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Gravity-B proves Einstein right

To within one per cent, anyway

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The first data from Gravity Probe-B has confirmed that Einstein was a pretty clever chap who knew what he was talking about when it came to space, time, and the universe.

The team working on the mission have demonstrated the so-called geodetic effect, the amount by which the Earth's mass distorts local space time.

So far, the figures are fairly broad brush, but the scientists say by the end of the year they expect to have significantly refined their results.

Einstein described both the geodetic effect and a phenomenon known as frame-dragging, which is the amount by which the rotating Earth drags the local space-time along with it as it spins, in his general theory of relativity.

The theory predicts that over the course of a year, a free-spinning gyroscope in orbit around Earth will change the direction in which it points by 6.606 arc seconds in the plane of the orbit, thanks to the geodetic effect. The frame dragging will shift the gyroscope by an even smaller 0.039 arc seconds in the plane of the Earth's equator. These tiny effects can be measured relative to a fixed point, such as a star.

Gravity Probe-B carried with it four near-perfect gyroscopes, and the super-sensitive kit capable of tracking these minute changes.

Analysis of the data has revealed that the geodetic effect does indeed exist, and has been confirmed to an accuracy of within one per cent. The frame-dragging, however, is a much (170 times) smaller effect, and further analysis is needed to pick the details out of the data.

Professor Francis Everitt, a Stanford University physicist and principal investigator on the mission, said: "It's fascinating to be able to watch the Einstein warping of space-time directly in the tilting of these GP-B gyroscopes - more than a million times better than the best inertial navigation gyroscopes."

Although the final results of the experiment are not expected until the end of this year, the team behind the craft has made this interim announcement based on a year of collected data and eight months of analysis.

GP-B program manager William Bencze said with eight more months of analysis the team should be able to reduce the uncertainty from the 0.1 to 0.05 arc seconds they are currently working with to something better than 0.005 arc seconds per year.

"Understanding the details of this science data is a bit like an archaeological dig," he added. "A scientist starts with a bulldozer, follows with a shovel, and then finally uses dental picks and toothbrushes to clear the dust away from the treasure. We are passing out the toothbrushes now."

Everitt was reluctant to speculate about the final analysis, saying: "Always be suspicious of the news you want to hear." ®

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