Quantum Reduction the Ultimate Outcome of Decoherence ?

Whereas decoherence has been much studied theoretically, little attention was given to the difference between the full quantum density matrix and the decoherent one. Although inaccessible to experiment, it never vanishes and keeps in principle a phase memory of everything in the past. Its investigation is applied here to the influence of the external universe on a macroscopically isolated system, showing the possibility that it could generate changes in quantum probabilities. When this result is combined with previous work on Brownian reduction, it suggests a conceivable though far-fetched reduction effect, which would result directly from the basic principles of quantum mechanics. The main non-speculative conclusion is however that the inaccessible part of the state of the universe stands as the main open question in