Phys.org  June 16, 2023 Among the four fundamental forces, only gravity does not couple to particle spins according to the general theory of relativity. Researchers in China tested this principle by searching for an anomalous scalar coupling between the neutron spin and the Earth’s gravity on the ground. They developed an atomic gas comagnetometer to measure the ratio of nuclear spin-precession frequencies between 129Xe and 131Xe, and searched for a change of this ratio to the precision of 10−9 as the sensor is flipped in Earth’s gravitational field. The null results of this search set an upper limit on the […]