DEFLECTION OF STARLIGHT:
A total eclipse of the sun provides a chance to test relativistic against classical physics. Newtonian physics says that a stream of photons from a star grazing the limb of the sun will be deflected inward 0.875 arc second. General relativity theory predicts that since space-time in the sun's vicinity is warped, a ray of light traveling along the shortest path in this curved space is deflected by 1.75 arc seconds. This is exactly twice the amount predicted by Newtonian theory.
How can we test these predictions? During a total solar eclipse bright stars can be seen on a photograph of the darkened sky around the eclipsed sun. Another photograph of the same area can be made at night (a few months earlier or later) with the same telescope when the sun is in a different place in the sky. Star posi-
tions on the two photographs are then compared to show the stars around the eclipsed sun shifted away from the sun. The amount of deflection decreases with distance from the sun's limb. This test favors the answer given by general relativity theory although the difficulties of measuring keep this from being a definitive test. This observational test has become almost a standard procedure at every total eclipse of the sun.
Bending of radio waves near the sun has also been checked in recent years by radio interferometry. The apparent position of a pointlike radio source about to be occulted or
grazed by the sun can be found relative to nearby sources. Bending of the radio waves passing through the solar corona (caused by refraction by the corona) must be separated from that caused by the curvature of space-time in the sun's vicinity. The measured gravitational deflection agrees within 1.5 percent of the values predicted by Einstein's general theory.
when Mars was near conjunction with the sun from ate November to mid-December, 1976, radio signals from the Viking orbiters grazing the sun on their way to the earth were bent an slightly delayed due to the wraping of space around the sun. Mars was then about 321 million kilometers from the earth, and radio signals took about 42 minutes for the round trip. The difference in the travel time of the signals when Mars was near the sun compared with that when Mars was well separated from the sun weeks later, 0.0002 second, was in exact agreement with that predicted by the general theory of relativity.
