Perpetual motion machine
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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.