Drop a glass and it shatters. Spill milk and it spreads across the floor. Watch a campfire and it dies down to embers, never up to flames. These everyday observations all point at the same physical law: in a closed system, disorder always increases. This is the second law of thermodynamics.

It is, strangely, the only physical law that distinguishes past from future. Drop the same glass on a film played in reverse and the shards leap up and reassemble. Every other law of physics — gravity, electromagnetism, quantum mechanics — works the same forwards as backwards. Only entropy has a direction.

The physical quantity behind the law is called entropy: a measure of how many possible microscopic arrangements correspond to the same macroscopic state. A neat row of dominos has very few arrangements. A pile of fallen dominos has astronomically many. So the universe drifts from neat to messy because messy is statistically overwhelming.

This rule is unforgiving. Every star burns hydrogen because nuclear fusion increases entropy. Every cup of coffee cools because heat flows from concentrated to spread out. Every living thing eats and excretes because organisms are temporary entropy reducers — surrounded by a bigger entropy increase.

Eventually, in roughly 10^100 years, the universe runs out of usable temperature differences. Stars stop forming. Black holes evaporate. The temperature of everywhere becomes the same — a fraction of a degree above absolute zero. No engine can run, because engines need a hot side and a cold side to work.

Physicists call this the heat death of the universe. The lights, eventually, go out.

The arrow of time only points one way.