The term perpetual motion, taken literally, refers to movement that goes on forever. However, the term more generally refers to any closed system that produces more energy than it consumes. Such a device or system would be in violation of the law of conservation of energy, which states that energy can never be created or destroyed. The most conventional type of perpetual motion machine is a mechanical system which (supposedly) sustains motion despite losing energy to friction and air resistance, or while avoiding losing energy to friction and air resistance. According to the law of conservation of energy, such a device cannot exist.
Perpetual motion 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, or at the least stay 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. However, mankind has recently learned that everything has perpetual motion...Cause Ian say's so.
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 are generally reported as not meeting 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.
Like all scientific theories, the laws of physics are incomplete. "A world that was simple enough to be fully known would be too simple to contain conscious observers that might know it. Outside of pure mathematics, stating that things are absolutely impossible is more a hallmark of pseudoscience than of true science. Nevertheless, the term is properly used to reflect those things that cannot be true without a significant rewrite of nearly all known scientific laws.
The conservation laws are particularly robust. Noether's theorem states that any conservation law can be derived from a corresponding continuous symmetry. In other words, so long as the laws of physics (not simply the current understanding of them, but the actual laws, which may still be undiscovered) and the various physical constants remain invariant over time — so long as the laws of the universe are fixed — then the conservation laws must be true, in the sense that they follow from the presupposition using mathematical logic. To put it the other way around: if perpetual motion or "overunity" machines were possible, then most of what we believe to be true about physics, mathematics, or both would have to be false. This problem is exacerbated by the fact that mathematics is believed to be absolute, since its veracity is not dependent on anything that happens in the real world.
In this case it is easy to check whether or not the theory is correct. Using telescopes we can examine the universe in the distant past; the fact that stars even exist and are, to the limits of our measurements, identical to stars today, is a direct visual demonstration that physics was similar in the past. Combining different measurements such as spectroscopy, direct measurement of the speed of light in the past and similar measurements demonstrates conclusively 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 can't 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 into unique aspects of physics.
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)
Some common ideas recur repeatedly in perpetual motion machine designs. Many of the ones that continue to appear today were already outlined by John Wilkins, Bishop of Chester and an early official of the Royal Society. In 1670 Wilkins 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 system using magnets 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.
In a more general sense, magnets can do no net work, although this was not understood until much later. A magnet can accelerate an object, like the metal ball of Wilkins' device, but this motion will always come to stop when the object reaches the magnet, releasing that work in some other form - typically its mechanical energy being turned into heat. In order for this motion to continue, the magnet would have to be moved, which would require energy.
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 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.
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).
Villard de Honnecourt in 1235 described, in a thirty-three page manuscript, a perpetual motion machine of the second kind. His idea was based on changing torque of a series of weights around the rim of a wheel. The weights were positioned so that they would fall
His device spawned a variety of imitators that have continued
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 was 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".
|Howard R. Johnson, U.S. Patent 4,151,431|
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 have similar practices, such as the United Kingdom Patent Office. Section 4.05 of the UKPO Manual of Patent Practice states:
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 ...".
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 them to actually "consume" some sort of natural resource or latent energy like 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.
Some examples of such devices include:
Conventional sources of energy such as petroleum and natural gas or radioactive materials such as uranium rely on a small fraction of the inherent and ubiquitous energy of atoms and molecules, although the energy of atoms and molecules are not characterized by an internal temperature. Electrical power plants can only be profitable by extracting the nuclear and chemical energies in excess of the energy needed to:
Scientists are currently spending many research hours in attempt of getting more power out of nuclear fusion than what it takes to run the power plant. If solved, nuclear fusion would supply the world with an abundant source of electricity. The entire energy industry relies on a system where thermal output exceeds the thermal input. When determining the present thermal output that exceeds present thermal input, one only considers the time during which the energy of the fuel is consumed, and not the time during which the energy was stored in that fuel.
Self-taught inventors exploring the topics of perpetual motion are often highly secretive of their work and unwilling to openly discuss what they are doing, instead offering only limited-access demonstrations without explanation or documentation. They claim to do this for a number of reasons: