Photons move in a gravitational field and either
undergo acceleration (e.g. their speed becomes
c'>c=300000km/s) or do not undergo acceleration (that
is, their speed remains c=300000km/s). If they undergo
acceleration the frequency shift detected by the
receiver is due to the variable speed of light, in
accordance with the formula c'=Lf', where L is
wavelength and f is frequency. If the photons do not
undergo acceleration the frequency shift detected by
the receiver is due to gravitational time dilation and
variable wavelength, in accordance with the formula
c=L'f'. It is easy to see that c'=Lf' and c=L'f' are
the only possibilities. Roughly speaking, either
variable speed of light and no gravitational time
dilation, or gravitational time dilation and constant
speed of light.

Initially Einstein chose c'=Lf':
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 / c2 ) 
where V is the gravitational potential relative to the
point where the speed of light c0 is measured."

However later Einstein had to camouflage the fact that
the frequency shift is due to variable speed of light
and introduced gravitational time dilation - a concept
extremely dangerous for human rationality. Two
identical clocks in identical conditions (identical
gravitational fields) allegedly have different rates.
Rationality is immediately destroyed and the victim
starts worshipping both the miracle and its creator.

Pentcho Valev


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