A thought experiment

Graphic - maninlab

The Principle of Relativity says that the same laws of physics apply irrespective of the ‘perspective’ (frame of reference) of the observer. According to this principle, then, Maxwell’s equations (effectively the Laws of Electricity and Magnetism) must hold (see sketch) for both the experimentalist in his laboratory, and his colleague speeding through it. For both, the speed of the light must be given in terms of the fundamental constants of electromagnetism (the permittivity and permeability of free space, ϵ0 and μ0: but dont fret if you’ve not met them!). Nothwithstanding their relative motion, both must thus measure the speed of those photons flying past to be

\[ c= 1/\sqrt{\epsilon_0 \mu_o} = 3 \times 10^8\,ms^{-1} \]

The Principle of Relativity thus implies the ‘universality’ of c.