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One thing I’m always asked when I talk about interstellar topics is how long it would take a spacecraft like Voyager to get to the nearest star. After explaining how far away Proxima Centauri and the slightly farther Centauri A and B really are, I tell the audience that Voyager, if headed in that direction, would be facing a travel time of over 70,000 years. That usually shifts the conversation considerably, because many people assume that if we can get to the outer planets, the nearest stars can’t be that far behind. If only it were so.

The Centauri stars are, of course, only the closest known (and who knows, perhaps there’s a brown dwarf a bit closer). Assume a space technology able to travel at close to the speed of light and you’re still dealing with travel times that amount to years, although time for the crew would be shorted according to those interesting Einsteinian effects that cause the crew of a vehicle traveling at 86 percent of lightspeed to experience half the elapsed time felt by those left behind. Getting to anything but the closest stars at such speeds is a long haul for any crew.

Seth Shostak talks about this issue in a recent essay, noting that 61 Cygni, the first star whose distance was correctly measured (in 1838, at the same time that Thomas Henderson was measuring the distance to the Centauri trio) is eleven light years away. To understand the distance, we play the analogy game: A ping-pong ball representing the Sun, placed in New York, would be matched by a smaller ball, representing 61 Cygni, sitting in Denver. We’re talking tens of trillions of miles.

Shostak’s point is to examine what he calls the ‘one percent’ rule. The Romans could hold an empire together as long as travel times to connect the empire were no longer than about one percent of the lifetime of the average centurion. Apply that to a space ’empire,’ even one moving at close to light speed, and you run into problems:

Even if we could move people around at nearly the speed of light, this “one percent rule” would still limit our ability to effectively intervene – our radius of control – to distances of less than a light-year, considerably short of the span to even the nearest star other than Sol. Consequently, the Galactic Federation is a fiction (as if you didn’t know). Despite being warned that Cardassian look-alikes were wreaking havoc and destruction in the galaxy’s Perseus Arm, you couldn’t react quickly enough to affect the outcome. And your conscripts would be worm feed long before they arrived on the front lines anyway.

Lively discussion, but what about communications? Information exchange usually takes place quickly, with our idea of maximum delay often limited to the amount of time it might take an overseas letter to arrive. That time is clearly shortening — we live in a world where a one-day delay in returning an e-mail can be perceived as mystifying, and the generation now being raised on iPods and iPhones, texting away at each other at whim, is unlikely to accept anything but instantaneous communications.

This may be useful, at least for those of us who worry about METI — the idea of sending messages to nearby solar systems rather than listening for signals from them. Do we as a civilization have the long-term approach needed, even if we decided such a thing were benign, to mount a continuing world-wide attempt to communicate with a civilization hundreds of light years away? The attempts made thus far have been sporadic. Will they become more than that?

Our cultural patterns argue against the idea, and although I am a champion of long-term approaches in most respects, in this case I defer to impatience. Because until we understand what we are doing and have an informed consensus on the matter, shouting to the cosmos could have implications we have yet to understand. Let’s put METI on hold.

A greatly enlarged public debate on METI is needed, one that incorporates a wide variety of disciplines, before further signals are sent. Meanwhile, that Great Silence that Fermi speculated about, and which we now seem to be encountering in our SETI searches, may simply imply that other cultures are much like ours, knowledgable about the distances involved and unwilling to make the generational commitment to a kind of communication that may never pay off. Shostak puts it this way: “…while the cosmos could easily be rife with intelligent life – the architecture of the universe, and not some Starfleet Prime Directive, has ensured precious little interference of one culture with another.” That may not be such a bad thing, at least until we have sound reasons for making our presence known in a cosmos we are only beginning to understand.