Here Be Dragons

SETI
believe that there are unexplained phenomena," she says. "I also believe
that they have nothing whatsoever to do with little green men in
flying saucers, because there's no tangible evidence to that end; and if
you show me the evidence, I'll be happy to change my belief, because
that's the difference between science and religion or fact and fiction—
the difference is the evidence."
If one discounts the UFO claims, yet still believes that there are
many technological civilizations in the galaxy, why have they not visited
us? Drake's answer is straightforward: "High speed interstellar
travel is so demanding of resources and so hazardous that intelligent
civilizations don't attempt it." And why should they attempt it, when
radio communication can supply all the information they might want?
At first glance, Drake's argument seems very persuasive. The distances
between stars are truly immense. To get from Earth to the nearest
star and back, traveling at 99 percent of the speed of light, would
take 8 years. And Barney Oliver showed that, to accelerate a spacecraft
to such a speed, to bring it to a stop, and to repeat the process in the reverse
direction, would take almost unimaginable amounts of energy.
"Barney got it wrong in two ways," says Zuckerman. "One is that, of
course, we're not going to travel at 99 percent of the speed of light;
and second, once you get there, you don't come back. It's like when
the Polynesians arrived in Hawaii: they didn't turn round and go back
to wherever they came from, they stayed there. So his calculations are
off by umpteen orders of magnitude."
Zuckerman's argument stands SETI on its head, because it has traditionally
been the SETI folk who have been enthusiastic about interstellar
travel. It was Drake, after all, who affixed the message plaques
to the Pioneer and Voyager spacecraft that are already winging their
way toward the stars, less than a lifetime from the beginning of the
space age. And it is Drake who claims that technological civilizations
can last a billion years. That's plenty of time, it would seem, to build
larger spacecraft that could house migrants for however long it takes
to find a hospitable planet—for hundreds of generations, if necessary.
15 Such migration would become a necessity for some very longlived
civilizations, Zuckerman has pointed out, because all stars eventually
burn out. 16
Even if the members of a distant civilization did not wish to travel in
person, surely they would send robotic vehicles to explore the galaxy.
This idea was put forward by Ronald Bracewell, the Stanford
University astronomer who also first proposed applying optical inter-
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