When the atom is at a "mirror", it is paused and held there while a potential is introduced. The potential is created by moving 2 massive objects, each with mass M , to the left and right sides of the upper mirror, a distance r away from the mirror. The masses are brought towards the upper mirror in a symmetric manner such that the gravitational field caused by the masses is 0 at the upper mirror. We assume that the upper mirror is far enough away from the lower mirror such that the masses create zero potential (and zero field) at the lower mirror, which means they create a gravitational potential difference of Δ Δ U = − − 2 G M r between the upper and lower mirrors. Despite this gravitational potential difference, the gravitational field at the upper and lower mirrors is 0, and the atom is never in any position with a nonzero gravitation field. Still, a time T spent at the mirrors with that potential difference causes a phase shift,