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Ask Dr. SETI ®

Chapter 4: Psychology

Listening on Pi GHz

Dear Dr. SETI:
Don't know if this is a dumb question, but is there any good reason to look for intelligently generated extra-terestrial emissions in the spectrum at Pi GHz or 3.141...GHz?

David, Pittsburgh

The Doctor Responds:
Not a dumb question at all, David; it forces us to confront our own geocentricities. The short answer is, "no." Now here's the long one:

When we look for "magic frequencies", they must be instantly obvious to any alien civilization, and be based upon information which we have in common. That immediately rules out any frequency based upon any number (even a known number) of GHz. GigaHertz means billioins of cycles per second, so it rules itself out for two reasons.

First, Hertz means cycles per second, and the second is an arbitrary Earth unit for reckoning time. If another civilization has a basic time unit which differs from ours by only one part in a million, and sends on pi GHz (using their concept of a "second"), we'll be fully a kiloHertz off (and thus probably never hear it) if we try listening on pi of our kinds of GHz.

Second is the problem of "Giga," which means ten to the ninth power, or thousand million. To even use the number "Giga" implies base ten math, which I believe is what all Earthlings use because we have ten fingers (even in Pittsburgh). What if the aliens have seven fingers, and use base seven math? Then their "Giga" would be seven to the ninth power, a very different number of cycles indeed.

So we have to get around these concepts of Earth time measures and Earth-based numerical expression. The Hydrogen and Hydroxyl lines qualify as valid "magic frequencies" because they can be observed from anywhere in the universe, and are in the same place in the electromagnetic spectrum no matter what time measure or number base a given race uses to describe them. Pi is also universal. Regardless of your system of measure, the ratio of circumference to diameter of a circle is the same (in fact, by definition, "ratio" makes it unitless). Combining the above two justifications, the idea of using a frequency of hydrogen times pi, as stolen in the film "Contact" from an actual SETI study performed by Dr. David Blair at Parkes in 1990, is also valid. We would call the frequency 4.462332 GigaHertz; "they" would call it 7.964305 UrmaPlatz, but we'd both be looking in the same place.

So there's nothing magic about listening at 3.14159265358979323846 GHz. It's just another point on the radio dial. On the other hand, there's no particularly good reason why they wouldn't transmit there, either. The only wrong setting for your SETI receiver is "off." So it couldn't hurt to look!

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