Talk Between Planets

A space traveler who happened to be standing on the dark side of the moon last week, in the mountains southeast of the crater Albategnius, would have been startled to see 13 brief red flashes flame up on the dark side of the distant earth. The unexpected spurts of light marked the position of Lincoln Laboratory near Lexington, Mass. They came from a ruby laser--a source of pure light of a single frequency--fitted into a 12-in. telescope.

Since laser light can be concentrated into a thin beam that barely spreads out at all, Professor Louis Smullin and Dr. Giorgio Fiocco, the M.I.T. engineers who performed the experiment, estimate that the laser's light diverged only about two-thirds of an inch for each mile of its quarter-million-mile journey to the moon. When it reached the moon's mountains, the laser beam lighted faintly a circular area only two miles in diameter.

Smullin and Fiocco estimate that each of its 1/2,000-sec. flashes squirted 2 X 10 21 (200 billion trillion) photons of light toward the moon. Most of these tiny bits of light got there, but those reflected by the moon's rough, dark surface scattered widely. Only a few of them bounced back to Lincoln Lab. Bunched together by a 48-in. telescope, the returning photons were sent through a filter that passed only light of the laser's wave length. Then the photons were picked up by a sensitive photocell.

The engineers figure that only a dozen photons from each flash completed the round trip. But they arrived after the proper time interval (about 2.5 sec.), which proved that they had actually gone to the moon and back. This was the first time a segment of space had been spanned by laser light. And it may well mark a milestone in space communication. When they learn to beef up their lasers, scientists hope to use them to talk between planets.

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