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From: cw@vaxwaller.UUCP (Carl Weidling)
Newsgroups: net.sf-lovers
Subject: Conservation of Momentum (and MT)
Message-ID: <161@vaxwaller.UUCP>
Date: Wed, 29-Aug-84 16:19:19 EDT
Article-I.D.: vaxwalle.161
Posted: Wed Aug 29 16:19:19 1984
Date-Received: Tue, 4-Sep-84 08:11:32 EDT
Organization: Varian, Walnut Creek, CA
Lines: 59


	This item was inspired partly by the talk on matter
transmission, but is not really about matter transmission.
I'll mention a tie in between the two at the end.
	One problem with interstellar travel that I've hardly ever
seen addressed (maybe because sf-writers aren't LOOKING for problems,
just trying to explain away the ones people are already likely to know)
is that of conservation of momentum.  Isaac Asimov did make some use of
the concept in 'The Gods Themselves', but otherwise it has been ignored,
even by people like John Campbell, who generally seems very conscientious
about scientific accuracy.  In one of his novels, I think it was
called 'The Mightiest Machine', his characters have a device that makes
the random motion of molecules in an object become un-random; it can be
used to make an object shoot of in a given direction, the energy being
provided by the heat energy of the object, which becomes extremely
cold. Nothing shoots off in the opposite direction however, so momentum
is not conserved.
	A SF writer doesn't need to go around destroying planets to provide
energy to move a space ship around all the time, he or she only needs
to be able to borrow some for the voyage, then put it back.
	If a rocket takes off for Alpha Centauri, it gets momentum by
providing momentum to exhaust gases in the opposite direction.  When the ship
gets where it is going it loses this momentum by giving it to more exhaust
gases.  Now the rocket ship is in a different place, but you have two
clouds of gas hurtling away from each other.  Physics majors can check
me on this but I do not believe that the center of mass of the universe
ever changes during all of this.
	Suppose somebody invents an anti-gravity device, and his space ship
"pushes" against a planet to get going for Centauri, then pushes against
another planet to stop.  Now these two planets are moving apart ever so
slightly, or not so slightly if the ship got up to extremely high speeds.
So far no energy is recovered or put back, but maybe the ship could use
its anti-grav in a reverse direction and pull back on the same planet
that it started from, which is what normal gravity does, the kinetic energy
of the two objects has to go somewhere, maybe it can be recovered and stored.
Now the ship is at point B, nothing is hurtling away from anything else
anymore than it was before, and presumably the center of mass of the
universe is still the same because the planet has moved over a little
bit to balance the greater displacement of the ship. How soon the
ship can arrive at a certain place is determined by how much energy it
can beg borrow or steal for the duration. If the ship put a lot of energy
into the motion, and got there in a short period of time, would the
distribution of mass be the same as if the voyage had been more leisurely?
	A similar situation would be a car with batteries and electric
motors/generators that could brake by recharging its battery.  Except
for friction losses it could start and stop and travel all around the
planet, momentum being transferred back and forth between planet and
car.  Thinking about this I can't help but wonder if momentum isn't
the most primitive concept in physics.
	What is the tie-in with matter transmission?  Well, one or maybe
two people pointed out that one way around the problems brought up about
matter transmission was that two places in the universe could be juxtaposed
by bending the universe through another dimension and having the person or
whatever just step across the now short space. Momentum could also be
conserved by this method, but would the center of mass of the universe
still be the same?  What other implications would there be?
				Regards,
				Carl Weidling