Should've read the other article linked on here (http://www.nature.com/news/2010/101124/full/news.2010.630.html http://www.nature.com/news/2010/101124/full/news

Should've read the other article linked on here (http://www.nature.com/news/2010/101124/full/news.2010.630.html http://www.nature.com/news/2010/101124/full/news.2010.630.ht...): The team could tell when the transition had occurred because the small number of photons in the BEC formed an intense beam of yellow light — like a laser — in the centre of the cavity, surrounded by the 'gas' of remaining normal photons. To double-check that they were seeing a BEC of light, the researchers repeated the experiment with different numbers of photons. In each case, once the transition had taken place, they measured the spectrum of light leaking from the cavity and found that it matched theoretical predictions for the corresponding BEC. (BEC == Bose-Einstein Condensate)