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Mark_Friedenbach comments on What are your contrarian views? - Less Wrong
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Care to explain this one?
Yes.
A big pie, rotating in the sky, should have apparently shorter circumference than a non-rotating one, and both with the same radii.
I can't swallow this. Not because it is weird, but because it is inconsistent.
There's a reason it's called special relativity. It only works in special cases. Eucludian geometry and Newtonian mechanics are inconsistent, btw. Special relativity solves these inconsistencies in the special contexts where they originally came up (predicting the Lorentz contraction and time dilation which is experimentally observed). It wasn't until the curved space of general relativity was discovered that we had a fully consistent model.
And yes, curved space of general relativity fully explains the rotating disc in a way that is self-consistent in in agreement with observed results (as proven by Gravity Probe B, among other things).
Special relativity is consistent. It just isn't completely accurate.
It's inconsistent with solid-body physics, but that's due to the oversimplifications inherent in solid-body physics, not the ones inherent in special relativity.
Trying to fit solid-body physics into general relativity is even worse. With special relativity, it works fine as long as it doesn't rotate or accelerate. Under general relativity, it can only exist on flat space-time, which basically means that nothing in the universe can have any mass whatsoever, including the object in question.
Twin paradox.
What about the twin paradox?
You need GR if you want to treat talk about the rotating reference frame of the disk. Otherwise SR is fine.
“claim[ing] that special relativity can't handle acceleration at all ... is like saying that Cartesian coordinates can't handle circles”
See http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html
But then again, the question whether the study of flat spacetime using non-inertial reference frames counts as SR depends on what you mean by SR. If you mean the limit of GR as G approaches 0, then it totally does.
Is it any Lorentz contraction visible in the case of around the galaxy rim?
Are all the Lorentzian shrinks just cancelled out?
I'd really like to know that.
You don't need GR for a rotating disk; you only need GR when there is gravity.
Rotation drags spacetime.
Only if the rotating object is sufficiently massive.
Only if the rotating object has any mass at all.
For a rotating object of sufficiently small mass, the mass can be ignored, and reasonably accurate results can be found with special relativity.
I don't disagree. This discussion was philosophical in the pejorative sense, being about absolutely exact results, not reasonable approximations.
The OP was claiming that special relativity was incoherent, not just that it wasn't absolutely exact.
If you want absolutely exact results, you'll need a theory of everything. There are quantum effects messing with spacetime.