In 1911 Einstein demonstrated the variability of the speed of light:
http://www.physlink.com/Education/AskExperts/ae13.cfm :
"So, it is absolutely true that the speed of light is _not_ constant in
a gravitational field [which, by the equivalence principle, applies as
well to accelerating (non-inertial) frames of reference]. If this were
not so, there would be no bending of light by the gravitational field
of stars. One can do a simple Huyghens reconstruction of a wave front,
taking into account the different speed of advance of the wavefront at
different distances from the star (variation of speed of light), to
derive the deflection of the light by the star.
Indeed, this is exactly how Einstein did the calculation in:
"On the Influence of Gravitation on the Propagation of Light," Annalen
der Physik, 35, 1911.
which predated the full formal development of general relativity by
about four years. This paper is widely available in English. You can
find a copy beginning on page 99 of the Dover book "The Principle of
Relativity." You will find in section 3 of that paper, Einstein's
derivation of the (variable) speed of light in a gravitational
potential, eqn (3). The result is,
c' = c0 ( 1 + V / c^2 )
where V is the gravitational potential relative to the point where the
speed of light c0 is measured."
Later the formula c' = c0 ( 1 + V / c^2 ) was confirmed experimentally
when Pound and Rebka measured the frequency shift:
f = f' (1 + V / c^2)
Finally, the application of the equivalence principle converts c' = c0
( 1 + V / c^2 ) into
c' = c + v
where c=300000km/s is the initial speed of light relative to the light
source and v is the relative speed of the light source and the
observer, in the absence of a gravitational field.
Of course, in a world where the portrait of Einstein brings more money
than Elvis Presley's songs a falsification of Einstein's theory of
relativity is impossible by definition. Accordingly, Einstein's formula
c' = c0 ( 1 + V / c^2 ) acts like the face of Medusa the Gorgon - on
seeing it, Einsteinians get petrified and remain so until the danger is
over. Yet recently one of them - Relative Perseus (aka Paul Andersen) -
slew the Gorgon Medusa by declaring c' = c0 ( 1 + V / c^2 ) wrong and
f = f' (1 + V / c^2) non-existent:
http://groups.google.com/group/sci.physics.relativity/browse_frm/thread/fe1cc502b31251d2?
Pentcho Valev
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