The source of the gravitational potential in their experiment was a single 1.25 kg tungsten mass. The test masses were rubidium -87 atoms. The tungsten mass was fixed, so the gravitational field caused by the tungsten mass was not zero everywhere along the paths of the Rb atoms. This means that the phase shift of the rubidium atoms between the 2 paths was not caused by a gravitational potential energy difference alone, but also by a difference in the gravitational force felt by the atoms in the 2 paths. By detecting a difference in the phase shift between when the tungsten mass is present and when it is not present, they observed a phase shift consistent with that predicted by the Aharonov-Bohm effect.