wex@mcc.arpa (10/30/85)
From: Alan Wexelblat <wex@mcc.ARPA> Thanks to those who replied to my earlier posting on time-travel. One thing still puzzles me though: 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? People seem to talk about our universe as expanding, but exapanding away from what? --Alan Wexelblat WEX@MCC.ARPA
demillo@uwmacc.UUCP (Rob DeMillo) (11/05/85)
> From: Alan Wexelblat <wex@mcc.ARPA> > > Thanks to those who replied to my earlier posting on time-travel. > One thing still puzzles me though: 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? People > seem to talk about our universe as expanding, but exapanding away from > what? > --Alan Wexelblat > WEX@MCC.ARPA If we are to believe Einsteinian physics...there is no center of the Universe. When astrophysicists speak of the Universe expanding, they are not talking about galaxies fleeing away from a common center. What is being talked about is the fabric of spacetime (one word, please) being expanded, and everything on it is going along for the ride. The common analogy that is used is: consider the Universe as the dough for raisin bread. The spacetime is the dough, and the raisins are everything else. (i.e. galaxies, stars, people, small marine mammals, whatever) As you let the dough rise, or expand, it appears (regardless of what raisin you happen to be) that all the other raisins are rushing away from you. The result: everyone is rushing away from everyone else, but there is no common center. A subtle, but important, difference.. -- --- Rob DeMillo Madison Academic Computer Center ...seismo!uwvax!uwmacc!demillo "...I suppose you think the concept of a robot with an artificial leg is amusing?" -- Marvin, the Paranoid Android
#d22%ddathd21.BITNET@WISCVM.ARPA (11/08/85)
From: <#d22%ddathd21.BITNET@WISCVM.ARPA> (Ralf Bayer) >From: Alan Wexelblat <wex@mcc.ARPA> > >Thanks to those who replied to my earlier posting on time-travel. >One thing still puzzles me though: Is it the case that the center of >mass of the universe doesn't move? Is it (theoretically) possible >to calculate our position/velocity w.r.t. this non-moving point? >People seem to talk about our universe as expanding, but expanding >away from what? > >Alan Wexelblat As far as I always understood that ``expanding universe''-thing, you can't calculate a center, because it's not part of our universe. Just think of a balloon, steadyly blown up. In this picture, the surface of the balloon is the universe. The relative speed of any two points on this surface is proportional to their distance (guess what, that's exactly what the astronomers found out for our universe). Just imagine the whole thing with one dimesion more, our universe being the 3-dimensional surface of a 4-dimensional balloon. (Ever blown up a 4-D balloon?) From this it should also be clear that we can't find the non-moving center inside our universe, because (in the example) the center of the balloon is also no part of it's surface. But, to put some speculation in the time travel thing: Think of time being the fourth dimension. So as time goes by, we (our universe) gets alway farther away from the center of our 4-D balloon. And if we should travel in time, we should be able to stay always on the same radius of that sphere, and by this stay at the same place in space. So the problem is not the big universe-wide movement of everthing expanding, but the small scale movement of earth around the sun, sun around the center of our galaxy, and of course the movement of two or more galaxies around each other. But I don't know what to do about that problem. But, as I also don't know how to travel thru time, I don't bother. :-) Ralf #d22%ddathd21.bitnet@wiscvm.wisc.EDU (from ARPAnet) #d22@ddathd21 (from BITNET / EARN) (Beware of the number sign - it's part of my User-ID)