No one can accuse the Cassini spacecraft of getting a free ride. En route to a 2004 rendezvous with Saturn, the craft has already been put to work, verifying a key prediction of Albert Einstein’s general theory of relativity. Cassini met that challenge with findings 50 times as accurate as previous measurements.
A cornerstone of Einstein’s theory is the mind-bending concept that gravity is equivalent to the curvature of space-time. Without gravity, space-time is like a flat rubber sheet, and objects travel in a straight line. But put a heavy object onto the sheet and it sags, causing bodies moving nearby to take curved pathways. According to Einstein, even a light beam passing near a heavy object such as the sun will take a slightly longer path than if the massive body wasn’t there. That alteration also shifts the radiation to a lower frequency.
Using Cassini’s communication antenna, Bruno Bertotti of the University of Pavia in Italy and his colleagues measured the frequency shift of radio waves beamed between Earth and the craft when the two were on opposite sides of the sun and the radio waves traveled close to the sun.
Most of the frequency shift that the scientists measured came from radio noise from the sun’s corona and the relative motion of the antennas on Cassini and Earth. After accounting for these contributions, the researchers found a residual frequency shift that agrees to within 20 parts in 1 million with Einstein’s predictions, they report in the Sept. 25 Nature.
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