I'm not sure what this would even mean. Since 1983, SI has defined the length of the meter in terms of the speed of light and the length of the second. So the speed of light in a vacuum as a constant by definition. Before that, the meter was defined in terms of a wavelength emitted by krypton. The second was then, as it is now, defined in terms of a frequency emitted by cesium. So the constancy of the speed of light then depended on whether the relative behaviors of those two elements varied. Before that, the meter was defined as the length of a specific object in Paris. If the speed of light seemed to vary then, it could be that the size of the atoms in that object varied. Farfetched? Not really. The sizes of atoms depend on the values of various fundamental physical constants, including the speed of light. I think it's only meaningful to speak of changes in dimensionless numbers. For instance the fine structure constant, whose value is the same in all possible systems of units. It depends on the speed of light, Planck's constant, and the charge of the electron. It's meaningful to talk about the fine structure constant changing. If it does change, I think it's completely arbitrary whether that change is attributed to a change in the speed of light, Planck's constant, the charge of the electron, or some combination of these. (On the other hand, it seems to me that if Earth were to suddenly shrink, apparent gravity would drop as the shrinkage began, and would increase when it ended. Obviously, there would be no perceived change in gravity if Earth "shrank" only due to a redefinition of the meter. (The numerical value of the gravity might change due to redefinition of the meter. But readings on scales would remain unchanged until the scales were recalibrated or replaced.)) Can anyone find an online copy of the original study that makes this claim? From the news coverage, I can't even tell whether they claim to have measured an effect, or whether this is entirely theoretical. Warren D Smith <warren.wds@gmail.com>

Does this contradict the observed fact that light from halfway across the universe, hitting the left side of your telescope mirror, is still perfectly in phase with light hitting right side of your telescope mirror, allowing interferometry?

No, so long as the speed varies smoothly with time and location, rather than there being any abrupt transitions. Exactly as happens with curvature of space-time due to gravity. In fact, this alleged variability would be indistinguishable from such curvature. Indeed, it's arbitrary to say that space-time is curved near masses rather than to say that c is slower there and that objects there are shrunken and slowed by just enough that they don't notice the slowdown. One possible difference is that space-time curvature near masses is always equivalent to slower light, not faster. If this new effect allows light to go faster than c, it might allow causality violations, i.e. a way to send messages back through time. On second thought, space-time curvature near *negative* masses would also be equivalent to faster light. Maybe that's why there are no negative masses. Unless the Casimir effect counts. That's a very weak effect, and probably any speedup of light due to the negative mass-energy between the plates would be swamped by the positive mass-energy *of* the plates.