Almost all the physical laws we have met are reversible: if you film a planet orbiting and play the film backwards, you still see a possible planetary motion; if you film a ball bouncing elastically and play it backwards, you notice nothing strange.
But if you film a drop of ink diffusing in a glass of water and play it backwards — a dark speck spontaneously re-forming out of nothing — you immediately recognise that the video is running backwards. Why? Because the entropy of the universe can only increase.
In a universe governed only by Newton and Maxwell, time would have no privileged direction: the equations work equally well in both directions. It is we who perceive the past as different from the future because we live immersed in a myriad of irreversible processes — bodies cooling, gases mixing, structures degrading — and the law that governs all these processes, giving them a single direction, is the second law of thermodynamics. The arrow of time, in other words, is the arrow of increasing entropy.
Collegamenti
Argomenti: Entropy and the second law Concetti: Second law of thermodynamics · Entropy
Esercizi collegati: Worked exercise — the coffee cools down · Hot shower: ΔS of the universe · Heat from hot to cold: proof