On Sun, 21 May 2006, Dave Tweed wrote:

> However, because of the expansion of the universe overall, there are
> objects that are moving away from us at speeds greater than the speed
> of light. You're right, we can't see or interact with them in any way.

This begs a nasty question: Assuming the universe is expanding and 
contracting periodically and that it is closed in a sufficient number of 
dimensions (bubble or moebius bottle or simply an elastic string in a 
circle), and two particles start from the same point in opposite 
directions, both with speed c, and meet later elsewhere, what will their 
relative speeds be (assume the universe was expanding when they departed 
and is contracting when they meet) ? Come to think of it, what is the 
relative speed of two photons when they collide head on ?

Peter
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