Seth Shostak, Senior Astronomer, SETI
Dec. 04, 2000
by Seth Shostak - Senior Astronomer
Movie aliens are often like distant relatives: they resemble us in an unpleasant sort of way. This is hardly a surprise. Hollywood creates characters that audiences can identify with, and that’s why its aliens are so anthropomorphic (and why Donald Duck looks more like a human than a duck.)
But appearances aside, cinema aliens have another implausible attribute: they’re nearly always at our level of technical sophistication. We frequently trade gunfire with them or chase them around in dogfights. This is silly, of course. Any beings capable of bridging the vast distances between the stars would be able to clean our clock when it comes to science and engineering. Visitors from other worlds – should any appear – would be enormously ahead of us from a technological viewpoint.
It may surprise you to learn that the same is true for any aliens we might tune in with our SETI experiments.
Why is that? Why will our listening experiments – if they succeed – find only highly advanced aliens?
The reason is this: our chance of detecting societies that are sending high-powered radio signals or intense laser beams our way depends on their average longevity – how many years they stay "on the air." Imagine for a moment that the city you live in is the Milky Way Galaxy. Let’s further assume that you’re looking for signs of life in this urban agglomeration. It’s raining and you can’t walk the streets, so you decide to hunt for company from home by monitoring a short-wave radio. You hope that some of the life out there is on the air, either as radio hams, CB users or whatever.
Clearly, the longer the average time that the broadcasters spend blathering into the microphone, the greater the chance that you’ll stumble into a transmission as you twist your antenna and tune the dial. In SETI circles, this dependence on a society’s technological lifetime is represented by the factor "L" in the famous Drake Equation.
Fair enough. But now take a deep breath and consider some representative numbers. In the next two decades, the SETI Institute hopes to use new telescopes (including the Allen Telescope Array) to check out as many as a million Sun-like stars. If we find a signal in that sample, then we’ll know that approximately one in a million solar-type stars has a planet where broadcasters are active. Suppose that sometime in its 10 billion-year lifetime, every Sun-like star eventually cooks up some sophisticated beings. (This is highly unlikely, of course, but hold on…) Since each of these stars will last for roughly 10 billion years, then every civilization has to be on the air for 10,000 years – on average – in order for one in a million of them to be broadcasting now. If we suppose that only one in 10 of the Sun-like stars eventually produces a transmitting society, then they have to be on the air for 100,000 years for one in a million of them to be broadcasting now. And so forth.
In other words, even if we are wildly optimistic about the fraction of Sun-like stars that produce sophisticated beings, our chances for SETI success in the near term depend on broadcasting activities that last for at least 10,000 years, and undoubtedly much longer.
On Earth, we’ve been transmitting high-powered, high-frequency signals for 50 years or so. That’s 0.5 percent of 10,000 years. In other words, the chances are better (probably much better) than 99.5 percent that any ET we detect in the next decade will be ahead of us, scientifically. Ahead of us by many thousands of years. To them we will be the technological equivalent of club-wielding Neanderthals.
So forget about movie aliens, who always seem keen to engage us in aerial combat or invite us aboard their ships for unpleasant personal experiments. Any aliens we overhear will be considerably beyond that.