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In physics, **hidden-variable theories** are proposals to provide explanations of quantum mechanical phenomena through the introduction of unobservable hypothetical entities. The existence of fundamental indeterminacy for some measurements is assumed as part of the mathematical formulation of quantum mechanics; moreover, bounds for indeterminacy can be expressed in a quantitative form by the Heisenberg uncertainty principle. Most hidden-variable theories are attempts at a deterministic description of quantum mechanics, to avoid quantum indeterminacy, but at the expense of requiring the existence of nonlocal interactions.

- Motivation
- "God does not play dice"
- Early attempts at hidden-variable theories
- Declaration of completeness of quantum mechanics, and the Bohr–Einstein debates
- EPR paradox
- Bell's theorem
- Bohm's hidden-variable theory
- Recent developments
- See also
- References
- Bibliography
- External links

Albert Einstein objected to the fundamentally probabilistic nature of quantum mechanics,^{ [1] } and famously declared "I am convinced God does not play dice".^{ [2] }^{ [3] } Einstein, Podolsky, and Rosen argued that quantum mechanics is an incomplete description of reality.^{ [4] }^{ [5] } Bell's theorem would later suggest that local hidden variables (a way for finding a complete description of reality) of certain types are impossible. A famous non-local theory is the De Broglie–Bohm theory.

Per its mathematical formulation, quantum mechanics is non-deterministic, meaning that it generally does not predict the outcome of any measurement with certainty. Instead, it indicates what the probabilities of the outcomes are, with the indeterminism of observable quantities constrained by the uncertainty principle. The question arises whether there might be some deeper reality hidden beneath quantum mechanics, to be described by a more fundamental theory that can always predict the outcome of each measurement with certainty: if the exact properties of every subatomic particle were known the entire system could be modeled exactly using deterministic physics similar to classical physics.

In other words, it is conceivable that quantum mechanics is an incomplete description of nature. The designation of variables as underlying "hidden" variables depends on the level of physical description (so, for example, "if a gas is described in terms of temperature, pressure, and volume, then the velocities of the individual atoms in the gas would be hidden variables"^{ [6] }). Physicists supporting De Broglie–Bohm theory maintain that underlying the observed probabilistic nature of the universe is a deterministic objective foundation/property—the hidden variable. Others, however, believe that there is no deeper deterministic reality in quantum mechanics.^{[ citation needed ]}

A lack of a kind of realism (understood here as asserting independent existence and evolution of physical quantities, such as position or momentum, without the process of measurement) is crucial in the Copenhagen interpretation. Realistic interpretations (which were already incorporated, to an extent, into the physics of Feynman^{ [7] }), on the other hand, assume that particles have certain trajectories. Under such view, these trajectories will almost always be continuous, which follows both from the finitude of the perceived speed of light ("leaps" should rather be precluded) and, more importantly, from the principle of least action, as deduced in quantum physics by Dirac. But continuous movement, in accordance with the mathematical definition, implies deterministic movement for a range of time arguments;^{ [8] } and thus realism is, under modern physics, one more reason for seeking (at least certain limited) determinism and thus a hidden-variable theory (especially that such theory exists: see De Broglie–Bohm interpretation).

Although determinism was initially a major motivation for physicists looking for hidden-variable theories, non-deterministic theories trying to explain what the supposed reality underlying the quantum mechanics formalism looks like are also considered hidden-variable theories; for example Edward Nelson's stochastic mechanics.

In June 1926, Max Born published a paper, "Zur Quantenmechanik der Stoßvorgänge" ("Quantum Mechanics of Collision Phenomena") in the scientific journal * Zeitschrift für Physik *, in which he was the first to clearly enunciate the probabilistic interpretation of the quantum wave function, which had been introduced by Erwin Schrödinger earlier in the year. Born concluded the paper as follows:

Here the whole problem of determinism comes up. From the standpoint of our quantum mechanics there is no quantity which in any individual case causally fixes the consequence of the collision; but also experimentally we have so far no reason to believe that there are some inner properties of the atom which conditions a definite outcome for the collision. Ought we to hope later to discover such properties ... and determine them in individual cases? Or ought we to believe that the agreement of theory and experiment—as to the impossibility of prescribing conditions for a causal evolution—is a pre-established harmony founded on the nonexistence of such conditions? I myself am inclined to give up determinism in the world of atoms. But that is a philosophical question for which physical arguments alone are not decisive.

Born's interpretation of the wave function was criticized by Schrödinger, who had previously attempted to interpret it in real physical terms, but Albert Einstein's response became one of the earliest and most famous assertions that quantum mechanics is incomplete:

Quantum mechanics is very worthy of respect. But an inner voice tells me this is not the genuine article after all. The theory delivers much but it hardly brings us closer to the Old One's secret. In any event, I am convinced that

Heis not playing dice.^{ [3] }^{ [9] }

Niels Bohr reportedly replied to Einstein's later expression of this sentiment by advising him to "stop telling God what to do."^{ [10] }

Shortly after making his famous "God does not play dice" comment, Einstein attempted to formulate a deterministic counter proposal to quantum mechanics, presenting a paper at a meeting of the Academy of Sciences in Berlin, on 5 May 1927, titled "Bestimmt Schrödinger's Wellenmechanik die Bewegung eines Systems vollständig oder nur im Sinne der Statistik?" ("Does Schrödinger's wave mechanics determine the motion of a system completely or only in the statistical sense?").^{ [11] }^{ [12] } However, as the paper was being prepared for publication in the academy's journal, Einstein decided to withdraw it, possibly because he discovered that, contrary to his intention, it implied non-separability of entangled systems, which he regarded as absurd.^{ [13] }

At the Fifth Solvay Congress, held in Belgium in October 1927 and attended by all the major theoretical physicists of the era, Louis de Broglie presented his own version of a deterministic hidden-variable theory, apparently unaware of Einstein's aborted attempt earlier in the year. In his theory, every particle had an associated, hidden "pilot wave" which served to guide its trajectory through space. The theory was subject to criticism at the Congress, particularly by Wolfgang Pauli, which de Broglie did not adequately answer. De Broglie abandoned the theory shortly thereafter.

Also at the Fifth Solvay Congress, Max Born and Werner Heisenberg made a presentation summarizing the recent tremendous theoretical development of quantum mechanics. At the conclusion of the presentation, they declared:

[W]hile we consider ... a quantum mechanical treatment of the electromagnetic field ... as not yet finished, we consider quantum mechanics to be a closed theory, whose fundamental physical and mathematical assumptions are no longer susceptible of any modification.... On the question of the 'validity of the law of causality' we have this opinion: as long as one takes into account only experiments that lie in the domain of our currently acquired physical and quantum mechanical experience, the assumption of indeterminism in principle, here taken as fundamental, agrees with experience.

^{ [14] }

Although there is no record of Einstein responding to Born and Heisenberg during the technical sessions of the Fifth Solvay Congress, he did challenge the completeness of quantum mechanics during informal discussions over meals, presenting a thought experiment intended to demonstrate that quantum mechanics could not be entirely correct. He did likewise during the Sixth Solvay Congress held in 1930. Both times, Niels Bohr is generally considered to have successfully defended quantum mechanics by discovering errors in Einstein's arguments.

The debates between Bohr and Einstein essentially concluded in 1935, when Einstein finally expressed what is widely considered his best argument against the completeness of quantum mechanics. Einstein, Podolsky, and Rosen had proposed their definition of a "complete" description as one that uniquely determines the values of all its measurable properties.^{ [15] } Einstein later summarized their argument as follows:

Consider a mechanical system consisting of two partial systems

AandBwhich interact with each other only during a limited time. Let theψfunction [i.e., wavefunction] before their interaction be given. Then the Schrödinger equation will furnish theψfunction after the interaction has taken place. Let us now determine the physical state of the partial systemAas completely as possible by measurements. Then quantum mechanics allows us to determine theψfunction of the partial systemBfrom the measurements made, and from theψfunction of the total system. This determination, however, gives a result which depends upon which of the physical quantities (observables) ofAhave been measured (for instance, coordinates or momenta). Since there can be only one physical state ofBafter the interaction which cannot reasonably be considered to depend on the particular measurement we perform on the systemAseparated fromBit may be concluded that theψfunction is not unambiguously coordinated to the physical state. This coordination of severalψfunctions to the same physical state of systemBshows again that theψfunction cannot be interpreted as a (complete) description of a physical state of a single system.^{ [16] }

Bohr answered Einstein's challenge as follows:

[The argument of] Einstein, Podolsky and Rosen contains an ambiguity as regards the meaning of the expression "without in any way disturbing a system." ... [E]ven at this stage [i.e., the measurement of, for example, a particle that is part of an entangled pair], there is essentially the question of an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system. Since these conditions constitute an inherent element of the description of any phenomenon to which the term "physical reality" can be properly attached, we see that the argumentation of the mentioned authors does not justify their conclusion that quantum-mechanical description is essentially incomplete."

^{ [17] }

Bohr is here choosing to define a "physical reality" as limited to a phenomenon that is immediately observable by an arbitrarily chosen and explicitly specified technique, using his own special definition of the term 'phenomenon'. He wrote in 1948:

As a more appropriate way of expression, one may strongly advocate limitation of the use of the word

phenomenonto refer exclusively to observations obtained under specified circumstances, including an account of the whole experiment."^{ [18] }^{ [19] }

This was, of course, in conflict with the definition used by the EPR paper, as follows:

If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.[Italics in original]^{ [4] }

In 1964, John Bell showed through his famous theorem that if local hidden variables exist, certain experiments could be performed involving quantum entanglement where the result would satisfy a Bell inequality. If, on the other hand, statistical correlations resulting from quantum entanglement could not be explained by local hidden variables, the Bell inequality would be violated. Another no-go theorem concerning hidden-variable theories is the Kochen–Specker theorem.

Physicists such as Alain Aspect and Paul Kwiat have performed experiments that have found violations of these inequalities up to 242 standard deviations^{ [20] } (excellent scientific certainty). This rules out local hidden-variable theories, but does not rule out non-local ones. Theoretically, there could be experimental problems that affect the validity of the experimental findings.

Gerard 't Hooft has disputed the validity of Bell's theorem on the basis of the superdeterminism loophole and proposed some ideas to construct local deterministic models.^{ [21] }

Assuming the validity of Bell's theorem, any deterministic hidden-variable theory that is consistent with quantum mechanics would have to be non-local, maintaining the existence of instantaneous or faster-than-light relations (correlations) between physically separated entities. The currently best-known hidden-variable theory, the "causal" interpretation of the physicist and philosopher David Bohm, originally published in 1952, is a *non-local hidden-variable theory*. Bohm unknowingly rediscovered (and extended) the idea that Louis de Broglie had proposed in 1927 (and abandoned) – hence this theory is commonly called "de Broglie-Bohm theory". Bohm posited *both* the quantum particle, e.g. an electron, and a hidden 'guiding wave' that governs its motion. Thus, in this theory electrons are quite clearly particles—when a double-slit experiment is performed, its trajectory goes through one slit rather than the other. Also, the slit passed through is not random but is governed by the (hidden) guiding wave, resulting in the wave pattern that is observed. Since the location where the particles start in the double-slit experiment is unknown, the initial position of the particle is the hidden variable.

Such a view does not contradict the idea of local events that is used in both classical atomism and relativity theory as Bohm's theory (and quantum mechanics) are still locally causal (that is, information travel is still restricted to the speed of light) but allow non-local correlations. It points to a view of a more holistic, mutually interpenetrating and interacting world. Indeed, Bohm himself stressed the holistic aspect of quantum theory in his later years, when he became interested in the ideas of Jiddu Krishnamurti.

In Bohm's interpretation, the (non-local) quantum potential constitutes an implicate (hidden) order which organizes a particle, and which may itself be the result of yet a further implicate order: a *superimplicate order* which organizes a field.^{ [22] } Nowadays Bohm's theory is considered to be one of many interpretations of quantum mechanics which give a realist interpretation, and not merely a positivistic one, to quantum-mechanical calculations. Some consider it the simplest theory to explain quantum phenomena.^{ [23] } Nevertheless, it *is* a hidden-variable theory, and necessarily so.^{ [24] } The major reference for Bohm's theory today is his book with Basil Hiley, published posthumously.^{ [25] }

A possible weakness of Bohm's theory is that some (including Einstein, Pauli, and Heisenberg) feel that it looks contrived.^{ [26] } (Indeed, Bohm thought this of his original formulation of the theory.^{ [27] }) It was deliberately designed to give predictions that are in all details identical to conventional quantum mechanics.^{ [27] } Bohm's original aim was not to make a serious counter proposal but simply to demonstrate that hidden-variable theories are indeed possible.^{ [27] } (It thus provided a supposed counterexample to the famous proof by John von Neumann that was generally believed to demonstrate that no deterministic theory reproducing the statistical predictions of quantum mechanics is possible.) Bohm said he considered his theory to be unacceptable as a physical theory due to the guiding wave's existence in an abstract multi-dimensional configuration space, rather than three-dimensional space.^{ [27] } His hope was that the theory would lead to new insights and experiments that would lead ultimately to an acceptable one;^{ [27] } his aim was not to set out a deterministic, mechanical viewpoint, but rather to show that it was possible to attribute properties to an underlying reality, in contrast to the conventional approach to quantum mechanics.^{ [28] }

In August 2011, Roger Colbeck and Renato Renner published a proof that any extension of quantum mechanical theory, whether using hidden variables or otherwise, cannot provide a more accurate prediction of outcomes, assuming that observers can freely choose the measurement settings.^{ [29] } Colbeck and Renner write: "In the present work, we have ... excluded the possibility that any extension of quantum theory (not necessarily in the form of local hidden variables) can help predict the outcomes of any measurement on any quantum state. In this sense, we show the following: under the assumption that measurement settings can be chosen freely, quantum theory really is complete".

In January 2013, Giancarlo Ghirardi and Raffaele Romano described a model which, "under a different free choice assumption [...] violates [the statement by Colbeck and Renner] for almost all states of a bipartite two-level system, in a possibly experimentally testable way".^{ [30] }

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 of numerous proposed interpretations of 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.

The **Einstein–Podolsky–Rosen paradox** is a thought experiment proposed by physicists Albert Einstein, Boris Podolsky and Nathan Rosen (EPR), with which they argued that the description of physical reality provided by quantum mechanics was incomplete. In a 1935 paper titled "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?", they argued for the existence of "elements of reality" that were not part of quantum theory, and speculated that it should be possible to construct a theory containing them. Resolutions of the paradox have important implications for the interpretation of quantum mechanics.

**Quantum entanglement** is a physical phenomenon that occurs when a group of particles is generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.

In quantum mechanics, **Schrödinger's cat** is a thought experiment that illustrates a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead as a result of its fate being linked to a random subatomic event that may or may not occur.

**Wave–particle duality** is the concept in quantum mechanics that every particle or quantum entity may be described as either a particle or a wave. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects. As Albert Einstein wrote:

It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.

The **de Broglie–Bohm theory**, also known as the *pilot wave theory*, **Bohmian mechanics**, **Bohm's interpretation**, and the **causal interpretation**, is an interpretation of quantum mechanics. In addition to the wavefunction, it also postulates an actual configuration of particles exists even when unobserved. The evolution over time of the configuration of all particles is defined by a guiding equation. The evolution of the wave function over time is given by the Schrödinger equation. The theory is named after Louis de Broglie (1892–1987) and David Bohm (1917–1992).

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, 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 stochastic, which elements of quantum mechanics can be considered real, and what is the nature of measurement, among other matters.

**Bell's theorem** proves that quantum physics is incompatible with local hidden-variable theories. It was introduced by physicist John Stewart Bell in a 1964 paper titled "On the Einstein Podolsky Rosen Paradox", referring to a 1935 thought experiment that Albert Einstein, Boris Podolsky and Nathan Rosen used to argue that quantum physics is an "incomplete" theory. By 1935, it was already recognized that the predictions of quantum physics are probabilistic. Einstein, Podolsky and Rosen presented a scenario that, in their view, indicated that quantum particles, like electrons and photons, must carry physical properties or attributes not included in quantum theory, and the uncertainties in quantum theory's predictions were due to ignorance of these properties, later termed "hidden variables". Their scenario involves a pair of widely separated physical objects, prepared in such a way that the quantum state of the pair is entangled.

**John Stewart Bell** FRS was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden variable theories.

In physics, **action at a distance** is the concept that an object can be moved, changed, or otherwise affected without being physically touched by another object. That is, it is the non-local interaction of objects that are separated in space.

In quantum physics, a **measurement** is the testing or manipulation of a physical system in order to yield a numerical result. The predictions that quantum physics makes are in general probabilistic. The mathematical tools for making predictions about what measurement outcomes may occur were developed during the 20th century and make use of linear algebra and functional analysis.

**Nathan Rosen** was an American-Israeli physicist noted for his study on the structure of the hydrogen atom and his work with Albert Einstein and Boris Podolsky on entangled wave functions and the EPR paradox. The Einstein–Rosen bridge, later named the wormhole, was a theory of Nathan Rosen.

In quantum mechanics, the **measurement problem** considers how, or whether, wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer.

In Bell tests, there may be problems of experimental design or set-up that affect the validity of the experimental findings. These problems are often referred to as "loopholes". See the article on Bell's theorem for the theoretical background to these experimental efforts. The purpose of the experiment is to test whether nature is best described using a local hidden variable theory or by the quantum entanglement theory of quantum mechanics.

A **Bell test**, also known as **Bell inequality test** or **Bell experiment**, is a real-world physics experiment designed to test the theory of quantum mechanics in relation to Albert Einstein's concept of local realism. The experiments test whether or not the real world satisfies local realism, which requires the presence of some additional local variables to explain the behavior of particles like photons and electrons. To date, all Bell tests have found that the hypothesis of local hidden variables is inconsistent with the way that physical systems behave.

In theoretical physics, the **pilot wave theory**, also known as **Bohmian mechanics**, was the first known example of a hidden-variable theory, presented by Louis de Broglie in 1927. Its more modern version, the de Broglie–Bohm theory, interprets quantum mechanics as a deterministic theory, avoiding troublesome notions such as wave–particle duality, instantaneous wave function collapse, and the paradox of Schrödinger's cat. To solve these problems, the theory is inherently nonlocal.

In physics, **complementarity** is a conceptual aspect of quantum mechanics that Niels Bohr regarded as an essential feature of the theory. The complementarity principle holds that objects have certain pairs of complementary properties which cannot all be observed or measured simultaneously. An example of such a pair is position and momentum. Bohr considered one of the foundational truths of quantum mechanics to be the fact that setting up an experiment to measure one quantity of a pair, for instance the position of an electron, excludes the possibility of measuring the other, yet understanding both experiments is necessary to characterize the object under study. In Bohr's view, the behavior of atomic and subatomic objects cannot be separated from the measuring instruments that create the context in which the measured objects behave. Consequently, there is no "single picture" that unifies the results obtained in these different experimental contexts, and only the "totality of the phenomena" together can provide a completely informative description.

In quantum mechanics, the **Kochen–Specker** (**KS**) **theorem**, also known as the **Bell–Kochen–Specker theorem**, is a "no-go" theorem proved by John S. Bell in 1966 and by Simon B. Kochen and Ernst Specker in 1967. It places certain constraints on the permissible types of hidden-variable theories, which try to explain the predictions of quantum mechanics in a context-independent way. The version of the theorem proved by Kochen and Specker also gave an explicit example for this constraint in terms of a finite number of state vectors.

In theoretical physics, **quantum nonlocality** refers to the phenomenon by which the measurement statistics of a multipartite quantum system do not admit an interpretation in terms of a local realistic theory. Quantum nonlocality has been experimentally verified under different physical assumptions. Any physical theory that aims at superseding or replacing quantum theory should account for such experiments and therefore must also be nonlocal in this sense; quantum nonlocality is a property of the universe that is independent of our description of nature.

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.

- ↑
*The Born-Einstein letters: correspondence between Albert Einstein and Max and Hedwig Born from 1916–1955, with commentaries by Max Born*. Macmillan. 1971. p. 158., (Private letter from Einstein to Max Born, 3 March 1947: "I admit, of course, that there is a considerable amount of validity in the statistical approach which you were the first to recognize clearly as necessary given the framework of the existing formalism. I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance.... I am quite convinced that someone will eventually come up with a theory whose objects, connected by laws, are not probabilities but considered facts, as used to be taken for granted until quite recently".) - ↑ This is a common paraphrase of a sentence in a private letter from Einstein to Max Born, 4 December 1926, Albert Einstein Archives reel 8, item 180
- 1 2 The Collected Papers of Albert Einstein, Volume 15: The Berlin Years: Writings & Correspondence, June 1925-May 1927 (English Translation Supplement), p. 403
- 1 2 Einstein, A.; Podolsky, B.; Rosen, N. (1935). "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?".
*Physical Review*.**47**(10): 777–780. Bibcode:1935PhRv...47..777E. doi: 10.1103/PhysRev.47.777 . - ↑ "The debate whether Quantum Mechanics is a complete theory and probabilities have a non-epistemic character (i.e. nature is intrinsically probabilistic) or whether it is a statistical approximation of a deterministic theory and probabilities are due to our ignorance of some parameters (i.e. they are epistemic) dates to the beginning of the theory itself". See: arXiv:quant-ph/0701071v1 12 Jan 2007
- ↑ Senechal M, Cronin J (2001). "Social influences on quantum mechanics?-I".
*The Mathematical Intelligencer*.**23**(4): 15–17. doi:10.1007/BF03024596. S2CID 120478477. - ↑ Individual diagrams are often split into several parts, which may occur beyond observation; only the diagram as a whole describes an observed event.
- ↑ For every subset of points within a range, a value for every argument from the subset will be determined by the points in the neighbourhood. Thus, as a whole, the evolution in time can be described (for a specific time interval) as a function, e.g. a linear one or an arc. See Continuous function#Definition in terms of limits of functions
- ↑
*The Born–Einstein letters: correspondence between Albert Einstein and Max and Hedwig Born from 1916–1955, with commentaries by Max Born*. Macmillan. 1971. p. 91. - ↑ This is a common paraphrasing. Bohr recollected his reply to Einstein at the 1927 Solvay Congress in his essay "Discussion with Einstein on Epistemological Problems in Atomic Physics", in
*Albert Einstein, Philosopher–Scientist*, ed. Paul Arthur Shilpp, Harper, 1949, p. 211: "...in spite of all divergencies of approach and opinion, a most humorous spirit animated the discussions. On his side, Einstein mockingly asked us whether we could really believe that the providential authorities took recourse to dice-playing ("*ob der liebe Gott würfelt*"), to which I replied by pointing at the great caution, already called for by ancient thinkers, in ascribing attributes to Providence in everyday language." Werner Heisenberg, who also attended the congress, recalled the exchange in*Encounters with Einstein*, Princeton University Press, 1983, p. 117,: "But he [Einstein] still stood by his watchword, which he clothed in the words: 'God does not play at dice.' To which Bohr could only answer: 'But still, it cannot be for us to tell God, how he is to run the world.'" - ↑ The Collected Papers of Albert Einstein, Volume 15: The Berlin Years: Writings & Correspondence, June 1925-May 1927 (English Translation Supplement), p. 512
- ↑ Albert Einstein Archives reel 2, item 100
- ↑ Baggott, Jim (2011).
*The Quantum Story: A History in 40 Moments*. New York: Oxford University Press. pp. 116–117. - ↑ Max Born and Werner Heisenberg, "Quantum mechanics", proceedings of the Fifth Solvay Congress.
- ↑ Einstein, A.; Podolsky, B.; Rosen, N. (1935). "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?".
*Physical Review*.**47**(10): 777–780. Bibcode:1935PhRv...47..777E. doi: 10.1103/physrev.47.777 . - ↑ Einstein A (1936). "Physics and Reality".
*Journal of the Franklin Institute*.**221**. - ↑ Bohr N (1935). "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?".
*Physical Review*.**48**(8): 700. Bibcode:1935PhRv...48..696B. doi: 10.1103/physrev.48.696 . - ↑ Bohr N. (1948). "On the notions of causality and complementarity".
*Dialectica*.**2**(3–4): 312–319 [317]. doi:10.1111/j.1746-8361.1948.tb00703.x. - ↑ Rosenfeld, L. (). 'Niels Bohr's contribution to epistemology', pp. 522–535 in
*Selected Papers of Léon Rosenfeld*, Cohen, R.S., Stachel, J.J. (editors), D. Riedel, Dordrecht, ISBN 978-90-277-0652-2, p. 531: "Moreover, the complete definition of the phenomenon must essentially contain the indication of some permanent mark left upon a recording device which is part of the apparatus; only by thus envisaging the phenomenon as a closed event, terminated by a permanent record, can we do justice to the typical wholeness of the quantal processes." - ↑ Kwiat P. G.; et al. (1999). "Ultrabright source of polarization-entangled photons".
*Physical Review A*.**60**(2): R773–R776. arXiv: quant-ph/9810003 . Bibcode:1999PhRvA..60..773K. doi:10.1103/physreva.60.r773. S2CID 16417960. - ↑ G 't Hooft,
*The Free-Will Postulate in Quantum Mechanics*;*Entangled quantum states in a local deterministic theory* - ↑ David Pratt: "David Bohm and the Implicate Order". Appeared in
*Sunrise magazine*, February/March 1993, Theosophical University Press - ↑ Michael K.-H. Kiessling: "Misleading Signposts Along the de Broglie–Bohm Road to Quantum Mechanics",
*Foundations of Physics*, volume 40, number 4, 2010, pp. 418–429 (abstract) - ↑ "While the testable predictions of Bohmian mechanics are isomorphic to standard Copenhagen quantum mechanics, its underlying hidden variables have to be, in principle, unobservable. If one could observe them, one would be able to take advantage of that and signal faster than light, which – according to the special theory of relativity – leads to physical temporal paradoxes." J. Kofler and A. Zeiliinger, "Quantum Information and Randomness",
*European Review*(2010), Vol. 18, No. 4, 469–480. - ↑ D. Bohm and B. J. Hiley,
*The Undivided Universe*, Routledge, 1993, ISBN 0-415-06588-7. - ↑ Wayne C. Myrvold (2003). "On some early objections to Bohm's theory" (PDF).
*International Studies in the Philosophy of Science*.**17**(1): 8–24. doi:10.1080/02698590305233. S2CID 10965929. Archived from the original on 2014-07-02. - 1 2 3 4 5 David Bohm (1957).
*Causality and Chance in Modern Physics*. Routledge & Kegan Paul and D. Van Nostrand. p. 110. ISBN 0-8122-1002-6. - ↑ B. J. Hiley:
*Some remarks on the evolution of Bohm's proposals for an alternative to quantum mechanics*, 30 January 2010 - ↑ Roger Colbeck; Renato Renner (2011). "No extension of quantum theory can have improved predictive power".
*Nature Communications*.**2**(8): 411. arXiv: 1005.5173 . Bibcode:2011NatCo...2E.411C. doi:10.1038/ncomms1416. PMC 3265370 . PMID 21811240. - ↑ Giancarlo Ghirardi; Raffaele Romano (2013). "Onthological models predictively inequivalent to quantum theory".
*Physical Review Letters*.**110**(17): 170404. arXiv: 1301.2695 . Bibcode:2013PhRvL.110q0404G. doi:10.1103/PhysRevLett.110.170404. PMID 23679689. S2CID 197479.

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- Einstein, Albert; Podolsky, Boris; Rosen, Nathan (1935). "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?".
*Physical Review*.**47**(10): 777–780. Bibcode:1935PhRv...47..777E. doi: 10.1103/physrev.47.777 . - Bell, John Stewart (1964). "On the Einstein–Podolsky–Rosen paradox".
*Physics Physique Физика*.**1**(3): 195–200. doi: 10.1103/physicsphysiquefizika.1.195 . Reprinted in*Speakable and Unspeakable in Quantum Mechanics*. Cambridge University Press. 2004. - Bohm, D.; Hiley, B. J. (1993).
*The Undivided Universe*. Routledge. - Pauli, Wolfgang (1988). "Letter to M. Fierz dated 10 August 1954".
*Beyond the Atom: The Philosophical Thought of Wolfgang Pauli*. Translated by Laurikainen, K. V. Berlin: Springer-Verlag. p. 226. - Heisenberg, Werner (1971).
*Physics and Beyond: Encounters and Conversations*. Translated by Pomerans, A. J. New York: Harper & Row. pp. 63–64. - Cohen-Tannoudji, Claude; Diu, Bernard; Laloë, Franck (1982).
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