cw@vaxwaller.UUCP (Carl Weidling) (08/29/84)
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
STEINBERG@RUTGERS.ARPA (09/06/84)
From: Louis Steinberg <STEINBERG@RUTGERS.ARPA> Carl Weidling note that matter transmission does not necessarily imply violating consevation of momentum, but notes that classical conservation of momentum implies that the center of mass of the the universe is not altered while matter transmission does seem to alter it. I have two comments: If momentum is conserved, then you come out of the matter transmitter with the same momentum you went into it with. Since your mass doesn't change (we hope!), your velocity doesn't change. The problem is that the place you transmit to may be moving with a quite different velocity. E.g. if you transmit from one point on the equator to another point on the equator exactly on the other side of the earth, you will come out moving about 2000 mph relative to the ground. Interplanetary MT is even worse. I vaguely remember one of the series of short stories on MT used this device to limit the range of MT for the first few stories. Eventually a method was developed that allowed the momentum to be transfered to some large mass somewhere in the process of transmitting you. (This was the series including the story Flash Crowd.) While conservation of momentum is a well established law, the question about center of mass of the universe is not a real issue. In fact, the term "center of mass of the universe" does not even make much sense in a relativistic universe. Presumably one defines this by taking the position and mass of every particle at some instant. The problem is that there is no consistent way to define "the same instant in time" for two particles that are moving relative to each other at some noticable fraction of the speed of light. -------
hutch@shark.UUCP (Stephen Hutchison) (09/06/84)
<transmitting bug killer now!> | 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. | | Carl Weidlin Well, actually, Larry Niven dealt with the conservation of momentum in all the Known Space series. Matter transmission is defined as impractical outside a gravity well, so interplanetary is out. Then he has the transmitters take all the momentum relative to the receiver and transmit it instead to a large set of heavy barges set in the middle of Lake Michigan. If momentum has to be added, it jerks the barge one way, if it has to be removed then it jerks it another way. Simple. Another series dealing with conservation of momentum is the series "The Journeys of McGill Feighan" whose author I forget the name of. Matter transmission is psionic (read magic for you materialists) and limited by several physical factors. First, a person with the knack for sending matter has a 914 kg limitation on the mass that can be moved. Second, kinetic energy has to be equalized and this is done by drawing or dumping momentum from a "hyperspace" which the transport apparently deals with. Third, the power MUST be trained. If you aren't trained and you have the power, it eventually kills you when you flip to someplace without equalizing kinetic energy. Fourth, you cannot "transmit" to anyplace you haven't been to. So, there is a fleet of ships travelling sublight, reaching various planets VERY SLOWLY. I don't know if you can transmit to a ship or not, the author didn't say. Anyway, most authors I've read who deal with matter transmission toss off conservation of momentum as something that's already been worked out. Hutch