In terms of astronomy and physics, the speed of light in the near perfect vacuum of space is pretty simple -- 299,792,458 meters per second (or 186,282 miles p/s), generally. This is something astronomers call "the speed of light." This constant speed means that it takes light more than four years to travel from the sun to Proxima Centauri, the red-dwarf star that is a member of the Alpha Centauri system (our closest celestial neighbors). It also means that it takes more than 8 minutes for light to travel from the sun's photosphere to Earth. Though that number can vary a bit here on Earth, depending upon whether or not a photon's path is interrupted by something like water, as a random example.
So says the team, which was lead by Marcel Urban from the University of Paris-Sud, that claims to have found that the speed of light does vary ever-so-slightly, only by as little as 50 quintillionths of a second per square meter. While that number is small, it is still enough to point to a new underlying type of physics, which is dictated by the properties of the particles that inhabit the vacuum -- something that has been heavily debated for years now, after it was discovered that seemingly empty space is not empty at all, but full of virtual particles (such as electron-positron or quark-antiquark pairs) that pop in and out of existence at their own leisure. Generally when a partner particle is sucked into a singularity, thus causing their energy values to fluctuate.
According to the team, these fluctuations may sometimes impact the speed that light travels through space. Furthermore, light speed and other cosmological constants: "are not fundamental constants but observable parameters of the quantum vacuum." What does this mean? Well apparently, in another paper published by Gerd Leuchs and Luis L. Sánchez-Soto, who hail from the Max Planck Institute for the Physics of Light in Erlangen, Germany, the impedance of free space (another supposed electromagnetic constant, whose values hinge on the speed of light) does not depend on the mass of particles in space, but on their electric charge -- something that answers a long standing question posed in physics...why DOES light travel at a fixed speed? Furthermore, what dictates the speed? In short: It depends on the total number of charged particles in the universe. This may also mean that it is possible to estimate the total number of charged elementary particles per unit volume that exist in the universe.
It states, "… there is a theoretical possibility that the speed of light is not fixed, as conventional physics has assumed. But it could fluctuate at a level independent of the energy of each light quantum, or photon, and greater than fluctuations induced by quantum level gravity. The speed of light would be dependent on variations in the vacuum properties of space or time."
If correct, this may actually be testable with the help of ultra-short pulse lasers. It would also explain the magnetization and polarization of the vacuum, something that is referred to as vacuum permeability and permittivity. We shall see, but it appears as if experimental results support this hypothesis. What a universe we live in!
M. Urban et al. (2013), "The quantum vacuum as the origin of the speed of light," European Physical Journal D, DOI 10.1140/epjd/e2013-30578-7
Gerd Leuchs and Luis L. Sánchez-Soto (2013), "A sum rule for charged elementary particles," European Physical Journal D, DOI 10.1140/epjd/e2013-30577-8