From: PHARABOD@FRCPN11.IN2P3.FR
Subject: Question about SETI
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Organization: [via International Space University]
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Fri, 8 Jan 1993 14:59:51 GMT

In answer to Francois Yergeau (8 Jan 93 04:10:48 GMT):

>>                                                           A laser
>>crams all its energy into just one specific wavelength. If you are
>>receiving the signal, you split the light into a spectrum. Now stretch
>>out the spectrum. The whitish light from the star is diluted more and
>>more as it is stretched, while the single narrow spectral line from the
>>laser keeps its intensity. With enough stretching of the spectrum, the
>>laser will eventually stand out clearly. (N. Henbest)

>Wrong.  No laser has an infinitely narrow linewidth.  Even if you could
>build one, you would induce a finite linewidth by modulating it to
>carry your message.  Thus spectral dispersion stops being beneficial at
>some point, and if you haven't recovered your signal by then, you're
>dead. (F. Yergeau)

Rather obvious. I think (at least, I hope) that Nigel Henbest was making
some kind of journalistic approximation. Most New Scientist's readers
are not professional scientists, just amateurs.

>> Laser beams are
>>also narrow: whereas a radio signal spreads out as it travels through
>>space, diluting its power all the way, you can use comparatively
>>little power with a laser because it does not spread out. (N. Henbest)

>Completely bogus.  All electromagnetic beams diffract.  In fact, you
>can make a radio beam just as well collimated as a given laser beam;
>you just need a much larger antenna. (F. Yergeau)

IMHO, Nigel Henbest knows that too, since he wrote previously in
his article: "Television is increasingly being transmitted [by cable
or] by satellites that beam only onto a small region of the Earth,
with no wastage into space." But maybe he contradicts himself in
the article (as journalists do too often).

>Francois Yergeau (yergeau@phy.ulaval.ca) |  De gustibus et coloribus
>Centre d'Optique, Photonique et Laser    |  non disputandum
>Departement de Physique                  |     -proverbe scolastique
>Universite Laval, Ste-Foy, QC, Canada    |

It seems that you have about the same credentials as Stuart Kingsley:

<     Stuart A. Kingsley is a fiber optic consultant and Director of the
< world's first Amateur Optical SETI Observatory.  He received a B.Sc.
< Honors and Ph.D. in Electronic and Electrical Engineering from The
< City University, London (1972) and University College London (1984),
< respectively (England).  He worked at Battelle Columbus Division as
< Principal Research Scientist and then as Senior Research Scientist from
< 1981 to 1987.  He has been involved in producing a variety of fiber-
< optic sensors, including fiber-optic rotation sensors.  He invented
< the fiber-optic line-stretcher and fiber-optic line-squeezer phase
< modulators that are now important components in fiber-optic sensor
< systems.  He shared a 1984 Rank Prize in Optoelectronics for pioneering
< work on fiber-optic sensing.

It seems also that you disagree with him (see EJASA, January 1993).

J. Pharabod
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