Editing Counterfactual definiteness (section)

==Overview==

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: [[Principle of locality|locality]] (no "[[spooky action at a distance]]"), [[No-conspiracy assumption|no-conspiracy]] (called also "asymmetry of time"),<ref>Gábor Hofer-Szabó, Miklós Rédei, László E. Szabó, "The principle of the common cause" (Cambridge 2013), Sect. 9.2 "Local and nonconspiratorial common cause systems".</ref><ref>T.N. Palmer "Bell's conspiracy, Schrödinger's black cat and global invariant sets", Philosophical Transactions of the Royal Society A, 2015, vol. 373, issue 2047.</ref> or counterfactual definiteness (or "non-contextuality").

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.<ref>Christoph Saulder, "Contextuality and the Kochen-Specker Theorem", p. 11. Available from the author at: http://www.equinoxomega.net/files/studies/quantenphysik_Handout.pdf</ref>

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."<ref>Angel G. Valdenebro, "Assumptions Underlying Bell's Inequalities," p. 6.</ref>

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 < #a<sub>i</sub> > which unambiguously determine the measurement outcome < a<sub>i</sub> >, given that the corresponding measurement '''a''' is performed."<ref>Internet Encyclopedia of Philosophy, "The Einstein-Podolsky-Rosen Argument and the Bell Inequalities," section 3.</ref>

As a noun, "counterfactual" may refer to an inferred effect or consequence of an unobserved macroscopic event. An example is [[counterfactual quantum computation]].<ref name="Bradford">Rick Bradford, "The Observability of Counterfactuals" p.1. "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." http://www.rickbradford.co.uk/QM13Counterfactuals.pdf</ref>