A hypothetical device that produces energy indefinitely without external input.A perpetual motion machine is a hypothetical device that operates indefinitely without an external energy source, either by producing more energy than it consumes (perpetual motion of the first kind) or by perfectly converting heat into work without losses (perpetual motion of the second kind). Such machines violate the laws of thermodynamics—specifically, the conservation of energy and the increase of entropy. Despite centuries of attempts, no perpetual motion machine has ever been built, and their impossibility is a cornerstone of modern physics.
The idea dates back to at least the 8th century in India, with designs like the overbalanced wheel. Formal impossibility was established in the 19th century through the work of Sadi Carnot, Rudolf Clausius, and Lord Kelvin, who formulated the laws of thermodynamics. The U.S. Patent Office has famously refused to examine perpetual motion applications since 1918.
Proposed designs often involve clever arrangements of magnets, wheels, or fluids that seem to create a self-sustaining cycle. For example, a wheel with magnets might be arranged to pull itself around, or a capillary action might be used to lift water indefinitely. However, analysis always reveals hidden energy losses—friction, air resistance, or the energy required to reset the system—that prevent net energy gain. In the case of the second kind, even a perfect mechanical system cannot extract heat from a single reservoir and convert it entirely to work without a colder reservoir to dump entropy.
None—perpetual motion machines are impossible. However, the study of why they fail has led to a deeper understanding of thermodynamics and energy efficiency, inspiring research into heat engines and refrigeration cycles.
The U.S. Patent Office requires working models for perpetual motion applications, but no such model has ever been submitted that actually works.
