Editing Complementarity (physics) (section)

==History==
=== 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.

=== Bohr's lectures ===
Niels Bohr apparently conceived of the principle of complementarity during a skiing vacation in Norway in February and March 1927, during which he received a letter from [[Werner Heisenberg]] regarding an as-yet-unpublished result, a [[Heisenberg's microscope|thought experiment about a microscope using gamma rays]]. This thought experiment implied a tradeoff between uncertainties that would later be formalized as the [[uncertainty principle]]. To Bohr, Heisenberg's paper did not make clear the distinction between a position measurement merely disturbing the momentum value that a particle carried and the more radical idea that momentum was meaningless or undefinable in a context where position was measured instead. Upon returning from his vacation, by which time Heisenberg had already submitted his paper for publication, Bohr convinced Heisenberg that the uncertainty tradeoff was a manifestation of the deeper concept of complementarity.<ref name="Baggott2011">{{cite book|title=The Quantum Story: A History in 40 moments|last=Baggott|first=Jim|publisher=Oxford University Press|year=2011|isbn=978-0-19-956684-6|series=Oxford Landmark Science|location=Oxford|page=97|author-link=Jim Baggott}}</ref> Heisenberg duly appended a note to this effect to his paper, before its publication, stating:

<blockquote>Bohr has brought to my attention [that] the uncertainty in our observation does not arise exclusively from the occurrence of discontinuities, but is tied directly to the demand that we ascribe equal validity to the quite different experiments which show up in the [particulate] theory on one hand, and in the wave theory on the other hand.</blockquote>

Bohr publicly introduced the principle of complementarity in a lecture he delivered on 16 September 1927 at the International Physics Congress held in [[Como, Italy]], attended by most of the leading physicists of the era, with the notable exceptions of [[Albert Einstein|Einstein]], [[Erwin Schrödinger|Schrödinger]], and [[Paul Dirac|Dirac]]. However, these three were in attendance one month later when Bohr again presented the principle at the [[Solvay Congress|Fifth Solvay Congress]] in [[Brussels, Belgium]]. The lecture was published in the proceedings of both of these conferences, and was republished the following year in ''Naturwissenschaften'' (in German) and in ''Nature'' (in English).<ref name="Bohr1928English">{{cite journal |last=Bohr |first=N. |title=The Quantum Postulate and the Recent Development of Atomic Theory |journal=[[Nature (journal)|Nature]] |volume=121 |issue=3050 |pages=580–590 |year=1928 |doi= 10.1038/121580a0|bibcode =   1928Natur.121..580B|doi-access=free }}  Available in the collection of Bohr's early writings, ''Atomic Theory and the Description of Nature'' (1934).</ref>

In his original lecture on the topic, Bohr pointed out that just as the finitude of the speed of light implies the impossibility of a sharp separation between space and time (relativity), the finitude of the [[Planck constant|quantum of action]] implies the impossibility of a sharp separation between the behavior of a system and its interaction with the measuring instruments and leads to the well-known difficulties with the concept of 'state' in quantum theory; the notion of complementarity is intended to capture this new situation in epistemology created by quantum theory. Physicists F.A.M. Frescura and [[Basil Hiley]] have summarized the reasons for the introduction of the principle of complementarity in physics as follows:<ref>{{cite journal|first1=F. A. M. |last1=Frescura |author2-link=Basil Hiley |first2=B. J. |last2=Hiley |url=http://www.bbk.ac.uk/tpru/BasilHiley/P12FrescandHiley3.pdf |title=Algebras, quantum theory and pre-space |journal=Revista Brasileira de Física |volume=Special volume "Os 70 anos de Mario Schonberg" |pages=49–86, 2 |date=July 1984}}</ref>
{{blockquote|In the traditional view, it is assumed that there exists a reality in space-time and that this reality is a given thing, all of whose aspects can be viewed or articulated at any given moment. Bohr was the first to point out that quantum mechanics called this traditional outlook into question. To him the "indivisibility of the quantum of action" [...] implied that not all aspects of a system can be viewed simultaneously. By using one particular piece of apparatus only certain features could be made manifest at the expense of others, while with a different piece of apparatus another complementary aspect could be made manifest in such a way that the original set became non-manifest, that is, the original attributes were no longer well defined. For Bohr, this was an indication that the principle of complementarity, a principle that he had previously known to appear extensively in other intellectual disciplines but which did not appear in classical physics, should be adopted as a universal principle.}}

=== Debate following the lectures ===
{{Main | Bohr–Einstein debates}}

Complementarity was a central feature of Bohr's reply to the [[EPR paradox]], an attempt by Albert Einstein, [[Boris Podolsky]] and [[Nathan Rosen]] to argue that quantum particles must have position and momentum even without being measured and so quantum mechanics must be an incomplete theory.<ref name=":0">{{cite journal|first=Christopher A. |last=Fuchs |title=Notwithstanding Bohr: The Reasons for QBism |journal=Mind and Matter |volume=15 |pages=245–300 |year=2017 |arxiv=1705.03483 |bibcode=2017arXiv170503483F}}</ref> The [[thought experiment]] proposed by Einstein, Podolsky and Rosen involved producing two particles and sending them far apart. The experimenter could choose to measure either the position or the momentum of one particle. Given that result, they could in principle make a precise prediction of what the corresponding measurement on the other, faraway particle would find. To Einstein, Podolsky and Rosen, this implied that the faraway particle must have precise values of both quantities whether or not that particle is measured in any way. Bohr argued in response that the deduction of a position value could not be transferred over to the situation where a momentum value is measured, and vice versa.<ref>{{cite book|last=Jammer|first=Max|title=The Philosophy of Quantum Mechanics|publisher=John Wiley and Sons|year=1974|isbn=0-471-43958-4|author-link=Max Jammer}}</ref>

Later expositions of complementarity by Bohr include a 1938 lecture in [[Warsaw]]<ref name=":1">{{cite book|first=Niels |last=Bohr |author-link=Niels Bohr |chapter=The causality problem in atomic physics |title=New theories in physics |publisher=International Institute of Intellectual Co-operation |location=Paris |year=1939 |pages=11–38}}</ref><ref name="chevalley1999">{{cite book|first=Catherine |last=Chevalley |chapter=Why Do We Find Bohr Obscure? |title=Epistemological and Experimental Perspectives on Quantum Physics |editor-first1=Daniel |editor-last1=Greenberger |editor-first2=Wolfgang L. |editor-last2=Reiter |editor-first3=Anton |editor-last3=Zeilinger |publisher=Springer Science+Business Media |doi=10.1007/978-94-017-1454-9 |isbn=978-9-04815-354-1 |year=1999 |pages=59–74}}</ref> and a 1949 article written for a [[festschrift]] honoring Albert Einstein.<ref name="Bohr1949">{{cite book|title=Albert Einstein: Philosopher-Scientist|last=Bohr|first=Niels|publisher=Open Court|year=1949|editor=Schilpp|editor-first=Paul Arthur|editor-link=Paul Arthur Schilpp|chapter=Discussions with Einstein on Epistemological Problems in Atomic Physics|author-link=Niels Bohr}}</ref> It was also covered in a 1953 essay by Bohr's collaborator [[Léon Rosenfeld]].<ref>{{Cite journal|last=Rosenfeld|first=L.|date=1953|title=Strife about Complementarity|url=https://www.jstor.org/stable/43414997|journal=Science Progress (1933- )|volume=41|issue=163|pages=393–410|jstor=43414997 |issn=0036-8504}}</ref>