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
> On earth, that’s just the planet’s gravity. If you did the same experiment on space the objects wouldn’t roll down.
Except they would. Things in space fall. The Moon is falling towards the Earth (the Earth’s gravity is pulling it down), satellites and space stations are falling towards the Earth. The Sun is falling towards the Earth (as is the Earth towards the Sun).
The pull of gravity extends to infinity (with some qualifiers). Stars pull each other towards themselves to form spiralling galaxies, galaxies are pulled together to form clusters, gravity is everywhere. Anything with mass (and so energy, as the two are equivalent) pulls anything else with mass (or energy) towards it.
Gravity is just very weak. So it takes really huge things (like planets) for it to be noticeable, and then accelerations it creates can be fairly small (think how easy it is to create 1g of acceleration or “ge-force” by other means).
> So how is this an explanation of gravity as a curvature, when it requires a force to work?
Gravity can be modelled as curvature of spacetime. It isn’t the only way; the force model mostly works as well. And no one has quite found a solid way to combine the curvature model with quantum mechanics, so there may be some stuff missing.
The thing you may be missing with the rubber sheet analogy (and remember that analogies, like cars, tend to break down eventually) is that the “sinking” in the sheet extends forever (in this model). Spacetime isn’t a small sheet with defined boundaries that are held in place. So placing a small weight on one part of the sheet – distorting it there – will still create a distortion all the way to infinity. Even if that distortion will get infinitely small, and will quickly be drowned out by the distortions caused by other objects.
The weakness of the analogy is exactly what you’ve spotted. The rubber sheet thing only works *because of gravity* pulling the ball down (and so the sheet with it), and causing other objects to “stick” to the sheet (because they want to fall through). With actual gravity there is no “force” pulling things down, and spacetime isn’t pulled in some extra dimension. Instead spacetime is squished together.
If it helps, instead of thinking of spacetime as being distorted or curved through some extra direction, think of it as being bunched up; imagine two points in space that are 10m apart from the outside. There should be 10m of space between them. But if you try to walk from one point to the other (in a ‘straight’ line) you end up walking 16m. Because there is more space per space. Space is bunched up so that on the inside the points are 16m apart, even if from the outside they should be 10m apart. In this case, if the bunching is local, you might find that going around the distortion is a shorter distance than going through (so if you went around in a circle, you would walk 15.7m assuming no distortions happened there).
And it turns out this happens. Objects by default travel in “straight lines”; except a “straight line” in this context means the shortest distance between two points. So moving objects will curve around distortions in spacetime (parts where spacetime is bunched up).
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