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1. Invisible matter
2. Some force other than gravity acting on the interstellar scale.
3. Gravitational theories are entirely correct on the small scale, but need some adjustment that kicks in on larger scales.
4. Gravity as we know it only applies to the solar system.
I don't see why the last reason is any less implausible than the first three. And gravity is pretty fundamental physics, so why should all the other fundamentals of physics work elsewhere too?
I'd say it's less plausible because, as you say, it's "pretty fundamental physics;" makes more sense to look for less-fundamental reasons. Occam's razor and all that. Your 3) is really the more plausible case of 4), I think--why throw out the entire theory of gravity when fine-tuning might suffice?
Also, observing the trajectories of other stars, particularly those with close neighbors, gives us a way to check the local behavior of gravity nearby, and AFAIK it seems to meet our expectations. (Although obviously we can't do sensitive experiments and detect small deviations from light-years away.) You don't see big deviations from theoretical predictions until you look at the galaxy as a whole.
And because physics seems to go a little strange on the interstellar scale, we can't say for sure: Yes absolute zero is definitely the same in Andromeda', simply because we can't say that the simple laws of motion work there for certain.
Well, we still can say absolute zero's the same, simply because absolute zero is defined as an absence of atomic or molecular motion. If stuff there is moving, it's not at absolute zero.
One might ask, though, whether you can get as close to absolute zero in one part of space as to another, since in general the ground state energy of a system is nonzero. And there may certainly be discrepancies in ground state energies of similar systems in different regions of space; I dunno if anyone's tested for that yet. |
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