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SUMMARY:Operationalism\, Causality\, and Quantum Theory: a time symmetric 
 perspective - Lucien Hardy (Perimeter Institute)
DTSTART:20250529T131500Z
DTEND:20250529T141500Z
UID:TALK231379@talks.cam.ac.uk
CONTACT:Adrian Kent
DESCRIPTION:Standard textbook Operational Quantum Theory is not formulated
  in a time symmetric way.  For example\, the space of states is a cone wi
 th a flat top\, whilst the space of effects is a double cone (with the ide
 ntity effect at the apex).  Does this time asymmetry point to a fundament
 al time asymmetry in nature?  In fact\, we will see that it is possible t
 o formulate Operational Quantum Theory in a time symmetric manner and\, fu
 rther\, this time symmetric theory is fully equivalent to the usual time f
 orward textbook formulation.  We can simply think of changing perspective
  between a time forward and a time symmetric point of view (and\, indeed\,
  a time backward point of view).  Arguably the time symmetric perspective
  is more fundamental.  To set up the time symmetric theory\, we have to g
 o back to the basic building block of Operational Theories - the operation
  - and make it time symmetric.  We do this by adding "incomes" which are 
 the time reverse of "outcomes" (whereby the results of measurements are re
 corded).  Operations are subject to two types of constraints (i) positivi
 ty and (ii) causality.   These constraints dictate the space of allowed 
 operations (such as the space of states and effects mentioned above).  Po
 sitivity is the constraint that closed circuits have non-negative probabil
 ity.  In the time symmetric theory\, causality consists of two parts: for
 ward causality (that it is impossible to send information back in time wit
 hout conditioning on the future)\, and backward causality (that it is impo
 ssible to send information forward in time without conditioning on the pas
 t).    If I have time I will also speak about causally complex operation
 s (as opposed to causally simple operations for which inputs are strictly 
 to the past of outputs).   Thes can be a model for the physics in arbitr
 ary shaped regions of spacetime.  
LOCATION:Center for Mathematical Sciences\, Lecture room MR2
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