http://www.relativitycalculator.com/...ht_English.pdf
On the Influence of Gravitation on the Propagation of Light, by A. Einstein. Annalen der Physik, 35, pp. 898-908, 1911: "...If we call the velocity of light at the origin of coordinates c_o, then the velocity of light c at a location with the gravitation potential phi will be given by the relation c = c_o(1+phi/c^2)."

Einstein had plagiarized this (correct) equation from Newton's emission theory of light - his purpose in the 1911 paper was to camouflage this fact and confuse the whole issue by introducing his idiotic "gravitational time dilation".

Then in the final 1915 version of general relativity the speed of light in a gravitational field became two times more variable than the speed of ordinary matter - the correct equation c = c_o(1+phi/c^2) was replaced by the absurd equation c = c_o(1+2phi/c^2). This greater variability of the speed of light predicted by general relativity is a grand secret between knowledgeable Einsteinians - I have found only a few references on internet, e.g.. this one:

http://arxiv.org/pdf/gr-qc/9909014v1.pdf
Steve Carlip: "It is well known that the deflection of light is twice that predicted by Newtonian theory; in this sense, at least, light falls with twice the acceleration of ordinary "slow" matter."

Most Einsteinians neither know nor care about the greater variability of the speed of light predicted by Einstein's general relativity. They just teach that the speed of light in a gravitational field is constant and that's it, Divine Einstein, yes we all believe in relativity, relativity, relativity:

http://www.oapt.ca/newsletter/2004-0...Searchable.pdf
Richard Epp: "One may imagine the photon losing energy as it climbs against the Earth's gravitational field much like a rock thrown upward loses kinetic energy as it slows down, the main difference being that the photon does not slow down; it always moves at the speed of light."

http://www.amazon.com/Brief-History-.../dp/0553380168
Stephen Hawking, A Brief History of Time, Chapter 6: "A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; a photon, however, must continue upward at a constant speed..."

http://www.amazon.com/Why-Does-mc2-S.../dp/0306817586
Brian Cox, Jeff Forshaw, p. 236: "If the light falls in strict accord with the principle of equivalence, then, as it falls, its energy should increase by exactly the same fraction that it increases for any other thing we could imagine dropping. We need to know what happens to the light as it gains energy. In other words, what can Pound and Rebka expect to see at the bottom of their laboratory when the dropped light arrives? There is only one way for the light to increase its energy. We know that it cannot speed up, because it is already traveling at the universal speed limit, but it can increase its frequency."

Pentcho Valev