““The principles of relativity and quantum mechanics are almost incompatible with each other and can coexist only in a limited class of theories. In the nonrelativistic quantum mechanics of the 1920s we could imagine almost any kind of force among electrons and nuclei, but as we shall see, this is not so in a relativistic theory: forces between particles can arise only from the exchange of other particles. Furthermore, all these particles are bundles of the energy, or quanta, of various sorts of fields. A field like an electric or magnetic field is a sort of stress in space, something like the various sorts of stress that are possible within a solid body, but a field is a stress in space itself. There is one type of field for each species of elementary particle; there is an electron field in the standard model, whose quanta are electrons; there is an electromagnetic field (consisting of electric and magnetic fields) , whose quanta are the photons; there is no field for atomic nuclei, or for particles (known as protons and neutrons) of which the nuclei are composed, but there are fields for various types of particles called quarks, out of which the proton and neutron are composed; and there are a few other fields I need not go into right now. The equations of a field theory like the standard model deal not with particles but with fields; the particles appear as manifestations of these fields. The reason that ordinary matter is composed of electrons, protons, and neutrons is simply that all the other massive particles are violently unstable. The standard model qualifies as an explanation because it is not merely what computer hackers call a kludge, an assortment of odds and ends thrown together in whatever way works. Rather, the structure of the standard model is largely fixed once one specifies the menu of fields that it should contain and the general principles (like the principles of relativity and quantum mechanics) that govern their interactions.””
