In Search of Alien Lasers

Jun. 14, 2001

by Seth Shostak - Senior Astronomer

Could galactic civilizations be pulsing our planet with high-powered lasers? The idea of communicating with flashes of light may sound archaic, conjuring up visions of cowboys signaling with hand-held mirrors or sailors wielding ship-to-ship telegraphs. And indeed, schemes for using bright lights to establish cosmic contact date back a while. In the mid-nineteenth century, both Karl F. Gauss, the famous German mathematician, and the French polymath, Charles Cros, suggested projects for getting the attention of Martians using lanterns and mirrors.

In fact, what makes a beam scheme attractive today is the possibility of using intensely powerful, pulsed lasers. Scientists at Lawrence Livermore National Laboratory, in California, have built a laser capable of putting out light pulses with a power of 1,000 trillion watts, although the pulses are brief: only a trillionth of a second long. It is called Nova, and its not your daddys laser pointer.

Imagine mounting this beefy beast on a 10 m mirror to tightly focus its output (as discussed in my last article [LINK]), and taking aim at a star system 50 light-years away. You can work out the numbers yourself, but let me save you the time. Every pulse of this transmitting setup will eventually deliver about 10 photons per square meter onto the planets of the neighboring star system, every pulse.

Compare that with the natural brilliance of our Sun that is, of course, also shining on those distant planets. Old Sol puts out a lot of lumens, as youve no doubt noticed: about 4 1026 watts, in fact. Naturally, this sunlight spreads out in all directions (unlike our beamed laser), but it still pumps 250 million photons per second onto every square meter of our target star system. That seems to dwarf the mega-gigawatt lasers punch, but of course it doesnt during the trillionth of a second when the laser pulse arrives. During that brief pulse period, the laser is supplying 8 photons per square meter, but the sunlight only amounts to 0.00025 photons per square meter.

In other words, during the laser transmitters short duty cycle, it outshines the Sun by a factor of thirty thousand.

What do optical SETI researchers do? They point their instruments at nearby stars and count the photons arriving during short slices of time typically a billionth of a second. The steady shower of photons from the targeted sun will occasionally cause the counters to register a hit or two. No more. But if someone has rigged up a laser transmitter similar to our example, then the counter will sometimes hit the bell detecting dozens of photons per time slice. It would be clear evidence of alien cowboys flashing signals our way. Its a great way to establish contact.

This type of experiment is being deployed by several SETI Institute researchers and academics at a half-dozen universities. Theyve already checked out a few hundred stars for bright lights in the sky. Next time, well elaborate on their techniques and the results.