Light Up My World

What would it take to send and receive a simple light signal? In answering this question, the STWG drew inspiration from a high-powered laser at the Lawrence Livermore National Laboratory. This leviathan light source (intended for triggering nuclear fusion rather than communicating with aliens) produces pulses that are a trillionth of a second long, brief indeed. But during that short pulse, the laser puts out a million billion watts. Note that the electric bill for this formidable flasher needn't be large. When pulsing once per second, its average power output is a measly kilowatt.

We could see a pulsed laser signal this bright beamed by a large telescope a few dozen light-years away, and a bigger brother could be used for communicating across a sizable chunk of the galaxy. For example, imagine a device 1,000 times more powerful than the Livermore laser affixed to a 10-meter telescope. If it packed infrared pulses into a billionth of a second, it would flash 25 photons per pulse into another 10-meter telescope 10,000 light-years away. Such a heavy-duty communication device could reach billions of stars! Even this little handful of photons, bunched into a single nanosecond, would stand out as artificial were anyone looking for it. Nature just doesn't do things like that.

Of course, flashing a billion stars would be a major effort. But even a modest device could be useful for hailing to the stellar neighborhood. Imagine an automated beacon consisting of a laser ten or a hundred times heftier than the Livermore model feeding a computer-steered mirror system. This system might be tasked with sequentially "pinging" a thousand, or even a million, of the most promising stars. Assuming the system is supple enough to take aim in a tenth of a second, it could repeat its pings every two minutes if a thousand stars were being flashed, and once a day if the targets numbered a million.

For laser beacons, the aiming accuracy has to be very good. If we assume that a reasonable target size to include any inhabited planets has a diameter of 10 astronomical units (the size of Jupiter's orbit) centered on a star, the pointing accuracy needs to be 0.03 arcsecond at 1,000 light-years. So the alien broadcasters will have to loft their laser beacons into orbit, where the atmosphere of their planet won't mess up the beams. In addition, they'll need to know the precise distance and motion of each star they're trying to ping in order to "lead" their targets by the right amount. But such aiming and tracking requirements are neither difficult nor particularly onerous. The aliens could do it.

Suppose they have. Suppose such pulsed laser beacons exist? Shouldn't we look for them?

Some people have. Stuart Kingsley in Columbus, Ohio, was busy pioneering optical searches for a decade, using a backyard 10-inch telescope and high-speed photomultipliers to look for pulses. A few years ago Paul Horowitz of Harvard and Dan Werthimer at Berkeley entered the optical SETI fray after participating in the STWG. They've geared up clever searches that "piggyback" on two medium-size telescopes that were already engaged in other stellar research. Starlight collected by the telescopes that would otherwise be wasted is sent through a beamsplitter and fed to a pair of photomultiplier tubes; a coincidence detector looks for very short flashes appearing simultaneously in each tube. (The use of two detectors is essential to reduce false alarms caused by cosmic rays, scintillation, and radioactive decays in the photomultipliers' glass itself.) Such detectors are simple and cheap. Amateurs can fit them to their scopes.

Optical SETI is gaining adherents. Part of the appeal is that the telescope doesn't have to make good images. Paul Horowitz is building a 72-inch (1.8-meter) optical SETI telescope around a cheap "light bucket" of a mirror. The light will go through a beamsplitter to two arrays of 1,024 pulse detectors, each with nanosecond speed, covering a 1.6-by-0.2-degree rectangle of sky. Only recently have such arrays become available. This instrument, says Horowitz, will be able to examine every point on more than half the celestial sphere (not just selected stars) for at least 48 seconds every 200 clear nights — roughly once a year, considering the weather. It will sweep the whole sky from declination +60 degrees to ­20 degrees, a region that includes more than half the visible Milky Way. [It's up and running as of spring 2006; see our article.].

One followup to the STWG's work is the development of an improved detector for targeted optical SETI. Remington Stone of Lick Observatory has teamed up with Frank Drake (SETI Institute), Shelley Wright (University of California, Santa Cruz), Richard Treffers, and Dan Werthimer to build a device that should help keep observers' blood pressure low. It does so by reducing the number of false alarms. In a two-detector system, noise and electronic glitches still typically produce one false alarm per night, complicating the search. By using three photomultiplier tubes instead of two, the new experiment should only have about one false alarm per year.

The new instrument is already checking out stars using Lick Observatory's 1-meter Nickel Telescope. "It's great," says Drake. "No terrestrial interference, such as complicates radio SETI. The only drawback is that you do require that the extraterrestrials are deliberately targeting you with their lasers."

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