Scharnhorst effect

The Scharnhorst effect is a hypothetical phenomenon in which the speed of light is slightly greater between two closely-spaced conducting plates than it is in a normal vacuum. It was predicted by Klaus Scharnhorst of the Humboldt University of Berlin, Germany, and Gabriel Barton of the University of Sussex in Brighton, England. Scharnhorst was able to mathematically analyze quantum electrodynamics to show how the effect might arise.

Explanation

Due to Heisenberg's uncertainty principle, an empty space which appears to be a true vacuum is actually filled with virtual subatomic particles. These are called vacuum fluctuations. As a photon travels through a vacuum, it is thought to interact with these virtual particles, and may be absorbed by them to give rise to a real electron-positron pair. This pair is unstable, and quickly annihilates to produce a photon like the one which was previously absorbed. It was recognized that the time that the photon's energy spends as an electron-positron pair would seem to effectively lower the observed speed of light in a vacuum, as the photon would have temporarily transformed into subluminal particles.

A prediction made by this assertion is that the speed of a photon will be increased if it travels between two Casimir plates. [ [http://www.nat.vu.nl/~scharnh/m16newsc.htm New Scientist: Can photons travel 'faster than light'?] ] Due to the limited amount of space between the two plates, some virtual particles present in vacuum fluctuations will have wavelengths that are too large to fit between the plates. This causes the effective density of virtual particles between the plates to be lower than that outside the plates. Therefore, a photon that travels between these plates will spend less time interacting with virtual particles because there are fewer of them to slow it down. The ultimate effect would be to increase the apparent speed of that photon. The closer the plates are, the lower the virtual particle density, and the higher the speed of light.

The effect, however, is predicted to be minuscule. A photon travelling between two plates that are 1 micrometer apart would increase the photon's speed by only about one part in 10³⁶. [ [http://www.nat.vu.nl/~scharnh/m16scine.htm Science News: Secret of the vacuum: Speedier light] ] This change in light's speed is too small to be detected with current technology, which prevents the Scharnhorst effect from being tested at this time.

Causality

The possibility of superluminal photons has caused concern because it might allow for the violation of causality by sending information faster than "c". Liberati, Sonego and Visser have argued S. Liberati, S. Sonego and M. Visser, "Faster-than-c signals, special relativity, and causality", Annals Phys. 298, 167-185 (2002) [http://arxiv.org/abs/gr-qc/0107091 preprint] .] that the Scharnhorst effect cannot be used to create causal paradoxes.

References