The search for extraterrestrial intelligence rests on a fascinating combination of concrete fact and scientific theory. Dr. Seth Shostak of the SETI Institute discusses some of the most basic issues behind listening for signals from advanced civilizations in the vast sea of space.

Q. Why doesn't SETI transmit?

A. It's not for paranoid reasons... not because someone's afraid that if we make our presence known, the aliens will come to Earth to steal our chlorophyll or our women. After all, I Love Lucy is already announcing our presence to neighborhood extraterrestrials.

The reason we don't broadcast is far simpler. Suppose the nearest civilization is 100 light-years away (not so far, astronomically speaking). Our "message" would take 100 years to get to the aliens, and if they deign to reply, their answer would take another 100 years to make the return trip to Earth. Total elapsed time: two centuries. By that time, all the scientists involved with the project will have lost interest and, probably, funding!


Q. So how many star systems has I Love Lucy already reached?

A. I Love Lucy was popular in the fifties, so the earliest shows have travelled 40 light-years into space. There are about 100 stars within that distance, and if there are any inhabited planets encircling these nearby stellar sites, they might be watching Lucy and Desi if they've bothered to build a very large antenna capable of working at the relatively low broadcast frequencies of television (about 100 MHz).


Q. How powerful would the aliens' transmitters have to be in order for us to hear them?

A. This depends on two things: how far away are the extraterrestrials, and how large a transmitting antenna are they using? As a typical example, suppose the nearest cosmic civilization is 100 light-years distant (there are about a thousand stars within that distance, incidentally). And further suppose that their transmitting antenna is comparable in size to the antennas we use for receiving -- for SETI -- here on Earth, a few hundred feet in diameter. Then they would need a 500,000-watt transmitter for us to hear their call. That's not very much; there are radars and TV stations that burn up that many kilowatts here on Earth.


Q. Would the aliens be friendly?

A. Obviously no one knows the answer to this. If we pick up a signal from an alien society, that civilization will almost surely be far in advance of our own. They will presumably have survived the aggressive instincts in their own society, and may have a benevolent view towards others.

On the other hand, aliens that undertake interstellar travel and land in our backyard might be of a different sort. The history of such expeditions on Earth has always been that it is better to be the visitor than the visitee. Consider the Indians of North and South America; their societies didn't survive contact with the Europeans, even in those few instances when the latter weren't deliberately malicious.


Q. Why would any real, detected extraterrestrials be much more advanced then the familiar aliens from sci-fi films?

A. We won't hear anything from aliens that are less technically advanced than we are, that's obvious. But what are the chances that they have just invented radio in the past 100 years, as we have? That's highly unlikely. It would be like getting on the freeway and finding that the first car that passes you has the same license plate number as your own, except incremented in the last digit. It could happen, but most probably won't. Any aliens we overhear will be thousands to millions of years more advanced than our own civilization.


Q. Could we ever understand anything we pick up? If so, could we short-circuit a million years of history, and leap into the future?

A. If the aliens are sending deliberate broadcasts for the benefit of emerging societies, such as ours, then they will make the messages easy to understand. In that case, we might grasp their meaning.

If, on the other hand, we merely happen to "eavesdrop" on internal traffic, there's little chance we'll ever be able to make anything of it. It would be like giving a Neanderthal the output from your modem. He might have considerable cranial capacity, but he'd never understand a bit of it!


Q. What about UFOs? Are the aliens already here? Or stacked up somewhere by the government?

A. The answer is no. This would be the biggest science story of the millennium. If scientists thought there was even the slightest chance that this was true, thousands of them would be working on the problem. They're not!


Dr. Seth Shostak is the Public Programs Scientist at the SETI Institute. He holds a B.A. in Physics from Princeton University and a Ph.D. in Astrophysics from the California Institute of Technology. His research background is in Radio Astronomy, dynamics of galaxies, image processing, and missing matter (!).