Perpetual motion

Perpetual motion describes the movement of bodies that continues forever in an unperturbed system. A perpetual motion machine is a hypothetical device that could perform work indefinitely without any external energy source. While the concept is fascinating, such machines are physically impossible. Their existence would violate the fundamental laws of thermodynamics. These laws apply to all systems, regardless of how large or small they are. Even machines that seem to run forever, like those powered by ocean currents, are not truly perpetual. They rely on energy from the Sun, which will eventually burn out.

To understand the impossibility of these machines, we must look at the laws of thermodynamics. The first law is the law of conservation of energy. This principle states that energy cannot be created in an isolated system. Therefore, a machine cannot produce more energy than it consumes. The second law involves how heat moves and how efficient engines can be. Heat spontaneously flows from hotter places to colder places. In every macroscopic process, some energy is lost to friction or other dissipative forces. This means the thermal efficiency of a heat engine can never be one. In fact, the maximum possible efficiency is limited by the Carnot efficiency, which is always less than one.

Scientists classify perpetual motion machines into three distinct types based on which law they violate. A machine of the first kind attempts to produce work without any energy input. This directly violates the law of conservation of energy. A machine of the second kind tries to spontaneously convert thermal energy into mechanical work. This would require only one heat reservoir, cooling it down without transferring heat to a colder area. This violates the second law of thermodynamics. Finally, a machine of the third kind aims to eliminate all friction and dissipative forces. It would use mass inertia to maintain motion forever. However, it is impossible to completely eliminate dissipation in any mechanical system.

The history of these attempts stretches back to the Middle Ages. For millennia, humans were unsure if such devices could exist. It was not until the development of modern thermodynamics that their impossibility was proven. Despite this, many inventors have continued to try to build them into modern times. Some proponents use terms like "over unity" to describe their inventions. This term suggests a system that produces more energy than it takes in.

Mathematical proofs provide even deeper reasons for why these machines fail. In 1915, Emmy Noether proved a significant theorem. Noether's theorem states that conservation laws are derived from continuous symmetries in physical systems. The conservation of energy is specifically linked to time invariance. This means that if the laws of physics do not change over time, energy must be conserved. Scientists use telescopes to check if physics has changed over billions of years. They examine ancient stars to see if they match stars today. So far, all evidence shows that the laws of physics remain the same.

Many clever designs have been proposed throughout history to trick the laws of physics. One famous example is the overbalanced wheel, seen in designs from the 12th century.

This design uses weights that move further from the center on one side of a wheel. The idea was that the greater torque would keep the wheel spinning. However, the side with more torque also has fewer weights, which balances the forces. Another common idea involves using magnets to pull objects up a ramp. As early as 1670, John Wilkins outlined such ideas involving "magnetical virtues." Most magnetic motors fail because the magnet is either too strong to let the object fall or too weak to lift it.

Even modern theoretical attempts cannot bypass these rules. Some suggest using electromagnetic levitation in a vacuum to remove friction. While this reduces resistance, the machine still cannot perform useful work. Work requires the object to cause motion in something else, which introduces energy loss. While these machines are impossible, they still serve a purpose in science. They provide instructive challenges for physicists. Analyzing why a complex design fails can help scientists understand subtle aspects of physics. For example, the Brownian ratchet was a thought experiment that helped scientists understand the exact mechanisms of failure.