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General Relativity makes the surprising prediction that clocks are affected by the gravitational field that they experience. For example, an atomic clock at the top of the CN Tower would not keep pace with one at the base (which would run faster?).

One technological advance in which this phenomenon is important is the design of global positioning systems, which depend for their accuracy on very precise time keeping. If the theory of General Relativity was not incorporated into their design, these systems would not function.

More information on Global Positiong Systems (GPS).

GPS

GPS satellite in orbit.