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C00002 00002	Dear Ron:
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Dear Ron:

	I found your book very readable, and I think it will do  well
for  a  popular  audience.   There  are a few places where at first I
thought the mathematical exposition was superfluous  and  might  lose
the  unmathematical  reader,  but now I think these should be left as
they are as a sign that these matters can be discussed rigorously and
are  not  just  matters  of  opinion.   Enclosed with this note are a
number of remarks tied to specific places in the book. Besides  this,
I have a number of more general comments:

	1.  It  is  difficult  to get a clear idea of a more advanced
civilization than ours, but one way to try is to  imagine  what  ours
may  be  like after many thousands of years of progress. There may be
radically new  laws  of  physics  discovered  that  permit  presently
impossible  technical devices.  Thus, an advanced civilization may be
able travel faster than light or to examine distant planets in detail
without  travelling.   However,  while some radically new discoveries
are  likely,  it  is  pointless  to  base  speculation  on   specific
possibilities  for  which  there  is  no present evidence such as the
above examples.  Therefore, I shall consider an  advanced  technology
based on present science.

	The  second  major  question  is whether the present level of
unity  of  human  civilization  will  increase  or  diminish.    Most
speculations including yours seem to assume that either nation states
will vanish or will remain similar  to  each  other  in  goals.  This
allows   reference  to  what  humanity  decides  to  do.   The  other
possibility is that space travel will allow a renewed  fragmentation.
When  I  rad books like Rawls's \F1Theory of Justice\F0, I think that
if his super-equalitarian society came about, people who think  as  I
do  would  flee  to space and even launch interstellar expeditions to
avoid its uniformity, moralism, and dullness. After the \F1Shuttle\F0
becomes  available,  I  can  imagine  small, privately financed space
efforts that would produce extremely austere space  colonies  fleeing
socialism.

	If  society  fragments,  then  your  question is not what the
average will do about interstellar communication and travel, but what
the most adventurous will do.

	In  my  opinion, interstellar colonizing expeditions from the
earth are likely within 200 years.  The most plausible technology  to
me is not Dyson's Orion rocket, but a nuclear powered ion rocket with
an exhaust  velocity  increasing  during  the  journey  in  order  to
optimize  the  use  of power and exhaust mass.  Such rockets can also
reach the nearer stars in a few hundred  years  with  plausible  mass
ratios  and  should  be  buildable  for  a few billion dollars within
twenty years and a few hundred million later.  They  require  no  new
science  and  very  little  new  technology.  They do require a small
population to pass  on  its  traditions  and  goals  for  ten  or  so
generations, but this is within the capacity of many tribal societies
today, and these  expeditions  will  have  much  greater  information
resources  to  help  them.   The  only  thing  likely to stop it is a
uniform society on earth  that  will  make  secessionist  expeditions
illegal.

	If   occurs,  then  secondary  pioneering  might  occupy  the
habitable  stars  in  the  galaxy,   barring   contact   with   other
civilizations,  within  a  few  million  years.   Once it starts, the
galaxy  would  be  dominated  by  those  societies  with   the   most
expansionist values.

	From  that point of view, it is worthwhile to ask what is the
highest technically feasible rate of expansion. This would  arise  as
follows:  from  an  initial  planet,  expansion into the solar system
occurs as fast as possible utilizing the matter of the  solar  system
and  organizing  it  into  people  and  machines.   As this expansion
occurs, a  substantial  part  of  the  resources  are  put  into  the
manufacture of spaceships that will travel as fast as possible to new
solar systems to continue the process.  These spaceships  would  have
very  high mass ratios and would travel at as large a fraction of the
velocity of light as the technology  permitted.   When  such  a  ship
reached  a  star  it  would  decelerate  and  put in orbit as small a
machine as would be capable of self-reproduction and taking over that
solar system.  Such a machine might well be smaller than a human, and
therefore, humans might go along only as fertilized ova, or even more
abstractly as genetic information.

	It  is  hard  to  imagine that there would be no expansionist
races or races that might create expansionist machines.   This  leads
me to the belief that humanity may well be very early in spite of the
\F1a priori\F0 improbability of that proposition.

	2. Imagine a very powerful civilization with  a  very  strong
desire  to  communicate.  The best way to do this that I can think of
is to modulate the light of a very small bright star, e.g. a  pulsar.
This  can  be  done  by  englobing  it with satellites in the form of
Venetian blinds.  Modulation can be done at  frequencies  of  several
hundred bits per second if the individual slats are small and this is
consistent with the smearing caused by the 20 mile (say) diameter  of
the  system  and the velocity of light.  The distance at which such a
signal would be detectable depends on the telescopes of the receiving
civilizations.   The  limit  is  when not enough quanta arrive at the
collecting mirrors of the receivers.  Perhaps the technique is better
employed  at radio frequencies.  Considering how what fraction of the
people want to be astronomers, we can imagine  a  billion  dollars  a
year  being  spent on star surveys within a few hundred years if some
fraction of our civilization  chose  to  expand  into  interplanetary
space.  How far could they then carry out a pulsar survey in how much
time? Our determined communicators could enhance their  detectability
by  shooting off a peculiar supernova every few thousand or every few
million years depending on their resources and their impatience. This
would  cause  moderately  advanced civilizations to concentrate their
light collecting power in the vicinity of the  peculiarity  and  make
probable  their  detection  of  the  modulated  neutron  star. As you
suggest the signal put out by the neutron star would  be  time-shared
among   signals   merely   demonstrating   artificiality,   a  signal
establishing a communication language, and signals of various lengths
containing  the  messages.   Of  course, for communication within the
galaxy, supernovas are not necessary, and  there  may  be  a  cheaper
anomaly  sufficient to establish artificiality.  If the communicators
are aiming at the second millenium of  a  technological  civilization
rather  than  the first, they can depend on the recipient having very
good star catalogs and having very  sharp  criteria  for  recognizing
artificiality.  My memory for this is not very good, but I think that
we would have to go about 1000  light  years  to  find  a  pulsar  to
modulate  and  that  a galaxy wide catalog of pulsars not obscured by
dust should be available within  a  century.   If  the  communicators
found  themselves  in  a  dust  cloud,  they  might  have  to  use an
exclusively radio means of communication.