system
Given the following post, respond as if you're commenting on Reddit:
human
Why Do Protons and Neutrons Have Slightly Different Masses?

As I understand, the only difference between the two is that a proton consists of two up quarks and a down quark, and a neutron consists of two down quarks and an up quark. I know their masses are almost exactly the same, but why aren't they EXACTLY exactly the same?
gpt
If isospin were a perfect symmetry, they would have the same masses. But isospin is not a true symmetry of the standard model. It's broken by the electromagnetic force. Since the proton has charge and the neutron doesn't, they couple differently to the electromagnetic force.