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Perpetual Motion Machine
"This 'perpetual motion' machine that she made today is a joke — it just keeps going faster and faster."
Homer Simpson, The Simpsons, "The PTA Disbands"

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 energynote .

Subtrope of Applied Phlebotinum. A Mad Scientist may work on this.

A subtrope of this: Hyper Destructive Bouncing Ball. See also Perpetual-Motion Monster. Not to be confused with Eternal Engine, which is a type of Video Game Settings.

Examples:

    open/close all folders 

    Advertising 
  • 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.

    Comic Books 
  • Gaston Lagaffe once invented one of the "weak" type. It doesn't do much, it just hops around (and gets on his co-workers' nerves).

    Film 
  • 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.

    Literature 
  • 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 the Vorkosigan Saga novel 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 [1].
  • 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.
  • 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.

    Video Games 
  • 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.
  • In The Incredible Machine, a perpetual motion machine can be easily constructed. 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) - voilą! Perpetual motion.
  • 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.

    Web Original 
  • 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.

    Western Animation 
  • 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 one episode of Recess, Gretchen invented one for a school project, and it was immediately confiscated by the government.

    Real Life 
  • 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 nation patent offices can and will reject any on 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.

Perilous Power SourceApplied PhlebotinumPhilosopher's Stone
Parasol ParachuteArtistic License - PhysicsPlasma Cannon
Parasol ParachuteJustForFun/Tropes Examined by the Myth BustersPinball Projectile

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