The VSL Discussion: What Does Variable Speed of Light Mean and Should we be Allowed to Think About ?

Abstract

In the past years, variable speed of light (VSL) theories have been of growing interest but also a subject of controversial discussion. They have been accused both for tautologies and for violating special relativity, and concerns have been expressed about the validity of such approaches in general (e.g. Ellis, astro-ph/0703751). Without trying completeness on the issue, the example of Einstein's VSL attempts (1911) and Dicke's `electromagnetic' theory (1957) are urges to give some comments on the above criticism.