Length contraction from three experimentally verified effects

Length contraction has reappeared, quite recently, after more than a century of intermittent debate about its relevance as a physical effect. We start with the verified constancy of the two-way speed of light requiring the same round-trip travel time measured by any inertial observer. By analysing a configuration with a stationary and moving system and incorporating the verified effects of the Sagnac asymmetry and time dilation, we show that, these two effects alone are insufficient to account for the invariance of the round-trip speed of light in the moving system. Consequently, length contraction—lacking a direct experimental verification—, supported by the theory, emerges as a necessary physical phenomenon. On the same footing as time dilation, length contraction thus becomes a real effect, a direct consequence of three experimental pillars: the invariance of the round-trip speed of light, time dilation, and the Sagnac effect.