Hugh Pickens writes writes: "Eoin O'Carroll reports in the Christian Science Monitor that a pair of studies suggest that the speed of light might not be so constant after all. In the first study, Marcel Urban from the University of Paris-Sud and his team found that the speed of light in a vacuum varies ever so slightly. Urban's paper suggests that the speed of light and other constants "are not fundamental constants but observable parameters of the quantum vacuum." In other words, the speed of light emerges from the properties of particles in the vacuum. Urban 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. In the other paper, physicists Gerd Leuchs and Luis L. Sánchez-Soto, from the Max Planck Institute for the Physics of Light, hypothesize how this emergence occurs. They suggest that the impedance of a vacuum – another electromagnetic 'constant' whose value depends on the speed of light – itself depends only on the electric charge of the particles in the vacuum, and not their masses. If their hypothesis is correct, it answers our question of where the speed of light comes from: it emerges from the total number of charged particles in the universe. The fluctuations of the photon propagation time are estimated to be on the order of 50 attoseconds per square meter of crossed vacuum, which might be testable with the help of new ultra-fast lasers."