A direct experimental test for local commutativity (i.e., if and are associated with mutually spacelike regions) is proposed. It consists of a comparison of the distribution of outcomes of measurements in the two cases where alone is measured (no measurement apparatus for present) and where both are measured on the same sample, in short, a test of the compatibility of the two observation procedures for and . The implication compatibility → commutativity (substantially proved by von Neumann, but questioned by Park and Margenau) is proved in a stronger form, with close attention paid to the distinction between different procedures corresponding to a given self-adjoint operator.
- Received 25 February 1977
©1977 American Physical Society