Explanatory Strain in Wheeler’s Delayed Choice

“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. … 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. … experiments…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.”

— Gribbin, Six Impossible Things, pp. 35, 37

Wheeler’s delayed choice experiment stretches classical temporality like taffy. According to the Copenhagen Interpretation (CI):

• The photon passes through the double-slit screen.

• Only after passing, a choice is made to monitor it.

• The observed pattern depends on this future choice.

On the surface, this reads as if the future affects the past, an apparent retrocausal paradox. The explanatory strain is immediate: classical intuition about cause and effect is in tension with what is observed.

Copenhagen’s manoeuvre

Copenhagen resolves the tension without explanation, by:

• Treating the photon as a probability wave, not a classical object.

• Allowing the interference pattern to depend on measurement context rather than particle trajectory.

• Restoring classical intuition at detection via collapse, which occurs only when measurement is made.

The result is a juggling act: particle-like language persists (“the photon only goes through one hole”), classical causality seems strained, and collapse is invoked as a narrative prop rather than a mechanism.

The locus of strain

The reader is asked to hold three incompatible ideas simultaneously:

1. The photon behaves as a wave in transit.

2. Measurement choice after passage determines past behaviour.

3. Collapse restores classical outcomes at detection.

This is explanatory strain in its temporal form: the theory is narratively coherent but conceptually over-stretched.

The relational perspective

From a relational ontology:

• The photon does not travel with a determinate path.

• The interference pattern is an instantiation of potential, defined by the perspectival cut of the experiment.

• Measurement does not reach back in time; it simply actualises one of the possible outcomes.

• Classical causality is preserved; the apparent retrocausality dissolves when we stop insisting that potential trajectories are physical histories.

Takeaway

Wheeler’s delayed choice demonstrates that explanatory strain is not a quirk, but a structural feature of the Copenhagen narrative. The theory pressures classical intuitions about time, trajectory, and particle behaviour to fit inherently relational phenomena. Collapse and wave descriptions act as props that smooth the story, but the underlying tension remains. Viewed relationally, no paradox exists: the photon is never forced to know or decide after the fact — the experiment defines the possible and actual outcomes through the cut.