Perpetual Motion
The persuance of perpetual motion is to physics and mechanical engineering what alchemy is to modern chemistry, which is to say that it is seeking after the unrealizable. The earliest written description of a perpetual motion machine dates to the fifth century A.D. Manuals and scientific treatises abounded throughout the Middle Ages and the Renaissance that described various perpetual motion devices. As late as the early 1800s, a miller in Ohio poured his life savings into building a three-story mill house that was supposed to run perpetually by means of a closed system powered by 500 gal (1,890 L) of water. It didn't work.
Perpetual motion, when defined by its literal meaning, is an impossibility according to Classical physics. A perpetual motion device, when set in motion, never stops and never requires further input of energy. Another way of saying that is, it puts out more energy than it consumes. If such a machine could be built on a big enough scale, it might promise limitless kinetic or electrical energy.
The first two laws of thermodynamics are what prevent perpetual motion from working. The first law, which addresses the conservation of energy, prevents a system from continuing to create its own power. The first law negates the possibility of a perfectly efficient system, and requires that some energy always be lost as heat or friction. Conservation of energy dictates that work and energy seek to reach equilibrium, and thus, as energy is dissipated, work slows to a stop. This is why perpetual motion machines that utilize falling water or overbalanced wheels always grind to a halt, even if they power themselves for a startlingly long time.
The second law of thermodynamics gives us entropy, which says that all closed systems eventually break down. It is this principle that disallows perpetual motion devices that attempt to self-sustain a system based on condensation/evaporation cycles of liquids and gasses in sealed boilers and similar apparatus. Over time, the finite energy within the system is lost as heat, and the system ceases to work.
The laws of thermodynamics are statistical in nature. This is just due to the fact that while it is impossible to measure the motion of every molecule in a system, it is possible to measure the average of these motions. Statistically then, the individual motions are just treated as random fluctuations around the average. Though exceedingly rare, this collective randomness does open up the possibility that the molecules of a system will not precisely follow the predictions of thermodynamics. That means there is a statistical possibility that a pan of water placed on a roaring fire will freeze. It follows that a closed system has a theoretical chance to operate without energy loss. This "loophole" in the physical impediment to the possibility of perpetual motion offers an intriguing, if totally academic twist to this 1,500-year-old quest. Since such a happenstance would be totally unpredictable, it is likewise totally unharnessable. It is also highly unlikely. Chemist Henry Bent, who coined the now famous analogy about a group of monkeys typing out the works of Shakespeare (as a comparison to the likelihood of a local reversal of entropy), also claimed to have calculated the odds of a reversal of entropy resulting in usable energy. He said it would be akin to the monkeys turning out Shakespeare's plays 15 quadrillion times without error.
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