`Measurement of Quantum Mechanical Operators' Revisited

Leon Loveridge, UBC
Event Date and Time: 
Fri, 2012-09-28 14:00 - 15:00
Hennings 318
Local Contact: 
Robert Raussendorf
Intended Audience: 
In 1952 Wick, Wightman and Wigner argued that relativistic symmetry dictates the impossibility of measuring a relative phase factor in a superposition of bosonic and fermionic states. The ensuing reduction of the set of observables is called a superselection rule. I'll review some history and contrasting perspectives on the subject, other occurences of superselection rules, and various arguments in their favour. For instance, positions or electromagnetic phases should be thought of as relative objects, and one cannot assign meaning to corresponding absolute quantities. I'll provide some simple models that highlight the generic structure of attempts to circumvent superselection rules and, by introducing an explicit means for constructing relative observables, discuss the conditions under which the statistics of permissible and forbidden observables may become close. In this context I'll discuss various attempts to overcome a superselection rule by introduction of a (quantum) reference frame. I'll finish with some open questions in this area.
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