fundamental interactions; unification (Section 2.9)
mass and rest energy (Section 26.7)
Mastering the Concepts
Protons and neutrons are not fundamental particles; they contain quarks.
According to the standard model, the fundamental particles are the six quarks (up, down, strange, charm, bottom, and top), the six leptons (electron, muon, tau, and the three kinds of neutrinos), the antiparticles of the quarks and leptons, and the exchange particles for the strong, weak, and electromagnetic interactions.
Isolated quarks are not observed; quarks are always confined by the strong force to colorless groups. Color charge plays a role in the strong interaction similar to, but more complicated than, that of electric charge in the electromagnetic interaction.
Only the first generation of quarks and leptons (up, down, electron, and electron neutrino) are found in ordinary matter.
Just after the Big Bang, there was only a single interaction. First gravity split off, then the strong interaction; finally the weak and electromagnetic interactions split, giving the four fundamental interactions we now recognize.
New particle accelerators at higher energies will put the standard model, as well as theories competing to be its successor, to the test.
