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From: El Kabong <twang@the.noodle>
Newsgroups: talk.origins
Subject: Re: New SETI search
Date: Tue, 27 Aug 2024 10:04:24 -0700
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RonO wrote:
> On 8/27/2024 8:27 AM, El Kabong wrote:
> > 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.
> 
> This example is looking at other galaxies.
> 
> Ron Okimoto

Yes, I missed that obvious statement in the article.
Between that, and the wide field nature of the receiving
antenna, the aliens would have to broadcast a signal with
the power of a star to reach us.  I wish them luck.