The history of the concept of the photon in twentieth-century physics is far from a simple story opening with Einstein’s vision of light as a collection of indivisible particles whose energy and momentum are conserved during its interaction with matter, and reaching closure with the wave-particle duality as an accomplishment of quantum mechanics. Since then there has been an intermittent debate on the need for and adequacy of such a concept, even if this debate has been absent from the literature on the history of physics and from physics teaching. This paper analyzes a major event which led to the revival of this debate, namely, the experiment carried out by Robert Hanbury Brown and Richard Quentin Twiss (HBT) in 1956 in the context of low-intensity interferometry. As part of their work to build a new kind of interferometer to measure the diameter of optical stars, their results seemed to suggest that photons split through two different channels and detectors. These results stirred up a debate involving Edward Purcell, Eric Brannen, Harry Ferguson, Peter Fellgett, Richard Sillitto, Lajos Jánossy, Leonard Mandel, and Emil Wolf, in addition to Hanbury Brown and Twiss themselves. The building of this device in astronomy thus renewed the old controversy about the nature of light. Later on, with the invention of lasers, the HBT experimental results played a role in developments leading to the creation of quantum optics and currently play a role in various fields in physics.

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