Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site caip.RUTGERS.EDU Path: utzoo!linus!philabs!cmcl2!seismo!caip!#d22%ddathd21.BITNET From: #d22%ddathd21.BITNET@WISCVM.ARPA Newsgroups: net.sf-lovers Subject: RE: Time travel, take 2 Message-ID: <382@caip.RUTGERS.EDU> Date: Thu, 7-Nov-85 22:37:45 EST Article-I.D.: caip.382 Posted: Thu Nov 7 22:37:45 1985 Date-Received: Sat, 9-Nov-85 06:12:50 EST Sender: daemon@caip.RUTGERS.EDU Organization: Rutgers Univ., New Brunswick, N.J. Lines: 48 From: <#d22%ddathd21.BITNET@WISCVM.ARPA> (Ralf Bayer) >From: Alan Wexelblat> >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)