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The traditional, standard approach to quantum theory is to assume that thetheory ``really'' contains only unitary physical dynamics--i.e., that the onlyphysically quantifiable evolution is that given by the time-dependentSchrodinger equation. This leads to two distinct classes of interpretations forthe standard theory in its orthodox form: (i) an Everettian-type approachassuming that all mutually exclusive outcomes occur in different ``branches''of the universe; or (ii) single-outcome approaches that assume a ``projectionpostulate'' (PP) with no accompanying physical account within quantum theory. Acontrasting, unorthodox approach is to suggest forms of quantum theory thatinvolve physical non-unitarity; these are called ``objective collapse models.''Among these are Penrose's theory of gravitation-induced collapse and theTransactional Interpretation. The primary focus of this paper is an exampledemonstrating that standard quantum theory (with or without the projectionpostulate) can in-principle yield empirically consequential inconsistencies.Thus, it appears that for quantum theory to be viable in a realist sense (asopposed to being an instrumentalist protocol in which inconsistencies areevaded by changing the protocol), it must possess genuine, physicalnon-unitarity yielding well-defined single outcomes. This leads to theconclusion that objective collapse models should be more seriously considered.

Quantum Theory Needs (And Probably Has) Real Reduction