Shapiro delay

The Shapiro delay (or Shapiro time delay) is a specific kind of gravitational time delay due to EMR slowing while it passes close to a massive object, from general relativity (GR) effects. The effect was confirmed in the 1960s in an experiment bouncing radar off Venus at a time when such a delay was calculated to be measurable due to the mass of the Sun. This was a GR confirmation, and also demonstrated it must be taken into account regarding precise measurements of EMR transients, such as in pulsar timing array (PTA) studies.