Delayed Choice Experiment

Physics: Wheeler's delayed-choice (1978): the decision to observe wave (interference) or particle (which-path) behavior is made AFTER the photon has already entered the interferometer. Without BS2: which-path → particle behavior, no interference. With BS2: wave → interference fringes. Quantum mechanics: superposition collapses only at measurement — the photon doesn't "decide" its nature in advance. Experiments: Hellmuth 1987, Kim 2000 (quantum eraser), Jacques 2007 (spacelike separation). Cosmic version: quasar lensed by galaxy, photon path chosen after billion-year journey.