The MICROSCOPE Satellite Result on the Equivalence Principle, Read as a Limit on Torsional Differential Acceleration

The MICROSCOPE satellite compared the free fall of titanium and platinum test masses in Earth orbit and found the Eotvos parameter to be -1.50e-15 with a combined uncertainty of 2.75e-15 (Touboul et al. 2022) - no violation of the equivalence principle at the level of parts in 10^15. The two-standard-deviation bound is 6.99e-15, corresponding to a differential acceleration no larger than 5.52e-14 metres per second squared at the orbital acceleration scale. In a frame where torsion couples to spin content, two materials with different nuclear spin and composition would in general fall differently; the satellite result limits any such torsional differential acceleration to below the stated figure and thereby limits the coupling itself when combined with the composition factors computed in the companion analysis. Computed from published results, not measured.