Polchinski’s Paradox

In 1990, physicist Joseph Polchinski described a puzzling situation: Suppose a billiard ball were sent through a traversable wormhole into the past, where it knocked its earlier self off course, away from the wormhole. This is a version of the famous grandfather paradox: How can we explain the ball s presence where it seems to have prevented its own arrival? Fortunately, Caltech students Fernando…