When most I wink, then do my eyes best see
Einstein wondered what would happen if the Sun were to suddenly explode. Since the Sun is so far away that it takes light eight minutes to travel to Earth, we wouldn’t know about the explosion straight away. For eight glorious minutes we’d be completely oblivious to the terrible thing that was about to happen.
But what about gravity? The Earth moves in an ellipse around the Sun, due to the Sun’s gravity. If the Sun wasn’t there, it would move off in a straight line. Einstein’s puzzle was when that would happen: straight away, or after eight minutes? According to Newton’s theory, the Earth should know immediately that the Sun had disappeared. But Einstein said that couldn’t be right. Because, according to him, nothing can travel faster than the speed of light — not even the effects of gravity. […]
Before Einstein people thought of space as stage on which the laws of physics play out. We could throw in some stars or some planets and they would move around on this stage.
Einstein realised that space isn’t as passive as that. It is dynamic and it responds to what’s happening within it. If you put something heavy in space — let’s say a planet like Earth — then space around it gives a little. The presence of the planet causes a small dent in space (and in fact, in time as well). When something else moves close to the planet — say the Moon — it feels this dent in space and rolls around the planet like a marble rolling in a bowl. This is what we call gravity. […] Stars and planets move, causing space to bend in their wake, causing other stars and planets to move, causing space to bend in their wake. And so on. This is Einstein’s great insight. Gravity is the manifestation of the curvature of space and time.
