Experimental test of equivalence principle with polarized masses

A torsion pendulum having masses with ~10²² and ~10²³ polarized electrons is used to search for an anomalous spin interaction of macroscopic range. Competition from magnetic forces is reduced by using Dy-Fe masses (which exhibit orbital compensation of the electron intrinsic spin), combined with light magnetic shielding, so that the sensitivity is better than one-tenth of a percent of the gravitational force. Fluctuations set the overall experimental limit at about 8 times this level. Interpretation of our null result sets limits on electron spin interactions and on moments which are not of electromagnetic origin. In terms of a standard dipole-dipole form the result is (1.6±{}6.9) ×{} 10^-12 of the interaction strength between the magnetic moments of the electrons. Comparisons are made with theoretical predictions for very light exchange particles.