Measurements according to Consistent Histories Elias Okon a,n , Daniel Sudarsky b a Instituto de Investigaciones Filosóficas, Universidad Nacional Autónoma de México, Mexico City, Mexico b Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico article info Article history: Received 3 September 2013 Received in revised form 8 August 2014 Accepted 22 August 2014 Keywords: Consistent Histories Consistent Quantum Theory Measurement problem abstract We critically evaluate the treatment of the notion of measurement in the Consistent Histories approach to quantum mechanics. We find such a treatment unsatisfactory because it relies, often implicitly, on elements external to those provided by the formalism. In particular, we note that, in order for the formalism to be informative when dealing with measurement scenarios, one needs to assume that the appropriate choice of framework is such that apparatuses are always in states of well defined pointer positions after measurements. The problem is that there is nothing in the formalism to justify this assumption. We conclude that the Consistent Histories approach, contrary to what is claimed by its proponents, fails to provide a truly satisfactory resolution for the measurement problem in quantum theory. & 2014 Elsevier Ltd. All rights reserved. When citing this paper, please use the full journal title Studies in History and Philosophy of Modern Physics 1. Introduction The Consistent Histories (CH) approach to quantum mechanics (also known as Decoherent Histories or Consistent Quantum Theory) is an interpretation of quantum theory that, according to its proponents, overcomes the measurement problem. The form- alism is supposed to assign probabilities to histories of all kinds of systems, microscopic or macroscopic, using a single and universal machinery, and without any reference to measurements. As a result, CH is claimed to require neither a separation of the situation under consideration into classical and quantum parts, nor a special treatment for the measurement situations, thus solving the measurement problem. In this work, however, we challenge such assertions. To do so, we evaluate in detail its treatment of the notion of measurement. We find such treatment unsatisfactory because it relies, often implicitly, on elements external to those provided by the formalism. Furthermore, we argue that the introduction of these external notions invalidates the claim that CH solves the measurement problem. We conclude, then, that the CH approach fails to provide a truly satisfactory solution to the measurement problem of quantum mechanics. Our paper is organized as follows: in Section 2, we introduce the measurement problem afflicting the standard formulation of quantum mechanics and we describe the CH formalism. In Section 3, we evaluate the treatment of measurements in CH and we present our main arguments against the claim that CH solves the measurement problem; we also discuss possible objections to our arguments. In Section 4, we comment on a couple of possible replies of a more general scope against our criticism, regarding the necessity for, and the existence of alternatives to CH, and in Section 5, we present our conclusions. 2. The measurement problem and the Consistent Histories approach The measurement problem, as commonly understood, is the fact that, even though the standard formulation of quantum mechanics depends crucially on the notion of measurement (to decide when to use the unitary evolution and when the reduction postulate), such notion is never formally defined within the theory. Then, in order to apply the formalism, one needs to know, by means external to quantum mechanics, what constitutes a measurement, when a measurement is taking place, and what it is one is measuring. An alternative way to present the problem is as a mismatch between experience and some predictions of unitary quantum Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/shpsb Studies in History and Philosophy of Modern Physics http://dx.doi.org/10.1016/j.shpsb.2014.08.011 1355-2198/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: eokon@filosoficas.unam.mx (E. Okon), sudarsky@nucleares.unam.mx (D. Sudarsky). Studies in History and Philosophy of Modern Physics 48 (2014) 7–12