I often hear that gravity is not a force, but simply the curvature of spacetime. A common example given is having a heavy ball placed on a rubber sheet. The ball sinks, distorting the sheet. Therefore, other “orbiting” objects will also tend to roll towards the sunken part.
What I can’t understand is, what causes the objects to “fall” anyway? On earth, that’s just the planet’s gravity. If you did the same experiment on space the objects wouldn’t roll down. So how is this an explanation of gravity as a curvature, when it requires a *force* to work? Is there a better explanation? Am I just missing something?
In: Physics
The gravity of earth extends beyond just the objects on earth. It keeps the moon in orbit, for example. These together also act on the sun and other planets, which in turn act on other solar systems and galaxies etc. Space itself doesn’t have gravity, just objects with mass. So any mass has a gravitational force, and they all influence one another.
Instead of imagining a trampoline with one object in the center, you might imagine space as a really large stretchy sheet with many objects. Any place there is a planet it will make a dimple in the fabric. As the objects move around their dimples cause the other objects to be affected.
Let’s say you have an object like a bowling ball, it will have a large dimple. Now imagine rolling a marble towards it. (The marble creates its own small dimple). If the marble isn’t going very fast, it will fall into the bowling ball dimple and stay there. But if you roll it fast enough along the edge of the bowling ball it will change direction and come out the other side and keep going (slingshot).
As to why objects distort this space time? It’s not entirely clear. We don’t know yet. Like you said, the trampoline model works because of earths gravity. It’s just an illustration that makes it easy for us to visualize. You are intended to ignore the gravity of earth on the trampoline. The object itself is what distorts the fabric.
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