Instant Genius - Gravity, with Marcus Chown

Episode Date: November 22, 2021

Science writer Marcus Chown tells us everything we need to know about gravity, from Isaac Newton’s apple falling from a tree to Einstein’s general relativity. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Produced by the team behind BBC Science Focus Magazine. Visit our website: sciencefocus.com Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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In this week's episode, I talk to science writer Marcus Chown. He tells me everything I need to know about gravity, from Isaac Newton's apple falling from a tree to Einstein's general relativity. So let's start at the very beginning. beginning with what most people would understand as gravity, which is what physicists like to call Newtonian gravity, or just gravity as Isaac Newton described it. So what would Isaac Newton say that gravity is? Interestingly, Newton did not know what gravity was. All he did was describe it. And from the

motion of the moon around the earth and the planets around the sun, which had actually been recorded by Johann Kepler, he was able to deduce the way gravity behave. So he imagined that there was an invisible tether connecting the earth for the sun, which kept the earth trapped orbiting the sun, and that was the gravitational force. And he was able to work out that it fell off or weakened with what's known as an inverse square law. So if you had two masses and you moved them twice as far apart, the poor, the attractive gravity between them would be four times less. And if they were three times as far apart, it would be nine times less. So basically, gravity to Newton was an attractive force between any masses. So it was called universal gravity. So every mass pulls on every

other mass with an attractive force that weakens with the inverse square of the distance. So actually, incredibly, there's a gravitational force between. you and somebody you pass by on the street, there's a gravitational pull between you and the coins in your pocket, and there's a gravitational pull between the moon and the earth. But the gravitational traction grows with the amount of mass. Okay, so as you get more and more mass, it becomes stronger. And it turns out that the point between you and a parcel and the pool between you and the coins in your pocket is too weak to measure or to notice. But it's only when you get to astronomical size bodies that it becomes appreciable. So why is gravity only attractive? Why does it pull and not push?

Well, that's a very interesting question. And in fact, I would actually say your question is incorrect because since 1990, since 1998, we've actually discovered that most of the stuff in the universe has repulsive gravity. So in 1998, the dark energy was discovered. So we believe the universe began in a big bang about 13.8 billion years ago. And the galaxies, the building blocks of the universe, I mean, we live in one of them, the Milky Way, there are two trillion of them, have been flying apart ever since like pieces of cosmic trapnel. And so we assumed that the only force operating was gravity, an attractive force between all the galaxies. So you can imagine it's like an invisible web of elastic between all the galaxies.

And so it should be slowing down or breaking their expansion. But in 1998, contrary to all expectations, we found that the expansion is actually speeding up. And we've had to postulate the existence of stuff called dark energy, which is invisible, fills all of space and has repulsive gravity. And it accounts for two-thirds of the mass energy of the universe. So incredibly, until 1998, which is what's that, 23 years ago, we've missed the major mass component of the universe. And that stuff, and we've got no idea what it is, has repulsivity. So your question, why does attract most of the stuff in the university is actually repelling with gravity? And interesting, if you look at Einstein's theory of gravity, the source of gravity, in Newton's theory,

the source was mass, absolutely straightforward mass, right? But in Einstein's theory, the source is energy. Okay. So basically, that means that light energy has gravity, heat energy has gravity, sound energy has gravity, and mass energy is the most compact form of energy we know. So that's what Newton thought that was a source of gravity. So all these things are the source of gravity. Even gravitational energy itself has gravity. And that's, of course, why we know that the gravity near the sun is stronger than Newton would have predicted,

because not only is there the gravity of the mass of the sun, but there's the gravity of the gravitational energy of the gravitational field around the sun. So that makes gravity stronger near the sun than you would have expected. So incredibly, gravity creates gravity. Anyway, to get to your point, it turns out that energy is not the only source of gravity

in Einstein's theory. It's actually energy plus, I think, three times the pressure Okay, now this pressure term was considered to be of no consequence by Einstein because normal matter doesn't really have much pressure. I mean, a gas has pressure. If you inflate a balloon, you know, the plastic or the rubber of the balloon is being pushed into a sphere by the pressure of the air that you've blown the balloon. That pressure is negligible compared to the energy content of matter.

So it was considered to be of no consequence. But now we think that term actually is important. And that is the term that is delivering the dark energy. So because if you have negative pressure, right, which is the opposite of what you'd expect. So if you can imagine if you normal pressure is the pressure of air pushing outwards to keep a balloon inflated, negative pressure would be sucking inwards. if you have negative pressure and it's big enough, the energy term which generates gravity can be reversed

because the pressure term, energy plus pressure, the pressure term can negate the energy and give you repulsive gravity. So this is what we think is happening. So we think there is a kind of stuff in the universe, invisible, fills all of space, and has very large negative pressure. It's unlike anything that we've ever experienced.

experienced or known about before. And that negative pressure, incredibly, is giving it repulsive gravity, completely the opposite of what you'd expect. Wow. So is this what we might think of in science fiction as anti-gravity? Well, in a sense, yes. The only caveat is that the dark energy, well, it's in the room where you are now. You know, it's on the earth. It's everywhere, right? but we believe that its energy density is incredibly tiny. So we don't notice its effect. So basically, if you double the amount of this stuff, it has double the repulsion.

You know, if you triple the amount, you have triple the repulsion. But this energy is so small that within the solar system, it never builds up to anything that we can actually, you know, we can't actually see its effect. And it's only on very large scales, cosmological scales, when there's enough of this stuff that it's repulsions,

effect overcomes gravity and dominates gravity and causes the expansion of the universe. So this stuff is incredibly weak. Kind of, yeah, you're right. It does give you, it does give you anti-gravity, but practically not very useful on the human scale. And now I think maybe we've got a bit ahead of ourselves. So let's just take a step back and talk about Einstein's theory of gravity or what we like to call relativity.

So what did Einstein have to say about gravity? Well, basically, he was looking for a theory. Basically, in 1905 in his miraculous year, he was able to work out what the world would look like, what people or what a person or observer would look like if they were flying past you at constant speed. And he found if that speed was close to the speed of light, then that person would appear to be moving in slow motion because their time would slow down. They would appear to flatten in the direction of motion because their space was shrink. So these were two effects. of special relativity. But he realized that motion at constant speed was a bit of a bit special. I mean, really, most of the motion we see around is acceleration. You know, a car accelerates away from traffic lights, you know, a rocket accelerates. So he wanted to generalize his theory to find out what it would look like, you know, what the world looked like for an observer who was accelerated.

And he discovered that this was exactly the same as finding a theory of gravity. So this was his most wonderful. He said the most wonderful thought of his life. He discovered that it was the same. Basically, the theory is based on an observation that had been made by Galileo three or four hundred years before, 300 years before. And that observation, we're probably all aware of this, is that all masses fall at the same rate.

And this was demonstrated on the moon by, I think, was it Apollo 17 astronaut? He dropped a feather and golf ball. or whatever, and they both hit the lunar, you know, dropped them from the same height. They hit the lunar soil at exactly the same time, and you can see a puff of moon dust. So that was correct. So the question is, why does that happen? Okay, because normally you would expect that the gravity on the bigger, on the more massive body, would be stronger, you know, because it's got more mass, more attraction, so it should fall faster.

How could this be? and he came up with a really, really simple explanation of why. And that really is that gravity as a force pretty much doesn't exist. And so he imagined being in that craft far away from the gravity of any planet, so zero gravity, and the spacecraft is accelerating at 1G, so exactly the acceleration of, you know, we experience, on the Earth's surface. He imagined just having, say, a feather and a golf ball in your hands and letting go of them. And what would actually happen if you were the astronaut in that cabin, you would see the feather and the golf ball fall to the floor at the same time.

But if you were looking from outside, if the spacecraft were transparent, you'd see that there's a really simple explanation for that. And that is that the feather and the golf ball just hang in space. They don't move at all. But the floor of the spacecraft accelerates up at 1G and meets both of them at the same time. How can it not? Because there's only one floor. It's got to meet them at the same time. So this is really his explanation that gravity is an apparent force because we don't realize that we're actually accelerating. And he later discovered that space itself was curved. The reason we don't know we were accelerating is because it's curved in higher dimensions, in four dimensions that we can't actually see. So for instance, we think of that the, Newton thought that the earth was

tethered in effect to the sun by this invisible force. But Einstein realized that's not pace. What actually happens is the mass of the sun, curves that the space or space time around the sun creates a valley. And in effect, the earth rolls around the edge of this valley, around the top of this valley, like a roulette ball in a roulette world. Okay, so what we think of as gravity doesn't actually exist. The ball is just going, or the earth is just going along the path, the only path it can go, go along, and it looks to us because it's going around in the circle, that there must be a force trapping it in the vicinity of the sun. So does this mean, if Einstein came up with this brand new theory, does that mean that Isaac Newton was wrong

about gravity? Newton was not wrong about gravity, but he was basically describing gravity in a special case. Einstein was a more general theory. So because there are a couple of problems with special relativity and gravity. So in 1905, when Einstein came up with the simpler theory,

the theory which special relativity, which is really about the motion of observers moving at constant speed relative to each other, when he came up with that, he instantly knew that there was something was wrong or wasn't quite correct. And that was because the theory of special relativity was based on the assumption that the speed of light was uncatchable. So the speed of light, and that means that speed of light is actually a constant, that you can deduce that the speed of light has to be constant. from that. And so nothing could travel fast and speed of life. So of course, Newton was assuming that there's this gravitational force between the sun and the earth. And he's assuming it's

instantaneous. You know, it's just there. So he's assuming that it's, it's, it basically gravitational influence moves at infinite speed. What Einstein knew was that the gravity itself would have to travel at the speed of light. And therefore, Newton's picture couldn't be correct. So Newton would have said, if the sun were to vanish, then the Earth would instantaneously fly off a tangent towards nearest star, because suddenly it would not be under the gravitational pull of the sun. But Einstein knew that if the sun were to vanish, it would be eight and a half minutes before the Earth recognized this. Okay, so it continues circling around the sun or orbiting the sun for eight and a half minutes, because that's the time it would take for the Earth. to realize that the gravitational influence had vanished because there's an upper speed limit. And the other thing that Einstein discovered in his 1905 theory of special relativity,

which was E equals MC squared, that mass is a form of energy. And so he realized that energy itself had to be the source of gravity. It wasn't simply just mass, any form of energy. So those two modifications were needed for his next theory. So yeah, in the sense, he went beyond Newton, but Newton's theory is actually works incredibly well within our solar system, so well that you really need to use Einstein's theory to send a space probe to Titan or Saturn or something like that. You can just use Newton's theory. It's a really, really good approximation. But when gravity becomes very strong around objects like black holes and things like that, or very, very close to the sun, you can just see the effect of Einstein's theory near the sun,

because said gravitational energy has gravity. And so nearer the sun, there are two sources of gravity, the mass of the sun, plus its own gravitational energy. So gravity is slightly stronger. So yeah, yeah, it's a marginal effect within the solar system, but around objects where there's strong gravity, you really do need Einstein's theory. Right. So are there any times on Earth when we need to use general relativity? Pretty much every minute of every day if you've got a smartphone or you use GPS because the, basically what the software in your phone does is it works out your position on the surface of the Earth relative to global positioning satellites. At any one time there could be, I don't know, five or six about the horizon.

So it works out your position relative to those. And those satellites, they actually carry clocks, very, very accurate clocks. And one of the consequences of Einstein's theory of gravity of 1950, which is general relativity, is that it's not just space that is warped by the presence of mass, it's space time. So time itself is warped as well. So you find that when something is in strong gravity, its time flows more slowly. So what we find with these global positioning satellites is they're in highly elliptical orbits, elongated objects. And when they swing in close to the earth, they're in stronger gravity.

The clock slowed down. And when they swing back out again, the clock speed up. So the software in your smartphone or GPS system has to compensate for that effect. And if it didn't, you would get your position on the earth's surface wrong by an extra 50 metres every day. Wow. So general relativity sounds like quite a powerful theory. But does it explain everything? Does it have, all of gravity completely well explained or are there any holes in it? General relativity is an interesting theory in that it actually contains the seeds of its own destruction. So within it, as you just rightly said, there are holes. They were known about by Einstein. So it shows its own limits. So what it predicts is that if mass,

a large mass shrinks under its own gravity to form what we now call a black hole. Everything skyrockets to infinity. The density, the temperature, everything becomes infinite, what we call a singularity. And a singularity is an indication that your theory is wrong, okay, because it's a nonsense. It's a complete nonsense. And we also know that when we, if we were to run the expansion of the universe backwards, in our imagination, back to the Big Bang, we would also meet another singularity, which is nonsense. So we know that the theory breaks down in those two instances.

So we know that even general relativity is an approximation of a deeper theory. So that's, although it's fantastic, we know that it breaks down. Right. Okay. But we can be pretty confident that black holes exist. I mean, we've taken a picture of one just a few years ago. Where have we gone wrong with the physics of black holes? What is it about black holes that general relativity can't explain? Well, general relativity doesn't tell us what happens at the center of a black hole because general relativity predicts something which is nonsense, which is a singularity.

Basically, all bets are off. That's not mathematically, your theory is just wrecked. So in effect it breaks down. So we know that something else has to happen at the centre of a black hole. Something has to form, you know, maybe some dense nugget of material, but not a singularity. And we do know that there is another theory. The two towering achievements of 20th century physics are general relativity, that theory of gravity, Einstein theory of gravity, and quantum theory.

And quantum theory generally, a theory of very small things, you know, like atoms and their constituents and general relativity, because gravity only gets strong on the big scale, tends to be a theory of big things. And on the whole, there's no real conflict because, you know, there's the theory of big things, stars and galaxies and the universe, and it's the theory of small things, you know, the atoms and things in your smartphone, and they don't really overlap. But when you get to the near the center of a black hole as matter shrinks down to that or the singularity at the beginning of the universe, something very big becomes,

very small. So, you know, we think that the Big Bang, the entire universe was smaller than an atom. So you have to unite these two theories. If you want to describe what was actually happening, you have to unite quantum theory and general relativity. And that's proved very, very hard, because the two theories are fundamentally incompatible. If you take general relativity and the rest of physics, it describes things like the path of the planet, you know, the, you're the, of a planet. That's the thing that you can describe. But quantum theory, there's no such thing in quantum theory. An atom does not follow a single path. It follows, it can follow any of a number of different paths through space, each of which has a certain chance of happening. So even the idea of

a path of a single path through space, which is fundamental to general relativity, is not even allowed in quantum theory. So to cut a long story short, I don't know. I think of gravity is a theory of certainty. You know, we certainly know where the moon will be tomorrow. We certainly know what it would be the day afterwards, whereas quantum theory is a theory of uncertainty. So how you unite a theory of certainty with a theory of uncertainty? I mean, that's really boggled the brains of the best physicists. Just going back to black holes for a minute. Everyone sort of knows that black holes have really, really strong gravity. And the sort of common phrase is that the gravity is so strong that not even light can escape. But how is that possible? How is it possible for light to get trapped by gravity?

In Einstein's picture, what actually happens, a mass actually warps time around it. So it kind of valley in a space time. We can't actually see it because space time is a four-dimensional thing. And And of course, that's why it took the genius of Einstein to realize. But if you imagine compressing that mass, gravity becomes more and more intense. So that little valley becomes deeper and deeper and deeper. And eventually, if you compress that mass to a critical amount, the valley becomes infinitely deep. So it becomes like an infinite well, an infinite pit. And really what actually happens is light cannot actually climb out of that pit.

because as it climbs out, it loses energy. And by the time it actually gets to the top of that deep well, it's lost all of its energy. So that that's really what's happening. It's not that light is actually getting trapped because it can't reach enough speed to get out. It's simply that it's just cannot, it's sacked of all energy trying to get out.

It can't escape. So that's why a black hole is black. But interestingly, the bigger the black hole, the less extreme the conditions around it. And in the centres of galaxies, including our own Milky Way, and we've got no idea why this is the case. There is a supermassive black hole in pretty much every galaxy,

some of them up to about 30 or 50 billion times the mass of the south. These holes, you could quite happily wander into the black hole without any ill effects, because the gravitational forces are not enough to do you any damage, wherever. I mean, you can never get out again, but they're not as, but the most extreme conditions are near the smaller mass black holes, like the stellar mass black holes. So it's a bit like chili peppers. The smaller it is the more powerful it is. I've never heard that analogy, but you're completely right. Yes, absolutely. I mean, we call this,

I mean, you probably know that the reason that they are so dangerous to smaller ones is because gravity, the gravitational force changes rapidly as you get close to the black hole. and if you were an astral and you were falling feet first, you would find that the gravitational pull on your feet was a lot greater than on your head and you'd be ripped apart. However, that force or that difference in force, which we call a tidal force, is not so great with really big black holes. And so you could fall in and probably not even notice that you've actually fallen into a supermassive black hole.

But of course you can never get out again, unfortunately. I've heard that that effect where your feet are pulled much more strongly by the black hole than your head and you get stretched out as a result. I've heard that that's called spaghettification. Yeah, that's the word. So if you're playing Scrabble this Christmas, that's a good word to use. Yeah, there are extraordinary things. I mean, the other thing that happens is that when you get close to a black hole, there is a distance from a black hole relatively close to it, where light is. is bent so much in this kind of valley around the black hole, that that light actually orbits a black hole. So if you were to be near, you know, at that point, you could look sideways,

not directly at the black hole and see the back of your own head. These are the effects of extreme curvature of space time that you get around a black hole. And just to finish up this episode, could you please just tell us three things you think we all need to know about gravity? Things we need to know about gravity. Okay, the first thing we need to know, and nobody knows this, is why there are four fundamental forces which glue together everything in the universe, you and me, and gravity is one followed by 40 zeros weaker. So a factor of one followed by 40 zeros weaker than the others.

I mean, no one knows why that is. I mean, it's such an incredibly huge factor. That's the first thing. Nobody knows that. We also need to know how to unite quantum theory and Einstein theory of gravity. Nobody knows how to do that. Only one framework even attempts to do that, and that is called string theory, but unfortunately string theory doesn't seem to predict anything that we can go out and measure,

and it tells us that there are six invisible dimensions, which we don't see. So there are some problems with that. And the other thing, the last thing that would be absolutely fantastic, to know is why the hell is there a supermassive black hole in the heart of every galaxy? We've got no idea how they could have got there. When we look back in our telescopes, you know, back in time to almost the big bang, we can see that these giant black holes existed. So they must have formed very, very quickly.

But how did billions of solar masses accumulate and form these supermassive black holes? And then, sorry, I'm adding a fourth question now. And that is what came first, the supermassive black holes or galaxies, you know, the chicken and egg question. So did supermassive black holes form very quickly in some way we've got no idea how after the Big Bang and form the seeds that sucked in the material that became the stars of galaxies? Or did the galaxies form first and then maybe material fall towards their centre and create these supermassive black holes? I mean, we've got no idea. But the interesting thing is that although for most of the 20th century physicists were trying to prove that black holes didn't exist, when they were forced to admit that they did, they thought they would definitely have to be some kind of inconsequential thing, you know, on the frontier of physics, not central at all.

But the fact that there's a supermassive black hole in the center of every galaxy is saying, telling us that they are absolutely key and central to our understanding of the universe. but we've got no idea why. Thank you for listening to this episode of Instant Genius. That was Science Writer Marcus Chal. To hear him tell me all about the physics of gravitational waves, head over to Instant Genius Extra, available only on Apple Podcasts. The November issue of BBC Science Focus magazine is out now.

Pick up a copy in store or visit ScienceFocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist intended. Discover more at name audio.com.

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