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Space Topics: Search for Extraterrestrial Intelligence

What to Expect when E.T. Calls

by Amir Alexander and Eric Heien

If you are like most SETI@home users, you know this fantasy well. There you are, working at your desk, while SETI@home silently crunches data on the computer screen beside you. Suddenly, from the corner of your eye you notice some unusual graphics on the screen. You look up, and there it is! An unmistakable signal from an alien civilization, clearly visible on your computer screen. Right there on your home computer you have discovered ET! Your place in history is assured…

But wait a minute: how will you recognize ET's signal when you see it? If you don't know what to look for, that signal you've been waiting for will pass through your screen unnoticed, and you will miss your rendezvous with destiny. Therefore you must be ready and know in advance what to look for in an alien transmission. For those of you who want to be well prepared for "The Moment" we offer this page.

E.T. Watch
E.T. Watch
An anxious SETI@home user awaiting a signal from the stars.

What will an alien signal be like? The truth is we don't know. The first extraterrestrial civilization to make contact will likely be far more technologically advanced than humans. We can scarcely imagine their thought processes, or their reasons and methods for communicating with us. All we can do is make some educated guesses based on our own knowledge and technology. And even if we are wrong about the aliens' reasoning, we can still hope that they will try to tailor their signal to our own naïve expectations.

Our first assumption is that the signal will be received at or around the frequency of 1420 MHz. This is the spectral frequency of hydrogen, the most common element in the universe, and therefore a universal reference point for different intelligent species. The arguments about this frequency have raged ever since Philip Morrison and Giuseppe Cocconi first proposed it in 1959. But although we don't know for sure that the aliens will hail us at that frequency, it does seem like a plausible place to start. Most SETI projects have proceeded on this assumption.

Our second assumption is that the signal will be focused on a very narrow frequency band. This is because a wide band signal is unnecessarily wasteful in energy that could be better used to enhance the strength and range of the signal: the wider the frequency band, the weaker the signal. In addition, a narrow band transmission will be easily recognized as artificial, as against a naturally occurring signal. No known natural radio signals are narrow band.

Finally, we assume that the signal will be initially weak, gain in strength, and then fade away, all in the span of 12 seconds. Its power curve will resemble the bell-shaped graph known as a "gaussian." This is because it takes 12 seconds for the Arecibo radio telescope's "beam" to scan any given point in the sky. During that time span an alien signal will initially be at the edge of the reception beam, and therefore weak; gain in strength and ultimately peak when the beam is aimed directly at the signal's source; and finally fade away as Arecibo moves on to search other points in the sky

A perfect SETI@home gaussian
A perfect SETI@home gaussian
A narrow band transmission with a gaussian power curve at 1420 MHz, as it would appear on SETI@home. Click on the image to view an AVI movie of what it would look like on your screen. Note the the SETI@home program analyses the signal several times over at different resolutions. Credit: The Planetary Society

Has a narrow band transmission at 1420 MHz with a gaussian power curve ever been detected by SETI? Yes, at least once. It was recorded at the Big Ear radio telescope at Ohio State University on the night of August 15, 1977. Jerry Ehman, a volunteer and a professor at a local university who was checking the computer logs the following morning was so startled by what he saw that he jotted "Wow!" next to the signal printout. The signal, the most promising radio transmission ever detected by SETI, became enshrined as the "Wow!" signal.

No satisfactory explanation has ever been found for the "Wow!" signal, and repeated attempts to recapture it have failed. It may have been Earthly radio interference coincidentally mimicking a message from space, or it may have been an unpublicized, possibly military spacecraft launched by one of the superpowers during the cold war. Or it might have been the "real thing" - a genuine message from an alien civilization.

Although we may never know what the "Wow!" signal really was, it is still our best indication of what an alien transmission might be like. When it was detected in 1977, the "Wow!" signal was recorded as a dry series of numbers and letters - 6EQUJ5 to be exact. But what would it look like if it were detected today at Arecibo, and sent out to SETI@home users? What would you see on your screen if the "Wow!" signal passed through it? Well, something like this:

The
The "Wow! Signal as it would appear on SETI@home
Click on the image to view an AVI movie of what the signal would look like on your screen.
Note that in reality the "Wow!" signal could never be captured on SETI@home, becuse it originated in a part of the sky that is not covered by the Arecibo radio telescope. Furthermore, the signal bears the mark of the radio telescope that detected it, Big Ear. Whereas Arecibo's beam takes 12 seconds to cross any given point in the sky, Big Ear, with a wider beam, takes 37 seconds. As a result, the "Wow!" signal's gaussian is three times as wide as a genuine SETI@home signal would be.

As you can see in the graphics, the "Wow!" signal is so strong that it completely suppresses the normal transmissions recorded around its frequency. The signal is, in fact, so much stronger than the background noise that it doesn't even look much like a gaussian, but rather like enormous spikes towering above the background signals.

So be prepared, SETI@home users! If those spikes show up on your computer screen, you may be looking at a new "Wow!" signal. And this time, it just might prove to be "the real thing..."