An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments. However, there exist a number of contending schools of thought over their interpretation. These interpretations differ on such fundamental questions as whether quantum mechanics is deterministic or stochastic, local or nonlocal, which elements of quantum mechanics can be considered real, and what the nature of measurement is, among other matters.
While some variation of the Copenhagen interpretation is commonly presented in textbooks, many other interpretations have been developed. Despite a century of debate and experiment, no consensus has been reached among physicists and philosophers of physics concerning which interpretation best "represents" reality.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.
History
The definition of quantum theorists' terms, such as wave function and matrix mechanics, progressed through many stages. For instance, Erwin Schrödinger originally viewed the electron's wave function as its charge density smeared across space, but Max Born reinterpreted the absolute square value of the wave function as the electron's probability density distributed across space; the Born rule, as it is now called, matched experiment, whereas Schrödinger's charge density view did not.
The views of several early pioneers of quantum mechanics, such as Niels Bohr and Werner Heisenberg, are often grouped together as the "Copenhagen interpretation", though physicists and historians of physics have argued that this terminology obscures differences between the views so designated. Copenhagen-type ideas were never universally embraced, and challenges to a perceived Copenhagen orthodoxy gained increasing attention in the 1950s with the pilot-wave interpretation of David Bohm and the many-worlds interpretation of Hugh Everett III.
The physicist N. David Mermin once quipped, "New interpretations appear every year. None ever disappear." (Mermin also coined the saying "Shut up and calculate" to describe many physicists' attitude to interpretations of quantum theory, a remark which is often misattributed to Richard Feynman.) As a rough guide to development of the mainstream view during the 1990s and 2000s, a "snapshot" of opinions was collected in a poll by Schlosshauer et al. at the "Quantum Physics and the Nature of Reality" conference of July 2011. 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: "The Copenhagen interpretation still reigns supreme here, especially if we lump it together with intellectual offsprings such as information-based interpretations and the QBism 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."
Some concepts originating from studies of interpretations have found more practical application in quantum information science.
Interpretive challenges
Abstract, mathematical nature of quantum field theories: the mathematical structure of quantum mechanics is abstract and does not result in a single, clear interpretation of its quantities. Apparent 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 (understood as an interaction with a given state) has a special role in the theory, as it is the sole process that can cause a nonunitary, irreversible evolution of the state. Role of the observer in determining outcomes. Copenhagen-type interpretations imply that the wavefunction is a calculational tool, and represents reality only immediately after a measurement performed by an observer. Everettian interpretations grant that all possible outcomes are real, and that measurement-type interactions cause a branching process in which each possibility is realised. 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 by action at a distance.La nouvelle cuisine, by John S. Bell, last article of Speakable and Unspeakable in Quantum Mechanics, second edition. 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. 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, fail even for local systems. Also, physical principles such as Leibniz's Principle of the identity of indiscernibles no longer apply in the quantum domain, signaling that most classical intuitions may be incorrect about the quantum world. Role of time: time enters the formalism of quantum mechanics as an external parameter rather than as an observable; by an argument due to Wolfgang Pauli, no self-adjoint time operator exists when the Hamiltonian is bounded below. Statements such as "instantaneous collapse" therefore refer to this parameter in a chosen reference frame rather than to a measurable simultaneity; which distant events count as simultaneous with a collapse is frame-dependent.
Influential interpretations Copenhagen interpretation
The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics principally attributed to Niels Bohr and Werner Heisenberg. It is one of the oldest attitudes towards quantum mechanics, as features of it date to the development of quantum mechanics during 1925–1927, and it remains one of the most commonly taught.Bell, John S. (1987), Speakable and Unspeakable in quantum Mechanics (Cambridge: Cambridge University Press) There is no definitive historical statement of what is the Copenhagen interpretation, and there were in particular fundamental disagreements between the views of Bohr and Heisenberg. For example, Heisenberg emphasized a sharp "cut" between the observer (or the instrument) and the system being observed, while Bohr offered an interpretation that is independent of a subjective observer or measurement or collapse, which relies on an "irreversible" or effectively irreversible process that imparts the classical behavior of "observation" or "measurement"...
Features common to Copenhagen-type interpretations include the idea that quantum mechanics is intrinsically indeterministic, with probabilities calculated using the Born rule, and the principle of complementarity, which states certain pairs of complementary properties cannot all be observed or measured simultaneously. Moreover, properties only result from the act of "observing" or "measuring"; the theory avoids assuming definite values from unperformed experiments. Copenhagen-type interpretations hold that quantum descriptions are objective, in that they are independent of physicists' mental arbitrariness. The statistical interpretation of wavefunctions due to Max Born differs sharply from Schrödinger's original intent, which was to have a theory with continuous time evolution and in which wavefunctions directly described physical reality.
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. More precisely, the parts of the wavefunction describing observers become increasingly entangled with the parts of the wavefunction describing their experiments. Although all possible outcomes of experiments continue to lie in the wavefunction's support, the times at which they become correlated with observers effectively "split" the universe into mutually unobservable alternate histories.
Quantum information theories
Quantum information approaches view the quantum state as a representation of information and measurement as the process of updating that information.
These approaches are subdivided into two main categories"Let us call the thought that information might be the basic category from which all else flows informational immaterialism." Information, Immaterialism, Instrumentalism: Old and New in Quantum Information. Christopher G. Timpson:
The first category consists of information ontologies, exemplified by J. A. Wheeler's "it from bit" concept, which have been described as a revival of immaterialism.
The second category comprises epistemic interpretations which propose that quantum mechanics describes an observer's knowledge of the world rather than the world itself. Within these frameworks, wavefunction collapse (also known as the reduction of the state vector) is often interpreted as an observer acquiring information from a measurement rather than as an objective physical event. As summarised by James Hartle: 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.A program associated with these approaches is “quantum reconstruction” which seeks to derive the formalism of quantum mechanics from such information-theoretic axioms.
Relational quantum mechanics