Picture book launch on April 20, 2004 from Vandenberg Air Force Base (Source: Stanford University)
A spoon spooned in a jar of honey pulls the sticky honey near it. Similarly, a rotating mass should take space-time a little with it. This is what the two Austrian physicists Joseph Lense and Hans Thirring concluded in 1918 from Einstein's General Theory of Relativity. This prediction of the theory of relativity is one of the last, which has not yet been confirmed beyond doubt. This will now be done by the satellite Gravity Probe B (GP-B), which NASA put into orbit last week. Although there have been promising indications for the accuracy of this prediction for some years (see bulletins), these are either indirect inferences or not? in the case of a satellite measurement about seven years ago? for measurements on the edge of the measuring accuracy. The accuracy of GP-B measurements, on the other hand, will be beyond doubt? assuming that the mission continues to be as successful as the picture book launch on April 20 would hope.

But how do you measure the twist of spacetime? At first glance, the answer is astonishingly simple: with gyros. The axis of rotation of a gyro, on which no external forces act, retains its orientation in space. According to this principle, gyroscopes have been working for a long time? for example in aviation? to be used for navigation. But if the room itself rotates, then the axis of rotation of the gyro must move accordingly.

The problem is the required precision. In addition to several gyroscopes, there is a telescope aboard GP-B. At the beginning of the one-year measurement phase, which will begin after checking all instruments in about three months, the telescope and the gyroscopes will be aligned exactly. The telescope will target the star IM Pegasus about 300 light-years away, while the task of the gyroscopes is to maintain their orientation with respect to the local space.

The General Theory of Relativity now predicts two effects that will ensure that the telescope and the gyroscopes will no longer point in the same direction over time. The one is a purely geometric effect: the earth bends because of their mass, the space in their environment. Because the gyroscopes maintain their orientation in the orbit of the earth with respect to the local space, their orientation at the end of the one-year measurement phase depends on the path they have traveled that year. display

A simple example is intended to illustrate this effect: We assume that a man carries an arrow that always points to the front. In addition, the man, no matter where he moves, should never turn. That is, if he wants to go to the right or left, then he has to go sideways. If he wants to go back then he has to go backwards. The effect of this rule is: No matter where the man goes and what path he takes, his arrow will always point in the same direction.

Now suppose that the man does not move on a flat plane, but on the curved surface of the earth. His starting point is the North Pole. It goes along the zeroth longitude to the equator, the arrow always points to the front, to the south. Arrived at the equator, he goes sideways to the ninetieth degree of longitude and then backwards to the North Pole. The result: The direction of the arrow has turned ninety degrees with respect to the initial direction.

At GP-B will this effect? transferred to the three-dimensional curved space? after one year make an angle of about two thousandths of a degree, which arises between the gyroscopes and the telescope. The actual goal of the mission, however, is to measure the much smaller Lense-Thirring effect that results from twisting the space through the Earth's rotation. The theoretical prediction for this angle is about twelve millionths of a degree. The accuracy of the instruments on GP-B surpasses this tiny size another hundred times, so that you will be able to prove the Lense-Thirring effect with a maximum error of one percent? or refute what at the same time would bring the General Theory of Relativity in dire trouble.

Axel Tilleman

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