Sounds absolutely ridiculous, for vacuum is no 'material' in strict sense. But special relativity tells me that length may be contracted or expanded. I find no problem in extending this idea, that space itself may be compressed or expanded, and if it's so, why can't we devise a piston and cylinder like setup where piston moving in and out compresses/decompresses the vacuum/empty space, atleast this is possible theoretically to build a heat engine with vacuum acting as the working material. Is this even possible? If yes, then in what domain , I mean, if not classical then in relativistic or quantum physics? Moreover, will it be an ordinary heat engine or will it supercede , in terms of efficiency? Any suggestions are welcome.

  • $\begingroup$ Vacum has no specific heat, i.e. temperature changes won't produce heat. $\endgroup$ – TheAverageHijano Feb 14 at 9:16

Maybe, if you consider light pressure, vacuum HAS a temperature; light pressure from a thermal background could, ideally, expand a volume (by pushing a light sail) while lessening (by Doppler-shifting the thermal light toward the red) the temperature of the vacuum.

The practical problem, though, is that equilibrium of radiation in a vacuum doesn't happen on short timescales. No human lifespan will ever see the photon/photon interactions in any real volume cause a return to equilibrium spectrum after a disturbance. You cannot expect to ever see a second cycle of such an engine, because unlike an 'ideal' gas of weakly-interacting particles, a vacuum is a gas of noninteracting photons.

The temperature of 'the vacuum' is measurable, and that tells us of the conditions at the creation of the universe, because it hasn't been re-heated since. Dust can be heated by starlight, but not vacuum.


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