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An estimate of the fine structure constant


Quantum electrodynamics, despite its many successes, has left many questions unanswered. Why is electrodynamics so different from other interactions? Why do all particles have commensurate electric charges? Why is the value of the fine-structure constant what it is? Today, we have an apparently correct and essentially complete description of all of elementary particle physics in terms of a gauge theory1. If the underlying gauge group is simple2,3, all three elementary particle interactions—strong, weak, and electromagnetic—are parts of one unified theory. (The earlier attempt to synthesise all three interactions obtained both charge quantisation and baryon number violation. It was not based on a simple gauge group, nor did it incorporate conventional quantum chromodynamics.) Electromagnetism is no longer so unique. Apparent differences among the interactions reflect the pattern of spontaneous symmetry breakdown. Charge quantisation is forced on us: all fields have integer mutiples of one-third the electron's charge2,3; all observable particles have integer multiples. (We assume that colour is a truly hidden variable, that quarks are fractionally charged and never liberated. For another point of view, see ref. 4.) The fine-structure constant cannot be chosen arbitrarily. For unified theories in which the proton is unstable, we find an upper limit to its strength, α <0.04. We derive this result here and survey the context in which it arises.

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Glashow, S., Nanopoulos, D. An estimate of the fine structure constant. Nature 281, 464–465 (1979).

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