{"id":1003,"date":"2014-11-10T18:44:02","date_gmt":"2014-11-10T16:44:02","guid":{"rendered":"https:\/\/webs.uab.cat\/giq\/seminar\/operational-quantum-theory-without-predefined-time\/"},"modified":"2014-11-10T18:44:02","modified_gmt":"2014-11-10T16:44:02","slug":"operational-quantum-theory-without-predefined-time","status":"publish","type":"seminar","link":"https:\/\/webs.uab.cat\/giq\/seminar\/operational-quantum-theory-without-predefined-time\/","title":{"rendered":"Operational quantum theory without predefined time"},"content":{"rendered":"<p><span>The current operational formulation of quantum theory is<\/span><br \/><span>based on the concept of operation with an input and an output system,<\/span><br \/><span>which assumes a prior notion of time and is asymmetric under time<\/span><br \/><span>reversal. But in certain contexts, such as those involving gravity,<\/span><br \/><span>time is expected to be dynamical and not predefined. In this talk, I<\/span><br \/><span>will describe a proposal for an operational formulation of quantum<\/span><br \/><span>theory without any predefined notion of time. After critically<\/span><br \/><span>examining the implicit assumptions underlying the standard concept of<\/span><br \/><span>operation, I will propose a generalization of that concept based on an<\/span><br \/><span>epistemic approach: an operation is a description of knowledge about<\/span><br \/><span>the events in a given region, which can be updated conditionally on<\/span><br \/><span>information obtained from that region. First assuming a background<\/span><br \/><span>time, I will show how this approach leads us to an operationally<\/span><br \/><span>time-symmetric quantum theory. I will make precise the notion of time<\/span><br \/><span>reversal symmetry and clarify several misconceptions. I will present a<\/span><br \/><span>generalization of Wigner&#8217;s theorem which puts time-reversal symmetry<\/span><br \/><span>on operational grounds, and will show that the developed operationally<\/span><br \/><span>time-symmetric theory allows for more general symmetry transformations<\/span><br \/><span>than those assumed before. I will explain how the time-reversal<\/span><br \/><span>asymmetry we observe can be understood as due to boundary conditions<\/span><br \/><span>and will establish a link between that asymmetry and the fact that we<\/span><br \/><span>remember the past but not the future. I will then introduce a<\/span><br \/><span>mathematical formalism that allows us to completely drop the<\/span><br \/><span>assumption of predefined time. In the resultant formulation of quantum<\/span><br \/><span>theory, operations are associated with regions with boundary systems<\/span><br \/><span>and can be connected in networks with no directionality assumed for<\/span><br \/><span>the connections, generalizing the standard circuit picture. The events<\/span><br \/><span>associated with an operation are described by positive semidefinite<\/span><br \/><span>operators on the Hilbert spaces of the boundary systems, while the<\/span><br \/><span>connections between regions are described by entangled states that<\/span><br \/><span>encode a nontrivial physical symmetry. A simple rule provides the<\/span><br \/><span>joint probabilities for the events in a network of operations. I will<\/span><br \/><span>discuss how it may be possible to understand the emergence of a causal<\/span><br \/><span>structure in space-time from properties of the operators on the<\/span><br \/><span>boundaries of compact space-time regions. The framework allows for<\/span><br \/><span>indefinite causal order, timelike loops, and other acausal structures.<\/span><\/p>\n<p><span>Based on:&nbsp;<\/span><a href=\"http:\/\/arxiv.org\/abs\/1406.3829\">http:\/\/arxiv.org\/abs\/1406.3829<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>The current operational formulation of quantum theory isbased on the concept of operation with an input and an output system,which assumes a prior notion of time and is asymmetric under timereversal. But in certain contexts, such as those involving gravity,time is expected to be dynamical and not predefined. In this talk, Iwill describe a proposal [&hellip;]<\/p>\n","protected":false},"author":20,"featured_media":0,"template":"","class_list":["post-1003","seminar","type-seminar","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/webs.uab.cat\/giq\/wp-json\/wp\/v2\/seminar\/1003","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/webs.uab.cat\/giq\/wp-json\/wp\/v2\/seminar"}],"about":[{"href":"https:\/\/webs.uab.cat\/giq\/wp-json\/wp\/v2\/types\/seminar"}],"author":[{"embeddable":true,"href":"https:\/\/webs.uab.cat\/giq\/wp-json\/wp\/v2\/users\/20"}],"wp:attachment":[{"href":"https:\/\/webs.uab.cat\/giq\/wp-json\/wp\/v2\/media?parent=1003"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}