But as we search for signals from extraterrestrials, perhaps bearing tutorials in mathematics, perhaps with encoded messages describing alien analogues of music or art, objective time is not enough. Just as crucial to sustaining a search that may take decades or centuries is an appreciation of our subjective experience of time. The metronome may offer a precise measure of time, but its pace has its origins in the beating of the heart. And for humans, the best of hearts stop long before a laboratory-grade cesium clock.

We humans are familiar with the back-and-forth of face-to-face contactsomething we will not have in an interstellar conversation. The timescale of a human life may well not be enough for a meaningful dialogue with another species. Interstellar dialogue may make sense only across generations.

For now, SETI scientists solve the problem by placing the burden of transmitting on the other civilization. Whereas we are guaranteed not to receive a reply for many years, if ever, by listening we may succeed tomorrow. As Jill Tarter, leader of the SETI Institutes Project Phoenix noted recently, "Because transmitting is harder than receiving (in terms of both cost and cultural commitment), and must be a long-term effort to have any chance of success, it is reasonable to place the burden of transmission onto the more advanced technologies."

Rather than conceiving of SETI as a two-way conversation, some have proposed viewing it as an analogue to learning about great civilizations of the past. For example, according to Philip Morrison, Professor Emeritus of Physics at MIT, "the SETI enterprise can best be understood as a kind of exercise in the archeology of the future. We know that it is possible that some other civilization, who wants to, can bring us in. Of course it will be their past, but our future, which we are investigating to some degree." SETI examines that hypothesis, consistent with Morrisons view that "When you've got the spade it seems very wrong not to dig."

To date, Project Phoenix has examined fewer than 1000 stars. As technologies on Earth increase, that number will explode in the next decade or two. For example, the Allen Telescope Array, now being designed by the SETI Institute, will increase the number of stars observed to 100,000 or even a million.

If in the course of that search we do detect another civilization, should we respond? Uncertain about whether we can exist as a civilization for even another century, should we embark on a conversation that may take a millennium? If our past is any indication, if Earthly time capsules and monuments to the future provide any lessons, it seems likely that at least some people will try.

As SETI pioneer Carl Sagan noted in his preface to a book on the interstellar recordings borne by the Voyager spacecraft, "For those who have done something they consider worthwhile, communication to the future is an almost irresistible temptation, and it has been attempted in virtually every human culture. In the best of cases, it is an optimistic and far-seeing act; it expresses great hope about the future; it time-binds the human community; it gives us a perspective on the significance of our own actions at this moment in the long historical journey of our species."