The model usually shown to demonstrate how gravity works is a ball (planet) sitting on top of a mesh plane (space-time) and the mesh plane bending at the bottom of the ball. But what about the space above the ball? How is it that an object sits "on top" of space-time? Isn't it amerced in space? Isn't it more like a ball (planet) being submerged in water (space)? And isn't the ball displacing water and not bending it? Why is that not the case with a planet displacing the space it occupies? Lastly, when it is said that space bends, is it the molecules and other tiny objects in space that are being bent or space itself?
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The problem is that our universe is (at least) 4 dimensional. We have the 3 spatial dimensions (length, height, width) and time. Now, it's difficult enough to show a 3D image. It's impossible to show a 4D object.
So what you see in these types of animations is an object bending a 2D surface whereas, in fact, objects bend space-time which is 4 dimensional. We humans have serious difficulties trying to imagine a 4D object which is why we rely on the mathematics to describe it.
It's the actual physical space (and time) that are bent by objects. Or more precisely by the mass of those objects. What that means in practice is that if you watched a metre rule moving in that region, you'd see it physically change length. If you threw a clock into that region you'd see it physically slow down.
GPS satellites have to take the slight slowing down of their clocks relative to the clocks on earth to work out your position. Also, light traveling in a straight line can be bent around massive objects like stars (gravitational lensing) because the space-time through which it moves is distorted by the mass of that object. So this isn't just some theoretical calculation ... it's observed experimentally and we have to take it into account in things like GPS.
So what you see in these types of animations is an object bending a 2D surface whereas, in fact, objects bend space-time which is 4 dimensional. We humans have serious difficulties trying to imagine a 4D object which is why we rely on the mathematics to describe it.
It's the actual physical space (and time) that are bent by objects. Or more precisely by the mass of those objects. What that means in practice is that if you watched a metre rule moving in that region, you'd see it physically change length. If you threw a clock into that region you'd see it physically slow down.
GPS satellites have to take the slight slowing down of their clocks relative to the clocks on earth to work out your position. Also, light traveling in a straight line can be bent around massive objects like stars (gravitational lensing) because the space-time through which it moves is distorted by the mass of that object. So this isn't just some theoretical calculation ... it's observed experimentally and we have to take it into account in things like GPS.