The Scharnhorst effect is a hypothetical phenomenon in which light signals travel slightly faster than c between two closely spaced conducting plates. It was predicted by Klaus Scharnhorst of the Humboldt University of Berlin, Germany, and Gabriel Barton of the University of Sussex in Brighton, England. They showed using quantum electrodynamics that the effective refractive index, at low frequencies, in the space between the plates was less than 1 (which by itself does not imply superluminal signaling). They were not able to show that the wavefront velocity exceeds c (which would imply superluminal signaling) but argued that it is plausible.
Scharnhorst effect Wikipedia
Owing to the Dirac sea, 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 interacts with these virtual particles, and is absorbed by them to give rise to a virtual electron–positron pair. This pair is unstable, and quickly annihilates to produce a photon like the one which was previously absorbed. The time the photon's energy spends as subluminal electron–positron pairs lowers the observed speed of light in a vacuum.
A prediction made by this assertion is that the speed of a photon will be increased if it travels between two Casimir plates. Because of 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. 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.
The possibility of superluminal photons has caused concern because it might allow for the violation of causality by sending information faster than c. However, several authors (including Scharnhorst) argue that the Scharnhorst effect cannot be used to create causal paradoxes.