Wheeler's delayed choice experiment

'''Wrong: must also consider double slit with delayed choice. + must consider putting measurement device on the path. Bottom line - take time to reread all accessible materials to understand what is the meaning of these experiments.'''

One of interpretations (ppt presentation) says that Wheeler's experiment demonstrates "retro-causality", that is by detecting a photon after it passes double-slit we may "make it" to pass via one of the slits in the past(!), whilst if we do not try to detect a photon, it is considered to pass via both slits. I do not like such an interpretation and I do not think this experiment demonstrates any kind of retro-causality. But such an interpretation does exist..

Wheeler's delayed choice experiment attempts to resolve quite a subtle question of whether the wave-like behavior of a particle exists in a case when an experiment is also completely explicable with a particle-like behavior.

For instance, suppose that a half-silvered mirror directs a photon into one of the two paths, then this photon may be discovered by a detector. Is that description correct, or should we rather believe that the photon in this experiment is directed in both directions (superposed state) and then collapses on one of two detectors?

The resolution of this dilemma, according to Wheeler, is to insert a second half-mirror in the end of the trajectory, effectively creating a Mach–Zehnder interferometer, and verifying whether interference appears (it does). Such insertion is done fast at the very last moment (therefore the "delayed choice" name), so that photons cannot "know" of the final configuration of the experimental apparatus when they start their way (unless we allow faster than light information transfer speed).

In other words, we either conclude that the photon existed in superposed state until the very last moment, or we should assume faster than light communication (btw, not completely unthincable as it does not transfer "classical" information).

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One extreme implementation of this experiment is observing interference from the two beams coming from a star subjected to gravitational lensing. Such interference means either that the wave nature of photons existed from their departure from the star and until the very last moment, or, alternatively, that the information (of the nature of observation) could be transferred to the past (thousands of years back). Of course, more sophisticated options may be conceived, like superdeterminism.