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Perpetual motion is the motion of bodies that continues forever in an unperturbed system. A perpetual motion machine is a hypothetical machine that can do work infinitely without an external
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
source. This kind of machine is impossible, as it would violate either the
first First or 1st is the ordinal form of the number one (#1). First or 1st may also refer to: *World record, specifically the first instance of a particular achievement Arts and media Music * 1$T, American rapper, singer-songwriter, DJ, and rec ...
or
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
or both. These
laws of thermodynamics The laws of thermodynamics are a set of scientific laws which define a group of physical quantities, such as temperature, energy, and entropy, that characterize thermodynamic systems in thermodynamic equilibrium. The laws also use various paramet ...
apply regardless of the size of the system. For example, the motions and rotations of celestial bodies such as planets may appear perpetual, but are actually subject to many processes that slowly dissipate their kinetic energy, such as
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
,
interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
resistance,
gravitational radiation Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
and
thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
, so they will not keep moving forever. Thus, machines that extract energy from finite sources will not operate indefinitely, because they are driven by the energy stored in the source, which will eventually be exhausted. A common example is devices powered by ocean currents, whose energy is ultimately derived from the Sun, which itself will eventually burn out. Machines powered by more obscure sources have been proposed, but are subject to the same inescapable laws, and will eventually wind down. In 2016, new states of matter,
time crystal In condensed matter physics, a time crystal is a quantum system of particles whose lowest-energy state is one in which the particles are in repetitive motion. The system cannot lose energy to the environment and come to rest because it is alre ...
s, were discovered in which on a microscopic scale the component atoms are in continual repetitive motion, thus satisfying the literal definition of "perpetual motion". However, these do not constitute perpetual motion machines in the traditional sense or violate thermodynamic laws because they are in their quantum
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
, so no energy can be extracted from them; they exhibit motion without energy.


History

The history of perpetual motion machines dates back to the Middle Ages. Lynn Townsend White, Jr. (April 1960). "Tibet, India, and Malaya as Sources of Western Medieval Technology", ''The American Historical Review'' 65 (3), p. 522-526. For millennia, it was not clear whether perpetual motion devices were possible or not, but the development of modern theories of thermodynamics has shown that they are impossible. Despite this, many attempts have been made to construct such machines, continuing into modern times. Modern designers and proponents often use other terms, such as "over unity", to describe their inventions.


Basic principles

There is a
scientific consensus Scientific consensus is the generally held judgment, position, and opinion of the majority or the supermajority of scientists in a particular field of study at any particular time. Consensus is achieved through scholarly communication at confe ...
that perpetual motion in an
isolated system In physical science, an isolated system is either of the following: # a physical system so far removed from other systems that it does not interact with them. # a thermodynamic system enclosed by rigid immovable walls through which neither m ...
violates either the
first law of thermodynamics The first law of thermodynamics is a formulation of the law of conservation of energy, adapted for thermodynamic processes. It distinguishes in principle two forms of energy transfer, heat and thermodynamic work for a system of a constant amoun ...
, the
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
, or both. The first law of thermodynamics is a version of the law of
conservation of energy In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means th ...
. The second law can be phrased in several different ways, the most intuitive of which is that
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
flows spontaneously from hotter to colder places; relevant here is that the law observes that in every macroscopic process, there is friction or something close to it; another statement is that no
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
(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 A Carnot heat engine is a heat engine that operates on the Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded by Benoît Paul Émile Clapeyron in 1 ...
operating between the same two temperatures. In other words: # In any isolated system, one cannot create new energy (law of conservation of energy). As a result, the
thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a he ...
—the produced work power divided by the input heating power—cannot be greater than one. # The output work power of heat engines is always smaller than the input heating power. The rest of the heat energy supplied is wasted as heat to the ambient surroundings. The thermal efficiency therefore has a maximum, given by the Carnot efficiency, which is always less than one. # The efficiency of real heat engines is even lower than the Carnot efficiency due to
irreversibility In science, a process that is not reversible is called irreversible. This concept arises frequently in thermodynamics. All complex natural processes are irreversible, although a phase transition at the coexistence temperature (e.g. melting of i ...
arising from the speed of processes, including friction. Statements 2 and 3 apply to heat engines. Other types of engines that convert e.g. mechanical into electromagnetic energy, cannot operate with 100% efficiency, because it is impossible to design any system that is free of energy dissipation. Machines that 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 Cox's timepiece is a clock developed in the 1760s by James Cox. It was developed in collaboration with John Joseph Merlin (with whom Cox also worked on developing automata). Cox claimed that his design was a true perpetual motion machine, but as ...
, 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. Even machines that extract energy from long-lived sources - such as ocean currents - will run down when their energy sources inevitably do. They are not perpetual motion machines because they are consuming energy from an external source and are not isolated systems.


Classification

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 Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal tr ...
without the input of
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
. It thus violates the first law of thermodynamics: the
law of conservation of energy In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means that ...
. * A perpetual motion machine of the second kind is a machine that 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 The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
in a cyclic process (see also
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
). 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 perpetual motion machine of the third kind is 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 The third law of thermodynamics states, regarding the properties of closed systems in thermodynamic equilibrium: This constant value cannot depend on any other parameters characterizing the closed system, such as pressure or applied magnetic fiel ...
). It is impossible to make such a machine, as dissipation can never be completely eliminated in a mechanical system, no matter how close a system gets to this ideal (see examples in the
Low Friction Low or LOW or lows, may refer to: People * Low (surname), listing people surnamed Low Places * Low, Quebec, Canada * Low, Utah, United States * Lo Wu station (MTR code LOW), Hong Kong; a rail station * Salzburg Airport (ICAO airport code: LO ...
section).


Impossibility

" 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 Noether's theorem or Noether's first theorem states that every differentiable symmetry of the action of a physical system with conservative forces has a corresponding conservation law. The theorem was proven by mathematician Emmy Noether in ...
, which was proven mathematically in 1915, states that any conservation law can be derived from a corresponding continuous symmetry of the
action Action may refer to: * Action (narrative), a literary mode * Action fiction, a type of genre fiction * Action game, a genre of video game Film * Action film, a genre of film * ''Action'' (1921 film), a film by John Ford * ''Action'' (1980 fil ...
of a physical system. The symmetry which is equivalent to conservation of energy is the time invariance of physical laws. Therefore, if the laws of physics do not change with time, then the conservation of energy follows. For energy conservation to be violated to allow perpetual motion would require that the foundations of physics would 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 Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
, 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 time spanning
billion Billion is a word for a large number, and it has two distinct definitions: *1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is its only current meaning in English. * 1,000,000,000,000, i.e ...
s 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 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 experiment A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. History The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anci ...
s and often shed light upon certain aspects of physics. So, for example, the thought experiment of a
Brownian ratchet In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as a ...
as a perpetual motion machine was first discussed by
Gabriel Lippmann Jonas Ferdinand Gabriel Lippmann (16 August 1845 – 13 July 1921) was a Franco-Luxembourgish physicist and inventor, and Nobel laureate in physics for his method of reproducing colours photographically based on the phenomenon of interference. ...
in 1900 but it was not until 1912 that
Marian Smoluchowski Marian Smoluchowski (; 28 May 1872 – 5 September 1917) was a Polish physicist who worked in the Polish territories of the Austro-Hungarian Empire. He was a pioneer of statistical physics, and an avid mountaineer. Life Born into an upper-c ...
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. In the mid-19th-century
Henry Dircks Henry Dircks FRSE FCS (26 August 1806– 17 September 1873) was an English engineer who is considered to have been the main designer of the projection technique known as ''Pepper's ghost'' in 1858. It is named after John Henry Pepper who impleme ...
investigated the history of perpetual motion experiments, writing a vitriolic attack on those who continued to attempt what he believed to be impossible:


Techniques

Some common ideas recur repeatedly in perpetual motion machine designs. Many ideas that continue to appear today were stated as early as 1670 by
John Wilkins John Wilkins, (14 February 1614 – 19 November 1672) was an Anglican clergyman, natural philosopher, and author, and was one of the founders of the Royal Society. He was Bishop of Chester from 1668 until his death. Wilkins is one of the fe ...
,
Bishop of Chester The Bishop of Chester is the Ordinary of the Church of England Diocese of Chester in the Province of York. The diocese extends across most of the historic county boundaries of Cheshire, including the Wirral Peninsula and has its see in the C ...
and an official of the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
. He outlined three potential sources of power for a perpetual motion machine, " Extractions", "Magnetical Virtues" and "the Natural Affection of Gravity". The seemingly mysterious ability of
magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
s 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. However, if the magnet is to be strong enough to pull the ball up the ramp, it cannot then be weak enough to allow gravity to pull it 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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
, it was thought that the wheel would rotate forever. However, since the side with weights further from the center has fewer weights than the other side, at that moment, the torque is balanced and perpetual movement is not achieved. The moving weights may be hammers on pivoted arms, or rolling balls, or mercury in tubes; the principle is the same. Another theoretical machine involves a frictionless environment for motion. This involves the use of
diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...
or
electromagnetic levitation Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic force is used to counteract the effects of the gravitational force and any other forces. The ...
to float an object. This is done in a
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
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 Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomeno ...
, 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 Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter Maxwell called the demon a "finite being", while the ' ...
) is ''unidirectionality''. Only molecules moving fast enough and in the right direction are allowed through the demon's trap door. In a
Brownian ratchet In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as a ...
, 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 (requiring 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 Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
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.


Patents

Proposals for such inoperable machines have become so common that the
United States Patent and Trademark Office The United States Patent and Trademark Office (USPTO) is an agency in the U.S. Department of Commerce that serves as the national patent office and trademark registration authority for the United States. The USPTO's headquarters are in Alexa ...
(USPTO) has made an official policy of refusing to grant
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
s for perpetual motion machines without a working model. The USPTO Manual of Patent Examining Practice states: And, further, that: 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 o
Perpetual Motion Gimmicks
The
United Kingdom Patent Office The Intellectual Property Office of the United Kingdom (often referred to as the UK IPO) is, since 2 April 2007, the operating name of The Patent Office. It is the official government body responsible for intellectual property rights in the UK ...
has a specific practice on perpetual motion; Section 4.05 of the UKPO Manual of Patent Practice states: 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 The European Classification (ECLA) is a former patent classification system maintained by the European Patent Office (EPO). The ECLA classification system contains 134 000 subdivisions. It was introduced mainly as an extension of the International ...
(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

As "perpetual motion" can exist only in isolated systems, and since true isolated systems do not exist, there are no real "perpetual motion" devices. However, there are concepts and technical drafts that propose "perpetual motion", but on closer analysis it is revealed that they actually "consume" some sort of natural resource or latent energy, such as the phase changes of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
or other fluids or small natural temperature gradients, or simply cannot sustain indefinite operation. In general, extracting work from these devices is impossible.


Resource consuming

Some examples of such devices include: * The
drinking bird Drinking birds, also known as insatiable birdies, dunking birds, drinky birds, water birds, dipping birds, and “Sippy Chickens” are toy heat engines that mimic the motions of a bird drinking from a water source. They are sometimes incorrectly ...
toy functions using small ambient temperature gradients and evaporation. It runs until all water is evaporated. * A
capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
-based water pump functions using small ambient temperature gradients and
vapour pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
differences. With 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. * A
Crookes radiometer The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more i ...
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 In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
around it, such as a solar-powered motor. * Any device powered by changes in air pressure, such as some clocks (
Cox's timepiece Cox's timepiece is a clock developed in the 1760s by James Cox. It was developed in collaboration with John Joseph Merlin (with whom Cox also worked on developing automata). Cox claimed that his design was a true perpetual motion machine, but as ...
,
Beverly Clock The Beverly Clock is a clock in the 3rd-floor lift foyer of the Department of Physics at the University of Otago, Dunedin, New Zealand. The clock is still running despite never having been manually wound since its construction in 1864 by Arthur ...
). The motion leeches energy from moving air which in turn gained its energy from being acted on. * A
heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing h ...
due to it having a COP above 1. * The
Atmos clock Atmos is the brand name of a mechanical torsion pendulum clock manufactured by Jaeger-LeCoultre in Switzerland which does not need to be wound manually. It gets the energy it needs to run from temperature and atmospheric pressure changes in the en ...
uses changes in the vapor pressure of ethyl chloride with temperature to wind the clock spring. * A device powered by
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
from an isotope with a relatively long
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
; such a device could plausibly operate for hundreds or thousands of years. * The
Oxford Electric Bell The Oxford Electric Bell or Clarendon Dry Pile is an experimental electric bell, in particular a type of bell that uses the electrostatic clock principle that was set up in 1840 and which has run nearly continuously ever since. It was one of th ...
and the driven by
dry pile upright=1.2, Schematic diagram of a copper–zinc voltaic pile. The copper and zinc discs were separated by cardboard or felt spacers soaked in salt water (the electrolyte). Volta's original piles contained an additional zinc disk at the bottom, ...
batteries.


Low friction

* In
flywheel energy storage Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a c ...
, "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 satellite Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle/satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, et ...
s, etc.—dissipate energy very slowly, allowing them to spin for long periods.
Tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
s on Earth are dissipating the gravitational energy of the Moon/Earth system at an
average In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7, ...
rate of about 3.75
terawatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wat ...
s. * In certain quantum-mechanical systems (such as
superfluidity Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two i ...
and
superconductivity Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
), very low friction movement is possible. However, the motion stops when the system reaches an equilibrium state (e.g. all the
liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...
arrives at the same level.) Similarly, seemingly entropy-reversing effects like superfluids climbing the walls of containers operate by ordinary
capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
.


Thought experiments

In some cases a
thought In their most common sense, the terms thought and thinking refer to conscious cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, a ...
(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 Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter Maxwell called the demon a "finite being", while the ' ...
: This was originally proposed to show that the
Second Law of Thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
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 Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
. *
Brownian ratchet In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as a ...
: In this thought experiment, one imagines a paddle wheel connected to a ratchet.
Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of random fluctuations in a particle's position insi ...
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 The laws of thermodynamics are a set of scientific laws which define a group of physical quantities, such as temperature, energy, and entropy, that characterize thermodynamic systems in thermodynamic equilibrium. The laws also use various paramet ...
. *
Vacuum energy Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum. The effects of vacuum energy can be experimental ...
and
zero-point energy Zero-point energy (ZPE) is the lowest possible energy that a quantum mechanical system may have. Unlike in classical mechanics, quantum systems constantly Quantum fluctuation, fluctuate in their lowest energy state as described by the Heisen ...
: In order to explain effects such as
virtual particles A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in the perturbat ...
and the
Casimir effect In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of the field. It is named after the Dutch physicist Hendrik Casimir, who pr ...
, many formulations of
quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...
include a background energy which pervades empty space, known as vacuum or zero-point energy. The ability to harness zero-point energy for useful work is considered
pseudoscience Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or falsifiability, unfa ...
by the scientific community at large. Inventors have proposed various methods for extracting useful work from zero-point energy, but none have been found to be viable,
Martin Gardner Martin Gardner (October 21, 1914May 22, 2010) was an American popular mathematics and popular science writer with interests also encompassing scientific skepticism, micromagic, philosophy, religion, and literatureespecially the writings of Lewis ...

"'Dr' Bearden's Vacuum Energy"
, ''
Skeptical Inquirer ''Skeptical Inquirer'' is a bimonthly American general-audience magazine published by the Committee for Skeptical Inquiry (CSI) with the subtitle: ''The Magazine for Science and Reason''. Mission statement and goals Daniel Loxton, writing in ...
'', January/February 2007
no claims for extraction of zero-point energy have ever been validated by the scientific community, and there is no evidence that zero-point energy can be used in violation of conservation of energy. * Ellipsoid paradox: This paradox considers a perfectly reflecting cavity with two black bodies at points ''A'' and ''B''. The reflecting surface is composed of two elliptical sections ''E''1 and ''E''2 and a spherical section ''S'', and the bodies at ''A'' and ''B'' are located at the joint foci of the two ellipses and ''B'' is at the center of ''S''. This configuration is such that apparently black body at ''B'' heat up relative to ''A'': the radiation originating from the blackbody at ''A'' will land on and be absorbed by the blackbody at ''B''. Similarly, rays originating from point ''B'' that land on ''E''1 and ''E''2 will be reflected to ''A''. However, a significant proportion of rays that start from ''B'' will land on ''S'' will be reflected back to ''B''. This paradox is solved when the black bodies' finite sizes are considered instead of punctual black bodies.


Conspiracy theories

Despite being dismissed as
pseudoscientific Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...
, perpetual motion machines have become the focus of
conspiracy theories A conspiracy theory is an explanation for an event or situation that invokes a conspiracy by sinister and powerful groups, often political in motivation, when other explanations are more probable.Additional sources: * * * * The term has a nega ...
, alleging that they are being hidden from the public by corporations or governments, who would lose economic control if a power source capable of producing energy cheaply was made available.


See also

*
Anti-gravity Anti-gravity (also known as non-gravitational field) is a hypothetical phenomenon of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to ...
* Faster than the speed of light *
Incredible utility In United States patent law, incredible utility is a concept according to which, in order for an invention to be patentable, it must have some credible useful function. If it does not have a credible useful function despite the assertions of the ...
*
Johann Bessler Johann Ernst Elias Bessler (''ca''. 1680 – 30 November 1745), known as Orffyreus or Orffyré, was a German entrepreneur who claimed to have built several perpetual motion machines. Those claims generated considerable interest and controversy a ...
*
Pathological science Pathological science is an area of research where "people are tricked into false results ... by subjective effects, wishful thinking or threshold interactions."Irving Langmuir, "Colloquium on Pathological Science," held at the Knolls Research Lab ...
*
Time machine Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a w ...


Notes


References


External links

*
The Museum of Unworkable Devices
*
"Perpetual Motion - Just Isn't."
''Popular Mechanics'', January 1954, pp. 108–111.
In Our Time: Perpetual Motion
BBC discussion with Ruth Gregory, Frank Close and Steven Bramwell, hosted by Melvyn Bragg, first broadcast 24 September 2015. {{DEFAULTSORT:Perpetual Motion Pseudoscience