Particles as Collective Stationary States
The hypothesis of Yukawa which connected the nuclear forces with some kind of strongly interacting particles, has led in a rather straight line of reasoning to the modern concept of an elementary particle. The atomic nucleus may be considered as a compound system of nucleons and a field of force; this field is according to Yukawa equivalent to virtual pions (or as we now know: ρ mesons etc.). This ‘virtual’ existence of pions in the nucleus is a consequence of the localisation of the nuclear field which—on account of the uncertainty relations—allows to consider the pions ‘off their mass shell’. If this situation is described by saying that the nucleus potentially consists of nucleons and pions, it should be equally permitted to say that any system (atom, nucleus or elementary particle) potentially consists of any other particles producing together the same symmetry as the system concerned. In this way the particles appear generally as collective states1) resembling the excited states of a solid body or a liquid, like phonons, excitons, polarons etc. The solid body in quantum mechanics should in elementary particle physics be replaced by the ground state ‘world’ or ‘universe’, but else the analogy should be rather close.
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