"We have no better way of describing elementary particles than quantum field theory. A quantum field in general is an assembly of an infinite number of interacting harmonic oscillators. Excitations of such oscillators are associated with particles. The special importance of the harmonic oscillator follows from the fact that its excitation spectrum is additive, i.e. if E1 and E2 are energy levels above the ground state then E1 + E2 will be an energy level as well. It is precisely this property that we expect to be true for a system of elementary particles."