Why Is Gravity Always Attractive?

[PeterDonis]

For the same reason time always moves from past to future.

This can't be right, because gravity is still attractive if you reverse the direction of time. There are physical processes we know of that violate time reversal invariance, but gravity is not one of them.

 

[PeterDonis]

The Galileo experiment, and astronaut Scott's demonstration of it, was not accurate enough to show variations in gravity due to mass distribution.

I assume you are talking about tidal gravity here? The general law that acceleration of a test object is independent of its mass (and mass distribution) assumes that tidal gravity is negligible. If tidal gravity is not negligible then you are right that this general law is no longer exactly correct.

 

[PeterDonis]

If he had dropped another moon what would have happened?

This isn't really a comparable situation, because there is no way to drop another moon towards the moon while ignoring the effects of tidal gravity, whereas, as I noted in another post just now, the general law under discussion is only correct if tidal gravity is negligible. Also, another moon would not qualify as a "test object", and the general law under discussion only applies to test objects. Test objects, by definition, have negligible gravitational effects themselves, and that is obviously not true for another moon.

 

[Andrew Mason]

I assume you are talking about tidal gravity here? The general law that acceleration of a test object is independent of its mass (and mass distribution) assumes that tidal gravity is negligible. If tidal gravity is not negligible then you are right that this general law is no longer exactly correct.

Yes. Tidal forces. I assume that is what you mean by tidal gravity. Tidal forces, due to differences in the gravitational forces/unit mass acting on different parts of a body due to differences in position in the gravitational field of the other gravitating body, have the tendency to pull a body apart, which is the opposite of how one thinks of gravity.

AM

 

[PeterDonis]

Tidal forces. I assume that is what you mean by tidal gravity.

Yes.

Tidal forces, due to differences in the gravitational forces/unit mass acting on different parts of a body due to differences in position in the gravitational field of the other gravitating body, have the tendency to pull a body apart

Not always. Radial tidal forces in the field of a spherical mass do this, but tangential ones don't; they have a tendency to push a body together. But both of these are different from the effect of the Newtonian "force" of gravity, which has no effect at all on the internal structure of a body, neither pulling apart nor pushing together.