In physics, and more specifically, quantum mechanics, the Afshar experiment is an optical experiment, devised by Shahriar S. Afshar in 2004, that its proponents claim disproves Niels Bohr's principle of complementarity. Since this principle is a central idea of quantum mechanics, the proper interpretation of the experiment has engendered some controversy in the physics community; there have been response papers have been critical of Afshar's interpretation. The controversy has appeared online in blogs, at physics colloquia and academic conferences, and in arXiv e-print archives. Papers by Afshar on the experiment have been published in the American Institute of Physics and SPIE conference proceedings; however, as of May 4, 2006, neither a description of the experiment, nor any discussion of its theoretical interpretation, has been published in a refereed physics journal. Physics (from the Greek, (phúsis), nature and (phusiké), knowledge of nature) is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space and time. ... Fig. ... Table of Opticks, 1728 Cyclopaedia Optics ( appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... In the scientific method, an experiment (Latin: ex-+-periri, of (or from) trying), is a set of actions and observations, performed in the context of solving a particular problem or question, to support or falsify a hypothesis or research concerning phenomena. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Niels (Henrik David) Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics. ... Complementarity is a concept in a number of fields: Economics In economics is a concept similar to that of externality. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... An academic conference is a conference for researchers (not always academics) to present and discuss their work. ... arXiv (pronounced archive, as if the X were the Greek letter χ) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and biology which can be accessed via the internet. ... The American Institute of Physics (AIP) is a professional body representing American physicists and publishing physics related journals. ... SPIE - The International Society for Optical Engineering (or SPIE) is a not-for-profit society that has become the largest international force for the exchange, collection and dissemination of knowledge in optics, photonics, and imaging engineering. ... May 4 is the 124th day of the year in the Gregorian calendar (125th in leap years). ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ...

Overview

The principle of complementarity states that two complementary physical observables cannot both be measured for any given quantum particle without one measurement disturbing the other. For example, a particle's position and momentum cannot be observed at the same time: this is Werner Heisenberg's uncertainty principle. In physics, particularly in quantum physics, a system observable is a property of the system state that can be determined by some sequence of physical operations. ... Look up position in Wiktionary, the free dictionary. ... In classical mechanics, momentum (pl. ... Werner Karl Heisenberg (December 5, 1901 – February 1, 1976) was a celebrated German physicist and Nobel laureate, one of the founders of quantum mechanics, and acknowledged to be one of the most important physicists of the twentieth century. ... In quantum physics, the Heisenberg uncertainty principle or the Heisenberg indeterminacy principle — the latter name given to it by Niels Bohr — states that when measuring conjugate quantities, which are pairs of observables of a single elementary particle, increasing the accuracy of the measurement of one quantity increases the uncertainty of...

One of Afshar's assertions is that, in his experiment, it is possible to check for interference fringes of a photon stream (a momentum measurement) while at the same time observing the photon's path (a position measurement). Afshar's experiment attempts to do this using a variant of the classic Thomas Young double-slit experiment. Such interferometer experiments typically have two "arms" or paths a photon may take. Afshar attempts to both preserve interference, and to determine "which way" or "Welcher Weg" the photon went. Many of the claims associated with this experiment cut across several conventional ideas in quantum mechanics. Interference fringes are sinusoidal intensitiy fluctuations, caused by interference, typically found in Youngs double slit experiment. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, microwaves, radio waves, and visible light are all forms of light. ... Thomas Young, English scientist Thomas Young (*June 14, 1773 – †May 10, 1829) was an English scientist, researcher, physician and polymath. ... The double-slit experiment consists of letting light diffract through two slits producing fringes on a screen. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ...

History

Shahriar S. Afshar's experimental work was done initially at the Institute for Radiation-Induced Mass Studies (IRIMS)^[1] in 2001 and later reproduced at Harvard University in 2003, while he was a Research Scholar there, where he presented his results in March 2004, entitled Waving Copenhagen Good-bye: Were the founders of Quantum Mechanics wrong?^[2], published as conference proceeding by the International Society for Optical Engineering (SPIE)^[3]. The experiment was featured as the cover story in the July 24, 2004 edition of New Scientist[13] Harvard University (incorporated as The President and Fellows of Harvard College) is a private university in Cambridge, Massachusetts. ... SPIE - The International Society for Optical Engineering (or SPIE) is a not-for-profit society that has become the largest international force for the exchange, collection and dissemination of knowledge in optics, photonics, and imaging engineering. ... July 24 is the 205th day (206th in leap years) of the year in the Gregorian Calendar, with 160 days remaining. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... New Scientist is a weekly international science magazine covering recent developments in science and technology for a general English-speaking audience. ...

Afshar presented his work at the American Physical Society meeting in Los Angeles, in late March of 2005.^[4] The American Physical Society was founded in 1899 and is the worlds second largest organization of physicists. ...

The New Scientist feature article itself generated many responses, including various letters to the editor that appeared in the August 7 and August 14, 2004 issues, arguing against the conclusions being drawn by Afshar, with Cramer's response[14]. August 14 is the 226th day of the year in the Gregorian Calendar (227th in leap years), with 139 days remaining. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ...

Afshar claims that his experiment invalidates the complementarity principle and has far-reaching implications for the understanding of quantum mechanics, challenging the Copenhagen interpretation. According to John Cramer, Afshar's results support Cramer's own transactional interpretation of quantum mechanics and challenges the many-worlds interpretation of quantum mechanics.^[5] The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... John Cramer is a Professor of Physics at the University of Washington in Seattle, United States. ... The transactional interpretation of quantum mechanics (TIQM) by Professor John Cramer is an unusual interpretation of quantum mechanics that describes quantum interactions in terms of a standing wave formed by retarded (forward in time) and advanced (backward in time) waves. ... The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that averts the special role played by the measurement process in the Copenhagen interpretation by proposing several key ideas. ...

Experimental setup and Afshar's interpretation

Fig.1 Experiment without obstructing wire grid

Fig. 2 Experiment with obstructing wire grid and one pinhole covered

Fig. 3 Experiment with obstructing wire grid and both pinholes open

The experiment uses a setup similar to that for the double-slit experiment. In Afshar's variant, light generated by a laser passes through two closely spaced circular pinholes (not slits). After the dual pinholes, a lens refocuses the light so that the image of each pinhole is received by a separate photo-detector (Fig. 1). In this setup, a photon that goes through pinhole number one impinges only on detector number one, and similarly, if it goes through pinhole two. Therefore, if observed at the image plane, the setup is such that the light behaves as a stream of particles and can be assigned to a particular pinhole. Ashfar experiment setup File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Ashfar experiment setup File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Ashfar experiment setup with wires File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Ashfar experiment setup with wires File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Ashfar experiment setup with wires so-called interference File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Ashfar experiment setup with wires so-called interference File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The double-slit experiment consists of letting light diffract through two slits producing fringes on a screen. ... // Experiment using a (likely argon) laser. ...

When the light acts as a wave, because of interference one can observe that there are regions that the photons avoid, called dark fringes. Afshar now places a grid of thin wires just before the lens (Fig. 2). These wires are placed in previously measured positions of the dark fringes of an interference pattern which is produced by the dual pinhole setup when observed directly. If one of the pinholes is blocked, the interference pattern can no longer be formed, and some of the light will be blocked by the wires. Consequently, one would expect that the image quality is reduced, as is indeed observed by Afshar. Afshar then claims that he can check for the wave characteristics of the light in the same experiment, by the presence of the grid. At this point, Afshar compares the results of what is seen at the photo-detectors when one pinhole is closed with what is seen at the photo-detectors when both pinholes are open. When one pinhole is closed, the grid of wires causes some diffraction in the light, and blocks a certain amount of light received by the corresponding photo-detector. When both pinholes were open, however, the effect of the wires is minimized, so that the results are comparable to the case in which there are no wires placed in front of the lens (Fig.3). Afshar's conclusion is that the light exhibits a wave-like behavior when going through the wires, since the light goes through the spaces between the wires when both slits were open, but also exhibits a particle-like behavior after going through the lens, with photons going to a given photo-detector. To meet Wikipedias quality standards, this article or section may require cleanup. ...

This behavior, Afshar argues, contradicts the principle of complementarity, since it shows both complementary wave and particle characteristics in the same experiment for the same photons; although not at the same time, some critics point out, and hence compatible with an interpretation of complementarity more closely tied to the uncertainty principle. Afshar asserts this experiment has also been conducted with single photons and the results are identical to the high flux experiment, although these results were not available at the time of the talk at Harvard. In quantum physics, the Heisenberg uncertainty principle or the Heisenberg indeterminacy principle — the latter name given to it by Niels Bohr — states that when measuring conjugate quantities, which are pairs of observables of a single elementary particle, increasing the accuracy of the measurement of one quantity increases the uncertainty of...

Ongoing debate

Bohr's principle of complementarity is a philosophical system applied in experimental physics. Mathematical axioms have been developed to embody this philosophical system. The philosophical system is not immune to assumptions. It is such assumptions the Afshar experiment exposes. The historical record consists of texts from which the principle of complementarity, and its assumptions, can be reconstructed, such as "Discussion with Einstein on Epistemological Problems in Atomic Physics", in Albert Einstein: Philosopher-Scientist, P Schilpp ed (Open Court, La Salle, IL, 1949).[15][16]

The Afshar experiment is the subject of ongoing heated interpretation and discussion.

A general idea of such can be read in the following:

• Niels Bohr stated ". . . we are presented with a choice of either tracing the path of the particle, or observing interference effects . . ."[17]. Afshar's experiment does not yield which which way information and demonstrate interference effects for any individual particle (the photon), any more than the classic double slit experiment does, since we already know the photon propagates according to a wave-equation between the slits and any screen. The claim of complementarity violation here is a statistical argument that applies only to large numbers photons, not to individuals (cf "the particle" above in Bohr's statement is a reference to a single photon, not to groups of photons) ^[6].

An interference effect is an ensemble (or statistical) effect. Neither Bohr nor Afshar require interference to be observed in any way other than a statistical manner. For the simple reason that such is impossible. But contrary to Bohr's statement, we are not actually prevented from observing an interference effect (the statistical effect) and tracing the path of individual photons that otherwise contribute to that effect. Niels (Henrik David) Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics. ... The double-slit experiment consists of letting light diffract through two slits producing fringes on a screen. ...

• Niels Bohr stated "an adequate tool for a complementary way of description is offered precisely by the quantum-mechanical formalism" ^[7] If the photons in the experiment obey the precise mathematical laws of quantum mechanics (the formalism), how can Bohr's principle of complementarity be violated by the experiment? ^[8][9][10]

Although Bohr had faith in the quantum-mechanical formalism as an adequate expression of the principle, it is precisely the experiment's obedience to the formalism that throws into question Bohr's faith in the formalism as an expression of complementarity. Niels (Henrik David) Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics. ... Scientific formalism is a possible term for two aspects of the presentation of science, particularly relevant to the physical sciences. ...

Specific critiques

The following is a partial list of specific critiques of Afshar's experimental design and analysis. Afshar's rebuttals are available on his Q&A archive^[11] and FAQ.^[12]

• Bill Unruh, Professor of Physics at University of British Columbia^[13] has claimed that Afshar's experiment is equivalent to a simplified version which can be shown to obey complementarity, and thus Afshar's cannot break complementarity. Afshar argues that the Unruh experiment is not the same because Unruh uses half silvered mirrors which destroy the information on which hole the photon passed through, whereas Afhsar uses a lens which retains this information.
• Lubos Motl, Assistant Professor of Physics, Harvard University.^[14]
• Ruth Kastner, Committee on the History and Philosophy of Science, University of Maryland, College Park.^[15][16]
• Aurelien Drezet, University of Graz Institut of experimental physics, Austria, ^[6]
• Ole Steuernagel, School of Physics, Astronomy and Mathematics, University of Hertfordshire.^[17]

William G. Unruh (born August 28, 1945) is a Canadian physicist at the University of British Columbia, Vancouver, who discovered the Unruh effect. ... The University of British Columbia (UBC) is a public university with its main campus located at Point Grey, in the University Endowment Lands adjacent to Vancouver, British Columbia, Canada and another smaller campus known as UBC Okanagan located in Kelowna, British Columbia. ... Luboš Motl in a restaurant Luboš Motl (born 1973) is a Czech theoretical physicist who works on string theory and conceptual problems of quantum gravity. ... Harvard University (incorporated as The President and Fellows of Harvard College) is a private university in Cambridge, Massachusetts. ... The University of Maryland, College Park (also known as UM, UMD, or UMCP) is a public university located in the city of College Park, in Prince Georges County, Maryland, just outside Washington, D.C., USA. Founded in 1856, the University of Maryland is considered to be a Public Ivy...

References and notes

1. ^ Afshar's IRIMS paper [1]
2. ^ Harvard seminar announcement, March 23, 2004,[2][3]
3. ^ Afshar S, Proc. SPIE 5866, 229-244, July 2005 [4]
4. ^ Afshar APS presentation[5]
5. ^ Image from Kathy Cramer[6]
6. ^ ^{a b} "To conclude, in spite of Afshar's claim we still need two experiments in order to exploit the totality of the phenomenon. As pointed out originally by Bohr, we can not use information associated with a same photon event to rebuild in a statistical way (i.e. by an accumulation of such events) the two complementary distributions of photons in the image plane and in the interference plane. The hypothesis of Afshar that we only need some partial information concerning the interference pattern in order to reconstruct the complete interference is only based on the idea that the fringes already exist. The whole reasoning is circular and for this reason misleading."[7]
7. ^ Niels Bohr, "Discussion with Einstein on Epistemological Problems in Atomic Physics", in Albert Einstein: Philosopher-Scientist, P Schilpp ed (Open Court, La Salle, IL, 1949).[8][9]
8. ^ "There is absolutely nothing mysterious about Afshar's experiment." "And of course, the conventional quantum mechanics is compatible with the principle of complementarity." Lubos Motl at [10]
9. ^ Bill Unruh proposes what he claims is an equivalent set-up to Afshar's and states: "Bohr would have had no problem whatsoever with this experiment within his interpretation. Nor would any other interpretation of quantum mechanics. It is simply another manifestation of the admittedly strange, but utterly comprehensible (it can be calculated with exquisite precision), nature of quantum mechanics." Bill Unruh at [11]
10. ^ "It was claimed that this experiment could be interpreted as a demonstration of a violation of the principle of complementarity in quantum mechanics. Instead, it is shown here that it can be understood in terms of classical wave optics and the standard interpretation of quantum mechanics." Ole Steuernagel at [12]
11. ^ Afshar's weblog
12. ^ Afshar's FAQ
13. ^ "Shahriar Afshar--Quantum Rebel?": Unruh's counter experiment
14. ^ Luboš Motl's interpretation of Afshar's experiment
15. ^ Ruth Kastner "Why the Afshar Experiment Does Not Refute Complementarity"
16. ^ Ruth Kastner "The Afshar Experiment and Complementarity"
17. ^ Ole Steuernagel "Afshar's Experiment does not show a Violation of Complementarity"

March 23 is the 82nd day of the year in the Gregorian Calendar (83rd in Leap years). ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... The American Physical Society was founded in 1899 and is the worlds second largest organization of physicists. ... Niels (Henrik David) Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics. ... Luboš Motl in a restaurant Luboš Motl (born 1973) is a Czech theoretical physicist who works on string theory and conceptual problems of quantum gravity. ... William G. Unruh (born August 28, 1945) is a Canadian physicist at the University of British Columbia, Vancouver, who discovered the Unruh effect. ... William G. Unruh (born August 28, 1945) is a Canadian physicist at the University of British Columbia, Vancouver, who discovered the Unruh effect. ...

Further reading

• Berthold-Georg Englert, Marlan O. Scully & Herbert Walther, Quantum Optical Tests of Complementarity , Nature, Vol 351, pp 111-116 (9 May 1991) and (same authors) The Duality in Matter and Light Scientific American, pg 56-61, (December 1994). Demonstrates that complementarity is enforced, and quantum interference effects destroyed, by decoherence (irreversible object-apparatus correlations), and not, as was previously popularly believed, by Heisenberg's uncertainty principle itself.

Marlan Scully is a theoretical physicist best known for his work in quantum optics. ... May 9 is the 129th day of the year in the Gregorian Calendar (130th in leap years). ... 1991 (MCMXCI) was a common year starting on Tuesday of the Gregorian calendar. ... In physics, complementarity is a basic principle of quantum theory, and refers to effects such as the wave-particle duality, in which different measurements made on a system reveal it to have either particle-like or wave-like properties. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... Quantum decoherence is the general term for the consequences of irreversible quantum entanglement. ... Irreversibility is that property of an event which makes reverting back to the state before the occurrence of the event impossible. ... The framework of quantum mechanics requires a careful definition of measurement, and a thorough discussion of its practical and philosophical implications. ... In quantum physics, the Heisenberg uncertainty principle or the Heisenberg indeterminacy principle — the latter name given to it by Niels Bohr — states that when measuring conjugate quantities, which are pairs of observables of a single elementary particle, increasing the accuracy of the measurement of one quantity increases the uncertainty of...

See also

• Wheeler's delayed choice experiment
• Delayed choice quantum eraser

Wheelers delayed choice experiment was originally a thought experiment proposed by John Archibald Wheeler in 1983 (Quantum Theory and Measurement, edited by J.A. Wheeler and W.H. Zurek, Princeton Univ. ... A delayed choice quantum eraser is a combination between a quantum eraser experiment and Wheelers delayed choice experiment. ...

External links

• Shahriar S. Afshar, "Sharp complementary wave and particle behaviours in the same welcher weg experiment", (2003) IRIMS www.irims.org/quant-ph/030503/; Proc. SPIE 5866 (2005) 229-244; AIP Cof. Proc. 810, (2006) 294-299. (Crossed beam experiment).
• John G. Cramer, "A Farewell to Copenhagen?" (2005), Analog Science Fiction and Fact. (A non-technical discussion in a popular forum)
• Marcus Chown, "Afshar's Quantum Bombshell", "Quantum rebel" (July 24, 2004) New Scientist magazine; "A great leap forward", [18] (October 6, 2004) The Independent.
• Ira Flatow, "Einstein, Bohr and the Nature of Light", [19], (July 30, 2004) Science Friday radio prgram, NPR.
• Paul O'Hara, "Entanglement and quantum interference"