Models of wavefunction collapse, underlying theories, and experimental tests
Abstract
Quantum mechanics is an extremely successful theory that agrees with every experimental test. 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 explain why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule. In this article a review is given of an experimentally falsifiable phenomenological proposal, known as continuous spontaneous collapse: a stochastic nonlinear modification of the Schrödinger 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 gravityinduced collapse. As the macroscopic scale is approached, 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 test the validity of linear superposition for large systems, are reviewed here, and their technical challenges, current results, and future prospects summarized. 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.
 Publication:

Reviews of Modern Physics
 Pub Date:
 April 2013
 DOI:
 10.1103/RevModPhys.85.471
 arXiv:
 arXiv:1204.4325
 Bibcode:
 2013RvMP...85..471B
 Keywords:

 03.65.Ta;
 03.65.Ud;
 03.65.Yz;
 42.50.Xa;
 Foundations of quantum mechanics;
 measurement theory;
 Entanglement and quantum nonlocality;
 Decoherence;
 open systems;
 quantum statistical methods;
 Optical tests of quantum theory;
 Quantum Physics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Theory
 EPrint:
 130 pages, 4 figures, shortened to meet journal page limit requirement, original Appendix on Stochastic Processes now appears as separate article on arXiv [quantph], accepted for publication in Reviews of Modern Physics [http://rmp.aps.org/accepted/4b07bE82S9a1ad0a00765c81730c64affbedd06b2]