Speed of light (c)

The speed of light c = 2.99792458 × 10⁸ m/s is the propagation speed of electromagnetic waves in vacuum, derived from Maxwell's equations as c = 1/√(μ₀ε₀). Since the 1983 redefinition of the metre, c is not measured — it is defined to be exactly 299,792,458 m/s, and the metre is defined as the distance light travels in 1/c seconds. The first terrestrial measurement was Fizeau's 1849 toothed-wheel experiment (≈ 3.15 × 10⁸ m/s, 5% high); Foucault's rotating-mirror method in 1862 reached 0.3% accuracy; Michelson's late-nineteenth-century work pushed to 0.001%. All subsequent refinements confirmed Maxwell's value.

The significance of c extends far beyond electromagnetism. In special relativity (Einstein 1905), c is the invariant speed: every inertial observer measures the same c for light propagation, regardless of their motion. This frame-independence forces the Lorentz transformations, time dilation, length contraction, and mass–energy equivalence E = mc². c also appears in general relativity as the maximum signal-propagation speed — nothing carrying information travels faster. In media, the phase velocity of light can exceed c (this is why the refractive index n can be less than 1 near absorption resonances) and the group velocity can too under certain conditions, but the signal velocity — the speed at which a genuine information-carrying front propagates — is always ≤ c.