That is what is so unsettling about T-symmetry. What you describe is merely <i>improbable</i>, not impossible. With appropriate starting conditions, it is certain. Basically we&#x27;d require all the particles in the comet to bounce into each other in such a way that their heat motion cancels and organizes into momentum in one direction, away from the planet. This is physically permitted in either time-direction, but we only see it in one.