# Did Einstein ever refer to the coordinate speed/velocity of light?

In modern parlance we talk about two different speeds of light in general relativity. We distinguish between the local speed of light, which is always $c$, and the coordinate speed of light, which can have any value from zero upwards without limit.

In Einstein's early papers he simply refers to the speed of light and by this he means what we would today call the coordinate speed of light. I'm curious to know if at any point in his published work he started making the same distinction that we do today, and if so whether he used the modern terminology.

I've attempted to investigate this using the Einstein Digital Archive, but with 5,000 documents it's proving a bit impenetrable. I wonder if someone more familiar with Einstein's key papers would be able to answer this more easily.

If it is of interest there is a related question in the Physics Stack Exchange: GR. Einstein's 1911 Paper: On the Influence of Gravitation on the Propagation of Light

• IMO the first paragraph presents a false assumption, that a certain distinction is important and widely recognized. The way a relativist would normally look at this is that coordinates and coordinate velocities are of little interest and have no direct physical interpretation. Since Einstein designed GR to have general covariance, I think he'd see it the same way. A modern relativist would seldom have any need to write the symbol c, because they would work in units where c=1, and they would be conscious of the fact that calling it the "speed of light" is basically a historical mistake.
– user466
Dec 13 '16 at 23:56