Protons are in very close proximity with each other in a nucleus. This means there is constant electromagnetic interaction, of which the exchange particle is a photon. What determines the wavelength of this exchange photon? How do they exist in the nucleus: constantly being emitted, or staying inside the nucleus somehow? Also, how can a neutral pion be an exchange particle in, say, a proton-neutron collision. Being a strong interaction, aren't gluons the gauge bosons? I have a hard time thinking about this kind of stuff.