Over the past forty years, astronomers have intermittently observed the heavens with radio telescopes, looking for signs of intelligent life around distant stars. During its early decades, this search was limited by the amount of time available on major telescopes, as well as signal processing capability.

Only recently, with more powerful search programs such as the SETI Institute's Project Phoenix, have we conducted more comprehensive searches. Even so, the most sensitive searches proceed star by stara process requiring considerable patience. Given these constraints, it is hardly surprising that we have not yet discovered intelligence beyond Earth.

One of the most important questions that humankind would need to answer in a post-contact world is, "Should we reply?" To be sure, a dialogue with extraterrestrials would be no ordinary conversation. It could easily take hundreds or thousands of years for a message to travel from Earth to the inhabitants of another star system, and equally long for their reply. Given such immense timescales, we are under no obligation to make a hasty replyif we decide to reply at all.

But knowing human nature, we should expect that some would be eager to initiate a two-way dialogue, even when that conversation would be across generations. Our species seems to have an innate need to exploreto make contact. And if some day we detect extraterrestrials, it would be surprising if no one on Earth wanted to reply.

Astronomers involved in the Search for Extraterrestrial Intelligence (SETI) are currently only listening, not transmitting. The biggest reason for a listening strategy is simple: by listening, we might succeed tomorrow. If we transmit and then wait for a reply, success would take yearspotentially, hundreds or thousands of years.

But once we know that life is out there, the rules of the game might change. Certainly, listening programs would proliferate, as we would search for yet other civilizations around other stars. But once we know where to point our transmitters, it becomes increasingly easy to engage in an interstellar dialogue.

What would we gain by pondering these issues ahead of time? Wont we have time enough to start thinking about reply messages after we detect ET?

Probably not. At least not if we want our decisions to be carefully considered and well-informed. By planning ahead, before contact, we can consider the pros and cons of the many facets of interstellar exchange.

For example, if we do reply, how much should we say about ourselves? In the interstellar recordings onboard two of the Voyager spacecraft, we portrayed the positive side of Earth. Absent were images of poverty, disease, and nuclear mushrooms. Is that our best strategyto put our best foot forward, even if it means downplaying our faults and frailties?

Once weve decided what sorts of things we want to say about ourselves, we also need to decide how much to say. Should we think of interstellar dialogue, for example, as a kind of economic exchange? In this scenario, one motivation for interstellar communication might be to learn something new about the universe and its denizens. In this case, should we share as much as possible initially, in the hope that ET will reciprocate? Or should we parcel out tidbits about our cultural idiosyncrasies, stockpiling a few choice details for our ancestors to use in future rounds of interstellar trading?

And of course, once we know what we want to say, and how much of it wed like to share, we face the challenge of determining how best to say it. Within the SETI community, most proposals for interstellar messages have been based on math and science. The rationale is straightforward: if extraterrestrials can build sophisticated radio telescopes, then they probably know some of the basics of math, physics, and chemistry.

Although math and science might provide a sort of "Cosmic Rosetta Stone," allowing civilizations to understand one another because of partially shared knowledge, would they provide a good foundation for communicating something about ourselves and our cultures? Recently, the SETI Institute launched a series of workshops to answer this question. For instance, at a meeting in Paris, artists and scientists discussed ways that we might communicate some of our aesthetic sensibilities in interstellar messages, perhaps by using basic concepts from math and science.

Given the many unknowns in interstellar communication, I suspect there will always be room for debate on questions such as "What should we say to ET?", "How much should we say?", "How should we say it?", and indeed, "Should we say anything at all?" By encouraging in-depth discussion on these topics now, we are much more likely to act with the care and wisdom befitting an event like the detection of intelligent life beyond Earth.