G'day Paul,



On 30-05-95 22:01, Paul Edwards said to Jeff Green



 PE> Thanks, and it is quite obvious that the CR doesn't help your

 PE> system wake up any faster.  Hmmmm, what I need is a good theory.



Ok. Here you go :-)



>>



The theory of relativity is a single, all-encompassing theory of
space-time, gravitation, and mechanics.  It is popularly viewed, however,
as having two separate, independent theoretical parts--special relativity
and general relativity. One reason for this division is that Einstein
presented special relativity in 1905, while general relativity was not
published in its final form until 1916.  Another reason is the very
different realms of applicability of the two parts of the theory:  special
relativity in the world of microscopic physics, general 

relativity in the world of astrophysics and cosmology.



A third reason is that physicists accepted and understood special
relativity by the early 1920s.  It quickly became a working tool for
theorists and experimentalists in the then-burgeoning fields of atomic and
nuclear physics and quantum mechanics.  This rapid acceptance was not,
however, the case for general relativity.  The theory did not appear to
have as much direct connection with experiment as the special theory;  most
of its applications were on astronomical scales, and it was apparently
limited to adding minuscule corrections to the predictions of Newtonian
gravitation theory;  its cosmological impact would not be felt for another
decade.  In addition, the mathematics of the theory were thought to be
extraordinarily difficult to comprehend.  The British astronomer Sir Arthur
Eddington, one of the first to fully understand the theory in detail, was
once asked if it were true that only three people in the world understood
general relativity.  He is said to have replied, "Who is the
third?"



This situation persisted for almost 40 years.  General relativity was
considered a respectable subject not for physicists, but for pure
mathematicians and philosophers. Around 1960, however, a remarkable
resurgence of interest in general relativity began that has made it an
important and serious branch of physics and astronomy.  (By 1977,
Eddington's remark was recalled at a conference on general relativity
attended by more than 800 researchers in the subject.) This growth has its
roots, first, beginning around 1960, in the application of new mathematical
techniques to the study of general relativity that significantly
streamlined calculations and that allowed the physically significant
concepts to be isolated from the mathematical complexity, and second, in
the discovery of exotic astronomical phenomena in which general relativity
could play an important role, including quasars (1963), the 3-kelvin
microwave background radiation (1965), pulsars (1967), and the possible
discovery of black holes (1971).  In addition, the rapid technological
advances of the 1960s and '70s gave experimenters new high-precision tools
to test whether general relativity was the correct theory of gravitation.



The distinction between special relativity and the curved space-time of
general relativity is largely a matter of degree. Special relativity is
actually an approximation to curved space-time that is valid in
sufficiently small regions of space-time, much as the overall surface of an
apple is curved even though a small region of the surface is approximately
flat.  Special relativity thus may be used whenever the scale of the
phenomena being studied is small compared to the scale on which space-time
curvature (gravitation) begins to be noticed.  For most applications in
atomic or nuclear physics, this approximation is so accurate that
relativity can be assumed to be exact;  in other words, gravity is assumed
to be completely absent.  From this point of view, special relativity and
all its consequences may be "derived" from a single simple
postulate.  In the presence of gravity, however, the approximate nature of
special relativity may manifest itself, so the principle of equivalence is
invoked to determine how matter responds to curved space-time.  Finally, to
learn the extent that space-time is curved by the presence of matter,
general relativity is applied.



>>



Regards...Jeff (jeff{at}grntrs.dialix.oz.au)



... Draw your salary before spending it.


--- Sqed/32 0.97/r15123