Causality constraints on nonlocal measurements

Ying Zhang, Kan He

Lev Vaidman A

We report results of an investigation of relativistic causality constraints on the measurability of nonlocal variables. We show that measurability of certain nondegenerate variables with entangled eigenstates contradicts the principle of causality, but that there are other, certainly nonlocal, variables which can be measured without breaking causality. We show that any causal measurement of nonlocal variables must erase certain local information. For example, for a system of two spin-1/2 particles, even if we take the weakest possible definition of verification measurement, verification of an entangled state must erase all local information.

Lev Vaidman A

Sandu Popescu, Lev Vaidman A

Consequences of relativistic causality for measurements of nonlocal characteristics of composite quantum systems are investigated. It is proved that verification measurements of entangled states necessarily erase local information. A complete analysis of measurability of nondegenerate spin operators of a system of two spin-1/2 particles is presented. It is shown that measurability of certain projection operators which play an important role in axiomatic quantum theory contradicts the causality principle.

Amos Maley

This paper shows that apparent nonlocal causation arises from a category error: treating relations internal to a constraint envelope as if they were interactions between distinct envelopes. When admissibility and envelope non-contact are enforced, no coherent notion of nonlocal causal influence remains. The analysis demonstrates that correlations, instantaneous updates, and coordination at a distance are admissible only as internal re-descriptions within a single envelope. Interpreting them as causal connections between separated systems requires illicit scope transport and implicit global anchoring. Once these moves are excluded, the confusion dissolves without introducing hidden variables or additional postulates. The result removes the need for nonlocal causation as an explanatory primitive. No alternative mechanisms are proposed. The paper records the structural reason nonlocal causation cannot appear in any admissible representational framework governed by constraint envelopes.

Daegene Song