{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,12]],"date-time":"2025-10-12T03:57:39Z","timestamp":1760241459633,"version":"build-2065373602"},"reference-count":35,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2018,3,28]],"date-time":"2018-03-28T00:00:00Z","timestamp":1522195200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Entropy"],"abstract":"<jats:p>In this paper, we review the concept of entropy in connection with the description of quantum unstable systems. We revise the conventional definition of entropy due to Boltzmann and extend it so as to include the presence of complex-energy states. After introducing a generalized basis of states which includes resonances, and working with amplitudes instead of probabilities, we found an expression for the entropy which exhibits real and imaginary components. We discuss the meaning of the imaginary part of the entropy on the basis of the similarities existing between thermal and time evolutions.<\/jats:p>","DOI":"10.3390\/e20040231","type":"journal-article","created":{"date-parts":[[2018,3,29]],"date-time":"2018-03-29T05:26:37Z","timestamp":1522301197000},"page":"231","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["The Definition of Entropy for Quantum Unstable Systems: A View-Point Based on the Properties of Gamow States"],"prefix":"10.3390","volume":"20","author":[{"given":"Osvaldo","family":"Civitarese","sequence":"first","affiliation":[{"name":"Department of Physics, University of La Plata, and IFLP-CONICET, 49 y 115. c.c.67, La Plata 1900, Argentina"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Manuel","family":"Gadella","sequence":"additional","affiliation":[{"name":"Departamento de F\u00edsica Te\u00f3rica, At\u00f3mica y \u00d3ptica and IMUVA, Universidad de Valladolid, Paseo Bel\u00e9n 7, 47011 Valladolid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2018,3,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Reichl, L. 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