A Perpetual Motion Machine (also Perpetuum Mobile, Latin for "forever moving") is an old dream of mankind: A machine that creates more energy than it receives from the outside. (A weaker version just keeps moving on and on, without creating new energy you could put in use.) "Free Energy Device" is another name for it.
Obviously, a Perpetual Motion Machine contradicts the First Law of Thermodynamics, which states, roughly, "You can't get something from nothing." Of course, this didn't stop some people from believing in a PMM - they just insist that obviously, the First Law has to be wrong.
No Conservation of Energy applied to machines.
A perpetual motion machine of the second kind is a machine violating the second law of thermodynamics — that isolated system evolves toward maximum entropy. This involves transferring heat from colder to hotter objects without spending additional energy.note
- A Toyota commercial demonstrates a car with regenerative braking, which attempts to recapture a portion of the energy lost as the car brakes. The actor in the commercial imagines applying this same technology to a roller coaster to create a "self-sustaining amusement park." Unfortunately, he is talking about creating a perpetual motion machine. No matter how perfect the machine, heat, friction, gravity, and air resistance guarantee that this is impossible. The flaw in his idea comes from the fact that the energy expended to cause the roller coaster to start will always be greater than the energy regained from the brakes, in much the same way that hybrid cars need gas.
- In The Absent-Minded Professor and its remake Flubber, Flubber is a perpetual motion substance.
- In Battlefield Earth, the planet Psychlo has an atmosphere that spontaneously ignites in the presence of radiation. This means radioactive decay does not naturally occur on the planet, meaning the planet ignores the second law of thermodynamics and is effectively a perpetual motion machine.
- In the book, Psychlo's atmosphere has the same implausible property. But, there, it's stated that the Psychlos are actually from a different universe with different physical laws.
- The action-comedy Knight and Day revolves around three forces of people trying to secure a prototype Perpetual Energy battery.
- The train in Snowpiercer never stops, never needs refueling, and the engine does not appear to be a nuclear reactor. But while the engine is perpetual, its parts aren't, necessitating some creative replacements over time.
- The animated Space Pirate Captain Harlock (2013) has the Dark Matter Engine for the main ship. An alien technology that gifts the ship the ability to never refuel, rearm or repair.
- In the second Jim Button book by Michael Ende, the protagonists invent it. Essentially, their version is based on a magnet which you can switch on and off, which pulls their locomotive.
- Violating the Law of Conservation of Momentum as well.
- Discussed in Komarr. One of the physicists Miles calls in to consult determines that the device he's asking her about looks like a perpetual motion machine. Since she's a competent physicist who doesn't believe in such things, she concludes that it must be drawing energy from the deep structure of the wormholes it gets pointed at — because there's nowhere else it could be coming from.
- In Atlas Shrugged, John Galt's engine relies on perpetual motion.
- The main focus of the first episode of The Chronicles of Professor Jack Baling is what happens to an engineering professor when he encounters a working perpetual motion machine. Specifically, an overbalanced wheel.
- An 8th grade student in Yevgeni Veltistov's The New Adventures of Elektronic claims to have created a small device that causes a little light bulb to light up and never go off after you wind the crank once. When the titular android character is asked if such a thing is possible, he simply says that he doesn't know, but that the device doesn't have any moving parts (except, obviously, the crank). The device is put on a shelf in the classroom and forgotten. It's mentioned that it worked without anyone touching it for weeks, but the little light bulb burned out shortly after. Of course, this is the same book where another student from the same class proved Fermat's Last Theorem, only to rip up his proof a week later, as he did not want recognition.
- Isaac Asimov's short story "The Billiard Ball" (reprinted in Asimov's Mysteries) is about a zero-gravity device which, when the zero-g field is established, the field becomes a brightly-glowing cylinder of hard vacuum — because any air molecules in it lose all proper mass, and thus become incapable of movement at other than the speed of light, so they smash their way out of the field. It's explained by the main scientist character that they get the energy to do this (from nowhere) because in abolishing gravity, the field repeals the law of conservation of energy.
- In Robert Reed's An Exaltation of Larks, time travelers from the heat death of the universe have been steadily making their way back to the Big Bang (at 15 month intervals) in order to tweak the laws of physics to make the entire universe a perpetual motion machine - rather than slowly succumbing to entropy, the universe will periodically collapse and then expand again.
- Leo Tolstoy tells a folk story about a Russian peasant who tried to invent this, but failed. The man was quite capable of building mills and claimed to even have repaired mills where professional engineers failed, but lacked education, and so wouldn't know about the laws of thermodynamics.
- Discworld: Alluded to at the end of The Fifth Elephant. One of the valuable artifacts recovered is a pair of 1cm cubes that rotate in opposite directions about once a minute, no matter what. It's explained that ancient dwarven civilizations used this minimalist PMM in conjunction with a massive gear and pulley system to power absolutely everything.
- The Dream of Perpetual Motion. When Prospero Taligent diverts his company's resources to building a perpetual motion machine, his stock plummets as it's regarded as a sign he's become a literal Mad Scientist. Ten years later he claims to have succeeded, installing his only model in a Cool Airship designed never to land. The protagonist realises however that the airship is slowly losing power and will eventually crash.
- The titular Eternal Flame from the Greg Egan novel is a hypothetical chemical reaction that never exhausts itself, or at least one that continues for an absurdly long period of time, AND can be controlled and put to practical use, such as powering the Generation Ship. They spend an entire book trying to harness the explosive properties of orthogonal matter as fuel, only to develop an utterly unrelated type of engine that is just insanely efficient and runs on light. All of the (near-)perpetual motion, without the risk of catastrophic explosion.
- In the Girl Genius novel Agatha H. and the Clockwork Princess, it's mentioned in passing that one of the minor Sparks with the circus exhibited a perpetual motion machine, and was utterly mortified when Agatha proved that it needed a small push every ten years to keep going.
- In the Thursday Next series, Thursday's uncle Mycroft invents the "Nextahedron", a solid shape that is perpetually unbalanced. When unrestricted on a flat surface, the Nextahedron never stops wobbling, falling over onto a different face, wobbling again, and so on.
- In The Mouse and His Child, the eponymous characters seek to become self-winding, effectively making themselves this. Eventually, someone comes up with the clever idea of connecting two wind-up motors together so that they wind each other. Despite this being a book about sentient wind-up toys and talking animals, however, this ends up being treated realistically, as the motor still winds down over time - but it undeniably lasts much longer than it used to.
- One of the plot hinges of Angels & Demons is that the production of antimatter has not only been vastly improved in efficiency, so that large amounts can be made (and stored), but it can now be made at a cost of less energy than will be yielded by annihilating it.
- The Ransom Apparatus of The Rise of Ransom City, a machine that utilizes the Ransom Processnote to "[strip] away the world and [reveal] the energy that lies beneath," generating both light and heat in infinite quantities (among other things, like altering local gravity and creating "ghosts" of people from alternate universes). It only works because it incorporates a rune of the First Folk in its construction. Folk magic not being a toy, the Process can be very dangerous when it melts down. While Ransom intended to use it to provide free light to everyone in the West, everyone else is more interested in its applications as a Phlebotinum Bomb.
- Red Green attempted to make one in the "Handyman's Corner" segment of the Grand Finale of The Red Green Show, using only household items. He hooks up a lawnmower to an alternator that powers a ceiling fan located amongst some corn stalks. The fan chops off an ear of corn when the corn gets as high as an elephant's eye, sending the corn into a sink with a garbage disposal also powered by the alternator. The corn then falls into an oil drum where it decomposes into ethanol, where it gets siphoned back into the lawnmower. Meanwhile, the spare corn seeds gathered by the garbage disposal slide down a downspout to where the corn grows, so the corn keeps on coming. However, Red finds one problem in his "ultimate project". The lawnmower won't start.
- In Dwarf Fortress the mechanical energy needed to pump water up one story is only one tenth the amount generated when the water comes back down and powers a water wheel. Power the pump with the water wheel, prime it once with manual labor, and it will endlessly generate power.
- Portal fans have proposed several ideas for perpetual motion machines using the portal technology. Most of them revolve around the fact that if one portal is on the ceiling and the other is on the floor, any object thrown in would fall indefinitely.
- The Reapers in Mass Effect somehow work without fuel. In Mass Effect 3 Codex it is outright stated how strange and impossible this should be, as well as the fact that without need for resources and capable of replenishing their foot-soldiers from enemy ranks, the Reapers need absolutely no supply lines in war.
- The Incredible Machine has absolutely no concept of thermodynamics, making it easy to make perpetual motion machines in dozens of different forms. A power generator produces electricity from rotation, while an electric engine produces rotation from electricity. Place a generator, plug an engine into it, connect the wheels of the generator and the engine, add an initial source of energy (e.g. bellows and a windmill) - voilà! Perpetual motion.
- Try connecting a laser to a matching laser-powered outlet, and direct the beam so that it feeds itself. Now all you need is an outside laser to get the ball rolling.
- Probably the easiest perpetual motion machine you can construct is a Newton Motor with something like a tennis ball permanently set on top of it. The mouse inside the motor will continue running indefinitely so long as something is colliding with its box. Hook it up to a generator and you've got free electricity, too.
- There is one in the museum in Ultima VI. It's a set of gears connected by shafts; each gear transmits motion to the next, and the last one transmits it back to the first one. It has no relevance in the story, though.
- The Star Forge from Knights of the Old Republic is essentially this in both function and capability. By endlessly harvesting the practically infinite energy of a star, the Star Forge is able to create limitless amounts of droids, fighters and capital ships for those who control it. It, and everything made with the Star Forge's technology such as the Star Maps that lead to its location, are also capable of completely self-repairing themselves unless completely obliterated. Justified as it's a mechanical Eldritch Starship that is fueled by the Dark Side of the Force itself and heavily implied to be sentient.
- In Ilivais X, the titular mech is powered by one of these. How it works isn't explained, all we know is that it's so expensive that the Aztecs will throw as much military force as possible at recapturing it instead of just making another one, and that the limitless energy is the only reason the protagonist was capable of escaping in the first place. It's hinted that it may not be the energy they want, but rather something to do with the sheer fact that it shouldn't be possible, as with one rule of possibility broken, all the others can be as well.
- There's a whole gallery of plans for these machines — Donald Simanek's Museum of Unworkable Devices. Some visitors to this site misinterpret Simanek's motivation as "seeking to eliminate the flaws of such machines, so that they can be made workable"; of course, his actual motivation is to educate the public that those flaws are inherent and can never be eliminated.
- The ectoentropic SCPs of the SCP Foundation all technically count as Perpetual Motion Machines, though the ones that merely create matter from out of nothing don't really fit under this tropenote .
- The Simpsons, "The PTA Disbands": Lisa is going crazy while the teachers are on strike and creates a perpetual motion machine. Homer later told Lisa that no physics law should be broken in his home.
Homer: Lisa, get in here! In this house, we obey the laws of thermodynamics!
- In one episode of Chip 'n Dale Rescue Rangers, Gadget claims she once found a perpetual motion machine in the garbage can after a school science fair; of course, by then, it had stopped moving.
- In an episode of ChalkZone, Protagonist Rudy Tabootie creates one of these for the school science fair using his magic chalk. This wasn't intentional, as he was just in a rush to cheat and succeed, however the government immediately confiscates it upon its discovery in order to figure out how it works. Rudy's solution? Draw a power cord on it, making it so that it's no longer a perpetual motion machine.
- In one episode of Recess, Gretchen invented one for a school project. It was immediately confiscated by the government. The substitute teacher at the time gave her a passing grade.
- Subverted in the Gadget Boy's Adventures in History episode "These are a Few of my Favorite Flying Things". The villain Spydra succeeds in stealing Leonardo da Vinci's perpetual motion machine prototype. Leonardo takes the theft well because his machine was supposed to prove that perpetual motion is impossible. Cue Spydra yet again suffering a humiliating defeat when her flying machine crashes.
- On the Gravity Falls episode "A Tale of Two Stans", Grunkle Stan's brother and author of the journals made a perpetual motion machine for the science fair, which would have guaranteed him a college grant. Stan accidentally breaks it, which leads to his brother being denied the grant, causing a rift between the two.
- The Rocket Power episode "Great Sandcastle Race" has Oliver's team building one for the eponymous contest.
- In the frictionless vacuum of outer space it is possible to have an object that will move forever through inertia, as in planetary orbits (which do slow down, but by a negligible amount). Technically, due to conservation of Angular momentum, if you spun a planet in empty space and no force or friction acts against it at all, it would spin forever. However, this is just perpetual storage of an initial amount of energy. If you hook a generator onto your frictionlessly moving object to achieve the production of energy that's the whole idea of perpetual-motion machines, all it'll do is tap that initial amount of energy. The object will then slow down, and eventually stop.
- Some people claim that Zero Point Energy can be used to generate power out of nothing.
- People attempt to patent Perpetual Motion Machines. Most nations' patent offices can and will reject any at face value, but a few get patented if they are labelled as something else.
- There have been a few cases of seemingly successful perpetual motion machines, though careful examination has always shown either a lack of credible documentation, or an external power source. In fairness to the creators, some of these cases used energy sources not well understood at the time. One in particular, Orffyreus' Wheel, is still the subject of controversy as to its functioning; however, it is believed to be a deliberate fraud.
- Dan Brown sincerely claims that his scheme for using antimatter to create endless free energy (a major plot device in Angels & Demons) is realistic and will someday work, despite numerous corrections from experts. (Some have pointed out that the only reason we can burn wood or coal for fuel is that the enormous energy cost of growing those trees was paid by the Sun.)
- Subatomic particles such as electrons could be considered legitimate examples of objects in perpetual motion, although whether what they do really qualifies as "motion" and not a state of being in multiple places at once (but more so in some than in others) constitutes a whole branch of physics.