How to Measure Quantum Foam With a Tabletop Experiment: Bekenstein's goal is to move the block by a distance that is about equal to the Planck length. His method is simple: zap the block with a single photon.
The photon carries a small amount of moment and consequently pushes the block as it enters the glass, giving it some momentum. As the photon leaves the block, the block comes to rest.
So the result of the photon's passage is that it moves the block a small distance.
Bekenstein's idea is that if this distance is smaller than the Planck length, then the block cannot move and the photon cannot pass through it.
So the experiment involves measuring the number of photons that pass through the block. If the number is fewer than predicted by classical optics, then that proves the existence of quantum foam.