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Encyclopedia > Copenhagen interpretation
Quantum physics
Delta x , Delta p ge frac{hbar}{2}
Quantum mechanics

Introduction to...
Mathematical formulation of... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... Quantum mechanics (QM, or quantum theory) is a physical science dealing with the behaviour of matter and energy on the scale of atoms and subatomic particles / waves. ... The mathematical formulation of quantum mechanics is the body of mathematical formalisms which permits a rigorous description of quantum mechanics. ...

Fundamental concepts

Decoherence · Interference
Uncertainty · Exclusion
Transformation theory
Ehrenfest theorem · Measurement
Superposition · Entanglement In quantum mechanics, quantum decoherence is the mechanism by which quantum systems interact with their environments to exhibit probabilistically additive behavior - a feature of classical physics - and give the appearance of wavefunction collapse. ... For other uses, see Interference (disambiguation). ... In quantum physics, the outcome of even an ideal measurement of a system is not deterministic, but instead is characterized by a probability distribution, and the larger the associated standard deviation is, the more uncertain we might say that that characteristic is for the system. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ... The term transformation theory refers to a procedure used by P. A. M. Dirac in his early formulation of quantum theory, from around 1927. ... The Ehrenfest theorem, named after Paul Ehrenfest, relates the time derivative of the expectation value for a quantum mechanical operator to the commutator of that operator with the Hamiltonian of the system. ... The framework of quantum mechanics requires a careful definition of measurement, and a thorough discussion of its practical and philosophical implications. ... Quantum superposition is the application of the superposition principle to quantum mechanics. ... It has been suggested that Quantum coherence be merged into this article or section. ...

Experiments

Double-slit experiment
Davisson-Germer experiment
Stern–Gerlach experiment
Bell's inequality experiment
Popper's experiment
Schrödinger's cat Slit experiment redirects here. ... In 1927 at Bell Labs, Clinton Davisson and Lester Germer fired slow moving electrons at a crystalline Nickel target. ... In quantum mechanics, the Stern–Gerlach experiment, named after Otto Stern and Walther Gerlach, is a celebrated experiment in 1920 on deflection of particles, often used to illustrate basic principles of quantum mechanics. ... In quantum mechanics, Bells Theorem states that a Bell inequality must be obeyed under any local hidden variable theory but can in certain circumstances be violated under quantum mechanics (QM). ... Poppers experiment is an experiment proposed by the 20th century philosopher of science Karl Popper, to test the standard interpretation (the Copenhagen interpretation) of Quantum mechanics. ... Schrödingers Cat: When the nucleus (bottom left) decays, the Geiger counter (bottom centre) may sense it and trigger the release of the gas. ...

Equations

Schrödinger equation
Pauli equation
Klein-Gordon equation
Dirac equation For a non-technical introduction to the topic, please see Introduction to quantum mechanics. ... The Pauli equation is a Schrödinger equation which handles spin. ... The Klein-Gordon equation (Klein-Fock-Gordon equation or sometimes Klein-Gordon-Fock equation) is the relativistic version of the Schrödinger equation. ... In physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928 and provides a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity. ...

Advanced theories

Quantum field theory
Wightman axioms
Quantum electrodynamics
Quantum chromodynamics
Quantum gravity
Feynman diagram Quantum field theory (QFT) is the quantum theory of fields. ... In physics the Wightman axioms are an attempt of mathematically stringent, axiomatic formulation of quantum field theory. ... Quantum electrodynamics (QED) is a relativistic quantum field theory of electrodynamics. ... Quantum chromodynamics (abbreviated as QCD) is the theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons found in hadrons (such as the proton, neutron or pion). ... Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. ... In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ...

Interpretations

Copenhagen · Ensemble
Hidden variables · Transactional
Many-worlds · Consistent histories
Quantum logic
Consciousness causes collapse It has been suggested that Quantum mechanics, philosophy and controversy be merged into this article or section. ... The Ensemble Interpretation, or Statistical Interpretation of Quantum Mechanics, is an interpretation that can be viewed as a minimalist interpretation. ... In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ... The transactional interpretation of quantum mechanics (TIQM) by Professor John Cramer is an unusual interpretation of quantum mechanics that describes quantum interactions in terms of a standing wave formed by retarded (forward in time) and advanced (backward in time) waves. ... The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, many-universes interpretation, Oxford interpretation or many worlds), is an interpretation of quantum mechanics that claims to resolve all the paradoxes of quantum theory by allowing every possible outcome to every event to... In quantum mechanics, the consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ... In mathematical physics and quantum mechanics, quantum logic can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. ... Consciousness causes collapse is the name given to the claim that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. ...

Scientists

Planck · Schrödinger
Heisenberg · Bohr · Pauli
Dirac · Bohm · Born
de Broglie · von Neumann
Einstein · Feynman
Everett · Penrose · Others “Planck” redirects here. ... Schrödinger in 1933, when he was awarded the Nobel Prize in Physics Bust of Schrödinger, in the courtyard arcade of the main building, University of Vienna, Austria. ... Werner Karl Heisenberg (December 5, 1901 – February 1, 1976) was a celebrated German physicist and Nobel laureate, one of the founders of quantum mechanics and acknowledged to be one of the most important physicists of the twentieth century. ... Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ... This article is about the Austrian-Swiss physicist. ... Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪræk]) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. ... David Bohm. ... Max Born (December 11, 1882 in Breslau – January 5, 1970 in Göttingen) was a mathematician and physicist. ... Louis-Victor-Pierre-Raymond, 7th duc de Broglie, generally known as Louis de Broglie (August 15, 1892–March 19, 1987), was a French physicist and Nobel Prize laureate. ... For other persons named John Neumann, see John Neumann (disambiguation). ... “Einstein” redirects here. ... This article is about the physicist. ... Hugh Everett III (November 11, 1930 – July 19, 1982) was an American physicist who first proposed the many-worlds interpretation(MWI) of quantum physics, which he called his relative state formulation. ... Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the Mathematical Institute, University of Oxford and Emeritus Fellow of Wadham College. ... Below is a list of famous physicists. ...

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Early twentieth century studies of the physics of very small-scale phenomena led to the Copenhagen interpretation. The new experiments led to the discovery of phenomena that could not be predicted on the basis of classical physics, and to new empirical generalizations (theories) that described and predicted very accurately those micro-scale phenomena so recently discovered. These generalizations, these models of the real world being observed at this micro scale, could not easily be squared with the way objects are observed to behave on the macro scale of everyday life. The predictions they offered often appeared counter-intuitive to observers. Indeed, they touched off much consternation -- even in the minds of their discoverers. "What can we make of these uncanny experimental results?" The Copenhagen interpretation consists of attempts to explain the experiments and their mathematical formulations in ways that do not go beyond the evidence to suggest more (or less) than is actually there. Classical physics is physics based on principles developed before the rise of quantum theory, usually including the special theory of relativity and general theory of relativity. ...


The work of relating the remarkable experiments and the abstract mathematical and theoretical formulations that constitute quantum physics to the experience that all of us share in the world of everyday life fell first to Niels Bohr and Werner Heisenberg in the course of their collaboration in Copenhagen around 1927. Bohr and Heisenberg had stepped beyond the world of empirical experiments, pragmatic predictions of such phenomena as the frequencies of light emitted under various conditions and the observation that a discrete quantities of energy must be postulated in order to avoid the paradoxes to which classical physics inevitably led when it was pushed to extremes, and found a new world of quantums of energy, entities that fit neither the classical ideas of particles nor the classical ideas of waves, elementary particles that behaved in ways highly regular when many similar interactions were analyed yet highly unpredictable when one tried to predict things like individual trajectories through a simple physical apparatus. Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ... Werner Karl Heisenberg (December 5, 1901 – February 1, 1976) was a celebrated German physicist and Nobel laureate, one of the founders of quantum mechanics and acknowledged to be one of the most important physicists of the twentieth century. ... For other uses, see Copenhagen (disambiguation). ... Year 1927 (MCMXXVII) was a common year starting on Saturday (link will display full calendar) of the Gregorian calendar. ...


Not only did laboratory experiments disclose the fact, but the new theories predicted the consequences that elementary particles are neither wave nor particle, that knowing the position of a particle prevents us from knowing its direction and velocity (and vice-versa), that the very fact of observing whether a small object such as a photon or electron passes through an apparatus by one path or another can change the end result of the experiment when that small entity reaches a detection screen. The results of their own burgeoning understanding disoriented Bohr and Heisenberg. And their results seemed to some people to indicate, for instance, that the fact that a human being had observed some event changed the reality of the event.


The Copenhagen interpretion was a composite statement about what could and could not be legitimately stated in common language to complement the statements and predictions that could be made in the language of instrument readings and mathematical operations. In other words, it attempted to answer the question, "What do these amazing experimental results really mean?"

Contents

Overview

There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.[1]

There is no definitive statement of the Copenhagen Interpretation[2] since it consists of the views developed by a number of scientists and philosophers at the turn of the 20th Century. Thus, there are a number of ideas that have been associated with the Copenhagen interpretation. Asher Peres remarked that very different, sometimes opposite, views are presented as the Copenhagen interpretation by different authors.[3] Asher Peres (born 1934 and died January 1, 2005) was an Israeli physicist, considered a pioneer in quantum information theory. ...


Principles

  1. A system is completely described by a wave function ψ, which represents an observer's knowledge of the system. (Heisenberg)
  2. The description of nature is essentially probabilistic. The probability of an event is related to the square of the amplitude of the wave function related to it. (Max Born)
  3. Heisenberg's uncertainty principle states the observed fact that it is not possible to know the values of all of the properties of the system at the same time; those properties that are not known with precision must be described by probabilities.
  4. (Complementary Principle) Matter exhibits a wave-particle duality. An experiment can show the particle-like properties of matter, or wave-like properties, but not both at the same time.(Niels Bohr)
  5. Measuring devices are essentially classical devices, and measure classical properties such as position and momentum.
  6. The Correspondence Principle of Bohr and Heisenberg: the quantum mechanical description of large systems should closely approximate to the classical description.

Werner Heisenberg Werner Karl Heisenberg (December 5, 1901 – February 1, 1976) was a celebrated German physicist and Nobel laureate, one of the founders of quantum mechanics. ... Max Born (December 11, 1882 in Breslau – January 5, 1970 in Göttingen) was a mathematician and physicist. ... In quantum physics, the Heisenberg uncertainty principle, sometimes called the Heisenberg indeterminacy principle, expresses a limitation on accuracy of (nearly) simultaneous measurement of observables such as the position and the momentum of a particle. ... Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ... In physics, the correspondence principle is a principle, first invoked by Niels Bohr in 1923, which states that the behavior of quantum mechanical systems reduce to classical physics in the limit of large quantum numbers. ...

The meaning of the wave function

The Copenhagen Interpretation denies that any wave function is anything more than an abstraction, or is at least non-committal about its being a discrete entity or a discernible component of some discrete entity.


There are some who say that there are objective variants of the Copenhagen Interpretation that allow for a "real" wave function, but it is questionable whether that view is really consistent with positivism and/or with some of Bohr's statements. Niels Bohr emphasized that science is concerned with predictions of the outcomes of experiments, and that any additional propositions offered are not scientific but rather meta-physical. Bohr was heavily influenced by positivism. On the other hand, Bohr and Heisenberg were not in complete agreement, and held different views at different times. Heisenberg in particular was prompted to move towards realism.[4] Positivism is a philosophy that states that the only authentic knowledge is scientific knowledge, and that such knowledge can only come from positive affirmation of theories through strict scientific method. ... Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ... Contemporary philosophical realism, also referred to as metaphysical realism, is the belief in a reality that is completely ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc. ...


Even if the wave function is not regarded as real, there is still a divide between those who treat it as definitely and entirely subjective, and those who are non-committal or agnostic about the subject.


An example of the agnostic view is given by von Weiszacker, who, while participating in a colloquium at Cambridge, denied that the Copenhagen interpretation asserted: "What cannot be observed does not exist". He suggested instead that the Copenhagen interpretation follows the principle: "What is observed certainly exists; about what is not observed we are still free to make suitable assumptions. We use that freedom to avoid paradoxes."[5]


The subjective view, that the wave function is merely a mathematical tool for calculating probabilities of specific experiment, is a similar approach to the Ensemble interpretation. The Ensemble Interpretation, or Statistical Interpretation of Quantum Mechanics, is an interpretation that can be viewed as a minimalist interpretation. ...


The nature of collapse

All versions of the Copenhagen interpretation include at least a formal or methodological version of wave function collapse,[6] in which unobserved eigenvalues are removed from further consideration. (In other words, Copenhagenists have never rejected collapse, even in the early days of quantum physics, in the way that adherents of the Many-worlds interpretation do.) In more prosaic terms, those who hold to the Copenhagen understanding are willing to say that a wave function involves the various probabilities that a given event will proceed to certain different outcomes. But when one or another of those more- or less-likely outcomes becomes manifest the other probabilities cease to have any function in the real world. So if an electron passes through a double slit apparatus there are various probabilities for where on the detection screen that individual electron will hit. But once it has hit, there is no longer any probability whatsoever that it will hit somewhere else. Many-worlds interpretations say that an electron hits wherever there is a possibility that it might hit, and that each of these hits occurs in a separate universe. In quantum mechanics, quantum decoherence is the process by which quantum systems in complex environments exhibit classical behavior. ... In linear algebra, a scalar λ is called an eigenvalue (in some older texts, a characteristic value) of a linear mapping A if there exists a nonzero vector x such that Ax=λx. ... The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, many-universes interpretation, Oxford interpretation or many worlds), is an interpretation of quantum mechanics that claims to resolve all the paradoxes of quantum theory by allowing every possible outcome to every event to... Slit experiment redirects here. ...


An adherent of the subjective view, that the wave function represents nothing but knowledge, would take an equally subjective view of "collapse", understanding it as nothing more than an observer becoming informed about something that was previously not known.


Some argue that the concept of collapse of a "real" wave function was introduced by John Von Neumann and was not part of the original formulation of the Copenhagen Interpretation.[7] For other persons named John Neumann, see John Neumann (disambiguation). ...


Acceptance among physicists

According to a poll at a Quantum Mechanics workshop in 1997, the Copenhagen interpretation is the most widely-accepted specific interpretation of quantum mechanics, followed by the many-worlds interpretation.[8] Although current trends show substantial competition from alternative interpretations, throughout much of the twentieth century the Copenhagen interpretation had strong acceptance among physicists. Astrophysicist and science writer John Gribbin describes it as having fallen from primacy after the 1980s.[9] For the band, see 1997 (band). ... The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, many-universes interpretation, Oxford interpretation or many worlds), is an interpretation of quantum mechanics that claims to resolve all the paradoxes of quantum theory by allowing every possible outcome to every event to... It has been suggested that Quantum mechanics, philosophy and controversy be merged into this article or section. ... Dr. John Gribbin (1946 - ) is a British science writer and a visiting Fellow in astronomy at the University of Sussex. ...


Consequences

The nature of the Copenhagen Interpretation is exposed by considering a number of experiments and paradoxes.


1. Schrödinger's Cat - A cat is put in a box with a radioactive source and a radiation detector. There is a 50-50 chance that a particle will be emitted and detected by the detector. If a particle is detected, a poisonous gas will be released and the cat killed. Schrödinger set this up as what he called a "ridiculous case" in which "The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts." He resisted an interpretation that would "so naively accepting as valid a 'blurred model' for representing reality."[10] How can the cat be both alive and dead? Schrödingers Cat: When the nucleus (bottom left) decays, the Geiger counter (bottom centre) may sense it and trigger the release of the gas. ...


The Copenhagen Interpretation: The wave function reflects our knowledge of the system. The wave function (|deadrangle + |aliverangle)/sqrt 2 simply means that there is a 50-50 chance that the cat is alive or dead.


2. Wigner's Friend - Wigner puts his friend in with the cat. The external observer believes the system is in the state (|deadrangle + |aliverangle)/sqrt 2. His friend however is convinced that cat is alive. I.e. for him, the cat is in the state | alive > . How can Wigner and his friend see different wave functions? Wigners friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödingers cat experiment designed as a point of departure for discussing the mind-body problem as viewed by the Copenhagen interpretation of quantum mechanics. ...


The Copenhagen Interpretation: Wigner's friend highlights the subjective nature of probability. Each observer (Wigner and his friend) have different information and therefore different wave functions. The distinction between the "objective" nature of reality and the subjective nature of probability has led to a great deal of controversy. C.f. Bayesian versus Frequentist interpretations of probability. Bayesian probability is an interpretation of probability suggested by Bayesian theory, which holds that the concept of probability can be defined as the degree to which a person believes a proposition. ... Statistical regularity has motivated the development of the relative frequency concept of probability. ...


3. Double Slit Diffraction - Light passes through double slits and onto a screen resulting in a diffraction pattern. Is light a particle or a wave? Slit experiment redirects here. ... The intensity pattern formed on a screen by diffraction from a square aperture Diffraction refers to various phenomena associated with wave propagation, such as the bending, spreading and interference of waves passing by an object or aperture that disrupts the wave. ...


The Copenhagen Interpretation: Light is neither. A particular experiment can demonstrate particle (photon) or wave properties, but not both at the same time (Bohr's Complementary Principle).


The same experiment can in theory be performed with electrons, protons, atoms, molecules, viruses, bacteria, cats, humans, elephants and planets. In practice it has been performed for light, electrons, buckminsterfullerene, and some atoms. Matter in general exhibits both particle and wave behaviors. Buckminsterfullerene (C60) Fullerenes are molecules composed entirely of carbon, taking the form of a hollow sphere, ellipsoid, or tube. ...


4. EPR paradox. Entangled "particles" are emitted in a single event. Conservation laws ensure that the measured spin of one particle must be the opposite of the measured spin of the other, so that if the spin of one particle is measured, the spin of the other particle is now instantaneously known. The most discomfiting aspect of this paradox is that the effect is instantaneous so that something that happens in one galaxy could cause an instantaneous change in another galaxy. 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. ...


The Copenhagen Interpretation: Assuming wave functions are not real, wave function collapse is interpreted subjectively. The moment one observer measures the spin of one particle, he knows the spin of the other. However another observer cannot benefit until the results of that measurement have been relayed to him, at less than or equal to the speed of light.


Copenhagenists claim that interpretations of quantum mechanics where the wave function is regarded as real have problems with EPR-type effects, since they imply that the laws of physics allow for influences to propagate at speeds greater than the speed of light. However, proponents of Many worlds [11] and the Transactional interpretation [12] [13] maintain that their theories are fatally non-local. This article may be too technical for most readers to understand. ... The transactional interpretation of quantum mechanics (TIQM) by Professor John Cramer is an unusual interpretation of quantum mechanics that describes quantum interactions in terms of a standing wave formed by retarded (forward in time) and advanced (backward in time) waves. ...


The claim that EPR effects violate the principle that information cannot travel faster than the speed of light can be avoided by noting that they cannot be used for signaling because neither observer can control, or predetermine, what he observes, and therefore cannot manipulate what the other observer measures. Relativistic difficulties about establishing which measurement occurred first also undermine the idea that one observer is causing what the other is measuring.


In fact, it can be argued that the subjective variant of the Copenhagen interpretation, where the wave function is taken to represent only the observer's knowledge of the situation, is severely undermined by these effects. Suppose that the spins of a pair of entangled particles are fixed (but unknown) at the time of emission. Then the subjective interpretation is straightforward: the superposition of the wave function just represents lack of information on the part of the observers, and collapse represents their becoming aware. But this is a hidden variable approach; The information is ontologically present, but unknown. Moreover, it is a local hidden-variable approach, since the information is fixed at a precise space-time point, the event that creates the entangled pair. But such local hidden variables are ruled out by the Aspect experiment. Perhaps there are non-local hidden variables? But such non-local, yet ontologically real, phenomena are just what the subjective Copenhagenism is trying to rule out. A subjective approach is left with the claim that the observers have subjective knowledge that isn't knowledge of anything. Yet the knowledge of the observers is still correlated. It is just that without either an ontologically real wave function, or local hidden variables, the correlation cannot be explained. [citation needed] In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ... The term locality has different meanings in different disciplines: Geography In geography, a locality is a place. ... Bell test experiments serve to investigate the validity of the entanglement effect in quantum mechanics by using some kind of Bell inequality. ...


Criticisms

The completeness of quantum mechanics (thesis 1) was attacked by the Einstein-Podolsky-Rosen thought experiment which was intended to show that quantum physics could not be a complete theory. 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. ...


Experimental tests of Bell's inequality using particles have supported the quantum mechanical prediction of entanglement. In quantum mechanics, Bells Theorem states that a Bell inequality must be obeyed under any local hidden variable theory but can in certain circumstances be violated under quantum mechanics (QM). ... Bells theorem is the most famous legacy of the late John Bell. ...


The Copenhagen Interpretation gives special status to measurement processes without clearly defining them or explaining their peculiar effects. In his article entitled "Criticism and Counterproposals to the Copenhagen Interpretation of Quantum Theory," countering the view of Alexandrov that (in Heisenberg's paraphrase) "the wave function in configuration space characterizes the objective state of the electron." Heisenberg says,

Of course the introduction of the observer must not be misunderstood to imply that some kind of subjective features are to be brought into the description of nature. The observer has, rather, only the function of registering decisions, i.e., processes in space and time, and it does not matter whether the observer is an apparatus or a human being; but the registration, i.e., the transition from the "possible" to the "actual," is absolutely necessary here and cannot be omitted from the interpretation of quantum theory.

-- Heisenberg, Physics and Philosophy, p. 137

Many physicists and philosophers have objected to the Copenhagen interpretation, both on the grounds that it is non-deterministic and that it includes an undefined measurement process that converts probability functions into non-probabilistic measurements. Einstein's comments "I, at any rate, am convinced that He (God) does not throw dice."[14] and "Do you really think the moon isn't there if you aren't looking at it?" exemplify this. Bohr, in response, said "Einstein, don't tell God what to do". Not to be confused with physician, a person who practices medicine. ... A philosopher is a person who thinks deeply regarding people, society, the world, and/or the universe. ... “Einstein” redirects here. ...


Steven Weinberg in "Einstein's Mistakes", Physics Today, November 2005, page 31, said: Steven Weinberg (born May 3, 1933) is an American physicist. ... Physics Today magazine, created in 1948, is the flagship publication of The American Institute of Physics. ...

All this familiar story is true, but it leaves out an irony. Bohr's version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wave function (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?
Considerable progress has been made in recent years toward the resolution of the problem, which I cannot go into here. It is enough to say that neither Bohr nor Einstein had focused on the real problem with quantum mechanics. The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wave function, the Schrödinger equation, to observers and their apparatus.

The problem of thinking in terms of classical measurements of a quantum system becomes particularly acute in the field of quantum cosmology, where the quantum system is the universe.[15] Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) order + λογια (logia) discourse) is the study of the Universe in its totality, and by extension, humanitys place in it. ...


Alternatives

The Ensemble Interpretation is similar; it offers an interpretation of the wave function, but not for single particles. The consistent histories interpretation advertises itself as "Copenhagen done right". Consciousness causes collapse is often confused with the Copenhagen interpretation. The Ensemble Interpretation, or Statistical Interpretation of Quantum Mechanics, is an interpretation that can be viewed as a minimalist interpretation. ... In quantum mechanics, the consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ... Consciousness causes collapse is the name given to the claim that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. ...


If the wave function is regarded as ontologically real, and collapse is entirely rejected, a many worlds theory results. If wave function collapse is regarded as ontologically real as well, an objective collapse theory is obtained. Dropping the principle that the wave function is a complete description results in a hidden variable theory. This article may be too technical for most readers to understand. ... Objective collapse theories are an approach to the interpretational problems of quantum mechanics. ... In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ...


Many physicists have subscribed to the null interpretation of quantum mechanics summarized by the sentence "Shut up and calculate!". While it is sometimes attributed to Paul Dirac[16] or Richard Feynman, it's probably a misquotation[17]. Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪræk]) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. ... This article is about the physicist. ... A misquotation is an accidental or intentional misrepresentation of a persons speech or writing, involving one or more of: Omission of important context: The context can be important for determining the overall argument the quoted person wanted to make, for seeing whether the quoted statement was restricted or even...


A list of alternatives can be found at Interpretation of quantum mechanics. It has been suggested that Quantum mechanics, philosophy and controversy be merged into this article or section. ...


Notes

  1. ^ Aage Petersen paraphrasing Niels Bohr, Quantum Reality by Nick Herbert
  2. ^ In fact Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics, and none of them ever used the term “the Copenhagen interpretation” as a joint name for their ideas. Bohr once distanced himself from what he considered to be Heisenberg's more subjective interpretation Stanford Encyclopedia of Philosophy
  3. ^ "There seems to be at least as many different Copenhagen interpretations as people who use that term, probably there are more. For example, in two classic articles on the foundations of quantum mechanics, Ballentine (1970) and Stapp(1972) give diametrically opposite definitions of “Copenhagen.”", A. Peres, Popper's experiment and the Copenhagen interpretation, Stud. History Philos. Modern Physics 33 (2002) 23, preprint
  4. ^ "Historically, Heisenberg wanted to base quantum theory solely on observable quantities such as the intensity of spectral lines, getting rid of all intuitive (anschauliche) concepts such as particle trajectories in space-time [2]. This attitude changed drastically with his paper [3] in which he introduced the uncertainty relations – there he put forward the point of view that it is the theory which decides what can be observed. His move from positivism to operationalism can be clearly understood as a reaction on the advent of Schr¨odinger’s wave mechanics [1] which, in particular due to its intuitiveness, became soon very popular among physicists. In fact, the word anschaulich (intuitive) is contained in the title of Heisenberg’s paper [3]."Kiefer, C. On the interpretation of quantum theory – from Copenhagen to the present day
  5. ^ John Cramer on the Copenhagen Interpretation
  6. ^ "To summarise, one can identify the following ingredients as being characteristic for the Copenhagen interpretation(s)[...]Reduction of the wave packet as a formal rule without dynamical significance"Kiefer, C. On the interpretation of quantum theory – from Copenhagen to the present day
  7. ^ "the “collapse” or “reduction” of the wave function. This was introduced by Heisenberg in his uncertainty paper [3] and later postulated by von Neumann as a dynamical process independent of the Schrodinger equation"Kiefer, C. On the interpretation of quantum theory – from Copenhagen to the present day
  8. ^ The Many Worlds Interpretation of Quantum Mechanics
  9. ^ Gribbin, J. Q for Quantum
  10. ^ Erwin Schrödinger, in an article in the Proceedings of the American Philosophical Society, 124, 323-38.
  11. ^ Michael price on nonlocality in Many Worlds
  12. ^ Relativity and Causality in the Transactional Interpretation
  13. ^ Collapse and Nonlocality in the Transactional Interpretation
  14. ^ "God does not throw dice" quote
  15. ^ 'Since the Universe naturally contains all of its observers, the problem arises to come up with an interpretation of quantum theory that contains no classical realms on the fundamental level.'Kiefer, C. On the interpretation of quantum theory from Copenhagen to the present day
  16. ^ http://home.fnal.gov/~skands/slides/A-Quantum-Journey.ppt
  17. ^ "Shut up and calculate" quote.

Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ...

See also

In physics, and more specifically, quantum mechanics, the Afshar experiment is an optical experiment, devised by Shahriar S. Afshar in 2004, that its proponents claim disproves Niels Bohrs principle of complementarity. ... Niels Bohr with Albert Einstein at Paul Ehrenfests home in Leiden (December 1925) The Bohr-Einstein debates is a popular name given to what was actually a series of epistemological challenges presented by Albert Einstein against what has come to be called the standard or Copenhagen interpretation of quantum... Consciousness causes collapse is the name given to the claim that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. ... In quantum mechanics, the consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ... The Ensemble Interpretation, or Statistical Interpretation of Quantum Mechanics, is an interpretation that can be viewed as a minimalist interpretation. ... It has been suggested that Quantum mechanics, philosophy and controversy be merged into this article or section. ... Classical Newtonian physics has, formally, been replaced by quantum mechanics on the small scale and relativity on the large scale. ... Poppers experiment is an experiment proposed by the 20th century philosopher of science Karl Popper, to test the standard interpretation (the Copenhagen interpretation) of Quantum mechanics. ...

Further reading

  • G. Weihs et al., Phys. Rev. Lett. 81 (1998) 5039
  • M. Rowe et al., Nature 409 (2001) 791.
  • J.A. Wheeler & W.H. Zurek (eds) , Quantum Theory and Measurement, Princeton University Press 1983
  • A. Petersen, Quantum Physics and the Philosophical Tradition, MIT Press 1968
  • H. Margeneau, The Nature of Physical Reality, McGraw-Hill 1950
  • M. Chown, Forever Quantum, New Scientist No. 2595 (2007) 37.

Video Demonstration

External links


  Results from FactBites:
 
Copenhagen interpretation - Wikipedia, the free encyclopedia (1233 words)
Bohr and Heisenberg extended the probabilistic interpretation of the wavefunction, proposed by Max Born.
Their interpretation attempts to answer some perplexing questions which arise as a result of the wave-particle duality in quantum mechanics, such as the measurement problem.
The original formulation of the Copenhagen Interpretation has led to several variants; one of these is based on Consistent Histories and the concept of quantum decoherence that allows us to calculate the fuzzy boundary between the "microscopic" and the "macroscopic" world.
  More results at FactBites »

 
 

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