In Chap. 19 we used quantum mechanics to shed light on various behaviors of atoms, then molecules, then materials. But what if, instead of looking at ever-more complex collections of matter, we go in the other direction and investigate the nature of subatomic things such as photons, electronsElectron, and protonsProton? In that case, quantum mechanics still serves us well, but we also need a change in perspective; instead of quantum–mechanical particles, the most effective model involves quantum–mechanical fields—“quantum fields” for short. I did discuss quantum fields very briefly in Chap. 18 , but in this chapter they'll get the attention they deserve. There's a surprisingly rich world of phenomena at subatomic scales (including but not limited to nuclear physics), and quantum–mechanical fields appear to account for much if not all of it (In introductory textbooks, an emphasis on the fact that the most fundamental things in physics appear to be fields is somewhat unusual, but Art Hobson is one author who has been known for stressing this point [21]).