What is a proton gas? I was studying nuclear fusion reactions, then suddenly came nuclear fusion reactions of two protons. My book says when two protons in a proton gas fuse then something something happens. I understood everything and mathematics but not proton gas. Can someone please explain briefly what that is?
 A: It is hard to say what they meant, because proton gas has no standardised meaning. The most likely explanation is that they refer to a state similar to an electron gas, where the electrons are moving so freely that they can be described by gas equations from thermodynamics, even though they are not gaseous, just very weakly or non-interacting (free electron model). To achieve a proton gas, I assume that you need to start with pure hydrogen and separate the electrons from it, maybe by a combination of laser-induced ionisation with a magnetic field. Then only the cores, the protons remain. But then the question remains why the protons should not be interacting, maybe if the density is low enough, but if you just assume that there is a proton gas, then it is probably a collection of free protons that do not interact and can be described by gas equations from thermodynamics.
A: Proton gas can be considered as a type of degenerate matter so you can study it in reference to the degenerate matter. In such type, gravitational pressure is so high that quantum mechanical effects came into accounts.
One line conclusion can be said as: "these are non-interacting fermions".
