Quantum measurement

In physics, and more specifically, quantum mechanics, the Afshar experiment is an optical experiment, devised by Shahriar S. Afshar in 2004, that is claimed to have disproved the Niels Bohr's principle of complementarity. Since this principle is a central feature of quantum mechanics, the proper interpretation of the experiment has engendered some controversy in the physics community. This controversy has been mostly limited to blogs, physics colloquia, and arXiv e-print archives; as of 2006, neither a description of the experiment, nor any discussion of its theoretical interpretation, has been published in a refereed physics journal. ...more on Wikipedia about "Afshar experiment"

The series of increasingly sophisticated Bell test experiments has narrowed to a small group the critics who question results by pointing to loopholes (some hypothetical, others acknowledged), some of which bias the experimental results in favor of quantum mechanics. An overview of such loopholes can be found in Loopholes in optical Bell test experiments. ...more on Wikipedia about "Bell test experiments"

Bell's theorem is the most famous legacy of the late John Bell. It is famous for drawing an important line in the sand between quantum mechanics (QM) and the world as we know it intuitively. It is simple and elegant, and at the same time touches upon many of the fundamental philosophical issues that relate to modern physics. In its simplest form, Bell's theorem states: ...more on Wikipedia about "Bell's theorem"

The Bohm interpretation of quantum mechanics, sometimes called the Causal interpretation, the Ontological interpretation, or the de Broglie-Bohm theory, 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. ...more on Wikipedia about "Bohm interpretation"

The Bohr-Einstein debates on foundational aspects on quantum mechanics happened during the Solvay conferences. They consisted of analyses of thought experiments. Put simply, they were an attempt by Einstein to explain away the aspects of Bohr's interpretation of Quantum Mechanics that he disliked. Bohr attempted (and, most scholars agree, largely succeeded) to rebut these challenges. The Bohr-Einstein debates remain among the most important in the history of the philosophy of physics, and are certainly the most influential of their kind in the Twentieth Century. ...more on Wikipedia about "Bohr-Einstein debates"

In physics, the CHSH Bell test is an application of Bell's theorem, intended to distinguish between quantum mechanics (QM) and local hidden variable theories. It is named after John Clauser, Michael Horne, Abner Shimony and Richard Holt, who described it in a much-cited paper published in 1969 (Clauser, 1969). They derived the CHSH inequality, which, as with John Bell's original one (Bell, 1964), applies to a statistical property of counts of "coincidences" in a Bell test experiment that follows from the assumption that there exist underlying local hidden variables. The inequality must be obeyed under local realism but can be infringed under certain conditions by the QM formula. ...more on Wikipedia about "CHSH inequality"

(Clauser and Horne's 1974 Bell test) * Aspect, 1981: A. Aspect et al., Phys. Rev. Lett. 47, 460 (1981) ...more on Wikipedia about "Clauser and Horne's 1974 Bell test"

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Consciousness causes collapse is the speculative theory that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. It is an attempt to solve the Wigner's friend paradox by simply stating that collapse occurs at the first "conscious" observer. Supporters claim this is not a revival of substance dualism, since (in a ramification of this view) consciousness and objects are entangled and cannot be considered as distinct. The consciousness causes collapse theory can be considered as a speculative appendage to almost any interpretation of quantum mechanics and most physicists reject it as unverifiable and introducing unnecessary elements into physics. ...more on Wikipedia about "Consciousness causes collapse"

In quantum mechanics, 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. Some believe that this derives from the work by Hugh Everett and is a modern version of the many-worlds interpretation. Others strongly disagree. The theory is based on a consistency criterion that then allows the history of a system to be described so that the probabilities for each history obey the rules of classical probability while being consistent with the Schrödinger equation. ...more on Wikipedia about "Consistent histories"

The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. Bohr and Heisenberg extended the probabilistic interpretation of the wavefunction, proposed by Max Born. Their interpretation attempts to answer some perplexing questions which arise as a result of the wave-particle duality in quantum mechanics, such as the measurement problem. ...more on Wikipedia about "Copenhagen interpretation"

Counterfactual definiteness or CFD is a property of some interpretations of quantum mechanics but not others. ...more on Wikipedia about "Counterfactual definiteness"

In physics, the Elitzur-Vaidman bomb-testing problem is a thought-experiment in quantum mechanics, first proposed by Avshalom Elitzur and Lev Vaidman in 1993. It employs quantum superposition in order to construct a mechanism for ascertaining whether a measurement has taken place. ...more on Wikipedia about "Elitzur-Vaidman bomb-testing problem"

In quantum mechanics, the EPR paradox (Einstein-Podolsky-Rosen) is a thought experiment that demonstrates that the result of a measurement performed on one part of a quantum system can have an instantaneous effect on the result of a measurement performed on another part, regardless of the distance separating the two parts. This runs counter to the intuition of special relativity, which states that information cannot be transmitted faster than the speed of light. "EPR" stands for Albert Einstein, Boris Podolsky, and Nathan Rosen, who introduced the thought experiment in a 1935 paper to argue that quantum mechanics is not a complete physical theory. It is sometimes referred to as the EPRB paradox for David Bohm, who converted the original thought experiment into something closer to being experimentally testable. ...more on Wikipedia about "EPR paradox"

FAPP is an abbervation of "For All Practical Purposes", a pragmatic approach toward the problem of incompleteness of every scientific theory and the usage of asymptotical approximations. ...more on Wikipedia about "FAPP"

GHZ experiments are a class of experiments which arise in quantum mechanics, in discussion and experimental determination of whether local hidden variables are required for, or even compatible with, the representation of experimental results; and with particular relevance to the EPR experiment. ...more on Wikipedia about "GHZ experiment"

In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ...more on Wikipedia about "Hidden variable theory"

In physics, interaction-free measurement is a type of measurement in quantum mechanics that detects the position or state of an object without an interaction occurring between it and the measuring device. Examples include the Renninger negative-result experiment, the Elitzur-Vaidman bomb-testing problem, and certain double-cavity optical systems. ...more on Wikipedia about "Interaction-free measurement"

An interpretation of quantum mechanics is an attempt to answer the question: what exactly is quantum mechanics talking about? Quantum mechanics has been described as "the most precisely tested and most successful theory in the history of science" (c.f. Jackiw and Kleppner, 2000.), but the basic question as to what quantum mechanics means is still posed. Nor is it ever likely to receive a satisfactory answer, inasmuch as there are no macroworld analogs of the quantum world. ...more on Wikipedia about "Interpretation of quantum mechanics"

Is logic empirical? is the title of two articles that discuss the radical concept, that the empirical facts about quantum phenomena may provide grounds for revising classical logic. The replacement derives from the work of Garrett Birkhoff and John von Neumann on quantum logic. The algebraic properties of their proposed logic are somewhat different from those of classical propositional logic in that the principle of distributivity fails. The idea that the principles of logic might be susceptible to revision on empirical grounds has many roots, including the work of W. V. O. Quine and the foundational studies of Hans Reichenbach, presented in his 1944 book " The Philosophical Foundations of Quantum Mechanics". ...more on Wikipedia about "Is logic empirical?"

where $p_A(a, \lambda)$ is the probability of detection of particle A with hidden variable λ by detector A, set in direction a, and similarly p_B (b, λ) is the probability at detector B for particle B, sharing the same value of λ. The source is assumed to produce particles in the state λ with probability $\rho(\lambda)$ . ...more on Wikipedia about "Local hidden variable theory"

In scientific and fictional narrative, a parallel universe or alternate universe, sometimes popularly referred to incorrectly as an 'alternate dimension', is a hypothetical universe which exists separately from our own. Since speculating that a universe does not contain all that exists, the concept of a multiverse contains multiple (or infinite) universes. Some cosmological theories postulate the existence of multiple, possibly infinitely many, parallel universes, which depending on the theory, may or may not mutually interact. ...more on Wikipedia about "Many worlds and possible worlds in literature and art"

The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that accepts the concept of quantum superpositions at face value and proposes the existence of multiple superposed " parallel universes", all of which have the same physical laws and constants, but occupy different states. MWI was initially formulated by Hugh Everett as an alternative to wavefunction collapse used in the Copenhagen interpretation to explain non-deterministic processes (such as measurement) in quantum mechanics. ...more on Wikipedia about "Many-worlds interpretation"

The framework of quantum mechanics requires a careful definition of measurement, and a thorough discussion of its practical and philosophical implications. ...more on Wikipedia about "Measurement in quantum mechanics"

The measurement problem is the key set of questions that every interpretation of quantum mechanics must answer. The problem is that the wavefunction in quantum mechanics evolves according to the Schrödinger equation into a linear superposition of different states, but the actual measurements always find the physical system in a definite state, typically a position eigenstate. Any future evolution will be based on the system having the measured value at that point in time, meaning that the measurement "did something" to the process under examination. Whatever that "something" may be does not appear to be explained by the basic theory. ...more on Wikipedia about "Measurement problem"

In quantum mechanics, the Mott problem is a paradox that illustrates some of the difficulties of understanding the nature of wave function collapse and measurement in quantum mechanics. The problem was first formulated in 1929 by Sir Nevill Francis Mott and Werner Heisenberg, illustrating the paradox of the collapse of a spherically symmetric wave function into the linear tracks seen in a cloud chamber. In practice, the resolution of this paradox underpins all experimental high-energy research, and is experimentally observed out millions of times a day at particle collider laboratories. ...more on Wikipedia about "Mott problem"

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