Remember the story about Isaac Newton seeing an apple falling from a tree and discovering gravity? Of course, and ever since, it has seemed clear that there a force called gravity which pulls objects down towards the earth – the apple falls to the ground or when you jump you land back down. But it turns out there is no such thing as this force of gravity.
Back in 1915, Einstein published his theory of relativity which actually upended our understanding of gravity. But it is a bit complicated, so it never entered the popular imagination in the way that Newton’s apple did. Einstein’s thought experiment revolved around the idea of weightlessness or freefall.
Pulling down or pushing up
Imagine if you jumped off a tall building, at first you think that you were accelerating down to the ground. But if you closed your eyes (and assuming no wind resistance), you would feel weightless. It would feel the same as if you were in space – floating around. Now imagine you were on an elevator that was being pulled up at an accelerating speed (at 9.8m per second squared – the acceleration of gravity) – you would feel like you were zooming up. Back in space, if you were that rocket floating around but now the rocket switched on its booster rockets (at 9.8m/s^2), you would suddenly stop feeling weightless and you would think were stationary like on earth.
Einstein’s point was that on earth, rather than a force pulling you down, you could just as easily argue that there was a force pushing you up (to stop you free falling). Put another way – there was no way to tell the difference between acceleration and the force of gravity.
So, what was really going on? Einstein argued that rather than seeing space and time as fixed, we need to think of them as being dynamic. He argued that space-time could get curved by matter – so big objects like the earth bend space-time a lot more than small objects like people.
When you combine space and time – moving along the shortest route means moving on a curved path. Think of a plane flying from New York to Singapore – it will take the most direct path, but when you zoom out and look at the earth it is taking a curved path. Or imagine you have two people on the equator 1,000km apart, and then travel in what they think is a straight line to the North Pole. They will end up at the same point in the North Pole despite initially being 1,000km apart, the curvature of the earth meant that they are actually moving closer to each other.
In the same way, the earth curves space and time, so when you are free-falling from that tall building you are moving along the curved space-time created by the earth. The same can be said about Newton’s apple (see picture) And the earth is moving along the curved space-time created by the Sun and so on.
This idea was called general relativity and it made different predictions about how light would behave around large objects and black holes than Newton’s theory of gravity. Experiments since Einstein have proven his theories to be correct.
So, the next time someone says that gravity is pulling you down, you can say no it’s curved space-time!