> Hi All, I am still having difficulty getting my head around the gravity point. Now I accept, in principle, that due to relativity an intense gravity field will slow a clock. My problem is visualising where you will find this field. At the centre of this planet gravity (from the planet) is zero. This comes about by an elegant piece of calculus that shows that everywhere inside a hollow sphere the gravity is zero. So a clock in the centre of Earth runs at the same rate as one on the surface? or does it run faster because the one on the surface has the planetary gravity acting on it? I think that the one inside the Earth runs faster. But when you are between the Earth and Moon at a point where gravity forces are neutral we should have the same rate as centre of planet? Now the way to measure gravity is to measure the force on a test mass. If there is zero force there is zero gravity, except when you are in orbit. This can be tested with 3 crossed gyroscopes that show your angular velocity. If your angular velocity is negligible then the magnitude of the gravity field is proportional to the force on a test mass. Unless you are in free fall accelerating towards a mass. I guess my question really is" can you know that you are in a zero gravity field so your clock is running at the fastest rate?" Or is relativity relative. Does relativity only show up when there are two frames of reference being compared, so there is no ultimate reference frame with the fastest clock? Or can any clock have a single relativity correction applied to it? How do you measure the gravitational potential at any site? (ie the scalar quantity that would be used to correct your clock). Has anyone got a clear answer? cheers, neville Michie
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