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Hafele–Keating experiment
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==Historical and scientific background== In his original 1905 paper on [[special relativity]],<ref>{{cite journal |first=A. |last=Einstein |author-link=Albert Einstein |title=On the electrodynamics of moving bodies |journal=[[Annalen der Physik]] |volume=17 |issue=10 |page=891 |translator1-first=W. |translator1-last=Perrett |translator2-first=G.B. |translator2-last=Jeffery |date=1923 |publication-date=1905 |url=https://www.fourmilab.ch/etexts/einstein/specrel/www/|bibcode=1905AnP...322..891E |doi=10.1002/andp.19053221004 |doi-access=free }}</ref> [[Albert Einstein]] suggested a possible test of the theory: "Thence we conclude that a spring-clock at the equator must go more slowly, by a very small amount, than a precisely similar clock situated at one of the poles under otherwise identical conditions." Actually, it is now known that all clocks located at sea level on the Earth's surface tick at the same rate, regardless of latitude, because kinematic and gravitational time dilation effects cancel out (assuming that Earth's surface is an [[equipotential]] one).<ref>{{Cite web|url=https://physics.stackexchange.com/q/126919 |title=Does time move slower at the equator?|website=physics.stackexchange.com|access-date=2018-03-04}}</ref><ref>{{Cite journal|last=Drake|first=S.P.|date=January 2006|title=The equivalence principle as a stepping stone from special to general relativity|url=http://newt.phys.unsw.edu.au/einsteinlight/jw/2006AJP.pdf|journal=[[American Journal of Physics]] |volume=74|issue=1|pages=22–25|arxiv=gr-qc/0501034|bibcode=2006AmJPh..74...22D|doi=10.1119/1.2135316|s2cid=119539826}}</ref> The kinematic effect was verified in the 1938 [[Ives–Stilwell experiment]] and in the 1940 [[experimental testing of time dilation|Rossi–Hall experiment]]. General relativity's prediction of the gravitational effect was confirmed in 1959 by [[Pound–Rebka experiment|Pound and Rebka]]. These experiments, however, used subatomic particles, and were therefore less direct than the type of measurement with actual clocks as originally envisioned by Einstein.{{citation needed|date=June 2017}} Hafele, an assistant professor of physics at [[Washington University in St. Louis]], was preparing notes for a physics lecture when he did a back-of-the-envelope calculation showing that an atomic clock aboard a commercial airliner should have sufficient precision to detect the predicted relativistic effects.<ref name="New Scientist 1972">{{cite journal |last=Wick |first=Gerald |date=February 3, 1972 |title=The clock paradox resolved |url=https://books.google.com/books?id=BAIlXPgcsHwC&pg=PA261 |journal=[[New Scientist]] |pages=261–263}}{{dead link|date=November 2023}}</ref> He spent a year in fruitless attempts to get funding for such an experiment, until he was approached after a talk on the topic by Keating, an astronomer at the [[United States Naval Observatory]] who worked with atomic clocks.<ref name="New Scientist 1972"/> Hafele and Keating obtained $8000 in funding from the [[Office of Naval Research]]<ref>{{cite conference |last=Hafele |first=J. C. |author-link=Joseph C. Hafele |title=Performance and results of portable clocks in aircraft |conference=PTTI, 3rd Annual Meeting |year=1971 |conference-url=http://tycho.usno.navy.mil/ptti/ptti1971.html |url=http://tycho.usno.navy.mil/ptti/1971papers/Vol%2003_17.pdf |access-date=2017-03-31 |archive-url=https://web.archive.org/web/20170126213131/http://tycho.usno.navy.mil/ptti/1971papers/Vol%2003_17.pdf |archive-date=2017-01-26 |url-status=dead }}</ref> for one of the most inexpensive tests ever conducted of general relativity. Of this amount, $7600 was spent on the eight round-the-world plane tickets,<ref>Martin Gardner, Relativity Simply Explained, Dover, 1997, p. 117</ref> including two seats on each flight for "Mr. Clock." They flew eastward around the world, ran the clocks side by side for a week, and then flew westward. The crew of each flight helped by supplying the navigational data needed for the comparison with theory. In addition to the scientific papers published in ''[[Science (journal)|Science]]'',<ref name=paper1/><ref name=paper2/> there were several accounts published in the popular press and other publications.<ref name="New Scientist 1972"/><ref>{{cite news |magazine=[[Time (magazine)|Time]] |date=October 18, 1971 |title=Science: A Question of Time |url=http://www.time.com/time/magazine/article/0,9171,910115,00.html |archive-url=https://web.archive.org/web/20081221210138/http://www.time.com/time/magazine/article/0,9171,910115,00.html |url-status=dead |archive-date=December 21, 2008 |url-access=subscription}}</ref><ref>{{cite news |first=John |last=Pearson |url=https://books.google.com/books?id=iNQDAAAAMBAJ&pg=PA30 |title=Science Worldwide |magazine=[[Popular Mechanics]] |date=January 1972 |page=30}}</ref>
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