Study of Black Holes Backs Einstein’s Theory of Gravity

In a major confirmation of Einstein’s theory of gravity, astrophysicists have seen evidence that space gets dragged around by spinning objects like the train of a wedding dress circling a twirling bride. If correct, the findings--announced Thursday at a meeting in Colorado--pin down one of the final predictions of Einstein’s theory, which forms the bedrock of physicists’ understanding of all large-scale events in the universe.

Other seemingly preposterous predictions of the theory that are now all but proven true include the existence of galaxy-size gravity lenses that bend starlight, and black holes that suck in everything including light. Now, one of the last holdouts--the idea that space itself gets swirled by spinning objects--appears to have been seen by researchers at MIT and in Rome, 80 years after it was proposed.

“It’s taking what was science fiction and bringing it into everyday astronomy,” said Michael Nowak, astrophysicist at the University of Colorado.

According to Einstein’s theory of gravity, what people normally think of as empty space is actually a tightly woven fabric of space and time, interlocked like threads in a cloth. The space-time fabric gets warped by massive objects, just as a sheet would get warped into a deep well if an elephant were to sit on a bed. Anything that comes near the well naturally rolls in, and that “falling” is the force we perceive as gravity.

If the elephant twists around on the bed, his motion carries the sheet with him. And if Einstein’s theory is correct, so should space-time be dragged around massive objects. In the new work, the scientists report they were able to see evidence of this dragging by observing the behavior of cosmic behemoths such as black holes and spinning stars.

Black holes--by definition--cannot be seen directly, as no light escapes their grasp. However, a visible companion orbiting near the black hole slowly gets sucked in by its immense gravity. The dust and gas get ripped off the star and swirl down into the hole, heating up to a billion degrees in the process and sending out an X-ray signal that can be decoded. Using NASA’s Rossi X-Ray Timing explorer satellite, scientists believe they saw convincing evidence for “frame dragging.”

If the evidence for frame dragging bears out, it would give further strength to the belief that Einstein’s theory of gravity is right. “It’s probably the biggest effect of Einstein’s theories that hasn’t been detected,” said astrophysicist Lynn Cominsky of Sonoma State University, press spokeswoman for the meeting.

While Einstein’s theory has passed all experimental tests “with flying colors,” according to University of Illinois astrophysicist Frederick Lamb, “this is the first time we’re entering the regime of very strong gravity.”

Einstein’s space-time warp has left physicists an “outrageous legacy” of bizarre effects to plumb, in the words of Caltech physicist Kip Thorne. Respected physicists such as Thorne and Stephen Hawking have seriously explored the question of whether Einstein’s theory leads to wormholes through the fabric of space-time that could reverse time.

That Einstein correctly predicted so many strange and counterintuitive effects at a time when there was no hard evidence that he was right was a “gigantic intellectual achievement,” Lamb said.

The strongest gravitational fields encircle stars that have used up their nuclear fuel and collapse under their own weight. Some condense into neutron stars, so dense that a teaspoon made of the material would weigh millions of tons. Other, more massive stars get squeezed to such extremes that they bend space-time into a black hole, from which nothing can escape.

The Rossi X-Ray satellite was launched by NASA in 1995 specifically to study such objects. Unlike the Hubble Space Telescope, the Rossi telescope does not take visible images. Instead, it detects energetic X-ray light coming from very close to the center of collapsed stars--within about 500 miles of the center.

Over the past year, researchers discovered that black holes and neutron stars send out very regular X-ray signals, “like pure tones,” said Lamb. The tones are probably caused by dense blobs of gas swirling around the black holes or neutron stars hundreds or thousands of times per second, like a beacon orbiting a lighthouse. (By comparison, the Earth orbits the sun only once a year).

Unexpectedly, these pure signals gave researchers a way to detect subtle wobbles that would indicate that space-time is being dragged. Just like a top wobbles as it gets dragged down by friction, so the orbiting blobs of gas would wobble as they drag space-time along with them. Signals had been detected several years ago, but no one had looked for evidence of frame-dragging in them, partly because no one had thought it was possible to see them, said astrophysicist Luigi Stella of the Astronomical Observatory of Rome.

Stella and his colleagues calculated that if certain neutron stars were dragging space around them, their fluctuating X-ray signal should contain a second, slower signal superimposed on the first.

He saw such a motion in three of 15 stars he analyzed.

“The conclusion is that . . . we are getting the first observational evidence of an effect that has been searched for for 80 years,” Stella said. If it’s true, he said, “it confirms one of the deepest consequences of Einstein’s theory.”

After learning about Stella’s work, MIT astrophysicist Wei Cui applied a similar analysis to X-ray signals from black holes. Like Stella, he concluded that he was probably seeing evidence of frame dragging “because the numbers seem to work out so well.”

However, the evidence is “very indirect,” Wei cautioned. Along with Stella, he said he was looking forward to the results of an experiment to measure the dragging of the space-time frame around the Earth. The device involved--a nearly perfect gyroscope--is due for launch aboard the space shuttle in March 2000.

Because scientists know exactly how much the Earth weighs and how fast it spins, they can more accurately calculate the predicted effects of frame dragging. However, the effects will be millions of times more subtle than those possibly perceived in these experiments, because of the Earth’s relatively small mass.

Known as Gravity Probe B, that satellite “will be a direct and unambiguous measurement,” Wei said.

Many ideas that logically follow from Einstein’s theory of gravity seemed unbelievable when they were proposed, but most have stood the test of experiment. Gravity does bend light like a lens, producing multiple images of distant quasars. Time does slow down near very massive objects. Gravitational energy does appear to radiate away from rapidly spinning pairs of collapsed stars. Black holes are everyday objects of research.

Whether or not the dragging of space-time has actually been detected awaits further experiment, scientists said. However, “while we might argue about the exact mechanism that’s going on, we’re all convinced that this is probing the ideas of Einstein at the extremes,” Nowak said. “That’s very exciting.”