Article,
Models of wave-function collapse, underlying theories, and experimental tests
Reviews of Modern Physics, 85 (2): 471--527 (Apr 21, 2013)
DOI: 10.1103/revmodphys.85.471
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
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
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.
