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
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