# Why is gravity the only force the causes time dilation?

## Main Question or Discussion Point

We all know that in general relativity, there is the idea that the gravitational force and accelerating reference frames are very closely related. In GR, gravity acts through distortions of the space-time continuum, which causes phenomena such as redshift and time dilation.

My question is this: how come the electromagnetic and nuclear forces aren't thought of along the same lines? What experiment could we do to determine the difference between an accelerating frame and a frame acted on by a uniform electric field? How come the other forces aren't thought of as being caused by some distortion of space and time, yielding things such as electromagnetic time dilation? Does it have to do with gravity's ability to attract all things with energy, regardless of charge, flavor, color, etc.? Lastly, whatever this fundamental difference is that separates gravity from the rest in these regards, how are they handled in current attempts at unified field theories?

Thanks for humoring me!
- Sooth

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PeterDonis
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My question is this: how come the electromagnetic and nuclear forces aren't thought of along the same lines? What experiment could we do to determine the difference between an accelerating frame and a frame acted on by a uniform electric field?
An electric field does not impart the same acceleration to all objects; for example, it doesn't affect electrically neutral objects at all, so it is trivial to distinguish a uniform electric field from an accelerating frame, since neutral objects will behave differently in the two cases.

In other words, the fundamental difference between gravity and the other forces is that gravity imparts the same acceleration to all objects, because it couples to energy, which is also what determines inertia. That is, the property of objects which determines how strongly they respond to gravity is the same property that determines how strongly they resist being accelerated. That's not true for the other forces.

How come the other forces aren't thought of as being caused by some distortion of space and time
Because of the above difference between gravity and the other forces. It's the fact that gravity imparts the same acceleration to all objects that makes the interpretation of gravity as spacetime curvature possible.

Thanks, PeterDonis. I think I was trying to convince myself of what you just said in my original post, but I couldn't think of it quite this concisely. It didn't really seem like a very strong argument until I read this:

That is, the property of objects which determines how strongly they respond to gravity is the same property that determines how strongly they resist being accelerated. That's not true for the other forces.
That was kind of an "aha!" moment.