Greetings ! Here's an example by Steven Weinberg I read sometime ago (in SciAm I think): Imagine a light beam coming from a quasar billions of light years away. It encounters galaxy superclusters on the way and is split so that gravitational lensing is observed when we look at the quasar from the Earth. Now, let's take the uncollapsed WFs of photons in the beam. We now have the choice of detecting the photons in a 1 or 2 slit (as in the double-slit experiment) configuration. If we choose to use a single slit and aim at one of the images we'll collapse the WF of the photon (let's say we have just 2 images). If we use two slits we'll collapse it and get the ussual difraction pattern. If we try to interpret these results it appears abvious that we've decided upon the path - if any, that the photon took during all these billions of years. The above example is of course intresting in its own right, and was used by Weinberg for emphasis of some of the "conscious mind" stuff in various interpretations of QM. However, I'll be using it in this post in a somewhat different way, and want to inquire about physics rather than its interpretations (though separating the two is not that easy at this point). First let's suppose that our 2 WF splits are both trapped for a while in an orbit around two different black holes, before they reach us. Let's further assume that the light beam is extremely intense. So much that such an amount of light at a close orbit would be able to affect the BHs. Now let's play the "what if" game: The BH's are affected by the light (the WFs of some of the photons in the beam will have to collapse for that to happen, I guess) and we detect the shifts in its radiation due to these gravitational distortions before the quasar's beam reaches us. Now, let's say that some WFs did not collapse. I assume we'll agree that these will not affect the BHs initially ? And now, we run one of the above versions of the experiment. What will happen in each case ? Thanks ! Live long and prosper.