Debate:Why is a perpetual motion machine impossible?

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Machines that simply "move" forever, or a very long time

  • The phrase "perpetual motion" is old-fashioned language. It does not mean simply something that stays in motion forever. This might be theoretically possible—it at least does not violate conservation of energy. But the phrase "perpetual motion" uses "motion" in an older sense to mean motor or engine, and it means something that can deliver energy—to run an automobile, for example—without requiring any external source of energy, such as gasoline. It means something for nothing.
COUNTER: By "perpetual motion machine" I mean simply what the term says: remain in motion perpetually. I'm not proposing running "something for nothing," which is obviously impossible. I'm proposing running something indefinitely, like the clock you describe. Why is that impossible?--Aschlafly 21:48, 31 December 2006 (EST)
I don't think it is impossible. I don't understand enough about the entropy and the second law of thermodynamics to be absolutely sure. And the Clock of the Long Now isn't expected to run forwever, just for ten thousand years.
By the definition of 'perpetually moving' it is indeed possible: the simple hydrogen atom becomes a working prototype. But the term is generally used to describe a machine that violates the 2nd law of thermodynamics: in any closed system useful energy will dissipate. Indeed any engine more efficient than a Carnot Engine is by definition a perpetual motion machine (Carnot Cycle). Airor 22 Feb 2007 2:22PM PST
The "perpetual motion machines" or perpetuum mobiles—the gadgets that the patent office refuses to review—all were intended to generate energy from nothing. They got to be a problem because inventors were constantly claiming to have found it and lining up investors. Often there was deliberate fraud, but sometimes there was just innocent ignorance involved. Usually there was either a hidden source of energy (e.g. in frauds, like the Keeley Motor) or the inventor believe he was producing energy when he was just releasing stored energy.
  • For an interesting example of an attempt to build a machine that will stay in motion for a long time, see The Clock of the Long Now. This is a serious effort to encourage long-term thinking by solving the engineering and social problems involved in building a clock will actually run for ten thousand years.
  • a long time is not forever.
  • It is not impossible becuase The Lord could make a perpetual motion device if he so wished it. --Cranky Joe 00:32, 14 March 2007 (EDT)

  • Hum... Could he also create a rock so heavy that he himself could not lift it?--Sm355 17:07, 19 March 2007 (EDT)
  • I do think that the second law of thermodynamics states "The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.", which essentially means that every closed system will eventually lose the potential for energy until it will reach the closest it can reach above absolute zero without actually reaching it (reaching absolute zero violates the third law). I personally have no idea as to whether this means perpetual motion is impossible - although it does essentially mean a practical application for perpertual motion is, as the energy released would be so insignificant that it could not be put to any forseeable use -- Shikamablue
  • It is possible to establish a flow of current in a superconducting ring forever, but that's all you get, and if you draw some of the current off to do useful work, there's just less in the ring. Teresita 22:05, 7 April 2007 (EDT)
  • I'm not sure that a superconducting ring qualifies as a machine. A machine must do work and I can't think of any work that such a ring would do.
  • If you draw off any energy from a pmm, and use it to power something, you would be increasing the amount of energy in the universe - thus creating something from nothing -> breaks 2nd Law of Thermodynamics. Wikinterpreter
  • "a long time is not forever" - exactly, try thinking of a place where a superconducting ring or a hydrogen atom could last undisturbed for an arbitrarily long period of time. EmanresU 17:42, 26 June 2007 (EDT)

First and Second Type

Let's not forget that two types of perpetual motion machines are defined. The first type is one that doesn't satisfy the First Law of Thermodynamics, while the second type doesn't satisfy the Second Law of Thermodynamics. And it is possible to design machines which satisfy both and are perpetual. Any watch based on solar energy for example. SilvioB 17:07, 3 July 2008 (EDT)

But a solar-powered watch isn't a closed system. It gains energy from the sun. -CSGuy 18:01, 3 July 2008 (EDT)
Absolutely. Just giving an example of a machine moving perpetually. But a REAL perpetually moving machine is impossible by definition - as long as either the First or the Second Law of Thermodynamics isn't falsified. SilvioB 18:04, 3 July 2008 (EDT)