Shapiro delay

Also: gravitational time delay. In general relativity, not only are light rays deflected, in addition gravity can lead to light taking more time in its travels through space than in classical physics. This is called Shapiro effect of Shapiro delay. It has been measured numerous times for light signals in the solar system, for instance for radar waves sent from Earth to Venus and reflected back. These radar signals took measurably longer when their path led them closely by the massive sun.

Measuring this time delay is sometimes referred to as the "fourth test of general relativity", in addition to the three classical tests of that theory.