presumably traveled as a particle up to that moment, would the beam splitter let it pass through and manifest itself as would a particle were that second beam splitter not to be there? Or, would it behave as though the second beam-splitter had always been there? Would it manifest interference effects? And if it did manifest interference effects then to have done so it must have gone back in time and changed its "decision" about traveling as a particle to traveling as a wave. Note that Wheeler wanted to investigate several hypothetical statements by obtaining objective data. Albert Einstein did not like these possible consequences of quantum mechanics. However, when experiments were finally devised that permitted both the double-slit version and the interferometer version of the experiment, it was conclusively shown that a photon could begin its life in an experimental configuration that would call for it to demonstrate its particle nature, end up in an experimental configuration that would call for it to demonstrate its wave nature, and that in these experiments it would always show its wave characteristics by interfering with itself. Furthermore, if the experiment was begun with the second beam-splitter in place but it was removed while the photon was in flight, then the photon would inevitably show up in a detector and not show any sign of interference effects. So the presence or absence of the second beam-splitter would always determine "wave or particle" manifestation. Many experimenters reached an interpretation of the experimental results that said that the change in final conditions would retroactively determine what the photon had "decided" to be as it was entering the first beam-splitter. As mentioned above, Wheeler rejected this interpretation. Cosmic interferometer In an attempt to avoid destroying normal ideas of cause and effect, some theoreticians suggested that information about whether there was or was not a second beam-splitter installed could somehow be transmitted from the end point of the experimental device back to the photon as it was just entering that experimental device, thus permitting it to make the proper "decision." So Wheeler proposed a cosmic version of his experiment. In that thought experiment he asks what would happen if a quasar or other galaxy millions or billions of light years away from Earth passes its light around an intervening galaxy or cluster of galaxies that would act as a gravitational lens. A photon heading exactly towards Earth