How Can Humanity Decode Alien Transmissions

Mar. 27, 2003

by Seth Shostak - Senior Astronomer

Sorting deliberate signals from a gush of cosmic static requires discernment. Its somewhat akin to beachcombing: finding the occasional bauble on a shoreline littered with dead seaweed, desiccated crustaceans, and other unappealing, sea-borne dross.

Radio SETI experiments do this by hunting for narrow-band signals, in which much of the transmitter power has been crammed into a bit of spectrum thats 1 Hz or less in width. Such signals are not only very easily noticed, they are also markedly different from the wide-band splatter of quasars, pulsars, and other cosmic broadcasters. Narrow-band signals are, in other words, "artificial." (Official disclaimer: its possible that there exists some still-undiscovered, natural astrophysical phenomenon that could produce narrow-band signals. While finding such a phenomenon might not chill the spine to quite the same low temperature as finding E.T., it would still be tremendously interesting, and probably worth tenure at your local institution of higher learning.)

Optical SETI experiments use a different criterion for artificiality: they search for short, intense photon bursts. If you turn a moderate-size telescope in the direction of a star many dozen light-years distant, and then measure how much light arrives in a very short interval say, a billionth of a second you will typically count no more than one photon. But if it transpires that you measure a few dozen photons piling into your scope in that brief time slice, you will certainly have found something highly artificial: presumably a laser pulse shot your way by extraterrestrials with a yen for contact.

So the SETI criterion for artificiality is this: Narrow-band signals in the radio, and brief photon bursts in the optical.

Fine. Such signals would tell us that the aliens are out there. But what about the message?

As weve remarked in the previous articles in this series, a narrow-band radio signal is like a single note on a flute: obvious, but not interesting. Well, suppose you decide to switch such a radio tone rapidly on and off, to encode some sort of pithy messages. This will spread the transmitter power over a wider piece of the dial (think of the "music" made by your computer modem), and the resulting signal at any specific frequency will be weaker, and therefore far more difficult to detect. Indeed, many transmissions that we produce on Earth (including the cell phone calls that both keep you in touch and may be cooking your brain) are deliberately engineered to be wide-band, to maximize security and minimize interference. Such signals are hard to find unless you are privy to the encoding scheme or work for the phone company.

So how do we reconcile easily found, narrow-band signals with E.T.s presumed interest in conveying information? After all, there seems little point in constructing a transmitter with interstellar reach if it does no more than send a dull, empty signal.

One possibility is that E.T.s wide-band, information-rich message will lie in the same part of the band as the narrow-band "beacon," forming a type of composite broadcast thats similar to our own television signals. In this case, having found the beacon, we can hope to corral the wide-band message by simply erecting a larger antenna that will make it detectable. Note that this antenna might be thousands of times the size of the one that bags the initial discovery, and consequently many, many years may elapse between finding an alien signal and getting the message (let alone understanding it). This is just one more case in which Hollywoods view of SETI diverges from scientific reality.

Another possibility is that the beacon may have some slow code that "points" to the message by indicating where it is on the dial. In this case, the narrow-band signal is an electromagnetic buoy that would tell us where to dive deeper to find the real treasure.

What about snagging the message from an optical signal? Since this approach looks for short flashes of light, you might think that we could get E.T.s bountiful bit-stream right away: all we would need to do is write the incoming pulses to a hard disk, and then examine them at leisure to search for patterns that would contain the profound insights of distant beings.

Can we do that? The experiment being run jointly by the Lick Observatory (University of California, Santa Cruz) and the SETI Institute, which uses a telescope with a 1 meter diameter mirror, is looking for short (one billionth of a second) bursts of light coming from the directions of nearby stars. But the equipment can only record information about incoming pulses at a rate of ten times a second. If E.T. sends his encyclopedia our way at a speed of tens of millions of bits per second something that could be easily done on a beam of light then the Lick system will be too slow to sort it all out. It would be akin to flipping through "Moby Dick" at fifty pages a second: you would know there are words flying by, but the subtleties of Melvilles prose might elude you.

"Its true we wouldnt get all the bits," notes Rem Stone, a research astronomer at the Lick Observatory. "We cant record that fast. But we would know that something is going on right away." Put differently, the researchers would note that words are flying by. And then? "We could speed up our sampling by a factor of ten or a hundred essentially overnight. In a few weeks, we could do even better by cobbling together new hardware."

Theres no doubt that, whether that first discovered signal is a radio tone or a light burst, there would be tremendous incentive to move beyond the detection and find whatever message accompanies it. For light flashes pinging our planet, that might not take very long. Indeed, as Rem Stone says with a smile in his voice, "you can bet that we would do that real fast."