The Impossible Loop

The grandfather paradox is the most famous problem in time travel logic. Travel to the past and prevent your grandparents from meeting. You are never born. But then nobody travels back, so your grandparents do meet. You are born. You travel back. The loop has no stable state.

This is not just a story problem. It is a logical contradiction — a situation where a statement and its negation are both required to be true at the same time. If backward time travel is possible, the universe must have some way of preventing or resolving these contradictions.