In this episode Anna and Professor Halloway examine the delayed-choice experiment. The delayed choice experiment demonstrates that a "wave or particle" is not a property of the photon alone, it's a property of the entire experiment, including choices made later. Nothing travels backward; the universe simply doesn't assign a definite wave/particle history to the photon until the full measurement context is fixed. It remains one of the most vivid (and misinterpreted) demonstrations of quantum weirdness — and a core reason quantum mechanics rejects naive realism about what's "really happening" between measurements.