title: 1-1=Counterfactual: on the potency and significance of quantum non-events.
abstract: We study the unique role played in quantum mechanics by non-events or 'counterfactuals'. Our earlier analysis of 'quantum oblivion' has revealed some subtle stages in the measurement process, which may end up in self-cancellation. To these findings, we now add two insights derived by two time-symmetric interpretations of quantum mechanics. (i) Like all quantum interactions, the non-event is formed by the conjunction of forward-plus-backward-evolving wave functions. (ii) Then, it is another feature of such dual evolutions, namely the involvement of negative masses and energies, that enables Nature to make some events 'unhappen' while leaving causal traces.
authors.affiliation: Iyar, The Israeli Institute for Advanced Research, Rehovot, Israel avshalom@iyar.org.il.
authors.forename: A C
authors.identifier: http://orcid.org/0000-0002-4578-5986
authors.initials: AC
authors.lastname: Elitzur
authors.affiliation: Iyar, The Israeli Institute for Advanced Research, Rehovot, Israel H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: E
authors.initials: E
authors.lastname: Cohen
keywords: counterfactuals; interaction-free measurement; non-events; quantum oblivion; time symmetry