Entropy is the only quantity in the physical sciences that "picks" a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says that, the entropy of an isolated system will increase when no extra energy is consumed. Hence, from one perspective, entropy measurement is thought of as a kind of clock though measuring entropy does not accurately measure time. There is also the complication that, locally, entropy can decrease with time: living systems decrease their entropy by expenditure of energy at the expense of environmental entropy increase. By contrast, all physical processes occurring at the microscopic level, such as mechanics, do not pick out an arrow of time. Going forward in time, an atom might move to the left, whereas going backward in time, the same atom might move to the right; the behavior of the atom is not qualitatively different in either case. In contrast, it would be an astronomically improbable event if a macroscopic amount of gas that originally filled a container evenly, spontaneously shrunk to occupy only half the container.

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