Space: Einstein passes black hole test

Albert Einstein's general theory of relativity is 81 years old - but astronomers are still finding experimental ways to test it. The latest, from observations of five spinning black holes, demonstrates that one of the major pieces of scientific thinking of this century is correct. Charles Arthur, Science Editor, explains.

Observations have confirmed an essential tenet of Albert Einstein's general theory of relativity - that huge, spinning objects "drag" space and time around them, distorting the fabric of the universe.

A team at the Massachusetts Institute of Technology (MIT) announced yesterday that it has found evidence of "frame dragging", as the phenomenon is called, around five spinning black holes in our galaxy.

Einstein's theory suggests that any object distorts space and time around it, rather like a ball bearing placed on a stretched rubber sheet. Central to this theory is the idea that any observer moving at a constant velocity will have a self-consistent "frame of reference". So someone travelling almost at the speed of light will find that his watch runs normally and light travels at the normal 186,000 miles per second. But someone at rest compared to the traveller will observe that the traveller's watch seems to run more slowly, and that the traveller (and watch) are far more massive than those at rest. But light still travels at 186,000 miles per second.

These frames of reference can be distorted; near very massive objects, time and space do not behave as predictably they do in deep, empty space. A spinning object will drag space and time around with it, like a tornado, and this would affect those frames of reference. Thus the phenomenon is called "frame dragging".

The information is not directly useful, although some theoreticians have suggested that spinning black holes might open the way to some form of time travel.

Though the phenomenon was first predicted in 1918, it has taken nearly 70 years to be confirmed. First scientists had to demonstrate how objects massive enough to cause such a change could exist in our galaxy, then find them, and finally find evidence that backs up the theoretical prediction.

The existence of massive black holes - objects that exert such a strong gravitational pull that not even light can escape them - has been shown by a number of astronomical observations, though nobody has ever seen one directly.

The MIT team used observations from five black holes which pull in material from a neighbouring star. The material forms an "accretion disc" around the black hole, then as it falls into it, heats up and gives off X-rays.

It was these X-rays that the team measured to determine the existence of frame-dragging. Dr Wei Cui, head of the team, told the American Astronomical Society at a meeting in Colorado yesterday that predictions showed that the matter in the accretion disc should wobble - "much as a child's top wobbles when it slows down".

Dr Cui then deduced that this wobbling is evidence of frame-dragging: "the matter's orbit can only wobble if the space and time in which it exists are being dragged," he said.