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  #91 (permalink)  
Old 08-October-2009, 04:23 PM
Jeff Root Jeff Root is online now
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Location: Minneapolis
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Quote:
Originally Posted by billy2 View Post
Quote:
Originally Posted by AndrewJ
A rubber rope is staked to the ground and stretched at the other end:
the further from the stake the more the rubber stretches (the expansion
of space increases with distance). A little ant (photon) near the stretched
end of the rope walks at a set speed (speed of light) toward the stake
(your retina).
I never understood this. What about the Ant walking away from your
Retina on the same rubber rope towards another persons distant Retina.
What about it?

Andrew's discription may be misleading: When he says,
"the further from the stake the more the rubber stretches", he really
means, "the further from the stake, the faster that part of the rope
is moving away from the stake." The stretching is the same along the
entire length of the rope. There is no difference between the two
ends or between the two directions. The rope could be infinitely long,
with ants walking along the rope in both directions, passing over marks
made on the rope at intervals. An ant that walks from one mark to
another mark nearby will make progress toward the second mark from
the beginning of its trip. If it tries to walk to a mark which is far
away, it will initially be getting farther and farther from that mark
as the rope stretches. But as the ant progresses along the rope, there
is less and less stretching between it and the mark it is headed for,
so eventually the ant starts getting closer to it.

This works if the rope is stretching at a constant rate or a slowing
rate. Until 1998 it was assumed that the cosmic expansion was slowing
due to the gravitational attraction of everything in the Universe to
everything else in the Universe. In 1998 it was discovered that the
expansion has been accelerating for the last several billion years.
An increasing rate of stretching like that would make it impossible
for the ant to reach very distant marks on the rope because they
would always be moving away from the ant faster than the ant was
walking toward them.

-- Jeff, in Minneapolis
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  #92 (permalink)  
Old 08-October-2009, 07:28 PM
billy2 billy2 is offline
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Join Date: Jul 2009
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Quote:
Originally Posted by Jeff Root View Post
What about it?

Andrew's discription may be misleading: When he says,
"the further from the stake the more the rubber stretches", he really
means, "the further from the stake, the faster that part of the rope
is moving away from the stake." The stretching is the same along the
entire length of the rope. There is no difference between the two
ends or between the two directions.
-- Jeff, in Minneapolis
Understood. Thing is, and there is a lot of it on this forum, unless you already know in your head what someone's really saying trying to say, maybe from a previous thread, then you end up asking questions like mine. Taking it at face value what Andrew said couldn't be right. If a newbie had invented that analogy and posted it, he'd have been pulled up right away.
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