Mirages in space, just as Einstein said

One of the intriguing predictions of Einstein's general theory of relativity is that the gravitational field of a large mass can act like a giant lens system. It should produce multiple images of objects seen through it. After decades of speculation and searching, astronomers believe they have found such a cosmic mirage.

A little over a year ago, D. Walsh of the University of Manchester and colleagues reported what they thought were double images of a quasar knwon as 0957 plus 561.

A quasar (quasistellar) is a compact, very energetic object. Because of a number of uncertainties, scientists were unable to decide whether they were in fact seeing images of a single object or two distinct quasars. By mid-June, however, enough evidence had accumulated for a review in the journal Nature to conclude that the gravitational lens effect "is no longer in serious doubt." What is more, there may be three images formed, not just two.

Two weeks later, again in Nature, a research team led by Ray Weymann of the University of Arizona has reported finding a second case of the lens effect. This time, there definitely appear to be three images of a quasar cataloged as PG1115 plus 08.

Thus, some six decades after scientists found that gravity can indeed bend light as Einstein predicted, the corollary of that proposition -- that gravitational fields can act as lenses -- has also been verified.

What caught the astronomers' attention in each case is the fact that the multiple "objects" observed have identical spectra. This implies that astronomers are seeing images of a single object rather than two or three different objects. But identity of spectra is not enough to prove the lens effect. The images are not identical in all respects. Only if the differences were the kinds of distortions expected with a gravity lens could the issue be decided.

In particular, gravity lens theory predicts that the ratio of the energy radiating from two images should be constant at all wavelenghts. In the case of 0957 plus 561, there was some doubt about this. Even Walsh had wondered if there were really a gravitational lens involved. Meanwhile, a galaxy massive enough to do the job was found in a favorable location with respect to the images. Then, in June, P. M. Gondhalekar of Britain's Rutherford and Appleton Laboratory and R. Wilson of University of College London reported observations that show the images probably do follow the constant ratio "law." They call this "strong evidence in favor of that [gravity lens] hypothesis."

The studies of Weymann and his associates are not this advanced. However, with one gravity lens case already well established, they feel they are have some grounds for claiming a second one on the basis of identical spectra alone.

When observers saw the sun deflect light during the eclipse of May 29, 1919, the news of Einstein's successful prediction was heralded by the classic headline: "Light Caught Bending." Sixty-one years later, we can refine that and say: "Light Caught Focusing."