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Originally Posted by GUILLE PMMs are impossible, right? |
PMM is possible only if some laws of physics are broken.
Perpetual motion machines (the
Latin term
perpetuum mobile is not uncommon) are a class of
hypothetical machines which would produce useful
energy in a way which would violate the established
laws of
physics. It is generally accepted that perpetual motion machines cannot exist. In particular, perpetual motion machines would violate either the first or second
laws of thermodynamics. Perpetual motion machines are divided into two subcategories defined by which law of thermodynamics would have to be broken in order for the device to be a
true perpetual motion machine. These two subcategories are referred to as "perpetual motion of the first kind" and "perpetual motion of the second kind". Scientists and engineers accept the possibility that the current understanding of the laws of physics may be incomplete or incorrect; a perpetual motion device may not be
impossible, but overwhelming
evidence would be required to justify rewriting the laws of physics.
Any proposed perpetual motion design offers a potentially instructive challenge to physicists: We know it can't work (because of the laws of thermodynamics), so explain
how it fails to work. The difficulty (and the value) of such an exercise depend on the subtlety of the proposal; the best ones tend to arise from physicists' own
thought experiments.
Because the principles of thermodynamics are so well established, serious proposals for perpetual motion machines are met with disbelief on the part of physicists, which makes a discussion of the merits (if any) of the proposal difficult if not impossible.
Serious discussions of perpetual motion usually occur only when dealing with the topics of
open systems,
aether theories,
free energy, and
vacuum energy.