Good article. I have one question however: Why does it seem to be
assumed that the aliens are so much like us? -Steve
From space.com
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.