Nobody needs physics training to spot a film running backwards: shards gathering themselves into an intact glass simply look wrong. The strange part is that every individual molecular collision is reversible and no law forbids the reverse direction. What forbids it is statistics: disordered arrangements vastly outnumber tidy ones, so the chance that 10²³ gas molecules crowd back into half the container is about (1/2)^N, effectively zero. The second law of thermodynamics says the entropy of an isolated system never decreases, and that direction is the arrow of time.
Nobody needs physics training to spot a film running backwards: shards gathering themselves into an intact glass simply look wrong. The strange part is that every individual molecular collision is reversible and no law forbids the reverse direction. What forbids it is statistics: disordered arrangements vastly outnumber tidy ones, so the chance that 10²³ gas molecules crowd back into half the container is about (1/2)^N, effectively zero. The second law of thermodynamics says the entropy of an isolated system never decreases, and that direction is the arrow of time.
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