Entropy

:For other senses of the term entropy, see entropy (disambiguation).

The arrow of time

Entropy is the only quantity in the physical sciences that "picks" a particular direction for time, sometimes called an arrow of time. As we go "forward" in time, the Second Law of Thermodynamics tells us that the entropy of an isolated system can only increase or remain the same; it cannot decrease. In contrast, all physical processes occurring at the microscopic level, such as mechanics, do not pick out an arrow of time. Going forward in time, we might see an atom moving to the left, whereas going backward in time, we would see the same atom moving to the right; the behavior of the atom is not qualitatively different in either case. In contrast, we would be shocked if a gas that originally filled a container evenly, spontaneously shrinks to occupy only half the container.

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The reader may have noticed that the Second Law allows for the entropy remaining the same. If the entropy is constant in either direction of time, there would be no preferred direction. However, the entropy can only be a constant if the system is in the highest possible state of disorder, such as a gas that always was, and always will be, uniformly spread out in its container. The existence of a thermodynamic arrow of time implies that the system is highly ordered in one time direction, which would by definition be the "past".

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Unlike most other laws of physics, the Second Law of Thermodynamics is statistical in nature, and its reliability arises from the huge number of particles present in macroscopic systems. It is not impossible, in principle, for all 1023 atoms in a gas to spontaneously migrate to one half of container; it is only fantastically unlikely -- so unlikely that no macroscopic violation of the Second Law has ever been observed.

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In 1867, James Clerk Maxwell introduced a now-famous thought experiment that highlighted the contrast between the statistical nature of entropy and the deterministic nature of the underlying physical processes. This experiment, known as Maxwell's demon, consists of a hypothetical "demon" that guards a trapdoor between two containers filled with gases at equal temperatures. By allowing fast molecules through the trapdoor in only one direction and only slow molecules in the other direction, the demon raises the temperature of one gas and lowers the temperature of the other, apparently violating the Second Law. Maxwell's thought experiment was only resolved in the 20th century by Leó Szilárd, Charles H. Bennett, and others. It turns out that the information possessed by the demon can be considered a form of entropy, known as information entropy; as the demon operates, it gains entropy even as the gases lose entropy, so that the entropy of the system as a whole increases.

Related Topics:
1867 - James Clerk Maxwell - Thought experiment - Maxwell's demon - 20th century - Leó Szilárd - Charles H. Bennett - Information - Information entropy

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Many physicists believe that all phenomena that behave differently in one time direction can ultimately be linked to the Second Law of Thermodynamics. This includes the fact that ice cubes melt in hot coffee rather than assembling themselves out of the coffee, that a block sliding on a rough surface slows down rather than speeding up, and that we can remember the past rather than the future. (This last phenomenon, called the "psychological arrow of time", has deep connections with Maxwell's demon and the physics of information.) If the thermodynamic arrow of time is indeed the only arrow of time, then the ultimate reason for a preferred time direction is that the universe as a whole was in a highly ordered state at the Big Bang. The question of why this highly ordered state existed, and how to describe it, remains an area of research.

Related Topics:
Universe - Big Bang

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