© Science News, 1990
Science News 137:19 (1990)303, May 12, 1990


Secret of the vacuum: Speedier light

The notion that no particle or signal can travel faster than the speed of light in a vacuum is one of the cornerstones of modern physics. Now two physicists have challenged that well-established idea by uncovering a subtle quantum phenomenon that allows particle of light, or photons, under certain circumstances to travel at a slightly faster rate. That minuscule increase in the speed of light hinges on the peculiar effect of two parallel, conducting plates, or mirrors, on the properties of the vacuum.

In the Feb. 22 PHYSICS LETTERS B, Klaus Scharnhorst of the Humboldt-Universität zu Berlin in East Germany uses the theory of quantum electrodynamics, which describes the way photons interact with matter, to calculate what happens to electromagnetic waves (or photons) between a pair of closely spaced, parallel plates. Electromagnetic waves propagating at right angles to the plates would travel a little faster than light in the free vacuum, Scharnhorst reports. "This is simply the result of the change in the vacuum structure enforced by the plates."

Taking a somewhat different approach, Gabriel Barton of the University of Sussex in Brighton, England, comes to the same conclusion.

"One could say that between parallel mirrors, even at zero temperature, there is a disturbance of the electromagnetic field, and it is as if between the mirrors, the energy density of the electromagnetic field were less than zero," Barton says. "So is seemed to me that if a positive energy density makes light go slower, then in a sense, a negative energy density, such as you have between mirrors, would make light go faster." Barton's analysis appears in the March 22 PHYSICS LETTERS B.

Fundamental to both approaches is the theoretical picture of the vacuum as a turbulent sea of randomly fluctuating electromagnetic fields and short-lived pairs of electrons and positrons (the antimatter counterparts of electrons) that appear and disappear in a flash. According to quantum electrodynamics, light propagating through space interacts with these vacuum fields and electron-positron pairs, which influence how rapidly light travels through the vacuum.

The presence of a pair of mirrors modifies the vacuum so that certain types of interactions between photons and the phantom electron-positron pairs can no longer occur. This allows light to travel a little faster than it normally would. The same type of modification in vacuum properties is responsible for the so-called Casimir effect, which predicts that two such plates would also attract each other.

But the predicted increase in speed is exceedingly small and occurs only for light propagating perpendicular to the plates. For parallel plates just 1 micron apart, the change amounts to roughly on part in 1036.

"It's laughably small," Barton says. "The effects are too small by many orders of magnitude to be measured, but appear fascinating as matters of principle."

The results don't call into question anything basic about relativity theory, Barton argues. "All this says is that if you really had infinitely extended, parallel mirrors, then at right angles to these, there is still a maximum speed - in the same way that ordinary relativity says there is a maximum speed called c in empty space."

Nonetheless, the new findings suggest a number of technical questions worth exploring further. "The vacuum is certainly a most mysterious and elusive object that makes itself known by only the most indirect of hints." Stephen M. Barnett of the University of Oxford in England comments in the March 22 NATURE. "The suggestion that [the] value of the speed of light is determined by its structure is worthy of serious investigation by theoretical physicists."