A Theory of Quantum Observation and the Emergence of the Born Rule

Abstract: A realist description of our universe requires a twofold concept of locality. On one hand, there are the strictly Einstein-local interactions which generate the time evolution. On the other hand, the quantum state space calls for a non-local description of multi-particle states. This article uses a behavioristic approach to argue, that an observer in a universe like this has to rely on local interactions to learn about its properties and behavior. Such an observer is fundamentally restricted in his ability to understand and structurally reconstruct the individual local physical universe. We argue, that this reconstruction based on dynamically available information is the defining process of observation in quantum theory. The observer-centric view of the global quantum dynamics is shown to be non-unitary and non-linear in general, even if the universe itself evolves unitarily. Interactions with massless free particles are found to have great influence on observation, because of their special role in the light-cone structure of an Einstein-local universe. For a specific scattering process with a photon of unknown state, the observed outcome can be subjectively random and follow the Born statistic, while the output state is really determined by the photon polarization. Based on this result, a theory of quantum measurement is formulated, which describes a measurement device as a cascade of scattering events.