For most of the physicists, it is hard to conceive the idea of anything being faster than light. The problem lies in the rejection of the need for a medium for light to travel. Einstein thought that any medium would eventually slow the light down. Einstein could not conceive and idea of medium which is not a physical matter.
He said that space has curves in General Theory of Relativity but he never tried to join his special theory of relativity with it. If space can have curves then it can also have density. And if you consider the space as the medium of light then any change in the density of space will change the speed of light too.
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