Editing Complementarity (physics) (section)

=== 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in Norway in February and March 1927, during which he received a letter from [[Werner Heisenberg]] regarding an as-yet-unpublished result, a [[Heisenberg's microscope|thought experiment about a microscope using gamma rays]].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.}}