Perpetual motion

See also Perpetuum mobile as a musical term and Perpetual Motion as a solitaire game.

Perpetual motion is a condition of a body's changing position with respect to time, usually without a source of energy. Perpetual motion machines (the Latin term perpetuum mobile is not uncommon) are a class of hypothetical machines which would produce useful energy in a way which would violate the established laws of physics. It is generally accepted that perpetual motion machines cannot exist. 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.

Introduction

There are two subcategories of perpetual motion machines which 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 justify rewriting the laws of physics. Any proposed perpetual motion design offers a potentially instructive challenge to physicists: we know it can't work (because of the laws of thermodynamics), so explain how it fails to work. The difficulty (and the value) of such an exercise depend on the subtlety of the proposal; the best ones tend to arise from physicists' own thought experiments. 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 (if any) 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.

In short, the entire reason perpetual motion is possible in theory, but impossible in practice, is because friction exists when two entities make contact whilst moving. If friction were removed (which is impossible) then perpetual motion would be possible.

Criteria and violations

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, or at the least stays the same; another statement is that no heat engine (an engine which produces work while moving heat between two places) can be more efficient than a Carnot heat engine and that as a special case of this no machine operating in a closed cycle can do anything but transform thermal energy to work in a region of constant temperature. 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. By way of example, it is quite possible to design a clock or other low-power machine to run on the differences in barometric pressure or temperature between night and day. Such a machine has a source of energy, albeit one from which it is quite impractical to produce power in quantity.

First kind

A perpetual motion machine of the first kind produces at least as much energy as it uses. Therefore, once started, the machine could go on forever, which appears to violate 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 often can run on their own for a long time, as long as it's not required that they 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

Serious work in theoretical physics often involves thought experiments that test the boundaries of understanding of physical laws. Some such thought experiments involve apparent perpetual motion machines, and insight may be had from understanding why they either don't work or don't violate the laws of physics. For example:

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

One of the first descriptions of a perpetual motion machine was by Bhaskara in 1150. He described a wheel that he claimed would run forever. Villard de Honnecourt in 1235 described, in a thirty-three page manuscript, a perpetual motion machine of the second kind. In the 1760s, James Cox (with the help of John Joseph Merlin) developed a working perpetual motion machine (known as Cox's timepiece), though it doesn't violate the laws of science but is built on them. Cox was quite open with his machine, unlike many perpetual motion inventors. The clock's internal mechanisms are powered from changes in atmospheric pressure via a barometer. The clock still exists today [but was deactivated by the clock's relocation]. ^[1]

In the 19th century, the invention of perpetual motion machines became an obsession for many scientists. Many machines were designed based on electricity, but none of them lived up to their promises. Other early prospectors in this field included John Gamgee and Cromwell Varley. Gamgee developed the Zerometer, a perpetual motion machine of the second kind. Varley, though, did discover in 1867 that an electric generator did not need to be started with a conventional prime mover. He used the Earth's magnetic field to induce enough field strength in the stator windings to get a generator running. ^[2]

Other 19th century inventors such as Thomas Henry Moray, an admirer of Nikola Tesla and Tesla himself claimed to be able to tap into unknown sources of cosmic energy utilizing high frequency high voltage currents interacting with the aether. The energy would be derived from the "running river" of the aether. Several demonstrations by Moray were done where 50 kW of power were generated for several days from an antenna connected to a series of transformers, capacitors, and other components. However, all plans and knowledge were kept secret by Moray, demonstration was not verified, and patents were never granted.

Devising these 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. One reason for this concern is that a few "inventors" have waved a patent in front of potential investors, who may believe that said patent proves the machine works. The USPTO has 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. Some of 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.