Wheeler’s delayed‑choice experiment is one of those quantum ideas that feels like it was designed specifically to make classical intuition throw up its hands and storm out of the room.
At its core, it asks a deceptively simple question: when does a photon decide to behave like a particle or a wave?
A playful yet profound challenge to our everyday sense of time, causality, and the idea that the universe has a fixed history waiting to be observed.
Wheeler imagined a setup where that “decision” is not made at the moment when the photon travels through an interferometer wave-like or particle-like, but after it has already passed the point where the choice should matter, as if affected by a measurement of a decision made at a later time.
By changing the measurement setup at the last possible instant, the experiment suggests that present actions can determine how a particle behaved in the past.
However, information transfer into the past or retrocausal signaling, as opposed to influence without information transfer, remains controversial since it has yet to be demonstrated experimentally.
Wheeler’s concept has two equivalent paths between a source and detector. The experiment is run in two versions: one designed to detect wave interference and one designed to detect particles.
The new ingredient in Wheeler’s approach is a delayed-choice between these two experiments. The decision to measure wave interference or particle path is delayed until just before the detection.
The goal is to ensure that any traveling particle or wave will have passed the area of two distinct paths in the quantum system before the choice of experiment is made.