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Originally Posted by turbo-1
Lifted from John Baez's site:
"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." Einstein 1920.
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That quote comes from Einstein's 1916 book, Crown paperback edition, page 76, and the concept originated with his 1911 "gravitational redshift" theory. In the 1911 theory, light slows down in a gravity field, and it slows down more in a stronger gravity field, and this is what makes a beam of light curve as it passes a massive astronomical body.
This idea of light slowing down in a gravity field is discussed in several Einstein papers in the 1911-1914 era. See Volume 4 of "The Collected Papers of Albert Einstein."
Also, Max Abraham noted in 1912 that in the gradually developing and changing Einstein "relativity" theory, Einstein had gravity acting like an "ether" for light. In the developing Einstein theories from around 1907 through about 1920, a gravitational field is not required for light to propagate, but a gravitational field does have an effect on light by causing it to slow down in strong gravity.
What is missing from a lot of people's libraries are many of Einstein's important papers that are not available in the $9.95 book "The Principle of Relativity". I've noticed a lot of people on the internet are trying to say "what Einstein believed" while studying only the papers published in that 1952 book. But I think that people need to obtain copies of at least 3 issues of the "Collected Papers," at least Vols 2, 4, and 7. I think people don't buy these books because they are expensive. I think volume 7 is now up to $125 per copy, used. Apparently Princeton Press didn't print many copies of each volume and the prices for used copies are going up rapidly.
Volume 4 contains some great transitional papers, as he gradually transitioned from SR to GR, and I would recommend that everyone interested in Einstein buy a copy of Volume 4. Be sure to get the PAPERBACK English edition and NOT the Hardback (which is all in German). There are too many people on the internet trying to debate Einstein theory without reading many of his papers, while reading only the few papers that are published in the cheap Dover edition of "The Principle of Relativity". But he wrote a lot more than those papers, and some of his best other papers are published in English only in the Princeton Press "Collected Papers" series.