the arrow of time
The arrow of time is the deep puzzle of why time seems to run in one direction only — always from past to future, never the reverse. We remember yesterday but not tomorrow; a dropped glass shatters but the shards never reassemble; a film of an omelette being un-scrambled looks absurd. Yet the strange thing is that almost all the basic laws of physics work equally well forwards and backwards, so where does this one-way sense of time come from?
The answer physics gives is entropy and the second law of thermodynamics. The second law says the total entropy — the amount of disorder, or of energy spread out and made unavailable — of an isolated system almost never decreases; it climbs relentlessly toward the future. That steady increase is the one physical quantity that reliably distinguishes past from future, so the direction in which entropy grows IS the direction we call forward in time. The future is simply the direction of higher entropy.
This thermodynamic arrow points the same way for the whole observable universe because it started, at the Big Bang, in an extraordinarily low-entropy, highly ordered state, and has been running downhill into disorder ever since. The honest subtlety is that a single molecule's motion is perfectly time-reversible; the arrow is a statistical, collective effect — overwhelmingly likely for the huge numbers of particles in everyday objects, but not an absolute prohibition. It is why you can trust that spilled milk stays spilled.
Film a drop of ink spreading through a glass of water, then play the film backward: the ink gathering itself back into a single sharp drop looks obviously wrong. That wrongness is the arrow of time — the spreading raises entropy, and only the entropy-increasing direction matches reality.
The direction in which entropy increases is the direction we experience as forward in time.
The arrow of time is statistical, not an ironclad law for a few particles: a handful of atoms could momentarily grow more ordered, but for the countless atoms in everyday objects a spontaneous drop in entropy is so unlikely it never happens in practice.