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C REC  PAGE   DESCRIPTION
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C00002 00002	rocket[e87,jmc]		Black hole rockets
C00004 00003	Rearranging the universe
C00008 ENDMK
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rocket[e87,jmc]		Black hole rockets

	Consider a binary star --- performance will be best if
both components are black holes.  We wish to propel the system
through its galaxy and use it to collect matter to be moved
towards our galaxy.

	If we drop matter into the system from infinity with
the right phase and impact parameter, it will be expelled in
a fixed direction at a velocity a few times the mutual
orbital velocity of the stars.  (If one of the stars is small
compared to the other, the velocity will be about 3.1 times
the orbital velocity of the smaller around the larger).
Conservation of momentum assures that the system will be
propelled in the opposite direction.  The energy is at the
cost of the mutual orbital velocity.  This energy can be
repaired by dropping in additional matter so as to collide
with one of the bodies from behind, thus restoring its
energy in orbit.  Random phases for this matter assures that
it has no net momentum effect.  The amount of mass that
must be dropped in is of the same order of magnitude
as that of the stars if they are to obtain an increment of
velocity of the same order as their orbital velocity.
I haven't calculated the effect of this process on the eccentricity of the
orbit.
Rearranging the universe

	The question of whether the universe will continue to
expand indefinitely or will close up and collapse again might
be answerable with the question, ``Which would you prefer?''.
Our discussion of the question begins with reasons why humanity
might want to interfere with the evolution of the universe
and continues with scientific and technological questions about
how it might be done.  The conclusion is that most likely it
is both possible and desirable.  However, the time scale for
doing anything about it is billions of years.

	According to the cosmological theory that has the
present greatest consensus, the universe will expand indefinitely
and humanity will eventually perish from lack of available
energy.  However, like a person, the species will probably want
to live as long as possible, and this is very long indeed.
Present technology would probably keep humanity alive on this
planet for a few billion years, i.e. as long as our sun
maintains about its present output of energy.  However, most
of the energy in the universe is gravitational, and if we can
figure out how to tap that, we might last from $10↑30$ to $10↑60$
years, depending on how clever and determined our descendants
turn out to be in collecting this energy and in exercising
economy.

	Even $10↑30$ years is so large that it is hard to imagine what
humanity will do with it.  Also there are about $10↑11$ suns in a galaxy
and about $10↑11$ galaxies, so there might be $10↑22$ different
civilizations.  Moreover, we will be able to create new forms of
intelligence by biology and/or by computer science.  This means that there
may be enormous variations in what all those potential intellgences might
want to do in all that time.  Some of them might not be interested
in long term survival, but it seems likely that the long term survivors
will be those that are interested in survival.  This article is devoted
solely to the survival question, but the reader should keep in mind that
for most of the time, mere survival will only be one of the minor
interests and activities of our descendants --- biological and
artifactual --- even if they undertake the large scale rearrangements
to be discussed.