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From: josh@v1.UUCP (Josh Knight)
Newsgroups: net.astro
Subject: Re: Re: StarDate: March 7 Project Sentinel's Anniversary
Message-ID: <135@v1.UUCP>
Date: Sat, 9-Mar-85 18:47:00 EST
Article-I.D.: v1.135
Posted: Sat Mar  9 18:47:00 1985
Date-Received: Tue, 12-Mar-85 20:05:45 EST
References: <1065@utastro.UUCP> <23031@lanl.ARPA>
Organization: IBM Research, Yorktown Heights, N.Y.
Lines: 37

> I ask
> because it occurs to me that frequencies such as "the radiation frequency
> of neutral hydrogen atoms", while attractive as a physical constant,
> seem to me to be as unlikely for use in everday communications by an
> "extra-terrestrial" as they are here on earth.

The Search for Extra-Terrestrial Intelligence (SETI) has received a great deal
of attention from some very knowledgeable folks.  A good compendium of this knowledge
(I may be a bit out of touch) is NASA SP-419 "The Search for Extraterrestrial
Intelligence" with a section on why one would look at the "water hole" (another
popular place to look besides the neutral hydrogen frequency) by Bernard Oliver:
"The Rationale of a Preferred Frequency Band:  The Water Hole".  The frontspiece
says it is available from the Superintentdent of Documents, U.S> Government Printing
Office, Washington, D.C. 20402, stock number 033-000-00696-0.  Eugene Miya might
be able to help more, the document was "prepared" at NASA Ames.  There was also an
earlier study of how to look for extra-terrestrial life: "Project Cyclops: A
Design Study for Detecting Extraterrestrial Intelligent Life", also from NASA Ames,
CR 114445.  It claims to be available to the general public from Dr. John Billingham,
NASA/Ames Research Center, Code LT, Moffett Field, California 94305, but that was
10-12 years ago now.

The basic reason for looking at one small frequency range is that you need to know
where to look.  You have to look at lots of places in the sky and, if you are to be
able to detect the signal, it needs to be a relatively narrow frequency band.  Radio
signals propagating through interstellar space propagate at a rate that depends on
frequency so signals with a large frequency band "disperse".  The amount of dispersion
depends on the distance travelled, so that if you weren't using narrow band signals,
you'd need to know the distance to the source as well as the direction.  There is also
"noise" (signals from sources besides extra-terrestrials) that needs to be avoided,
and it turns out that you want to look in the range of 1-10 GHZ (the neutral hydrogen
frequency is about 1.42 GHZ = 1,420 MHZ).

The opinions expressed are my own, not those of my employer.

		Josh Knight, IBM T.J. Watson Research
    josh at YKTVMX on BITNET, josh.yktvmx.ibm on CSnet,
    ...!philabs!v1!josh