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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Could the displacement of star images near the sun be caused by refraction in the atmosphere of the Sun, not by general relativity?

No. Long wavelength electromagnetic radio waves are, in fact, refracted by the plasma in the solar photosphere, chromosphere and corona, but this effect can be accounted for, leaving a frequency-independent bending of the amount predicted by general relativity.

In 1974-75 a series of radio observations were made of the occultation by the sun of the quasars 3C273 and 3C279 by astronomers Fomalont and Sramek. The measurements were made at 2.7 and 8.1 gigacycles. Because refraction from the solar corona varies with the square of the observing frequency as n^2 - 1, where n is the plasma index of refraction, it is possible from a 2-frequency observation to eliminate most of the effects caused by refraction in the solar atmosphere. General relativity predicts that the 'lensing' of light by a gravitational field does NOT depend on the frequency of the light, unlike lensing of light by optical means.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.