Irwin I. Shapiro

Irwin Ira Shapiro was born in New York City in 1929. He took a bachelor's degree in mathematics from Cornell in 1950 and a PhD in physics from Harvard in 1955, then joined MIT's Lincoln Laboratory, the radar and electronics research center founded in the wake of World War II. It was an unlikely place to do fundamental gravitational physics — Lincoln Laboratory existed to track objects and study the ionosphere — but the marriage of precise radar timing and a sharp theoretical mind turned out to be exactly what general relativity needed for its first new experimental test in a generation.

In 1964 Shapiro realized that the curved geometry around the Sun, already known to deflect starlight, must also delay a radar pulse that grazes the solar limb: the round-trip travel time measured by a distant observer should be longer than flat-space geometry predicts, by about 200 microseconds for a signal passing near the Sun on its way to Venus or Mercury. He published the prediction as the 'Fourth Test of General Relativity' in Physical Review Letters that December. Between 1966 and 1971 his group used the Haystack and Millstone Hill radars to bounce signals off Mercury and Venus near superior conjunction and traced out the predicted logarithmic delay peak, confirming the effect that now bears his name.

Shapiro spent the rest of his career sharpening that test and inventing new ones. He pioneered very-long-baseline interferometry (VLBI) for high-precision astrometry and geodesy, used spacecraft transponders rather than planetary surfaces as radar targets to push the delay measurement to the 0.1% level with Viking in the 1970s, and helped establish the techniques that would eventually confirm the parametrized-post-Newtonian parameter gamma to one part in 10^5 with the Cassini spacecraft in 2002.

From 1982 to 2004 he directed the Harvard-Smithsonian Center for Astrophysics, one of the largest astronomical research institutions in the world, while remaining a Harvard professor. He is widely regarded as one of the architects of precision experimental gravitation, the discipline that turned general relativity from an elegant theory tested at the few-percent level into one verified to parts per million.