For other uses, see Perpetual motion (disambiguation).

Perpetual motion refers to a condition in which an moves forever without the expenditure of any limited internal or external source of energy.

As well as being poetically descriptive of motions beyond the scale of human lifetimes, for example in the phrase "the stars in perpetual motion wheeled overhead", the term is commonly used to refer to actual attempts to build machines which display this phenomenon.

In the real world, friction is always larger than zero, so an object in perpetual motion implies work being continuously done to overcome this natural retarding process.

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 work. Specifically, 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.

Basic principles

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 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. As a special case of this, any machine operating in a closed cycle cannot only 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.

Classification

It is customary to classify perpetual motion machines as follows:

1. A perpetual motion machine of the first kind produces strictly more energy than it uses, thus violating the law of conservation of energy.
2. A machine that produces (in still-usable form) exactly as much energy as it uses is a perpetual motion machine of the second kind, which continues running forever (not necessarily doing any usable work) by converting its waste heat back into mechanical work. This need not violate the law of conservation of energy, but does violate the less fundamental second law of thermodynamics (see also entropy). More generally, any device that converts heat into work without loss can be considered a perpetual motion of the second kind, since it could be used to make something that moves perpetually.

By minimizing friction and other causes of dissipation, it is possible to produce a good approximation to a perpetual motion machine of the second kind. Planetary systems (such

In an otherwise completely empty Newtonian universe, a single particle could travel for ever at constant velocity with no violation of the laws of physics -- though of course no energy could be extracted from it without slowing it down. For example, an electron can spin around a nucleus in an atom of matter indefinitely unless it or the atom is disrupted in some way.

Just how impossible is impossible?

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.

Though perpetual motion may be impossible, near-perpetual motion - motion lasting thousands to billions of years - may be possible in a machine. Near-perpetual motion is a reality in nature: in the rotation of planets, stars and electrons, for example. If such a thing could be duplicated in a lab, man-made near-perpetual motion could become a reality. Planets and electrons have no significant drag/friction because the momentum of the object is thousands of times stronger than the slowing effect caused by the friction in the system.

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

Techniques

Some ideas recur repeatedly in perpetual motion machine designs. For instance:

The seemingly mysterious ability of magnets to influence motion at a distance without any apparent energy source has long appealed to inventors. Unfortunately, a constant magnetic field does no work because the force it exerts on any particle is always at right angles to its motion; a changing field can do work, but requires energy to sustain. A "fixed" magnet can do work, but energy is dissipated in the process, typically weakening the magnet's strength over time. Thus, when a magnet does work by lifting an iron weight, some of the work that was put into magnetizing the magnet is being used to lift the weight, and the strength of the magnet is reduced correspondingly. When the weight is removed from the magnet, the work required to do this restores the strength of the magnet, minus losses due to friction.

Gravity also acts at a distance, without an apparent energy source. But to get energy out of a gravitational field (for instance, by dropping a

Gravity and magnetism are an attractive combination indeed, and a frequently rediscovered design has a ball pulled up by a magnetic field and then rolling down under the influence of gravity, in a cycle. (At the highest point, the ball is supposed to have acquired enough speed to escape the magnet's influence.)

To extract work from heat, thus producing a perpetual motion machine of the second kind, the most common approach (dating back at least to Maxwell's demon) is unidirectionality. Only molecules moving fast enough and in the right direction are allowed through the demon's trap door. In a Brownian ratchet, forces tending to turn the ratchet one way are able to do so while forces in the other direction aren't. A diode in a heat bath allows through currents in one direction and not the other. These schemes typically fail in two ways: either maintaining the unidirectionality costs energy (Maxwell's demon needs light to look at all those particles and see what they're doing), or the unidirectionality is an illusion and occasional big violations make up for the frequent small non-violations (the Brownian ratchet will be subject to internal Brownian forces and therefore will sometimes turn the wrong way).

Inventions and patents

Main article: History of perpetual motion machines

The recorded history of perpetual motion machines dates at least as far back as the 8th century.

• An early description 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 of sorts: a clock (known as Cox's timepiece) powered by changes in atmospheric pressure. Cox was quite open about the workings of his machine, unlike many perpetual motion inventors. The clock still exists today (but was deactivated by the clock's relocation).

• Johann Bessler (also known as Orffyreus) created a series of claimed perpetual motion machines in the 18th Century.

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.

• Other 19th century inventors such as Thomas Henry Moray, an admirer of Nikola Tesla, claimed to be able to tap into radiant energy sources 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 used official patents to convince gullible potential investors that their machine is "approved" by the Patent Office. 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)),
• Flynn; Charles J., US6246561 "Methods for controlling the path of magnetic flux from a permanent magnet and devices incorporating the same", July 31, 1998
• Patrick, et al., "Motionless electromagnetic generator" , March 26, 2002

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

Perpetual motion in pop culture

In "The PTA Disbands" episode of The Simpsons, Lisa builds a perpetual motion machine when there was no school due to a teachers' strike; after seeing the machine, her father, Homer observes that, "it just keeps going faster and faster," before yelling at her, "..in this house we obey the laws of thermodynamics!".

In the Playstation 2 video games Xenosaga I & II, and in the Playstation 1 video game Xenogears, the device, called the Zohar, is a form of a perpetual motion machine. It is briefly described as a Pseudo-Perpetual Infinite Energy Engine.

In the computer game The Sims, a complicated (and very expensive) perpetual motion machine can be bought as a household decoration.

See also

• Main: History of perpetual motion machines, Open system, Vacuum energy, Radiant energy, Free energy, Negative resistance, Brownian ratchet, Axletree Dynamo
• Science: Conservation of energy, Thought experiment, Thermodynamic entropy, Second law of thermodynamics, Kirchhoff's law of thermal radiation
• People: George Westinghouse, Josef Hoëné-Wronski, Joseph Newman, John E.W. Keely
• Other: Perpetuum mobile, List of disputed theories
• Science:

References

1.   Ord-Hume, Arthur W. J. G., "Perpetual Motion: The History of an Obsession". New York, St. Martin's Press. 1977. ISBN 0-312-60131-X
2.   Bunch, Bryan, and Alexander Hellemans, "The History of Science and Technology: A Browser's Guide to the Great Discoveries, Inventions, and the People Who Made Them from the Dawn of Time to Today". ISBN 0618221239

Historic

• Richard Clegg, "", richardclegg.org.

Research

• "". 2003.
• Vlatko Vedral's (PDF)
•  : PPM fueled by entropy.
• Xin Yong Fu and Zi Tao Fu, "". Shanghai Jiao Tong University, Shanghai, P.R. China (arXiv.org)
• by John Walker.

Patents

• USPTO page Perpetuum mobile

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