Finding the Message in a SETI Signal
by Seth Shostak - Senior Astronomer Dec. 27, 2000
In the summer of 1924, Mars was in opposition, which is to say that it was opposite the Sun in our skies. This situation, which only occurs every few years, brings the Red Planet within three dozen million miles of Earth --virtually spitting distance. Scientists of the time thought this was a perfect opportunity to get in touch with the Martians using the new technology of radio.
Sensitive receivers were turned on, and some large military transmitters were switched off to lessen interference. The famous Army cryptologist William F. Friedman stood by, ready to decipher whatever message our Red Planet buddies might choose to send.
The Martians didnt send anything, of course. There may be life on Mars, but if so, its probably amoeba-like, and not very accomplished in radio technology. William Friedman had a quiet day.
Still, the idea of "deciphering" alien messages remains popular. Modern SETI searches are looking for technically sophisticated aliens around other star systems. Do the research teams that run these experiments also have cryptologists standing by?
They dont, and this isnt just a matter of the high salaries such experts draw. Its primarily because most SETI experiments "integrate" or average incoming radio noise in order to improve sensitivity. This strategy is very similar to what you do when taking a photograph at night. By making a time exposure, and keeping the shutter open for a long time, your photo will show even faint city lights. Similarly, radio astronomers keep their electronic shutters open for seconds or minutes, in order that weak signals have a chance to stand out against the random static produced by their equipment.
Indeed, the ability to integrate for many minutes distinguishes the targeted SETI searches (such as the SETI Institutes Project Phoenix) from sky surveys. By holding open the electronic shutters, a targeted scrutiny of a nearby star system can easily be 15 to 20 times as sensitive as a sky survey, even if the same antenna is used for both experiments.
Every SETI researcher values sensitivity. But the price paid for "integration" is that complicated messages would be averaged out too and lost. About the only sort of information that would survive integration would be a really slow, pulsing signal -- something like Morse code sent by a beginner. Project Phoenix could find such lethargic messages if they were sent at 1 bit per second or less. Advanced aliens might find such laid-back transmitting rates entirely too tedious. At 1 bit per second, it would take several weeks to send a Tom Clancy novel (or its alien equivalent). For comparison, conventional television blasts out bits at 10 million times this rate. (Nonetheless, we still complain that TV is boring.)
Clearly, todays radio SETI experiments are optimized to find the signal itself, and not the message. Optical SETI efforts which look for flashes from high-powered extraterrestrial lasers could, in principle, find complex messages if the detection equipment was hooked up to high-speed data recorders. But so far, its not.
Of course, irrespective of whether weve set up our equipment for messages or not, we fully expect that any aliens interested in transmitting would accompany their broadcasts with a lot of information. Why bother otherwise? This is one of the reasons why I find the brouhaha about crop circles so silly. Crop circles, in case you dont know, are geometric patterns that routinely "appear" in the wheat fields of southern England.
These grain graffiti supposedly the work of advanced extraterrestrials are attractive, but consist of rather simple patterns. And a simple symbol can only convey a simple message. A crop circle is comparable to a single Egyptian hieroglyph. The ancients would cover walls with such hieroglyphs when describing even the most mundane thoughts. Can you imagine that sophisticated aliens would traverse hundreds, perhaps thousands of light-years to offer us one or two of these characters?
The bottom line? You need not wonder if SETI scientists are sitting around fretting over how theyll decode messages. Theyre not yet. The cryptographers will come later.
Job One is to find the signal.