--- title: "Aspect changes the analyzers while photons are in flight" description: "What did rapidly changing the optical path add to the 1982 Bell experiment, and what remained assumed?" canonical_url: "https://sandboxphysics.com/stories/quantum-evidence/aspect-switching-spacetime-records/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/quantum-evidence/aspect-switching-spacetime-records/" last_modified: "2026-10-24" --- [Quantum physics: reading the evidence](https://sandboxphysics.com/stories/quantum-evidence/) Nobel A · Evidence and control # Aspect changes the analyzers while photons are in flight Sandbox Physics · 2026-10-16 What did rapidly changing the optical path add to the 1982 Bell experiment, and what remained assumed? ## Change the setting while the photons are in flight A fixed polarizer leaves a long time for information about its setting to cross the apparatus. In 1982 Aspect, Dalibard and Roger made the measurement setting change while the photons were still in flight. Each arm held an optical switch that directed light toward one of two fixed polarizers. Changing the route changed the polarization question, without having to turn a mechanical part. The aim was to make the setting change too late for a light-speed signal to reach the distant measurement in time. The key comparison was therefore a time against a distance: how often did the switches change, and how long would light take to travel between the two sides? If two events are farther apart than light can travel during the interval between them, they are spacelike separated. In the simplified condition below, L is their distance, delta-t is their time difference in one inertial frame, and c is the speed of light. The paper compared switching intervals of roughly ten nanoseconds with a light-travel time of roughly forty nanoseconds between the two sides. A real timing check has to define each event and include detector delays, path lengths and safety margins with their uncertainties. The moment an electrical command starts is not necessarily the moment a photon meets the chosen analyzer. Collapsing all these events into one convenient timestamp can reverse the conclusion. $$ c\,|\Delta t|