In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. This was stated as follows by Albert Einstein in his article "Quantum Mechanics and Reality" ("Quanten-Mechanik und Wirklichkeit", Dialectica 2:320-324, 1948): It has been suggested that this article or section be merged with Memory locality. ... This is a discussion of a present category of science. ... “Einstein” redirects here. ...

Local realism is the combination of the principle of locality with the "realistic" assumption that all objects must objectively have pre-existing values for any possible measurement before these measurements are made. Einstein liked to say that the Moon is "out there" even when no one is observing it. The names Alice and Bob are commonly used placeholders for archetypal characters in fields such as cryptography and physics. ... Contemporary philosophical realism, also referred to as metaphysical realism, is the belief in a reality that is completely ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc. ...

Realism in the sense used by physicists does not directly equate to realism in metaphysics.^[1] The latter is the claim that there is in some sense a mind-independent world. Even if the results of a possible measurement do not pre-exist the measurement, that does not mean they are the creation of the observer (as in the consciousness causes collapse interpretation of quantum mechanics). Furthermore, a mind-independent property does not have to be the value of some physical variable such as position or momentum. A property can be dispositional, i.e, it can be a tendency, in the way that glass objects tend to break, or are disposed to break, even if they do not actually break. Likewise, the mind-independent properties of quantum systems could consist of a tendency to respond to certain measurements with certain values with some probability.^[2] Such an ontology would be metaphysically realistic without being realistic in the physicist's sense of "local realism" (which would require that single value be produced with certainty). Contemporary philosophical realism, also referred to as metaphysical realism, is the belief in a reality that is completely ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc. ... Plato (Left) and Aristotle (right), by Raphael (Stanza della Segnatura, Rome) Metaphysics is the branch of philosophy concerned with explaining the ultimate nature of reality, being, and the world. ... Consciousness causes collapse is the theory that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. ... In classical mechanics, momentum (pl. ... In philosophy, physiology, and psychology, a disposition is a habit, a preparation, a state of readiness, or a tendency to act in a specified way. ...

Local realism is a significant feature of classical mechanics, general relativity and Maxwell's theory, but quantum mechanics largely rejects this principle due to the presence of distant quantum entanglements, most clearly demonstrated by the EPR paradox and quantified by Bell's inequalities.^[3] Any theory, like quantum mechanics, that violates Bell's inequalities must abandon either local realism or counterfactual definiteness. (The vast majority of physicists believe that experiments have demonstrated Bell's violations, but some local realists dispute the claim, in view of the recognised loopholes in the tests.) Different interpretations of quantum mechanics reject different parts of local realism and/or counterfactual definiteness. For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... Maxwells equations are the set of four equations, attributed to James Clerk Maxwell (written by Oliver Heaviside), that describe the behavior of both the electric and magnetic fields, as well as their interactions with matter. ... Fig. ... It has been suggested that Quantum coherence be merged into this article or section. ... In quantum mechanics, the EPR paradox is a thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. ... This article may be too technical for most readers to understand. ... This article may be too technical for most readers to understand. ... Counterfactual definiteness or CFD is a property of some interpretations of quantum mechanics but not others. ... Violation of Bells inequality is, as John Bell realised (see Bells theorem), a straightforward consequence of quantum mechanics. ... It has been suggested that Quantum mechanics, philosophy and controversy be merged into this article or section. ... Counterfactual definiteness or CFD is a property of some interpretations of quantum mechanics but not others. ...

In most of the conventional interpretations, such as the version of the Copenhagen interpretation and the interpretation based on Consistent Histories, where the wavefunction is not assumed to have a direct physical interpretation of reality it is realism that is rejected. The actual definite properties of a physical system "do not exist" prior to the measurement and the wavefunction has a restricted interpretation as nothing more than a mathematical tool used to calculate the probabilities of experimental outcomes, in agreement with positivism in philosophy as the only topic that science should discuss. The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... 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. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ... Positivism is a philosophy developed by Auguste Comte in the beginning of the 19th century, which stated that the only authentic knowledge is scientific knowledge. ...

In the version of the Copenhagen interpretation where the wavefunction is assumed to have a physical interpretation of reality (the nature of which is unspecified), the principle of locality is violated during the measurement process via wavefunction collapse. This is a non-local process because Born's Rule, when applied to the system's wave function, yields a probability density for all regions of space and time. Upon measurement of the physical system, the probability density vanishes everywhere instantaneously, except where (and when) the measured entity is found to exist. This "vanishing" would be a real physical process, and clearly non-local (faster-than-lightspeed), if the wave function is considered physically real and the probability density converged to zero at arbitrarily far distances during the finite time required for the measurement process. The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ... 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. ... The Born rule (also called the Born law, Borns rule, or Borns law) is a law of quantum mechanics which gives the probability that a measurement on a quantum system will yield a given result. ...

The Bohm interpretation always wants to preserve realism, and it needs to violate the principle of locality to achieve the required correlations. The Bohm interpretation of quantum mechanics, sometimes called the Bohmian Mechanics or Ontological interpretation is an interpretation postulated by David Bohm in 1952, which was an extension of the de Broglie-pilot-wave theory of 1927. ...

In the many-worlds interpretation realism and locality are retained but counterfactual definiteness is rejected by the extension of the notion of reality to allow the existence of parallel universes. The many-worlds interpretation of quantum mechanics or MWI (also known as the relative state formulation, theory of the universal wavefunction, many-universes interpretation, Oxford interpretation or many worlds), is an interpretation of quantum mechanics that claims to resolve all the paradoxes of quantum theory by allowing every possible outcome... Counterfactual definiteness or CFD is a property of some interpretations of quantum mechanics but not others. ... A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of physical reality. ...

Because the differences between the different interpretations are mostly philosophical ones (except for the Bohm and many-worlds interpretations), the physicists usually use the language in which the important statements are independent of the interpretation we choose. In this framework, only the measurable action at a distance - a superluminal propagation of real, physical information - would usually be considered in violation of locality by the physicists. Such phenomena have never been seen, and they are not predicted by the current theories (with the possible exception of the Bohm theory). Faster-than-light (also superluminal or FTL) communications and travel are staples of the science fiction genre. ...

Locality is one of the axioms of relativistic quantum field theory, as required for causality. The formalization of locality in this case is as follows: if we have two observables, each localized within two distinct spacetime regions which happen to be at a spacelike separation from each other, the observables must commute. Alternately, a solution to the field equations is local if the underlying equations are either Lorentz invariant or, more generally, generally covariant or locally Lorentz invariant. Quantum field theory (QFT) is the quantum theory of fields. ... Causality describes the relationship between causes and effects, and is fundamental to all natural science, especially physics. ... 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 the context of special relativity, space-like separated points (or events) in spacetime have a spacetime interval less than 0 (see sign convention). ... Lorentz covariance is a term in physics for the property of space time, that in two different frames of reference, located at the same event in spacetime but moving relative to each other, all non-gravitational laws must make the same predictions for identical experiments. ... This article or section is in need of attention from an expert on the subject. ...

Notes

1. ^ Norsen, T. - Against "Realism"
2. ^ Ian Thomson's dispositional quantum mechanics
3. ^ Ben Dov, Y. Local Realism and the Crucial experiment.

See also

• EPR paradox
• Local hidden variable theory
• Nonlocality