Perpetual motion describes "motion that continues indefinitely without any external source of energy; impossible in practice because of friction." It can also be described as "the motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear". There is a scientific consensus that perpetual motion in an isolated system is impossible as it would violate the first or second law of thermodynamics.
Cases of apparent perpetual motion can exist in nature, but either are not truly perpetual or cannot be used to do work without changing the nature of the motion. For example, the motion of a planet around its star may appear "perpetual," but interplanetary space is not completely frictionless, so planets' orbital motion is very gradually slowed over time. The fly-by of a space probe past a planet can be used to speed up the probe but in doing so alters the motion and reduces the energy of the planet in its orbit around the Sun. The flow of current in a superconducting loop can be used as an energy storage medium, but just as with a battery, using it to power a device will remove the equivalent amount of energy from the current in the loop.
Machines which extract energy from seemingly perpetual sources—such as ocean currents—are capable of moving "perpetually" (for as long as that energy source itself endures), but they are not considered to be perpetual motion machines because they are consuming energy from an external source and are not isolated systems (in reality, no system can ever be a fully isolated system). Similarly, machines which comply with both laws of thermodynamics but access energy from obscure sources are sometimes referred to as perpetual motion machines, although they also do not meet the standard criteria for the name.
Despite the fact that successful isolated system perpetual motion devices are physically impossible in terms of the current understanding of the laws of physics, the pursuit of perpetual motion remains popular.
Basic principles 
There is a scientific consensus that perpetual motion in an isolated system violates either the first law of thermodynamics, the second law of thermodynamics, or both. The first law of thermodynamics is essentially a statement of conservation of energy. The second law can be phrased in several different ways, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known statement is that entropy tends to increase (see entropy production), or at the least stay the same; another statement is that no heat engine (an engine which produces work while moving heat from a high temperature to a low temperature) can be more efficient than a Carnot heat engine.
In other words:
- In any isolated system, one cannot create new energy (first law of thermodynamics)
- The output power of heat engines is always smaller than the input heating power. The rest of the energy is removed as heat at ambient temperature. The efficiency (this is the produced power divided by the input heating power) has a maximum, given by the Carnot efficiency. It is always lower than one
- The efficiency of real heat engines is even lower than the Carnot efficiency due to irreversible processes.
The statements 2 and 3 only apply to heat engines. Other types of engines, which convert e.g. mechanical into electromagnetic energy, can, in principle, operate with 100% efficiency.
Machines which comply with both laws of thermodynamics by accessing 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, clocks and other low-power machines, such as Cox's timepiece, have been designed to run on the differences in barometric pressure or temperature between night and day. These machines have a source of energy, albeit one which is not readily apparent so that they only seem to violate the laws of thermodynamics.
Machines which extract energy from seemingly perpetual sources - such as ocean currents - are indeed capable of moving "perpetually" until that energy source runs down. They are not considered to be perpetual motion machines because they are consuming energy from an external source and are not isolated systems.
One classification of perpetual motion machines refers to the particular law of thermodynamics the machines purport to violate:
- A perpetual motion machine of the first kind produces work without the input of energy. It thus violates the first law of thermodynamics: 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. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However it does violate the more subtle second law of thermodynamics (see also entropy). The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics.
- A more obscure category is a perpetual motion machine of the third kind, usually (but not always) defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). Third in this case refers solely to the position in the above classification scheme, not the third law of thermodynamics. Although it is impossible to make such a machine, as dissipation can never be 100% eliminated in a mechanical system, it is nevertheless possible to get very close to this ideal (see examples in the Low Friction section). Such a machine would not serve as a source of energy but would have utility as a perpetual energy storage device.
Use of the term "impossible" and perpetual motion 
"Epistemic impossibility" describes things which absolutely cannot occur within our current formulation of the physical laws. This interpretation of the word "impossible" is what is intended in discussions of the impossibility of perpetual motion in a closed system.
The conservation laws are particularly robust from a mathematical perspective. Noether's theorem, which was proven mathematically in 1915, states that any conservation law can be derived from a corresponding continuous symmetry of the action of a physical system. For example, if the true laws of physics remain invariant over time then the conservation of energy follows. On the other hand, if the conservation laws are invalid, then the foundations of physics would need to change.
Scientific investigations as to whether the laws of physics are invariant over time use telescopes to examine the universe in the distant past to discover, to the limits of our measurements, whether ancient stars were identical to stars today. Combining different measurements such as spectroscopy, direct measurement of the speed of light in the past and similar measurements demonstrates that physics has remained substantially the same, if not identical, for all of observable history spanning billions of years.
The principles of thermodynamics are so well established, both theoretically and experimentally, that proposals for perpetual motion machines are universally met with disbelief on the part of physicists. Any proposed perpetual motion design offers a potentially instructive challenge to physicists: one is almost completely certain that it cannot work, so one must 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 and often shed light upon certain aspects of physics. So, for example, the thought experiment of a Brownian ratchet as a perpetual motion machine was first discussed by Gabriel Lippmann in 1900 but it was not until 1912 that Marian Smoluchowski gave an adequate explanation for why it cannot work. However, during that twelve-year period scientists did not believe that the machine was possible. They were merely unaware of the exact mechanism by which it would inevitably fail.
The law that entropy always increases, holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.—Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)
In the mid 19th-century Henry Dircks investigated the history of perpetual motion experiments, writing a vitriolic attack on those who continued to attempt what he believed to be impossible:
"There is something lamentable, degrading, and almost insane in pursuing the visionary schemes of past ages with dogged determination, in paths of learning which have been investigated by superior minds, and with which such adventurous persons are totally unacquainted. The history of Perpetual Motion is a history of the fool-hardiness of either half-learned, or totally ignorant persons."—Henry Dircks, Perpetuum Mobile: Or, A History of the Search for Self-motive (1861)
||This section needs additional citations for verification. (August 2010)|
|“||One day man will connect his apparatus to the very wheelwork of the universe [...] and the very forces that motivate the planets in their orbits and cause them to rotate will rotate his own machinery.||”|
Some common ideas reoccur repeatedly in perpetual motion machine designs. Many ideas that continue to appear today were stated as early as 1670 by John Wilkins, Bishop of Chester and an official of the Royal Society. He outlined three potential sources of power for a perpetual motion machine, "Chymical Extractions", "Magnetical Virtues" and "the Natural Affection of Gravity".
The seemingly mysterious ability of magnets to influence motion at a distance without any apparent energy source has long appealed to inventors. One of the earliest examples of a magnetic motor was proposed by Wilkins and has been widely copied since: it consists of a ramp with a magnet at the top, which pulled a metal ball up the ramp. Near the magnet was a small hole that was supposed to allow the ball to drop under the ramp and return to the bottom, where a flap allowed it to return to the top again. The device simply could not work: any magnet strong enough to pull the ball up the ramp would necessarily be too powerful to allow it to drop through the hole. Faced with this problem, more modern versions typically use a series of ramps and magnets, positioned so the ball is to be handed off from one magnet to another as it moves. The problem remains the same.
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) one has 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 in the 12th century, 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.
Yet another theoretical machine involves a frictionless environment for motion. This involves the use of diamagnetic or electromagnet levitation to float an object. This is done in a vacuum to eliminate air friction and friction from an axle. The levitated object is then free to rotate around its center of gravity without interference. However, this machine has no practical purpose because the rotated object cannot do any work as work requires the levitated object to cause motion in other objects, bringing friction into the problem. Furthermore, a perfect vacuum is an unattainable goal since both the container and the object itself would slowly vaporize, thereby degrading the vacuum.
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 are not. 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 (It would require Maxwell's demon to perform more thermodynamic work to gauge the speed of the molecules than the amount of energy gained by the difference of temperature caused) 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).
Buoyancy is another frequently-misunderstood phenomenon. Some proposed perpetual-motion machines miss the fact that to push a volume of air down in a fluid takes the same work as to raise a corresponding volume of fluid up against gravity. These types of machines may involve two chambers with pistons, and a mechanism to squeeze the air out of the top chamber into the bottom one, which then becomes buoyant and floats to the top. The squeezing mechanism in these designs would not be able to do enough work to move the air down, or would leave no excess work available to be extracted.
Invention history 
The 8th century Bavarian "magic wheel" was a disc mounted on an axle powered by lodestones, claimed to be able to rotate forever.
Villard de Honnecourt in 1235 described, in a 33 page manuscript, a perpetual motion machine of the first kind. His idea was based on the changing torque of a series of weights attached with hinges to the rim of a wheel. While ascending they would hang close to the wheel and have little torque, but they would topple after reaching the top and drag the wheel down on descent due to their greater torque during the swing. His device spawned a variety of imitators who continued to refine the basic design.
Following the example of Villard, Peter of Maricourt designed a magnetic globe which when mounted without friction parallel to the celestial axis would rotate once a day and serve as an automatic armillary sphere.
In 1607 Cornelius Drebbel in "Wonder-vondt van de eeuwighe bewegingh" dedicated a Perpetuum motion machine to James I of England. It was described by Heinrich Hiesserle von Chodaw in 1621. Also in the 17th century, Robert Boyle's proposed self-flowing flask purports to fill itself through siphon action and Blaise Pascal introduced a primitive form of roulette and the roulette wheel in his search for a perpetual motion machine.
In the 18th century, Johann Bessler (also known as Orffyreus) created a series of claimed perpetual motion machines. In 1775 the Royal Academy of Sciences in Paris issued the statement that the Academy "will no longer accept or deal with proposals concerning perpetual motion".
In the 19th century, the invention of perpetual motion machines became an obsession for many scientists. Many machines were designed based on electricity. John Gamgee developed the Zeromotor, a perpetual motion machine of the second kind. Devising these machines is a favourite pastime of many eccentrics, who often devised elaborate machines in the style of Rube Goldberg or Heath Robinson. Such designs appeared to work on paper, though various flaws or obfuscated external energy sources are eventually understood to have been incorporated into the machine (unintentionally or intentionally).
Proposals for such inoperable machines have become so common 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 Manual of Patent Examining Practice 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.
And, further, 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.
The filing of a patent application is a clerical task, and the USPTO will not refuse filings for perpetual motion machines; the application will be filed and then most probably rejected by the patent examiner, after he has done a formal examination. Even if a patent is granted, it does not mean that the invention actually works, it just means that the examiner believes that it works, or was unable to figure out why it would not work.
The USPTO maintains a collection of Perpetual Motion Gimmicks.
In 1979, Joseph Newman filed a US Patent application for his "energy machine" which unambiguously claimed over-unity operation, where power output exceeded power input; the source of energy was claimed to be the atoms of the machine's copper conductor. The Patent Office rejected the application after the National Bureau of Standards measured the electrical input to be greater than the electrical output. Newman challenged the decision in court and lost.
Other patent offices around the world, such as the United Kingdom Patent Office, have similar practices. Section 4.05 of the UKPO Manual of Patent Practice states:
Processes or articles alleged to operate in a manner which is clearly contrary to well-established physical laws, such as perpetual motion machines, are regarded as not having industrial application.
Examples of decisions by the UK Patent Office to refuse patent applications for perpetual motion machines include:
- Decision BL O/044/06, John Frederick Willmott's application no. 0502841
- Decision BL O/150/06, Ezra Shimshi's application no. 0417271
The European Patent Classification (ECLA) has classes including patent applications on perpetual motion systems: ECLA classes "F03B17/04: Alleged perpetua mobilia ..." and "F03B17/00B: [... machines or engines] (with closed loop circulation or similar : ... Installations wherein the liquid circulates in a closed loop; Alleged perpetua mobilia of this or similar kind ...".
Apparent perpetual motion machines 
Even though they fully respect the laws of thermodynamics, there are a few conceptual or real devices that appear to be in "perpetual motion." Closer analysis reveals that they actually "consume" some sort of natural resource or latent energy, such as the phase changes of water or other fluids or small natural temperature gradients. In general, extracting large amounts of work using these devices is difficult to impossible.
Resource consuming 
Some examples of such devices include:
- The drinking bird toy functions using small ambient temperature gradients and evaporation.
- A capillarity based water pump functions using small ambient temperature gradients and vapour pressure differences.
- A Crookes radiometer consists of a partial vacuum glass container with a lightweight propeller moved by (light-induced) temperature gradients.
- Any device picking up minimal amounts of energy from the natural electromagnetic radiation around it, such as a solar powered motor.
- Any device powered by changes in air pressure, such as some clocks. The motion leeches energy from moving air and while may appear 'perpetual', but it will eventually stop when Earth's atmosphere disappears.
- The Atmos clock uses changes in the vapor pressure of ethyl chloride with temperature to wind the clock spring.
- A device powered by radioactive decay from an isotope with a relatively long half-life; such a device could plausibly operate for hundreds or thousands of years.
- The Oxford Electric Bell and Karpen Pile driven by dry pile batteries.
- Cox's timepiece and the Beverly Clock powered by changes in atmospheric pressure.
Low friction 
- In flywheel energy storage, "modern flywheels can have a zero-load rundown time measurable in years."
- Once spun up, objects in the vacuum of space—stars, black holes, planets, moons, spin-stabilized satellites, etc.—continue spinning almost indefinitely with no further energy input. Tide on Earth is dissipating the gravitational energy of the Moon/Earth system at an average rate of about 3.75 terawatts.
- In certain quantum-mechanical systems (such as superfluidity and superconductivity), dissipation-free "motion" is possible.
Thought experiments 
In some cases a thought (or "gedanken") experiment appears to suggest that perpetual motion may be possible through accepted and understood physical processes. However, in all cases, a flaw has been found when all of the relevant physics is considered. Examples include:
- Maxwell's Demon: This was originally proposed to show that the Second Law of Thermodynamics applied in the statistical sense only, by postulating a "demon" that could select energetic molecules and extract their energy. Subsequent analysis (and experiment) have shown there is no way to physically implement such a system that does not result in an overall increase in entropy.
- Brownian Ratchet: In this thought experiment, one imagines a paddle wheel connected to a ratchet. Brownian motion would cause surrounding gas molecules to strike the paddles, but the ratchet would only allow it to turn in one direction. A more thorough analysis showed that when a physical ratchet was considered at this molecular scale, Brownian motion would also affect the ratchet and cause it to randomly fail resulting in no net gain. Thus, the device would not violate the Laws of thermodynamics.
This is a gallery of some of the perpetual motion machine plans.
The "Capillary Bowl". It was thought that the capillary action would keep the water flowing in the tube, but since the cohesion force that draws the liquid up the tube in the first place holds the droplet from releasing into the bowl, the flow is not perpetual.
See also 
- Entropy production
- History of perpetual motion machines
- Incredible utility
- Pathological science
- Free energy suppression
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- Manual of Patent Practice, Section 4. United Kingdom Patent Office
- See also, for more examples of refused patent applications at the United Kingdom Patent Office (UK-IPO), UK-IPO gets tougher on perpetual motion, IPKat, 12 June 2008. Consulted on June 12, 2008.
- "Patents Ex parte decision (O/044/06)" (PDF). Retrieved 2013-03-04.
- ECLA classes F03B17/04 and F03B17/00B. Consulted on June 12, 2008.
- WO application 2008037004, Kwok, James, "An energy storage device and method of use", published 2008-04-03
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- Perpetual motion at the Open Directory Project
- The Museum of Unworkable Devices
- Vlatko Vedral's Lengthy discussion of Maxwell's Demon (PDF)
- "Perpetual Motion - Just Isn't." Popular Mechanics, January 1954, pp. 108–111.