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My understanding is that without acceleration the "movement" of a body is a relative concept, i.e. we can choose an inertial frame of reference where the body is at rest and there is no property or experiment that can tell us that the body is in movement, because it's a meaningless question.

In the same way, can I say that the movement of earth is arbitrary, just choosing a non-inertial frame of reference? The fact that I need to include fictitious forces to explain for example the movement of a Foucault pendulum, means that the earth rotation is "absolute"? The law of physics should stay the same if I choose the earth as frame of reference, but does that mean that there is nothing absolute about the movement of the earth?

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    $\begingroup$ Yes. We think that earth is orbiting the sun, but as well we could think of the sun orbiting earth. The (huge) difference is that in the later case the equations of movement would be much more complicated. $\endgroup$
    – Gyro Gearloose
    Commented Nov 7, 2023 at 19:06

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Rotation is absolute. It may be that rotation is relative, but relative to all the gravitational interactions with the rest of the universe, in which case the absolute character of rotation is an emergent quality peculiar to the configuration of the real universe. One cannot isolate a system from gravitation and the rest of the universe does in fact exist, and one cannot spin the universe, so it could be true (but un-testable) that a truly isolated system in some hypothetical otherwise empty universe couldn't tell if it was rotating. It may just be true that rotation is absolute. We don't yet have a theory which produces a testable hypothesis which could distinguish between the two options.

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  • $\begingroup$ This answer is worded in a somewhat confusing way? Perhaps you could focus on the question of whether a reference frame in which the earth is stationary is physically acceptable? $\endgroup$
    – Eletie
    Commented Nov 7, 2023 at 22:07
  • $\begingroup$ You can pick whatever coordinate system you want, but if you pick one in which Earth is not rotating in all the ways that it is rotating, you have picked one in which the laws of physics are strongly position-dependent. Adding centrifugal, Coriolis, and Euler forces works well enough in the local neighborhood and Newtonian velocity and distance domains, but try calculating e.g. the translational velocity of the Andromeda galaxy in an earth-stationary reference frame. $\endgroup$
    – g s
    Commented Nov 7, 2023 at 23:35
  • $\begingroup$ The statement that "The law of physics should stay the same if I choose the earth as frame of reference" is itself false - they have to be changed by the addition of the aforementioned fictitious forces; and at large distances (hence large velocities from east to west) the definition of velocity has to be changed because large velocities do not concatenate by arithmetic addition the way that small ones do. $\endgroup$
    – g s
    Commented Nov 7, 2023 at 23:44
  • $\begingroup$ This seems at odds with the comment from Gyro Gearloose, could you clarify? Saying that calculations are difficult in certain systems of coordinates doesn't imply they cannot be used. $\endgroup$
    – Eletie
    Commented Nov 8, 2023 at 12:15

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