Editing Complementarity (physics) (section)

=== Background ===
Complementarity as a physical model derives from Niels Bohr's 1927 lecture during the [[Como Conference]] in Italy, at a scientific celebration of the work of [[Alessandro Volta]] 100 years previous.<ref>{{Cite book |last=Baggott |first=J. E. |title=The quantum story: a history in 40 moments |date=2013 |publisher=Oxford Univ. Press |isbn=978-0-19-965597-7 |edition=Impression: 3 |location=Oxford}}</ref>{{rp|103}} Bohr's subject was complementarity, the idea that measurements of quantum events provide complementary information through seemingly contradictory results.<ref name=BohrComo>{{Cite journal |doi = 10.1038/121580a0|title = The Quantum Postulate and the Recent Development of Atomic Theory|year = 1928|last1 = Bohr|first1 = N.|journal = Nature|volume = 121|issue = 3050|pages = 580–590|bibcode = 1928Natur.121..580B|doi-access = free}}</ref> While Bohr's presentation was not well received, it did crystallize the issues ultimately leading to the modern wave-particle duality concept.<ref name=Kumar2011>{{cite book | last =Kumar | first =Manjit | title =Quantum: Einstein, Bohr, and the Great Debate about the Nature of Reality | publisher =W. W. Norton & Company | edition =Reprint | year =2011 | pages =[https://archive.org/details/quantumeinsteinb00manj/page/242 242, 375–376] | isbn =978-0-393-33988-8 | url =https://archive.org/details/quantumeinsteinb00manj/page/242 }}</ref>{{rp|315}} The contradictory results that triggered Bohr's ideas had been building up over the previous 20 years.

This contradictory evidence came both from light and from electrons.
The [[wave theory of light]], broadly successful for over a hundred years, had been challenged by [[Max Planck|Planck]]'s 1901 model of [[blackbody radiation]] and [[Albert Einstein|Einstein]]'s 1905 interpretation of the [[photoelectric effect]]. These theoretical models use discrete energy, a [[quantum]], to describe the interaction of light with matter. Despite confirmation by various experimental observations, the [[photon]] theory (as it came to be called later) remained controversial until [[Arthur Compton]] performed a [[Compton effect|series of experiments]] from 1922 to 1924 demonstrating the momentum of light.<ref name="Whittaker">{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 2: The modern theories, 1900 - 1926 |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |location=New York}}</ref>{{rp|211}} The experimental evidence of particle-like momentum seemingly contradicted other experiments demonstrating the wave-like interference of light.

The contradictory evidence from electrons arrived in the opposite order. Many experiments by [[J. J. Thomson|J. J. Thompson]], [[Robert Millikan]], and [[Charles Thomson Rees Wilson|Charles Wilson]], among others, had shown that free electrons had particle properties. However, in 1924, [[Louis de Broglie]] proposed that electrons had an associated wave and [[Erwin Schrödinger|Schrödinger]] demonstrated that wave equations accurately account for electron properties in atoms. Again some experiments showed particle properties and others wave properties.

Bohr's resolution of these contradictions is to accept them. In his Como lecture he says: "our interpretation of the experimental material rests essentially upon the
classical concepts."<ref name=BohrComo/> Direct observation being impossible, observations of quantum effects are necessarily classical. Whatever the nature of quantum events, our only information will arrive via classical results. If experiments sometimes produce wave results and sometimes particle results, that is the nature of light and of the ultimate constituents of matter.