Collapse theories stand in opposition to many-world theories. They hold that a process of [[wavefunction collapse]] curtails the branching of the wavefunction and removes unobserved behaviour.<ref>[http://plato.stanford.edu/entries/qm-collapse/ Stanford Encyclopedia of Philosophy on Collpase Theories]</ref> Objective collapse theories differ from the [[Copenhagen interpretation]] in regarding both the wavefunction and the process of collapse as ontologically objective. The Copenhagen interpretation includes collapse, but it is non-committal about the objective reality of the wave function, and because of that it is possible to regard Copenhagen-style collapse as a subjective or informational phenomenon. In objective theories, collapse occurs randomly ("spontaneous localization"), or when some physical threshold is reached, with observers having no special role.
Objective collapse theories differ from the [[Copenhagen interpretation]] in regarding both the wavefunction and the process of collapse as ontologically objective. In objective theories, collapse occurs randomly ("spontaneous localization"), or when some physical threshold is reached, with observers having no special role. Thus, they are realistic, indeterministic, no-hidden-variables theories. The mechanism of collapse is not specified by standard quantum mechanics, which needs to be extended if this approach is correct, and thus Objective Collapse is more of a theory than an interpretation. Examples include the [[Ghirardi-Rimini-Weber theory]]<ref>[http://www.romanfrigg.org/writings/GRW%20Theory.pdf Frigg, R. GRW theory]</ref>
Objective collapse theories regard the present formalism of quantum mechanics as incomplete, in some sense. (For that reason it is more correct to call them theories than interpretations). They subdivide into two subtypes, depending on how the hypothesised mechanism of collapse stands in relation to the unitary evolution of the wavefunction. One subtype of theory seeks to find collapse ''within'' the evolution of the wavefunction, often by modifying the equations to introduce small amount s of non-linearity. A well-known example is the [[Ghirardi-Rimini-Weber theory]]<ref>[http://www.romanfrigg.org/writings/GRW%20Theory.pdf Frigg, R. GRW theory]</ref>. The other subtype leaves the evolution of the wavefunction unchanged, and adds (or at least posits) an additional collapse process ("Objective reduction"). A well known example is the [[Penrose interpretation]], which links collapse to gravitational stress in general [[relativistic]] [[spacetime]], with the threshold value being one graviton.<ref>[http://www.thymos.com/mind/penrose.html Review of Penrose's Shadows of the Mind]</ref>
and the [[Penrose interpretation]]. <ref>[http://www.thymos.com/mind/penrose.html Review of Penrose's Shadows of the Mind]</ref>
