Vacuum Birefringence Ellipticity as a Tabletop Limit on Torsion-Induced Optical Anisotropy

This paper examines whether cavity-enhanced vacuum birefringence searches already constrain optical anisotropy that an axial torsion background would induce. The system considered is a 1064 nm laser in a high-finesse cavity with an effective path near 100 km, and the observable is ellipticity, refractive-index difference and the corresponding torsion scale. Evaluating the ordinary behaviour of the system and the spin-torsion contribution under explicit assumptions about polarisation and uniformity, and inverting the comparison at the stated laboratory precision, gives a quantitative bound rather than a qualitative assertion of smallness. The computed record is: ellipticity = 1.000e-08; delta_n_bound = 3.387e-20; torsion_bound_per_m = 2.000e-13; wavelength_m = 1.064e-06; effective_path_m = 1.000e+05. The torsion-related contribution is far below the level that would alter the ordinary behaviour of the system at current precision, and the value that would have produced a visible residual is stated so that future improvements can be assessed against it directly. No new measurement is reported. This study is simulated and computed from stated assumptions, not measured.