Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.

The importance of the speed of light to physics can hardly be overstated. The number 2.997 × 108 m/s governs our lives, even if we seldom notice it. It forms the speed limit of the universe; the c in the famous equation e=mc2, and also defines the way we measure distance. High school physics teaches us that this speed is not quite universal – when travelling through water or glass, light slows down; an effect put to good use in lenses and prisms.

For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, in a paper on arXiv, Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.

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You can expect Creationists to make a great deal of this.

Have we resolved yet the speed of dark? in vacuo or any other medium?

Does this indicate that many cosmological measurements will have to be reviewed?

> Light can change speed, even in a vacuum, a new paper reports.

Not a new idea, not a new paper.

The idea is as old as Newton, and has gone through reincarnations through many of the unsuccessful attempts at an unsuccessful relativity. It was only when the Bianchi identities were rediscovered that the constancy of the speed of light became assured.

Then, for a long time after that, many different alternatives to General Relativity were invented that had a non-constant speed of light, in particular the variation of the speed of light with wavelength is predicted by many of these. But all of the alternatives fell to better experimentation.

In particular, the constancy of the speed of light is one of the components of experiments to test the PPN (parametric post-Newtonian) accuracy of alternatives to General Relativity. When last I checked, it was signals from the Cassini spacecraft that gave us the greatest certainty about the constancy of the speed of light in a vacuum. The results from the Oklo nuclear reactor, from Lunar Laser Ranging, from spectral lines in molecular clouds between us and distant quasars, and from the spin-down rate of pulsars both alone and in binary systems. All of these give us confidence in the fact that the speed of light has not changed significantly since the time of the cosmic microwave background radiation (CMB).

Every now and then some further idiot comes along and say that the speed of light may have changed. Either because his experiments aren’t accurate enough to distinguish between change and no change. Or because of some ad hoc hypothesis that, for instance, the speed of light changed before the CMB. This last idea was wiped out within a couple of months of its proposal by the discovery of slow-roll inflationary cosmology.

This new one, though takes a different tack. Light can travel faster than the speed of light through the process of “tunnelling”. It travels more slowly through transparent solids, liquids, gases and plasma. It can be stopped and even be made to reverse direction in some media. In addition, because the speed that light travels depends on the permittivity of free space and because that can be changed by altering the quantum vacuum, for instance between metal plates (an application of the Casimir effect), the speed that light travels can be altered even in a vacuum. But great care is needed in distinguishing between the phase velocity and the group velocity at which light travels.