Measuring the general relativistic curvature of wave-fronts

Einstein's general theory of relativity predicts that an initially plane wave-front will curve because of gravity. This effect can now be measured using Very Long Baseline Interferometry (VLBI). A wave-front from a distant point source will curve as it passes the gravitational field of the Sun. We propose an experiment to directly measure this curvature, using four VLBI stations on earth, separated by intercontinental distances. Expressed as a time delay, the size of the effect is a few hundred picoseconds and may be measureable with present technology.