In this first lecture, David Wallace examines the justification for interpreting the superposition states as multiplicities.

The Life of Psi: More on the Superposition Principle

The Many-Worlds Interpretation (MWI) of quantum mechanics holds that quantum theory - our best current theory of physics - is correctly understood as claiming that there are many worlds that exist in parallel to our own and which branch constantly off our own. The existence of the other worlds makes it possible to make sense of physics without action-at-a-distance, objective randomness, or any strange role for "observer" or "consciousness". During this weekend school we shall be discussing this interpretation of quantum mechanics and asking what would follow from its being the correct one.