Einstein's general relativity: it's a drag.

Let's do the twist! According to Einstein, everything does.

Astronomers announced last week that they may have found the first evidence that spinning bodies drag or twist space and time along with them. An inescapable consequence of Einstein's Theory of General Relativity, this seemingly bizarre concept was described nearly 80 years ago, but until now, researchers lacked the equipment to test it.

Two teams of astronomers base their conclusion on periodic variations in the intensity of X rays emitted by the gas that circles neutron stars and suspected black holes in our galaxy. Gas pulled toward these superdense objects forms a disk around them, and as material in the disk gets compressed and heated, it emits X rays.

If the gaseous disk happens to orbit at an angle to the plane in which the neutron star or black hole is spinning, the dragging of space-time predicted by relativity theory will have a specific effect: It will cause the disk to wobble like a top.

The wobble, in turn, will induce periodic oscillations in the intensity of the radiation emitted by the gas. Such oscillations, occurring as rapidly as 300 times per second, are precisely what astronomers saw when they analyzed X-ray data recorded by the Rossi X Ray Timing Explorer. Launched 2 years ago, the craft was designed to measure rapid variations in high-energy radiation.

Wei Cui of the Massachusetts Institute of Technology and his colleagues examined X rays from the vicinity of five candidate black holes. In accordance with relativity, they found faster oscillations--evidence of greater dragging--in X rays emitted by the gas surrounding the more rapidly spinning holes.

Another team, led by Luigi Stella of the Astronomical Observatory of Rome and Mario Vietri of the University of Rome, found telltale oscillations in the intensity of X rays from several rapidly spinning neutron stars. The teams reported their findings at a meeting of the American Astronomical Society's High Energy Astrophysics Division in Estes Park, Colo.

Detection of the oscillations, notes theorist Cole Miller of the University of Chicago, only hints that scientists have glimpsed the effect of relativistic drag. He says that even if gas in the disk initially orbited at an angle, several forces are likely to compel the gas to align itself with the plane in which a black hole or neutron star spins. In such a case, the disk would not wobble, and the variations in X-ray emissions could not arise from drag.

"Right now, what we have is a suggestion, which is very exciting precisely because it is possibly a signature of a dramatic strong gravity-general relativity effect that we otherwise have no evidence for," says Miller. "It would be great if it were true, but we have to be cautious because the physical mechanisms by which this would be produced are not evident at the moment."

Miller and other researchers note that the drag effect is a direct outcome of relativity theory. Einstein replaced the traditional concept of gravity as a mysterious force that acts at a distance with the notion that matter and space profoundly influence each other. The geometry of space acts on mass and energy, telling them how to move. Similarly, mass and energy act on space, telling it how to curve.

If space is likened to a rubber sheet, then concentrations of mass create dimples in it. As a result, particles traveling along the sheet follow curved paths rather than straight lines. If the mass rotates, it will impart a twist to the surrounding space.

Any spinning body, regardless of its mass, should produce this effect. In 2 years, NASA plans to launch Gravity Probe-B, a suite of four gyroscopes that will attempt to measure the drag generated by Earth (SN: 6/10/95, p. 367).