Perpetual motion machine

A perpetual motion machine is a fictional isolated system capable of having internal motion indefinitely. Many attempts to build such a system have failed.

The only argument that a perpetual motion machine is impossible is based on an interpretation of the Second Law of Thermodynamics. If entropy is always increasing, even in closed (and isolated) systems, then indefinite motion is impossible because an increase in the disorder of the system will inevitably disrupt the motion.

Those who deny that entropy is always increasing, even in closed (and isolated) systems, are left without a persuasive argument that perpetual motion machines are impossible.

---

This is not the usual meaning of the term "perpetual motion."

The perpetual motion, then, which has been the subject of such anxious and laborious search, is not a mere motion which is continued indefinitely. If it were, the diurnal and annual motion of the earth, and the corresponding motions of the other planets and satellites in the solar system, as well as the rotation of the sun upon its axis, would be all perpetual motions....

In short, a perpetual motion would be a watch or clock which would go for so long as its mechanism would endure without being wound up: it would be a mill which could grind corn or work machinery without the action upon it of water, wind, steam, animal power, or any other moving force external to it.

—Dionysis Lardner, 1857, Natural Philosophy for Schools, p. 70

Perpetual motion is of course possible, as is stated in the first law of motion, "Motion continues in a straight line undiminished in velocity unless acted on by some external force." A top set spinning under conditions where there is no friction will never cease to spin. These conditions are very nearly realized in the motions of the planets and stars. The statement that energy cannot be destroyed asserts this possibility.

But by "perpetual motion" is generally meant a machine which will do work and keep going though energy equivalent to the work done is not supplied to it. Many attempts have been made to construct such a machine. They have all failed. The statement that energy cannot be created denies the possibility of such a machine.

—S. Lawrence Bigelow (1912), "Theoretical and Physical Chemistry," Theoretical and Physical Chemistry By S Lawrence Bigelow p. 26