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Electromagnetic Signals

So, the search today is for electromagnetic signals. To engage in such a search it is necessary to answer the basic question: What sort of electromagnetic radiation does technology produce? In order to answer this question it is also necessary to determine what sort of electromagnetic radiation nature appears never to produce.

The possible answers could be sought in compressions in the frequency and in the time domains that allow high signal-to-noise ratios. In the frequency domain, nature does not seem to be able to become a coherent emitter.

We have looked at natural radiation and the narrowest feature in the spectrum that we have been able to find at radio wave lengths is associated with an hydroxyl (OH) maser and occupies 300 hertz of the spectrum. There is no electromagnetic radiation found in nature that is more narrow. This is already non-thermal radiation, a non-linear amplification that narrows the frequency down to less than thermal. Yet, with contemporary technologies, we produce signals cheaply that are a fraction of a hertz wide all the time. These are very good detectable signals.

This, then, is where in the spectrum the search for SETI is taking place. Any electromagnetic radiation that is more narrow than 300 hertz is fair game for us to begin thinking about technology.

Now, it may be that what in fact we find is a natural phenomenon and leads to a whole new branch of coherent astrophysics that we did not think was possible. That, of course, in itself would be a good thing.

In terms of the time duration, we have not been able to find a comparable niche. Nature exhibits pulsed emission on a large number of time scales. We cannot find anything that would naturally say to us, “Technology.” So, in place of time scales as such, we are attempting to detect various pulse repetition rates and durations. We have to build our equipment to be flexible enough to look for lots of different temporal signal signatures.

Contributed by: Dr. Jill Tarter and Jim Miller

Cosmic Questions

Are We Alone? Topic Index
What is SETI?

Electromagnetic Signals

Life in the Universe
Detecting Intelligence
Sky Surveys: Serendip IV, BETA and Argus
Targeted Search: Project Phoenix
A Pseudo-interferometer
Plans for the Future


Jill Tarter

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