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> []
> Steve Aldrich writes
> 
> > With all the recent discussion on the "Center of the {Known} Universe,"
> > and/or 3D(+) Space-Time, I thought I'd pose a question of my own.
> > 
> >  Namely WHAT, to the best of current knowledge, exactly is a dimension?
> > How are they defined, and what seperates them from the others? Is there
> > a "perceptable" border across dimensions, and if so what constitutes the
> > "barrier region" between them? Is it a "state of energy/matter" or am I
> > completely off base?
> 
> A dimension is a continuous parameter necessary to uniquely define an
> event in space-time.  "Space has three dimensions" means that on
> any spatial hypersurface three numbers are needed.  A particular direction
> associated with a dimension is a matter of convention (in the above example).
> "Space-time has four dimensions" just means to add another number (the date).
> The direction of the time direction is not *completely* arbitrary, but
> is substantially so.  Observers moving at different speeds will define
> different time "directions" in space-time.
> 
> Science fiction writers have used the word in a slightly different sense.
> The idea is that space-time might have umpteen dimensions, only four of
> which are readily apparent.  Then something that is near us *in an
> unusual direction* is referred to as another "dimension",  (as in
> "The Giant Cockroach from the Twelfth Dimension that Ate Detroit").
> 
> "Don't argue with a fool.      Ethan Vishniac
>  Borrow his money."            {charm,ut-sally,ut-ngp,noao}!utastro!ethan
>                                Department of Astronomy
>                                University of Texas
> 

To put it in less abstract terms, to demonstrate we really *do* need
four "dimensions" to specify things in our universe, suppose I say I'll
meet you in the National Bank building, corner of 5th (1D) and main st (2D)
on the 13th floor (3D).  You might find it necessary to invoke the fourth
dimension to be sure we'll meet:  "When?" (4D).

"It's a *beautiful* theory.  I don't believe a word of it."

-- 
Ed Nather
Astronony Dept, U of Texas @ Austin
{allegra,ihnp4}!{noao,ut-sally}!utastro!nather
nather%utastro.UTEXAS@ut-sally.ARPA