User:Afshar

Shahriar S. Afshar

 * Professor of Physics & Principal Investigator at Rowan University in NJ
 * Chief Scientist at IRIMS, Boston.
 * Performed a demostration experiment at Harvard University in March of 2004, originally conducted at IRIMS in 2001. The experiment, briefly discussed in Afshar experiment, puts the Copenhagen Interpretation of Quantum Mechanics in doubt. Specifically, his experiment violates Bohr's Complementarity Principle, and has important implications for better understanding Measurement in quantum mechanics.

Shahriar Afshar's experiment
In March 2004, Shahriar S. Afshar announced at Harvard University the results of a variation on the similar two-pin-hole "which-way" experiment, in which he claims to have disproved Bohr's Principle of Complementarity, also reported in the July 24 edition of New Scientist.  

Using his experiment, it is possible to detect interference fringes even when observing the path of a photon stream, indicating that the wavefunction does not collapse. If his results are verified, it has far-reaching implications for the understanding of the quantum world, and invalidates the Copenhagen interpretation. It would also seem to falsify the Many-worlds interpretation, which predicts that there should be no interference between wave functions in universes that are physically distinguishable, although some physicists explain this by saying the wave functions interfere as long as the quantum state of the photon remains coherent, which is the case in the Afshar's experiment. However, it lends support to the Transactional interpretation, which is consistent with the results.

Unruh's rebuttal and Afshar's response
On August 7, 2004, Bill Unruh presented an argument in which he claims to disprove Afshar's results. Because of this, some in the scientific community question the validity of Afshar's experiment.

Unruh offers a simplified version of Afshar's experiment using a series of half-silvered mirrors, a setup which he claims to be equivalent to Shariar Afshar's. He demonstrates that his experiment is consistent with the Copenhagen interpretation and, on that basis, argues that Afshar's results are incorrect. 

However, there is a major difference between Unruh's experiment and Afshar's experiment. Whereas Afshar uses a lens to focus two light sources onto two detectors in a way that allows him to exactly identify the path of any photon, the design of Unruh's experiment destroys the path information at the second mirror.

Although Unruh demonstrates that he can detect the path of a photon when one path is blocked, this is tautological: if photons are admitted on only one path, the detectors the beam reaches will agree with the path of the beam. When light travels along both paths, two beams are combined into a single beam with a single direction, and so Unruh's argument is invalid.

Afshar's response, found in his FAQ, compares Unruh's experiment to one in which the wings are removed from an airplane. The experimenter might find that a wingless airplane cannot fly, but such an experiment would not prove anything about the flight capabilities of an intact plane.

External links, references, and resources

 * "The Double Slit Experiment". [physicsweb.org]


 * Analog: A Farewell to Copenhagen?


 * Shahriar S. Afshar, "Waving Copenhagen Good-bye: Were the founders of Quantum Mechanics wrong?"
 * "Variation on the (similar two-pin-hole "which-way" experiment". (reported in New Scientist; July 24)
 * www.irims.org/quant-ph/030503/ (Preprint)

Category:Wave mechanics Category:Quantum mechanics Category:Physics experiments

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