Six Impossible Things: The Mystery of the Quantum World

by John Gribbin

This works as a method of calculating quantum behavior, as if things like electrons really did behave like this. But it also poses many puzzles. One of the most puzzling is a so-called “delayed choice” experiment, dreamed up by the physicist John Wheeler. He started from the fact that when photons are fired one at a time through the experiment with two holes, they still build up an interference pattern on the detector screen. But according to the CI, if a device is placed between the two holes and the detector screen to monitor which hole the photon goes through, the interference pattern will vanish, showing that each photon really did go through just one of the holes. The “delayed choice” comes in because we can decide whether or not to monitor the photons after they have passed the screen with two holes. Of course, human reactions are not fast enough to do this. But experiments have been carried out with automatic monitoring devices to do exactly this, switching the monitors on or off after the photons have passed the holes. They show that the interference pattern does indeed disappear when the photons are monitored, meaning that each photon (or the probability wave) only goes through one hole—even though the decision to monitor the photon was made only after it had passed the holes.