Editing Complementarity (physics) (section)

{{Short description|Quantum physics concept}}
{{Quantum mechanics}}
In [[physics]], '''complementarity''' is a conceptual aspect of [[quantum mechanics]] that [[Niels Bohr]] regarded as an essential feature of the theory.<ref>{{Cite magazine|last=Wheeler|first=John A.|author-link=John Archibald Wheeler|date=January 1963|title="No Fugitive and Cloistered Virtue"—A tribute to Niels Bohr|magazine=[[Physics Today]] |volume=16 |issue=1 |page=30 |bibcode=1963PhT....16a..30W |doi=10.1063/1.3050711}}</ref><ref name="Howard 2004">{{cite journal |title=Who invented the Copenhagen Interpretation? A study in mythology |year=2004 |last1=Howard |first1=Don |journal=Philosophy of Science |pages=669–682 | jstor=10.1086/425941 |volume=71 |issue=5 |doi=10.1086/425941|url=http://www.nd.edu/~dhoward1/Copenhagen%20Myth%20A.pdf |citeseerx=10.1.1.164.9141 |s2cid=9454552 }}</ref> The complementarity principle holds that certain pairs of complementary properties cannot all be [[measurement in quantum mechanics|observed or measured]] simultaneously. For example, position and momentum, frequency and lifetime, or optical phase and photon number.<ref>{{Cite web |title=Number-phase Uncertainty Relations |url=https://www.optica-opn.org/home/articles/volume_4/issue_12/features/number-phase_uncertainty_relations/ |access-date=2025-04-01 |website=www.optica-opn.org}}</ref> In contemporary terms, complementarity encompasses both the [[uncertainty principle]] and [[wave-particle duality]].

Bohr considered one of the foundational truths of quantum mechanics to be the fact that setting up an experiment to measure one quantity of a pair, for instance the position of an [[electron]], excludes the possibility of measuring the other, yet understanding both experiments is necessary to characterize the object under study. In Bohr's view, the behavior of atomic and subatomic objects cannot be separated from the measuring instruments that create the context in which the measured objects behave. Consequently, there is no "single picture" that unifies the results obtained in these different experimental contexts, and only the "totality of the phenomena" together can provide a completely informative description.<ref name="Bohr1996a">{{cite book |first1=Niels |last1=Bohr |author-link1=Niels Bohr |author-link2=Léon Rosenfeld |first2=Léon |last2=Rosenfeld |title=Foundations of Quantum Physics II (1933–1958) |chapter-url=https://books.google.com/books?id=yet5P7f_63oC&pg=PA284 |series=Niels Bohr Collected Works |volume=7 |year=1996 |publisher=Elsevier |isbn=978-0-444-89892-0 |pages=284–285 |chapter=Complementarity: Bedrock of the Quantal Description}}</ref>