« on: 03/08/2020 10:46:31 »
Quote from: Jarek Duda
Is CPT analogue of laser possible?I don't think so.
The clue is in the metastable state that lases.
- The laser medium has a ground state
- You have to pump the laser medium up to an energy level which is too unstable to lase (it doesn't hang around long enough)
- But this unstable state decays into a metastable state which doesn't readily decay by itself. This decay is dissipated as heat.
- Most of the laser medium has to be in this metastable state before lasing can occur (a population inversion)
- If too many atoms are in the ground state, they will just absorb any laser output before it has time to build up a coherent resonance.
- This part of the process is certainly reversible!
- The metastable state stays around long enough to be triggered by a passing photon of the right frequency into emitting a photon of identical frequency and phase (stimulated emission of radiation).
The reason that illuminating a laser medium with light does not produce electricity + a cooling effect is:
- The incoming laser photons have the right energy to pump electrons from the ground state to the metastable state
- But they lack the energy to pump the electrons to the higher energy state - this would involve turning heat into orderly energy, which violates entropy
- Different lasers have different mechanisms for producing the metastable state- some laser diodes inject holes and electrons into the laser medium; to reverse this, thermal energy would have to cause random holes and electrons to undergo an orderly diffusion in opposite directions, again violating entropy.
- In general, turning a source of heat into useful energy has to battle the Carnot equation, which was the original basis of thermodynamics.
In general, I think that interactions on a Feynman diagram with 2 particles on each side might be reversible.
- But by the time you get 3 or more particles on one side of the Feynman diagram, entropy holds the trump card.
- Because it is statistically unlikely that all 3 components would appear in the right place at the right time with the right energy to reverse the process.
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