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.
During most of the 20th century, collapse theories were clearly the mainstream view, and the question of interpretation of quantum mechanics mostly revolved around how to interpret "collapse". Proponents of either "pilot-wave" (de Broglie-Bohm-like) or "many-worlds" (Everettian) interpretations tend to emphasize how their respective camps were intellectually marginalized throughout 1950s to 1980s. In this (historical) sense, all non-collapse theories are (historically) "minority" interpretations.
The term 'Copenhagen interpretation' suggests some definite set of rules for interpreting the mathematical formalism of quantum mechanics. However, no such text exists, apart from some informal popular lectures by Bohr and Heisenberg, which contradict each other on several important issues.
It appears that the term "Copenhagen interpretation", with its more definite sense, was coined by Heisenberg in the 1950s,[1] while criticizing "unorthodox" interpretations such as that of David Bohm.[2][3][4]
Before the book was released for sale, Heisenberg privately expressed regret for having used the term, due to its suggestion of the existence of other interpretations, that he considered to be "nonsense".[5]
Since the 1990s, there has been a resurgence of interest in non-collapse theories. Interpretations of quantum mechanics now mostly fall into the categories of collapse theories (including the Copenhagen interpretation), hidden variables ("Bohm-like"), many-worlds ("Everettian") and quantum information approaches. While collapse theories continue to be seen as the default or mainstream position, there is no longer any clear dichotomy between "orthodox" and "unorthodox" views. The Stanford Encyclopedia as of 2015 groups interpretations of quantum mechanics into five classes (all of which contain further divisions): "Bohmian mechanics" (pilot-wave theories),[6] "collapse theories",[7] "many-worlds interpretations",[8] "modal interpretations"[9]
and "relational interpretations".[10]
Some of the historically relevant approaches to quantum mechanics have now themselves become "minority interpretations", or widely seen as obsolete. In this sense, there is a variety of reasons for why a specific approach may be considered marginal: because it is a very specialized sub-variant of a more widely known class of interpretations, because it is seen as obsolete (in spite of possible historical significance), because it is a very recent suggestion that has not received wide attention, or because it is rejected as flawed.
"Everettian" (many-worlds) interpretations as a whole were long a "minority" field in general, but they have grown in popularity. Multiple variants and offshoots of Everett's original proposal exist, which have sometimes developed the basic ideas in contradictory ways.[12][13] Interpretations of an Everettian type include the following.
"Cosmological interpretations", such as that proposed by Anthony Aguirre and Max Tegmark, in which the wavefunction for a quantum system describes not an imaginary ensemble of possibilities for what the system might be doing, but rather the actual spatial collection of identical copies of the system that exist in the infinite space that is, hypothetically, generated by eternal inflation.[16]
Relational quantum mechanics treats the state of a quantum system as being observer-dependent, that is, the state is the relation between the observer and the system. While a relational conception of quantum states dates back at least to Grete Hermann in 1935,[17] in modern usage "relational quantum mechanics" refers to an interpretation delineated by Carlo Rovelli in 1996. It uses some ideas from Wheeler about quantum information.[18]
The ensemble interpretation, or statistical interpretation can be viewed as a minimalist approach;[29] The wave function in this interpretation is not a property of any individual system, it is by its nature a statistical description of a hypothetical "ensemble" of similar systems. This is the interpretation historically advocated by Albert Einstein.[30]
Modal interpretation (van Fraassen 1972)[31] Van Fraassen's proposal is "modal" because it leads to a modal logic of quantum propositions. Since the 1980s, a number of authors have developed other "realist" proposals which can in retrospect be classed with van Fraassen's "modal" proposal.
Superdeterminism (Bell 1977),[32] the idea that the universe is completely deterministic, and thus Bell's theorem does not apply, as observers are not free to make independent choices in their measurements, rather everything is predetermined from the Big Bang.
Consistent histories (Dowker and Kent 1995),[33] based on a consistency criterion that then allows probabilities to be assigned to various alternative histories of a system.
"Montevideo interpretation" (Gambini and Pullin 2009),[34][35] suggesting that quantum gravity makes for fundamental limitations on the accuracy of clocks, which imply a type of decoherence.[36]
"Psychophysical interpretation"(Pradhan 2012)[39] based on Psychophysical parallelism takes the conjugate quantities to represent psychic counterparts to the corresponding physical aspects such as states and observables.[40]
The interpretation from a bundle-theoretic view of objective idealism (Korth 2022), based on the idea that quantum 'weirdness' follows from objects being bundles of universals.[41]
Quantum mysticism is a set of metaphysical beliefs and associated practices that seek to relate consciousness, intelligence, spirituality, or mystical worldviews to the ideas of quantum mechanics and its interpretations. Quantum mysticism is considered by most scientists to be pseudoscience or quackery.
^H. Kragh, Quantum generations: A History of Physics in the Twentieth Century, Princeton University Press, 1999, p. 210. ("the term 'Copenhagen interpretation' was not used in the 1930s but first entered the physicist’s vocabulary in 1955 when Heisenberg used it in criticizing certain unorthodox interpretations of quantum mechanics.")
^Lectures with the titles 'The Copenhagen Interpretation of Quantum Theory' and 'Criticisms and Counterproposals to the Copenhagen Interpretation', that Heisenberg delivered in 1955, are reprinted in the collection Physics and Philosophy.
^Olival Freire Jr., "Science and exile: David Bohm, the hot times of the Cold War, and his struggle for a new interpretation of quantum mechanics", Historical Studies on the Physical and Biological Sciences, Volume 36, Number 1, 2005, pp. 31–35. ("I avow that the term ‘Copenhagen interpretation’ is not happy since it could suggest that there are other interpretations, like Bohm assumes. We agree, of course, that the other interpretations are nonsense, and I believe that this is clear in my book, and in previous papers. Anyway, I cannot now, unfortunately, change the book since the printing began enough time ago.")
^Goldstein, Sheldon, "Bohmian Mechanics", The Stanford Encyclopedia of Philosophy (Spring 2013 Edition).
^Ghirardi, Giancarlo, "Collapse Theories", The Stanford Encyclopedia of Philosophy (Winter 2011 Edition).
^Laudisa, Federico and Rovelli, Carlo, "Relational Quantum Mechanics", The Stanford Encyclopedia of Philosophy (Summer 2013 Edition)
^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. S2CID55537196.
^Wheeler, J. A.: "Information, physics, quantum: The search for links"; in Zurek, W., ed.: "Complexity, Entropy and the Physics of Information"; pp 3–28; Addison-Wesley; 1990, p. 3.
^"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: Philosopher-Scientist, ed. P.A. Schilpp (Harper & Row, New York)
^J. S. Bell, Free variables and local causality, Epistemological Letters, Feb. 1977. Reprinted as Chapter 12 of J. S. Bell, Speakable and Unspeakable in Quantum Mechanics (Cambridge University Press 1987)
^Shafieea, Afshin; Jafar-Aghdamib, Maryam; Golshanic, Mehdi (2006). "A critique of Mohrhoff's interpretation of quantum mechanics". Studies in History and Philosophy of Science. 37 (2): 316–329. Bibcode:2006SHPMP..37..316S. doi:10.1016/j.shpsb.2005.10.003.
^Korth, Martin (2022-08-24). "A new interpretation of quantum theory, based on a bundle-theoretic view of objective idealism". arXiv:2208.10964 [quant-ph].