The Schrodinger’s Cat thought experiment was originally put forward as a demonstration reductio ad absurdum that quantum theory had a problem. Surely a cat cannot exist in superposition state of being both alive and dead until a concerned pet owner checks in on its status. And yet, unitary evolution of the Schrodinger equation predicts that the superposition state even persists after the cat is observed, and that the human also ends up in a superposition of bereavement and happiness. This simplest version of quantum theory, that of uninterrupted unitary evolution, is often called the Many Worlds Interpretation. However, the Copenhagen Interpretation, wherein an extra wave function collapse postulate is added, currently enjoys greater popularity among physicists. I will consider unitary evolution of an interferometer experiment in which microscopic and macroscopic observers obtain “which path” information. A qualitative difference between microscopic and macroscopic observations arises for reasons closely related to the emergence of an arrow of time from underlying time-symmetric equations. Unitary evolution readily predicts all experimentally observed behavior, including decoherence and apparent (but not real) collapse. Thus, the Copenhagen Interpretation would be better called the Copenhagen Approximation.
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