Why are perpetual motion machines impossible?
In: Physics
I’m afraid there isn’t really a “why”. The laws of thermodynamics aren’t derived from something deeper – they’re just based on observations. We think that energy is always conserved (the first law) because that consistently happens in experiments. We think that systems can’t be perfectly efficient (which is roughly the second law) because that also consistently happens in experiments. These two laws together imply that you can’t keep something running *forever* without putting energy into it – though plenty of things keep running for an extremely long time, such as the motion of planets.
There is a theoretical connection – a particular case of Noether’s theorem – between conservation of energy and the fact that the laws of physics don’t change over time. If one is true, then the other must be true. But again, we don’t have any deep reason to believe the laws of physics don’t change over time, we just have lots of experimental evidence.
On a very small scale – like that of individual atoms – the laws of thermodynamics become more of a statistical thing – they’re very likely to apply, but not always. But there are a lot of unanswered questions about how the laws of thermodynamics operate in microscopic systems, and this is an active area of research. A big question is whether it’s possible to create devices that can exploit repeated violations of the laws on a very small scale to create violations on a big scale. On the one hand, you’d think if it were possible we would see examples somewhere in nature, but on the other hand you can come up with ideas for such devices (such as “Maxwell’s demon”) and it’s not exactly clear why they couldn’t work.
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