Besides forbidding the construction of perpetual-motion machines, the second law defines what Planck’s predecessor Rudolf Clausius named entropy: because energy dissipates as heat whenever work is done—heat that cannot be collected back into useful, organized form—the universe must slowly run down to randomness.89 This vision of increasing disorder means that the universe is one-way and not reversible; the second law is the expression in physical form of what we call time.
