Professor Hannah Fry (3:07)

So, okay, here is a little glimpse of what is to come from our podcast. And if it sparks something unexplainable for you, then you can join us every Tuesday and Thursday for new episodes of the Rest is Science, and we'll figure it out together.

Michael Stevens (3:21)

How would you describe gravity to an alien from another universe that had never experienced gravity?

Professor Hannah Fry (3:27)

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?

Michael Stevens (3:43)

And at this point, the alien goes, what? That is so odd. Right, and what do you mean by an object?

Professor Hannah Fry (3:50)

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 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.

Michael Stevens (4:09)

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 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, it's.

Professor Hannah Fry (4:47)

How does.

Michael Stevens (4:48)

How does astrology work?

Professor Hannah Fry (4:49)

Something, something, something. Pisces, Right?

Michael Stevens (4:52)

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.

Professor Hannah Fry (5:05)

Because otherwise, birth ain't working.

Michael Stevens (5:07)

That's why. Yeah. People are like, oh, so you're an Aquarius? And I'm like, no, I'm a Schnit cookie. Because Dr. Schnit cookie was there, influencing me to catch you at a physical level. Yeah. Not just the catchy. Not just the physical touch, but the gravitational attraction to his mass.

Professor Hannah Fry (5:22)

Right.

Michael Stevens (5:23)

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. 9 trillionths the width of a proton. Oh.

Professor Hannah Fry (6:03)

Which is, by my calculation, small.

Michael Stevens (6:06)

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

Professor Hannah Fry (6:12)

But they are coming to meet each other.

Michael Stevens (6:14)

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

Professor Hannah Fry (6:18)

Yeah.

Michael Stevens (6:18)

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.

Professor Hannah Fry (6:33)

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 gravity in that, in that statement itself. Right. But if you take something that has more matter, the amount that the Earth would move would change too.

Michael Stevens (6:51)

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.

Professor Hannah Fry (7:04)

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? 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 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 it 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 like, let's say you took the sun and you had like a magic wand that made the Sun Disappear instantaneously. It would take 8, 9 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?

Michael Stevens (8:32)

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

Professor Hannah Fry (8:37)

Speed of light.

Michael Stevens (8:38)

Speed of light? No, faster.

Professor Hannah Fry (8:39)

Well, because it's the universal speed limit.

Michael Stevens (8:43)

Yeah. Certainly it's not instantaneous.

Professor Hannah Fry (8:46)

Absolutely. Which means that if the sun suddenly vanished, we wouldn't know about it at all.

Michael Stevens (8:51)

But was that a problem for Newton?

Professor Hannah Fry (8:53)

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. 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 it doesn't trace out the same ellipse. Every single time it orbits the sun, that ellipse is moving around. It's called the perihelion 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? 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 like, maybe we've just made a miscalculation. It's sort of a bit. I don't know.

Michael Stevens (9:57)

And this was for a long time. Hundreds of years.

Professor Hannah Fry (9:59)

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. Is literally bending and warping space time 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 gonna 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. Had this great intuition. We worked out all the calculations for it. And one of the very first things that he turned his equations to was the prohelion 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.

Michael Stevens (11:04)

Nailed it.

Professor Hannah Fry (11:05)

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

Michael Stevens (11:14)

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.

Professor Hannah Fry (11:34)

Gigantic.

Michael Stevens (11:35)

Because space time is such a bizarrely abstract thing. It's, 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?

Professor Hannah Fry (12:09)

Exactly.

Michael Stevens (12:10)

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

Professor Hannah Fry (12:18)

Exactly right. It's the crumpling of the curtain. That's really, that's a really nice way to do it.

Michael Stevens (12:22)

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

Professor Hannah Fry (12:29)

Totally.

Michael Stevens (12:30)

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.

Professor Hannah Fry (12:42)

But here's the thing, right. The implications of this idea that space time 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.

Michael Stevens (13:11)

And what is it in Greenwich?

Professor Hannah Fry (13:13)

9.812.

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Professor Hannah Fry (13:15)

I've got higher gravitational effect than you.

Michael Stevens (13:17)

Yeah. So you are more attracted to the center of Earth than I am in Boulder because I'm further away. Yep. And the inverse square law says exactly further away, gravitational effect diminishes.

Professor Hannah Fry (13:29)

Except that what that means is, 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 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.

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