THE RAPID DEVELOPMENT of electronic technology has given man the ability to control electrical events in the time domain with extraordinary precision, just as the mechanical revolution of the last century improved, by orders of magnitude, our power to control linear dimensions of physical objects. Perhaps the only failure in the analogy is quantitative; linear dimensions are now controllable to parts in whereas time sequences can be generated and measured to parts in The modern radio equivalent of Michelson's stellar interferometer is an interesting example of the revival of old mechanical principles in new electronic forms. The first motivation for building radio interferometers was the same as Michelson's: the angular resolution of sources of electromagnetic radiation. The principal difference lay in the limitation of the conventional techniques, for angular resolution of stars in the visible region of the electromagnetic spectrum is limited to a second of arc or so by the irregular refracting properties of the atmosphere, and the diffraction limit of a few minutes of arc for conventional radio telescopes is set by the size of the federal budget.
Radio telescopes separated by many thousands of kilometers can be operated in pairs to obtain very high angular resolution, suitable for studies of quasars and hydroxyl‐line emitters, for geodesy, and for a test of general relativity.