Applications of Intensity Interferometry in Physics and Astronomy

Criteria as to when an intensity interferometer is competitive with a Michelson interferometer are established with the help of a simple example. There do not appear to be any cases in the field of frequency measurement where this is true. In radio astronomy it is argued that intensity interferometry might still have a role in angular measurements of the very smallest sources but it is clear that the main application still lies in visual astronomy in measurements in hot single stars and close spectroscopic binaries. A discussion is given as to how far the method could be pushed both from the technical as well as from the observational aspect. The triple correlation interferometer of Gamo is considered but because of its extremely low sensitivity it seems to have no application at least in visual astronomy and in this field the use of lasers is also rejected partly because of the various serious effects of atmospheric 'seeing', and partly because of the excessively tight tolerances on the performance of the very large mirrors needed to obtain adequate sensitivity. The possibility of laboratory applications also seem remote, in particular the idea that intensity interferometry could be used at x-ray wavelengths to measure the phase of scattering amplitudes is shown to be ruled out by inadequate sensitivity which could only be overcome by the development of coherent x-ray sources.