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ZL2VAL > SETI 24.10.03 19:28l 132 Lines 6749 Bytes #999 (0) @ WW
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Subj: Can Aliens Find Us?
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Sent: 031024/0850Z @:ZL2AB.#46.NZL.OC #:28785 [New Plymouth] FBB7.00g $:7621-ZL
From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SETI@WW
By Seth Shostak
Senior Astronomer, SETI Institute
posted: 07:00 am ET
23 October 2003
It's a legend about as popular, and generally believed, as the reputed
presence of alligators in the sewers of New York; namely, that the only
human-made edifice that astronauts can see from space is the Great Wall
of China.
Well, forget it. The Great Wall is about 15 feet wide, which even from
as little as 200 miles up (Shuttle cruising altitude) subtends an angle
of only about one-twentieth of a minute of arc. The human eye can see
detail down to one minute of arc, which is obviously far too poor for
Wall watching.
Still, with a really nice pair of binoculars, the Wall (not to mention
less romantic constructions, such as interstate highways) does become
visible from orbit. Any curious aliens that made it to within a few
hundred miles of Earth would have no trouble seeing the artifacts of our
civilization. They would know, without doubt, that technologically
competent beings roamed our world.
But how visible are we to aliens that are farther away? In the early
nineteenth century, the Austrian physicist Joseph von Littrow is said to
have suggested digging giant geometric shapes in the Sahara Desert as
signaling devices. The excavations would be filled with water and
kerosene, and set afire at night to get the attention of our Martian
brethren. The desert figures were to be roughly 20 miles across. So to
make out these patterns from the Red Planet would require a 10-meter
Keck-size telescope perched on top of, say, Olympus Mons (where the
effects of atmospheric "seeing" would be minimal). If sophisticated
Martians existed, they could presumably build such an instrument and
admire von Littrow's flaming trench work.
This is just one of many early attempts to flirt with nearby aliens, but
it boils down to this: if intelligent beings were hanging out just about
anywhere in the solar system, it would be a piece of technological cake
for them to detect modern Homo sapiens, Saharan trenches or no.
OK. But what about aliens that inhabit other worlds, around other stars?
How easy would it be for them to learn of our existence? If they've
already built planet-finding telescopes, comparable to, or slightly
better than, the one that NASA will be hefting into orbit in the next
dozen years, then they could detect the Earth. With substantially larger
telescopes, they could find our planet from hundreds or even thousands
of light-years distance. Not only that, but they could also
spectroscopically sample the light reflected from our atmosphere, and
learn that it has large quantities of oxygen and methane, tell-tale
markers of biology.
In other words, aliens -- even relatively distant aliens -- could make
straightforward astronomical observations that would prove that the
third planet from the Sun hosts life. If biology is common in the
cosmos, then Earth might be just another entry in a long list of "living
worlds" compiled by some alien graduate student. Its discovery might not
excite the extraterrestrials very much.
But proof of intelligence on this planet might.
So how could the aliens learn that high IQ creatures crawl the Earth?
For them to see the Great Wall of China, the lights from our cities, or
even the cities themselves, would be extremely difficult. But as
virtually every reader of these columns knows, our radio signals are
dead giveaways of terrestrial technology. The aliens could "hear" us far
more easily than they could see us.
Radio was invented in the 19th century, and large-scale broadcasting
began in the 1920s. Alas, these early broadcasts were of low power, and
at low frequency. The difficulty with low frequency transmissions, such
as AM radio, is that they are refracted by Earth's ionosphere, and have
difficulty making it into space. However, beginning in the 1950s, we
started to construct high-power, high frequency transmitters - for
radar, for FM radio, and for television. These signals leaked off the
planet, and headed for the stars.
A modern TV transmitter can put out as much as a megawatt of power. It's
not very tightly focused, so even though much of the broadcast energy
spills into space, it's fairly weak by the time it reaches another star
system. Consider one of our early TV programs just washing over a planet
that's 50 light-years away. To detect the "carrier" signal from this
broadcast in a few minutes time would require about 3,000 acres of
rooftop antennas connected to a sensitive receiver. That's a lot of
antennas, and an unsightly concept. But it's not hard to build, and the
aliens could conceivably do it. If the extraterrestrials were unwise
enough to actually want to see the program, then they'd need an antenna
about 30,000 times greater in area (roughly the size of Colorado).
Ambitious, but possible.
A rather easier task would be to detect our military radars. The bigger
ones typically boast a megawatt of power, and are focused into beams
that are a degree or two across. There are enough such radars that, at
any given time, they cover a percent of the sky or so. The signal from
the most powerful of these could be found at 50 light-years? distance in
a few minutes time with a receiving antenna 1,000 feet in diameter.
Indeed, these military radars are the only signals routinely transmitted
from Earth that are intense enough to be detectable at interstellar
distances with setups equivalent to our own SETI experiments.
Bottom line? With radio technology slightly more advanced than our own,
Homo sapiens is detectable out to a distance of roughly 50 light-years.
Within that distance are about 5,000 stars, all of which have had the
enviable pleasure of receiving terrestrial television. And each day, a
fresh stellar system is exposed to signals from Earth.
But even if you believe in highly optimistic estimates regarding the
prevalence of cosmic intelligence, it's unlikely that another
civilization exists within 50 light-years. That's too small a distance.
We're no doubt listed in some alien grad student's data tables as a
world with life, but without the footnote indicating intelligent life.
We are the new kids on the block, and so far it's a safe bet that none
of the other kids know we're here.
==============================
73 de Alan
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Brain Cramps
~~~~~~~~~~~~
"Half this game is ninety percent mental."
- Philadelphia Phillies manager, Danny Ozark
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