From: yergeau@phy.ulaval.ca (Francois Yergeau)
Newsgroups: sci.space
Subject: Re: Question about SETI
Date: 8 Jan 93 04:10:48 GMT
Distribution: sci
Organization: Universite Laval, Quebec

In article <C0Ho0o.H4G.1@cs.cmu.edu> PHARABOD@FRCPN11.IN2P3.FR writes:
>From "New Scientist", 12 December 1992:
>
>WHEN WILL EARTHLINGS SEE THE LIGHT?
>
>Nigel Henbest believes that NASA's search for ET is on the wrong wavelength

[ long discussion of SETI at radio frequencies deleted ]

>  And if you want to send an interstellar message, radio, as a medium,
>has one overriding drawback. Its frequencies are so low that you can
>transmit information only at quite a slow rate.

Using the 21 cm line of hydrogen, you can transmit about 100 Mbits/s.
Not bad.  Other factors are going to limit you to much, much lower
speed, especially over interstellar distances.

>So let me rewrite the
>history books a bit. Suppose the idea of SETI had come along a decade
>later than it did. The leading edge of technology is now the laser.
>Scientists regard the laser as the ideal mean of communication.

In a fiber, it's pretty good.  But it doesn't go through clouds.

>  But surely a laser's light would be overwhelmed by the brilliance of
>starlight - especially by the light from the sun of the civilisation
>sending the message? In fact, that is not a problem either. 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.

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.

>  Laser communication has two great advantages. Due to its high
>frequency, you can send a lot of information very quickly.

Only if you have enough power available.  To transmit a bit of
information, you need _at least_ a few photons to overcome the shot
noise.  But photons in the near IR have about 100,000 more energy than
photons at 21 cm, so much more power needs to reach the receiver for
_the same_ bit rate.

> 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.

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.

>  For these reasons, NASA spacecraft engineers are planning to use
>lasers to communicate on its future missions to the outer parts of the
>Solar System. Lasers are small, too, so spacecraft will not need large
>radio antennas to communicate with Earth.

It will need a sizeable, delicate and precisely figured focusing mirror
instead.  And downlinks will be cut off when it's cloudy.  But lasers
have advantages for satellite-to-satellite communications.

>This will avoid some
>embarrassing debacles: the Galileo spacecraft, for example, on its way
>to Jupiter, is gagged because its umbrella-like antenna has not

-- 
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    |
Newsgroups: sci.space