Models of Wave-function Collapse, Underlying Theories, and Experimental Tests

TL;DR

This paper reviews the Continuous Spontaneous Collapse model, a modification of quantum mechanics that explains the absence of macroscopic superpositions and discusses experimental tests that could confirm or refute this theory within the next two decades.

Contribution

It provides a comprehensive review of the phenomenological collapse models, underlying theories, and the experimental efforts to test their validity.

Findings

Collapse models align with quantum results at microscopic scales

Experimental tests are ongoing and could verify or falsify the models

Predictions diverge from quantum mechanics at macroscopic scales

Abstract

Quantum mechanics is an extremely successful theory that agrees with every experiment. However, the principle of linear superposition, a central tenet of the theory, apparently contradicts a commonplace observation: macroscopic objects are never found in a linear superposition of position states. Moreover, the theory does not really explain as to why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule. In this article we review an experimentally falsifiable phenomenological proposal, known as Continuous Spontaneous Collapse: a stochastic non-linear modification of the Schr\"{o}dinger equation, which resolves these problems, while giving the same experimental results as quantum theory in the microscopic regime. Two underlying theories for this phenomenology are reviewed: Trace Dynamics, and gravity induced collapse. As one approaches the macroscopic scale, the predictions of this proposal begin to differ appreciably from those of quantum theory, and are being confronted by ongoing laboratory experiments that include molecular interferometry and optomechanics. These experiments, which essentially test the validity of linear superposition for large systems, are reviewed here, and their technical challenges, current results, and future prospects summarized. We conclude that it is likely that over the next two decades or so, these experiments can verify or rule out the proposed stochastic modification of quantum theory.