Can Aliens Find Us?

Oct. 23, 2003
by Seth Shostak - Senior Astronomer

Its 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 Littrows 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 theyve 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 Earths 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. Its not very tightly focused, so even though much of the broadcast energy spills into space, its fairly weak by the time it reaches another star system. Consider one of our early TV programs just washing over a planet thats 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. Thats a lot of antennas, and an unsightly concept. But its 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 theyd 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, its unlikely that another civilization exists within 50 light-years. Thats too small a distance. Were no doubt listed in some alien grad students 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 its a safe bet that none of the other kids know were here.