Yea and those happen when other forces than gravity come into play. When matter starts colliding.

Every object has been subject to forces other gravity at some point.

And the point is that sometimes comets do indeed fall into the Sun. If you object to people calling that a comet that’s fine - we can use whatever name you want.

The point is that the gravity interactions are time reversible. Not so with friction etc.

Sure, nothing in the laws of physics prevents a celestial body from distancing itself from the Sun or from the Moon. But it would like suspicious! Wouldn't you suspect that you were watching a reversed video?

You don't understand my point. If we watch purely gravity interactions, the video can be reversed and you wouldn't be able to detect it.

If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible. Ie it's not only gravity.

That's the point.

In other words, if a comet approached the moon at high speed, missed and slingshotted in another direction, it would be traveling away from the moon, but the video would be time reversible and you couldn't be able to detect it. Gravity only interaction.

I think you don’t understand my point.

Someone wrote “If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video?”

I replied “If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.”

I don’t know what part do you object to (if any).

> If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible.

If a meteor (or an apple) is still in a crashing trajectory when you stop recording there are no effects other than gravity. The video is reversible - but weird when you play it in reverse.

Well yes for example if there was a cannon firing an apple from the moon, then it would travel the same trajectory, just in reverse.

But we know there are no apple firing cannons on the moon.

Ie if an object was coming from the moon and its past trajectory intersected with the moon's surface, you could say "this is reverse video".

That was exactly my point, that an apple coming from a black hole may seem problematic but so does an apple coming from the Sun. I don't see an essential difference regarding the "reversibility" of the physical process (but I could be wrong).

Maybe this thought experiment makes it clear. There's a cannon firing straight up from the lunar pole. One cannot observe the explosion charge. The ball goes straight up, and then falls down and goes back to the barrel. The cannon is filmed from afar firing and you can see the whole ball travel.

Then you are shown two films, the normal and a reversed one. Would you be able to tell which one is which, and if so, how?

> Maybe this thought experiment makes it clear.

Everything was already clear, I think.