Isn't two-way mirror Maxwell's demon for photons?

[Duda Jarek]

Everybody has seen two-way mirror - transparent from one side, reflective from the second ... isn't that Maxwell's demon for photons?

Ok - it's not perfect - it absorbs some photons increasing own heat

and emits thermal photons - so it can stay in thermal equilibrium with

environment.

 

Let's take a container for photons (covered with mirrors), now place

two-way mirror in thermal equilibrium with photon gas inside, dividing

container into two parts.

The density of photons on the reflective side should be larger than on

the second - so it would reduce entropy?

[Kyrisch]

No. Precisely because it is not perfect. The only reason Maxwell's demon was a paradox because he introduced said supernatural demon. It was later resolved with the conclusion that whatever sorts the particles itself would balance the entropy, i.e. the "demon" would introduce more entropy judging the temperature and then opening the door than the entropy lost by the system of particles by itself.

[Duda Jarek]

Maxwell's demon is something that creates spontaniously ('from nothing') gradient of temperature/pressure/concentration - reducing entropy.

It doesn't have to be perfect: if one side of the mirror would just a bit more likely reflect photons - it will enforce pressure gradient.

 

The slightest pressure gradient it would spontaneously create can be used to create work (from energy stored in heat).

For example we could connect both parts to constantly equilibrate their pressure.

Through this connection would dominate direction from higher to lower pressure, which we can use to create work (from heat) - for example placing there something like water wheel but made of mirrors.

 

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I completely agree that we usually don't observe entropy reductions, but maybe it's because such reductions has usually extremely low efficiency, so they are usually just imperceptible, shadowed by general entropy increase... ?

 

2nd law is statistical mathematical property of model with assumed physics.

But it was proven for extremely simplified models!

And still for such simplified models was used approximation - while introducing functions like pressure, temperature we automatically forget about microscopic correlations - it's mean field approximation.

Maybe these ignored small scale interactions could be use to reduce entropy...

For example thermodynamics assumes that energy quickly equilibrate with environment ... but we have eg.ATP, which stores own energy in much more stable form then surrounding molecules, be converted into work...

 

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I apologies for the two-way mirror example, I generally feel convinced now, that they work only because the difference in amount of light - the effect while looking on dark glasses could be explained for example by their curvature.

When I was thinking about it, I had a picture of destructive interference from anti-reflective coating.

 

But let's look at such coating...

http://en.wikipedia.org/wiki/Anti-reflective_coating

Let say: thick layer of higher refractive index material and thin of lower.

The destructive interference in thin layer happen only from anti-reflective side (thin layer) - shouldn't it reflect a bit smaller amount of photons than the second side? ... create gradient of pressure in photon containment - reducing entropy.

Edited September 11, 2008 by Duda Jarek
multiple post merged

[CaptainPanic]

Maxwell's demon is something that creates spontaniously ('from nothing') gradient of temperature/pressure/concentration - reducing entropy.

 

While the entropy might be reduced locally, the light will come from a source. In some cases entropy will be increased in that source (the sun for example) or at a distant power generator (like a coal powered plant or nuclear installation). In any case, the entropy of the whole thing will increase, reflective mirror or not.

[Gilded]

Everybody has seen two-way mirror - transparent from one side, reflective from the second ... isn't that Maxwell's demon for photons?

 

No. Two-way mirrors allow equal amounts of light through from both sides (is everyone missing this?). The illusion of it being transparent on one side and a mirror on the other side is because of the lighting in interrogation situations for example; one side is brightly lit and the observer side is dark.

 

Actually, I think a real two-way mirror would be in violation of some physical law... can't remember which though.

Edited September 11, 2008 by Gilded