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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 would produce useful energy "from nowhere". The existence of a perpetual motion machine is generally accepted as being impossible according to our current understanding of the laws of physics. In particular, perpetual motion machines would violate either the first or second laws of thermodynamics. Perpetual motion machines are divided into two subcategories defined by which law of thermodynamics would have to be broken in order for the device to be a true perpetual motion machine. These two subcategories are referred to as "perpetual motion of the first kind" and "perpetual motion of the second kind". Scientists and engineers accept the possibility that the current understanding of the laws of physics may be incomplete or incorrect; a perpetual motion device may not be impossible, but overwhelming evidence would be required to regeneralize the laws of physics.
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 produces more
energy than it uses. Therefore, once started, the machine can go on forever. This is prohibited by the Law of Conservation of Energy. Many of these designs utilize
magnets as some kind of free energy source, and assume frictionless surfaces. However, while several of these designs are not perpetual motion machines, they oftentimes can run on their own for a long time, as long as they're not asked to do any real
work to the external environment.
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
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:
- Johnson, Howard R., US4151431 "Permanent Magnet Motor", April 24, 1979
- Baker, Daniel, US4074153 "Magnetic propulsion device", February 14, 1978
- Hartman; Emil T., US4215330 "Permanent magnet propulsion system", December 20, 1977 (this device is related to the Simple Magnetic Overunity Toy (SMOT))
Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines.
See also
- Main: History of perpetual motion machines, Open system, Vacuum energy, Radiant energy, Free energy, Negative resistance, Brownian ratchet
- Science: Conservation of energy, Thought experiment, Thermodynamic entropy, Second law of thermodynamics, Kirchhoff's Laws
- People : George Westinghouse, Josef Hoëné-Wronski
- Other: Perpetuum mobile, Mad scientist, List of disputed theories
External links
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