Stephen Hawking:
http://www.hawking.org.uk/lectures/dice.html
"Both Mitchell and Laplace thought of light as consisting of particles, rather like cannon balls, that could be slowed down by gravity, and made to fall back on the star. But a famous experiment, carried out by two Americans, Michelson and Morley in 1887, showed that light always travelled at a speed of one hundred and eighty six thousand miles a second, no matter where it came from. How then could gravity slow down light, and make it fall back."
Einstein never abandoned the principle of VARIABILITY of speed of light, according to relativists (other than Stephen Hawking):
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."
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html
"Einstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so."
At the end of his career (in 1954) Einstein predicts a possible death of physics:
"I consider it quite possible that physics cannot be based on the field concept,i.e., on continuous structures. In that case, nothing remains of my entire castle in the air, gravitation theory included, [and of] the rest of modern physics."
Where does the despair come from? The choice Einstein had to make between the concept of light as a continuous field and the concept of light as discontinuous particles (photons) is rarely mentioned in the literature but still there are eloquent quotations:
http://www.pbs.org/wgbh/nova/einstein/genius/ :
"Genius Among Geniuses" by Thomas Levenson
"And then, in June, Einstein completes special relativity, which adds a twist to the story: Einstein's March paper treated light as particles, but special relativity sees light as a continuous field of waves. Alice's Red Queen can accept many impossible things before breakfast, but it takes a supremely confident mind to do so. Einstein, age 26, sees light as wave and particle, picking the attribute he needs to confront each problem in turn. Now that's tough."
http://www.powells.com/cgi-bin/biblio?inkey=4-0486406768-0 :
"Relativity and Its Roots" by Banesh Hoffmann:
(I do not have the text in English so I am giving it in French)
Banesh Hoffmann, "La relativite, histoire d'une grande idee", Pour la Science, Paris, 1999, p. 112:
"De plus, si l'on admet que la lumiere est constituee de particules, comme Einstein l'avait suggere dans son premier article, 13 semaines plus tot, le second principe parait absurde: une pierre jetee d'un train qui roule tres vite fait bien plus de degats que si on la jette d'un train a l'arret. Or, d'apres Einstein, la vitesse d'une certaine particule ne serait pas independante du mouvement du corps qui l'emet! Si nous considerons que la lumiere est composee de particules qui obeissent aux lois de Newton, ces particules se conformeront a la relativite newtonienne. Dans ce cas, il n'est pas necessaire de recourir a la contraction des longueurs, au temps local ou a la transformation de Lorentz pour expliquer l'echec de l'experience de Michelson-Morley. Einstein, comme nous l'avons vu, resista cependant a la tentation d'expliquer ces echecs a l'aide des idees newtoniennes, simples et familieres. Il introduisit son second postulat, plus ou moins evident lorsqu'on pensait en termes d'ondes dans l'ether."
Clearly, the particle model of light finds its support in the negative result of Michelson-Morley experiment. It is also consistent with the third equation of Maxwell (Faraday's induction law) as implied at the beginning of Einstein's 1905 paper:
http://www.fourmilab.ch/etexts/einstein/specrel/www/
(The "customary view" Einstein refers to is the ether model of Maxwell that Maxwell himself abandoned in the end; the fact that the particle model of light naturally contradicts the ether model by no means implies that the particle model is inconsistent with the Faraday's induction law, although the mythology says otherwise.)
Pentcho Valev