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Summary (Aug 14, 2003): Finding an otherwordly signal turns out to be different than what most sky surveys offer, or even what eventually might be required to decode a signal once found.
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Job One Tuning a signal to decode its message is different from finding the signal in the first place.
It's important to understand that we could easily trip over ET's signals without immediately knowing we have done so.
Radio SETI observers look for narrow-band emissions, similar to the carrier signals used for radio and television. These are the pure-tone squeals which your receiver uses to capture the attention-grabbing aspects of the broadcast, namely the sound or picture (the accompanying figure shows that a TV signal actually has two of these carriers, one for video and another - less powerful - for audio.)
Carriers are commonplace, but they're not the only way to transmit. Your cell phone uses another technology, known as spread-spectrum, which is carrier-free. Spread-spectrum techniques, while useful for minimizing interference when lots of transmitters are present, is a wasteful way to make your presence known. It takes more power, and requires that both transmitters and receivers agree in advance on the method of encoding information.
If extraterrestrials wish to get in touch with distant, unknown beings that won't have access to alien technical manuals, then narrow-band signals make a lot more sense. They put all the transmitter power into a tiny spot on the radio dial, easing the task of discovery.
 | | Allen Telescope Array (ATA) | It's 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 Institute's 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.)
 | | Arecibo. World's largest dish, radio telescope. Puerto Rico. |
Clearly, today's 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, it's not.
Of course, irrespective of whether we've 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 don't know, are geometric patterns that routinely "appear" in the wheat fields of southern England.
These examples of "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 they'll decode messages. They're not - yet. The cryptographers will come later.
Job One is to find the signal.
Related Web Pages How To Find An Extrasolar Planet
SIM (NASA's Space Interferometry Mission
GAIA - The Galactic Census Project
FAME: Full-sky Astrometric Mapping Explorer
Allen Telescope Array Capabilities
SETI Institute
Note: Phoenix
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