Polchinski's Paradox should be focused on more. In the scenario given, a billiard ball is struck, and goes through a wormhole at just the precise angle to were it goes back in time to where it had been struck, and it collides with itself causing it to avoid the wormhole. This is more significant than the grandfather paradox because it mentions the use of wormholes, and an object that is not in control of where it is to go once it leaves the wormhole. This would mean that the chances of this happening are either extremely low meaning that wormholes are nearly impossible to be found in existence and take us through time to the precise time, or impossible because if the ball was to go through the wormhole as it did in the example, it would be incapable of going through in the first place.