Friday, June 26, 2020

A First Take On A NYT Story About Collapse Theory In Quantum Mechanics

In story in yesterday's New York Times talked about someone who is developing an approach to quantum mechanics called "objective collapse models" and is portrayed as a radical rebel fighting scientific orthodoxy.

There is a lot of local color and personal profiling in the story, but the scientific core of it is less radical or rebellious than the atmospherics in the story would suggest.

In quantum mechanics there are lots of phenomena in which something behaves like a spread out wave until it is observed, after which when observed, they behave like point-like objects with more specific properties. Quantum mechanics is rather vague about exactly what it is about a measurement that triggers this "collapse of the wave function".

Angelo Bassi's "objective collapse models" are an attempt to put more definition on what triggers the collapse of the wave function. He does not try to dispute parts of quantum mechanics that are well established experimentally, as so many others crackpot theorists (mostly not professional physicists), who just don't like the realities that quantum mechanics reveals, do.

Wikipedia has this to say in its introduction to its Objective Collapse Theory page:
Objective-collapse theories, also known as models of spontaneous wave function collapse or dynamical reduction models, were formulated as a response to the measurement problem in quantum mechanics, to explain why and how quantum measurements always give definite outcomes, not a superposition of them as predicted by the Schrödinger equation, and more generally how the classical world emerges from quantum theory. The fundamental idea is that the unitary evolution of the wave function describing the state of a quantum system is approximate. It works well for microscopic systems, but progressively loses its validity when the mass / complexity of the system increases. 
In collapse theories, the Schrödinger equation is supplemented with additional nonlinear and stochastic terms (spontaneous collapses) which localize the wave function in space. The resulting dynamics is such that for microscopic isolated systems the new terms have a negligible effect; therefore, the usual quantum properties are recovered, apart from very tiny deviations. Such deviations can potentially be detected in dedicated experiments, and efforts are increasing worldwide towards testing them. 
An inbuilt amplification mechanism makes sure that for macroscopic systems consisting of many particles, the collapse becomes stronger than the quantum dynamics. Then their wave function is always well localized in space, so well localized that it behaves, for all practical purposes, like a point moving in space according to Newton’s laws.
In this sense, collapse models provide a unified description of microscopic and macroscopic systems, avoiding the conceptual problems associated to measurements in quantum theory.
The most well-known examples of such theories are: 
Ghirardi–Rimini–Weber (GRW) model 
* Continuous Spontaneous Localization (CSL) model 
Diósi–Penrose (DP) model 
Collapse theories stand in opposition to many-worlds interpretation theories, in that they hold that a process of wave function collapse curtails the branching of the wave function and removes unobserved behaviour.
The Stanford Encyclopedia of Philosophy has a nice entry on Collapse Theories that puts Bassi's work in context. This entry explains that experimental tests of his proposals have been proposed, but the differences in the theoretically expected experimental outcomes between these models and the mainstream alternatives are so subtle, that it is hard to do an experiment that definitively distinguishes between them. We can, however, do experiments that are close to being good enough to see these distinctions, and improvements in instrumental technologies should make it possible to confirm or  rule out this class of theories in the medium term future.

Bassi is also part of the LHCb collaboration jointly publishing papers such as this one experimentally measuring one of the Standard Model's weak force related constants, which governs the probability of a b quark transitioning to a charm quark in a W boson mediated interaction.

Bassi's most recent preprint on this subject is this one (which was eventually published):
Collapse models implement a progressive loss of quantum coherence when the mass and the complexity of quantum systems increase. We will review such models and the current attempts to test their predicted loss of quantum coherence.
Matteo Carlesso, Angelo Bassi, "Current tests of collapse models: How far can we push the limits of quantum mechanics?" arXiv (January 27, 2020) published in Quantum Information and Measurement (QIM) V: Quantum Technologies; OSA Technical Digest (Optical Society of America, 2019), paper S1C.3

Another paper exploring the line of research mentioned in the New York Times article is this September 25, 2019 preprint examining the question of whether there is a "minimum measurement time" for quantum phenomena. This paper references the "Continuous Spontaneous Localization" (CSL) model which is one of the objective collapse models that he works with in his research.

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