How big is a photon? According to Louis de Broglie, a founding father of quantum physics, an individual quantum “fills all space” (see his statement quoted later). He was talking about electrons, but the same principle applies to photons. Quantum theory implies that every free (unconstrained) quantum is extended indefinitely in space and is not contained within any region of finite volume. So any talk of the size of a photon or of any other quantum must refer to some finite region within which the photon has a large probability (say, 99%) of interacting with a detector such as a viewing
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