In quantum mechanics, wavefunction collapse is one of two processes by which quantum systems apparently evolve. It is also called collapse of the state vector. As of March 2004, wavefunction collapse appears to have been disproven.
In general, quantum systems exist in a superposition of basis states, and evolve according to the time dependent Schrödinger equation, which is one of the two processes. The contribution of each basis state to the overall wavefunction is called the amplitude. However, when the wavefunction collapses, which is the other process, from an observer's perspective the state seems to "jump" to one of the basis states and uniquely acquire the value of the property being measured that is associated with that particular basis state.
Upon performing measurement of an observable A, the probability of collapsing to a particular eigenstate of A is directly proportional to the square modulus of the (generally complex) amplitude associated with it. Hence, in experiments such as the double-slit experiment each individual photon arrives at a discrete point on the screen, but as more and more photons are accumulated, they form an interference pattern overall. After the collapse, the system begins to evolve again according to the Schrödinger equation.
The cluster of phenomena described by the expression wavefunction collapse is a fundamental problem in the interpretation of quantum mechanics known as the measurement problem. The problem is not really confronted by the Copenhagen interpretation which simply postulates that this is a special characteristic of the "measurement" process. The Everett many-worlds interpretation deals with it by discarding the collapse-process, thus reformulating the relation between measurement apparatus and system in such a way that the linear laws of quantum mechanics are universally valid, that is, the only process according to which a quantum system evolves is governed by the Schrödinger equation. Often tied in with the many-worlds interpretation but not limited to it is the physical process of decoherence, which causes an apparent collapse.
Note that a general description of the evolution of quantum mechanical systems is possible by using density operators and quantum operations. In this formalism (which is closely related to the C*-algebraic formalism) the collapse of the wave function corresponds to a non-unitary quantum operation.
Shahriar Afshar's experiment
In March 2004, Shahriar Afshar announced at Harvard University the results of a variation on the two-pin-hole "which-way" experiment (similar to the double-slit experiment) in which he claims to have disproved Bohr's Principle of Complementarity, also reported in the July 24 edition of New Scientist. [1] [2] [3]
![[1]](http://www.sciencefriday.com/images/shows/2004/073004/AfsharExperimentSmall.jpg){kind=link}
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 invalidate the Many-worlds interpretation which predicts that there should be no interference between wave functions in universes that are physically distinguishable.
Unruh has claimed that Afshar's experiment does not prove or disprove any of the interpretations. Basically, the counter-argument is that Afshar's apparatus can be analyzed by analogy to a pair of cascaded interferometers and that this elucidates why the contradiction claimed is not real.
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.