THE development of atomic time standards and high-speed recording techniques has allowed the extension of radio interferometry to baselines extending thousands of miles. The amplitude, rather than the power, of the received radiation is recorded separately at two or more stations with accurate time control, and the interference pattern is later recovered by cross-correlating the signals by digital1,2 or analogue3 methods. The measurements are usually described in classical terms, for the observations are well outside the domain of quantum phenomena. A discussion of the technique in quantum terms, however, raises an interesting form of the quantum interference paradox that Einstein, Bohr and others debated so vigorously4 40 years ago, in the discussion of the two-slit interference pattern shown in Fig. la. Any attempt to determine which slit a photon or an electron passes through must result in the destruction of the interference pattern.