I’m not quite sure of the biology behind it, but for my question let’s assume that spinning of any kind would cause dizziness as a more dense object would sink to the outside of the body showing that centrifugal force is present. If the astronaut, alone in a black universe with no reference point, was spinning would he be able to tell? (My guess is no). By an extension of that, would he feel dizzy. If he was spinning he would feel dizzy, and if he wasn’t spinning he wouldn’t feel dizzy. Would there be any way to know as the perspective of spin is relative? Would the denser fluids inside him be pushed to the outside?
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Qmechanic
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RadChad139
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This is essentially Mach's principle https://en.wikipedia.org/wiki/Mach%27s_principle – fqq Mar 20 '20 at 23:46
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Define alone. – Qmechanic Mar 21 '20 at 01:21
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If the astronaut or the spaceship or both were spinning, the astronaut would know it because of the nerve signals coming from the semicircular canals in his or her inner ear. Because of his or her weightless state, those spin signals would likely give the astronaut a case of motion sickness!
niels nielsen
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Yes. Even if there's no material reference point, the spacetime metric would look different whether he's rotating or not in respect to the inertial frame. Flat metric is invariant under Lorentz groop, (translations, linear boosts and rotations), but this group does not include angular boosts, that is changing to a rotating frame changes the observed behavior of matter (including the astronaut's body).
You just couldn't tell out front which frame is inertial; for that you need to observe the movement of the matter, or at least light. If there were no other matter in the universe thant the observer's bofy, he could only find out which frame is inertial by inspecting the ractions of his body.
Adam Latosiński
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You seem to imply that there is an inertial frame, but I’m confused on what this is as implying an inertial frame for which the universe and the matter in it (the astronaut) would be judged against doesn’t make sense. – RadChad139 Mar 21 '20 at 00:41
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@An inertial frame is a frame in which free bodies will travel along straight lines with constant speeds. If you start rotating, you see that the lines along which these bodies travel are no longer straight, so the rotating frame is not inertial. Since in an empty univverse you don't have any bodies, you can't see if the lines are straight or not, but it remains the fact that in an inertial frame they would be straight, if you'd put some test particles there. And eventually, the astronaut's body is testing it, whether the frame he's placed in is inertial or not. – Adam Latosiński Mar 21 '20 at 09:32