Editing Gravity Probe A (section)

==Results==
The goal of the experiment was to measure the rate at which time passes in a higher gravitational potential, so to test this the maser in the probe was compared to a similar maser that remained on Earth.<ref name="Vessot_1980" group="p">{{cite journal |last1=Vessot |first1=R.F.C. |title=Test of Relativistic Gravitation with a Space-Borne Hydrogen Maser |journal=[[Physical Review Letters]] |date=1980 |volume=45 |issue=26 |pages=2081–2084 |doi=10.1103/PhysRevLett.45.2081 |bibcode=1980PhRvL..45.2081V}}</ref> Before the two clock rates could be compared, the Doppler shift was subtracted from the clock rate measured by the maser that was sent into space, to correct for the relative motion between the observers on Earth and the motion of the probe. The two clock rates were then compared and further compared against the theoretical predictions of how the two clock rates should differ. The stability of the maser permitted measurement of changes in the rate of the maser of 1 part in {{10^|14}} for a 100-second measurement.

The experiment was thus able to test the [[equivalence principle]]. Gravity Probe A confirmed the prediction that deeper in the gravity well, the flow of time is slower,<ref>{{cite web|url=http://news.nationalgeographic.com/news/2011/05/110505-einstein-theories-confirmed-gravity-probe-nasa-space-science/|archive-url=https://web.archive.org/web/20110507220326/http://news.nationalgeographic.com/news/2011/05/110505-einstein-theories-confirmed-gravity-probe-nasa-space-science/|url-status=dead|archive-date=May 7, 2011|title=Einstein Theories Confirmed by NASA Gravity Probe|last=Than|first=Ker|publisher=[[National Geographic Society]]|date=May 5, 2011|access-date=May 5, 2013}}</ref> and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.