The 3.8-cm-wide metal balls, or rotors, inside Gravity Probe B鈥檚 gyroscopes exhibit electrostatic effects called 鈥減atches鈥� that change the orientation of the gyroscopes, mimicking relativistic effects. The team hopes to understand these effects well enough to improve the precision of their measurements by a factor of 20 by December
A NASA mission that took 40 years to get off the drawing board has finally measured how the Earth dents the fabric of space-time.
The first result from the Gravity Probe B satellite confirms a prediction of Einstein鈥檚 general theory of relativity to a precision of better than 1%. 鈥淔or the first time, we have seen one of Einstein鈥檚 effects directly,鈥� says mission leader Francis Everitt of Stanford University in California, US.
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Launched in April 2004, the satellite used four precision-engineered gyroscopes to measure two effects predicted by Einstein鈥檚 theory of general relativity.
One, called the geodetic effect, predicts that the Earth鈥檚 mass leaves a dent in space-time that should tilt each gyroscope by 0.0018掳, or 6606 milliarcseconds, over the course of a year. A second, more subtle effect, called frame dragging, predicts how much the Earth鈥檚 rotation drags space-time around with it.
Previously, astronomers have measured both effects by firing laser beams at mirrors left on the Moon by the Apollo astronauts. 鈥淭he Moon鈥檚 orbit acts as a gyroscope,鈥� says Clifford Will, an expert on general relativity at Washington University in St Louis, Missouri, US. 鈥淏ut lunar ranging provides indirect measurements. Gravity Probe B provides a direct measure that鈥檚 unique and new.鈥�
Now, the spacecraft team has reported a measurement of the geodetic effect that falls within the value predicted by Einstein but is still not as precise as the lunar laser experiments.
鈥楪limpses鈥� of frame dragging
鈥淭he results aren鈥檛 quite what we鈥檇 hoped for at this stage,鈥� admits Bill Bencze who is programme manager for the mission. That is due in part to a series of solar flares in March 2005 that interrupted the satellite鈥檚 observations and will limit the final accuracy of the experiment.
However, the biggest challenge for the mission team is to correct for some unexpected torques on the gyroscopes that change their orientation and can mimic relativistic effects. Bencze is confident that by December the team will understand these effects well enough to improve the precision of their measurement by a factor of 20, eventually achieving an accuracy of better than 0.01%.
So far, Everitt says the team has seen 鈥済limpses鈥� of frame dragging but is not yet in a position to report a figure for the effect. They hope to do that in December as well. 鈥淔rame dragging is the name of the game,鈥� says Will. 鈥淨uestion is: what will GP-B achieve at the end of the day?鈥�
The predictions of general relativity fall well within the probe鈥檚 precision, though that might change when the team announces far more accurate results. 鈥淚t鈥檚 not a done deal that general relativity is safe,鈥� says Bencze, who points out that the team鈥檚 job is to make the best measurements, rather than confirm general relativity. 鈥淚f Einstein is right, good for him,鈥� says Bencze. 鈥淚f not, too bad.鈥�
The results were presented over the weekend at a meeting of the American Physical Society in Jacksonville, Florida, US.
