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Perpetual motion

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See also perpetuum mobile as a musical term

Perpetual motion machines (the latin term perpetuum mobile is not uncommon) are a class of hypothetical machines which produce useful energy "from nowhere." The existence of a perpetual motion machine is generally accepted as being impossible according to current known laws of physics. In particular, perpetual motion machines would violate either the first or second laws of thermodynamics. According to which law is violated, perpetual motion machines are divided into two subcategories, referred to as perpetual motion of the first kind and perpetual motion of the second kind. There is always a chance that the accepted laws of physics are wrong, but a lot of evidence is needed to regeneralize these.

Physicists may try to test their knowledge of physics by proving, without using thermodynamics, that a proposed perpetual motion machine cannot work. Also, sometimes physicists will discover "apparent" perpetual motion in thought experiments. Such "paradoxes" expose misunderstandings of the meaning of accepted physical theories and are considered quite instructive.

Because the principles of thermodynamics are so well established, serious proposals for perpetual motion machines are met with disbelief on the part of physicists, which makes a discussion of the merits of the proposal difficult if not impossible.

Serious discussions of perpetual motion usually occur only when dealing with the topics of open systems, aether theories, free energy, and vacuum energy.

Criteria

Perpetual motion machines violate one or both of the following two laws of physics: the first law of thermodynamics and the second law of thermodynamics. The first law of thermodynamics is essentially a statement of conservation of energy. The second law has several statements, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known is that disorder (entropy) always increases; another statement is that no engine can be more efficient than a Carnot heat engine. See the respective articles, and thermodynamics, for more information.

Machines which claim not to violate either of the two laws of thermodynamics but rather claim to generate energy from unconventional sources are sometimes referred to as perpetual motion machines, although they do not meet the standard criteria for the name.

Kinds of Violations

First Kind

A perpetual motion machine of the first kind is one which produces power without energy uptake. Such a machine would, once started, operate indefinitely. This is forbidden by the law of conservation of energy.

Note that this explicitly prohibits the existence of devices which produce more energy than their input energy, as they can trivially be converted to a perpetual motion machine of the first kind by diverting part of their output energy back to their input.

Note that heat engines with an 'efficiency' greater than one do not violate this rule: the 'efficiency' in this case is defined as the ratio of heat output to work input -- the total energy input (heat + work) is still equal to the total energy output (heat).

Second Kind

A perpetual motion machine of the second kind is one which converts heat completely into other forms of energy. Such a device would violate the second law of thermodynamics (see also entropy) and would be viewed with great skepticism.

Thought experiments

As stated in the introduction, serious work in theoretical physics often involves thought experiments that test the boundaries of understanding of physical laws. An incomplete list of such includes:

  • Maxwell's demon: a thought experiment which led to physicists considering the interaction between entropy and information
  • Feynman's "Brownian Ratchet": a "perpetual motion" machine which extracts work from thermal fluctuations and appears to run forever but only runs as long as the environment is warmer than the ratchet
  • "Cosmic background space drive": where redshift/blueshift of the background radiation is used to drive a rocket's engine

An incomplete list of proposed experiments:

  • Casimir Cones: the basis for almost perpetual motion machine fueled by entropy, which exploits the Casimir effect

Inventions and Patents

Main article: History of perpetual motion machines

The invention of perpetual motion machines is a favourite pastime of many eccentrics, who often come up with elaborate machines in the style of Rube Goldberg or Heath Robinson. These designs may appear to work on paper at first glance, but have various flaws or obfuscated external power sources that render them useless in practice.

This sort of "invention" has become common enough that the United States Patent and Trademark Office (USPTO) has made an official policy of refusing to grant patents for perpetual motion machines without a working model. The USPTO granted a few patents for motors that are claimed to run without net energy input. These patents were issued because it was not obvious from the patent that a perpetual motion machine was being claimed. These are:

Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines.

See also

External links, resources, references

Historic

Research


Patents

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