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Perpetual motion

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For other uses, see Perpetual motion (disambiguation).
Perpetuum Mobile of Villard de Honnecourt (about 1230)

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 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 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

Robert Boyle's self-flowing flask appears to fill itself

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 as the earth and moon) could be considered an example. Planets, moons, and stars spin without any fuel, batteries, muscle, cost, or other limited power. Planets don't rotate forever however; energy in such system is dissipated via tidal forces, by friction with the dust and gas in space, and perhaps also very slowly via gravitational waves.

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.

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.

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 (actually to slow it down to the frame of cosmic background).

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 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).

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".

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:

"Free energy" devices

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). [3] Another early prospector in this field included Cromwell Varley. Varley 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. [4]

Some 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. Hermann Plauson, an Estonian engineer and inventor, also investigated the production of energy and power via atmospheric electricity.

Some free energy devices are devices that absorbs ambient electromagnetic fields (known as radiant energy) and converts the incoming energy into a useful form of power or function. Here the term is categorised more as renewable energy. Other "free energy devices" are solar cells and thermocouples which do the same for light and heat. Of course "free energy" here is something of a misnomer, it is simply that the energy used is generated elsewhere. These devices are not perpetual motion machines in the strict sense of breaking thermodynamic laws and being unworkable.

An early "free energy" device that was widely used was the crystal radio, which consisted of a solenoid coil made of insulated wire and a galena crystal. It used no batteries. Over the course of history, powerful versions of these "wireless" machines were built by Mahlon Loomis, David Edward Hughes and Nikola Tesla.

The Testatika is an electromagnetic generator based on the 1898 Pidgeon electrostatic machine which includes an inductance circuit, a capacitance circuit, and a thermionic rectification valve. Allegedly a perpetual motion machine, the Testatika resembles in some respects a Wimshurst machine. It was built by German engineer, Paul Suisse Baumann, and promoted by a Swiss community, the Methernithans.

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," before yelling at her, 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 complicated (and very expensive) perpetual motion machine can be bought as a household decoration.

The Discovery Channel program MythBusters attempted to build a perpetual motion machine consisting of several propane tanks arranged in an overbalanced wheel, supposedly to draw energy from a heat difference between two ends of the device (with the lower end moving through water heated by the sun). Technically it did work, but its movement was barely perceptible and created so little electricity the hosts declared it a failure. Since it relied on the sun to heat the water beneath it, the device was essentially an overly complicated solar power generator. They also examined other methods of gathering "free energy". These methods generally failed, were not cost-effective, or were too unwieldy to be feasible. They did generate a spark that zapped one of the show's host during the investigation of a radio based device.

See also

References

  1. ^ 608.03 Models, Exhibits, Specimens [R-3 - 600 Parts, Form, and Content of Application]
  2. ^ 706.03(a) Rejections Under 35 U.S.C. 101 [R-3 - 700 Examination of Applications II. UTILITY]
  3. ^ 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
  4. ^ 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

Manufacturers of purported perpetual motion machines

Historic

Research

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