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Quantum mechanics
\Delta x \, \Delta p \ge \frac{\hbar}{2}
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In quantum mechanics, the EPR paradox is a thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain Quantum mechanics (QM or quantum theory) is a physical science dealing with the behavior of Matter and Energy on the scale of Atoms The mathematical formulation of quantum mechanics is the body of mathematical formalisms which permits a rigorous description of Quantum mechanics. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons A thought experiment (from the German Gedankenexperiment) is a proposal for an Experiment that would test a Hypothesis or Theory "EPR" stands for Einstein, Podolsky, and Rosen, who introduced the thought experiment in a 1935 paper to argue that quantum mechanics is not a complete physical theory. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical Boris Podolsky ( Борис Подольский) born into a Jewish family in 1896 Taganrog, Russia - died 1966 U Prof Nathan Rosen (Hebrew נתן רוזן Born into a Jewish family ( March 22, 1909, Brooklyn, New York &ndash December 18, 1995 Year 1935 ( MCMXXXV) was a Common year starting on Tuesday (link will display full calendar of the Gregorian calendar.

The EPR experiment yields a dichotomy. A dichotomy is any splitting of a whole into exactly two non-overlapping parts Either

  1. The result of a measurement performed on one part A of a quantum system has a non-local effect on the physical reality of another distant part B, in the sense that quantum mechanics can predict outcomes of some measurements carried out at B; or. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings . .
  2. Quantum mechanics is incomplete in the sense that some element of physical reality corresponding to B cannot be accounted for by quantum mechanics (that is, some extra variable is needed to account for it. )

Although originally devised as a thought experiment that would demonstrate the incompleteness of quantum mechanics, actual experimental results refute the principle of locality, invalidating the EPR trio's original purpose. Incompleteness of quantum physics is the assertion that the state of a physical system as formulated by Quantum mechanics, does not give a complete description for The Bell test experiments serve to investigate the validity of the entanglement effect in Quantum mechanics by using some kind of Bell inequality. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings The "spooky action at a distance" that so disturbed the authors of EPR consistently occurs in numerous and widely replicated experiments, though the validity of these experiments does remain in debate. Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object In Bell test experiments, there may be experimental problems that affect the validity of the experimental findings Einstein never accepted quantum mechanics as a "real" and complete theory, struggling to the end of his life for an interpretation that could comply with relativity without implying that "God plays dice. "

The EPR paradox is a paradox in the following sense: if one takes quantum mechanics and adds some seemingly reasonable (but actually wrong, or questionable as a whole) conditions (referred to as locality, realism, counter factual definiteness, and completeness; see the above-mentioned articles on the Bell inequality and Bell test experiments), then one obtains a contradiction. A physical Paradox is an apparent contradiction in physical descriptions of the Universe. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings Contemporary philosophical realism is the belief in a Reality that is completely Ontologically independent of our conceptual schemes linguistic practices beliefs In some interpretations of Quantum mechanics, Counterfactual definiteness ( CFD) is the ability to speak meaningfully about the definiteness of the results of measurements Bell's theorem is a theorem that shows that the predictions of Quantum mechanics (QM are not intuitive and touches upon fundamental philosophical issues that relate to modern The Bell test experiments serve to investigate the validity of the entanglement effect in Quantum mechanics by using some kind of Bell inequality. In Classical logic, a contradiction consists of a logical incompatibility between two or more Propositions It occurs when the propositions taken together yield However, quantum mechanics by itself does not appear to be internally inconsistent, nor — as it turns out — does it contradict relativity. As a result of further theoretical and experimental developments since the original EPR paper, most physicists today regard the EPR paradox as an illustration of how quantum mechanics violates classical intuitions.

Contents

Quantum mechanics and its interpretation

During the twentieth century, quantum theory proved to be a successful theory, which describes the physical reality of the mesoscopic and microscopic world. In Physics and Chemistry, the mesoscopic scale refers to the length scale at which one can reasonably discuss the properties of a material or phenomenon without having Up to now, no method has been found to contradict the predictions made by quantum theory. This is remarkable, since measurement accuracy has increased, and the size of the systems under consideration has decreased at a fast pace.

Quantum mechanics was developed with the aim of describing atoms and to explain the observed spectral lines in a measurement apparatus. The fact that quantum theory allows for an accurate description of reality is clear from many physical experiments and has probably never been seriously disputed. Interpretations of quantum phenomena are another story.

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 backgrounds. An interpretation of quantum mechanics is a statement which attempts to explain how Quantum mechanics informs our Understanding of Nature.

Quantum theory and quantum mechanics do not account for single measurement outcomes in a deterministic way. According to an accepted interpretation of quantum mechanics known as the Copenhagen interpretation, a measurement causes an instantaneous collapse of the wave function describing the quantum system, and the system after the collapse is random. The Copenhagen interpretation is an interpretation of Quantum mechanics. A wave function or wavefunction is a mathematical tool used in Quantum mechanics to describe any physical system

The most prominent opponent of the Copenhagen interpretation was Albert Einstein. Einstein did not believe in the idea of genuine randomness in nature, the main argument in the Copenhagen interpretation. In his view, quantum mechanics is incomplete and suggests that there had to be 'hidden' variables responsible for random measurement results. Historically in Physics, hidden variable theories were espoused by a minority of Physicists who argued that the statistical nature of Quantum mechanics

The famous paper "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?"[1], authored by Einstein, Podolsky and Rosen in 1935, condensed the philosophical discussion into a physical argument. The year 1935 in Science and Technology involved some significant events listed below They claim that given a specific experiment, in which the outcome of a measurement could be known before the measurement takes place, there must exist something in the real world, an "element of reality", which determines the measurement outcome. They postulate that these elements of reality are local, in the sense that they belong to a certain point in spacetime. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS This element may only be influenced by events which are located in the backward light cone of this point in spacetime. In Special relativity, a light cone (or null cone) is the pattern describing the temporal evolution of a flash of Light in Minkowski spacetime Even though these claims sound reasonable and convincing, they are founded on assumptions about nature which constitute what is now known as local realism. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings

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. The Institute for Advanced Study, located in Princeton New Jersey, United States is a center for theoretical research 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. "[1]

Description of the paradox

The EPR paradox draws on a phenomenon predicted by quantum mechanics, known as quantum entanglement, to show that measurements performed on spatially separated parts of a quantum system can apparently have an instantaneous influence on one another. Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object This effect is now known as "nonlocal behavior" (or colloquially as "quantum weirdness" or "spooky action at a distance"). In Physics, nonlocality is a direct influence of one object on another distant object in violation of Principle of locality. In order to illustrate this, let us consider a simplified version of the EPR thought experiment put forth by David Bohm. David Joseph Bohm ( December 20 1917, Wilkes-Barre Pennsylvania – October 27 1992, London) was an American

Measurements on an entangled state

We have a source that emits pairs of electrons, with one electron sent to destination A, where there is an observer named Alice, and another is sent to destination B, where there is an observer named Bob. The names Alice and Bob are commonly used placeholders for archetypal characters in fields such as Cryptography and Physics. The names Alice and Bob are commonly used placeholders for archetypal characters in fields such as Cryptography and Physics. According to quantum mechanics, we can arrange our source so that each emitted electron pair occupies a quantum state called a spin singlet. In Quantum physics, a quantum state is a mathematical object that fully describes a quantum system. In Theoretical physics, a singlet usually refers to a one-dimensional representation (e This can be viewed as a quantum superposition of two states, which we call state I and state II. Quantum superposition is the fundamental law of Quantum mechanics. In state I, electron A has spin pointing upward along the z-axis (+z) and electron B has spin pointing downward along the z-axis (-z). In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin In state II, electron A has spin -z and electron B has spin +z. Therefore, it is impossible to associate either electron in the spin singlet with a state of definite spin. The electrons are thus said to be entangled. Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object

The EPR thought experiment, performed with electrons. A source (center) sends electrons toward two observers, Alice (left) and Bob (right), who can perform spin measurements.
The EPR thought experiment, performed with electrons. A source (center) sends electrons toward two observers, Alice (left) and 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 quantum mechanics, the quantum state of the system collapses into state I. In certain interpretations of quantum mechanics, wave function collapse is one of two processes by which Quantum systems apparently evolve according to the laws of (Different interpretations of quantum mechanics have different ways of saying this, but the basic result is the same. An interpretation of quantum mechanics is a statement which attempts to explain how Quantum mechanics informs our Understanding of Nature. ) 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, he will obtain -z with 100% probability. Similarly, if Alice gets -z, Bob will get +z.

There is, of course, nothing special about our choice of the z-axis. For instance, suppose that Alice and Bob now decide to measure spin along the x-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. We'll call these states Ia and IIa. In state Ia, Alice's electron has spin +x and Bob's electron has spin -x. In state IIa, Alice's electron has spin -x and Bob's electron has spin +x. Therefore, if Alice measures +x, the system collapses into Ia, and Bob will get -x. If Alice measures -x, the system collapses into IIa, and Bob will get +x.

In quantum mechanics, the x-spin and z-spin are "incompatible observables", which means that there is a Heisenberg uncertainty principle operating between them: a quantum state cannot possess a definite value for both variables. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain 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. Furthermore, it is fundamentally impossible to predict which outcome will appear until Bob actually performs the measurement.

So how does Bob's electron know, at the same time, which way to point if Alice decides (based on information unavailable to Bob) to measure x and also how to point if Alice measures z? Using the usual Copenhagen interpretation rules that say the wave function "collapses" at the time of measurement, there must be action at a distance or the electron must know more than it is supposed to. To make the mixed part quantum and part classical descriptions of this experiment local, we have to say that the notebooks (and experimenters) are entangled and have linear combinations of + and – written in them, like Schrödinger's Cat. Schrödinger's cat is a Thought experiment, often described as a Paradox, devised by Austrian physicist Erwin Schrödinger in 1935

Incidentally, although we have used spin as an example, many types of physical quantities — what quantum mechanics refers to as "observables" — can be used to produce quantum entanglement. The original EPR paper used momentum for the observable. In Classical mechanics, momentum ( pl momenta SI unit kg · m/s, or equivalently N · s) is the product Experimental realizations of the EPR scenario often use photon polarization, because polarized photons are easy to prepare and measure. Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave

Reality and completeness

We will now introduce two concepts used by Einstein, Podolsky, and Rosen (EPR), which are crucial to their attack on quantum mechanics: (i) the elements of physical reality and (ii) the completeness of a physical theory.

The authors (EPR) did not directly address the philosophical meaning of an "element of physical reality". Philosophy is the study of general problems concerning matters such as existence knowledge truth beauty justice validity mind and language Instead, they made the assumption that if the value of any physical quantity of a system can be predicted with absolute certainty prior to performing a measurement or otherwise disturbing it, then that quantity corresponds to an element of physical reality. Note that the converse is not assumed to be true; there may be other ways for elements of physical reality to exist, but this will not affect the argument.

Next, EPR defined a "complete physical theory" as one in which every element of physical reality is accounted for. The aim of their paper was to show, using these two definitions, that quantum mechanics is not a complete physical theory.

Let us see how these concepts apply to the above thought experiment. Suppose Alice decides to measure the value of spin along the z-axis (we'll call this the z-spin. ) After Alice performs her measurement, the z-spin of Bob's electron is definitely known, so it is an element of physical reality. Similarly, if Alice decides to measure spin along the x-axis, the x-spin of Bob's electron is an element of physical reality after her measurement.

We have seen that a quantum state cannot possess a definite value for both x-spin and z-spin. If quantum mechanics is a complete physical theory in the sense given above, x-spin and z-spin cannot be elements of reality at the same time. This means that Alice's decision — whether to perform her measurement along the x- or z-axis — has an instantaneous effect on the elements of physical reality at Bob's location. However, this violates another principle, that of locality.

Locality in the EPR experiment

The principle of locality states 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 information can never be transmitted faster than the speed of light without violating causality. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial Information as a concept has a diversity of meanings from everyday usage to technical settings Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in It is generally believed that any theory which violates causality would also be internally inconsistent, and thus deeply unsatisfactory.

It turns out that the usual rules for combining quantum mechanical and classical descriptions violate the principle of locality without violating 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. Randomness is a lack of order Purpose, cause, or predictability Furthermore, Bob is only able to perform his measurement once: there is a fundamental property of quantum mechanics, known as the "no cloning theorem", which makes it impossible for him to make a million copies of the electron he receives, perform a spin measurement on each, and look at the statistical distribution of the results. The no cloning theorem is a result of Quantum mechanics which forbids the creation of identical copies of an arbitrary unknown quantum state 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.

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". In Physics, action at a distance is the Interaction of two objects which are separated in Space with no known mediator of the interaction The conclusion they drew was that quantum mechanics is not a complete theory.

In recent years, however, doubt has been cast on EPR's conclusion due to developments in understanding locality and especially quantum decoherence. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings In Quantum mechanics, quantum decoherence is the mechanism by which quantum systems interact with their environments to exhibit probabilistically additive behavior The word locality has several different meanings in physics. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings For example, in quantum field theory "locality" means that quantum fields at different points of space do not interact with one another. In quantum field theory (QFT the forces between particles are mediated by other particles 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. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings 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. In certain interpretations of quantum mechanics, wave function collapse is one of two processes by which Quantum systems apparently evolve according to the laws of In Quantum mechanics, quantum decoherence is the mechanism by which quantum systems interact with their environments to exhibit probabilistically additive behavior Since the underlying behaviour doesn't violate local causality, it follows that neither does the additional effect of wavefunction collapse, whether real or apparent. Therefore, as outlined in the example above, neither the EPR experiment nor any quantum experiment demonstrates that faster-than-light signaling is possible.

Resolving the paradox

Hidden variables

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. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain Such a theory is called a hidden variable theory. Historically in Physics, hidden variable theories were espoused by a minority of Physicists who argued that the statistical nature of Quantum mechanics

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 electron 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, of course, there is an (uncountably) infinite number of axes along which Alice and Bob can perform their measurements, so there has 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 very broad class of hidden variable theories (the local hidden variable theories). Bell's theorem is a theorem that shows that the predictions of Quantum mechanics (QM are not intuitive and touches upon fundamental philosophical issues that relate to modern Year 1964 ( MCMLXIV) was a Leap year starting on Wednesday (link will display full calendar of the 1964 Gregorian calendar. John Stewart Bell ( June 28 1928 &ndash October 1 1990) was a Physicist, and the originator of Bell's Theorem, one of the Roughly speaking, quantum mechanics predicts much stronger statistical correlations between the measurement results performed on different axes than the hidden variable theories. In Probability theory and Statistics, correlation, (often measured as a correlation coefficient) indicates the strength and direction of a linear These differences, expressed using inequality relations known as "Bell's inequalities", are in principle experimentally detectable. In Mathematics, an inequality is a statement about the relative size or order of two objects or about whether they are the same or not (See also equality Later work by Eberhard showed that the key properties of local hidden variable theories that lead to Bell's inequalities are locality and counter-factual definiteness. In Physics, the principle of locality is that distant objects cannot have direct influence on one another an object is influenced directly only by its immediate surroundings In some interpretations of Quantum mechanics, Counterfactual definiteness ( CFD) is the ability to speak meaningfully about the definiteness of the results of measurements Any theory in which these principles hold produces the inequalities. A. Fine subsequently showed that any theory satisfying the inequalities can be modeled by a local hidden variable theory.

After the publication of Bell's paper, a variety of experiments were devised to test Bell's inequalities. (As mentioned above, these experiments generally rely on photon polarization measurements. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations ) All the experiments conducted to date have found behavior in line with the predictions of standard quantum mechanics.

However, Bell's theorem does not apply to all possible philosophically realist theories, although a common misconception touted by new agers is that quantum mechanics is inconsistent with all notions of philosophical realism. Contemporary philosophical realism is the belief in a Reality that is completely Ontologically independent of our conceptual schemes linguistic practices beliefs Realist interpretations of quantum mechanics are possible, although as discussed above, such interpretations must reject either locality or counter-factual definiteness. Mainstream physics prefers to keep locality while still maintaining a notion of realism that nevertheless rejects counter-factual definiteness. Examples of such mainstream realist interpretations are the consistent histories interepretation and the transactional interpretation. In Quantum mechanics, the consistent histories approach is intended to give a modern Interpretation of quantum mechanics, generalising the conventional Copenhagen The transactional interpretation of Quantum mechanics ( TIQM) is an Interpretation of quantum mechanics that describes quantum interactions in terms of a Fine's work showed that taking locality as a given there exist scenarios in which two statistical variables are correlated in a manner inconistent with counter-factual definiteness and that such scenarios are no more mysterious than any other despite the inconsistency with counter-factual definiteness seeming 'counter-intuitive'. Violation of locality however is difficult to reconcile with special relativity and is thought to be incompatible with the principle of causality. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial On the other hand the Bohm interpretation of quantum mechanics instead keeps counter-factual definiteness while introducing a conjectured non-local mechanism called the 'quantum potential'. The Bohm interpretation of Quantum mechanics, sometimes called Bohmian mechanics, the ontological interpretation, or the causal interpretation 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 such theory has been produced that can reproduce all the results of quantum mechanics. In Bell test experiments, there may be experimental problems that affect the validity of the experimental findings

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. David Joseph Bohm ( December 20 1917, Wilkes-Barre Pennsylvania – October 27 1992, London) was an American

"Acceptable theories", and the experiment

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

In the EPR paper (1935) the authors realized that quantum mechanics was non-acceptable in the sense of their above-mentioned assumptions, and Einstein thought erroneously that it could simply be augmented by 'hidden variables', without any further change, to get an acceptable theory. He pursued these ideas until the end of his life (1955), i. e. over twenty years.

In contrast, John Bell, in his 1964 paper, showed "once and for all" that quantum mechanics and Einstein's assumptions lead to different results, different by a factor of \sqrt{2}, for certain correlations. John Stewart Bell ( June 28 1928 &ndash October 1 1990) was a Physicist, and the originator of Bell's Theorem, one of the So the issue of "acceptability", up to this time mainly concerning theory (even philosophy), finally became experimentally decisable.

There are many Bell test experiments hitherto, e. The Bell test experiments serve to investigate the validity of the entanglement effect in Quantum mechanics by using some kind of Bell inequality. g. those of Alain Aspect and others. Alain Aspect (born 15 June 1947 in Agen) is a French Physicist and alumnus of the École Normale Supérieure de Cachan They all show that pure quantum mechanics, and not  Einstein's "local realism", is acceptable. Thus, according to Karl Popper these experiments falsify Einstein's philosophical assumptions, especially the ideas on "hidden variables", whereas quantum mechanics itself remains a good candidate for a theory, which is acceptable in a wider context. Sir Karl Raimund Popper ( July 28 1902  &ndash September 17 1994) was an Austrian and British Philosopher and a professor

But apparently an experiment, which would also classify Bohm's non-local quasi-classical theory as non-acceptable, is still lacking.

Implications for quantum mechanics

Most physicists today believe that quantum mechanics is correct, and that the EPR paradox is only a "paradox" because classical intuitions do not correspond to physical reality. How EPR is interpreted regarding locality depends on the interpretation of quantum mechanics one uses. An interpretation of quantum mechanics is a statement which attempts to explain how Quantum mechanics informs our Understanding of Nature. In the Copenhagen interpretation, it is usually understood that instantaneous wavefunction collapse does occur. The Copenhagen interpretation is an interpretation of Quantum mechanics. However, the view that there is no causal instantaneous effect has also been proposed within the Copenhagen interpretation: in this alternate view, measurement affects our ability to define (and measure) quantities in the physical system, not the system itself. In the many-worlds interpretation, a kind of locality is preserved, since the effects of irreversible operations such as measurement arise from the relativization of a global state to a subsystem such as that of an observer. The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, parallel universes,

The EPR paradox has deepened our understanding of quantum mechanics by exposing the fundamentally non-classical characteristics of the measurement process. The measurement problem is the key set of questions that every Interpretation of quantum mechanics must address Prior to the publication of the EPR paper, a measurement was often visualized as a physical disturbance inflicted directly upon the measured system. 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 explanations, which are still encountered in popular expositions of quantum mechanics, are debunked by 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.

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. Quantum cryptography, or quantum key distribution (QKD uses Quantum mechanics to guarantee secure communication Eavesdropping is the act of surreptitiously listening to a private conversation 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. A quantum computer is a device for Computation that makes direct use of distinctively Quantum mechanical Phenomena, such as superposition Parallel computing is a form of computation in which many instructions are carried out simultaneously operating on the principle that large problems can often

Mathematical formulation

The above discussion can be expressed mathematically using the quantum mechanical formulation of spin. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin The spin degree of freedom for an electron is associated with a two-dimensional Hilbert space H, with each quantum state corresponding to a vector in that space. This article assumes some familiarity with Analytic geometry and the concept of a limit. The operators corresponding to the spin along the x, y, and z direction, denoted Sx, Sy, and Sz 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 stands for Planck's constant divided by 2π. The Pauli matrices are a set of 2 × 2 complex Hermitian and unitary matrices. The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta.

The eigenstates of Sz are represented as

\left|+z\right\rang \leftrightarrow \begin{bmatrix}1\\0\end{bmatrix}, \quad \left|-z\right\rang \leftrightarrow \begin{bmatrix}0\\1\end{bmatrix}
With qubits it looks:
|0\rang=\begin{bmatrix}1\\0\end{bmatrix},\quad |1\rang=\begin{bmatrix}0\\1\end{bmatrix}

and the eigenstates of Sx are represented as

\left|+x\right\rang \leftrightarrow \frac{1}{\sqrt{2}} \begin{bmatrix}1\\1\end{bmatrix}, \quad \left|-x\right\rang \leftrightarrow \frac{1}{\sqrt{2}} \begin{bmatrix}1\\-1\end{bmatrix}
With qubits it looks:
|+\rang={1\over\sqrt{2}}(|0\rang+|1\rang)=\frac{1}{\sqrt{2}}\begin{bmatrix}1\\1\end{bmatrix},\quad |-\rang={1\over\sqrt{2}}(|0\rang-|1\rang)=\frac{1}{\sqrt{2}}\begin{bmatrix}1\\-1\end{bmatrix}

The Hilbert space of the electron pair is H \otimes H , the tensor product of the two electrons' Hilbert spaces. In Mathematics, given a Linear transformation, an of that linear transformation is a nonzero vector which when that transformation is applied to it changes A qubit is not to be confused with a Cubit, which is an ancient measure of length A qubit is not to be confused with a Cubit, which is an ancient measure of length In Mathematics, the tensor product, denoted by \otimes may be applied in different contexts to vectors matrices, Tensors Vector The spin singlet state is

\left|\psi\right\rang = \frac{1}{\sqrt{2}} \bigg(\left|+z\right\rang \otimes \left|-z\right\rang - \left|-z\right\rang \otimes \left|+z\right\rang \bigg)
With qubits it looks:
|\psi\rang=\frac{1}{\sqrt{2}}(|01\rang-|10\rang)

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

\left|\psi\right\rang = \frac{1}{\sqrt{2}} \bigg(\left|+ -\right\rang - \left|- +\right\rang \bigg)
With qubits it looks:
\left|\psi\right\rang = \frac{1}{\sqrt{2}} \bigg(\left|+ -\right\rang - \left|- +\right\rang \bigg)=\frac{1}{\sqrt{2}}\bigg({1\over\sqrt{2}}(|0\rang+|1\rang){1\over\sqrt{2}}(|0\rang-|1\rang)-{1\over 2}(|0\rang-|1\rang)(|0\rang+|1\rang)\bigg)=

=\frac{1}{\sqrt{2}}({1\over 2}(|00\rang-|01\rang+|10\rang-|11\rang)-{1\over 2}(|00\rang+|01\rang-|10\rang-|11\rang))=\frac{1}{\sqrt{2}}(|10\rang-|01\rang)

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

\left|\psi\right\rang = \frac{-1}{\sqrt{2}} \bigg(\left|+x\right\rang \otimes \left|-x\right\rang - \left|-x\right\rang \otimes \left|+x\right\rang \bigg)
With qubits it looks:
|\psi\rang=-\frac{1}{\sqrt{2}}\bigg(\left|+\right\rang \otimes \left|-\right\rang - \left|-\right\rang \otimes \left|+\right\rang \bigg)=\frac{-1}{\sqrt{2}}(|10\rang-|01\rang)=\frac{1}{\sqrt{2}}(|01\rang-|10\rang)

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 Sz (or Sx), Bob's value of Sz (or Sx) is uniquely determined, and therefore corresponds to an "element of physical reality". This follows from the principles of measurement in quantum mechanics. When Sz is measured, the system state ψ collapses into an eigenvector of Sz. 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 H
With qubits it looks:
\left| 0 \right\rangle \otimes \left| \phi\right\rangle \quad \phi \in H

For the spin singlet, the new state is

\left| +z \right\rangle \otimes \left| -z \right\rangle.
With qubits it looks:
\left| 0 \right\rangle \otimes \left| 1 \right\rangle.

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

\left| -z \right\rangle \otimes \left| \phi\right\rangle \quad \phi \in H
With qubits it looks:
\left| 1 \right\rangle \otimes \left| \phi\right\rangle \quad \phi \in H

which means that the new state is

\left|-z\right\rangle \otimes \left|+z\right\rangle
With qubits it looks:
\left|1\right\rangle \otimes \left|0\right\rangle

This implies that the measurement for Sz for Bob's electron is now determined. It will be -z in the first case or +z in the second case.

It remains only to show that Sx and Sz 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. In Mathematics, given a Linear transformation, an of that linear transformation is a nonzero vector which when that transformation is applied to it changes More generally, one may use the fact that the operators do not commute,

\left[ S_x, S_z\right] = - i\hbar S_y \ne 0

along with the Heisenberg uncertainty relation

\lang (\Delta S_x)^2 \rang \lang (\Delta S_z)^2 \rang \ge \frac{1}{4} \left|\lang \left[ S_x, S_z\right] \rang \right|^2

See also

References

Selected papers

Notes

  1. ^ Quoted in Kaiser, David. "Bringing the human actors back on stage: the personal context of the Einstein-Bohr debate," British Journal for the History of Science 27 (1994): 129-152, on page 147.

Books

External links

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