Editing Hafele–Keating experiment (section)

==Repetitions==
A more complex and precise experiment of this kind was performed by a research group at the [[University of Maryland]] between September 1975 and January 1976. Three atomic clocks were brought to an altitude of 10&nbsp;km above [[Chesapeake Bay]] in Maryland, and three other atomic clocks were at the ground. A [[turboprop]] plane was used, flying at only 500&nbsp;km/h, in order to minimize the velocity effect. The plane was steadily observed using radar, and its position and velocity were measured every second. Five flights were carried out, each of 15 hours duration. Special containers protected the clocks from external influences such as vibrations, magnetic fields, or temperature variations. The time difference was measured by direct clock comparison at the ground before and after the flight, as well as during the flight by laser pulses of 0.1&nbsp;ns duration. Those signals were sent to the plane, reflected, and again received at the ground station. The time difference was observable during the flight, before later analysis. An overall difference of 47.1&nbsp;ns was measured, which consisted of the velocity effect of −5.7&nbsp;ns and a gravitational effect of 52.8&nbsp;ns. This agrees with the relativistic predictions to a precision of about 1.6%.<ref>{{Cite journal|author=Alley, C.O.|title=Relativity and Clocks |journal=Proceedings of 33rd Annual Symposium on Frequency Control|year= 1979 |pages=4–39|doi=10.1109/FREQ.1979.200296}}</ref><ref>{{Cite journal|author=Alley, C.O.|title=Introduction to some fundamental concepts of general relativity and to their required use in some modern timekeeping systems|journal=Proceedings of the Precise Time and Time Interval Systems and Applications Meeting|volume=13|year=1981|pages=687–727|url=http://www.pttimeeting.org/archivemeetings/ptti1981.html|url-status=dead|archive-url=https://web.archive.org/web/20120826103801/http://www.pttimeeting.org/archivemeetings/ptti1981.html|archive-date=2012-08-26}}</ref>

A reenactment of the original experiment by the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] took place in 1996 on the 25th anniversary of the original experiment, using more precise atomic clocks during a flight from [[London]] to [[Washington, D.C.]] and back again. The results were verified to a higher degree of accuracy. A time gain of {{val|39|2|u=ns}} was observed, compared to a relativistic prediction of 39.8&nbsp;ns.<ref>NPL Metromnia, [http://resource.npl.co.uk/docs/publications/newsletters/metromnia/issue18_einstein.pdf Issue 18 - Spring 2005]</ref> In June 2010, the National Physical Laboratory again repeated the experiment, this time around the globe (London - [[Los Angeles]] - [[Auckland]] - [[Hong Kong]] - London). The predicted value was {{val|246|3|u=ns}}, the measured value {{val|230|20|u=ns}}.<ref>NPL news, [http://www.npl.co.uk/news/time-flies Time flies, 1 Feb. 2011]</ref>

Because the Hafele–Keating experiment has been reproduced by increasingly accurate methods, there has been a consensus among physicists since at least the 1970s that the relativistic predictions of gravitational and kinematic effects on time have been conclusively verified.<ref name="Wolfgang Rindler 1979, p. 45">Wolfgang Rindler, Essential Relativity: Special, General, and Cosmological, Springer-Verlag, 1979, p. 45</ref> Criticisms of the experiment  did not address the subsequent verification of the result by more accurate methods, and have been shown to be in error.<ref>Roberts and Schleif, [http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Twin_paradox What is the experimental basis of Special Relativity?]</ref>