I was reading this article recently, which summarizes a couple of new studies into the speed of light.
In one paper, Marcel Urban from the University of Paris-Sud, located in Orsay, France and his colleagues identified a quantum level mechanism for interpreting vacuum as being filled with pairs of virtual particles with fluctuating energy values.
As a result, the inherent characteristics of vacuum, like the speed of light, may not be a constant after all, but fluctuate.
Meanwhile, in another study, Gerd Leuchs and Luis L. Sánchez-Soto, from the Max Planck Institute for the Physics of Light in Erlangen, Germany, suggest that physical constants, such as the speed of light and the so-called impedance of free space, are indications of the total number of elementary particles in nature.
Now, my knowledge and understanding of electromagnetic waves is a bit rusty, but I guess I don't follow: how does fluctuating energy values affect the speed of light?