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How does gravity work? How does gravity keep things on the ground? And how does it keep planets in orbit?

Answers written on January 27th, 2015

Question Date: 2015-01-27
Answer 1:

Awesome question! The answer involves understanding Einstein's theory of general relativity, which is the best theory of gravity we currently have (coincidentally, Einstein came up with his theory in 1915, so this year is the 100th birthday of general relativity!).

We're used to thinking of space as something we move through - you can go up and down, forward and back, left and right - but the space itself just sits there. Similarly, we're used to thinking of time as some universal thing that keeps track of how everything changes from one moment to the next - the steady ticking of all clocks. But here's Einstein's mind-blowing insight: space and time are actually part of the same thing, called spacetime. Even crazier, spacetime doesn't just sit there - it can warp, bend, curve, and even tear!

But what happens when an object (like a person, or a planet) tries to move through this warped spacetime? Well, in ordinary flat (or unbent) spacetime, stuff just moves in straight lines. But when you bend the spacetime, that makes objects move along curved paths. If that's confusing, here's a nice analogy: imagine that spacetime is like a trampoline. If you roll a marble along the flat trampoline, the marble just travels in a straight line. But what happens if you bend the trampoline by putting a weight in the middle of it? If you try to roll a marble along it, the marble will move in a curved path, since the surface of the trampoline (the "space" along which it's moving) is curved.

General relativity says that this is what gravity is: gravity is just the curvature of spacetime caused by heavy objects. The reason we feel pulled down towards the Earth isn't because the Earth is really pulling on us; instead, the Earth is bending the spacetime around it, and we feel that curved spacetime as something pulling us towards the center of the Earth (that is, something pulling us "down"). Similarly, just like the marble on the trampoline, if an object tries to move by the Earth (say, like a satellite or the moon), the curvature of spacetime causes that object's path to change towards the Earth. That's what creates orbits, and what keeps planets in their orbits.

I hope this helped!

Answer 2:

Gravity exerts force on objects in proportion to their mass, i.e. the more massive the object, the more force it exerts on other objects, and the more force other objects exert on it. Planets are already moving around fast enough that all gravity is able to do is to keep them from flying off in a straight line, which would lead into deep space away from the sun. Things on Earth have are typically not moving, so gravity is the only force acting on them, so they fall downward until they are on the ground.

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