Interpretations of quantum mechanics

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An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics "corresponds" to reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments (not one prediction from quantum mechanics is found to be contradicted by experiments), there exist a number of contending schools of thought over their interpretation. (These views on interpretation differ on such fundamental questions as whether quantum mechanics is deterministic or random, which elements of quantum mechanics can be considered "real", and what is the nature of measurement, among other matters.)

Quantum mechanics branch of physics dealing with phenomena at scales of the order of the Planck constant

Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles.

Reality is the sum or aggregate of all that is real or existent, as opposed to that which is merely imaginary. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, reality is the totality of the universe, known and unknown. Philosophical questions about the nature of reality or existence or being are considered under the rubric of ontology, which is a major branch of metaphysics in the Western philosophical tradition. Ontological questions also feature in diverse branches of philosophy, including the philosophy of science, philosophy of religion, philosophy of mathematics, and philosophical logic. These include questions about whether only physical objects are real, whether reality is fundamentally immaterial, whether hypothetical unobservable entities posited by scientific theories exist, whether God exists, whether numbers and other abstract objects exist, and whether possible worlds exist.

Determinism is the philosophical idea that all events, including moral choices, are determined completely by previously existing causes. Determinism is at times understood to preclude free will because it entails that humans cannot act otherwise than they do. It can also be called as hard determinism from this point of view. Hard determinism is a position on the relationship of determinism to free will. The theory holds that the universe is utterly rational because complete knowledge of any given situation assures that unerring knowledge of its future is also possible. Some philosophers suggest variants around this basic definition. Deterministic theories throughout the history of philosophy have sprung from diverse and sometimes overlapping motives and considerations. The opposite of determinism is some kind of indeterminism. Determinism is often contrasted with free will.

Contents

Despite nearly a century of debate and experiment, no consensus has been reached amongst physicists and philosophers of physics concerning which interpretation best "represents" reality. [1]

Philosophy of physics

In philosophy, philosophy of physics deals with conceptual and interpretational issues in modern physics, and often overlaps with research done by certain kinds of theoretical physicists. Philosophy of physics can be very broadly lumped into three main areas:

History

Main quantum mechanics interpreters

The definition of quantum theorists' terms, such as wave functions and matrix mechanics , progressed through many stages. For instance, Erwin Schrödinger originally viewed the electron's wave function as its charge density smeared across the field, whereas Max Born reinterpreted the absolute square value of the wave function as the electron's probability density distributed across the field.

Wave function mathematical description of the quantum state of a system; complex-valued probability amplitude, and the probabilities for the possible results of measurements made on the system can be derived from it

A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements made on the system can be derived from it. The most common symbols for a wave function are the Greek letters ψ or Ψ.

Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925.

Erwin Schrödinger 20th-century Austrian physicist

Erwin Rudolf Josef Alexander Schrödinger, sometimes written as Erwin Schrodinger or Erwin Schroedinger, was a Nobel Prize-winning Austrian physicist who developed a number of fundamental results in the field of quantum theory: the Schrödinger equation provides a way to calculate the wave function of a system and how it changes dynamically in time.

Although the Copenhagen interpretation was originally most popular, quantum decoherence has gained popularity. Thus the many-worlds interpretation has been gaining acceptance. [2] [3] Moreover, the strictly formalist position, shunning interpretation, has been challenged by proposals for falsifiable experiments that might one day distinguish among interpretations, as by measuring an AI consciousness [4] or via quantum computing. [5]

The Copenhagen interpretation is an expression of the meaning of quantum mechanics that was largely devised from 1925 to 1927 by Niels Bohr and Werner Heisenberg. It remains one of the most commonly taught interpretations of quantum mechanics.

Quantum decoherence loss of quantum coherence

Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. Coherence is preserved under the laws of quantum physics, and this is necessary for the functioning of quantum computers. However, when a quantum system is not perfectly isolated, coherence is shared with the environment and appears to be lost with time, a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics.

Many-worlds interpretation interpretation of quantum mechanics which denies the collapse of the wavefunction

The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction and denies the actuality of wavefunction collapse. The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics. Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world". In layman's terms, the hypothesis states there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes. The interpretation is also referred to as MWI, the relative state formulation, the Everett interpretation, the theory of the universal wavefunction, many-universes interpretation, multiverse theory or just many-worlds.

As a rough guide development of the mainstream view during the 1990s to 2000s, consider the "snapshot" of opinions collected in a poll by Schlosshauer et al. at the 2011 "Quantum Physics and the Nature of Reality" conference of July 2011. [6] The authors reference a similarly informal poll carried out by Max Tegmark at the "Fundamental Problems in Quantum Theory" conference in August 1997. The main conclusion of the authors is that "the Copenhagen interpretation still reigns supreme", receiving the most votes in their poll (42%), besides the rise to mainstream notability of the many-worlds interpretations:

Max Tegmark Swedish-American cosmologist

Max Erik Tegmark is a Swedish-American physicist and cosmologist. He is a professor at the Massachusetts Institute of Technology and the scientific director of the Foundational Questions Institute. He is also a co-founder of the Future of Life Institute and a supporter of the effective altruism movement, and has received donations from Elon Musk to investigate existential risk from advanced artificial intelligence.

"The Copenhagen interpretation still reigns supreme here, especially if we lump it together with intellectual offsprings such as information-based interpretations and the Quantum Bayesian interpretation. In Tegmark's poll, the Everett interpretation received 17% of the vote, which is similar to the number of votes (18%) in our poll."

It is noteworthy that only Cramer's transactional interpretation, published in 1986, assigns physical basis to Max Born's assertion that the absolute square of the wave function is a probability density. [7]

The transactional interpretation of quantum mechanics (TIQM) takes the psi and psi* wave functions of the standard quantum formalism to be retarded and advanced waves that form a quantum interaction as a Wheeler–Feynman handshake or transaction. It was first proposed in 1986 by John G. Cramer, who argues that it helps in developing intuition for quantum processes. He also suggests that it avoids the philosophical problems with the Copenhagen interpretation and the role of the observer, and also resolves various quantum paradoxes. TIQM formed a minor plot point in his science fiction novel Einstein's Bridge.

Max Born physicist

Max Born was a German-Jewish physicist and mathematician who was instrumental in the development of quantum mechanics. He also made contributions to solid-state physics and optics and supervised the work of a number of notable physicists in the 1920s and 1930s. Born won the 1954 Nobel Prize in Physics for his "fundamental research in quantum mechanics, especially in the statistical interpretation of the wave function".

Nature

More or less, all interpretations of quantum mechanics share two qualities:

  1. They interpret a formalism —a set of equations and principles to generate predictions via input of initial conditions
  2. They interpret a phenomenology —a set of observations, including those obtained by empirical research and those obtained informally, such as humans' experience of an unequivocal world

Two qualities vary among interpretations:

  1. Ontology—claims about what things, such as categories and entities, exist in the world
  2. Epistemology—claims about the possibility, scope, and means toward relevant knowledge of the world

In philosophy of science, the distinction of knowledge versus reality is termed epistemic versus ontic . A general law is a regularity of outcomes (epistemic), whereas a causal mechanism may regulate the outcomes (ontic). A phenomenon can receive interpretation either ontic or epistemic. For instance, indeterminism may be attributed to limitations of human observation and perception (epistemic), or may be explained as a real existing maybe encoded in the universe (ontic). Confusing the epistemic with the ontic, like if one were to presume that a general law actually "governs" outcomes—and that the statement of a regularity has the role of a causal mechanism—is a category mistake.

In a broad sense, scientific theory can be viewed as offering scientific realism—approximately true description or explanation of the natural world—or might be perceived with antirealism. A realist stance seeks the epistemic and the ontic, whereas an antirealist stance seeks epistemic but not the ontic. In the 20th century's first half, antirealism was mainly logical positivism, which sought to exclude unobservable aspects of reality from scientific theory.

Since the 1950s, antirealism is more modest, usually instrumentalism, permitting talk of unobservable aspects, but ultimately discarding the very question of realism and posing scientific theory as a tool to help humans make predictions, not to attain metaphysical understanding of the world. The instrumentalist view is carried by the famous quote of David Mermin, "Shut up and calculate", often misattributed to Richard Feynman. [8]

Other approaches to resolve conceptual problems introduce new mathematical formalism, and so propose alternative theories with their interpretations. An example is Bohmian mechanics, whose empirical equivalence with the three standard formalisms—Schrödinger's wave mechanics, Heisenberg's matrix mechanics, and Feynman's path integral formalism—has been demonstrated.

Challenges

  1. Abstract, mathematical nature of quantum field theories: the mathematical structure of quantum mechanics is mathematically abstract without clear interpretation of its quantities.
  2. Existence of apparently indeterministic and irreversible processes: in classical field theory, a physical property at a given location in the field is readily derived. In most mathematical formulations of quantum mechanics, measurement is given a special role in the theory, as it is the sole process that can cause a nonunitary, irreversible evolution of the state.
  3. Role of the observer in determining outcomes: the Copenhagen Interpretation implies that the wavefunction is a calculational tool, and represents reality only immediately after a measurement, perhaps performed by an observer; Everettian interpretations grant that all the possibilities can be real, and that the process of measurement-type interactions cause an effective branching process. [9]
  4. Classically unexpected correlations between remote objects: entangled quantum systems, as illustrated in the EPR paradox, obey statistics that seem to violate principles of local causality. [10]
  5. Complementarity of proffered descriptions: complementarity holds that no set of classical physical concepts can simultaneously refer to all properties of a quantum system. For instance, wave description A and particulate description B can each describe quantum system S, but not simultaneously. This implies the composition of physical properties of S does not obey the rules of classical propositional logic when using propositional connectives (see "Quantum logic"). Like contextuality, the "origin of complementarity lies in the non-commutativity of operators" that describe quantum objects (Omnès 1999).
  6. Rapidly rising intricacy, far exceeding humans' present calculational capacity, as a system's size increases: since the state space of a quantum system is exponential in the number of subsystems, it is difficult to derive classical approximations.
  7. Contextual behaviour of systems locally: Quantum contextuality demonstrates that classical intuitions in which properties of a system hold definite values, independent of the manner of their measurement, fails even for local systems. Also, physical principles such as Leibnitz's Principle of the identity of indiscernibles no longer apply in the quantum domain, signalling that most classical intuitions may be incorrect about the quantum world.

Summaries

Classification adopted by Einstein

An interpretation (i.e. a semantic explanation of the formal mathematics of quantum mechanics) can be characterized by its treatment of certain matters addressed by Einstein, such as:

To explain these properties, we need to be more explicit about the kind of picture an interpretation provides. To that end we will regard an interpretation as a correspondence between the elements of the mathematical formalism M and the elements of an interpreting structure I, where:

The crucial aspect of an interpretation is whether the elements of I are regarded as physically real. Hence the bare instrumentalist view of quantum mechanics outlined in the previous section is not an interpretation at all, for it makes no claims about elements of physical reality.

The current usage of realism and completeness originated in the 1935 paper in which Einstein and others proposed the EPR paradox. [11] In that paper the authors proposed the concepts element of reality and the completeness of a physical theory. They characterised element of reality as a quantity whose value can be predicted with certainty before measuring or otherwise disturbing it, and defined a complete physical theory as one in which every element of physical reality is accounted for by the theory. In a semantic view of interpretation, an interpretation is complete if every element of the interpreting structure is present in the mathematics. Realism is also a property of each of the elements of the maths; an element is real if it corresponds to something in the interpreting structure. For example, in some interpretations of quantum mechanics (such as the many-worlds interpretation) the ket vector associated to the system state is said to correspond to an element of physical reality, while in other interpretations it is not.

Determinism is a property characterizing state changes due to the passage of time, namely that the state at a future instant is a function of the state in the present (see time evolution). It may not always be clear whether a particular interpretation is deterministic or not, as there may not be a clear choice of a time parameter. Moreover, a given theory may have two interpretations, one of which is deterministic and the other not.

Local realism has two aspects:

A precise formulation of local realism in terms of a local hidden-variable theory was proposed by John Bell.

Bell's theorem, combined with experimental testing, restricts the kinds of properties a quantum theory can have, the primary implication being that quantum mechanics cannot satisfy both the principle of locality and counterfactual definiteness.

It should be noted that regardless of Einstein's concerns about interpretation issues, Dirac and other quantum notables embraced the technical advances of the new theory while devoting little or no attention to interpretational aspects.

Copenhagen interpretation

The Copenhagen interpretation is the "standard" 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 originally by Max Born. The Copenhagen interpretation rejects questions like "where was the particle before I measured its position?" as meaningless. The measurement process randomly picks out exactly one of the many possibilities allowed for by the state's wave function in a manner consistent with the well-defined probabilities that are assigned to each possible state. According to the interpretation, the interaction of an observer or apparatus that is external to the quantum system is the cause of wave function collapse, thus according to Paul Davies, "reality is in the observations, not in the electron". [12] In general, after a measurement (click of a Geiger counter or a trajectory in a spark or bubble chamber) it ceases to be relevant unless subsequent experimental observations can be performed.

Many worlds

The many-worlds interpretation is an interpretation of quantum mechanics in which a universal wavefunction obeys the same deterministic, reversible laws at all times; in particular there is no (indeterministic and irreversible) wavefunction collapse associated with measurement. The phenomena associated with measurement are claimed to be explained by decoherence, which occurs when states interact with the environment producing entanglement, repeatedly "splitting" the universe into mutually unobservable alternate histories—effectively distinct universes within a greater multiverse.

Consistent histories

The consistent histories interpretation generalizes the conventional Copenhagen interpretation and attempts to provide a natural interpretation of quantum cosmology. The theory is based on a consistency criterion that allows the history of a system to be described so that the probabilities for each history obey the additive rules of classical probability. It is claimed to be consistent with the Schrödinger equation.

According to this interpretation, the purpose of a quantum-mechanical theory is to predict the relative probabilities of various alternative histories (for example, of a particle).

Ensemble interpretation

The ensemble interpretation, also called the statistical interpretation, can be viewed as a minimalist interpretation. That is, it claims to make the fewest assumptions associated with the standard mathematics. It takes the statistical interpretation of Born to the fullest extent. The interpretation states that the wave function does not apply to an individual system for example, a single particle but is an abstract statistical quantity that only applies to an ensemble (a vast multitude) of similarly prepared systems or particles. Probably the most notable supporter of such an interpretation was Einstein:

The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.

Einstein in Albert Einstein: Philosopher-Scientist, ed. P.A. Schilpp (Harper & Row, New York)

The most prominent current advocate of the ensemble interpretation is Leslie E. Ballentine, professor at Simon Fraser University, author of the graduate level text book Quantum Mechanics, A Modern Development. An experiment illustrating the ensemble interpretation is provided in Akira Tonomura's Video clip 1. [13] It is evident from this double-slit experiment with an ensemble of individual electrons that, since the quantum mechanical wave function (absolutely squared) describes the completed interference pattern, it must describe an ensemble. A new version of the ensemble interpretation that relies on a reformulation of probability theory was introduced by Raed Shaiia. [14] [15]

De Broglie–Bohm theory

The de Broglie–Bohm theory of quantum mechanics (also known as the pilot wave theory) is a theory by Louis de Broglie and extended later by David Bohm to include measurements. Particles, which always have positions, are guided by the wavefunction. The wavefunction evolves according to the Schrödinger wave equation, and the wavefunction never collapses. The theory takes place in a single space-time, is non-local, and is deterministic. The simultaneous determination of a particle's position and velocity is subject to the usual uncertainty principle constraint. The theory is considered to be a hidden-variable theory, and by embracing non-locality it satisfies Bell's inequality. The measurement problem is resolved, since the particles have definite positions at all times. [16] Collapse is explained as phenomenological. [17]

Relational quantum mechanics

The essential idea behind relational quantum mechanics, following the precedent of special relativity, is that different observers may give different accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, "collapsed" eigenstate, while to another observer at the same time, it may be in a superposition of two or more states. Consequently, if quantum mechanics is to be a complete theory, relational quantum mechanics argues that the notion of "state" describes not the observed system itself, but the relationship, or correlation, between the system and its observer(s). The state vector of conventional quantum mechanics becomes a description of the correlation of some degrees of freedom in the observer, with respect to the observed system. However, it is held by relational quantum mechanics that this applies to all physical objects, whether or not they are conscious or macroscopic. Any "measurement event" is seen simply as an ordinary physical interaction, an establishment of the sort of correlation discussed above. Thus the physical content of the theory has to do not with objects themselves, but the relations between them. [18] [19]

An independent relational approach to quantum mechanics was developed in analogy with David Bohm's elucidation of special relativity, [20] in which a detection event is regarded as establishing a relationship between the quantized field and the detector. The inherent ambiguity associated with applying Heisenberg's uncertainty principle is subsequently avoided. [21]

Transactional interpretation

The transactional interpretation of quantum mechanics (TIQM) by John G. Cramer is an interpretation of quantum mechanics inspired by the Wheeler–Feynman absorber theory. [22] It describes the collapse of the wave function as resulting from a time-symmetric transaction between a possibility wave from the source to the receiver (the wave function) and a possibility wave from the receiver to source (the complex conjugate of the wave function). This interpretation of quantum mechanics is unique in that it not only views the wave function as a real entity, but the complex conjugate of the wave function, which appears in the Born rule for calculating the expected value for an observable, as also real.

Stochastic mechanics

An entirely classical derivation and interpretation of Schrödinger's wave equation by analogy with Brownian motion was suggested by Princeton University professor Edward Nelson in 1966. [23] Similar considerations had previously been published, for example by R. Fürth (1933), I. Fényes (1952), and Walter Weizel (1953), and are referenced in Nelson's paper. More recent work on the stochastic interpretation has been done by M. Pavon. [24] An alternative stochastic interpretation [25] was developed by Roumen Tsekov.

Objective collapse theories

Objective collapse theories differ from the Copenhagen interpretation by regarding both the wave function and the process of collapse as ontologically objective (meaning these exist and occur independent of the observer). In objective theories, collapse occurs either randomly ("spontaneous localization") or when some physical threshold is reached, with observers having no special role. Thus, objective-collapse theories are realistic, indeterministic, no-hidden-variables theories. Standard quantum mechanics does not specify any mechanism of collapse; QM would need to be extended if objective collapse is correct. The requirement for an extension to QM means that objective collapse is more of a theory than an interpretation. Examples include

Consciousness causes collapse (von Neumann–Wigner interpretation)

In his treatise The Mathematical Foundations of Quantum Mechanics, John von Neumann deeply analyzed the so-called measurement problem. He concluded that the entire physical universe could be made subject to the Schrödinger equation (the universal wave function). He also described how measurement could cause a collapse of the wave function. [29] This point of view was prominently expanded on by Eugene Wigner, who argued that human experimenter consciousness (or maybe even dog consciousness) was critical for the collapse, but he later abandoned this interpretation. [30] [31]

Variations of the consciousness causes collapse interpretation include:

Subjective reduction research
This principle, that consciousness causes the collapse, is the point of intersection between quantum mechanics and the mind/body problem; and researchers are working to detect conscious events correlated with physical events that, according to quantum theory, should involve a wave function collapse; but, thus far, results are inconclusive. [32] [33]
Participatory anthropic principle (PAP)
John Archibald Wheeler's participatory anthropic principle says that consciousness plays some role in bringing the universe into existence. [34]

Other physicists have elaborated their own variations of the consciousness causes collapse interpretation; including:

Many minds

The many-minds interpretation of quantum mechanics extends the many-worlds interpretation by proposing that the distinction between worlds should be made at the level of the mind of an individual observer.

Quantum logic

Quantum logic can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. This research area and its name originated in the 1936 paper by Garrett Birkhoff and John von Neumann, who attempted to reconcile some of the apparent inconsistencies of classical boolean logic with the facts related to measurement and observation in quantum mechanics.

Quantum information theories

Quantum informational approaches [35] have attracted growing support. [36] [6] They subdivide into two kinds [37]

The state is not an objective property of an individual system but is that information, obtained from a knowledge of how a system was prepared, which can be used for making predictions about future measurements. ...A quantum mechanical state being a summary of the observer's information about an individual physical system changes both by dynamical laws, and whenever the observer acquires new information about the system through the process of measurement. The existence of two laws for the evolution of the state vector...becomes problematical only if it is believed that the state vector is an objective property of the system...The "reduction of the wavepacket" does take place in the consciousness of the observer, not because of any unique physical process which takes place there, but only because the state is a construct of the observer and not an objective property of the physical system [40]

Modal interpretations of quantum mechanics were first conceived of in 1972 by B. van Fraassen, in his paper "A formal approach to the philosophy of science." However, this term now is used to describe a larger set of models that grew out of this approach. The Stanford Encyclopedia of Philosophy describes several versions: [41]

Time-symmetric theories

Several theories have been proposed which modify the equations of quantum mechanics to be symmetric with respect to time reversal. [42] [43] [44] [45] [46] [47] (E.g. see Wheeler-Feynman time-symmetric theory). This creates retrocausality: events in the future can affect ones in the past, exactly as events in the past can affect ones in the future. In these theories, a single measurement cannot fully determine the state of a system (making them a type of hidden-variables theory), but given two measurements performed at different times, it is possible to calculate the exact state of the system at all intermediate times. The collapse of the wavefunction is therefore not a physical change to the system, just a change in our knowledge of it due to the second measurement. Similarly, they explain entanglement as not being a true physical state but just an illusion created by ignoring retrocausality. The point where two particles appear to "become entangled" is simply a point where each particle is being influenced by events that occur to the other particle in the future.

Not all advocates of time-symmetric causality favour modifying the unitary dynamics of standard quantum mechanics. Thus a leading exponent of the two-state vector formalism, Lev Vaidman, highlights how well the two-state vector formalism dovetails with Hugh Everett's many-worlds interpretation. [48]

Branching space-time theories

BST theories resemble the many worlds interpretation; however, "the main difference is that the BST interpretation takes the branching of history to be a feature of the topology of the set of events with their causal relationships... rather than a consequence of the separate evolution of different components of a state vector." [49] In MWI, it is the wave functions that branches, whereas in BST, the space-time topology itself branches. BST has applications to Bell's theorem, quantum computation and quantum gravity. It also has some resemblance to hidden-variable theories and the ensemble interpretation: particles in BST have multiple well defined trajectories at the microscopic level. These can only be treated stochastically at a coarse grained level, in line with the ensemble interpretation. [49]

Other interpretations

As well as the mainstream interpretations discussed above, a number of other interpretations have been proposed which have not made a significant scientific impact for whatever reason. These range from proposals by mainstream physicists to the more occult ideas of quantum mysticism.

Comparison

The most common interpretations are summarized in the table below. The values shown in the cells of the table are not without controversy, for the precise meanings of some of the concepts involved are unclear and, in fact, are themselves at the center of the controversy surrounding the given interpretation. For another table comparing interpretations of quantum theory, see reference. [50]

No experimental evidence exists that distinguishes among these interpretations. To that extent, the physical theory stands, and is consistent with itself and with reality; difficulties arise only when one attempts to "interpret" the theory. Nevertheless, designing experiments which would test the various interpretations is the subject of active research.

Most of these interpretations have variants. For example, it is difficult to get a precise definition of the Copenhagen interpretation as it was developed and argued about by many people.

InterpretationAuthor(s) Determin­istic? Real
wave­function
?
Unique
history?
Hidden
variables
?
Collapsing
wave­functions
?
Observer
role?
Local
dynamics
?
Counterfactual
definiteness
?
Extant
universal
wavefunction
?
Ensemble interpretation Max Born, 1926AgnosticNoYesAgnosticNoNoNoNoNo
Copenhagen interpretation Niels Bohr, Werner Heisenberg, 1927NoNo 1 YesNoYes 2 CausalNoNoNo
de Broglie–Bohm theory Louis de Broglie, 1927, David Bohm, 1952YesYes 3 Yes 4 YesPhenomenologicalNoNo 15 YesYes
Quantum logic Garrett Birkhoff, 1936AgnosticAgnosticYes 5 NoNoInterpretational 6 AgnosticNoNo
Time-symmetric theories Satosi Watanabe, 1955YesYesYesYesNoNoYesNoYes
Many-worlds interpretation Hugh Everett, 1957YesYesNoNoNoNoYesIll-posedYes
Consciousness causes collapse Eugene Wigner, 1961NoYesYesNoYesCausalNoNoYes
Stochastic interpretation Edward Nelson, 1966NoNoYesYes 14 NoNoNoYes 14 No
Many-minds interpretation H. Dieter Zeh, 1970YesYesNoNoNoInterpretational 7 YesIll-posedYes
Consistent histories Robert B. Griffiths, 1984NoNoNoNoNoNoYesNoYes
Transactional interpretation John G. Cramer, 1986YesYesYesNoYes 8 NoNo 12 YesNo
Objective collapse theories Ghirardi–Rimini–Weber, 1986,
Penrose interpretation, 1989
NoYesYesNoYesNoNoNoNo
Relational interpretation Carlo Rovelli, 1994AgnosticNoAgnostic 9 NoYes 10 Intrinsic 11 Yes [51] NoNo
QBism Christopher Fuchs, Ruediger Schack, 2010NoNo 16 Agnostic 17 NoYes 18 Intrinsic 19 YesNoNo

See also

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In physics, hidden-variable theories are held by some physicists who argue that the state of a physical system, as formulated by quantum mechanics, does not give a complete description for the system. An example would be that quantum mechanics is ultimately incomplete, and that a complete theory would provide descriptive categories to account for all observable behavior and thus avoid any indeterminism. The existence of indeterminacy for some measurements is a characteristic of prevalent interpretations of quantum mechanics; moreover, bounds for indeterminacy can be expressed in a quantitative form by the Heisenberg uncertainty principle.

In physics, the principle of locality states that an object is directly influenced only by its immediate surroundings. A theory which includes the principle of locality is said to be a "local theory". This is an alternative to the older concept of instantaneous "action at a distance". Locality evolved out of the field theories of classical physics. The concept is that for an action at one point to have an influence at another point, something in the space between those points such as a field must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, carrying the influence.

The measurement problem in quantum mechanics is the problem of how wave function collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer. The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states, but actual measurements always find the physical system in a definite state. Any future evolution is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the system that is not obviously a consequence of Schrödinger evolution.

Objective-collapse theories, also known as quantum-mechanical spontaneous-localization models (QMSL), are an approach to the interpretational problems of quantum mechanics. They are realistic and indeterministic and reject hidden variables. The approach is similar to the Copenhagen interpretation, but more firmly objective.

There is a diversity of views that propose interpretations of quantum mechanics. They vary in how many physicists accept or reject them. An interpretation of quantum mechanics is a conceptual scheme that proposes to relate the mathematical formalism to the physical phenomena of interest. The present article is about those interpretations which, independently of their intrinsic value, remain today less known, or are simply less debated by the scientific community, for different reasons.

In physics, the observer effect is the theory that simply observing a situation or phenomenon necessarily changes that phenomenon. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner. A commonplace example is checking the pressure in an automobile tire; this is difficult to do without letting out some of the air, thus changing the pressure. Similarly, it is not possible to see any object without light hitting the object, and causing it to reflect that light. While the effects of observation are oftentimes negligible, the object still experiences a change. This effect can be found in many domains of physics, but can usually be reduced to insignificance by using different instruments or observation techniques.

The two-state vector formalism (TSVF) is a description of quantum mechanics in terms of a causal relation in which the present is caused by quantum states of the past and of the future taken in combination.

Quantum Bayesianism

In physics and the philosophy of physics, quantum Bayesianism is an interpretation of quantum mechanics that takes an agent's actions and experiences as the central concerns of the theory. This interpretation is distinguished by its use of a subjective Bayesian account of probabilities to understand the quantum mechanical Born rule as a normative addition to good decision-making. Rooted in the prior work of Carlton Caves, Christopher Fuchs, and Rüdiger Schack during the early 2000s, QBism itself is primarily associated with Fuchs and Schack and has more recently been adopted by David Mermin. QBism draws from the fields of quantum information and Bayesian probability and aims to eliminate the interpretational conundrums that have beset quantum theory. The QBist interpretation is historically derivative of the views of the various physicists that are often grouped together as "the" Copenhagen interpretation, but is itself distinct from them. Theodor Hänsch has characterized QBism as sharpening those older views and making them more consistent.

The von Neumann–Wigner interpretation, also described as "consciousness causes collapse [of the wave function]", is an interpretation of quantum mechanics in which consciousness is postulated to be necessary for the completion of the process of quantum measurement.

References

  1. Murray Gell-Mann - Quantum Mechanics Interpretations - Feynman Sum over Histories - EPR Bertlemann's https://www.youtube.com/watch?v=f-OFP5tNtMY Richard P Feynman: Quantum Mechanical View of Reality 1 (Part 1) https://www.youtube.com/watch?v=72us6pnbEvE
  2. Vaidman, L. (2002, March 24). Many-Worlds Interpretation of Quantum Mechanics. Retrieved March 19, 2010, from Stanford Encyclopedia of Philosophy: http://plato.stanford.edu/entries/qm-manyworlds/#Teg98
  3. Frank J. Tipler (1994). The Physics of Immortality: Modern Cosmology, God, and the Resurrection of the Dead. Anchor Books. ISBN   978-0-385-46799-5. A controversial poll mentioned in found that of 72 "leading cosmologists and other quantum field theorists", 58% including Stephen Hawking, Murray Gell-Mann, and Richard Feynman supported a many-worlds interpretation ["Who believes in many-worlds?", Hedweb.com, Accessed online: 24 Jan 2011].
  4. Quantum theory as a universal physical theory, by David Deutsch, International Journal of Theoretical Physics, Vol 24 #1 (1985)
  5. Three connections between Everett's interpretation and experiment Quantum Concepts of Space and Time, by David Deutsch, Oxford University Press (1986)
  6. 1 2 Schlosshauer, Maximilian; Kofler, Johannes; Zeilinger, Anton (2013-01-06). "A Snapshot of Foundational Attitudes Toward Quantum Mechanics". Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics. 44 (3): 222–230. arXiv: 1301.1069 . Bibcode:2013SHPMP..44..222S. doi:10.1016/j.shpsb.2013.04.004.
  7. The Transactional Interpretation of Quantum Mechanics, R.E.Kastner, Cambridge University Press, 2013, ISBN   978-0-521-76415-5, p35
  8. For a discussion of the provenance of the phrase "shut up and calculate", see Mermin, N. David (2004). "Could feynman have said this?". Physics Today. 57 (5): 10–11. Bibcode:2004PhT....57e..10M. doi:10.1063/1.1768652.
  9. Guido Bacciagaluppi, "The role of decoherence in quantum mechanics", The Stanford Encyclopedia of Philosophy (Winter 2012), Edward N Zalta, ed.
  10. La nouvelle cuisine, by John S Bell, last article of Speakable and Unspeakable in Quantum Mechanics, second edition.
  11. Einstein, A.; Podolsky, B.; Rosen, N. (1935). "Can quantum-mechanical description of physical reality be considered complete?". Phys. Rev. 47 (10): 777–780. Bibcode:1935PhRv...47..777E. doi:10.1103/physrev.47.777.
  12. http://www.naturalthinker.net/trl/texts/Heisenberg,Werner/Heisenberg,%20Werner%20-%20Physics%20and%20philosophy.pdf
  13. "An experiment illustrating the ensemble interpretation". Hitachi.com. Retrieved 2011-01-24.
  14. Shaiia, Raed M. (9 February 2015). "On the Measurement Problem". International Journal of Theoretical and Mathematical Physics. 4 (5): 202–219. doi:10.5923/j.ijtmp.20140405.04.
  15. "Publications - Raed Shaiia". sites.google.com.
  16. Maudlin, T. (1995). "Why Bohm's Theory Solves the Measurement Problem". Philosophy of Science. 62 (3): 479–483. doi:10.1086/289879.
  17. Durr, D.; Zanghi, N.; Goldstein, S. (Nov 14, 1995). "Bohmian Mechanics as the Foundation of Quantum Mechanics ". arXiv: quant-ph/9511016 . Also published in J.T. Cushing; Arthur Fine; S. Goldstein (17 April 2013). Bohmian Mechanics and Quantum Theory: An Appraisal. Springer Science & Business Media. pp. 21–43. ISBN   978-94-015-8715-0.
  18. "Relational Quantum Mechanics (Stanford Encyclopedia of Philosophy)". Plato.stanford.edu. Retrieved 2011-01-24.
  19. For more information, see Carlo Rovelli (1996). "Relational Quantum Mechanics". International Journal of Theoretical Physics . 35 (8): 1637–1678. arXiv: quant-ph/9609002 . Bibcode:1996IJTP...35.1637R. doi:10.1007/BF02302261.
  20. David Bohm, The Special Theory of Relativity, Benjamin, New York, 1965
  21. See relational approach to wave-particle duality. For a full account see Zheng, Qianbing; Kobayashi, Takayoshi (1996). "Quantum Optics as a Relativistic Theory of Light" (PDF). Physics Essays. 9 (3): 447. Bibcode:1996PhyEs...9..447Z. doi:10.4006/1.3029255. Also, see Annual Report, Department of Physics, School of Science, University of Tokyo (1992) 240.
  22. "Quantum Nocality – Cramer". Npl.washington.edu. Archived from the original on 2010-12-29. Retrieved 2011-01-24.
  23. Nelson, E (1966). "Derivation of the Schrödinger Equation from Newtonian Mechanics". Phys. Rev. 150 (4): 1079–1085. Bibcode:1966PhRv..150.1079N. doi:10.1103/physrev.150.1079.
  24. Pavon, M. (2000). "Stochastic mechanics and the Feynman integral". J. Math. Phys. 41 (9): 6060–6078. arXiv: quant-ph/0007015 . Bibcode:2000JMP....41.6060P. doi:10.1063/1.1286880.
  25. Roumen Tsekov (2012). "Bohmian Mechanics versus Madelung Quantum Hydrodynamics". Ann. Univ. Sofia, Fac. Phys. SE: 112–119. arXiv: 0904.0723 . Bibcode:2012AUSFP..SE..112T. doi:10.13140/RG.2.1.3663.8245.
  26. "Frigg, R. GRW theory" (PDF). Retrieved 2011-01-24.
  27. "Review of Penrose's Shadows of the Mind". Thymos.com. 1999. Archived from the original on 2001-02-09. Retrieved 2011-01-24.
  28. Arthur Jabs: A conjecture concerning determinism, reduction, and measurement in quantum mechanics, Quantum Studies: Mathematics and Foundations, vol. 3, issue 4, p. 279-292 (2016), DOI 10.1007/s40509-016-0077-7, arXiv:1204.0614
  29. von Neumann, John. (1932/1955). Mathematical Foundations of Quantum Mechanics. Princeton: Princeton University Press. Translated by Robert T. Beyer.
  30. [Michael Esfeld, (1999), "Essay Review: Wigner's View of Physical Reality", published in Studies in History and Philosophy of Modern Physics, 30B, pp. 145–154, Elsevier Science Ltd.]
  31. Zvi Schreiber (1995). "The Nine Lives of Schrödinger's Cat". arXiv: quant-ph/9501014 .
  32. Dick J. Bierman and Stephen Whitmarsh. (2006). Consciousness and Quantum Physics: Empirical Research on the Subjective Reduction of the State Vector. in Jack A. Tuszynski (Ed). The Emerging Physics of Consciousness. p. 27-48.
  33. Nunn, C. M. H.; et al. (1994). "Collapse of a Quantum Field may Affect Brain Function. '". Journal of Consciousness Studies. 1 (1): 127–139.
  34. "- The anthropic universe". Abc.net.au. 2006-02-18. Retrieved 2011-01-24.
  35. "In the beginning was the bit". New Scientist. 2001-02-17. Retrieved 2013-01-25.
  36. Kate Becker (2013-01-25). "Quantum physics has been rankling scientists for decades". Boulder Daily Camera. Retrieved 2013-01-25.
  37. Information, Immaterialism, Instrumentalism: Old and New in Quantum Information. Christopher G. Timpson
  38. Timpson, Op. Cit.: "Let us call the thought that information might be the basic category from which all else flows informational immaterialism."
  39. "Physics concerns what we can say about nature". (Niels Bohr, quoted in Petersen, A. (1963). The philosophy of Niels Bohr. Bulletin of the Atomic Scientists, 19(7):8–14.)
  40. Hartle, J. B. (1968). "Quantum mechanics of individual systems". Am. J. Phys. 36 (8): 704–712. Bibcode:1968AmJPh..36..704H. doi:10.1119/1.1975096.
  41. "Modal Interpretations of Quantum Mechanics". Stanford Encyclopedia of Philosophy. Science.uva.nl. Retrieved 2011-01-24.
  42. Watanabe, Satosi (1955). "Symmetry of physical laws. Part III. Prediction and retrodiction". Reviews of Modern Physics. 27 (2): 179–186. Bibcode:1955RvMP...27..179W. doi:10.1103/revmodphys.27.179. hdl:10945/47584.
  43. Aharonov, Y.; et al. (1964). "Time Symmetry in the Quantum Process of Measurement". Phys. Rev. 134 (6B): B1410–1416. Bibcode:1964PhRv..134.1410A. doi:10.1103/physrev.134.b1410.
  44. Aharonov, Y. and Vaidman, L. "On the Two-State Vector Reformulation of Quantum Mechanics." Physica Scripta, Volume T76, pp. 85-92 (1998).
  45. Wharton, K. B. (2007). "Time-Symmetric Quantum Mechanics". Foundations of Physics. 37 (1): 159–168. Bibcode:2007FoPh...37..159W. doi:10.1007/s10701-006-9089-1.
  46. Wharton, K. B. (2010). "A Novel Interpretation of the Klein–Gordon Equation". Foundations of Physics. 40 (3): 313–332. arXiv: 0706.4075 . Bibcode:2010FoPh...40..313W. doi:10.1007/s10701-009-9398-2.
  47. Heaney, M. B. (2013). "A Symmetrical Interpretation of the Klein–Gordon Equation". Foundations of Physics. 43 (6): 733–746. arXiv: 1211.4645 . Bibcode:2013FoPh...43..733H. doi:10.1007/s10701-013-9713-9.
  48. Yakir Aharonov, Lev Vaidman: The Two-State Vector Formalism of Quantum Mechanics: an Updated Review. In: Juan Gonzalo Muga, Rafael Sala Mayato, Íñigo Egusquiza (eds.): Time in Quantum Mechanics, Volume 1, Lecture Notes in Physics 734, pp. 399–447, 2nd ed., Springer, 2008, ISBN   978-3-540-73472-7, doi : 10.1007/978-3-540-73473-4_13, arXiv : quant-ph/0105101, p. 443
  49. 1 2 Sharlow, Mark; "What Branching Spacetime might do for Physics" p.2
  50. Olimpia, Lombardi; 1979-, Fortin, Sebastian; Federico, Holik; Cristian, López (2017). "Interpretations of Quantum Theory: A Map of Madness". What is quantum information?. pp. 138–144. arXiv: 1509.04711 . doi:10.1017/9781316494233.009. ISBN   9781107142114. OCLC   965759965.
  51. Smerlak, Matteo; Rovelli, Carlo (2007-03-01). "Relational EPR". Foundations of Physics. 37 (3): 427–445. arXiv: quant-ph/0604064 . Bibcode:2007FoPh...37..427S. doi:10.1007/s10701-007-9105-0. ISSN   0015-9018.

Sources

Further reading

Almost all authors below are professional physicists.

  1. de Muynck, Willem M (2002). Foundations of quantum mechanics: an empiricist approach. Klower Academic Publishers. ISBN   978-1-4020-0932-7 . Retrieved 2011-01-24.