Fwd = Shostak: Galaxies home to many advanced societies
- Forwarded by: fwestra@... (Frits Westra)
Original Date: Sun, 30 Jun 2002 14:35:29 +0200 (CEST)
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SETI Institute astronomer says hotly debated
topic should be decided by evidence, not opinion
By Seth Shostak
June 27 -- When it comes to alien activities, visiting Earth seems to
be pretty high on the "to do" list. But does that make sense?
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?
55 YEARS OF ALIEN INVASION
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
* 2. The aliens routinely visit Earth.
* 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.
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