The Quantum Stellar Interferometer of Hanbury Brown

Abstract

The conception of photons that we have expounded in Chapter 18 is illustrated in the stellar interferometer of R. Hanbury Brown. Photons of a given frequency emanating from the whole surface of a distant star and converging to the point of observation on Earth have a small lateral spread of momentum, depending on the size of the star and its distance from us. By the uncertainty principle, this gives the arriving photons a transverse dimension, which may be as small as a dinner plate or as large as many meters in diameter. Just as the convergence of the photons in Feynman’s two-slit experiment give rise to an interference pattern at a detector, so do those from the star. Provided that two detectors are set up at a distance from each other less than the radius of the photons, they will show the interference pattern. In this way, the transverse dimensions of the photon can be determined, and from them, the diameter of the star can be deduced. The smaller (and more distant) the star, the greater the photon’s lateral dimension will be.