[tux1968]

> A planet orbiting the sun is accelerating toward the sun.

How do we know that the sun isn't orbiting the fixed earth, and accelerating toward the earth? Everything would look _exactly_ the same to everyone.

Yes, the math works out much easier and more neatly, so that is likely what is happening in reality. But from a purely relativistic framework.. you can consider any point in space the frame of reference, and all motion relative to that "fixed" point.

It makes me think that relativity is actually describing a more subjective experience, rather than the objective reality where we "know" the earth orbits around the sun.

[tsimionescu]

> How do we know that the sun isn't orbiting the fixed earth, and accelerating toward the earth? Everything would look _exactly_ the same to everyone.

That wouldn't work out. You can take the earth as a fixed point and describe the Sun's motion relative to it, and it would be perfectly valid, but it wouldn't look as the Sun orbiting the Earth in any kind of almost constant speed elliptical orbit, it would look like a very different kind of motion.

[yongjik]

Well, maybe, but I think you're mixing the everyday definition of "relativity" (i.e., things are relative) with what physicists mean by "relativity" (i.e., the Theory of Special or General Relativity, both theorized by Einstein).

[random314]

Einstein's theory of relativity is exactly what you are referring to as "everyday definition of relativity ".

[tux1968]

> but I think you're mixing the everyday definition of "relativity"

Well, yes.. because Einstein's theory beautifully makes all the math work out, explaining one interpretation of motion as the real one. But what it did at the same time was show that there is no "fixed grid" of space independent of the objects themselves. Which is what leads to my uneasy feeling of how we ever can say one interpretation is more real than another -- i admit it may just be a nonsensical perspective.

As for what all physicists mean by relativity in general... I can highly recommend this old series from the National Science Foundation:

https://www.youtube.com/watch?v=Y75kEf8xLxI