The variable speed of light (VSL) concept states that the speed of light in a vacuum, usually denoted by c, may not be constant in most cases. In most situations in condensed matter physics when light is traveling through a medium, it effectively has a slower speed. Virtual photons in some calculations in quantum field theory may also travel at a different speed for short distances; however, this doesn't imply that anything can travel faster than light. While it is usually thought that no meaning can be ascribed to a dimensional quantity such as the speed of light varying in time (as opposed to a dimensionless number such as the fine structure constant), in some controversial theories in cosmology, the speed of light also varies by changing the postulates of special relativity. However, this would require a rewrite of much of modern physics to replace the current system which depends on a constant c.
Read more about Variable Speed Of Light: Varying c in Classical Physics, Varying c in Quantum Theory, Varying c in Time, The Varying Speed of Light Cosmology
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