SETI Why is it so quiet at the "water hole"?

Larry Klaes (
Wed, 20 Oct 1999 12:20:29 -0400

>X-Authentication-Warning: majordomo set sender to using -f
>Date: Tue, 19 Oct 1999 22:30:03 -0400
>From: (Bob Keeter)
>Subject: SETI Why is it so quiet at the "water hole"?
>I apologize for the "cross posting" but I posted this question on both the
>SCI.ASTRO and ALT.SCI.SETI newsgroups and did not get any answers (a couple
>of "me too's" but no answers), so I thought Id try another forum.
>"Im still struggling with the fascination for the "water hole" in the
>SETI efforts. Its a cute newsbite, "Look for alien life at the water
>hole!", but is there really any reason to look there. I keep hearing
>the explaination that the background noise level is oh-so conveniently,
>much lower at or near the 1.4 ghz "hydrogen and hydroxyl" lines.
>Thats worried me for quite a while. In an environment where there are
>EM signals all over the place, why was this particular (and
>particularly convenient!) band so quiet.
In trying to wriggle around
>with the concepts, I came up with this dingy idea and would like some
>critique from others.
>Unless I am greatly mistaken, the emission line for hydrogen is exactly
>the same as the absorption line (as is the case for the hydroxy radical
>and other elements). Hydrogen is also the most abundant element in the
>universe. If I launch an electromagnetic signal out into space at a
>frequency where the most abundant matter that it might encounter looks
>like a little dipole antenna, the signal will get absorbed and
>re-radiated all over the place (sort of like a bright light shining
>into a cloud bank). Some small portion of this RF energy is lost in
>each absorption and reradiation, so. . . . . would not this in itself
>make the water hole a bad place to go look for ET?

>Along the same lines, if I take any given element signature, i.e. the
>sodium line, and move out far enough the Hubble constant would imply
>that this signal would be shifted into the "water hole".
>using the same faulty logic as the paradox about "why is a night sky
>filled with an infinite number of stars not brightly lit", but I can not
>understand why even the red-shifted spectra of different elements are
>not all over the "water hole", bringing its "noise level" up to the
>other frequency bands. The only explaination that I can see is that
>the local area hydrogen (out to a couple of Light Years) is scattering,
>attenuating or absorbing even these red shifted signals that have the
>unfortunate circumstance to be "red shifted" into the water hole.
>If any of this cock-eyed theory is correct, why are we looking in the
>water hole for the very low level signals of an extrasolar

>Thanks in advance for any ideas!