[PeterisP]

If it wasn't a joke, then that was simply a misleading false statement.

Let's take the simple example of earth orbiting around the sun. Playing time backwards gets you a orbit in the opposite direction, while gravity becoming antigravity would mean that earth would get repelled by the sun and thus go off to infinity.

[robotresearcher]

That's interesting. Playing time backwards long enough would see the earth disassembled into rocks, dust and gas, repelling each other and indeed flying off into <far away>. Same with the sun. But the short term orbit example challenges the intuition. Perhaps the answer is that the time-forward orbit is (conventional) downhill in spacetime, and the time-backward orbit is uphill in spacetime, but both trajectories are seen in conventional space as a curved path around the center of gravity.

[PeterisP]

> Playing time backwards long enough would see the earth disassembled into rocks, dust and gas, repelling each other and indeed flying off into <far away>.

No, playing time backwards long enough would see a hot earth exploding into rocks, dust and gas that are attracting each other - just the initial velocity is large enough and attraction is not strong enough to stop them from flying out into <far away>. They would be slowing down when flying off, not accelerating as if they were repelling each other.

They would then be joined by the dissolving sun and form a cloud of dust which some time later (i.e. earlier) would converge (because the dust is attracting itself) into some earlier massive star(s) out of whose remains our solar system was formed.

If an asteroid hits the earth, the gravitational potential energy (of an attractive gravity) gets turned into kinetic energy as it accelerates when approaching the earth and afterwards into heat as it impacts it; playing time backwards, the heat gets turned into kinetic energy, which then gets turned into gravitational potential as it distances itself from earth.