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entropy

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Entropy is a measure of disorder, and in a closed system it always increases — that's the second law of thermodynamics. Things spread out and mix; they don't spontaneously tidy themselves up. Strangely, the basic laws of physics don't actually forbid time running backward — nothing in the math rules it out. Why we only ever see time move forward, from order to disorder, is still one of physics' biggest open questions.