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The 100th Anniversary of Einstein's Theory of General Relativity: Wyoming’s Connection

December 8, 2015
head portrait of an older man
UW Professor Glen Rebka in 2008 at a major conference named in his honor. Rebka and Robert Pound in 1960 published a paper that proved one of Albert Einstein’s most significant theories. (UW Photo)

As the world commemorates the 100th anniversary of Albert Einstein’s celebrated Theory of General Relativity, the University of Wyoming Department of Physics and Astronomy calls attention to a former professor who is credited with proving one of Einstein’s most significant predictions of that theory.

In 1960, Harvard University’s Glen Rebka and Robert Pound finally verified Einstein’s prediction that gravity could change light’s frequency. The Pound-Rebka effect, Rebka’s Ph.D. thesis, was the first measurement of Einstein’s general relativity prediction of a gravitational redshift. Physicists credit this finding as being essential to modern navigational technology and the global positioning system (GPS).

Rebka, who died earlier this year, was a UW professor from 1970-1997. In 2008, a conference at UW was named in his honor.

Before he worked out the general theory of relativity, Einstein had already deduced that gravity must affect a light wave’s frequency and wavelength. Light moving upward from Earth’s surface, for example, shifts to longer wavelength and lower frequency, as gravity saps some of its energy. But the effect is tiny in Earth’s modest gravity.

As described on the website Physics, Pound and Rebka in 1960 finally succeeded in testing this crucial prediction. Pound, stationed at the top of a tower in a Harvard physics building, communicated by phone with Rebka in the basement during calibrations for their experiment. The team verified Einstein’s prediction of the so-called gravitational redshift, essential for understanding the cosmos and operating GPS.

“Pound and Rebka placed an emitter at the top of a tower in the Jefferson Physical Laboratory at Harvard and installed a detector 74 feet below,” the website describes. “By measuring the detection rate as they jiggled the emitter up and down slightly, the researchers could find the velocity difference between source and detector that compensated for the gravitationally induced change of frequency.

“By reversing their experimental setup to also measure the frequency shift of gamma rays going up the tower, Pound and Rebka could eliminate several sources of experimental error. The difference between the up and down measurements -- a frequency change of only a few parts in a thousand trillion (1015) that represented the pure gravitational effect and matched Einstein’s prediction to 10 percent accuracy. By 1964 they had improved the measurement, reaching an agreement to within 1 percent.”

This was a “major scientific achievement,” wrote Clifford Will of Washington University in St. Louis, not only because it was a classic test of relativity, but because of the ingenious experimental design. And there is a practical consequence, he adds. The satellite-borne clocks of the GPS navigational system must be regularly corrected for changes induced by gravitational redshift. So relativity calculations keep every freighter and fighter jet on course.

At Harvard University, Rebka graduated Phi Beta Kappa in 1953 and completed his Ph.D. in 1961. He then spent 10 years at Yale University before joining the UW faculty in 1970. He was the department chair from 1983-1991, where he helped build the infrared astrophysics program to national prominence.

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