Which American physicist proposed the Shapiro time delay in 1964?

Irwin Shapiro proposed the Shapiro time delay in 1964.

Shapiro was an American astrophysicist whose work connected precise astronomical measurements with tests of gravitation. In his 1964 paper, “Fourth Test of General Relativity,” he argued that radar pulses traveling near the Sun should experience an additional delay before returning to Earth. The effect arises because spacetime around a massive body changes the measured travel time of light.

His proposed experiment used radar reflections from inner planets, especially Venus and Mercury. When Earth, the Sun and a target planet were favorably aligned, a radar signal could pass close to the Sun, where the predicted delay would be largest. Shapiro estimated about 200 microseconds for a particularly favorable Earth–Venus round trip.

The first successful tests used MIT’s Haystack radar antenna in 1966 and 1967. Shapiro later held major academic and research roles, including leadership of the Center for Astrophysics | Harvard & Smithsonian from 1982 to 2004. A common mix-up is crediting Einstein personally: Einstein’s general theory predicts the underlying phenomenon, but Shapiro proposed this specific 1964 observational test and the effect bears his name.