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“”Oh ye seekers after perpetual motion, how many vain chimeras have you pursued? Go and take your place with the alchemists.
|—Leonardo Da Vinci, 1494|
A perpetual motion or over-unity device is a device that, barring mechanical breakdown, is capable of running for an arbitrarily long period of time with no outside intervention or energy input. Such a device has never been built since the concept violates the laws of thermodynamics. Essentially, even in an "ideal" machine, with 100% efficiency, it is only possible to get enough power out to power the machine itself and no more. An advanced form of a perpetual motion machine is a device with an output energy MORE than the input, known as an Electron Pump. However, in the real world there will always be some inefficiency to deal with, such as friction and the load of the machine itself, meaning that it is impossible to get even 100% efficiency.
In the imaginations of some pseudoscientists and outright frauds, however, such problems don't exist. Perpetual motion machines have been patented by the US Patent Office, much to the amusement of physicists.
Perpetual motion machines fall into two categories - first order and second order:
- First Order machines break the first law of thermodynamics, i.e. conservation of energy - they generate more useful energy than is put into them (their efficiency >100%).
- Second Order machines break the second law of thermodynamics - they break even on their energy in/out efficiency (their efficiency =100%).
 How to spot a perpetual motion fraud
In short, the way to spot if a perpetual motion machine is fake and won't live up to its claim is simple: it's labeled as a perpetual motion machine. Any claim of perpetual motion is fraudulent on its face due to basic physics. At best, it's someone seeing only an illusion because they haven't tested it properly, or at worst they're engaging in active deceit and tricking people into believing the machine somehow works, sometimes in order to sell it.
Despite this, many inventors attempt to rationalize their perpetual motion invention with ad hoc explanations. For example, Joe Newman claims that his energy machine actually consumes the components of its motors by direct matter-to-energy conversion. A popular option for the extremely common magnet powered motors found on PESWiki is that they somehow take their energy by depolarising the magnet. Of course, such energy sources are not perpetual, and there is also the question of whether they can do a worthwhile amount of work. It is therefore necessary to insist on seeing the actual specifications and design of the device in question. Chances are, if the inventor won't share, the idea is wrong and may be a complete put-on.
Keep in mind that with a properly balanced device, it's possible to simulate perpetual motion for as long as you need to keep the audience focused. Of course, such devices are always con jobs, since they obviously are not perpetual devices internally, just very, very fuel efficient, and probably not capable of all that much real work.
 Energy cycles
The laws of conservation of energy basically say that any energy you get from a system is limited by how much energy you put into it. In the case of burning fossil fuels, the energy was put into the fuels (a mixture of hydrocarbons) by sunlight, which was converted via photosynthesis. All we are doing when burning fuels is to take this product that has some energy and releasing it, by reaction with oxygen. Similarly, with hydrogen fuel cells we are splitting water (using electricity which itself must come from somewhere) to form oxygen and hydrogen, and then reacting it to get the electrical energy back at another point in time. The energy in and out of the system balances at all times. The energy from sunlight, used to convert water and carbon dioxide into a fossil fuel is equal to what we get out from burning them completely back to water and carbon dioxide. The energy from a hydrogen fuel cell reaction is the same as the energy put into the electrolysis reaction to split the water molecule in the first place. Perpetual motion, and most other forms of free energy theory, insist that this "energy" in stage either doesn't need to happen or that the in and out doesn't need to balance. More on-the-ball proponents who understand conservation of energy may attempt to replace it with some form of magic, either magnetic depolarisation, gravitational work, just plain handwaved as "unexplained" or something far more exotic involving nanotechnology or zero-point energy.
Because the energy in and out of a system balances, a perpetual motion machine is possible in theory or as a thought experiment. This is providing that all of the energy it produces goes into feeding the machine again. Where the problem occurs is in attempting to get the machine to do any work. Extracting this energy causes the machine to slow down, reducing the energy output, and so on until it draws to a complete stop. This is why it needs to be fed something in the form of fuel, whether that be unburned hydrocarbons, hydrogen and oxygen or nuclear energy or even light. Even Heron's fountain will eventually run out as the water level settles to an equilibrium at the lowest point, and it must be "recharged" by physically lifting the water back to a higher level. This gives the water the potential energy (the energy gained by working against gravity) to move again - the "perpetual" nature of Heron's fountain is just a short-term illusion. But remember that perpetual motion is in theory only, in practice there are always inefficiencies and energy is always going to be lost from the machine, either via friction or even air resistance. This takes energy out of the system constantly, rendering it impossible.
 Physical Examples
Designs of alleged perpetual motion machines that can actually be built (as opposed to ones that work only on paper) tend to fall into one of two categories.
"Over unity" machines have a clear and undisguised energy source, but it is claimed that their output exceeds their input. For many devices this can be shown to be false very easily, but highly efficient devices or ones with a very convincing illusion of motion, very sensitive and observant measurements can be required. However, if the output genuinely exceeds the input, the question arises why their output can't be fed back in, eliminating the need for an outside energy source entirely. Sometimes, though, claims of "over unity" are simply based on blatantly faulty science, such as claiming that a machine that has an input of 5 amps and an output of 10 amps is "over unity".
The second type, ambient energy machines, do in fact continuously put out energy, but they do so by drawing on ambient energy sources: for instance, a clock might run off of the daily fluctuations of air pressure. Some perpetual motion designs that appear on the likes of PESWiki often claim to be of this type, where they claim to "depolarise" a magnetic field to draw their energy without violation of energy conservation, but in reality this is not the case. An alleged perpetual motion machine might have a purported design, but have a hidden energy source not disclosed, or at least not explicitly stated, in those plans. These fall into a category known as "fraudulent".
 What about planetary orbits?
Proponents of perpetual motion are quick to respond with the fact that the Earth orbits around the sun and rotates seemingly forever. However, the Earth seems to orbit forever because space is near-frictionless and there is no counter-momentum that stops them from orbiting. If a planet of equivalent size crashed at an identical speed, then it would stop or change course depending on the angle of collision. If you put a generator on a planet, it would start to slow down, although negligibly. Momentum machines in space can work, but cannot be harnessed for productive energy. Strictly speaking, planetary orbits are not perpetual motion. As the planet (and their star) rotate around their common center of gravity, they emit gravitational waves. Those gravitational waves drain the planet/star orbital system on energy so the planet eventually gets closer and closer to its star. In other words, planets DO slow down.
Now this orbital decay due to gravitational waves emission is ridiculously small so it has only been measured for extreme systems like binary neutron stars or black holes (which are heavy and may orbit each other within minutes, seconds or just a fraction of a second). Our planet Earth is subject to gravitational wave emission as well, but the orbital decay is so small that it in practice won't affect Earth within the Sun's lifetime; instead of Earth spiraling into and being devoured by our Sun, it will rather be our Sun (turning into a red giant) which extends beyond the current orbit of Earth and thus devours it.
 Reactionless thrusters
One highly desirable feature of a thruster for use in spacecraft is for it to use no fuel. Several ideas have been proposed, though few - if any - actually work in practice. In many cases, these devices do consume energy, but like perpetual motion machines violate conservation of momentum and can create an illusion of working even when they don't.
An oscillation thruster consists of a drive which attempts to create motion by sliding a mass around at different speeds. If you can imagine a mass being slowly drawn back then fired forward at speed, then the momentum from that would arise from this would transfer to the drive itself and motion will be created. However, this is a clear violation of the conservation of momentum and energy; the force required to draw this weight backwards is equal to what will be produced forwards, and both will cancel. Also, the force to push the mass in a direction will have an equal and opposite force on the device trying to push it. Like many perpetual motion engines, this particular drive can create the illusion of motion by exploiting coefficients of friction. The force from the slow drawing back of weight is proportionally lower (but over a longer period of time) and is not great enough to overcome friction. The force from the faster motion going forward is strong enough to overcome friction and a net motion occurs. This is actually a trivial (and somewhat inefficient) method of creating net motion and is known and exploited for certain tasks, but would not apply in the frictionless environment where such devices are expected to work.
 Classic examples
While hooking up a generator to its own motor is a popular modern variant on perpetual motion (which definitely doesn't work because of the horrendous inefficiencies in electricity generation), there are several classic examples made up over time. Even today, people are trotting out only minor variations of these long disproved models.
 Capillary bowl (Boyle's Self-flowing Fluid)
The capillary bowl, or Boyle's Flask (after Robert Boyle) makes use of a few "paradoxes" of hydrostatics. Particularly it's related to Pascal's Vases, where water remains at the same level regardless of the shape of the flask, so an apparently small volume of water can resist a large volume of water - demonstrating that the level of a fluid is dependent on depth and doesn't act like a set of weighing scales. The theory behind the flask is that capillary action, which is responsible for creating a meniscus and draws water through a small enough tube, would keep the water flowing constantly. It is not powered by gravity, as might be suggested by a quick glance at the hypothetical apparatus. However, the flask would fail to fill regardless as water tension (related to the same force that causes capillary action) would prevent the flow emerging from the capillary at the end. A droplet may form on the end of the capillary, but would be held in place by the surface tension of the fluid; you could shake the device, forcing it to drop and encourage the flow to start, but that would be adding in energy to the system and defy the point of perpetual motion.
In principle, Boyle's Flask will run perpetually using a superfluid as these have zero viscosity, and thus remove the main barrier that prevents the capillary action from flowing continuously in this setup. Demonstrations of near perpetual motion have been achieved using superfluids because they are frictionless (friction being the main barrier to producing real-world perpetual motion), although conditions required to keep something in a superfluidic state are very difficult to maintain. Superfluidic fountains demonstrate this principle quite readily, flowing as long as the chamber is at the right temperature and pressure for the effect to work. Perpetual motion also already exists in superconducting magnets, where electrons experience no electrical resistance, analogous to a frictionless environment, but again these need to be kept very cold for the conditions to be maintained. But don't all rush out to buy up the world's supply of helium-4 just yet; regardless of the perpetual nature of the motion in theory, it is still impossible to extract work from these devices and have them continue.
 Overbalanced wheel
The overbalanced wheel, sometimes referred to as Bhaskara's wheel, consists of a cog and several spokes with weights on the ends. Hinges allow the weights and spokes to move, altering the center of gravity of the device and causing it to spin. However, as it spins, the spokes at the top of the wheel flip down, adding to the momentum by keeping it off balance and cause the wheel to turn indefinitely. The hypothetical device should never come to an equilibrium position. However, a quick examination of the design shows that while there should be a clockwise torque caused by the extended spokes, this is compensated by the fact that there are more weights providing a counter-clockwise torque. Both forces are balanced at all times and the wheel would quickly fall into an equilibrium position. The principle prevents any device of this type from working whether it uses gravity or magnetism to "power" the wheel or even if the moving weights utilise mercury sloshing from the inside to the outside of the wheel as proposed by Bhaskara in the 12th Century.
To truly overbalance the wheel (so that torque in one direction is greater than the other) and cause motion, the radius of the spokes would have to be altered throughout the course of the wheel's motion. This would have to be done actively, thus consuming energy in the process—and so the machine would cease to be a perpetual motion engine. It's also important to consider the wheel as it moves, as it can be placed into an overbalanced position so that the math makes it appear that there is an overall torque. It is perfectly possible for the wheel to exert a motion if it is placed off balance (much in the same way a pendulum will swing if moved out of a perfectly vertical position), but that motion does not continue indefinitely and will eventually be counteracted.
The overbalanced wheel itself is a long-discredited mechanism, but the lessons it teaches about forces and axial motion are widely applicable to other proposed perpetual motion machines. In particular, magnetic motors where the magnets are placed in an "over-balanced" manner exhibit the same effect. The net forces in both directions balance exactly, sending the motor into an equilibrium position. Force-based motors like this that "work" are either fraudulent or illusions.
 Float belt
The float belt is also a common theme in perpetual motion, where buoyancy is exploited to extract perpetual energy. The balls are buoyant and float upwards, powering the machine. Even assuming the valve can be made watertight to prevent water leaking from the system this would fail as the water also provides resistance to any object trying to force their way into it. This can be measured simply with a device that tests the force required to push or pull and object through water—sensitive ones can calculate surface tension, although this isn't the main resistive force. If it didn't, boats wouldn't require engines to move (and wouldn't even float in the first place, for that matter). This force far exceeds the amount of force gained from buoyancy.
Sometimes this is combined with some mechanism to fill the falling vessels with water to make them drop with a greater force, however, the principle remains the same as whatever water comes down must be pushed up against gravity in the first place. The device cannot generate the energy required to keep it running, let alone produce usable excess energy to extract.
 See also
- Free energy (pseudoscience)
- Free energy suppression, a conspiracy theory
- Thermodynamic free energy[wp], a concept in science
- Zero-point energy
- The Museum of Unworkable Devices
- Ramiro Augusto Salazar La Rotta, Nueva y Revolucionaria Cuarta Ley de la Termodinámica
- ↑ http://news.nationalgeographic.com/news/2005/11/1111_051111_junk_patent.html
- ↑ Other industrial production methods involve steam reforming to get hydrogen from hydrocarbons, which, of course, requires a lot of thermal energy input.
- ↑ See the Wikipedia article on Heron's fountain.
- ↑ See the Wikipedia article on Beverly Clock. among other examples.
- ↑ Planets of red dwarf stars, that do not experiment a red giant phase, may suffer this fate in the very far future, however, assuming they're not ripped out of their orbits before because of passing stars.
- ↑ NASA Breakthrough Propulsion Physics Project - Common errors
- ↑ See the Wikipedia article on Stick-slip phenomenon.
- ↑ Pascal's Vases
- ↑ NIST on road to perpetual motion with 'superfluidity' demo
- ↑ Superfluid Fountain (video)
- ↑ Magical Machines - The Overbalanced Wheel
- ↑ PESWiki - Screw-Magnet motor