GRW Theory (Ghirardi, Rimini, Weber Model of Quantum Mechanics)

Roderich Tumulka

Sheldon Goldstein, Roderich Tumulka, Nino Zanghı̀

Charles Wesley Cowan, Roderich Tumulka

Among several possibilities for what reality could be like in view of the empirical facts of quantum mechanics, one is provided by theories of spontaneous wave function collapse, the best known of which is the Ghirardi–Rimini–Weber (GRW) theory. We show mathematically that in GRW theory (and similar theories) there are limitations to knowledge, that is, inhabitants of a GRW universe cannot find out all the facts true of their universe. As a specific example, they cannot accurately measure the number of collapses that a given physical system undergoes during a given time interval; in fact, they cannot reliably measure whether one or zero collapses occur. Put differently, in a GRW universe certain meaningful, factual questions are empirically undecidable. We discuss several types of limitations to knowledge and compare them with those in other (no-collapse) versions of quantum mechanics, such as Bohmian mechanics. Most of our results also apply to observer-induced collapses as in orthodox quantum mechanics (as opposed to the spontaneous collapses of GRW theory). 1 Introduction 1.1 Known examples of limitations to knowledge 1.2 Remarks2 Brief Review of GRW Theories 2.1 The GRW process 2.2 GRWm 2.3 GRWf3 First Examples of Limitations to Knowledge in GRW Theories4 Measurements of Flashes in GRWf, or of Collapses in GRWm 4.1 An example in which ψ is known 4.2 Other choices of ψ 4.3 Experiments beginning before t 2 4.4 If ψ is random 4.5 Optimal way of distinguishing two density matrices 4.6 If ψ is unknown5 Measurements of m(x) in GRWmAppendix

William Michael Kallfelz

Abstract I examine G.B. Baga’s arguments for the Ghirardi-Rimini-Weber (GRW) interpretation of non-relativistic quantum mechanics as ideally suited for Whitehead’s philosophy. Much of Bagci’s claims are in response to Michael Epperson, who argues in the same vein in favor of decoherence accounts (Omnès; Zureck). Pace Epperson, I do not think that decoherence is the final arbiter here, and instead I contrast GRW with several other accounts addressing foundational problems of quantum theory (Finkehtein; Green; Peres and Temo; etc.), which aho account for relativistic covariance, while GRW does not. I argue that such latter research programs align themselves in a more convincing manner with Whitehead’s scheme, in epistemic as well as metaphysical senses, than GRW.

David Z. Albert

Michael te Vrugt, Gyula I. Tóth, Raphael Wittkowski

Abstract Wigner functions, allowing for a reformulation of quantum mechanics in phase space, are of central importance for the study of the quantum-classical transition. A full understanding of the quantum-classical transition, however, also requires an explanation for the absence of macroscopic superpositions to solve the quantum measurement problem. Stochastic reformulations of quantum mechanics based on spontaneous collapses of the wavefunction are a popular approach to this issue. In this article, we derive the dynamic equations for the four most important spontaneous collapse models—Ghirardi–Rimini–Weber (GRW) theory, continuous spontaneous localization (CSL) model, Diósi-Penrose model, and dissipative GRW model—in the Wigner framework. The resulting master equations are approximated by Fokker–Planck equations. Moreover, we use the phase-space form of GRW theory to test, via molecular dynamics simulations, David Albert’s suggestion that the stochasticity induced by spontaneous collapses is responsible for the emergence of thermodynamic irreversibility. The simulations show that, for initial conditions leading to anti-thermodynamic behavior in the classical case, GRW-type perturbations do not lead to thermodynamic behavior. Consequently, the GRW-based equilibration mechanism proposed by Albert is not observed.