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Among its marvelous consequences, general relativity asserts that a stationary clock at Earth’s surface will run slower than one high in a tower where the gravitational potential is weaker; the phenomenon is called gravitational redshift (see the article by Neil Ashby, Physics Today, May 2002, page 41). Now Holger Müller (University of California, Berkeley and Lawrence Berkeley National Laboratory) and colleagues report that the redshift idea, first experimentally confirmed 45 years ago, has passed its strictest test yet.

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