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In quantum mechanics, Counterfactual definiteness (CFD) is the ability to speak meaningfully of the definiteness of the results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured). Counterfactual definiteness is a term used in discussions of physics calculations, especially those related to the phenomenon called quantum entanglement and those related to the Bell inequalities.〔Enrique J. Galvez, "Undergraduate Laboratories Using Correlated Photons: Experiments on the Fundamentals of Quantum Mechanics," p. 2f, says, "Bell formulated a set of inequalities, now known as 'Bell’s inequalities,' that would test non-locality. Should an experiment verify these inequalities, then nature would be demonstrated to be local and quantum mechanics incorrect. Conversely, a measurement of a violation of the inequalities would vindicate quantum mechanics’ non-local properties."〕 The single adjective "counterfactual" may also appear in physics discussions where it is frequently treated as a noun. The word "counterfactual" does not mean "characterized by being opposed to fact." Instead, it is used to characterize values that could have been measured but, for one reason or another, were not.〔Inge S. Helland, "A new foundation of quantum mechanics," p. 386: "Counterfactual definiteness is defined as the ability to speak with results of measurements that have not been performed (i.e., the ability to assure the existence of objects, and properties of objects, even when they have not been measured").〕〔W. M. de Muynck, W. De Baere, and H. Martens, "Interpretations of Quantum Mechanics, Joint Measurement of Incompatible Observables, and Counterfactual Definiteness" p. 54 says: "Counterfactual reasoning deals with nonactual physical processes and events and plays an important role in physical argumentations. In such reasonings it is assumed that, if some set of manipulations were carried out, then the resulting physical processes would give rise to effects which are determined by the formal laws of the theory applying in the envisaged domain of experimentation. The physical justification of counterfactual reasoning depends on the context in which it is used. Rigorously speaking, given some theoretical framework, such reasoning is always allowed and justified as soon as one is sure of the possibility of at least one realization of the pre-assumed set of manipulations. In general, in counterfactual reasoning it is even understood that the physical situations to which the reasoning applies can be reproduced at will, and hence may be realized more than once."Text was downloaded from: http://www.phys.tue.nl/ktn/Wim/i1.pdf 〕 Counterfactual measurement values are values derived by means other than direct observation or measure, such as by calculation on the basis of well-substantiated theory. If one knows an equation that permits deriving reliably expected values from a list of inputs to the physical system under investigation, then one has "counterfactual definiteness" in the knowledge of that system. Much of what humans regularly do in the field of engineering to design complex machines such as submarines or the Hubble Telescope depend on the use of physics theories to calculate desired specifications for structures, lenses, etc. The values used to grind a lens or fortify a bulkhead are not the ''ad hoc'' results of empirical measurements made to substantiate the theories of optics or the conclusions of materials scientists, but that does not make them in any sense unreliable. They are as reliable as all of the research done to create them, and all the instances of use in the real world in which they have not been disproven. ==Introduction== The subject of counterfactual definiteness receives attention in the study of quantum mechanics because it is argued that, when challenged by the findings of quantum mechanics, classical physics must give up its claim to one of three assumptions: locality (no "spooky action at a distance"), counterfactual definiteness, and no conspiracy.〔http://en.citizendium.org/wiki/Entanglement_%28physics%29#Counterfactual_definiteness〕 If physics gives up the claim to locality, it brings into question our ordinary ideas about causality and suggests that events may transpire at faster-than-light speeds.〔Christoph Saulder, "Contextuality and the Kochen-Specker Theorem", p. 11.〕 which also amounts to admitting the possibility of sudden "magical" intervention in human affairs from unknown and possibly unknowable sources of action. If physics gives up the "no conspiracy" condition, it becomes possible for "nature to force experimenters to measure what she wants, and when she wants, hiding whatever she does not like physicists to see."〔Angel G. Valdenebro, "Assumptions Underlying Bell's Inequalities," p. 6. 〕 If physics rejects the possibility that, in all cases, there can be "counterfactual definiteness," then it rejects some features that humans are very much accustomed to regarding as enduring features of the universe. "The elements of reality the EPR paper is talking about are nothing but what the property interpretation calls properties existing independently of the measurements. In each run of the experiment, there exist some elements of reality, the system has particular properties < #ai > which unambiguously determine the measurement outcome < ai >, given that the corresponding measurement a is performed."〔Internet Encyclopedia of Philosophy, "The Einstein-Podolsky-Rosen Argument and the Bell Inequalities," section 3.〕 Something else, something that may be called "counterfactuality," permits inferring effects that have immediate and observable consequences in the macro world even though there is no empirical knowledge of them. One such example is the Elitzur-Vaidman bomb tester.〔 Rick Bradford,"The Observability of Counterfactuals" p. 1 says: "Suppose something could have happened, but actually did not happen. In classical physics the fact that an event could have happened but didn't can make no difference to any future outcome. Only those things which actually happen can influence the future evolution of the world. But in quantum mechanics it is otherwise. The potential for an event to happen can influence future outcomes even if the event does not happen. Something that could happen but actually does not is called as counterfactual. In quantum mechanics counterfactuals are observable they have measurable consequences. The Elitzur-Vaidman bomb test provides a striking illustration of this. "See: http://www.rickbradford.co.uk/QM13Counterfactuals.pdf〕 These phenomena are not directly germane to the subject under consideration here. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「counterfactual definiteness」の詳細全文を読む スポンサード リンク
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