시간은 왜 한 방향으로만 흐를까

The Puzzle of Direction

A shattered cup never reassembles on its own, though the laws governing each atom would permit that reversal without objection. Films of billiard balls colliding look equally plausible run forward or backward, yet a film of milk unmixing from coffee looks absurd. This asymmetry poses one of physics' deepest puzzles: why does time appear to carry us in a single, irreversible direction? The answer, remarkably, does not lie in the fundamental equations of motion, which treat past and future as interchangeable partners. The equations run just as happily backward as forward, without any built-in preference for one direction. Nothing in Newton's laws, or in the rules of quantum mechanics, marks one moment as earlier and another as later. Yet our lived experience insists, stubbornly and universally, that the two directions are nothing alike. The resolution appears only when we look past individual particles toward the statistics of enormous crowds of them.