> Are gyroscopes entirely local or does the gravity of distant galaxies present a sort of universal reference frame? Or is it something else entirely?
Something else entirely. You don't need other galaxies or a universal reference frame. Even if the gyroscope were the only thing in the universe, it would be easy to tell if it were spinning: a very tiny person standing on the inside rim would be held down if it were spinning, or free floating if it were not.
Something else entirely. You don't need other galaxies or a universal reference frame. Even if the gyroscope were the only thing in the universe, it would be easy to tell if it were spinning: a very tiny person standing on the inside rim would be held down if it were spinning, or free floating if it were not.