Our best bet is to tweak their
interest, send them a signal. This too has been tried without success, but
astronomical discoveries could improve the odds by telling where to direct
beams. We could persist, sending a variety of messages.
Obviously the plan only works if extraterrestrials
are nearby, can answer our signals and are so inclined. About two hundred stars
lie within 25 light years of Earth. Since radio waves travel at the speed of
light, we could receive a reply from an associated planet less than 50 years
after we sent a signal, in less than nine years if Alpha Centauri answered.
Arguably, as many as four "habitable"
planets have been identified within 25 light years, and present methods would easily
miss others. There could be a dozen or a hundred, judging from estimates of
their frequency in the galaxy, and "habitable" moons too; I put habitable in
quotes because reconstructions of surface conditions are as yet very rough.
When SETI projects began, many enthusiasts
assumed that simple life readily evolves into intelligent life, which in turn
develops civilizations that eventually acquire technological capabilities far
beyond ours. Using that logic Carl Sagan once estimated our Milky Way galaxy contains
a million advanced civilizations, and that at any one time 10 -- 15 percent of
them would have radio communication. He had hopes for Alpha Centauri.
Critics concur that simple life is
likely common, but argue that intelligent life is rare. See the essay by
evolutionary biologist Ernst Mayr, and geologist Peter
Ward and astronomer Donald Brownlee's book, Rare Earth: Why Complex Life Is Uncommon in the Universe.
On Earth simple life
appeared early, and some forms survive in extreme conditions such as around
deep sea volcanic vents and in lakes below icecaps. With rare exceptions, complex
life forms demand a narrower range of conditions. It took more than three
billion years for them to evolve from simple life, and over all those years
Earth stayed habitable in a dangerous universe. Extinctions happened but were
incomplete; the more recent ones allowed complex life forms to survive and then
rapidly evolve into niches opened by the elimination of other species. Thus, the
death of the dinosaurs opened the door for mammals like us. We are lucky to
exist.
Mayr points out that only
one species out of billions that have lived on Earth has evolved the
intellectual capacity to produce a technologically advanced civilization, and
that took nearly four billion years. I would add that our scientific abilities
were a byproduct of the evolution of capabilities that were useful for our
hunting, gathering, and fishing ancestors, a remarkable outcome, not an
inevitable one.
Astronomers have taken the
point, to a point. Instead of Sagan's million civilizations, optimistic
estimates now put intelligent beings on a few thousand planets and moons in our
galaxy. If correct, they inhabit less than one out of a million "habitable"
planets. Rarer still, presumably, are extraterrestrials ready to engage us in conversation. The odds they live within
25 light years of Earth become vanishingly small.
Never Say Never
Let's
look beyond our little cosmic neighborhood. Unless Earth is divinely and
uniquely blessed, there must be intelligent, science-minded beings living on planets
and moons circling the million billion billion stars in the known universe. Through
astonishing coincidence and thanks to technology our science deems impossible,
they could show up here tomorrow.
In my daydream, people in modest
dress argue whether the visitors are angels or demons. Unitarians swarm past
Air Force guards and beg the visitors to speak at their churches. The pope sends
a cautiously worded welcome. Warned that the aliens come in three sexes, the
folks from Westboro Baptist carry signs that say, God Hates Omega Centauri B5.
Overhead, a mothership fills the sky,
and from it a voice echoes across mountains and plains, Y'all got everything wrong.
But I'll bet against it.
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