A possible experimental verification of retroactivity

We consider a variant of Young's double-slit experiment with polarized photons for which quantum theory predicts that different amounts of angular momentum will be absorbed at a certain location M according as we set up a revolving plate or a Nicol prism at M. This difference, which implies a retroactive effect because angular momentum is a conserved quantity, can in principle be established experimentally. The relevant experiment differs fundamentally from Wheeler's delayed-choice experiments.

Another experiment is also discussed in which retroactivity cannot be expected to manifest, though it is seemingly similar to the first one.

Finally, we consider an analogous experiment with spin particles, in which it can also be expected from quantum mechanics that a retroactive effect will appear. Here, too, experimental verification is in principle possible.