This thread is about my not understanding the conclusion that scientists came to that the photon makes a 'decision' on how to travel. In a simple interferometer experiment, a photon displays interference fringes only when a second beam splitter is present. According to Wiki, this led scientists to assert that the photon 'decides' to travels as a particle when there is no beam splitter and a wave when there is a beam splitter. The delayed choice version of the experiment demonstrates that the photon shows interference fringes, even if the second beam splitter is placed after the photon is already on it's way. Given the initial idea that the photon decides to travel as either a particle or wave depending on the apparatus, this led some scientists to conclude that the photon must act retroactively- if a 2nd beam splitter is placed, the photon goes back in time and decides to travel as a wave, whereas it was initially travelling as a particle (before the 2nd beam splitter was placed). Now, either I'm missing something very simple or these scientists were- I see no reason to assert that the photon makes any kind of decision in the first place. Going back to the simple interferometer experiment, I understand that the photon must be said to travel as a wave when the beam splitter is present- how else do the interference fringes appear- but I do not see any reason that it must be said to travel as a particle when the beam splitter is not present. Why not posit that it travels as a wave in both instances? I mean, in the simple interferometer exp, without the 2nd beam splitter you wouldn't expect interference fringes from a wave as the waves are travelling in perpendicular directions. If this is the case, then you can say that the photon travels as a wave in both instances and therefore there is no photon 'decision'. In the delayed choice version, this means there is no need to posit retroactivity. Where have I gone wrong?