Learn how gravity changes the color of light, in this episode of What the Physics?! from NOVA. Einstein predicted that light moving in a gravitational field would change color as it travels—it would undergo gravitational redshift or blueshift. In 1959, researchers Robert Pound and Glen A. Rebka Jr. tested Einstein’s hypothesis with thin sheets of iron-57, which only emits and absorbs a particular frequency of light. They punched holes through all the floors of a building to create a five-story path for light to travel downward from the attic to the sub-basement. Using a speaker to create a Doppler redshift to counteract the gravitational blueshift, they provided evidence that gravity does change the color of light. This resource is part of the NOVA: What the Physics?! Collection.
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