For years, I've taken the Pauli principle for granted, but now that I've taken a course on Subatomic Physics, I'm mystified again. The example is given in the course of Neutron Stars. Neutron stars are burning stars that experience an incredible compression, drawing a lot of matter in a very small space. Apparently, under these conditions protons can react and become neutrons, but neutrons cannot react and become protons, because of the Pauli principle - they are simply too close together. So, what we see here, is how the Pauli principle is behaving like some sort of 'absolute' (?) resistance : you simply can't have two quantum states with the same quantum numbers. So, the Pauli principle acts as some kind of potential energy which becomes ridiculously high at 'zero' distances in the 'quantum phase space'. This is what I'm trying to achieve here: is it possible to visualize the Pauli principle? Can it be explained as some kind of resistance in a 'quantum space'? Also, can someone try to explain why the Pauli principle works for protons, but not for neutrons in this example? Thank you.