Measuring the general relativistic curvature of wavefronts

Einstein's general theory of relativity predicts that an initially plane wavefront will curve because of gravity. This effect can now be measured using very long baseline interferometry (VLBI). A wavefront from a distant point source will curve as it passes the gravitational field of the Sun. We describe an idealized experiment to directly measure this curvature, using four VLBI stations on the Earth, separated by intercontinental distances. Expressed as a time delay, the size of the effect is a few hundred picoseconds and may be measurable with present technology. (letter to the editor)