Shapiro Time Delay

The Shapiro time delay effect, also known as the gravitational time delay effect, is one of the four classic Solar System tests of general relativity. It predicts that radar signals passing near a massive object take slightly longer to reach a target and to return than they would if the massive object were not present, an effect caused by the time dilation that increases the time light takes to travel a given distance as seen by an outside observer; it was described in a 1964 article by Irwin Shapiro.