If the Earth’s a big ball, why don’t we fall off the bottom of it?
Because Earth’s gravity doesn’t make things fall down – it makes things fall towards the middle of the planet. That goes for everything on Earth – its skies, its oceans and its people.
Oh, yeah. Of course. Er... what?
Right – let’s go back a bit. Isaac Newton told us how gravity works – like how and why things fall towards the ground when you drop them, and how fast you can expect them to go when you do. Agreed?
OK. Well he also told us that gravity wasn’t just about things falling towards the ground, but rather all objects falling towards – or attracting – each other. This force of attraction is stronger for larger objects, and Earth is by far the largest object on... well... Earth. So everything on the Earth is held on by the Earth’s super-strong gravity. A bit like a big ball-shaped magnet with bits stuck all over it – not just metals, but all kinds of things, like air, water, trees and people. That’s why none of it falls off.
OK – I get that. But magnets work by being magnetic, right? So why does gravity work just because something is big?
Well spotted. You’re right – there is a bit of the story missing here. So here’s the rest...
Newton’s explanation of gravity was all very clever – and we’ve since used it to figure out everything from orbiting planets to moon landings – but he still didn’t say what gravity actually is.
Where did the force come from? Why was it there at all? Newton didn’t know, couldn’t say.
So Albert Einstein bravely had a crack at these and other questions almost 200 years later, saying (more or less):
- Space is not empty, or even flat – it’s more like a fabric with lumps and dents in it
- The lumps and dents are distortions caused by objects (like stars and planets) in the fabric
- Gravity is just objects rolling into dents (or around lumps) in the fabric
Errr... having trouble with this one...
OK – try to picture it like this: imagine Space as a big sheet of rubber, and the Sun as a basketball plonked in the middle. The sheet will bend round the ball, making a big dent in it, right? Now imagine rolling a few marbles or tennis balls across the sheet: some will go straight across, but the ones that pass close to the basketball will get drawn into an arc round it as they dip into the dent. The marbles might even do a complete circle round the basketball before contacting it.
So this is why planets circle (or orbit) the Sun – not just planets, but asteroids and comets too. It’s also the reason why moons orbit planets, and why satellites and space shuttles can orbit the Earth.
Basically, big things make a dent in Space, and other things ‘roll’ or ‘fall’ towards the source of the dent. If they have enough speed and momentum, small things can circle round and round the dent forever. If they don’t have enough speed, they fall into the dent and eventually settle next to the object that made it.
So planets, comets and asteroids roll round a gravity dent made by the Sun. The Moon rolls round one made by the Earth. Rockets and satellites get launched to the edge of the gravity dent and roll round it until it’s time to come back down.
And the skies, oceans, deserts, glaciers, trees, animals and people of Earth all sit in the depths of a dent in Space created by the planet – stuck to its surface by the force of gravity.
Dents in Space, eh? Weird.
You said it.