What is Shapiro delay?
The '''Shapiro time delay''' is a [[physics]] [[experiment]]. It is one of the four classic [[solar system]] [[observation]]s or experiments which test [[general relativity]]. [[Radar]] [[signal]]s passing near a massive [[object]] take slightly longer to travel to a target and longer to return than it would if the [[mass]] of the object were not present. ==History== The time delay effect was first noticed in 1964, by [[Irwin I. Shapiro]]. Shapiro proposed an observational test of his prediction: bounce [[radar]] beams off the surface of [[Venus]] and [[Mercury (planet)|Mercury]], and measure the round trip travel time. When the [[Earth]], [[Sun]], and Venus are most favorably aligned, Shapiro showed that the expected time delay, due to the presence of the Sun, of a radar signal traveling from the Earth to Venus and back, would be about 200 microseconds,<ref>{{cite journal | author=Irwin I. Shapiro | title=Fourth test of general relativity | url=https://archive.org/details/sim_physical-review-letters_1964-12-28_13_26/page/n8 | journal=Physical Review Letters | year=1964 | volume=13 | issue=26 | pages=789–791 | doi=10.1103/PhysRevLett.13.789| bibcode=1964PhRvL..13..789S }}</ref> well within the limitations of 1960s era [[technology]]. The first tests, performed in 1966 and 1967 using the [[Massachusetts Institute of Technology|MIT]] [[Haystack Observatory|Haystack radar antenna]], were successful, matching the predicted amount of time delay.<ref>{{cite journal | author=Irwin I. Shapiro, Gordon H. Pettengill, Michael E. Ash, Melvin L. Stone, William B. Smith, Richard P. Ingalls, and Richard A. Brockelman | title=Fourth test of general relativity: preliminary results | journal=Physical Review Letters | year=1968 | volume=20 | issue=22 | pages=1265–1269 | doi=10.1103/PhysRevLett.20.1265| bibcode=1968PhRvL..20.1265S }}</ref> The experiments have been repeated many times since then, with increasing accuracy. == References == {{Reflist}} [[Category:Physics experiments]]