Fig. 2: Propagation of wavepacket and associated null geodesics in the Schwarzschild spacetime geometry.

a The null geodesics (dashed curves) for beams that propagate very close to the event horizon—we set the event horizon at 1. b Schematic of wavepacket propagation radially in the Schwarzschild spacetime geometry. Alice and Bob are located at ra and rb, respectively, while θ and φ are polar and azimuthal angles of the standard spherical coordinates.