Tarzan's Guide To The
Search For Alien Intelligence
By Dr. Jean-Marc Perelmuter

The SETI Institute focuses on the Search for Extraterrestrial Intelligence. There are two key words here: intelligent and extraterrestrial. If you utter them in the same breath you will be transformed into Dr. Jekyll and Mr. Hyde. At times they make you sound like a weirdo, and at other times like an insightful visionary.
Corbis Tarzan wasn't alone in the world, but he matured with no idea that people existed
This is because encountering extraterrestrial intelligence is highly unlikely in anyone's daily routine, and also highly suggestive of a person's imagination. For whatever the reason " psychological, socio-political or purely scientific " the issue of extraterrestrial intelligence resonates deeply in our minds. But how real is it?
Picture Tarzan in the middle of the African jungle with a vine, a fig leaf, and a tree house. His mission: to communicate with the nearest intelligent beings. So far, chimpanzees have paid him a measure of attention but he's run out of bananas. Exploration is impractical because of the density of the vegetation. He's not even certain there's anyone beyond his neighborhood trunk. So unless he can think of a brilliant idea, he's stuck babbling with the chimps.
Our situation in the cosmos isn't unlike Tarzan's. Humanity is uncertain whether other intelligent species exist beyond its own planet and exploring nearby systems takes too long compared to the human life span.
There are many trees in the jungle, thinks Tarzan, any one of them might host another tree house or a habitable cavity. Tarzan will find himself imagining a thousand different kind of tree trunks and the species living in them. Maybe he'll start counting the number of trees and speculate on the probability one is inhabited by intelligent life.
Astronomers are counting the number of stars in our galaxy. The latest estimate puts that number at two to three hundred billion stars. The probability that a stellar system hosts intelligent life is quite uncertain. The probability that an intelligent species " if any " is attempting to communicate with us is even more uncertain.
NASA A galaxy is capable of hosting millions of intelligent life forms
There are many unknowns: the percentage of stars with a planet, of those, how many support life or can, and of those, would the inhabitants be able to communicate or interested in communication, and so on. Estimates range from a dozen to a million possible intelligent extraterrestrial civilizations in the Milky Way Galaxy alone.
If Earth is any example, one planet out of nine hosts life, and one species out of billions has a higher intelligence. Given these numbers, even with 300 billion stars the estimate of advanced extraterrestrial civilizations is a handful, if that. In fact, given these numbers it seems highly unlikely that we would even exist. The strongest argument for intelligent extraterrestrial life is that we are here. This argument is moot if you believe humanity is special.
So we speculate on the numbers. One million advanced civilizations in the Milky Way implies an average separation on the order of 500 light-years. That is, at the speed of light (One light-year is the distance traveled in one year at the speed of light or 5.9 trillion miles. 1 trillion = 1,000 billion) it would take 500 years to send a message and another 500 to get a reply.
It is more efficient at this stage of the game to send waves rather than astronauts to far-flung systems. In fact, it is even more efficient to plainly listen. SETI is about listening efficiently and sending messages efficiently.
In the jungle Tarzan might light a fire, but there's little chance someone will see it because of the vegetation. He might try a thousand different things but his best bet is to hit a hollow tree trunk or scream audibly. The problem is how to distinguish his voice from all the jungle noise.
Since the recipient's unique trait compared to the environment is intelligence, then Tarzan's sound will have to be intelligent. Possibly, his timbre will follow a mathematical sequence or represent a universal constant (such as the speed of light). Tarzan opts for sound inflection, being smart enough to emit his message in the audible part of the spectrum.
Because hydrogen is one of the most common elements in the universe, and it naturally emits at a very specific radio-frequency, astronomers expect intelligent signals in the neighborhood of that frequency. So why haven't astronomers detected a greeting message yet?
Perhaps the answer lies in a recent discovery. In 1998, a previously unknown tribe of natives was spotted in the Brazilian jungle. Even with supersensitive satellites and industrial activity barely 60 miles away, we were unaware of their existence for almost the entire 20th century. Likewise, although bombarded by radio and television waves, they never detected them.
So whether we are the Amazon natives of the cosmos, or the advanced civilization we claim to be, one thing is for certain: It's not because we're alone, that someone isn't out there.