Abstract
Length contraction has reappeared, quite recently, after more than a century of intermittent debate about its mere existence. The verified constancy of the two-way speed of light, requiring the same round-trip travel time measured by an inertial observer, is considered. A configuration involving a stationary system, in which light propagation is assumed isotropic, and a uniformly moving system is analysed. By considering the independence of the SOL from the speed of the emitter and time dilation, those two effects are insufficient to account for the invariance of the round-trip speed of light in the moving system. By involving length contraction—so far lacking a direct experimental verification—the discrepancy disappears. Length contraction thus emerges as a real effect, a direct consequence of three experimentally verified effects: the invariance of the round-trip speed of light, time dilation, and the independence of the speed of light from the speed of the emitter.