Quantum Theory without Observers—Part Two

Albert Einstein believed in the possibility of a quantum theory without observers—a version of quan tum theory for which the notions of measurement, observation and observer are not invoked in its very formulation, but rather emerge from an analysis of more fundamental concepts. Niels Bohr believed that such a theory was “in principle” impossible. In part one of this article, I described one approach to such a theory, that of decoherent histories (DH). Although much progress has been made, it could be argued that this approach has not yet yielded a theory that is sufficiently well defined to provide decisive support for Einstein's view. The theories I discuss in this final part of the article are completely well defined and hence provide a conclusive refutation of Bohr's impossibility claims.