Dec. 9, 2004 —Eavesdropping on alien radio transmissions may be close to impossible, says a team of researchers who think alien signals would be indistinguishable from interstellar noise.

The problem, as first pointed out by Claude Shannon in the 1940s, is that any sort of optimally efficient encoded message has too much in common with background noise.

Researchers at the University of Michigan, Max Plank Institute and the University of New Mexico took another look at Shannon's work and found that the problem is particularly bad if you're looking at radio waves, the type of waves generally studied for signs of alien communication.

"If you are communicating with maximum efficiency, your signal looks like black body radiation," said physicist Mark Newman of the University of Michigan, referring to theoretical dark, warm heavenly bodies that transmit light in radio and infrared wavelengths.

Efficiently encoded messages look a lot like noise in the same way that email spam mimics real email — by containing strings of words that appear to contain a message, but are actually random jumbles. On the other hand, if something is non-random, it might contain a predictable string of symbols, like ... ~~~~~~~~~ ..., but contain little or no information.

"If something is non-random, then you can predict what's coming next," said Newman. Such a signal would be passed over as well since it lacks information, he said.

The problem is a familiar one to researchers involved in the Search for Extraterrestrial Intelligence (SETI), says SETI's Seth Shostak. The SETI project scans the cosmos for radio signals that could be messages from extraterrestrials, using primarily the Arecibo Radio Telescope in Puerto Rico.

"SETI assumes that the aliens will be as efficient as they want to be," said Shostak.

They would likely have more than our mere 80 years of experience making radio transmissions and would be much better at encoding messages, making their transmissions very hard to detect, he said.

"But if they have a signal that they want to be found they would put a flag on it," said Shostak. A flag would be something like one of our inefficient TV signals — a radio transmission with carrier waves, he said. "These are 10,000 times easier to find," he said.

SETI is also working on the assumption that an alien civilization would concentrate a radio signal that they want to be found on a very narrow radio band — sort of like a radio station does — so it would look unlike natural background radiation, which tends to be broadband.

"Narrow band is a lot of energy at one spot on the dial," said Shostak. "That's what we look for."