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Max Tegmark
Massachusetts Institute of Technology, Cambridge, USA

Why quantum observers find lower entropy after observation and in our early universe?



I describe how a tripartite subject-object-environment decomposition of a unitary quantum system combines decoherence and the Quantum Bayes Theorem into a simple unified picture where decoherence increases entropy while observation decreases it. This provides a rigorous quantum-mechanical version of the second law of thermodynamics, governing how the entropy of a system (the entropy of its density matrix, partial-traced over the environment and conditioned on what is known) evolves under general decoherence and observation, and it elegantly explains how cosmological inflation produces the low-entropy early universe that drives our perceived arrow of time.
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