Will we ever find a primer for decoding messages from extraterrestrials? Last month, anthropologists who gathered for a major conference in Atlanta, Georgia heard some news that will be sobering for SETI enthusiasts: it may be much more difficult to understand extraterrestrials than many scientists have thought before.

Among the sessions held during December's annual meeting of the American Anthropological Association was one called "Anthropology, Archaeology, and Interstellar Communication: Science and the Knowledge of Distant Worlds." The session included papers by scholars from such diverse fields as astronomy, archaeology, anthropology, and psychology. Is there a Cosmic Rosetta Stone, they asked, drawing parallels to Earth's own Rosetta Stone, which provided the key to decoding Egyptian hieroglyphics? Will we ever find a comparable primer for decrypting any messages we might receive some day from extraterrestrials?

Thirty years ago, a message was beamed to the stars from the world's largest radio telescope in Arecibo, Puerto Rico, written in the language of math and science. But could another civilization understand such a message? In the search for a universal language to overcome cultural differences between humans and extraterrestrials, many have emphasized knowledge that would be shared by extraterrestrial scientists. For example, the metal plaques borne on two Pioneer spacecraft, launched by NASA in the 1970s, indicate our location in the galaxy relative to prominent pulsars that slowly and systematically change frequency over time--locating the spacecrafts' launch in both time and space. And indeed, for an extraterrestrial civilization that values technical intelligence over social intelligence, such a description might be the start of an ideal message.

But not everyone is so sanguine about using science and math as universal languages. Anthropologist Ben Finney of the University of Hawai'i challenged the standard assumptions several years ago, by drawing on lessons learned from decoding Egyptian and Mayan hieroglypics--a story recounted in Atlanta. "SETI scientists reasoned that advanced ET would de-encrypt their messages through prime numbers, pi, the Planck constant and other presumed cosmic universals so that new members of the Galactic club could immediately begin deciphering them," Finney explained. "I questioned this reasoning on the basis of terrestrial experience in deciphering ancient Egyptian and Mayan inscriptions."

On closer examination of the process of decoding these scripts, Finney concluded that when initial assumptions are wrong, the decryption can be delayed for a long time. "These tasks were long held up by Plotinus' fallacy of treating each hieroglyph as an idea or concept in itself without reference to language, and were only accomplished with the aid of such keys as the Rosetta Stone and by studying modern forms of Egyptian and Mayan." The lesson for SETI, it seems, is to remain flexible in our initial interpretations of messages from other worlds.

"The archaeological search for peoples from another time and place offers some analogy to the search for extraterrestrial intelligence," said Paul Wason, an archaeologist at the John Templeton Foundation. "Without benefit of direct contact with living beings, without the aid of understandable written communications, ... prehistoric archaeologists rely on inferences drawn mainly from material traces of past activity."

Astronomer Frank Drake, who conducted the first SETI experiment in 1960, emphasized the ambiguities of communicating with those who encounter reality in very different ways. He cited examples from interstellar messages that have been drafted already, as well as initial planning to communicate with future generations of humans here on Earth - through radioactive waste site markers. According to Drake, "probably the most intensive and extensive efforts to communicate with intelligent creatures quite different from contemporary humans have been the program which placed a multi-media record on two of the Voyager spacecraft, and the plan to place fail-safe hazard markers at long-lived radioactive waste repositories."

As one of the designers of these two messages, Drake is well aware of the challenges of tapping into concepts that could be interpreted without error. "In both cases, the development of message content in these projects ran into enormous problems of potential misinterpretation," he explained. "The developers recognized that the interpretation of the message contents would surely take place in a context much different from the present, and unknowable to them. Attempting to construct messages which are unambiguous in such circumstances is extremely difficult and inevitably fraught with error."

As an example, Drake highlighted the ambiguity of the pictures of a man and woman etched onto the Pioneer plaques. "Is the depiction of a man holding an arm upraised to be taken as a friendly gesture of greeting, or as a threat of aggression?" Even supposing the recipients could discern that we are showing images of our species, how could they infer our intentions?

Perhaps SETI scientists can get some guidance in understanding other civilizations from social scientists. As archaeologists try to distinguish between rudimentary stone tools and rocks chipped by natural processes, their methods may provide insights that will help SETI scientists distinguish between naturally occurring cosmic static and signals from intelligent civilizations, whose purpose is to send intentional greetings.

"Archaeology regularly engages in the search for intelligence, intentionality, purpose and design broadly," Wason explained, "even as our deeper goal is recovering concrete detail concerning what, specifically, those intentions and purposes were." The resulting insights might inform the ways we structure messages, he suggested. "This is relevant for message construction from our end, reinforcing the view that we should give attention to all aspects of message form and context, not just its content. It is also relevant for identifying signs of purpose and intentionality beyond our planet."

The fields of anthropology and archaeology also offer analogies to understanding "the Other," beings radically different from ourselves. In the process, they may provide insights into communicating broader notions of culture, thereby increasing the likelihood that messages will be intelligible. Simulations based on anthropological models of first contact between terrestrial cultures, for example, might help evaluate the adequacy of current protocols that guide responses to detecting a signal from extraterrestrials.

And yet, the delay of centuries or millennia between each exchange makes interstellar dialogue impossible, except as a dialogue across generations. In this scenario, initial misunderstandings could go uncorrected for a long time. "Without the Other's dialogic input," said archaeologist Kathryn Denning of York University, "their responses to our misunderstandings, and answers to our questions, then the burden of self-correction falls to us. Perhaps then the challenge for both SETI and archaeology is that of recognizing and shedding enough of our own assumptions. Can we make ourselves unassumed, unfamiliar... indeed, alien?"