Imagine, if you will, a collection of many photons. Now imagine that they have mass, repulsive interactions, and number conservation. The photons will act like a gas of interacting bosonic atoms, and if cooled below a critical temperature, they will undergo a well-known phase transition: Bose–Einstein condensation. You will have a “superfluid of light.”1
Now imagine that you can choose the photons’ mass. Then, because the critical temperature for Bose–Einstein condensation depends on particle mass and density, you can create the condensed state even at room temperature.
That is not an idle dream. Condensed-matter physicists have a long history of inventing novel quasiparticles, such as massless electrons, particles with fractional charge, and particles with spins detached from their charges. Two decades ago researchers began to engineer hybrid particles of light and matter, called polaritons,2,3 that could be used to realize the Bose–Einstein condensates of light described...
