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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
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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
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