An international research team has found the first direct evidence that the Earth is dragging space and time around itself as it rotates, like a spoon spinning in honey. This swirling of space and time around a massive object was first suggested by German physicist Albert Einstein in his General Theory of Relativity.
Einstein stunned the scientific world in 1916 with his novel notions of space and time. One effect he calculated was that massive rotating bodies like planets distort space and time. He thought they pull space and time along with them as they spin. Physicist Michael Salamon of the U.S. space agency NASA elaborates on the analogy of the spoon in a honey jar.
"If you turn the spoon, you'll see that the honey gets twisted around with the spoon," he said. "The further you are away from the spoon, the less twisting there is of the honey. That's a similar process to what happens with space-time itself near the surface of the Earth. So as the Earth turns, it's actually twisting space-time around with it. Near the Earth, the twisting is greater."
Presumably, any spinning object would do the same, even a ballerina or a flywheel, but with a much smaller impact on space and time.
Einstein suggested a way of measuring the effect. He said a spinning flywheel would impart a twist to space and time that would affect a nearby gyroscope. If a frictionless gyro is placed near the flywheel, its axis of spin, the imaginary line around which the gyro revolves, will be dragged in the direction of the flywheel's rotation.
The flywheel's impact would be infinitesimal and hard to measure because it is not a massive object, so scientists at the University of Maryland and the University of Lecce in Italy devised a way to measure the rotating impact of something much bigger, Earth, on two orbiting satellites six-thousand kilometers high. Their results are published in the journal Nature.
To measaure the exact location and distances of the satellites, they arranged for laser beams to be bounced off them about 100 million times over 11 years from 50 locations around Earth. NASA's Michael Salamon says they were able to observe slight shifts in the satellites' orbits.
"This result is the first real, solid direct evidence we have for the twisting of space-time caused by a massive rotating body," said Mr. Salamon.
The satellites orbit Earth so that they cross each other's paths, making the shape of the letter "X." Each orbit has its own plane, the imaginary flat field around which the satellite circles. The two researchers determined the central plane of the combined two-satellite system. The University of Maryland member of the team, Erricos Pavlis, says the laser beam measurements of the satellites' location showed that the swirling space and time around Earth caused a shift in this plane of about two meters per year in the direction of the Earth's rotation. Mr. Pavlis says the measurement agrees 99 percent with what Einstein's theory predicted, with a margin of error of five to 10 percent.
"Now, obviously it would have been much greater news if we had proven Einstein was wrong, but I think we are very satisfied with the present result to a 99 percent correct prediction and confirmation of that prediction," said Mr. Pavlis.
To get this result, the scientists had to subtract the effect of Earth's gravity on the satellites' orbits, which is greater than that caused by twisting space-time and hides it. They could do this with high precision because recent extremely accurate models of Earth's gravitational field created from data sent back by another NASA satellite.
"We believe now that the estimate is a much more correct and robust estimate primarily because of the fact that the gravitational field is much, much more accurate today as it was six years ago," he added.
Physicists hope that this new result will soon be superseded by data from the U.S. Gravity Probe B satellite launched in April. It was designed to measure these tiny space-time disturbances with much greater accuracy. Like Einstein's suggested experiment with the flywheel, Gravity Probe B contains nearly frictionless gyroscopes whose axis of spin is expected to drift because of twisting space-time about one-tenth the width of a human hair over one year.
