Perpetual motion

From The Book of THoTH (Leaves of Wisdom)

Perpetual motion refers to a condition in which an object moves forever without being driven by an external source of energy.

The term is commonly used to refer to machines which display this phenomenon. In the macroscopic world, perpetual motion is not generally considered to be possible. 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. No genuine perpetual motion machine currently exists, and according to certain fundamental laws in physics they cannot exist. Specifically, perpetual motion machines would violate either the first or second laws of thermodynamics. Perpetual motion machines are divided into two subcategories (some physicists, including the noted professor of thermodynamics Mark W. Zemansky, include a third), defined by which law of thermodynamics would have to be broken in order for the device to be a true perpetual motion machine.

1 Basic principles
- 1.1 Classification
2 Just how impossible is impossible?
3 Thought experiments
4 Techniques
5 Invention history
- 5.1 Patents
6 Perpetual motion in pop culture
7 See also
8 References
9 External links
- 9.1 Historic
- 9.2 Research

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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 are claimed not to violate either of the two laws of thermodynamics but rather are claimed 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 (Cox's timepiece). Such a machine has a source of energy, albeit one from which it is quite impractical to produce power in quantity.

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Classification

It is customary to classify perpetual motion machines as follows:

A perpetual motion machine of the first kind produces strictly more energy than it uses, thus violating the law of conservation of energy.
A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. This need not violate the law of conservation of energy, since the thermal energy may be equivalent to the work done; however it does violate the more subtle second law of thermodynamics (see also entropy). Note that such a machine is different from real heat engines (such as car engines), which always involve a transfer of heat from a hotter reservoir to a colder one, the latter being warmed up in the process. The signature of a perpetual motion machine of the second kind is that there is only one single heat reservoir involved, which is being spontanously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible by the second law of thermodynamics.

In an otherwise completely empty Newtonian universe, a single particle could travel forever 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. Similarly, in an isolated system consisting of two objects orbiting each other gravitationally, the two objects will remain in orbiting motion forever, as long as they are not disturbed. However any attempt to extract useful work from this system would lead to a loss of energy. This would result in the objects slowing down and getting closer to each other, until at some point the objects would collapse together and no more energy would remain to extract.

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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 depends 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 often met with disbelief on the part of physicists.

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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 cosmic microwave background is used to drive a rocket's engine (actually to slow it down to the frame of cosmic background).

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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 not do 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, potential energy is converted to kinetic energy. Once the iron hits the magnet its kinetic energy is converted to heat and sound. In order to release further energy, the iron must be moved away from the magnet. This converts the energy of your arm to potential energy again. Since the energy of parting the magnet and iron is identical to the energy released as the magnet and iron come together, no net energy can be gained by changing the iron - magnet distance.

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 heavy object, producing kinetic energy as it falls) you have to put energy in (for instance, by lifting the object up), and some energy is always dissipated in the process. A typical application of gravity in a perpetual motion machine is Bhaskara's wheel, whose key idea is itself a recurring theme, often called the overbalanced wheel: Moving weights are attached to a wheel in such a way that they fall to a position further from the wheel's center for one half of the wheel's rotation, and closer to the center for the other half. Since weights further from the center apply a greater torque, the result is (or would be, if such a device worked) that the wheel rotates forever. The moving weights may be hammers on pivoted arms, or rolling balls, or mercury in tubes; the principle is the same.

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).

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Invention history

Main article: History of perpetual motion machines

The recorded history of perpetual motion machines dates at least as far back as the 8th century. Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines. 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 1775 Royal Academy of Sciences in Paris issued the statement that Academy "will no longer accept or deal with proposals concerning perpetual motion". 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. Another early prospector in this field included John Gamgee. Gamgee developed the Zeromotor, a perpetual motion machine of the second kind.

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. Usually, though, various flaws or obfuscated external power sources have been incorporated into the machine. Such activity has made them useless in the practice of "invention".

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Patents

Devising such inoperable machines 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, according to various skeptics, 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 states:

With the exception of cases involving perpetual motion, a model is not ordinarily required by the Office to demonstrate the operability of a device. If operability of a device is questioned, the applicant must establish it to the satisfaction of the examiner, but he or she may choose his or her own way of so doing. [1]

They state, though, that:

A rejection [of a patent application] on the ground of lack of utility includes the more specific grounds of inoperativeness, involving perpetual motion. A rejection under 35 U.S.C. 101 for lack of utility should not be based on grounds that the invention is frivolous, fraudulent or against public policy. [2]

The USPTO has granted a few patents for motors that are claimed to run without net energy input. These patents were issued because, skeptics claim, it was not obvious from the patent that a perpetual motion machine was being claimed. Some of these are:

Johnson, Howard R., U.S. Patent 4151431 "Permanent magnet motor", April 24, 1979
Baker, Daniel, U.S. Patent 4074153 "Magnetic propulsion device", February 14, 1978
Hartman; Emil T., U.S. Patent 4215330 "Permanent magnet propulsion system", December 20, 1977 (this device is related to the Simple Magnetic Overunity Toy (SMOT)),
Flynn; Charles J., U.S. Patent 6246561 "Methods for controlling the path of magnetic flux from a permanent magnet and devices incorporating the same", July 31, 1998
Patrick, et al., U.S. Patent 6362718 "Motionless electromagnetic generator" , March 26, 2002

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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 says: "This perpetual motion machine Lisa made is a joke, it just keeps going faster and faster," and afterwards yells at her saying "Lisa, get in here...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 complex (and very expensive) perpetual motion machine can be bought as a household decoration.

One episode of the Nickelodeon program Invader Zim showcased a Perpetual Energy Generator (or as its creator likes to call it, PEG) that is never activated due to the impatience of a member of the crowd present at the activation ceremony.

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