What does the quantum do after it is emitted

Planck did not assume anything more specific about the quantum of energy; in particular, he said nothing about how it is distributed in space. He had no reason at the time to picture it as anything other than a wave, dissipating in space, with energy spread out over the whole wave front. As he recalled in his Nobel lecture in 1920, 'What happens to the energy after emission? Does it spread out in all directions with propagation in the sense of Huygens' wave theory, in boundless progressive attenuation? Or does it fly out in one direction in the sense of Newton's emanation theory?

We will see in the next two chapters extraordinary and sometimes contradictory evidence on whether or not such a quantum is localised!

Much later, in 1926, Gilbert Newton Lewis (1875-1946) introduced the term 'photon' for the quantum of light. As this name is now in common use we will introduce it at this point, and use it freely for the remainder of this book, remembering that strictly speaking we are historically premature.

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