Volume 16: Pages 537-542, 2003

Quantum Mechanics and Contrafactuality

Joseph Di Rienzi

College of Notre Dame of Maryland, Baltimore, Maryland 21210 U.S.A.

Fundamental properties of quantum mechanics include superposition, entanglement, and decoherence. Contra/actuality or counter/actual reasoning arises in quantum mechanics involving statements of results of measurements not taken but implied by prevailing experimental conditions. This type of logic is commonly invoked in connection with the Einstein‐Podolsky‐Rosen (EPR) correlations or Bell's theorem‐related thought experiments to demonstrate quantum contradictions when quantum theory is combined with local realistic theories. It has been suggested that these counterfactual statements need to be applied very carefully. This paper demonstrates how contrafactuality is implicit in the properties of quantum mechanics mentioned above and how the interconnections of superposition, entanglement, and decoherence make contrafactuality an integral part of quantum‐theoretical formulations. What a reformulation of quantum theory would entail under a more restricted use of counterfactual statements is also suggested.

Keywords: contrafactuality, counterfactual reasoning, superposition, entanglement, decoherence, EPR, Hardy condition, physical realism

Received: August 8, 2003; Published online: December 15, 2008