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As the world turns, time flies.

As the world turns, time flies

One of the more interesting consequences of the theory of relativity is that moving clocks record a different time than stationary ones. It makes for a fun exercise for the imagination, but presents a problem when it comes to synchronizing clocks around the world to subnanosecond accuracy. This is the subject of a report in the April 5 SCIENCE by David W. Allan and Marc A. Weiss, both of the National Bureau of Standards in Boulder, Colo., and Neil Ashby of the University of Colorado in Boulder.

The synchronization problem is partly a result of the Sagnac effect, a phenomenon that causes clocks in a rotating system (in this case, the earth) to be out of sync with one another when viewed from a stationary frame. Theory predicts, and experimental data show, that when an eastbound clock completes a circle around the earth it will lag behind an earth-based clock, whereas one traveling west will lead that clock. In an experiment performed in 1971, researchers J.C. Hafele and R.E. Keating verified the theory by transporting atomic clocks by commercial jet around the globe in each direction.

In the recent experiment, rather than physically moving clocks, the Boulder researchers took earth-based clocks located in Boulder, Tokyo and Braunschweig, West Germany, to be the rotating frame of reference. Pairs of these earth stations simultaneously viewed electromagnetic signals from Global Positioning Satellites. Depending on the sequence in which they were observed, the signals in effect circumnavigated the globe in either an eastward or westward direction, and the time differences were recorded.

The results not only confirmed the theoretical prediction but, Allan asserts, had a higher degree of accuracy than any such experiment to date. The researchers conclude that "this measurement technique allows one . . . access to the most accurate clocks in the world at any other site without being limited by measurement uncertainties.'

Photo: This diagram shows the route of the three Global Positioning Satellites' signals to the earth timing stations: National Bureau of Standards (NBS) in Boulder; Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig; and Tokyo Astronomical Observatory (TAO).
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Title Annotation:Global Positioning Satellites send signals to earth timing stations
Author:Welch, Susan
Publication:Science News
Date:Apr 13, 1985
Words:352
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