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Professor M. Stoneham pointed out that in a dispersive medium there are three "c" s, phase speed, group speed, signal speed. Which one should we use in G/c^4?

I have used c' = c/n

n = index of refraction

so that would normally be phase speed. 

group speed is df/dk

phase speed is f/k

signal speed is more tricky.

Also here, and here.

Slow light "Researchers at the Rowland Institute for Science slowed light to 38 miles per hour in 1999"