  Quantum mechanics gives many versions of reality but we perceive only one.
One potential explanation for this, the one considered here, is that the wave
function collapses down to just one version. The experimental situation is
briefly reviewed, with no evidence found for collapse. The theoretical position
is also reviewed and found wanting. Collapse-by-observation schemes are
logically untenable. A mathematical theory of collapse must be nonlinear, a
significant departure from current quantum theory. In addition, the primary
candidate theory, the GRW-Pearle model, requires instantaneous, non-local
transmission of information. It also requires transmission of information
across versions of reality, which is forbidden in current quantum mechanics.
And there is no apparent physical quantity, such as particle number, that can
provide the mechanism for collapse in all cases. Further, these aspects, which
are in disagreement with current theory, seem to be necessary for any
mathematical theory of collapse. The conclusion is that the outlook for
mathematical collapse schemes is not encouraging.
