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From: ST401385@BROWNVM.BITNET
Newsgroups: net.space
Subject: Phase conjugation
Message-ID: <8601171730.AA20789@s1-b.arpa>
Date: Fri, 17-Jan-86 11:53:13 EST
Article-I.D.: s1-b.8601171730.AA20789
Posted: Fri Jan 17 11:53:13 1986
Date-Received: Sun, 19-Jan-86 03:47:30 EST
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What phase conjugation is particularly good for is correcting
errors due to atmospheric turbulance, eg., a laser propelling
a rocket being launched from the earth.  Typically this would
use a low power pilot beam from the spaceship to the launch
laser.  The pilot beam would be distorted by the atmosphere,
then phase conjugated and amplified into a reverse beam at high
power.  This power beam is then already be pre-distorted for the
effects of the atmosphere, and atmospheric turbulance exactly
cancels this distortion, resulting in a perfect beam.
This works because the travel time through the atmosphere (at
the speed of light) is small compared to the time scale of atmospheric
turbulance.
     This might also be the mechanism used for a ground-to-space
laser weapon, such as has been proposed for SDI, where a ground
based laser is bounced off a mirror in space, off another mirror
near the target, and then destroys ICBM's.
     For a space-to-space laser, either weapon or spacecraft propulsion,
phase conjugation is much less useful.  There is little or no
distortion due to interplanetary gas, and the distances are large
enough that sending a power signal down the reverse path of a pilot
beam would be pointless, since the ship would have moved in the
interim.
     In fact, there was an article in Analog sometime last year, in the
Alternate View column, discussing phase conjugation.
The author suggested that this process violates the second law
of thermodynamics.  Unfortunately, that is not the case:  while the
entropy of the reversed pilot beam is, in fact, decreased (since the
random components are removed and it is restored to the original,
low entropy state), the entropy of the two pump beams is increased by
the same amount.
     "You can't win, you can only break even at absolute zero, and
you can never reach absolute zero."
                    --GL