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In article <ksiew.720770120@munagin>, ksiew@mullian.ee.mu.OZ.AU (steven kok-hsien siew) writes:
> I have some physics questions which I would like some answers to.
> 
> 4) Why is the speed of light constant?
> 
> -----------------------------------------------------------------------------
> The only people who believe that unlimited | Steven Siew  tel 03 803 4662
> growth can take place in a finite system   | ksiew@munagin.ee.mu.oz.au
> are madmen or economists. Spaceship earth: | Don't call me! Gundam me! 
> -----------------------------------------------------------------------------

Short answer:  The invariance of c is an axiom of Einstein's special theory 
of relativity (1905).

Long answer:  This question is really concerned with the matter of how we 
choose (or design) standards for the measurement of space and time intervals.
It is little appreciated that this is a "catch 22" question:
One would like to have such standards a priori - for the purpose of
experimentation (both physical and gedanken) to seek direction in the 
construction of a physical theory;  yet one's choices of a  standard clock and 
meter stick already presuppose considerable understanding about the behavior 
of matter!  Ex: the choice of a standard clock already presupposes a theory
which will be committed to the conclusion that this particular mechanism ticks 
at a constant rate; logical consistency alone will force the theory to this
conclusion.  Choices among theories and choices among standards are 
inextricably intertwined.  

In particular, the choice of a clock defines the parameter "t" which will be 
used as the linear time base for the description of the evolution of 
phenomena.  We are comparing the course of all other phenomena to the 
succession of ticks of this clock.  Clearly, the choice of standards is a 
matter of definition.  The criterion is simply one of usefulness: which 
choices lead to the most "desirable" (you and/or current scientific culture 
must define this word) physical theory?  Historically, and logically, this 
is an iterative process, as we see more and more details of where 
the theory is leading.

Einstein was the first to capitalize upon this freedom of choice in a 
radical way. His definition of "desirable" was not mainstream.
To him the "desirable" theory must preserve the invariance of physical law (in 
particular Maxwell's equations) among inertial observers.  He dared to 
re-define the measurement of space and time intervals to make this so.  
One might expect (I'm sure many did) that such a re-definition would force 
irreconcilable weirdities into the theory as it struggled for acceptance.  
The remarkable result has been that the weirdities were only cultural and 
that physical phenomena have become more simply describable, even phenomena 
far removed from Maxwell's equations.

To get back to the original question (finally), Maxwell's equations 
(identical in all inertial frames) predict a definite velocity for E&M 
radiation in vaccuo, without reference to any medium.  

Bottom line answer: the speed of light is invariant because we have defined the
measurement of space and time intervals to make it so!

Bob Sciamanda
Physics
Edinboro Univ of PA