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Re: parallel branes, not parallel universes



On Sun, 13 Aug 2000, I wrote:

> We do not know, and do not care, whether our universe is
> embedded in a higher-dimensional space.


At 07:02 PM 8/13/00 -0500, Jack Uretsky wrote:
Ahh! One small correction:

A lot of people care a whole bunch.
In the Randall-Sundrum model, there may be extra large-dimensions
in which gravity, but nothing else, acts. Gravity at short distance is
modified; the scale of the modification varies inversely as the number of
extra dimensions. Two extra dimensions would be detectable at distances
of the order of a millimeter. Experiments are in progress at the U. of
Washington, and probably elsewhere.

For those of you who would like more detail on what Jack is talking about,
a not-very-technical introduction can be found at
http://dallasnews.com/science/12616_DIMENSIONS03.html

It seems to me the extra dimensions of which Jack speaks should be
considered part of our universe, not part of the embedding space. I am
quite aware of what some theories say about additional dimensions, and I
have tried hard in my recent posts to avoid assuming that our universe has
D=3+1.

I don't mind when Randall and Sundrum talk about parallel branes; that's
not the same as parallel universes.