Hello! I am reading (from Griffiths' book) about Casimir's trick in calculating the amplitude of an interaction. At a point it says that we need to average over all initial spin configurations and sum over all final spin configurations. Why is it a sum for the final states and not an average, too? Thank you!(adsbygoogle = window.adsbygoogle || []).push({});

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# I Casimir's Trick

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