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Misleading Statements about Optical SETI

Radobs 01

 
The literature on SETI is generally misleading in its rare references to the
optical approach.  Here are a few examples which will help to explain the
difficulty I face when confronting the establishment view.

     In Isaac Asimov's 1979 book on "Extra-Terrestrial Intelligence - The
     First Encounter" (page 263) he says:
     "WITH LASER LIGHT WE COME CLOSER TO A PRACTICAL SIGNALLING DEVICE THAN
     ANYTHING YET MENTIONED, BUT EVEN A LASER SIGNAL ORIGINATING FROM A SOME
     PLANET WOULD, AT GREAT DISTANCE, BE DROWNED OUT BY THE GENERAL LIGHT OF
     THE STAR THE PLANET CIRCLES".  He goes on to say "ONE POSSIBILITY THAT
     HAS BEEN SUGGESTED IS THIS--.  THE SPECTRA OF SUNTYPE STARS HAVE
     NUMEROUS DARK LINES REPRESENTING MISSING PHOTONS--PHOTONS THAT HAVE
     BEEN PREFERENTIALLY ABSORBED BY SPECIFIC ATOMS IN THE STARS'
     ATMOSPHERES.  SUPPOSE A PLANETARY CIVILIZATION SENDS OUT A STRONG LASER
     BEAM AT THE PRECISE ENERGY LEVEL OF ONE OF THE PROMINENT DARK LINES OF
     THE STAR'S SPECTRUM.  THAT WOULD BRIGHTEN THAT DARK LINE . . .". 
     Asimov also went on to imply that a laser system was complicated and
     that no civilization would be expected to use the harder method if a
     simpler (microwave) method is available.

I have shown that not only can large space-based telescopes and probably
ground based adaptive telescopes separate the laser light from the light of
nearby stars, i.e., those nearer than a few hundred light years, but that
working inside a so-called Fraunhofer dark line only produces a modest,
about 10 dB, increase in signal-to-noise ratio.  For a advanced technical
civilization (ATC), a laser transmitting telescope is only SLIGHTLY more
difficult to construct than a microwave transmitting dish, though Asimov
appears to think otherwise.


     A book by Edward Ashpole on "The Search For Extra-terrestrial
     Intelligence", published in the U.K. in 1989, and which contains a
     forward by our own Bob Dixon:

     On page 79 and 80 are several paragraphs devoted to "Signalling By
     Lasers".  At the top of page 80 is the following paragraph which I
     quote in full:
     "THE IDEA THAT ETIs MAY HAVE BEEN TRYING TO CONTACT US WITH POWERFUL
     LASERS HAS BEEN CRITICIZED BY SOME SCIENTISTS WHO POINT OUT THAT LASER
     BEAMS, UNLIKE RADIO BEAMS, ARE VISIBLE.  WE MIGHT THEREFORE HAVE
     EXPECTED OUR ANCESTORS IN HISTORICAL TIMES (WHO WERE A GOOD DEAL MORE
     AWARE OF THE NIGHT SKY THAN WE ARE) TO HAVE NOTICED BEAMS IN THE SKY. 
     YET THERE ARE APPARENTLY NO RECORDS OF ANY SUCH OBSERVATIONS."

This statement, like so many others I have seen is rubbish!  Didn't anyone
sit down to do some calculations to determine relative intensities in
sensible bandwidths?  As I have shown, a hugely powerful 1 GW alien
transmitter that could send "real-time" NTSC/PAL video of 10 light years, is
only about 0.6% as bright as the aliens' star.  The mistake in the above
statement is, as always, to assume signal detection with bandwidths
equivalent to that of the human eye.  From the beginning of my analysis I
have made it clear that we shouldn't expect to see extremely powerful
flashes of light - for what is very dim to our eyes represents a very strong
signal in conventional electronic communication bandwidths.


     A recent paper by Dr. Jill Tarter:

     Five lines from the top of page 192 she mentions the optical approach
     and dismisses it in two sentences.  It is not true for a beamed signal
     that "ANY OPTICAL COMMUNICATION SIGNAL COMING FROM A PLANET CIRCLING A
     DISTANT STAR WOULD HAVE TO OUTSHINE THE STAR ITSELF IN ORDER FOR US TO
     DETECT IT".

I suppose the problem is that some scientists have ventured an opinion
without having checked the numbers, and these statements have been taken up
by non-technical authors who don't have the wherewithal to check the
reasoning.  I have never come across a branch of science (SETI) before which
has been so burdened by so many "mistaken" conceptions.  I would hope that
in future if you see such statements in books (papers or proposals) that you
flag such statements as gross errors.  I would like to hear about other
citations with similar mistaken statements.


Composition Date: December 11, 1990
Revision Date: June 1, 1992
File: RADOBS.01
BBOARD No. 261


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