Irwin Shapiro

American astrophysicist at Harvard (PhD 1955), known for the Shapiro time delay - one of the four classic tests of general relativity.

In 1964 he predicted that radar signals bounced off Venus and Mercury would take slightly longer when they pass close to the Sun - the Sun's gravity slows light (bends spacetime). The prediction: about 200 microseconds extra, 'equivalent to 60 km in distance'. The first tests, in 1966-67 with the MIT Haystack radar antenna, confirmed it; it has been repeated ever since with increasing precision, and the effect is now corrected for in deep-space navigation. (Not to be confused with Ascher H. Shapiro, the MIT fluid dynamicist whose 1962 'Bath-Tub Vortex' paper shows the Coriolis effect is too weak to set your sink's swirl.)

Built on Einstein's general relativity (1915) and the radar tracking of the 1960s space age. Enabled the relativity-gps card (the same curved-spacetime physics that makes GPS clocks run 38 microseconds fast per day) and the radio-science tradition of measuring with signals.