Editing Copenhagen interpretation (section)

=== Double-slit experiment ===
{{main|Double-slit experiment}}
In the basic version of this experiment, a light source, such as a [[laser]] beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to [[interference (wave propagation)|interfere]], producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles. However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves); the interference pattern appears via the varying density of these particle hits on the screen. Furthermore, versions of the experiment that include detectors at the slits find that each detected [[photon]] passes through one slit (as would a classical particle), and not through both slits (as would a wave). Such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through.<ref name="Plotnitsky2012">{{cite book |last= Plotnitsky |first= Arkady |title= Niels Bohr and Complementarity: An Introduction |publisher= Springer |year= 2012 |location= US |pages= 75–76 |url= https://books.google.com/books?id=dmdUp97S4AYC&pg=PA75 |isbn= 978-1461445173}}</ref>{{Rp|73–76}}

According to Bohr's [[Complementarity (physics)|complementarity principle]], light is neither a wave nor a [[stream of particles]]. A particular experiment can demonstrate particle behavior (passing through a definite slit) or wave behavior (interference), but not both at the same time.<ref>{{Cite journal |last=Rosenfeld |first=L. |author-link=Léon Rosenfeld |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>

The same experiment has been performed for light, electrons, atoms, and molecules.<ref>{{cite journal |doi=10.1103/PhysRevLett.87.160401 |pmid=11690188 |title=Diffraction of Complex Molecules by Structures Made of Light |year=2001 |last1=Nairz |first1=Olaf |last2=Brezger |first2=Björn |last3=Arndt |first3=Markus |last4=Zeilinger |first4=Anton |author-link4=Anton Zeilinger |journal=[[Physical Review Letters]] |volume=87 |issue=16|pages=160401 |arxiv = quant-ph/0110012 |bibcode = 2001PhRvL..87p0401N |s2cid=21547361 }}</ref><ref>{{cite journal |doi=10.1103/PhysRevLett.88.100404 |title=Matter-Wave Interferometer for Large Molecules |year=2002 |last1=Brezger |first1=Björn |last2=Hackermüller |first2=Lucia |last3=Uttenthaler |first3=Stefan |last4=Petschinka |first4=Julia |last5=Arndt |first5=Markus |last6=Zeilinger |first6=Anton |author-link6=Anton Zeilinger |journal=[[Physical Review Letters]] |volume=88 |issue=10 |arxiv = quant-ph/0202158 |bibcode = 2002PhRvL..88j0404B |pmid=11909334 |pages=100404 |s2cid=19793304 }}</ref> The extremely small [[matter wave|de Broglie wavelength]] of objects with larger mass makes experiments increasingly difficult,<ref>{{Cite journal |last1=Arndt |first1=Markus |last2=Hornberger |first2=Klaus |date=April 2014 |title=Testing the limits of quantum mechanical superpositions |url=http://dx.doi.org/10.1038/nphys2863 |journal=Nature Physics |volume=10 |issue=4 |pages=271–277 |doi=10.1038/nphys2863 |arxiv=1410.0270 |bibcode=2014NatPh..10..271A |s2cid=56438353 |issn=1745-2473}}</ref> but in general quantum mechanics considers all matter as possessing both particle and wave behaviors.