The most accurate physical measurements are those of time and frequency made with atomic clocks. As a result such clocks are effectively used in exploring the universe, in testing fundamental physical laws and in other applications.

Although radioastronomy originally had much worse angular resolution than optical astronomy, with the use of atomic clocks arrays of distant radiotelescopes can be synthesized together in Very Long Baseline Interferometry (VLBI) to give angular resolutions of 250 micro arc sec which is 250 times better than the best optical telescopes, including the improved Hubble telescope. The VLBI also provides the most accurate measurements of distances along the surface of the earth and thereby aids in the study of the earth’s crustal dynamics. Atomic clocks are needed to measure the stability of pulsars, of the earths rotation and of other periodic phenomena. The constancy of the fundamental constants can be tested by comparing different kinds of clocks at different times. The Global Positioning System (GPS) and Differential GPS (DGPS) use atomic clocks to locate positions to within a meter and these systems are extensively used in navigation, geographical exploration, geology, archeology, paleontology, environmental studies, etc. Navigation in outer space also utilizes accurate atomic clocks. The unit of length is now defined in terms of time and the representation of the volt is in terms of frequency. Theories of relativity are tested by measurements on clocks in high altitude rockets, by the time delays of signals passing close to the sun and by measurements of small changes in the orbital periods of binary pulsars. The rates of change of the periods of the binary pulsars agree the Einstein form of general relativity to one half a percent, but do so only when one includes the radiation of gravity waves and strong field aspects of relativistic gravity.

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