Ask Dr. SETI: Why the "Water-Hole"?

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Ask Dr. SETI ®

Chapter 2: Biochemistry

Why the "Water-Hole"?

Dear Dr. SETI:
I am extremely interested in project ARGUS, however I do not understand a couple of things. First, why choose a frequency associated with water? After all, we are carbon-based lifeforms dependent on water, yet we use all types of high and low frequency transmission. You can't take a cell phone or any microwave transmission and directly relate that to water, so why pick the "Water Hole"?

Monty G.

The Doctor Responds:
The "water hole" frequencies are not directly related to water, Monty; that's just a poetic description. In the quietest part of the sky, in the microwave region where interstellar space is most transparent, are two radiation lines which can be seen from anywhere in the cosmos. One is from interstellar hydrogen, the other from hydroxyl. Hydrogen plus hydroxyl forms water under the proper conditions, hence the name. These radiation lines are signposts which are not geocentric, of which any civilization which has radioastronomy will be aware. Since we're already looking for astrophysical phenomena in this region, and it's very well suited to interstellar propagation, why not seek civilizations there as well?