http://www.space.com/searchforlife/seti_shostak_visit_020627.html
Would Aliens Visit?
By Seth Shostak
Senior Astronomer, Project Phoenix
posted: 07:00 am ET
27 June 2002
When it comes to alien activities, visiting Earth seems to be pretty high
on the "to do" list. But does that make sense?
SETI THURSDAY
TODAY'S DISCUSSION
What do you think of this story? >>Uplink your views
Approximately half the U.S. population suspects that extraterrestrials
have come to our planet. This is such a controversial (and emotional)
topic that its mere mention in one of these articles is usually
sufficient to guarantee a storm of Web chat and high-voltage e-mails. In
the end, of course, the matter of alien visitation will be decided by the
evidence, not by the intensity of opinion. While I certainly expect that
the Galaxy is home to many advanced societies, the quality of the
evidence has so far failed to convince me that any of them have
emissaries on our planet.
But let�s back off to our neutral corners for a moment and consider an
intimately related question: why would aliens be visiting now? According
to the most popular view of this matter, extraterrestrial craft have been
flitting across our skies since 1947. That�s 55 years in a planetary
history of 4,600,000,000 years. If we assume for the moment that these
claims are real, this chronology tells us immediately that either (1) we
are the beneficiaries of an enormously rare event (one chance in 100
million, or if you want to argue that no aliens would visit until they
detected oxygen in our atmosphere, one chance in 40 million), (2) the
aliens routinely visit Earth, or (3) our activities (nuclear tests,
environmental degradation, etc.) have attracted the aliens� attention,
and encouraged them to drop by.
The first possibility, that we just happened to luck out (being around
for the first and only alien encounter), is less probable than that you �
not someone, but you � will win next month�s lottery jackpot. It strains
credulity, to use polite vernacular.
The second possibility, that Earth hosts extraterrestrials on a routine
basis, and therefore a visit during your lifetime is not particularly
improbable, deserves a bit more scrutiny. The question is, how often do
they visit? If it�s only once in a few tens of millions of years, we�re
back to the first possibility, and the odds are highly stacked against
you being one of the lucky visitees. But some folk claim that aliens have
glissaded to Earth in historical times (five millennia ago, when the
pyramids were built, or one millennium ago, when the Nazca Indians
elected to decorate the Peruvian desert floor with glyphs of turkeys and
other of their favorite fauna). If any of this is true, it argues for
visits at least once every 1,000 years or so. The problem with this is
that � barring some reason for them to visit humans in particular (a
possibility we consider below) � it implies that there have been millions
of expeditions to Earth! We may send the occasional anthropological
research team to Borneo, but we don�t send millions. And it�s a lot
easier to get to Borneo than to traverse hundreds or thousands of
light-years. This, too, seems to be an unlikely explanation for visitors
now.
Finally, we consider door number three � we have enticed the aliens with
human activity. Let�s set aside the question of whether advanced galactic
societies would have the slightest interest in our wars, our pollution
problems, or our reproductive systems. The real question is, how would
they know about us at all?
In fact, there�s only one clear and persistent "signal" that Homo sapiens
has ever sent to the stars: our high-frequency radio transmissions,
including television and radar. The Victorians (let alone the Egyptians
or the Nazca Indians), despite all their technical sophistication, could
never have been spotted from light-years away. Humans have been making
their presence known to the universe only for the last 70 years or so.
And that�s a problem. It means that even if, after receiving an earthly
transmission, the aliens can immediately scramble their spacecraft and
fly to Earth at the speed of light, they can�t be farther than 8
light-years away to have arrived by 1947. There are four star systems
within this distance. Count �em, four. We�re back to winning the lottery.
What about warp drive? Maybe the aliens can create wormholes and get here
in essentially no time. It doesn�t matter. Our signals travel at the
speed of light, and this means that even with infinitely fast spacecraft,
the aliens can�t be farther off than 15 light-years to have reached our
lovely planet by 1947. The number of star systems within 15 light-years
is about three dozen. There would have to be 10 billion technically
sophisticated societies in the Galaxy to have a reasonable chance of
finding one camped out among the nearest three dozen stars. That�s
optimism of a high level indeed.
It�s nice to think that either Earth or its human inhabitants have not
only attracted the attention of galactic neighbors, but encouraged them
to visit. But frankly, the numbers don�t give much support to this
somewhat self-indulgent idea.
