Obituary for a Flea

Abstract

The Landsman-Reuvers proposal to solve the measurement problem from within quantum theory is extensively analysed. In favor of proposals of this kind, it is shown that the standard reasoning behind objections to solving the measurement problem from within quantum theory rely on counterfactual reasoning or mathematical idealisations. Subsequently, a list of objections/challenges to the proposal are made. Part of these objections are equally important for all attempts at solving the measurement problem, such as the problem of interpreting small numbers in the density matrix, the problem of reproducing the Born rule, the use of pure states as a tool to alleviate the interpretational issues of quantum states, and the necessity of introducing classical certainties which are not strictly present in quantum theory. The additional objections that are particular to the proposal, such as the physical interpretation/origin of the flea perturbation, the use of potentials to solve a dynamical problem, slow collapse times, the inability to handle unequal probabilities, and the dictatorial role of the flea perturbation, lead us to believe that the Landsman-Reuvers proposal is lacking in both physical grounding and theoretical promise. Finally, an overview is given of the challenges that were encountered in this attempt to solve the measurement problem from within quantum theory.

Bohrification: From classical concepts to commutative algebras

“…Moreover, all reference to the possible completeness of quantum states has also, implicitly, been bracketed under (c). The reason is that Maudlin's (and everyone else's) assumption 1(b), which forms the basis of all discussions of the measurement problem, is actually a counterfactual of the kind one should avoid in quantum mechanics ( van Heugten & Wolters, 2016). What this assumption says is that if ψ D were the initial state of Schrödinger's Cat, then it would evolve (linearly) according to the Schrödinger equation with given Hamiltonian h. If the initial state were ψ L , then it would evolve according to the same Hamiltonian h, and if it were (ψ D + ψ L )/ √ 2, then it would evolve according to h, too.…”

“…Having said this, many problems remain to be overcome since the first steps were taken in Landsman & Reuvers (2013) and Landsman (2013); see van Heugten & Wolters (2016).…”