Why does time have a direction? Release a gas into an empty space and it spreads out, never gathering back — entropy, and the second law, made visible.
Entropy & Free ExpansionLive
the second law in action
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Gas confined to one half rushes to fill the whole box the instant the partition is removed — and never spontaneously crowds back. That irreversibility is the second law: entropy, a measure of disorder, always increases. For doubling the volume the entropy rises by nR·ln2, purely because there are vastly more ways to be spread out than packed in.
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How it works
When a gas confined to half a box is freed, it rushes to fill the whole volume and never spontaneously returns. There are overwhelmingly more arrangements spread out than packed in, so entropy — the measure of that disorder — increases by nR·ln2 for a doubling of volume. This one-way behavior is the second law of thermodynamics and the origin of the arrow of time.
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