Question

I've recently read that what most people learned to think of as the 'speed of light' is actually the 'speed of causality', and that light just happens to travel at that speed (through free-space.) I'm also aware that light travels measurably slower through transparent materials such as water or glass.

This leads me to ask: is light traveling slower through such materials, because the speed of causality is reduced in such materials? And if so, what does this tell us about how the speed of causality is determined by permittivity and permeability of various materials?

Answer 1

Light travels slower in materials like water or glass due to interaction with atoms, not a reduced 'speed of causality'. The reduction in light's speed in a material is measured by its index of refraction, which depends on the material's electromagnetic properties.

Step-by-step explanation:

The speed of light is indeed slower in materials like water or glass compared to vacuum, not because the 'speed of causality' is reduced, but due to the interaction of light with the atoms within the material. In a vacuum, the speed of light is constant at approximately c = 3.00 × 108 m/s. However, this speed reduces when light passes through different media because of refraction, where light changes direction due to a change in speed when it enters a medium with a different density.

The extent to which light slows down is quantified by the medium's index of refraction (n), which describes how much the speed of light is reduced compared to its speed in a vacuum. The electromagnetic properties of a material, particularly its permittivity and permeability, determine this index of refraction. An important observation from Einstein's theory of relativity is that the speed of light in a vacuum (c) is the upper limit for the speed at which information can travel, also known as the 'speed of causality'.