A bird? A plane?
After more than eighty years modern physics has learned to live comfortably with one of its fundamental tenets: no particle, no signal, no causal connection - nothing except Superman - can travel faster than c, the speed of light in a vacuum. Indeed, so sure are physicists of the constancy of c that they now define it as 299,792,458 meters a second, thereby indirectly fixing the length of the meter in terms of the speed of light and the second. Now Klaus B. Scharnhorst, a physicist at Humboldt University in East Berlin, reports finding an exception to the rule. In a recent issue of Physics Letters B Scharnhorst describes conditions under which photons of light could inch past the universal speed limit.
"My gut reaction was that something must be wrong," notes Gabriel Barton, a physicist at the University of Sussex in Brighton, England. Nevertheless, through a thought experiment of his own, described in another issue of Physics Letters B, Barton confirms that the Scharnhorst effect is a perfectly legal outgrowth of quantum electrodynamics, the well-established quantum theory of electromagnetism.
The secret lies in the structure of the vacuum. Far from being empty, a vacuum is a turbulent sea of electrons and their positively charged counterparts, positrons. For quantum-mechanical reasons these particles rapidly pop in and out of existence as they interact with photons passing through the vacuum. A photon may spontaneously seem to disappear, for instance, leaving an electron-positron pair in its place; when the two particles collide, they annihilate one another, and the energy released by the collision is re-emitted as a photon. This process of continual absorption and re-emission slows down the photons as they travel through the vacuum, and it thus reduces the net speed of light.
Scharnhorst and Barton have suggested that if the vacuum fluctuations could be minimized, the photons would travel faster. When two conducting plates (or, in Barton's version, two mirrors) are placed parallel to each other a short distance apart, they effectively suppress all energies whose associated wavelengths are too long to fit between the plates. In the quantum theory, waves are associated with particles; fewer wavelengths therefore implies fewer pairs of particles to get in the way. Thus unhindered, photons moving perpendicularly to the plates are able to move slightly faster than they would in an unmodified vacuum.
At least in theory. Inducing the phenomenon and measuring it experimentally are other matters entirely. The effect increases as the separation between the plates is reduced. But even if the separation were as small as the diameter of an atom, the increase in speed would be extraordinarily minute - about one part in 1020. The implications for the length of the meter are not nil, should the International Council of Scientific Unions wish to recognize them, but they are not appreciable either: such a minuscule change in c implies that there is a discrepancy of roughly a millimeter over the distance from the earth to the nearest star.