Farsight wrote:People say the speed of light is constant, and Einstein said it. But it isn't true. Yes, Einstein started with this as a postulate in 1905, but in 1911 he wrote
On the Influence of Gravitation on the Propagation of Light, where we can see his ideas evolving. He gives the expression c = c
0 (1 + Φ/c²), which is c varying with gravitational potential. Then in 1912 he said it again when he wrote
"On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential". He repeated this in 1913 when he said this:
"I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis".
Wikipedia commons public domain image, Solvay 1911 crop uploaded by Fastfission
This wasn’t just some early thought that he later discarded, because there it is again in 1915 when he says
"the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned." That’s on page 259 of Doc 21, sorry, I’m not sure
what the original paper is called. He says it again in late 1915, on page 150 of Doc 30, within
The Foundation of the General Theory of Relativity. Einstein says
"the principle of the constancy of the velocity of light in vacuo must be modified.". He really spells it out in section 22 of the 1916 book
Relativity: The Special and General Theory where he says this:
"In the second place our result shows that, 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 and to which we have already frequently referred, 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. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)".
People tend to see the word
velocity in the translations without seeing the context and without noticing that he’s repeatedly referring to
“the principle”. They just don’t see the significance of
“laid in the dust”, and they skip over his reference to
"one of the two fundamental assumptions". They just don’t and won’t see that he's talking about the SR postulate, which is the constant
speed of light. Some will even huff and puff and cry
“out of context” and
“cherry picking” to dismiss what Einstein actually said. They refuse to accept that Einstein didn't speak English in 1916, and that what he actually said was
die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert. I’ve got the original German version. A German friend and an Austrian friend translated it for me. It translates into
the speed of light varies with the locality. The word “velocity” in the 1920 Methuen translation was the common usage, as in “high velocity bullet”, not the vector quantity that combines speed and direction. He was saying the
speed varies with position, hence the reference to the postulate, and hence it causes curvilinear motion. It causes the light to follow a curved path. like a car veers when the near-side wheels encounter mud at the side of the road.
