Figure 9.

When a neutral density ball falls on a viscous liquid (a) waves propagate outward from the ball (b,c,d) until finally the liquid, with the ball resting on it, is quiescent. If the ball is removed from the surface by some external means (a′), waves once again propagate outward from where the ball had been (b′,c′,d′), until finally the liquid is again quiescent. This backward mechanism is very different than the microscopic reverse mechanism (gray dashed arrows) for removal of the ball from the surface in which waves spontaneously propagate inward toward the ball (d_R,c_R,b_R) until finally the energy of the wave coalesces at the ball, propelling the ball away from the surface. The external removal of the ball corresponds molecularly to photo-dissociation,^[56] external changes to thermodynamic parameters^[57] (electric field strength, pressure, pH, or redox potential), or to computational disapparition.^[58] From the macroscopic or even mesoscopic perspective, the process d_R → c_R → b_R → a_R seems remarkably unlikely, requiring as it does energy to spontaneously concentrate from many degrees of freedom to the single degree of freedom of the ball. Even so, this is the most likely mechanism for thermally activated dissociation as required by the principle of microscopic reversibility.