I found Daniel Kleppner’s Reference Frame concerning the Hanbury Brown and Twiss (HBT) effect and its applications very interesting. In particular, Kleppner mentions recent beautiful experiments using the HBT effect to explore the physics of quantum gases. The applications have been even more important than it would appear from Kleppner’s piece, if one considers the role that time-dependent intensity correlations of scattered light have played in dynamical studies of critical phenomena and, more generally, of phase transitions in fluids, liquid crystals, and soft matter. Early contributions to such studies came from George Benedek (an MIT colleague of Kleppner’s) in the 1960s. Herman Cummins, working in those days at the Johns Hopkins University, also made relevant contributions. In the past 40 years, thousands of papers have described the applications of intensity-correlation spectroscopy not only to statistical physics but also to physical chemistry and biology.

Concerning spatial intensity correlations, I agree with Kleppner that until a few years ago no important applications had appeared, mainly because they simply reflect, in the far field, the geometric properties of the volume from which the light is collected. However, in addition to what Kleppner describes, recent work by Marzio Giglio and coworkers (University of Milan) on near-field scattering from colloidal particles 1 promises exciting new developments.

See, for instance,
Phys. Rev. Lett.