shiva@duts.UUCP (11/02/85)
> From: Alan Wexelblat <wex@mcc.ARPA> > .... Is it the case that the center of > mass of the universe doesn't move? Is is (theoretically) possible to > calculate our position/velocity w.r.t. this non-moving point? On the contrary, the center of mass of the universe must move, since all the masses in it are constantly moving. Mass is constantly being redistributed hence the center of mass has to move *with respect to any other mass*. It does not make sense to talk about the center of mass moving with respect to anything else, because there is no absolute frame of reference (thanks to Dr. Einstein). As to calculating our velocity with respect to the center of mass at any one instant, well, wouldn't you have to know the position of every other mass in the universe at that instant? You can't do this because of the speed of light (even assuming you could accumulate all that information in some computer) which will only tell you where any particular mass was sometime in the past. Now, having said all this, this is sf-lovers after all, so anything is possible...... Disclaimer: I am not a Theoretical Physicist, although I play one on TV..... "Anything is possible. Unfortunately, most of it is so damn improbable." -- me -- Shiva, Amdahl
ins_amap@jhunix.UUCP (Mark Aden Poling) (11/05/85)
> As to calculating our velocity with respect to the center of mass > at any one instant, well, wouldn't you have to know the position of > every other mass in the universe at that instant? You can't do > this because of the speed of light (even assuming you could accumulate > all that information in some computer) which will only tell you > where any particular mass was sometime in the past. > > Disclaimer: I am not a Theoretical Physicist, > although I play one on TV..... I may regret this, but I'm going to throw two pennies into this mess. If we are to bandy about relativity, then it must be recognised that any appreciable gravity well affects the "rate" at which time flows. Falling into a black hole has the same effect as nearing the speed of light. Things age slower with respect to us in our more normal inertial frame. Time travel will obviously involve mucking about with relativity, since changes in the rate of flow, or even removal from it, will also affect everything else in the inertial frame. If we assume that we can "break" relativity in a localized area, such a break would still have an effect on the frame around it. Therefore, I propose that the gravity well of earth would be sufficient to "capture" the effect produced by time travel. Basically, a traveler would remain in the same inertial frame, and wind up in the same general vicinity as he started. Any changes in the inertial frame from one time to the next would cause displacement from his point of departure. There is also the problem of the non- negligible effect of the moon on the earth-moon system's center of mass, which is really what would catch said traveler, but I'll assume it's solvable. Unfortunately, I still haven't come up with a good reason why any form of time travel shouldn't involve formation of a singularity, which we all know is a bad thing :-). Disclaimer: *Please* don't quote any of this to a physicist. And yes, I *am* using it in a story I'm presently writing. "Ogden understands. Ogden understands everything." -My girlfriend. Mark!