In certain interpretations of quantum mechanics, wavefunction collapse is one of two processes by which quantum systems apparently evolve according to the laws of quantum mechanics. It is also called collapse of the state vector. The existence of the wavefunction collapse is required in In a nontechnical sense, an interpretation of quantum mechanics is an attempt to answer the question: what exactly is quantum mechanics talking about? Quantum mechanics has been very successful in predicting experimental results. ... Fig. ... Fig. ... In the most restricted usage in quantum mechanics, the wavefunction associated with a particle such as an electron, is a complex-valued square integrable function ψ defined over a portion of space normalized in such a way that In Max Borns probabilistic interpretation of the wavefunction, the amplitude squared...

• the version of the Copenhagen interpretation where the wavefunction is real
• the so-called transactional interpretation
• in a "spiritual interpretation" in which consciousness causes collapse.

On the other hand, the collapse does not occur in The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... 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. ... Consciousness causes collapse is an interpretation of quantum mechanics based on the idea that conscious observers should be considered responsible for the wavefunction collapse into the forms or objects we see around us. ...

• the version of the Copenhagen interpretation where the wavefunction is not real
• the interpretation based on consistent histories
• the many-worlds interpretation
• the Bohm interpretation.

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 mentioned at the beginning of this article - a process included in all interpretations. 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. The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... The consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ... This article may be too technical for most readers to understand. ... The Bohm interpretation of quantum mechanics, sometimes called the Causal interpretation, or Ontological interpretation, is an interpretation postulated by David Bohm in which the existence of a non-local universal wavefunction (Schrödinger equation) allows distant particles to interact instantaneously. ... The term superposition can have several meanings: Quantum superposition Law of superposition in geology and archaeology Superposition principle for vector fields Superposition Calculus is used for equational first-order reasoning This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... In physics, the Schrödinger equation, proposed by the Austrian physicist Erwin Schrödinger in 1925, describes the time-dependence of quantum mechanical systems. ... Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation. ...

Upon performing measurement of an observable A, the probability of collapsing to a particular eigenstate of A is directly proportional to the squared 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. 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. ... In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ... This article is about proportionality, the mathematical relation. ... The complex numbers are an extension of the real numbers, in which all non-constant polynomials have roots. ... The double-slit experiment consists of letting light diffract through two slits producing fringes on a screen. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ...

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. Decoherence is also important for the interpretation based on Consistent Histories. The measurement problem is the key set of questions that every interpretation of quantum mechanics must answer. ... The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics, based on Hugh Everetts relative-state formulation. ... Quantum decoherence is the general term for the consequences of irreversible quantum entanglement. ... The consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ...

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. A density matrix, or density operator, is used in quantum theory to describe the statistical state of a quantum system. ... In quantum mechanics, a quantum operation is a mathematical formalism used to describe a broad class of transformations that a quantum mechanical system can undergo. ...

See also

• Afshar experiment
• Schrödinger's cat