The Born Rule is a feature of Superposition

Where does the Born Rule come from? We ask what is the simplest extension of
probability theory where the Born rule appears. This is answered by showing
that the Born Rule first appears as the square of the (normalized) vector
components of the notion of superposition events in the enriched probability
theory. The Superposition Principle requires the states to be represented as
(normalized) vectors in a vector space with positive, negative, or complex
components–so the rule of getting probabilities as the (absolute) squares of
normalized vector components generalizes from the simple case in the enriched
probability theory.

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