Speed of Light: Meaning, Units, and Physics

Speed of light as electromagnetic wave speed

The speed of light in vacuum, denoted c, is the propagation speed of electromagnetic waves. In classical physics, Maxwell’s equations predict that electromagnetic waves travel through vacuum at a speed determined by the vacuum’s electric and magnetic constants (ε0 and μ0), giving c = 1/√(ε0μ0). In modern quantum physics (QED), light is carried by photons, which are massless; special relativity then implies that massless particles and electromagnetic disturbances propagate at c in vacuum. Because c is invariant, it does not depend on the motion of the source or the inertial frame of the observer. This invariance makes c the upper limit for the speed at which information, matter, or energy can travel through space: particles with nonzero rest mass can approach c but cannot reach it. The finite value of c also explains observable delays (e.g., sunlight reaching Earth) and sets fundamental limits such as communication latency and time-of-flight distance measurements. In media, light generally travels more slowly than c, and different wave characteristics can have different velocities (phase, group, and front velocities). The refractive index describes how the phase velocity in a material compares to c, and it can vary with frequency and other properties. Apparent “faster-than-light” effects can occur in specific contexts (like phase velocity or projection effects), but they do not correspond to transmitting information faster than c.

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Exact vacuum value of c

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