The way this happens in the theory is elegant: the equation of Dirac determines the values a variable can take. Applied to the energy of Faraday’s lines, they tell us that this energy can take on only certain values and not others. Since the energy of the electromagnetic field can take on only certain values, the field behaves like a set of packets of energy. These are precisely the quanta of energy introduced by Planck and Einstein thirty years earlier. The circle closes, and the story is complete. The equations of the theory, written by Dirac, account for the granular nature of light, which
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