Abstract

Using the von Neumann theory of measurement, or one of its derivatives, one can attempt to describe the measurement process using the theoretical apparatus of quantum mechanics itself. Such attempts, which treat the measurement set-up as a quantum system, give rise to a family of difficulties and paradoxes which include the Measurement Problem and the Schrodinger's Cat Paradox. The root of the trouble is that the measuring apparatus, at the end of its interaction with the measured system, does not appear to be in a state in which it indicates a definite outcome to the measurement. This result is at odds with the Born interpretation and with everyday experience. ;Treating the measurement process within quantum mechanics naturally requires the quantum theory of composite systems, since the measurement set-up consists of the measuring apparatus and the measured system. This theory has the special feature that the state space of the composite system is represented in quantum theory as the tensor product of the state spaces of the component systems. In consequence, conventional quantum theory does not in general assign states to the subsystems of composite quantum systems. ;A number of authors, including H. P. Krips, B. van Fraassen and S. Kochen, have proposed versions of quantum theory whose central innovative feature is that pure states are assigned to subsystems of composite quantum systems even when the state of the composite does not factor as a direct product of subsystem states. By introducing this feature, these authors claim to be able to provide a straightforward account of the measurement process which is free from the traditional paradoxes. In this thesis, I examine the work of each of these authors and argue that none of them has successfully established his case. Following this, I provide a general argument whose conclusion is that no version of quantum theory which assigns pure states to the subsystems of composite systems can provide a straightforward account of the measurement process while remaining empirically equivalent to the more conventional quantum theory. ;In the final chapter of the thesis, I provide a critical account of the Many Worlds interpretation of quantum mechanics. The Many Worlds interpretation may be regarded as a member of the same group of interpretations mentioned above although, as I argue, it suffers from a different set of difficulties