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Posting-Version: version B 2.10.2 9/18/84; site watdcsu.UUCP
Path: utzoo!watmath!watnot!watdcsu!herbie
From: herbie@watdcsu.UUCP (Herb Chong - DCS)
Newsgroups: net.sf-lovers
Subject: Re: visible civilization
Message-ID: <1589@watdcsu.UUCP>
Date: Sun, 4-Aug-85 20:14:18 EDT
Article-I.D.: watdcsu.1589
Posted: Sun Aug  4 20:14:18 1985
Date-Received: Mon, 5-Aug-85 07:42:21 EDT
References: <3076@topaz.ARPA>
Reply-To: herbie@watdcsu.UUCP (Herb Chong - DCS)
Organization: U of Waterloo
Lines: 70
Summary: 

In article <3076@topaz.ARPA> milne@uci-icse writes:
>   I'm certainly no expert, but if Earth is brighter than the Sun, in any
>   band, I'll be very surprised.  You might double check with your source, 
>   if you can; also about competition with Jupiter, and possibly Saturn.

one of my first year physics project was to calculate how far away the
earth could be detected by a radio telescope the size of Aricebo.  it
turns out that a signal broadcast at the usual operating power of
Aricebo can be detected at a distance of some 100,000 light years by a
comparable instrument that is pointed toward the transmitter at the
right time.  the signal is good enough such that two way communications
with a 100,000 year delay is possible.

this has nothing to do with brightness per se, but correct
concentration of the broadcast energy.  the cone of transmission would
be about 20,000 or so light years across, but undetectable unless you
happened to be looking in the right direction at the right time.  Some
of the project Ceti transmissions using Aricebo were aimed at the
Hercules cluster some 13,000 light years away.  so, you might say
brightness is a relative thing.

>   There is also the problem of the angular distance of Earth from the Sun,
>   especially from very distant (ie much more than Alpha Centauri) stars.
>   I've neither the time nor the inclination to go through the math, but I
>   suspect it would be difficult, perhaps extremely so, to resolve Earth's
>   signals from the Sun's.

you have to remember that the signals from the sun are more or less
random (like black body radiation), while the signals from the earth
are anything but (contents exempt).  that alone would make anybody
watching take more interest.  something like the Aricebo transmission
would be like a supernova in our galaxy in terms of getting someone's
attention, but they have to be looking in the right direction.  another
thing too are the various nuclear tests that have been undertaken in
the last forty years.  remember that they generate a hefty
electromagnetic pulse that propogates in all directions.  because they
are effectively one time events with no set pattern, they would
probably be ignored as glitches in the instrumentation, but they are
detectable over very large distances, on the order of a few thousand
light years.

>   And remember, as I said in my first posting, to pick up any of Earth's
>   signals at all other than natural ones (and I don't know whether there are
>   any) you have to be within about 50 or 60 light years.  I'm sure no such
>   signals were generated anywhere on Earth more than 60 years ago, or perhaps
>   70, when the first crude recordings were done.  And at 70 light years'
>   radius, those signals, weak as they were to start with, must have
>   attenuated miserably.  So I would place 70 light years as the maximum 
>   radius at which Earth could be detected by radio telescopes, if it could 
>   be so detected at all.

the key thing is that our regular radio transmissions may be lost in
the overall electromagnetic transmissions from other things nearby, but
there are a lot of other things besides commercial broadcasts that
would be detectable from great distances.  the catch is that we've been
only noticeable for the last forty years or so, and only in the last
twenty or so have we actually tried to make ourselves known.  project
SETI spent about 6 months transmitting to various nearby stars with
such a signal power that even over the distance of thirty or forty
light years, a primitive radio would be capable of detecting the signal
if someone were listening at the right time in the right place.

Herb Chong...

I'm user-friendly -- I don't byte, I nybble....

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