jan 1, 1964 - First Observation of Gravitational Time Delay

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According to Irwin Shapiro, an American astrophysicist, if the general relativity is valid, then the radio waves will slow down because of sun's gravity as they bounce around the solar system. This effect was observed between 1966 and 1967 by bouncing radar beams off the surface of Venus and measuring the time taken for the signals to return to Earth. The measured delay of time agreed with Einstein's theory.

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