Science: Through a Lens Darkly

It was a coal-black night in March, the kind astronomers like best. At Arizona's Kitt Peak National Observatory, Princeton Astrophysicist Edwin Turner pointed the 158-in. reflecting telescope first at one distant pinpoint of light in the sky, then at a neighboring one. A few hours later, studying the results of his night's labors, Turner could hardly believe his eyes. "It was a big surprise," he says. "But a big surprise is always a clue you might be on the track of something."

Something indeed. After analyzing the light from the distant sources, Turner and seven other scientists concluded that they had apparently found evidence of the most massive object ever detected. That object, they surmise in a report published last week in Nature, could be a huge cluster of galaxies or a black hole far larger than any ever anticipated. More startling, it might be a "cosmic string," a bizarre, hypothetical remnant of the chaotic birth of the universe.

The Kitt Peak telescope had been aimed at what appeared to be two quasars, mysterious, intensely bright bodies so far away that the light they emit travels for billions of years before reaching the earth. Gathered by the telescope's parabolic mirror, the light from each of the quasars was converted into a spectrum, from which a quasar's characteristics and even its distance can be determined. Most scientists believe that each of the some 3,000 known quasars, and thus the spectrum of each, is unique. Says Charles Lawrence, a Caltech astronomer and a co-author of the Nature paper: "Quasar spectra are something like fingerprints, and no two are the same."

But as Turner confirmed, the two spectra recorded at Kitt Peak were virtually identical. This meant that if each were from a different quasar, the two objects would not only have identical chemical properties and temperatures but also would be the same distance (about 5 billion light-years, in this case) away--a highly unlikely coincidence. "If you get matching fingerprints," Turner says, "you could have images from the same quasar."

How does one quasar produce two images? The answer, astronomers say, lies in a "gravitational lens," an immense object with a powerful gravitational field located somewhere between the quasar and the earth. As light from the quasar approaches the object, it is diverted from its original path by the intense field (see diagram) and produces what earthbound observers see as multiple images.

As long ago as 1915, Albert Einstein predicted that as a consequence of his general theory of relativity, light rays would be bent if they passed through the intense gravitational field of a massive object. That prediction was confirmed by British Astronomer Arthur Eddington in 1919, when he traveled to an island off West Africa to observe a total solar eclipse. From there he was able to measure precisely the location of a star that became visible in the suddenly darkened sky near the edge of the sun. Because light from the star was bent by solar gravity as it passed the sun, the apparent position of the star in the sky was slightly displaced from its known position by the amount that Einstein had predicted.