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= Minority interpretations of quantum mechanics =
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Introduction
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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.
History
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The historical dichotomy between the "orthodox" Copenhagen
interpretation and "unorthodox" minority views developed in the 1950s
debate surrounding Bohmian mechanics.
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, while
criticizing "unorthodox" interpretations such as that of David Bohm.
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".
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),
"collapse theories", "many-worlds interpretations", "modal
interpretations"
and "relational interpretations".
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.
As a rough guide to a picture of what are the relevant "minority"
views, 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.
The authors reference a similarly informal poll carried out by Max
Tegmark at the "Fundamental Problems
in Quantum Theory" conference in August 1997.
In both polls, the Copenhagen interpretation received the largest
number of votes. In Tegmark's poll, many-worlds interpretations came
in second place, while in the 2011 poll, many-worlds was at third
place (18%), behind quantum information approaches in second place
(24%).
Other options given as "interpretation of quantum mechanics" in the
2011 poll were:
objective collapse theories (9% support), Quantum Bayesianism (6%
support) and Relational quantum mechanics (6% support),
besides consistent histories, de Broglie-Bohm theory, modal
interpretation, ensemble interpretation and transactional
interpretation which received no votes.
Many-worlds
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"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.
Interpretations of an Everettian type include the following.
* Many-minds interpretation
* Cosmological interpretation
* "Self-locating uncertainty" interpretation
Quantum information
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* QBism and other variants of Quantum Bayesianism
*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, 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.
Hidden variables
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"Bohm-like" (hidden variable) theories as a whole are a "minority
view" as compared to Copenhagen-type or many-worlds (Everettian)
interpretations.
*Popper's propensity-based interpretation
*Stochastic interpretation, the most well-known variant of which was
due to Edward Nelson, further elaborated upon by a conjecture of
Francesco Calogero
* Time-symmetric interpretations
*Transactional interpretation
*Zitterbewegung interpretation
Collapse theories
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*von Neumann-Wigner interpretation ("consciousness causes collapse"),
mostly of historical interest
*Objective-collapse theories: these are extensions of quantum
mechanics rather than "interpretations" in the narrow sense.
**Penrose interpretation
**Ghirardi-Rimini-Weber theory
Other
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*The ensemble interpretation, or statistical interpretation can be
viewed as a minimalist approach; 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.
*Modal interpretation (van Fraassen 1972) 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), 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), 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), suggesting
that quantum gravity makes for fundamental limitations on the accuracy
of clocks, which imply a type of decoherence.