The Billiards Paradox

I just read an interesting restatement of the Grandfather Paradox:

Imagine a pool table with a wormhole on it. The wormhole loops through time and space such that if you shoot a billiard ball into one end, it comes out the other end a second or so BEFORE it enters. So you'd see it emerge before it enters, and for a second or two, there are two of the same ball on the table. (We're ignoring whether this violates conservation of mass/energy.)

Now imagine aiming the ball at the wormhole so that when it comes out of the wormhole, it hits the version of itself rolling towards the wormhole, and deflects that one so it never enters the wormhole! Which means it doesn't come out of the wormhole, so it isn't deflected, so it does enter the wormhole, and it does deflect itself...and so on.


Yes, that's the kind of thing I find interesting.