Alien TV: Sorting Intelligent Signals from Witless Cosmic NoiseFeb. 20, 2003
by Seth Shostak - Senior Astronomer
SETI researchers are a bold lot. Theyve chosen to accept a mission that might dissuade Mr. Phelps. Year after year, they spin their telescopes to the sky, sifting through a broiling rumpus of cosmic static in hopes of finding a signal made by other beings.
But how will they know?
In our previous discussion we talked about the criterion of artificiality: some property of a transmission that would tag it as deliberately constructed the equivalent of finding stacked cannonballs in a field of stone heaps. It sounds easy, but pulsars, discovered in the late 1960s, showed how quickly we could be duped by a completely natural phenomenon. Since then, SETI researchers have expended considerable neural energy in considering what type of radio emissions would unequivocally qualify as artificial.
An obvious suggestion and one that well-meaning folks the world over love to send me in e-mails is to search for a signal that is branded with a mathematical label. For example, maybe the aliens will tag their transmission with the value of pi. That would clearly bespeak a middle school education, and would prove that the signal comes from thinking beings, rather than witless neutron stars or some other cosmic oddity. More numerate correspondents try to improve on pi. Perhaps the extraterrestrials will preface their message with a string of prime numbers, or maybe the first fifty terms of the ever-popular Fibonacci series.
Well, theres no doubt that such tags would convey intelligence. But what if the prime numbers are only broadcast at the start of a 100-hour interstellar screed, and we tune in somewhere in the middle? Wed miss the label.
In fact, we dont have to worry about this. Our TV signals, for instance, dont have repetitive headers with the value of pi attached, and yet we rightly suspect that they are recognizable as the product of a technically advanced society (were not talking program content here.) What is it about a TV signal that marks it as artificial? Surely, the picture and sound components make up a dynamic, and clearly non-random distribution of energy across the band. But these components are subtle, and would be rather difficult to detect at great distance.
However, one-third of the TV signal power is squeezed into a tiny part of the dial a narrow-band carrier thats only 1 Hz wide or so (the picture and sound are five million times wider.) You dont see this carrier on the screen, and you dont hear it from the speakers, thank goodness: your TV tuner uses it to decode the information, and then throws it away. But the point is that such narrow-band signals compress a lot of radio energy into a small bit of spectrum making them the easiest type of signal to pick out in a sea of static.
SETI Institute signal detection expert Kent Cullers, whose clear thinking routinely enlightens both novice and savant, describes the merits of narrow-band radio signals by comparing them to their audio counterparts.
"Imagine the roar of the ocean or the rustling of leaves in a high wind," he says. "For these natural events, the sound is produced simultaneously from many unsynchronized sources. If we plot the frequencies present in such natural events and compare them to artificial sounds, such as a tuning fork or an auto horn, a startling difference appears. Natural signals have a rather broad frequency spectrum, but the artificial ones usually dont."
Its the same with radio emissions. "Sure, if we want to, we can intentionally produce broad messy signals," Cullers notes. "Cell phones that use spread spectrum technology do this, and the military sometimes uses broad signals to intentionally hide them. But it seems that Nature cannot make a pure-tone radio signal."
So the point is simple and sharp. If we detect a signal from the Stygian depths of space that is the equivalent of a tuning forks sinusoidal whistle, then we can feel confident that we have found SETIs vaunted needle in a haystack: a message from another world.
Theres just one trouble with this. A perfect, narrow-band signal can have no message. A tuning forks steady note is not music. And it seems only reasonable to assume that if another civilization has troubled to build a transmitter, they wont waste the megawatts by merely sending an empty signal into space.
In the third and final lap of this journalistic marathon, well discuss how we can reconcile recognizable signals with the obvious need to make interesting ones; and whether its reasonable to hope that SETI can find not just cosmic whistles, but a whole celestial symphony.