Causal ambiguity is experimentally real
“Within the mathematical formalism of quantum theory, ambiguity about causation emerges in a perfectly logical and consistent way.”* This is an extraordinary statement made in an excellently written article by Philip Ball in the preeminent journal ‘Nature’, 28 June 2017.
The article explores experiments which put photons through pairs of optical gates, and find that it is impossible to state in which order the photons go through the gates. “It’s not that this information gets lost or jumbled — it simply doesn’t exist. The experimental arrangement enables information to be shared between two events in ways that are ruled out if there is a definite causal order – So that time seems to run in two directions at once.”
This links in directly with the issues around distantly separated entangled photons cooperating instantly when the property of one is measured. It is probable that quantum circuits incorporating causal ambiguity would offer a practical benefit of greater quantum-computing speed. But more important is the potential for theoretical understanding of causality and the obscure nature of quantum effects.
*I borrowed this extraordinary sentence as one of the chapter titles in my sci-fi novel.