Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site v1.UUCP Path: utzoo!linus!philabs!v1!josh 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