Is there still light in practical darkness? Do photons penetrate everywhere?

I always thought that, even in darkness, there would still be some light, making complete darkness, i.e. complete absence of photons as far as I know, just a theoretical thing. When I tried looking up whether or not this view was actually correct, I didn't find any clear information, however. Am I wrong?

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2 Answers

No, you are right. Anywhere you can go, you're surrounded by some kind of matter at some nonzero temperature. Though most matter is not quite a blackbody, it still does radiate when at finite temperature. However, at low temperatures the intensity of this radiation in the visible range vanishes proportional to $e^{-\frac1T}$, which means you normally have only few visible photons around, typically much less than those reflected in some indirect way from the sun or artificial light sources. You definitely can't see these photons with your eyes, because they are at a higher temperature as the surrounding and therefore have more thermal noise intensity.

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Yes, and with the typical temperatures they would radiate at infrared wavelengths, not something you can typically see unless the object is very hot (hence the red glow of very hot objects). – SMeznaric Oct 13 '12 at 23:35
@SMeznaric: yes, as I said: the intensity in the visible range is extremely weak. Still you can't really say "they would radiate at infrared wavelengths, not something you can typically see", since the Planck spectrum is continuous and in principle unbounded. Exponential decay is virtually like bounded support, but this was a principle question, not a pragmatic one. — But of course it's right to say "they radiate mainly at infrared wavelengths". – leftaroundabout Oct 14 '12 at 10:01
Right, I agree. – SMeznaric Oct 14 '12 at 21:17

A "darkroom" can be constructed to block external photons. The room itself and the space inside become the issues. All matter emits electromagnetic radiation when it has a temperature above absolute zero. Chilling the room to absolute zero would eliminate photons emitted by the walls. However, the uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well, even at absolute zero. The "empty space" inside has energy to potentially produce a photon. There is one major difference between zero-point electromagnetic radiation and ordinary electromagnetic radiation. Turning again to the Heisenberg uncertainty principle one finds that the lifetime of a given zero-point photon, viewed as a wave, corresponds to an average distance traveled of only a fraction of its wavelength. Such a wave "fragment" is somewhat different than an ordinary plane wave and it is difficult to know how to interpret this. Drilling down into ZPE and "antiphotons" may provide some further insight.

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