Experimental Measurement of Superluminal Propagation: Direct Test of Light-Pulse Arrival-Time Difference in Casimir Slit Fibers

This paper proposes an experimental scheme for directly measuring the arrival-time difference of light pulses, in order to test whether the speed of light in a Casimir-restricted vacuum differs from that in free vacuum. The experiment employs a slit waveguide fiber: a transverse nanoscale metal gap (plate separation d) provides the Casimir boundary condition, while a longitudinally ultra-long fiber (length L) independently amplifies the arrival-time difference. The reference path is free vacuum, and the experimental path is the Casimir slit waveguide fiber. Both paths have identical geometric lengths, and the pulse arrival-time difference Δ t is directly measured via femtosecond cross-correlation. If the pulse in the experimental path arrives earlier (Δ t>0), this constitutes direct experimental evidence for superluminal propagation; if it arrives later (Δ t<0), this constitutes direct experimental evidence for subluminal propagation; if no significant difference is observed within the sensitivity, an upper limit on δ(d) is obtained. The experiment does not presuppose the existence or non-existence of superluminal propagation, nor does it presuppose the validity or invalidity of any theory; the verdict is adjudicated by data. The Scharnhorst formula is adopted as candidate theory C1 to provide an order-of-magnitude reference, and four criteria (1/d⁴ scaling, polarization selectivity, L linearity, and disappearance upon plate separation) are designed to exclude systematic errors. The analysis shows that if the one-loop formal value is the physical value, there is a gap of four to six orders of magnitude between current fiber fabrication capabilities (single-span draw of about 110.6 km, field deployment of about 137.36 km) and the required length (on the order of 10⁶–10⁸\ km). Whether the experiment can be executed depends on a breakthrough in temporal resolution, an extension of waveguide length, or the actual effect being larger than the one-loop formal value; none of these three is excluded by theory, but none has been demonstrated by existing technology. The value of this scheme lies in providing a long-term technical target framework that is self-consistent in principle, has well-defined criteria, and has quantifiable engineering boundaries for the direct experimental test of superluminal propagation.