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How did physicists measure the gravitational redshift of light?

JERRY T. SEARCY, SCOTTSDALE, ARIZONA
It's one thing for a theorist like Einstein to make a prediction, but quite another for someone else to do an experiment that tests it. Nowhere is this more true than in the case of gravitational redshift. If you shoot a light beam straight up from Earth's surface to the top of a 100-foot (30.5 meter) tower, relativity predicts the light frequencies measured at the top and bottom of the tower will differ. Specifically, light at the top of the tower will be shifted into slightly longer, redder wa...

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