Hanbury Brown and Twiss: Important, anti-weird, beautiful Free

Kleppner replies: I thank Donald Morton for calling to my attention the measurements of a number of hot stars in the 1970s. My comment that the Hanbury Brown and Twiss (HBT) effect was never “particularly important” for astronomy reflected Hanbury Brown’s own assessment of his accomplishment. I believe that my comment is defensible if the impact of intensity interferometry on astronomy is compared with, for instance, the impact of very long baseline interferometry. Vittorio Degiorgio quite correctly points out that intensity correlations have long been a tool for studying critical phenomena. The early controversy about the HBT effect centered on two-particle correlations, however, and not on intensity correlations. The goal of my essay was to point out the interesting manifestations of two-particle correlations in atomic fluids.