Falling Forever

There are people living in a metal tube going around the Earth right now. They’ve been up there, in shifts, for about twenty-five years.

There’s no rope holding it up. There’s no rocket firing underneath it. There’s nothing beneath it at all.

So why doesn’t it fall down?

And here’s the other thing. Inside, the astronauts float. Their pens float. Their food floats. If they let go of a hammer it just hangs there in the air. Why?

Every single time, the ball fell. It never stopped falling. It just went further sideways before it got to the ground.

So here’s a question, and I don’t want you to answer it yet.

If I could throw it hard enough — ridiculously hard, harder than any person could ever throw — where would it land?

Look at that. Why is the water coming out?

Because it’s heavy, right? All that water is pressing down on the bottom of the cup, and it squeezes out through the hole. That’s why the stream is stronger when the cup is full and weaker when it’s nearly empty.

That looked like falling, didn’t it? It was going down. And the water came out the whole time. So going down isn’t the thing. My hand was still holding it.

It was going up. And the water still stopped coming out.

So put those two together. Going down with my hand on it: water pours. Going up with my hand off it: water stops.

“Falling” doesn’t mean going down. Falling means nothing is holding you.

Names for what the cup just did — and one idea we can stop believing.

And one idea we’re going to stop believing.

People say there’s no gravity up there. You’ll hear it on TV. That’s the wrong idea — the Earth is bending time just as hard 250 miles up as it is in this room.

What’s gone isn’t gravity. What’s gone is the floor.

Remember the question I wouldn’t answer? Isaac Newton asked himself the same one about three hundred and forty years ago, and he drew a picture of it.

He imagined a cannon on top of a mountain so tall it stuck out above the sky. And he imagined firing it sideways.

Fire it gently: the cannonball arcs over and lands nearby. Fire it harder: it lands further away. Harder still: further again — and now something interesting is happening, because the Earth is round, and the ground is starting to curve away underneath the ball while it flies.

So fire it harder still. And harder. Until you fire it so hard that the ground curves away exactly as fast as the ball falls.

Now the ball is falling — it’s falling the whole time, it never stops falling — but it never gets any closer to the ground. It goes all the way around the world and hits you in the back of the head.

It never stopped falling. It just kept missing.

And the astronauts inside? They’re the water in the cup.

Last lesson we said time runs slower down low, near heavy things. If that’s really true — and not just a nice story — then a clock way up in space should run faster than a clock down here. So let’s check.

The satellites that tell your phone where it is carry the most accurate clocks ever built. They’re 12,500 miles up, where gravity is weaker, so their clocks gain about 45 millionths of a second every day. They’re also moving very fast, and going fast makes clocks run slow, so that gives about 7 millionths back.

Add it up: those clocks gain about 38 millionths of a second every single day.

That sounds like nothing at all. But your phone works out where you are by timing radio signals travelling at the speed of light. And in 38 millionths of a second, light goes about seven miles.

So if nobody fixed it, every distance your phone worked out would be seven miles off after a single day — and worse every day after that. It would be useless before dinner.

Every one of those satellites is launched with its clock deliberately set to run slow — by exactly the right amount — so that when it gets up there, it runs right.

After this lesson we’ll sit with that properly — how the timing works, why seven miles, and the switch they built to check. You’ll be able to explain it to anyone.

Here’s my favourite thing in this whole Big Idea.

In 1977 the Americans were building the very first satellites for this system. They knew what Einstein’s equations said the clocks would do.

But somebody wasn’t sure.

We don’t know exactly who doubted it, and that’s alright — we know somebody did, because of what they built next.

And you can see why they’d doubt it. It’s one thing to write an equation on a blackboard. It’s another to bet a satellite on it — something that costs a fortune, that you launch on a rocket, and that you cannot go up and fix.

So they built a switch.

The satellite was called NTS-2. It went up in June 1977 carrying an atomic clock, and a little device that would correct the clock for Einstein’s effect — and that could be left switched off.

They launched it. They left the correction off. And they watched the clock.

Einstein’s equations predicted the clock would gain 446.5 parts in a trillion. It gained 442.5.

That’s a match. That’s as close as their clock could possibly tell. Getting within one part in a hundred of a number nobody had ever measured before is a bullseye.

They turned the switch on. It has been on ever since — in every satellite, every day, for nearly fifty years.

That’s what a test looks like. Not “we believe Einstein.” Not “the maths is beautiful.” A switch, a measurement, and a number that either matches or it doesn’t.

They built in a way to find out they were wrong. That’s the whole trick. That’s the thing.

Einstein’s picture has worked everywhere anyone has ever pointed it. Every test, for a hundred and ten years.

But there’s a problem, and it’s a big one.

There’s another set of rules, for very tiny things — atoms, and the bits inside atoms. Those rules work perfectly too. Every test.

And in one particular place, the two sets of rules don’t fit together. Anywhere something is both incredibly heavy and incredibly tiny, you’d have to use both at once — and then you get answers like infinity.

Infinity isn’t an answer. It’s the universe telling you you’ve asked the question wrong.

Nobody has fixed it. Not one person, in a hundred years of trying.

Which means nobody knows what happens at the exact middle of a black hole. Nobody knows what space and time are actually made of.

This is where the map runs out. Not a long time ago. Now. Today.

You have already felt this. You use it. The Moon has been doing it for as long as there has been a Moon.