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It is mentioned in this article It seems to be a Bose-Einstein condensate of some kind, but it is not exactly clear how one can create a BEC with just photons

Light consists of tiny indivisible portions, the photons. Under certain conditions, they, too, can condense, if they are cooled enough. Many thousands of these light packets then suddenly fuse into a kind of super-photon with unusual characteristics – a so-called Bose-Einstein condensate.

How do you "cool light"???

Read more at: http://phys.org/news/2016-04-capacity-condensed.html#jCp

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    $\begingroup$ People need to stop talking about a photon gas when they have matter in the cavity. It's an annoying misclassification of the physics that's going on here. $\endgroup$ – CuriousOne Apr 19 '16 at 14:06
  • $\begingroup$ @CuriousOne The only time I have seen reference to a "photon gas" has been in the design of fusion bombs. $\endgroup$ – user56903 Apr 19 '16 at 14:07
  • $\begingroup$ Any use of "photon" in the presence of matter is essentially wrong. Photons are vacuum states. Once matter is present, photons and matter interact and form new quasi-particle states. Those states may be showing BEC, the photons in a vacuum cavity will not, at least not under any experimentally realizable conditions in this universe. $\endgroup$ – CuriousOne Apr 19 '16 at 14:10
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It is not possible to get a Bose-Einstein condensate of photons in three-dimensional equilibrium. Since the photons have no mass gap and no chemical potential, they can just be absorbed by the walls. In this example, however, the experimenters used a gas that was out of equilibrium, with different effective temperatures for the motion in different directions.

They used a cavity to create an effectively two-dimensional photon gas, trapping light between two nearly flat surfaces. In the narrow dimension, normal to the surfaces, the temperature is relatively high, and the experimenters selected out the first TE mode in this direction. However, they cool the photons in the other two dimensions quite a bit more. Looking at the photons moving in the other two directions, they look like a two-dimensional gas of massive particles. The mass corresponds to the energy caught up in the motion in the third dimension. When sufficiently cooled, this two-dimensional gas can look like a condensate.

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    $\begingroup$ There ain't no photons here. The cavity is dye filled according to the abstract. $\endgroup$ – CuriousOne Apr 19 '16 at 14:12
  • $\begingroup$ @CuriousOne There's no mention of a dye in the linked abstract. Where did you run across the mention of a dye? $\endgroup$ – garyp Apr 19 '16 at 14:28
  • $\begingroup$ @garyp: Apologies, the dye filled cavity is shown in the actual body of the paper. Take the dye out and there is no experiment here. $\endgroup$ – CuriousOne Apr 19 '16 at 14:32
  • $\begingroup$ @CuriousOne Thanks. It's impossible to make sense of this from abstracts and news releases. I don't have immediate access to the article. $\endgroup$ – garyp Apr 19 '16 at 14:34

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