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Ask Dr. SETI ®

Chapter 1: Astrophysics

Hallmarks of Artificiality

Dear Dr. SETI:
My interest in SETI is very narrow. I am interested in how a signal from an intelligent source could be recognized. What sort of characteristics would one look for? I am particularly interested in any examples of theoretical signals that would certainly be generated by an intelligence.

I am not interested in how to filter out the noise and other known, naturally generated signals. I am only interested in how to identify an intelligent signal from what's left after the other stuff is filtered out. I would like some objective standards.

I would be grateful if you could direct me to where I could find this information. Thank you for any help you can give.

Kirk, California

The Doctor Responds:
If your interest is very narrow, Kirk, so is the signal we are seeking. The current thinking is that we believe the hallmark of artificiality to be narrow bandwidth. The narrowest known natural radiation spectra are hundreds of kiloHertz wide. A stable signal component occupying only a few Hz would be believed clearly artificial, and (we presume) intelligently generated. To determine its extra-terrestrial origin, we then look at its Doppler signature as viewed from two or more locations simultaneously (this test eliminates terrestrial, aircraft and satellite interference, as well as hoaxes). The technique was briefly discussed in a recent editorial.

Of course, we could be all wrong about this. SETI may well stumble across an extremely narrow-band source of natural radiation. Wouldn't the discovery of a previously unknown astrophysical phenomenon be almost as exciting as a valid SETI hit?

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