fry?

  Okay.

• 4:52Michael Stevens?

  And so it needs you to explain what it is.

• 4:54Hannah Fry?

  Okay, well, then I think, then I think that the, the, the simplest way to think of it is that in our universe, objects are attracted to each other. And if you, without any, 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.

• 5:13Michael Stevens?

  And at this point, the alien goes, what? That is so odd.

• 5:17Hannah Fry?

  Right.

• 5:17Michael Stevens?

  And what do you mean by an object?

• 5:19Hannah Fry?

  Anything with mass, anything with mass. Because I think that we, 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, uh, you, you get much smaller objects than planets and you put them in space, they're pulling each other and will come together.

• 5:39Michael Stevens?

  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. We 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 in the seats, but yet we are attracted. In fact, when you're born, right? You've got some, some zodiac constellation that's like, I don't know, it's, how, how does, how does astrology work?

• 6:19Hannah Fry?

  Something, something, something Pisces.

• 6:20Michael Stevens?

  Right. Okay, so, okay, you're a, 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.

• 6:35Hannah Fry?

  Cause otherwise, birth ain't working.

• 6:37Michael Stevens?

  That's why, yeah, people are like, oh, so you're an Aquarius? And I'm like, no, I'm a schnick cookie. Because Doctor Schnick Cookie was there influencing me.

• 6:45Hannah Fry?

  To catch you.

• 6:46Michael Stevens?

  At a physical level, yeah. Not just the catchy, not just the physical touch, but the gravitational attraction to his mass.

• 6:52Hannah Fry?

  Right.

• 6:52Michael Stevens?

  It's been with me my whole life.

• 6:55Hannah Fry?

  Where is he now?

• 6:56Michael Stevens?

  Now, I want to be this alien again, and I want to say, alright, so objects with mass enjoy each other's company. They come together.

• 7:03Hannah Fry?

  They do.

• 7:04Michael Stevens?

  What's mass?

• 7:05Hannah Fry?

  Oh, no, now we're getting into trouble, because, I mean, this very quickly gets into nasty quantum physics territories.

• 7:13Michael Stevens?

  It does, and so let me just posit some possible answers.

• 7:17Hannah Fry?

  Go on.

• 7:17Michael Stevens?

  One is that, look, when I say mass, in this context, I just mean the amount of matter something is made of.

• 7:24Hannah Fry?

  Yeah.

• 7:25Michael Stevens?

  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.

• 7:49Hannah Fry?

  Yeah.

• 7:50Michael Stevens?

  An object that's really hard to speed up and push around has a lot of inertia.

• 7:55Hannah Fry?

  Like resistance to motion.

• 7:56Michael Stevens?

  Resistance to motion change.

• 7:59Hannah Fry?

  Yes, absolutely.

• 8:00Michael Stevens?

  And gravitational attraction. They're both related to mass. They correlate with it. But are, is there still debate around whether they're the same thing?

• 8:11Hannah Fry?

  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.

• 8:34Michael Stevens?

  That's right. And to change what you're doing, something kind of forceful is required. That's the vocabulary that physics uses. If something, if something's motion changes, a force has acted on it.

• 8:46Hannah Fry?

  Look, Newton was just a lazy guy.

• 8:48Michael Stevens?

  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. But 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.

• 9:28Hannah Fry?

  Oh, which is by my calculation, small.

• 9:33Michael Stevens?

  It's very small. So the, the, the pen falls the remainder of that distance, which is still pretty much six feet.

• 9:39Hannah Fry?

  But they are coming to meet each other.

• 9:41Michael Stevens?

  But they're coming to meet each other somewhere in between.

• 9:45Hannah Fry?

  Yeah.

• 9:45Michael Stevens?

  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.

• 10:00Hannah Fry?

  I 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, of gravity in that, in that statement itself. Right. Uh, but, but if you take something that has, uh, more matter, the amount that the earth would move would change too.

• 10:18Michael Stevens?

  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.

• 10:31Hannah Fry?

  I tell you what, let, why don't we just clear up the, the question of what is gravity? According to what different people thought at different times.

• 10:39Michael Stevens?

  Yeah.

• 10:40Hannah Fry?

  Because, because 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, uh, forces, mass times acceleration was one of his, 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, he's got. Made, he's done it for us. That'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?

• 11:24Hannah Fry?

  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? So how can it be that we would feel the loss of the gravitational pull of the sun before the light switched out?

• 11:59Michael Stevens?

  Right. Yeah. And so we, we know for a fact today that gravity travels how fast?

• 12:04Hannah Fry?

  Speed of light.

• 12:05Michael Stevens?

  Speed of light, no faster.

• 12:06Hannah Fry?

  Well, it's, it's the, the universal speed limit.

• 12:10Michael Stevens?

  Yeah, certainly. Uh, it's not instantaneous.

• 12:13Hannah Fry?

  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, no, but as the time went on, people were like, there's something a bit fishy going on, something a bit weird. I'm not sure, 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. The 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, so the, the, it doesn't trace out the same ellipse every single time it orbits the sun. That ellipse is moving around.

• 12:48Michael Stevens?

  Yeah.

• 12:48Hannah Fry?

  It's called the perihelion of Mercury's orbit.

• 12:51Michael Stevens?

  Okay.

• 12:51Hannah Fry?

  Which sort of makes sense, right? Helion, helion meaning sun. And everyone was cool with that. Everyone was absolutely fine with that, that they knew that, you know, this, the, 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 acceler, objects accelerating towards each other. Something was off, right? It 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.

• 13:24Michael Stevens?

  And this was for a long time.

• 13:25Hannah Fry?

  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 had this great intuition that it's not just that objects are magically accelerating towards each other, but that spacetime itself has this curvature to it. So the sun, for instance, this giant gravitational force is literally bending and warping spacetime between us and it. And 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

• 14:10Hannah Fry?

  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 prohillion of Mercury's orbit to see if his new theory came up with a more accurate prediction than Newton's. And he absolutely nailed it.

• 14:31Michael Stevens?

  Nailed it.

• 14:32Hannah Fry?

  Level of precision. I mean, he said that he was happy for days after he looked at those calculations was like, I've absolutely got it. I found the missing piece to the puzzle.

• 14:40Michael Stevens?

  So two things. First, that leap from there's a force acting on things, maybe it's mediated by some particle or whatever, from to leap from there to actually, maybe gravity is just a change in the shape of spacetime is really gigantic.

• 15:01Hannah Fry?

  Gigantic.

• 15:02Michael Stevens?

  Because spacetime is such a bizarrely abstract thing. It's, it's, it's the canvas that we are on. If we were two dimensional, this would be easier. We could say, you know, a 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 gonna 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?

• 15:36Hannah Fry?

  Exactly.

• 15:37Michael Stevens?

  Even the shape of time and how quickly time runs for you. If we allow that to change, then Mercury's orbit makes sense.

• 15:45Hannah Fry?

  Exactly right. They're crumpling the curtain. That's a really nice way to do it.

• 15:49Michael Stevens?

  Yeah, I think you need analogies because we're just talking about things that are so outside of our normal day-to-day activities.

• 15:56Hannah Fry?

  Totally.

• 15:57Michael Stevens?

  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.

• 16:09Hannah Fry?

  But here's the thing, right? The implications of this idea that spacetime is like a crumpled curtain. It means that across the surface of the Earth even, the gravitational effects are slightly different. So I did some calculations. Boulder in Colorado, right? 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.

• 16:38Michael Stevens?

  And what is it in Greenwich?

• 16:40Hannah Fry?

  9.812.

• 16:42Michael Stevens?

  Wow.

• 16:42Hannah Fry?

  I've got higher gravitational effect than you.

• 16:44Michael Stevens?

  Yeah. So you are more attracted to the center of Earth than I am in Boulder.

• 16:48Hannah Fry?

  Yep.

• 16:49Michael Stevens?

  Because I'm further away?

• 16:50Hannah Fry?

  Yep.

• 16:51Michael Stevens?

  And the Inbersquare law says?

• 16:53Hannah Fry?

  Exactly. Further away, drops away.

• 16:55Michael Stevens?

  Yeah, diminishes.

• 16:56Hannah Fry?

  Except 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's doing is it's bending and warping spacetime. 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 what I will say is that you are aging faster than me.

• 17:29Michael Stevens?

  I I am, but relative to who? Because my proper time is gonna feel very normal to me.

• 17:35Hannah Fry?

  Sure.

• 17:35Michael Stevens?

  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.

• 17:44Hannah Fry?

  Yeah.

• 17:44Michael Stevens?

  I would notice that it was running behind.

• 17:47Hannah Fry?

  Uh huh.

• 17:48Michael Stevens?

  That's because you're closer to Earth's center of mass. Gravity is stronger where you are, so time runs more slowly.

• 17:55Hannah Fry?

  I mean, this stuff is so mind.

• 17:58Michael Stevens?

  It's mind blowing, it's mind warping, it's spacetime warping, but it's also experimentally confirmed.

• 18:04Hannah Fry?

  Absolutely.

• 18:05Michael Stevens?

  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 seeing that this exactly happens, that time runs more slowly on the lower clock.

• 18:26Hannah Fry?

  But what should also be said, we're not talking about like stopwatches here, right? We're talking about like atomic clocks.

• 18:32Michael Stevens?

  Yeah.

• 18:32Hannah Fry?

  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.

• 18:44Michael Stevens?

  If Einstein had had these ideas in the 1200s.

• 18:48Hannah Fry?

  Mm hmm.

• 18:49Michael Stevens?

  Confirmation would have eluded them for a long time.

• 18:54Hannah Fry?

  I mean, the idea of 5.6 microseconds a year is like, you just don't really notice it, do you?

• 18:59Michael Stevens?

  You wouldn't notice it.

• 19:00Hannah Fry?

  No.

• 19:00Michael Stevens?

  Yeah, he would have been seen as just a guy writing some cool fairy tales.

• 19:04Hannah Fry?

  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, it's not something that we're 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 to see how they behave and how they they react. And fish, in particular, something interesting that happens, if you take earthborn fish up into microgravity in space, you know, the ISS or whatever, they're completely disoriented, they 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

• 19:45Hannah Fry?

  breed them. When the babies are born, right, these kind of little fish eggs hatch, those babies, absolutely fine, totally chill about it.

• 19:53Michael Stevens?

  Interesting.

• 19:54Hannah Fry?

  Swim back normally.

• 19:55Michael Stevens?

  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.

• 20:01Hannah Fry?

  Well, I think that's the hypothesis, right? But they also did this with octopuses, octopi. And the the octopuses that were born in space seemed like they were fine. And then when they came back down, they really struggled. They struggled on earth. Yeah.

• 20:17Michael Stevens?

  I feel bad for those. And now here's like where you're supposed to be living and there's gravity. And it's just feels like an oppressive force squishing you all the time.

• 20:25Hannah Fry?

  Exactly.

• 20:26Michael Stevens?

  You're welcome.

• 20:27Hannah Fry?

  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.

• 20:34Michael Stevens?

  These poor like water animals that we're subjecting this to, it's harder for them. For for us, we need that gravitational force for our bones to develop properly. A human baby born in space in in the so called, you know, 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 it's very gruesome what would happen if you tried to grow a human in in 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 they they really need that land lover gravity.

• 21:16Hannah Fry?

  In order to form properly.

• 21:17Michael Stevens?

  Yeah.

• 21:18Hannah Fry?

  Yeah, I mean, some science experiments aren't that nice, are they?

• 21:21Michael Stevens?

  No, they were all OK, by the way. This was just like a a minor inconvenience and now they've got a good story to tell their friends.

• 21:29Michael Stevens?

  It's how I choose to look at it.

• 21:30Hannah Fry?

  It was worth it for the anecdote.

• 21:32Michael Stevens?

  Yeah, that's right.

• 21:33Hannah Fry?

  Yeah. These visualizations of warping of spacetime, 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 these extra dimensions.

• 21:59Michael Stevens?

  Yeah, I've never really liked the rubber sheet analogy because it doesn't. 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. That's why everything rolls down.

• 22:33Hannah Fry?

  Well, try and try and get a gravity free situation on earth in which to demonstrate.

• 22:37Michael Stevens?

  Yeah, and then also you can take a marble and you can you can throw it out tangentially towards towards the slope and it'll orbit the heavy object, go around and around. But it 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.

• 23:01Hannah Fry?

  Yeah, it's where the analogy is breakdown. The one I 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. If 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 or the corner between, you know, the ground and the wall is 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 gonna say, well, it's a straight line between the two, right? Which is like, yeah, great, well done, ant. But you could take that piece of paper and because we have an additional dimension,

• 23:46Hannah Fry?

  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 how 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.

• 24:12Michael Stevens?

  Yeah, and that analogy is great. But you can also see how hard it is to jump up to where we live. 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?

• 24:32Hannah Fry?

  Mm hmm.

• 24:33Michael Stevens?

  Some other dimension.

• 24:34Hannah Fry?

  Yeah.

• 24:35Michael Stevens?

  A fourth spatial.

• 24:36Hannah Fry?

  A fourth spatial dimension.

• 24:37Michael Stevens?

  Have you ever tried to visualize a fourth spatial dimension? A fourth line that could be perpendicular to three others?

• 24:46Hannah Fry?

  I mean, but you can't, right? You can only imagine slices of it. You can only imagine three dimensional slices.

• 24:52Michael Stevens?

  You don't think it's possible?

• 24:53Hannah Fry?

  I mean, you can describe it mathematically. That'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, four dimensional one, it has a corner at zero, zero, zero, zero.

• 25:16Michael Stevens?

  Yeah, you got four coordinates. Fantastic. I'm still not feeling it.

• 25:20Hannah Fry?

  You're never gonna feel it. You're never gonna see it.

• 25:23Michael Stevens?

  Why do you think I can't?

• 25:23Hannah Fry?

  I just don't think that your brain is capable. And that's not just you.

• 25:26Michael Stevens?

  This is, you know 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.

• 25:39Hannah Fry?

  You surprise me. All right, well, I hope we've suitably warped your minds. Should we go to a break?

• 25:45Michael Stevens?

  Yeah, let's do it.

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• 27:30Michael Stevens?

  All right, welcome back from the break. We've talked about Newton and Einstein's theories. Where are we now?

• 27:38Hannah Fry?

  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 favorite cosmologists of all time.

• 27:57Michael Stevens?

  It's a good name.

• 27:58Hannah Fry?

  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.

• 28:14Michael Stevens?

  Ah, very clever.

• 28:16Hannah Fry?

  I mean, he is a super smart guy.

• 28:17Michael Stevens?

  Yeah.

• 28:18Hannah Fry?

  Uh, anyway, he, uh, was studying this galaxy, the Coma Cluster. And, uh, he was like, well, you know what? Something's, 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 this, 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, uh, versions of gravity or undoing Newton's, they were like, well, they're, they, they, they look like they're right.

• 29:03Hannah Fry?

  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.

• 29:09Michael Stevens?

  We can't see.

• 29:09Hannah Fry?

  So it's like 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's good. Everyone can go home.

• 29:18Michael Stevens?

  And that's the birth of dark matter.

• 29:19Hannah Fry?

  That's the birth of dark matter and frankly, how dark matter has long remained.

• 29:23Michael Stevens?

  Wow. I didn't know that Zwicky was also part of the dark matter, uh, invention slash discovery. We know a lot more about dark matter today.

• 29:32Hannah Fry?

  We do, we do.

• 29:33Michael Stevens?

  We don't.

• 29:34Hannah Fry?

  Still haven't found it.

• 29:35Michael Stevens?

  What does it mean to find it? Like, what would it? I was gonna ask what it looks like. It doesn't look like anything. It doesn't interact with light. But could you have a jar full of it?

• 29:43Hannah Fry?

  Yeah, I mean, yeah. I mean, I'll be honest with you. Every jar you've ever had has probably got some dark matter in it.

• 29:48Michael Stevens?

  But there must be a small amount because there aren't a bunch of gravitational anomalies caused by this invisible matter in our day to day lives.

• 29:56Hannah Fry?

  Yeah, but it's here.

• 29:57Michael Stevens?

  So dark matter, though, the, the actual substance that's in the universe and around us is still very mysterious.

• 30:03Hannah Fry?

  Still very mysterious. And there's other things that are mysterious here too, right? There are other things that, that gravity doesn't totally explain. I mean, Newton, in a way, sort of wrapped up what gravity is doing at the scale of humans, right? That's sort of the scale of us wandering around.

• 30:15Michael Stevens?

  Had, had to write formulas about it and predict.

• 30:18Hannah Fry?

  Absolutely.

• 30:18Michael Stevens?

  What its effects will be.

• 30:19Hannah Fry?

  And you get really accurate predictions at the scale of kind of humans. Einstein then did it for the scale of galaxies, you know, the scale of solar systems, amazingly accurate predictions for that. But what we still don't really understand is how gravity works down at the level of particles, because there's some weird stuff about it, right?

• 30:41Michael Stevens?

  Right.

• 30:41Hannah Fry?

  You have all of these forces, you've got the electromagnetic force, you've got the strong nuclear force, which is the thing that sticks atomic nuclei together, you've got the weak nuclear force, which is what's responsible for radioactive decay. But each of those have a particle that does the job.

• 30:58Michael Stevens?

  Ah, right.

• 30:59Hannah Fry?

  So the electromagnetic force, for instance, it's the photon that does the job, like the photon physically travels between the emitter and the receiver. And that's what's doing the work of that force.

• 31:14Michael Stevens?

  Okay, which one is gluon?

• 31:15Hannah Fry?

  Gluon is the strong nuclear force, and then the boson is the weak nuclear force.

• 31:19Michael Stevens?

  Okay, so gravity?

• 31:21Hannah Fry?

  There isn't one, or at least we haven't found one.

• 31:24Michael Stevens?

  But is gravity a force in the same way?

• 31:26Hannah Fry?

  Well, who knows? So this is, I mean, we don't know for sure, right? It might be that, yes, gravity has a graviton, which is the sort of supposed name for this particle that's doing the job. And if that's right, then the theory is that that gravity sort of flows between things mediated by these particles in the same way that sort of, you know, water flows because of lots of H2O molecules that are moving. That might be the case, or it might be that gravity is actually this emergent property like temperature or pressure, and it's not this fundamental force of the universe, you know, as the the weak nuclear force, strong nuclear force and electromagnetic forces are. And we don't know, we don't know the answer, right? But if there is a graviton, and believe me, we've been looking for one.

• 32:11Michael Stevens?

  Yeah.

• 32:11Hannah Fry?

  A lot. They're really, really, really hard to find.

• 32:15Michael Stevens?

  How do you try to find one?

• 32:16Hannah Fry?

  I mean, you would need a particle accelerator that was basically on the scale of a galaxy.

• 32:21Michael Stevens?

  Let's get started.

• 32:22Hannah Fry?

  I would say hard, because the thing is, is that if you compare, you know, you take a magnet and a pin, right, and you can overcome the gravitational force.

• 32:32Michael Stevens?

  Gravity is very weak.

• 32:33Hannah Fry?

  Incredibly weak, right, like 10 to the minus 38 times weaker than the other nuclear forces.

• 32:40Michael Stevens?

  Yeah, right, like protons and neutrons are never like, whoa, make sure you don't fall out of the nucleus because of gravity.

• 32:45Hannah Fry?

  No, they don't care.

• 32:46Michael Stevens?

  Right.

• 32:47Hannah Fry?

  They don't care, they don't ignore that guy.

• 32:48Michael Stevens?

  So it's super weak.

• 32:48Hannah Fry?

  Super weak.

• 32:49Michael Stevens?

  Oh, right, so this is, this is where these theories of, well, maybe gravity is just as strong, but it's like, it leaks.

• 32:56Hannah Fry?

  Yes.

• 32:56Michael Stevens?

  And that's why it's so weak for us.

• 32:58Hannah Fry?

  Well, maybe gravity exists, inverted commas, in different dimensions.

• 33:03Michael Stevens?

  Right.

• 33:03Hannah Fry?

  Maybe, we're not in a three dimensional, four dimensional, if you include time, maybe we're not in this, this sort of three dimensional space at all. Maybe we're in a space with five dimensions, 11 dimensions, 27 dimensions. Maybe these extra dimensions are like us and the ants, right? Maybe they are so vast, so big, that we cannot possibly conceive of where they are. Or maybe we're the giants, maybe these extra dimensions are curled up really tightly in little, you know, this is the idea of string theory, basically, we've got all of these additional dimensions that we just can't experience because we're just, we're just way too big to.

• 33:38Michael Stevens?

  We're too big, yeah, it's like, you have this analogy of an ant on a tube.

• 33:42Hannah Fry?

  Hmm.

• 33:43Michael Stevens?

  And the ant, if the ant's big enough, it's basically just on a one dimensional surface.

• 33:47Hannah Fry?

  Absolutely.

• 33:47Michael Stevens?

  But smaller things would say, no, you can go around.

• 33:50Hannah Fry?

  Yeah.

• 33:51Michael Stevens?

  I can be underneath you, and he's like, what does that mean? Well, let me ask you this, do we know properties of gravity like, okay, electromagnetism can be like blocked, right? It can be absorbed, things can be opaque to light.

• 34:06Hannah Fry?

  Yeah.

• 34:06Michael Stevens?

  Can things be opaque to gravity?

• 34:08Hannah Fry?

  Doesn't look like it.

• 34:09Michael Stevens?

  Okay.

• 34:10Hannah Fry?

  Doesn't look like it, it's not like you can say, here's a gravity shield.

• 34:13Michael Stevens?

  Right.

• 34:13Hannah Fry?

  And just suddenly you don't feel the effect of the earth.

• 34:16Michael Stevens?

  And I guess then there's also not like a gravity reflector.

• 34:19Hannah Fry?

  No, there's no gravity mirror that we know of, but there is an electromagnetic mirror.

• 34:23Michael Stevens?

  Yeah, sure.

• 34:24Hannah Fry?

  I mean, to be fair, there isn't a shield for the, the weak or strong nuclear forces either.

• 34:31Michael Stevens?

  Oh, okay.

• 34:32Hannah Fry?

  You can't be like, stay out proton.

• 34:34Michael Stevens?

  Right, or right back at you neutron. Yeah, okay, okay. Here's the thing. When you've got stuff like dark matter and gravitons that we just don't know the nature of very much, they become wonderful seeds for wild speculation. Right? So if gravity is leaking into other dimensions that are just so small that we don't notice it, then, you know, maybe if we got close enough to an enormous concentration of matter, we would notice that like gravity was much stronger there. But because it's leaking into these like micron scaled dimensions, that by the time it reaches our scale and the scale of planets, it's just doing this inverse square thing.

• 35:18Hannah Fry?

  This tiny weak thing.

• 35:19Michael Stevens?

  And I know that we've tried this. We've tried to measure gravitational attraction between objects whose center of masses are just microns apart. And we haven't noticed a change in the equations of gravity. The problem is to get smaller. We've got to be able to concentrate mass in a smaller and smaller place.

• 35:40Hannah Fry?

  We need to get denser and denser and denser.

• 35:41Michael Stevens?

  We need to get denser and denser.

• 35:43Hannah Fry?

  Because otherwise gravity is just not doing anything.

• 35:45Michael Stevens?

  Yeah. I brought a bunch of dense things here, right? First of all, here, we'll just start with the densest one. Why work our way up? That is a pure cube of tungsten.

• 35:56Hannah Fry?

  Amazing.

• 35:56Michael Stevens?

  And it's two inches.

• 35:58Hannah Fry?

  Oh.

• 36:00Michael Stevens?

  Cube.

• 36:01Hannah Fry?

  It's that you feel like something's going on.

• 36:04Michael Stevens?

  Yeah.

• 36:05Hannah Fry?

  I mean, you feel like you're getting in the way of a magnetic pull.

• 36:07Michael Stevens?

  It's a bit like a spiritual crystal for me. I can just hold it and meditate on gravitational attraction. Like that wants to be with the earth.

• 36:18Hannah Fry?

  It wants to be with the earth.

• 36:19Michael Stevens?

  And the earth wants to be next to it. It feels like a magnetic kind of attraction.

• 36:24Hannah Fry?

  It really does.

• 36:25Michael Stevens?

  It's not just, oh, this is a lot of effort to lift. But it's like, where are you going? It's very, very cool. And I've got an inch cube of tungsten, which is not nearly as as heavy.

• 36:38Hannah Fry?

  Well, it's a ninth of the weight.

• 36:40Michael Stevens?

  But if you compare it to this cube, which is the same volume and yet made of steel, you really appreciate how much more dense tungsten is.

• 36:47Hannah Fry?

  Oh, yeah. Oh, my gosh. I was actually struggling to count it out for my hand, two of them together.

• 36:51Michael Stevens?

  I know, I know. And strange things happen when you get an enormous amount of mass concentrated in one place. The gravitational force, the curvature of spacetime, reaches an extent to which even the fastest thing in the universe can't escape.

• 37:07Hannah Fry?

  Namely, light.

• 37:08Michael Stevens?

  Namely, light. And if you don't have any light, you're dark, you're black, black holes, right?

• 37:14Hannah Fry?

  Absolutely.

• 37:15Michael Stevens?

  So another thing I brought along is this sphere of tungsten. And this sphere is 8.87 millimeters across. You know what's special about that number?

• 37:26Hannah Fry?

  Tell me. You want me to guess it?

• 37:29Michael Stevens?

  Yeah.

• 37:29Hannah Fry?

  Okay, wait, tell me, how far across is it?

• 37:32Michael Stevens?

  8.87 millimeters.

• 37:34Hannah Fry?

  8.87 millimeters. Okay, so what we're thinking is something extremely dense.

• 37:40Michael Stevens?

  Yeah, we're talking about black holes.

• 37:41Hannah Fry?

  Black holes. So if this was a black hole.

• 37:44Michael Stevens?

  Yeah.

• 37:45Hannah Fry?

  Is this the size of the Earth if you squished it down to the density of a black hole?

• 37:50Michael Stevens?

  That's right. If that was a black hole, its mass would be the same as the Earth.

• 37:53Hannah Fry?

  The entire Earth?

• 37:54Michael Stevens?

  Yeah, in the entire Earth.

• 37:56Hannah Fry?

  Everything you've ever known or experienced.

• 37:58Michael Stevens?

  Everything that's here. You don't throw any of it away. You just squish it down to that size. Then you'd be close enough to enough mass, you could never escape that. That is Earth's Schwarzschild radius.

• 38:10Hannah Fry?

  If you did manage to create a black hole this size, let's maybe not use the matter of the Earth to.

• 38:17Michael Stevens?

  No, no, no, leave us alone.

• 38:19Hannah Fry?

  I'd sort of, you know, prefer that everyone I've ever loved remained intact rather than compressed down into sort of infinite density. But let's say that you could manufacture a black hole this size, then experimentally, be way easier to find gravitons.

• 38:34Michael Stevens?

  Yes, it would.

• 38:35Hannah Fry?

  And then we might actually have an answer.

• 38:36Michael Stevens?

  And that's why I think someone should do it. I don't think a black hole that size would be very dangerous. I think it would evaporate away pretty quickly, and it would just fall right to the center of the Earth. Like, there'd be no way to contain it. In a laboratory.

• 38:48Hannah Fry?

  But hold on, wouldn't a lot of stuff swirl inside it on the way down?

• 38:51Michael Stevens?

  Um, very little stuff would swirl inside because the stuff wouldn't all fit. It would, it would, it would, it would crash into itself.

• 38:59Hannah Fry?

  Mm hmm.

• 39:00Michael Stevens?

  And so what you would wind up with is this immense energy, high density, um, ball of plasma surrounding it. Um, and so it would, it would be explosive, but it wouldn't eat up all the matter really quickly. All that matter would have to like work past itself to finally get in to make room for new. So it wouldn't suck everything in.

• 39:21Hannah Fry?

  No big deal, you're saying?

• 39:22Michael Stevens?

  No big deal.

• 39:23Hannah Fry?

  You are, you are playing fast and loose with the future of our planet.

• 39:27Michael Stevens?

  Let me show you something else, though. Uh, this is an EDC card that we made, an everyday carry card. I'm sure people have seen things like this. If you're just listening, this is a credit card sized piece of steel. It's, it looks just like the kinds that they sell that have little rulers on them and a bottle opener. So it's like a useful thing, but we wanted to make not a multi-tool, but a multi-fool. This is the most useless piece of steel you could ever carry with you. It doesn't even, it wasn't even cut at right angles.

• 39:52Hannah Fry?

  Amazing.

• 39:53Michael Stevens?

  This is 88 degrees, 92, 87, and 93.

• 39:57Hannah Fry?

  Amazing.

• 39:57Michael Stevens?

  There's a lot of silly things on here, but that hole is also 8.87 millimeters in radius, which is to say that it's, yeah. So if you're ever really like feeling down, you can just put this on the ground and if Earth doesn't fit through there, at least Earth hasn't become a black hole. You're welcome.

• 40:16Hannah Fry?

  Thank you. I hate it.

• 40:17Michael Stevens?

  It also has, uh, written on it, how to say hello in 10 languages no one speaks anymore.

• 40:22Hannah Fry?

  You are honestly the exact right amount of weird.

• 40:25Michael Stevens?

  I don't really see what's so weird about that.

• 40:28Hannah Fry?

  I guess on that note, we'll finish for the, uh, for this show. So, um, please do like and subscribe and send in your comment to [email protected].

• 40:38Michael Stevens?

  See you next time.

• 40:51Speaker 05

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