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

Billy Brown (
Wed, 20 Oct 1999 14:08:02 -0500

Larry Klaes wrote:
> >I apologize for the "cross posting" but I posted this question on both
> >SCI.ASTRO and ALT.SCI.SETI newsgroups and did not get any answers (a
> >of "me too's" but no answers), so I thought Id try another forum.
> >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.
> >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?

This is the reason why the water hole exists in the first place. Over long distances radio energy at this frequency tends to get absorbed, so the background noise is low. However, interstellar hydrogen is too tenuous to block signals over shorter distances (i.e. tens or hundreds of LY). Since SETI mostly concentrates on nearby stars it makes sense to check this frequency.

> >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".

The more red-shifted a signal is, the further away its origin lies. Signals from very distant sources pass through millions of LY of space, which serves to filter out any signal that hydrogen can absorb.

> >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
> >civilization?"

A better question would be "Why are we so intent on convincing ourselves that extrasolar civilizations would be hard to spot?"

But we just had that argument a few weeks ago, so its probably too soon to start another round.

Billy Brown, MCSE+I