Photons cooled down to room temperature, made into a blob of matter

To be a little more clear though (from the Nature article): "Bose-Einstein condensates (BECs) are a bizarre quantum phase of matter. "

I can email people the copy if they want, I won't post it though.

I believe that the preprint is at:

http://arxiv.org/abs/1007.4088

So reading more about this: http://en.wikipedia.org/wiki/Photon_gas , it looks like a scenario where a blackbody container was created, with a Boltzmann "temperature" peak equal to the energy level of the incident photons. Thus, all absorbed photons would be re-emitted at with the same distribution as the blackbody walls. And apparently, this phenomenon was tested in an ultra-low temperature scenario.

Still, I wonder. Given the low temperatures, the Boltzmann distribution suggests photons in the microwave or lower frequencies.

It's hard for me to say without reading the actual research article, but my guess is that temperature is referring to energy of each of the photons relative to one another. Meaning, if you were shining a laser beam down the z axis, typically the photons would also have some small velocity in the x and y directions (this would mean the light beam is diverging). So looking in a frame of reference moving along the z axis at the speed of the photons in the z direction, you would notice that most of the photons would be stationary in the z direction but are slowly moving in the x and y directions. The temperature is probably referring to the average kinetic energy of the photons in the x and y directions. So in order to get the photons all into a single state, you would want their x and y velocities to all be zero. That's my guess, I know sometimes temperature can refer to similar scenarios with a beam of atoms. Like with the LHC, they often refer to the atoms as having a very low temperature. However, the atoms are moving around in circles at very nearly the speed of light. With such a high kinetic energy, you would expect the atoms to have a high temperature. But really, the atoms are moving very slowly relative to one another, and thus temperature is referring to this kinetic energy.

Should've read the other article linked on here (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)

Not a physics major, but my guess is E=mc^2 still applies.

Given that the definition of "matter" is slightly ambiguous, it's not really wrong, but it's not really correct either. Traditionally, matter usually has to have mass, but if you just think of matter as a volume of particles, then a collection of photons could be defined loosely as "matter".

The photons that make up the condensate have no rest mass, which is why they travel at the speed of light whenever they have a non-zero amount of kinetic energy. Anything that has any form of energy has mass, since those are just two different words for the same property. Because of this, the photons can be called "massless" (i.e. no rest mass) but still have mass, and the condensate could in theory have mass without any of its constituents having mass. The photons have mass from their individual kinetic energies, and the condensate could have mass beyond that from any potential energy required to get it into that state. Although, if the condensate is stable in any sense, it probably has less mass than the sum of the energies of its photons, with the difference being the loss of potential energy it gains from being stable.

As a comparison, atomic nuclei are composed of particles with non-zero rest mass, and they tend to have less mass than the sum of their components because of their stability. The difference between the mass of the nuclei and its components is how much energy you have to dump into the nuclei to split it (plus enough kinetic energy so that they fly away before they reform).

As to the question of what constitutes "matter"... I'm not sure that's actually defined in a way that's 100% free of contention. My personal opinion would by any configuration of particles that's stable. This new stuff could qualify.

I don't really know what it means, but my understanding is that photons actually have a mass. E=mc^2 => m=E/c^2, and photons have energy...