![]() We now have a quantum superposition of light and excited matter states, so, although many people would call me pedantic, this isn't really light proper. When it interacts with matter, the speed of the disturbance arising from excitation by light can have different speeds. So experimentation tells us that light of all "kinds" and colors moves at $c$ and relativity gives us insight into how this comes about. ![]() No experiment so far has detected a measurable difference between the speed of light of any color and $c$. In general relativity, a massless particle's speed as measured locally is also always $c$. Special relativity says that such a massless particle's speed must always be observed to be $c$, irrespective of the observer's motion. ![]() ![]() As far as we know, light is mediated by a particle without rest mass. ![]()
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