Guido Bacciagaluppi

Non-equilibrium in Stochastic Mechanics

The notion of non-equilibrium, in the sense of a particle distribution other than rho equal psi squared, is imported into Nelson’s stochastic mechanics, and described in terms of effective wavefunctions obeying non-linear equations. These techniques are applied to the discussion of non-locality in non-linear Schroedinger equations.

Probability and time symmetry in classical Markov processes

Definitions of time symmetry and examples of time-directed behaviour are discussed in the framework of discrete Markov processes. It is argued that typical examples of time-directed behaviour can be described using time-symmetric transition probabilities. Some current arguments in favour of a distinction between past and future on the basis of probabilistic considerations are thereby judged to be unjustified.

Leggett’s Theorem without Inequalities

We prove a no‐go theorem for a class of hidden variables theories that satisfy parameter independence. Specifically, we show that, assuming two conditions, there are no non‐trivial hidden variables models of the quantum predictions for product measurements on two systems in any maximally entangled state in a Hilbert space of dimension at least 3×3. The two conditions are parameter independence and a condition that we call conditional parameter independence. The result is analogous to the recent no‐go theorems based on Leggett’s inequalities and their generalisations.

Bohr’s Slit and Hermann’s Microscope

The Heisenberg microscope and its analysis by Weizsacker are used by Grete Hermann in her 1935 essay on the foundations of quantum mechanics to argue her claims about causality in quantum mechanics. In this chapter, I wish to draw a comparison between Hermann’s use of the Heisenberg microscope and another famous use of a very similar thought experiment : Bohr’s analysis of the suspended single slit in his reply to EPR . I shall argue that Hermann’s use of different aspects of the classical pictures in the treatment of the Heisenberg microscope makes her treatment closer to Bohr’s discussion of the suspended slit than to Heisenberg’s own treatment of the microscope. This suggests that Hermann, who equally gave an extensive analysis of complementarity in her 1935 essay, may be an especially acute interpreter of Bohr’s views. I conclude by looking at Hermann’s (and Bohr’s) approach in the context of more general examples of measurement and at possible limitations of the approach.

Grete Hermann’s Lost Manuscript on Quantum Mechanics

Prior to her extended stay in Leipzig and her 1935 essay on the foundations of quantum mechanics, Grete Hermann had written a manuscript on ‘Determinism and Quantum Mechanics’, which she had sent to some of the main quantum physicists at the time but had since disappeared. This manuscript was rediscovered by us in the Dirac Archive and appears in translation as Chap. 14 of this volume. In this chapter we give a first analysis of this fascinating manuscript, in which Hermann criticises known arguments for the incompatibility of quantum mechanics and determinism, including the theorem in von Neumann’s 1932 book, and sketches what a completion of quantum mechanics might have to look like.

Review of The Emergent Multiverse

We review and discuss the recent monograph by DavidWallace on Everettian Quantum Mechanics. This book is a high point of two decades of work on Everett in both physics and philosophy. It is also a beautiful and welcome exemplar of a modern way of doing metaphysics. We discuss certain aspects more critically and take the opportunity to sketch an alternative pragmatist approach to probability in Everett, to be fully developed elsewhere.

A Critic Looks at QBism

This chapter comments on that by Chris Fuchs on qBism. It presents some mild criticisms of this view, some based on the EPR and Wigner’s friend scenarios, and some based on the quantum theory of measurement. A few alternative suggestions for implementing a subjectivist interpretation of probability in quantum mechanics conclude the chapter.