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Published online by Cambridge University Press: 08 January 2010
Forces produced by exchanges of messengers
The crucial idea that permitted our generation to unify electromagnetism and the weak nuclear forces (rather than electromagnetism and gravitation, as Faraday and Einstein wanted) was that both these forces (electromagnetism and the “weak” nuclear) have spin-one messengers and are “gauge” forces. (I shall define “gauge” forces presently.)
In quantum theory, all forces – whether of gauge or nongauge variety – are produced by an exchange of particles, which I shall call “messengers”. These “messengers” must always have integer intrinsic spins (0, 1ħ, 2ħ, etc.) as contrasted with the “source” particles of matter like electrons, neutrinos or protons and neutrons which are described by Dirac's equation and have intrinsic spins of ħ/2 each.
The fundamental property that characterises gauge forces (as opposed to nongauge forces) is that they are produced by an exchange of “messengers” of spin-one.
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