New Pound Data Verifies Einstein

Robert V. Pound, professor of Physics, has proved a fundamental postulate of Einstein's theory of relativity.

He has confirmed the Principle of Equivalence, which says that the observable effects of gravity and of acceleration outside a gravitational field are equivalent, within 0.003 per cent of Einstein's predictions. Pound and his collaborator, Joseph Snider, assistant professor of Physics, announced their results in an article published yesterday in the Physical Review Letters.

In 1960 Pound and Glen K. Rebka, a graduate student, obtained measurements of the effect only five per cent removed from Einstein's calculations, with a statistical uncertainty of ten per cent.

Einstein predicted that, if the Principle of Equivalence held, a single ray of light should change frequency when observed at different points in a gravitational field. In 1963, however, Pound's experimental results hovered consistently around a figure ten per cent lower than Einstein's predictions. The discrepancy was so great that Pound halted his experiments to check for errors in his equipment.

If Pound's data had been correct, they would have contradicted the Principle of Equivalence and hence destroyed the universal applicability of the theory of relativity. But refinement in equipment removed the discrepancy.

Red Shift

Einstein's prediction is also known as the gravitational red shift. This shift (the observed frequency changes in light waves) is similar to the Doppler effect for sound waves: the frequency of vibration of a light source moving toward an observer seems to increase just as the horn of an onrushing train apparently rises in pitch. To test this principle, Pound set up a source of radiation at the top of a 75-foot shaft in the Jefferson Physical Laboratory. At the bottom of the shaft was an absorber.

By measuring the difference between absorption at the top and the bottom of the shaft, Pound found the difference in frequency for a height of around 30 yards is approximately two parts per quadrillion.

Pound was not the only scientist to wonder about the Principle of Equivalence. In November, 1962, Pound received a letter from V. P. Lyadov of Kazan State University in Russia. Lyadov, who predicted a ten per cent deviation from Einstein's Theory, postulated that acceleration and gravitation are two different things. Pound's new results support Einstein.