A Second Look at Relativistic Wave Equations
Although it has been known for a long time that there exist more general theories besides the Klein-Gordon and Dirac equations, they were mainly considered a mathematical curiosity and no serious attempts were made to determine their physical content like mass spectra, magnetic moments or form factors. Such an attitude was permissible perhaps thirty years ago when only a few elementary particles were known and it still could be believed that only the simplest (=irreducible) mathematical structures should be used for their description. In the meantime it has been learned from experiment that most particles have a rather complicated internal structure and can exist in various excited states. Hence we see no cogent reason why, for example, only the Dirac equation should be used for the description of spin 1/2 — particles.
KeywordsWave Equation Irreducible Representation Dirac Equation Irreducible Component Lorentz Group
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