WE DIDN鈥橳 hold back with the superlatives on 17 May 1979, when describing observations made with the Multiple Mirror Telescope (MMT) just installed at the Mount Hopkins Observatory in Arizona. 鈥淭he instrument鈥檚 design represents the century鈥檚 most radical innovation in optical telescopes,鈥� we enthused, 鈥渁 carefully coordinated combination of six 1.8 metre telescopes, whose light-collecting power is equivalent to a single 4.5 metre mirror.鈥� That made it the third most powerful optical instrument in the world.
Despite only having operated for six nights, it had already come up with the goods: 鈥渢he most spectacular demonstration yet of light being deflected by gravity, as Einstein predicted in his theory of general relativity鈥�, we said.
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That story began in 1919, when two teams, promoted by astronomer Arthur Eddington, observed the positions of distant stars changing during a total solar eclipse, just as predicted by Einstein鈥檚 new theory. In 1933, Swiss astronomer Fritz Zwicky predicted that a sufficiently massive body, such as a galaxy, would bend light around itself to the point that objects behind it would appear twice in the sky when observed from Earth.
This was what, almost half a century on, the MMT saw for the first time. Analysis of the spectrographic signatures of two quasars 8.7 billion light years away, QSO 0957+561 A and QSO 0957+561 B, showed that they weren鈥檛 just suspiciously similar 鈥� they were identical. 鈥淭hat forces us to conclude that the images come from the same object,鈥� Smithsonian astronomer Fred Chaffee told us. The same light had bent two ways around an intervening object.
The pair have proved a gift that keeps on giving. In 1995, Johannes Pelt at the Tartu Observatory in Estonia and his colleagues at the Hamburg Observatory in Germany used the time lag between modulations in A and B鈥檚 brightness to calculate how fast the universe is expanding. From this, they determined that it is a couple of billion years older than previously thought. The following year, Rudy Schild at the Harvard-Smithsonian Center for Astrophysics and his colleagues spotted a momentary dip in the brightness of one quasar image. That is perhaps induced by the passing of the most distant planet we have yet detected, 4 billion light years away.
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