> A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position.<p>Einstein does not mean "speed" when he uses the word "velocity" here or anywhere. He means "a vector quantity consisting of a speed and direction".
Modern definition of distance is derived from speed of light. So by definition speed of light is CONSTANT.<p><a href="https://en.wikipedia.org/wiki/Metre#Timeline_of_definition" rel="nofollow">https://en.wikipedia.org/wiki/Metre#Timeline_of_definition</a>
> the speed of light is only guaranteed to have a value of 299,792,458 m/s when measured by an inertial observer in a vacuum.<p>This is the average speed of photons in vacuum, right? Because according to the quantum field theory, photons should have a probability of hitting virtual particles that popped into existence, which would slow it down a little.
Surprised no one has mentioned Vernor Vinge's wonderful, awesome, mind exploding <i>Zones of Thought</i> novels[1].<p>[1] <a href="http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_series" rel="nofollow">http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...</a>
> Does the speed of light change in air or water?<p>> Yes. Light is slowed down in transparent media such as air, water and glass.<p>[...]<p>> Special Relativity<p>> [...]<p>> The speed of light does not vary with time or place.<p>These two different ways of talking about speed of light caused megabytes of meta bickering on the Wikipedia page for speed of light, and a case for Arbitration, with topic bans and warnings. (Also the suggestion that you can <i>measure</i> the speed of light in SI units, because after 1983 the SI units use light to define them.)<p><a href="http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/Case/Speed_of_light" rel="nofollow">http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/...</a><p>The SoL page shows some of the problems of WP. Articles should have a simplistic lead (with caveats), a general introduction, and then higher level discussion of the oddities as currently understood. The WP page ignores all that, and leaps in at the deep end which means the article isn't much good for anyone.