The Shapiro Effect, also known as the Gravitational Time Delay is the slowdown of light in the presence of a gravitational field--an effect predicted by relativity theory.

Dr. Irwin I. Shapiro wrote in the Journal of Astrophysics in 1964: "...according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path." In other words, the theory of relativity predicts that the speed of light is reduced when it passes through a gravitational field.

In his letter, Dr. Shapiro further suggested that a test of relativity theory could be made by observing delay of radar signals returned from the surface of a planet in our solar system. He estimated that the effect of the sun's gravitational field on the radar beam would delay the returning signal. The maximum delay would occur at the beam's closest approach to the sun.

His idea was to bounce radar beams off the surface of Venus and Mercury and measure the total time it would take for the beams to return. Since the relative positions of the planets, the earth and the sun are known quite accurately, the expected travel time of the radar beam could be computed with great accuracy as well.

Relativity theory predicts that the total time for the radar beam to go from the earth to the planets and back, at the closest approach of the radar beam to the sun, would be increased by 200 microseconds compared to what would be expected if the sun were not there. This is an easy time difference to measure.

The first test, using the MIT Haystack radar, was successful, matching the predicted amount of time delay. Shapiro presented his results in 1968. The experiments have been repeated many times since, with increasing accuracy.

External links

Journal of Astrophysics and Astronomy