Explanation
Owing 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 interacts with these virtual particles, and is 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. 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.
Read more about this topic: Scharnhorst Effect
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