‎"POUND, R. V. & REBKA JR. G. A.‎
‎Gravitational Red-Shift in Nuclear Resonance. - [THE POUND-REBKA EXPERIMENT - THE TEST THAT PROVED TIME IS RELATIVE]‎

‎(New York), American physical Society, 1959. Lex8vo. Volume 3, No. 9, November 1, 1959 of ""Physical Review Letters"", In the original printed blue wrappers. A very nice and clean copy externally as well as internally, near mint. Pp. 439-441. [Entire issue: 411-457].‎

Reference : 43439

‎First publication of the Pound-Rebka experiment which is regarded as being the last of the classical test of general relativity proposed by Albert Einstein to be verified. It is a test of the general relativity prediction that clocks should run at different rates at different places in a gravitational field and is considered to be the experiment that ushered in an era of precision test of general relativity. Today the so called gravitational redshift is essential for understanding the cosmos and operating the Global Positioning System (GPS).""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 upwards from Earth's surface, for example, shifts to longer wavelength and lower frequency, as gravity saps it of some energy. But the effect is tiny in earth's modest gravity. In 1959 Robert Pound and Glen Rebka of Harvard University finally succeed in testing this crucial prediction, and they reported their results in [The present paper]."" (Physical Review Focus, 12 July 2005). Although the Global Positioning System (GPS) is not designed as a test of physics, it must account for the gravitational redshift in its timing system. When the first satellite was launched, some engineers neglected to predict that a noticeable gravitational time discrepancy would occur. So the first satellite was launched without the clock adjustment that was later built into satellites. It had a predicted time difference of 38 microseconds per day which, if not accounted for, could lead to hours of discrepancy. ‎