The Difference Between Paradoxes and Tautologies

Tautologies are not the same thing as paradoxes. There's nothing in physical law that prevents self-fulfilling prophecies. A billiard ball traveling through a wormhole into the past would never nudge its earlier self away from the mouth of the wormhole and always nudge itself into the wormhole. So you wind up with this causally circular structure rather than with any kind of paradox.