November 27, 2004
NASA Test Confirms an Einstein Prediction
A key prediction of Albert Einstein's general theory of relativity has been confirmed by an experiment showing that the Earth's rotation drags the surrounding fabric of space-time along with it.
The phenomenon, known as frame-dragging, was one of the last untested predictions of general relativity. A NASA satellite, Gravity Probe B, was launched this year to test the same effect.
"Frame-dragging is like what happens if a bowling ball spins in a thick fluid, such as molasses," said Errico C. Pavlis, of the Joint Center for Earth System Technology, a leader of the international team of researchers that measured the effect. "As the ball spins, it pulls the molasses around itself. ... Similarly, as the Earth rotates, it pulls space-time in its vicinity around itself."
Einstein's insight held that gravity was a product of how planets and stars warp space around them. It was predicted that spinning bodies in the same way would create drag around them. Measuring Earth's drag was difficult because it required separating the effect from the much larger effects of Earth's gravity.
Charting two National Aeronautics and Space Administration satellites, researchers concluded that frame-dragging altered their orbits by two meters per year.
"These guys have doggedly done a very good experiment," said Michael Salamon, a NASA physics expert who praised the team.