Does light gain energy when it falls under gravity?

Professor Ramanath Cowsik, Department of Physics, Washington University
November 11, 2017 at 10:00 am
300 Lab Sciences
Event Description 

When we drop a stone from a height it falls energetically onto the earth making noise and raising dust. The gain in energy is in proportion to the height from which it is dropped - greater the height from which it is dropped greater is the gain in its energy. What happens if we shine some light onto the earth from a high location; would the light gain energy? The answer to this question is ‘yes’ according to General Relativity developed by Einstein in 1915. In Physics, experiments stimulate theory, and theories suggest experiments that test them. The theory is accepted only when it is shown to be correct by the observations. In the year 1959, two scientists from Harvard, Pound and Rebka, conceived of a way to test this prediction of Einstein, which required a measurement with an accuracy of 0.000,000,000,000,004. By 1960 they completed the experiment to show that Einstein’s predictions were correct. This lecture is mainly devoted to a detailed recounting of their ingenious efforts that led to achieving such extraordinary sensitivity. During the intervening six decades the precision of the tests have increased 100 fold. We conclude with a brief description of the current theoretical perspectives and status of such experiments.