# General theory of relativity questions [duplicate]

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I do not understand how this theory (the theory of general relativity) would work because gravity is what makes things heavy and weight is what bends things. All of the proof makes it hard to question strongly though (with light bending and all) so simply put my question is how does the earth, sun and everything else "bend" space without gravity to pull it down into space? And if this is so how does the earth's bending of space attract us to the earth?

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• Stop thinking of the trampoline analogy. Nothing is “pulling” the earth down. The presence of matter is what sources spacetime curvature, and any test particle moving through space follows paths determined by the curvature (namely, the path of least “distance,” defined in a reasonable sense). The trampoline analogy is just there for a visually intuitive explanation, and should not be taken literally. – Bob Knighton Jun 24 at 17:10
• Then how does the sun "bend" space? – flying squirle Jun 24 at 17:20
• What kind of explanation are you looking for? Did you ask “how” Newtonian gravity worked, or did you just accept that we observe it to obey an inverse square law? Learning GR requires learning some new math in order to understand what “curved spacetime” means. – G. Smith Jun 24 at 17:24
• Gravity is caused by the "bending" of time, not space. We move forward in time, but since time is "bent" (dilated), this movement appears as falling. The reason time is "bent" by energy is the fact that time and energy are two sides of the same existence (Fourier conjugates). So more energy translates to less (slower) time and causes things to fall there. – safesphere Jun 24 at 20:56
• @BobKnighton The trampoline analogy is perfectly valid, but commonly misunderstood. It is a plot of time dilation in the reduced (by 1) number of dimensions. In other words, this analogy represents the curvature of time that causes gravity. Despite a common misconception, gravity is not caused by the curvature of space: "in space-time the curvature of space has nothing to do with gravity (the 'curvature of time’, one could say)" - en.wikipedia.org/wiki/Ellis_wormhole - You can use the Schwarzschild metric to plot a mathematically correct trampoline chart of gravitational potential. – safesphere Jun 25 at 13:10

## 1 Answer

Einstein's equation in GR describe quantitatively how space-time metric behaves, depending on the stress–energy tensor. This is how ("how much", if you wish). Nevertheless, I think that by "how" you meant "by which means", so it is indeed a "why". This one is easy to answer: no one knows. There is no proven physics theory going further that GR in understanding how gravity works. The most known "works in progress" are string theory (or its children) and quantum loop gravity (there exist others, less popular), but they haven't yet proved to hold any truth.

So, for the moment, the only answer is: "nobody knows".

• We do know why. Time and energy are Fourier conjugates (the "opposite sides" of the same), so "more" (concentrated) energy translates to "less" (slower) time that causes gravity. – safesphere Jun 25 at 12:43
• @safesphere The stress–energy tensor is the conserved Noether current associated with spacetime translations when the Hamiltonian doesn't depend on them. I don't think that you can use that to state "more energy translates to less time that causes gravity". By the way, curvature still exists outside of massive objects, in vacuum. – Matt Jun 25 at 13:28
• Sure I can, but otherwise you are correct. Things fall down releasing energy, so energy is not conserved in gravity, unless we invent a "renormaluzation" component and call it "potential energy". However in reality there is only curved spacetime, nothing else. The Noether relation that you mentioned is exactly the same time/energy symmetry in my comment. We are talking about the same thing. Same for gravity in vacuum - you can see it as cased by the energy of the "field", if you prefer, or by the energy of matter as the original source. There is no contradiction. – safesphere Jun 25 at 14:12