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From: El Kabong <twang@the.noodle>
Newsgroups: talk.origins
Subject: Re: New SETI search
Date: Tue, 27 Aug 2024 06:27:53 -0700
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RonO wrote:
> On 8/27/2024 2:11 AM, Ernest Major wrote:
> > On 27/08/2024 06:24, El Kabong wrote:
> >> Previous searches at Aricebo and other sites looked for
> >> alien signals at 1420 Mhz.  They picked that frequency
> >> because it is a hydrogen line.  The thinking is that
> >> aliens would more likely broadcast there than an
> >> arbitrary frequency.  It never made sense to me because
> >> the signal will be attenuated by any hydrogen lying in
> >> the path, and because if you tune in to the hydrogen
> >> line, you'll find... hydrogen noise!
> > 
> > The argument is that the 1420 MHz line is less arbitrary, and is not 
> > absorbed by interstellar dust.

Any radio signal is less absorbed by dust than optical
wavelengths.

1420 was probably a good pick, but it does have that
drawback.

> If the aliens had broadcast at 1420 MHz what would be the frequency that 
> we would detect in an expanding universe?  I realize that some galaxies 
> are moving towards us, but the red shift indicates that most things are 
> getting further away from us in all directions due to the Big Bang.

Any radio signal we receive will be redshifted.  But we
are not looking for extragalactic signals, they would be
way too faint.  We are looking for something here in our
own neighborhood of the Milky Way, around 10k lightyears
max.  At that distance the redshift is measurable but
unimportant, even for a narrow-band receiver.