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Dragging frames of relativity.

Einstein's general theory of relativity makes an essential connection among the force of gravity, the curvature of space and accelerated motions. One of the consequences of this connection is that frames of reference anchored on an object in accereated motion will appear to "drag," to lag or precess with respect to a frame fixed on bodies that are not accelerated. And this drag seriously complicates the description that a person fixed in one of these frames would make of a motion observed in the other.

The rotation of the earth is a constantly accelerated motion--physicists define; acceleration" as any change from motion in a straight line at uniform speed--and so a frame fixed on the rotation earth should drag compared with one fixed on distant stars. Detection of this drag and measurement of the amount would be a good test of the theory's accuracy.

Scientists had throught they needed to go to a fairly distant orbit to get a smooth enough motion and preserve the gyroscopes that are part of the experiment from spurious torques due to random glitches in the earth's surface motion. The advent of random glitches in the earth's surface motion. The advent of laser gyroscopes makes this no longer necessary, says Andrew Buffington of the University of California of San Diego. The experiment can now be done on the surface of the earth.

A laser gyroscope is a triangular arrangement of mirrors. Two laser beams--one clockwise, the other counterclockwise--are continually reflected around the triangle. After they make the circuit, the two beams are brough together, and the phase difference between them is measured by allowing them to interfere with each other. Any disturbance to the geometry of the triangle will cause the phase relation to change. Such a gyroscope, Buffington avers, is accurate enough not to be fooled by spurious torques.

Buffington would use the gyroscope to establish a direction in space, a line fixed to the rotating earth. A reference frame fixed on distant stars would be simultaneously determined by noting their passage across the meridian. Buffington intends to make the meridian measurements with a Ronchi telescope, to telescope with a grating in the eyepiece to ensure very precise determination of the instant of a given star's meridian passage. Optically or mechanically connecting the telescope and the gyroscope will allow a continuing comparison between the frames established by the two instruments, and so a calculation of any drift between them. Buffington is now looking for means to set up his Ronchi telescope.
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Title Annotation:measurement of drag due to earth's rotation
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Jan 25, 1986
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