Wave packets falling under a mirror

We depict and analyse a new effect for wave packets falling freely under a barrier or well. The effect appears for wave packets whose initial spread is smaller than the combination l_g³/|z₀|, between the gravitational length scale l_g = (ℏ²c ²/2m²g)^1/3 and the initial location of the packet z₀. It consists of a diffractive structure that is generated by the falling and spreading wave packet and the waves reflected from the obstacle. The effect is enhanced when the Gross-Pitaevskii interaction for positive scattering length is included. The theoretical analysis reproduces the essential features of the effect. Experiments emanating from the findings are proposed.