Clock in Outer Space

In the gloomy days of January, time seems to drag. According to the Einstein theory of relativity, it actually does--in a minute amount that means little to anyone except a scientist. During its annual elliptical trip through space, the earth reaches its maximum speed each January, when it makes its closest approach to the sun. According to Einstein, the earth's increased velocity, along with its passage through a more intense part of the sun's gravitational field, causes terrestrial time to slow down in relation to time outside the solar system.

A practical proof of January slow time has never been possible. How can the slowing of time be measured if all the available timing devices on earth are similarly affected by relativity? Last summer, when the regularly beeping signals of pulsars were first detected coming from outer space, Queens College Physicist Banesh Hoffmann figured that they might supply an answer. Though their source was unknown, the precisely spaced radio pulses coming from light-years away seemed to be the distant clock needed to measure earth time. In a letter to Nature, Hoffmann suggested that the pulse rate of pulsars be taken regularly from January through June, when the earth is farthest from the sun and slows to its minimum speed. Each time the pulse rate could be compared with an accurate timing device on earth.

Match with Cesium. If earth time does indeed slow down relative to the pulsar clock in January, and speed up correspondingly in June, the pulsar signals (which have blipped at a constant frequency since they were discovered) would appear to increase their repetition rates as earth clocks slowed down and decrease them as earth time speeded up. Hoffmann's plan was immediately snapped up by Dror Sadeh, a Tel Aviv University physicist currently attached to the U.S. Naval Research Laboratory at Washington, D.C.

Using the Navy's 150-ft. radio telescope at Sugar Grove, W. Va., Dr. Sadeh will attempt this month to establish the pulse rate of one or more of the pulsars to an accuracy of one part in 10 billion--the equivalent of a clock that would gain or lose only 1/300th of a second per year. Then, twice a month for the next half a year, he will match the rate of incoming pulses against a cesium clock, an atomic timer that is accurate to one part in 10 trillion.

Sadeh feels sure that by January he should be able to detect an apparent speedup in the pulsar clock when compared with its rate this month--a clear indication that earth time has slowed by the same amount. If Einstein was right, that observed slowdown will total about 1/ 100th of a second per year. "If our measurements are accurate and we don't get this result," says Hoffmann, "then we scientists--and the Einstein theory--are in trouble."

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