If space is distorted by the presence of massive objects, then light rays themselves must obey deviating here and there in response to invisible gravity wells. An immediate testing ground for this is astronomy, where a combination of great distances, large masses and luminous point sources provide a natural laboratory. This is what Einstein suggested, pointing that the apparent position of a given star should be shifted as seen from Earth, whenever a massive object like the Sun happens to be close to the line of sight. To test the hypothesis, one needs the kind of event provided by a total solar eclipse, so the apparent position of the star can be registered without the usual overwhelming glare, and then compared with the position observed at other times of the year when the Sun is not nearby.
This is exactly what Arthur Eddington did with the Hyades starfield in the constellation Taurus for the May 29, 1919 eclipse photographed from the island of Principe, off Africa's west coast. The apparent shift in position of about 1.61 arcseconds matched Einstein's prediction well within the photographic plate measurement uncertainties, providing the first conclusive experimental test of General Relativity and as such it established the physic's reputation forever.
Forward in time, the same kind of experiment was recently performed this time with radio waves - that are photons also but of a much lower frequency - when the Cassini spacecraft trajectory grazed the Sun in December / 2002. This time armed with state of the art instrumentation and computer resources, measurement of frequency shifts of radio waves broadcast by the probe matched to several decimal places predictions derived from General Relativity taking into account the Sun's gravitational influence.
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