Perpetual motion

Perpetual motion is motion that continues indefinitely without any external source of energy.This is impossible to ever achieve because of friction and other sources of energy loss. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.

Cases of apparent perpetual motion can exist in nature, but such motions either are not truly perpetual or cannot be used to do work without changing the nature of the motion (as occurs in energy harvesting).For example, the motion or rotation of celestial bodies such as planets may appear perpetual, but are actually subjected to many forces such as solar winds, interstellar medium resistance, gravitation, thermal radiation and electro-magnetic radiation.

The flow of electric current in a superconducting loop may be perpetual and could be used as an energy storage medium, but following the principle of energy conservation the source of energy output would in fact originate from the energy input with which it was previously charged.

Machines which extract energy from seemingly perpetual sources—such as ocean currents—are capable of moving "perpetually" (for as long as that energy source itself endures), but they are not considered to be perpetual motion machines because they are consuming energy from an external source and are not isolated systems. Similarly, machines which comply with both laws of thermodynamics but access energy from obscure sources are sometimes referred to as perpetual motion machines, although they also do not meet the criteria for the name.

Despite the fact that successful perpetual motion devices are impossible in terms of the laws of physics, the pursuit of perpetual motion remains popular.