EPR paradox

Part of a series on
Quantum mechanics
$i\hbar {\frac {\partial }{\partial t}}|\psi (t)\rangle ={\hat {H}}|\psi (t)\rangle$ Schrödinger equation
Introduction Glossary History
Background Classical mechanics Old quantum theory Bra–ket notation Hamiltonian Interference
Fundamentals Coherence Decoherence Complementarity Energy level Entanglement Hamiltonian Uncertainty principle Ground state Interference Measurement Nonlocality Observable Operator Quantum Quantum fluctuation Quantum foam Quantum levitation Quantum number Quantum noise Quantum realm Quantum state Quantum system Quantum teleportation Qubit Spin Superposition Symmetry (Spontaneous) symmetry breaking Vacuum state Wave propagation Wave function Wave function collapse Wave–particle duality Matter wave
Effects Zeeman effect Stark effect Aharonov–Bohm effect Landau quantization Quantum Hall effect Quantum Zeno effect Quantum tunnelling Photoelectric effect Casimir effect
Experiments Afshar Bell's inequality Davisson–Germer Double-slit Elitzur–Vaidman Franck–Hertz Leggett–Garg inequality Mach–Zehnder Popper Quantum eraser (delayed-choice) Schrödinger's cat Quantum suicide and immortality Stern–Gerlach Wheeler's delayed-choice
Formulations Overview Heisenberg Interaction Matrix Phase-space Schrödinger Sum-over-histories (path-integral) Hellmann–Feynman theorem
Equations Dirac Klein–Gordon Pauli Rydberg Schrödinger
Interpretations Overview Consistent histories Copenhagen de Broglie–Bohm Ensemble Hidden-variable Many-worlds Objective collapse Bayesian Quantum logic Relational Stochastic Scale relativity Transactional
Advanced topics Quantum annealing Quantum chaos Quantum computing Density matrix Quantum field theory Fractional quantum mechanics Quantum gravity Quantum information science Quantum machine learning Perturbation theory (quantum mechanics) Relativistic quantum mechanics Scattering theory Spontaneous parametric down-conversion Quantum statistical mechanics
Scientists Aharonov Bell Blackett Bloch Bohm Bohr Born Bose de Broglie Candlin Compton Dirac Davisson Debye Ehrenfest Einstein Everett Fock Fermi Feynman Glauber Gutzwiller Heisenberg Hilbert Jordan Kramers Pauli Lamb Landau Laue Moseley Millikan Onnes Planck Rabi Raman Rydberg Schrödinger Sommerfeld von Neumann Weyl Wien Wigner Zeeman Zeilinger

The Einstein–Podolsky–Rosen paradox (EPR paradox) is a thought experiment proposed by physicists Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) that they interpreted as indicating that the explanation 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 attempted to mathematically show that the wave function does not contain complete information about physical reality, and hence the Copenhagen interpretation is unsatisfactory; resolutions of the paradox have important implications for the interpretation of quantum mechanics.

The work was done at the Institute for Advanced Study in 1934, which Einstein had joined the prior year after he had fled Nazi Germany.

Albert Einstein

The essence of the paradox is that particles can interact in such a way that it is possible to measure both their position and their momentum more accurately than Heisenberg's uncertainty principle allows, unless measuring one particle instantaneously affects the other to prevent this accuracy, which would involve information being transmitted faster than light as forbidden by the theory of relativity ("spooky action at a distance"). This consequence had not previously been noticed and seemed unreasonable at the time; the phenomenon involved is now known as quantum entanglement.

History of EPR developments

The article that first brought forth these matters, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" was published in 1935. The paper prompted a response by Bohr, which he published in the same journal, in the same year, using the same title. There followed a debate between Bohr and Einstein about the fundamental nature of reality. Einstein had been skeptical of the Heisenberg uncertainty principle and the role of chance in quantum theory. But the crux of this debate was not about chance, but something even deeper: Is there one objective physical reality, which every observer sees from his own vantage? (Einstein's view) Or does the observer co-create physical reality by the questions he poses with experiments? (Bohr's view)

Einstein struggled to the end of his life for a theory that could better comply with his idea of causality, protesting against the view that there exists no objective physical reality other than that which is revealed through measurement interpreted in terms of quantum mechanical formalism. However, since Einstein's death, experiments analogous to the one described in the EPR paper have been carried out, starting in 1976 by French scientists Lamehi-Rachti and Mittig at the Saclay Nuclear Research Centre. These experiments appear to show that the local realism idea is false, vindicating Bohr.

Per EPR, the paradox demonstrated that quantum theory was incomplete, and needed to be extended with hidden variables. One modern resolution is as follows: for two "entangled" particles created at once (e.g., an electron-positron pair from a photon), measurable properties have well-defined meaning only for the ensemble system. Properties of constituent subsystems (e.g., the individual electron or positron), considered individually, remain undefined. Therefore, if analogous measurements are performed on the two entangled subsystems, there will always be a correlation between the outcomes, and a well-defined global outcome for the ensemble. However, the outcomes for each subsystem, considered separately, at each repetition of the experiment, will not be well defined or predictable. This correlation does not imply that measurements performed on one particle influence measurements on the other. This modern resolution eliminates the need for hidden variables, action at a distance, or other schemes introduced over time, in order to explain the phenomenon.

According to quantum mechanics, under some conditions, a pair of quantum systems may be described by a single wave function, which encodes the probabilities of the outcomes of experiments that may be performed on the two systems, whether jointly or individually. At the time the EPR article discussed below was written, it was known from experiments that the outcome of an experiment sometimes cannot be uniquely predicted. An example of such indeterminacy can be seen when a beam of light is incident on a half-silvered mirror. One half of the beam will reflect, and the other will pass. If the intensity of the beam is reduced until only one photon is in transit at any time, whether that photon will reflect or transmit cannot be predicted quantum mechanically.

The routine explanation of this effect was, at that time, provided by Heisenberg's uncertainty principle. Physical quantities come in pairs called conjugate quantities. Examples of such conjugate pairs are (position, momentum), (time, energy), and (angular position, angular momentum). When one quantity was measured, and became determined, the conjugated quantity became indeterminate. Heisenberg explained this uncertainty as due to the quantization of the disturbance from measurement.

The EPR paper, written in 1935, was intended to illustrate that this explanation is inadequate. It considered two entangled particles, referred to as A and B, and pointed out that measuring a quantity of a particle A will cause the conjugated quantity of particle B to become undetermined, even if there was no contact, no classical disturbance. The basic idea was that the quantum states of two particles in a system cannot always be decomposed from the joint state of the two, as is the case for the Bell state, $|\Phi ^{+}\rangle ={\frac {1}{\sqrt {2}}}\left(|00\rangle +|11\rangle \right).$

Heisenberg's principle was an attempt to provide a classical explanation of a quantum effect sometimes called non-locality. According to EPR there were two possible explanations. Either there was some interaction between the particles (Even though they were separated) or the information about the outcome of all possible measurements was already present in both particles.

The EPR authors preferred the second explanation according to which that information was encoded in some 'hidden parameters'. The first explanation of an effect propagating instantly across a distance is in conflict with the theory of relativity. They then concluded that quantum mechanics was incomplete since its formalism does not permit hidden parameters.

Violations of the conclusions of Bell's theorem are generally understood to have demonstrated that the hypotheses of Bell's theorem, also assumed by Einstein, Podolsky and Rosen, do not apply in our world. Most physicists who have examined the issue concur that experiments, such as those of Alain Aspect and his group, have confirmed that physical probabilities, as predicted by quantum theory, do exhibit the phenomena of Bell-inequality violations that are considered to invalidate EPR's preferred "local hidden-variables" type of explanation for the correlations to which EPR first drew attention.

Quantum mechanics and its interpretation

Main article: Interpretations of quantum mechanics

Since the early twentieth century, quantum theory has proved to be successful in describing accurately the physical reality of the mesoscopic and microscopic world, in multiple reproducible physics experiments.

Quantum mechanics was developed with the aim of describing atoms and explaining the observed spectral lines in a measurement apparatus. Although disputed especially in the early twentieth century, it has yet to be seriously challenged. Philosophical interpretations of quantum phenomena, however, are another matter: the question of how to interpret the mathematical formulation of quantum mechanics has given rise to a variety of different answers from people of different philosophical persuasions (see Interpretations of quantum mechanics).

Quantum theory and quantum mechanics do not provide single measurement outcomes in a deterministic way. According to the understanding of quantum mechanics known as the Copenhagen interpretation, measurement causes an instantaneous collapse of the wave function describing the quantum system into an eigenstate of the observable that was measured. Einstein characterized this imagined collapse in the 1927 Solvay Conference. He presented a thought experiment in which electrons are introduced through a small hole in a sphere whose inner surface serves as a detection screen. The electrons will contact the spherical detection screen in a widely dispersed manner. Those electrons, however, are all individually described by wave fronts that expand in all directions from the point of entry. A wave as it is understood in everyday life would paint a large area of the detection screen, but the electrons would be found to impact the screen at single points and would eventually form a pattern in keeping with the probabilities described by their identical wave functions. Einstein asks what makes each electron's wave front "collapse" at its respective location. Why do the electrons appear as single bright scintillations rather than as dim washes of energy across the surface? Why does any single electron appear at one point rather than some alternative point? The behavior of the electrons gives the impression of some signal having been sent to all possible points of contact that would have nullified all but one of them, or, in other words, would have preferentially selected a single point to the exclusion of all others.

Einstein's opposition

Einstein was the most prominent opponent of the Copenhagen interpretation. In his view, quantum mechanics was incomplete. Commenting on this, other writers (such as John von Neumann and David Bohm) hypothesized that consequently there would have to be 'hidden' variables responsible for random measurement results, something which was not expressly claimed in the original paper.

The 1935 EPR paper condensed the philosophical discussion into a physical argument. The authors claim that given a specific experiment, in which the outcome of a measurement is known before the measurement takes place, there must exist something in the real world, an "element of reality", that determines the measurement outcome. They postulate that these elements of reality are local, in the sense that each belongs to a certain point in spacetime. Each element may only be influenced by events which are located in the backward light cone of its point in spacetime (i.e., the past). These claims are founded on assumptions about nature that constitute what is now known as local realism.

Though the EPR paper has often been taken as an exact expression of Einstein's views, it was primarily authored by Podolsky, based on discussions at the Institute for Advanced Study with Einstein and Rosen. Einstein later expressed to Erwin Schrödinger that, "it did not come out as well as I had originally wanted; rather, the essential thing was, so to speak, smothered by the formalism." In 1936, Einstein presented an individual account of his local realist ideas.

Description of the paradox

The original EPR paradox challenges the prediction of quantum mechanics that it is impossible to know both the position and the momentum of a quantum particle. This challenge can be extended to other pairs of physical properties.

EPR paper

The original paper purports to describe what must happen to "two systems I and II, which we permit to interact ...", and, after some time, "we suppose that there is no longer any interaction between the two parts." As explained by Manjit Kumar (2009), the EPR description involves "two particles, A and B, [which] interact briefly and then move off in opposite directions." According to Heisenberg's uncertainty principle, it is impossible to measure both the momentum and the position of particle B exactly. However, it is possible to measure the exact position of particle A. By calculation, therefore, with the exact position of particle A known, the exact position of particle B can be known. Alternatively, the exact momentum of particle A can be measured, so the exact momentum of particle B can be worked out. Kumar writes: "EPR argued that they had proved that ... [particle] B can have simultaneously exact values of position and momentum. ... Particle B has a position that is real and a momentum that is real."

EPR appeared to have contrived a means to establish the exact values of either the momentum or the position of B due to measurements made on particle A, without the slightest possibility of particle B being physically disturbed.

EPR tried to set up a paradox to question the range of true application of Quantum Mechanics: Quantum theory predicts that both values cannot be known for a particle, and yet the EPR thought experiment purports to show that they must all have determinate values. The EPR paper says: "We are thus forced to conclude that the quantum-mechanical description of physical reality given by wave functions is not complete."

The EPR paper ends by saying:

While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible.

Measurements on an entangled state

Main article: Measurement in quantum mechanics

We have a source that emits electron–positron pairs, with the electron sent to destination A, where there is an observer named Alice, and the positron sent to destination B, where there is an observer named Bob. According to quantum mechanics, we can arrange our source so that each emitted pair occupies a quantum state called a spin singlet. The particles are thus said to be entangled. This can be viewed as a quantum superposition of two states, which we call state I and state II. In state I, the electron has spin pointing upward along the z-axis (+z) and the positron has spin pointing downward along the z-axis (−z). In state II, the electron has spin −z and the positron has spin +z. Because it is in a superposition of states it is impossible without measuring to know the definite state of spin of either particle in the spin singlet.

The EPR thought experiment, performed with electron–positron pairs. A source (center) sends particles toward two observers, electrons to Alice (left) and positrons to Bob (right), who can perform spin measurements.

Alice now measures the spin along the z-axis. She can obtain one of two possible outcomes: +z or −z. Suppose she gets +z. According to the Copenhagen interpretation of quantum mechanics, the quantum state of the system collapses into state I. The quantum state determines the probable outcomes of any measurement performed on the system. In this case, if Bob subsequently measures spin along the z-axis, there is 100% probability that he will obtain −z. Similarly, if Alice gets −z, Bob will get +z.

There is, of course, nothing special about choosing the z-axis: according to quantum mechanics the spin singlet state may equally well be expressed as a superposition of spin states pointing in the x direction. Suppose that Alice and Bob had decided to measure spin along the x-axis. We'll call these states Ia and IIa. In state Ia, Alice's electron has spin +x and Bob's positron has spin −x. In state IIa, Alice's electron has spin −x and Bob's positron has spin +x. Therefore, if Alice measures +x, the system 'collapses' into state Ia, and Bob will get −x. If Alice measures −x, the system collapses into state IIa, and Bob will get +x.

Whatever axis their spins are measured along, they are always found to be opposite. This can only be explained if the particles are linked in some way. Either they were created with a definite (opposite) spin about every axis—a "hidden variable" argument—or they are linked so that one electron "feels" which axis the other is having its spin measured along, and becomes its opposite about that one axis—an "entanglement" argument. Moreover, if the two particles have their spins measured about different axes, once the electron's spin has been measured about the x-axis (and the positron's spin about the x-axis deduced), the positron's spin about the z-axis will no longer be certain, as if (a) it knows that the measurement has taken place, or (b) it has a definite spin already, about a second axis—a hidden variable. However, it turns out that the predictions of Quantum Mechanics, which have been confirmed by experiment, cannot be explained by any local hidden variable theory. This is demonstrated in Bell's theorem.

In quantum mechanics, the x-spin and z-spin are "incompatible observables", meaning the Heisenberg uncertainty principle applies to alternating measurements of them: a quantum state cannot possess a definite value for both of these variables. Suppose Alice measures the z-spin and obtains +z, so that the quantum state collapses into state I. Now, instead of measuring the z-spin as well, Bob measures the x-spin. According to quantum mechanics, when the system is in state I, Bob's x-spin measurement will have a 50% probability of producing +x and a 50% probability of -x. It is impossible to predict which outcome will appear until Bob actually performs the measurement.

The paradox

One might imagine that, when Bob measures the x-spin of his positron, he would get an answer with absolute certainty, since prior to this he hasn't disturbed his particle at all. But it turns out that Bob's positron has a 50% probability of producing +x and a 50% probability of −x, meaning the outcome is not certain. It's as if Bob's positron "knows" that Alice has measured the z-spin of her electron, and hence his positron's own z-spin must also be set, but its x-spin remains uncertain.

Put another way, how does Bob's positron know which way to point if Alice decides (based on information unavailable to Bob) to measure x (i.e., to be the opposite of Alice's electron's spin about the x-axis) and also how to point if Alice measures z, since it is only supposed to know one thing at a time? The Copenhagen interpretation rules that say the wave function "collapses" at the time of measurement, so there must be action at a distance (entanglement) or the positron must know more than it's supposed to know (hidden variables).

It is one thing to say that physical measurement of the first particle's momentum affects uncertainty in its own position, but to say that measuring the first particle's momentum affects the uncertainty in the position of the other is another thing altogether. Einstein, Podolsky and Rosen asked how can the second particle "know" to have precisely defined momentum but uncertain position? Since this implies that one particle is communicating with the other instantaneously across space, i.e., faster than light, this is the "paradox".

Incidentally, Bell used spin as his example, but many types of physical quantities—referred to as "observables" in quantum mechanics—can be used. The EPR paper used momentum for the observable. Experimental realisations of the EPR scenario often use photon polarization, because polarized photons are easy to prepare and measure.

Locality in the EPR experiment

EPR describe the principle of locality as asserting that physical processes occurring at one place should have no immediate effect on the elements of reality at another location. At first sight, this appears to be a reasonable assumption to make, as it seems to be a consequence of special relativity, which states that energy can never be transmitted faster than the speed of light without violating causality.

However, it turns out that the usual rules for combining quantum mechanical and classical descriptions violate EPR's principle of locality without violating special relativity or causality. Causality is preserved because there is no way for Alice to transmit messages (i.e., information) to Bob by manipulating her measurement axis. Whichever axis she uses, she has a 50% probability of obtaining "+" and 50% probability of obtaining "−", completely at random; according to quantum mechanics, it is fundamentally impossible for her to influence what result she gets. Furthermore, Bob is only able to perform his measurement once: there is a fundamental property of quantum mechanics, the no cloning theorem, which makes it impossible for him to make an arbitrary number of copies of the electron he receives, perform a spin measurement on each, and look at the statistical distribution of the results. Therefore, in the one measurement he is allowed to make, there is a 50% probability of getting "+" and 50% of getting "−", regardless of whether or not his axis is aligned with Alice's.

Note that in this argument, we never assumed that energy could be transmitted faster than the speed of light. This shows that the results of the EPR experiment do not contradict the predictions of special relativity.

However, the principle of locality appeals powerfully to physical intuition, and Einstein, Podolsky and Rosen were unwilling to abandon it. Einstein derided the quantum mechanical predictions as "spooky action at a distance". The (incorrect) conclusion they drew was that quantum mechanics is not a complete theory.

The word locality has several different meanings in physics. For example, in quantum field theory "locality" means that quantum fields at different points of space do not interact with one another. However, quantum field theories that are "local" in this sense appear to violate the principle of locality as defined by EPR, but they nevertheless do not violate locality in a more general sense. Wavefunction collapse can be viewed as an epiphenomenon of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of its environment. Since the underlying behaviour doesn't violate causality or special relativity, it follows that neither does the additional effect of wavefunction collapse, whether real or apparent. Therefore, as mentioned above, neither the EPR experiment nor any quantum experiment demonstrates that superluminous signaling is possible.

Resolving the paradox

Hidden variables

Main article: Hidden-variable theory

There are several ways to resolve the EPR paradox. The one suggested by EPR is that quantum mechanics, despite its success in a wide variety of experimental scenarios, is actually an incomplete theory. In other words, there is some yet undiscovered theory of nature to which quantum mechanics acts as a kind of statistical approximation (albeit an exceedingly successful one). Unlike quantum mechanics, the more complete theory contains variables corresponding to all the "elements of reality". There must be some unknown mechanism acting on these variables to give rise to the observed effects of "non-commuting quantum observables", i.e., the Heisenberg uncertainty principle. Such a theory is called a hidden variable theory.

To illustrate this idea, we can formulate a very simple hidden variable theory for the above thought experiment. One supposes that the quantum spin-singlet states emitted by the source are actually approximate descriptions for "true" physical states possessing definite values for the z-spin and x-spin. In these "true" states, the positron going to Bob always has spin values opposite to the electron going to Alice, but the values are otherwise completely random. For example, the first pair emitted by the source might be "(+z, −x) to Alice and (−z, +x) to Bob", the next pair "(−z, −x) to Alice and (+z, +x) to Bob", and so forth. Therefore, if Bob's measurement axis is aligned with Alice's, he will necessarily get the opposite of whatever Alice gets; otherwise, he will get "+" and "−" with equal probability.

Assuming we restrict our measurements to the z- and x-axes, such a hidden variable theory is experimentally indistinguishable from quantum mechanics. In reality, there may be an infinite number of axes along which Alice and Bob can perform their measurements, so there would have to be an infinite number of independent hidden variables. However, this is not a serious problem; we have formulated a very simplistic hidden variable theory, and a more sophisticated theory might be able to patch it up. It turns out that there is a much more serious challenge to the idea of hidden variables.

Bell's inequality

Main article: Bell's theorem

In 1964, John Bell showed that the predictions of quantum mechanics in the EPR thought experiment are significantly different from the predictions of a particular class of hidden variable theories (the local hidden variable theories). Roughly speaking, quantum mechanics has a much stronger statistical correlation with measurement results performed on different axes than do these hidden variable theories. These differences, expressed using inequality relations known as "Bell's inequalities", are in principle experimentally detectable. After the publication of Bell's paper, a variety of experiments to test Bell's inequalities were devised. These generally relied on measurement of photon polarization. All experiments conducted to date have found behavior in line with the predictions of standard quantum mechanics theory.

Later work by Henry Stapp showed that a key property of local hidden variable theories which lead to Bell's inequalities was counterfactual definiteness. Building on Stapp's observations, P.H. Eberhard showed that any local counterfactual model results in Bell's inequality even without the assumption of there being hidden variables unknown to physics upon which the relevant observables depend. Arthur Fine subsequently showed that any theory satisfying the inequalities can be modeled by a local hidden variable theory. (Although Eberhard referred to his result as "Bell's theorem without hidden variables", Fine used a more general definition of "hidden variables" that includes the possibility of the observables being elementary.) Fine went on to show that any stochastic factorizable model leads to Bell's inequality. Itamar Pitowsky showed that Bell's inequality was a special case of an inequality discovered by George Boole which provides a consistency check on whether data can be represented by variables on a single classical probability space. He interpreted this to be an indication that the locality assumption prevented the data from being represented as events on such a space.

As Eberhard's proof made use of both locality and counterfactual definiteness it was assumed that an interpretation could reject either one of these to escape Bell's inequality. Violation of locality is difficult to reconcile with special relativity, and is thought to be incompatible with the principle of causality, nevertheless there was renewed interest in the Bohm interpretation of quantum mechanics which keeps counterfactual definiteness while introducing a conjectured non-local mechanism in the form of the 'quantum potential' that is defined as one of the terms of the Schrödinger equation. Mainstream physics preferred to keep locality and reject counterfactual definiteness. Fine's work showed that, taking locality as a given, there exist scenarios in which two statistical variables are correlated in a manner inconsistent with counterfactual definiteness, and that such scenarios are no more mysterious than any other, despite the fact that the inconsistency with counterfactual definiteness may seem counterintuitive.

Further insights resulted from the work of Lawrence J. Landau. Landau showed that if it is assumed that there is a single classical probability space underlying all the observables under consideration in the EPR experiment, Bell's inequality will result. Thus the fundamental issue is that Quantum mechanical probabilities cannot be modeled using classical (Kolmogorovian) probability regardless of whether Quantum Mechanics is considered a complete description of reality or not. Regarding Landau's proof Ray Streater notes that it shows that Bohmian mechanics is inconsistent with Quantum mechanics and succumbs to Bell's inequality despite claims to the contrary by its proponents. Streater notes that Landau's proof only requires the assumption of a single classical probability space (a condition still satisfied by Bohm's theory) and the fact that Bohmian mechanics additionally postulates the existence of a non-local mechanism, cannot prevent Bell's inequality from applying to it.

Similar observations have been made by Karl Hess, Walter, Philipp, Hans de Raedt and Kristel Michielsen, who note that in Bell's proof, Bell's assumption of a space of hidden variables behaving as a classical probability space is sufficient to produce a contradiction with the predications of Quantum mechanics via a consistency theorem of N. N. Vorob'ev, a statistician who had built on the same work of Boole used by Pitowsky. The additional assumption of locality used by Bell is redundant and indeed Fine's work had included a derivation of Bell's inequality that did not require the assumption of locality . Non-locality is not sufficient to escape Bell's inequality, any interpretation of Quantum mechanics needs to reject counterfactual definiteness to be consistent with the Quantum mechanical predications. The authors also produced a model of an EPR experiment that is local but which violates Bell's inequality, thus demonstrating that non-locality is also not necessary for escaping Bell's inequality. They also note a loophole regarding models of EPR experiments whereby even a counterfactual definite model can result in data that violates Bell's inequality if as in actual experiments there is a time window based post-selection of results due to the need to identify particles belonging to an emitted pair. Robert Griffiths has shown that according to a quantum mechanical analysis, the instrument settings for the measurement of one of the particles in the EPR scenario, does not influence subsequent measurement results on the second, thus ruling out non-locality as a viable explanation for the EPR correlations.

However, Bell's theorem does not apply to all possible philosophically realist theories. It is a common misconception that quantum mechanics is inconsistent with all notions of philosophical realism. Realist interpretations of quantum mechanics are possible, although as discussed above, such interpretations must reject counterfactual definiteness. Examples of such realist interpretations are the consistent histories interpretation and the transactional interpretation (first proposed by John G. Cramer in 1986). Griffiths notes that it is not "local realism" that is ruled out by quantum mechanics but "classical realism". Some workers in the field have also attempted to formulate hidden variable theories that exploit loopholes in actual experiments, such as the assumptions made in interpreting experimental data, although no theory has been proposed that can reproduce all the results of quantum mechanics.

Alternatives are still possible. A recent review article based on the Wheeler–Feynman time-symmetric theory rewrites the entire theory in terms of retarded Liénard–Wiechert potentials only, which becomes manifestly causal, and, establishes a conservation law for total generalized momenta held instantaneously for any closed system. The outcome results in correlation between particles from a "handshake principle" based on a variational principle applied to a system as a whole, an idea with a slightly non-local feature but the theory is nonetheless in agreement with the essential results of quantum electrodynamics and relativistic quantum chemistry.

There are also individual EPR-like experiments that have no local hidden variables explanation. Examples have been suggested by David Bohm and by Lucien Hardy.

Einstein's hope for a purely algebraic theory

The Bohm interpretation of quantum mechanics hypothesizes that the state of the universe evolves smoothly through time with no collapsing of quantum wavefunctions. One problem for the Copenhagen interpretation is to precisely define wavefunction collapse. Einstein maintained that quantum mechanics is physically incomplete and logically unsatisfactory. In "The Meaning of Relativity", Einstein wrote, "One can give good reasons why reality cannot at all be represented by a continuous field. From the quantum phenomena it appears to follow with certainty that a finite system of finite energy can be completely described by a finite set of numbers (quantum numbers). This does not seem to be in accordance with a continuum theory and must lead to an attempt to find a purely algebraic theory for the representation of reality. But nobody knows how to find the basis for such a theory." If time, space, and energy are secondary features derived from a substrate below the Planck scale, then Einstein's hypothetical algebraic system might resolve the EPR paradox (although Bell's theorem would still be valid). If physical reality is totally finite, then the Copenhagen interpretation might be an approximation to an information processing system below the Planck scale.

"Acceptable theories" and the experiment

According to the present view of the situation, quantum mechanics flatly contradicts Einstein's philosophical postulate that any acceptable physical theory must fulfill "local realism".

In the EPR paper (1935), the authors realised that quantum mechanics was inconsistent with their assumptions, but Einstein nevertheless thought that quantum mechanics might simply be augmented by hidden variables (i.e., variables which were, at that point, still obscure to him), without any other change, to achieve an acceptable theory. He pursued these ideas for over twenty years until the end of his life, in 1955.

In contrast, John Bell, in his 1964 paper, showed that quantum mechanics and the class of hidden variable theories Einstein favored would lead to different experimental results: different by a factor of 3/2 for certain correlations. So the issue of "acceptability", up to that time mainly concerning theory, finally became experimentally decidable.

There are many Bell test experiments, e.g., those of Alain Aspect and others. They support the predictions of quantum mechanics rather than the class of hidden variable theories supported by Einstein.

Implications for quantum mechanics

Most physicists today believe that quantum mechanics is correct, and that the EPR paradox is a "paradox" only because classical intuitions do not correspond to physical reality. How EPR is interpreted regarding locality depends on the interpretation of quantum mechanics one uses. In the Copenhagen interpretation, it is usually understood that instantaneous wave function collapse does occur. However, the view that there is no causal instantaneous effect has also been proposed within the Copenhagen interpretation: in this alternative view, measurement affects our ability to define (and measure) quantities in the physical system, not the system itself. In the many-worlds interpretation, locality is strictly preserved, since the effects of operations such as measurement affect only the state of the particle that is measured. However, the results of the measurement are not unique—every possible result is obtained.

The EPR paradox has deepened our understanding of quantum mechanics by exposing the fundamentally non-classical characteristics of the measurement process. Before the publication of the EPR paper, a measurement was often visualized as a physical disturbance that had to be inflicted directly upon the measured subsystem. For instance, when measuring the position of an electron, one imagines shining a light on it, thus disturbing the electron and producing the quantum mechanical uncertainties in its position. Such pat and convenient but unhelpful explanations of quantum mechanics remain commonplace today, but they fail to explain (among other things) the EPR paradox, which shows that a "measurement" can be performed on a particle without disturbing it directly, by performing a measurement on a distant entangled particle. In fact, Yakir Aharonov and his collaborators have developed a whole theory of so-called Weak measurement.

Technologies relying on quantum entanglement are now being developed. In quantum cryptography, entangled particles are used to transmit signals that cannot be eavesdropped upon without leaving a trace. In quantum computation, entangled quantum states are used to perform computations in parallel, which may allow certain calculations to be performed much more quickly than they ever could be with classical computers.

Mathematical formulation

The above discussion can be expressed mathematically using the quantum mechanical formulation of spin. The spin degree of freedom for an electron is associated with a two-dimensional complex vector space V, with each quantum state corresponding to a vector in that space. The operators corresponding to the spin along the x, y, and z direction, denoted S_x, S_y, and S_z respectively, can be represented using the Pauli matrices:

$S_{x}={\frac {\hbar }{2}}{\begin{bmatrix}0&1\\1&0\end{bmatrix}},\quad S_{y}={\frac {\hbar }{2}}{\begin{bmatrix}0&-i\\i&0\end{bmatrix}},\quad S_{z}={\frac {\hbar }{2}}{\begin{bmatrix}1&0\\0&-1\end{bmatrix}}$

where $\hbar$ is the reduced Planck constant (or the Planck constant divided by 2π).

The eigenstates of S_z are represented as

$\left|+z\right\rangle \leftrightarrow {\begin{bmatrix}1\\0\end{bmatrix}},\quad \left|-z\right\rangle \leftrightarrow {\begin{bmatrix}0\\1\end{bmatrix}}$

and the eigenstates of S_x are represented as

$\left|+x\right\rangle \leftrightarrow {\frac {1}{\sqrt {2}}}{\begin{bmatrix}1\\1\end{bmatrix}},\quad \left|-x\right\rangle \leftrightarrow {\frac {1}{\sqrt {2}}}{\begin{bmatrix}1\\-1\end{bmatrix}}$

The vector space of the electron-positron pair is $V\otimes V$, the tensor product of the electron's and positron's vector spaces. The spin singlet state is

$\left|\psi \right\rangle ={\frac {1}{\sqrt {2}}}{\bigg (}\left|+z\right\rangle \otimes \left|-z\right\rangle -\left|-z\right\rangle \otimes \left|+z\right\rangle {\bigg )}$

where the two terms on the right hand side are what we have referred to as state I and state II above.

From the above equations, it can be shown that the spin singlet can also be written as

$\left|\psi \right\rangle =-{\frac {1}{\sqrt {2}}}{\bigg (}\left|+x\right\rangle \otimes \left|-x\right\rangle -\left|-x\right\rangle \otimes \left|+x\right\rangle {\bigg )}$

where the terms on the right hand side are what we have referred to as state Ia and state IIa.

To illustrate how this leads to the violation of local realism, we need to show that after Alice's measurement of S_z (or S_x), Bob's value of S_z (or S_x) is uniquely determined, and therefore corresponds to an "element of physical reality". This follows from the principles of measurement in quantum mechanics. When S_z is measured, the system state ψ collapses into an eigenvector of S_z. If the measurement result is +z, this means that immediately after measurement the system state undergoes an orthogonal projection of ψ onto the space of states of the form

$\left|+z\right\rangle \otimes \left|\phi \right\rangle \quad \phi \in V$

For the spin singlet, the new state is

$\left|+z\right\rangle \otimes \left|-z\right\rangle$

Similarly, if Alice's measurement result is −z, the system undergoes an orthogonal projection onto

$\left|-z\right\rangle \otimes \left|\phi \right\rangle \quad \phi \in V$

which means that the new state is

$\left|-z\right\rangle \otimes \left|+z\right\rangle$

This implies that the measurement for S_z for Bob's positron is now determined. It will be −z in the first case or +z in the second case.

It remains only to show that S_x and S_z cannot simultaneously possess definite values in quantum mechanics. One may show in a straightforward manner that no possible vector can be an eigenvector of both matrices. More generally, one may use the fact that the operators do not commute,

$\left[S_{x},S_{z}\right]=-i\hbar S_{y}\neq 0$

along with the Heisenberg uncertainty relation

$\left\langle {\Delta S_{x}}^{2}\right\rangle \left\langle {\Delta S_{z}}^{2}\right\rangle \geq {\frac {1}{4}}\left|\left\langle \left[S_{x},S_{z}\right]\right\rangle \right|^{2}$

See also

• Bra–ket notation
• CHSH Bell test
• Coherence
• Correlation does not imply causation
• ER=EPR
• Ghirardi–Rimini–Weber theory
• GHZ experiment
• Measurement problem
• Penrose interpretation
• Philosophy of information
• Philosophy of physics
• Popper's experiment
• Quantum gravity
• Quantum information
• Quantum pseudo-telepathy
• Quantum teleportation
• Quantum Zeno effect
• Sakurai's Bell inequality
• Synchronicity
• Ward's probability amplitude
• Zero-point field

References

Selected papers

• P. H. Eberhard, Bell's theorem without hidden variables. Nuovo Cimento 38B1 75 (1977).
• P. H. Eberhard, Bell's theorem and the different concepts of locality. Nuovo Cimento 46B 392 (1978).
• A. Einstein, B. Podolsky, and N. Rosen, Can quantum-mechanical description of physical reality be considered complete? Phys. Rev. 47 777 (1935). [1]
• A. Fine, Hidden Variables, Joint Probability, and the Bell Inequalities. Phys. Rev. Lett. 48, 291 (1982).[2]
• A. Fine, Do Correlations need to be explained?, in Philosophical Consequences of Quantum Theory: Reflections on Bell's Theorem, edited by Cushing & McMullin (University of Notre Dame Press, 1986).
• L. Hardy, Nonlocality for two particles without inequalities for almost all entangled states. Phys. Rev. Lett. 71 1665 (1993).[3]
• M. Mizuki, A classical interpretation of Bell's inequality. Annales de la Fondation Louis de Broglie 26 683 (2001)
• Peres, Asher (2005). "Einstein, Podolsky, Rosen, and Shannon". Foundations of Physics. 35 (3): 511–514. arXiv:quant-ph/0310010. Bibcode:2005FoPh...35..511P. doi:10.1007/s10701-004-1986-6. ISSN 0015-9018.
• P. Pluch, "Theory for Quantum Probability", PhD Thesis University of Klagenfurt (2006)
• M. A. Rowe, D. Kielpinski, V. Meyer, C. A. Sackett, W. M. Itano, C. Monroe and D. J. Wineland, Experimental violation of a Bell's inequality with efficient detection, Nature 409, 791–794 (15 February 2001). [4]
• M. Smerlak, C. Rovelli, Relational EPR [5]

Books

• John S. Bell (1987) Speakable and Unspeakable in Quantum Mechanics. Cambridge University Press. ISBN 0-521-36869-3.
• Arthur Fine (1996) The Shaky Game: Einstein, Realism and the Quantum Theory, 2nd ed. Univ. of Chicago Press.
• Selleri, F. (1988) Quantum Mechanics Versus Local Realism: The Einstein–Podolsky–Rosen Paradox. New York: Plenum Press. ISBN 0-306-42739-7
• Leon Lederman, L., Teresi, D. (1993). The God Particle: If the Universe is the Answer, What is the Question? Houghton Mifflin Company, pages 21, 187 to 189.
• John Gribbin (1984) In Search of Schrödinger's Cat. Black Swan. ISBN 978-0-552-12555-0

External links

• The Einstein–Podolsky–Rosen Argument in Quantum Theory; 1.2 The argument in the text;
  http://plato.stanford.edu/entries/qt-epr/#1.2
• The original EPR paper.
• Internet Encyclopedia of Philosophy: "The Einstein-Podolsky-Rosen Argument and the Bell Inequalities".
• Abner Shimony (2004) "Bell’s Theorem."
• EPR, Bell & Aspect: The Original References.
• Does Bell's Inequality Principle rule out local theories of quantum mechanics? From the Usenet Physics FAQ.
• Theoretical use of EPR in teleportation.
• Effective use of EPR in cryptography.
• EPR experiment with single photons interactive.
• Spooky Actions At A Distance?: Oppenheimer Lecture by Prof. Mermin.