{"id":2670,"date":"2014-07-27T13:38:05","date_gmt":"2014-07-27T20:38:05","guid":{"rendered":"http:\/\/qpr.ca\/blogs\/?p=2670"},"modified":"2014-07-29T00:48:52","modified_gmt":"2014-07-29T07:48:52","slug":"measurement-in-qm","status":"publish","type":"post","link":"https:\/\/qpr.ca\/blogs\/2014\/07\/27\/measurement-in-qm\/","title":{"rendered":"Measurement in QM"},"content":{"rendered":"<p>This <a href=\"http:\/\/www.one-tab.com\/page\/FQyp8tqQTSSc77Z-d75LVA\">set of OneTab shared tabs<\/a> collects some recent blog activity on the measurement process in Quantum Mechanics. This reminds me of the fact that once as a graduate student (in the &#8217;70s), on hearing once too often that the measurement process was a mystery because unitary evolution cannot take a pure state into a mixed state, I thought I had something useful to say on the matter but was pointed to the discussion in von Neumann&#8217;s book and later elaborations by Jauch and Hepp that seemed to deal with the issue on the same lines (namely by modelling the measured and measuring systems together as a tensor product of a pure state of the former and a mixed state of the latter which could evolve unitarily in such a way that the marginal state in the measured system does evolve from pure to mixed).<\/p>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=plato.stanford.edu\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/plato.stanford.edu\/entries\/qm-copenhagen\/\">Copenhagen Interpretation of Quantum Mechanics (Stanford Encyclopedia of Philosophy)<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=putnamphil.blogspot.ca\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/putnamphil.blogspot.ca\/2014\/05\/the-measurement-problem-in-qm.html#comment-form\">Sardonic comment: The measurement problem in QM<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=adamilab.blogspot.ca\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/adamilab.blogspot.ca\/2014\/07\/on-quantum-measurement-part-2-some.html\">Spherical Harmonics: On quantum measurement (Part 2: Some history, and John von Neumann is confused)<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=adamilab.blogspot.ca\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/adamilab.blogspot.ca\/2014\/07\/on-quantum-measurement-part-3-no.html\">Spherical Harmonics: On quantum measurement (Part 3: No cloning allowed)<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=www.preposterousuniverse.com\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/www.preposterousuniverse.com\/blog\/2014\/02\/12\/the-many-worlds-of-quantum-mechanics\/#comment-7295910552604282177\">The Many Worlds of Quantum Mechanics | Sean Carroll<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=plato.stanford.edu\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/plato.stanford.edu\/entries\/qm-everett\/\">Everett&#8217;s Relative-State Formulation of Quantum Mechanics (Stanford Encyclopedia of Philosophy)<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=www.preposterousuniverse.com\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/www.preposterousuniverse.com\/blog\/2014\/05\/29\/quantum-mechanics-smackdown\/\">Quantum Mechanics Smackdown | Sean Carroll<\/a><\/div>\n<div style=\"padding-left: 24px; padding-top: 8px; position: relative; font-size: 13px;\"><img decoding=\"async\" style=\"vertical-align: middle; width: 16px; height: 16px;\" src=\"http:\/\/www.google.com\/s2\/favicons?domain=www.preposterousuniverse.com\" alt=\"\" \/><br \/>\n<a style=\"vertical-align: middle; padding-left: 10px; padding-right: 12px; text-decoration: none;\" href=\"http:\/\/www.preposterousuniverse.com\/blog\/2014\/07\/24\/why-probability-in-quantum-mechanics-is-given-by-the-wave-function-squared\/\">Why Probability in Quantum Mechanics is Given by the Wave Function Squared | Sean Carroll<\/a><\/div>\n","protected":false},"excerpt":{"rendered":"<p>This set of OneTab shared tabs collects some recent blog activity on the measurement process in Quantum Mechanics. This reminds me of the fact that once as a graduate student (in the &#8217;70s), on hearing once too often that the &hellip; <a href=\"https:\/\/qpr.ca\/blogs\/2014\/07\/27\/measurement-in-qm\/\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-2670","post","type-post","status-publish","format-standard","hentry","category-general"],"_links":{"self":[{"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/posts\/2670","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/comments?post=2670"}],"version-history":[{"count":4,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/posts\/2670\/revisions"}],"predecessor-version":[{"id":2678,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/posts\/2670\/revisions\/2678"}],"wp:attachment":[{"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/media?parent=2670"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/categories?post=2670"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/wp-json\/wp\/v2\/tags?post=2670"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}