The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics.[1] It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta, entities which fit neither the classical idea of particles nor the classical idea of waves. According to the interpretation, the act of measurement causes the set of probabilities to immediately and randomly assume only one of the possible values. This feature of the mathematics is known as wavefunction collapse. The essential concepts of the interpretation were devised by Niels Bohr, Werner Heisenberg and others in the years 1924–27.


Classical physics draws a distinction between particles and energy, holding that only the latter exhibit waveform characteristics, whereas quantum mechanics is based on the observation that matter has both wave and particle aspects and postulates that the state of every subatomic particle can be described by a wavefunction—a mathematical representation used to calculate the probability that the particle, if measured, will be in a given location or state of motion.

In the early work of Max Planck, Albert Einstein and Niels Bohr, the existence of energy in discrete quantities had been postulated, in order to avoid certain paradoxes that arise when classical physics is pushed to extremes. Also, while elementary particles showed predictable properties in many experiments, they became highly unpredictable in certain contexts, for example, if one attempted to measure their individual trajectories through a simple physical apparatus.

The Copenhagen interpretation is an attempt to explain the mathematical formulations of quantum mechanics and the corresponding experimental results. Early twentieth-century experiments on the physics of very small-scale phenomena led to the discovery of phenomena which could not be predicted on the basis of classical physics, and to the development of new models (theories) that described and predicted very accurately these micro-scale phenomena. These models could not easily be reconciled with the way objects are observed to behave on the macro scale of everyday life. The predictions they offered often appeared counter-intuitive and caused much consternation among the physicists—often including their discoverers.
Origin of the term

Werner Heisenberg had been an assistant to Niels Bohr at his institute in Copenhagen during part of the 1920s, when they helped originate quantum mechanical theory. In 1929, Heisenberg gave a series of invited lectures at the University of Chicago, explaining the new field of quantum mechanics. The lectures then served as the basis for his textbook, The Physical Principles of the Quantum Theory, published in 1930.[2] In the book's preface, Heisenberg wrote:

On the whole the book contains nothing that is not to be found in previous publications, particularly in the investigations of Bohr. The purpose of the book seems to me to be fulfilled if it contributes somewhat to the diffusion of that 'Kopenhagener Geist der Quantentheorie' [i.e., Copenhagen spirit of quantum theory] if I may so express myself, which has directed the entire development of modern atomic physics.

The expression 'The Copenhagen Interpretation' suggests something that is rather more than just a spirit, such as some definite set of rules for interpreting the mathematical formalism of quantum mechanics, presumably dating back to the 20s. However, no such text exists, apart from some informal popular lectures by Bohr and Heisenberg, which contradict each other on several important issues. It appears that the particular term with its more definite sense was coined by Heisenberg in the 1950s,[3] while criticizing alternate "interpretations" (e.g., David Bohm's[4]) that had been developed.[5] Lectures with the titles 'The Copenhagen Interpretation of Quantum Theory' and 'Criticisms and Counterproposals to the Copenhagen Interpretation', that Heisenberg delivered in 1955, are reprinted in the collection Physics and Philosophy.[6]

Because it consists of the views developed by a number of scientists and philosophers during the second quarter of the 20th Century, there is no definitive statement of the Copenhagen Interpretation.[7] Thus, various ideas have been associated with it; Asher Peres remarked that very different, sometimes opposite, views are presented as "the Copenhagen interpretation" by different authors.[8] Nonetheless, there are several basic principles that are generally accepted as being part of the interpretation:

A system is completely described by a wave function \psi, representing the state of the system, which grows gradually with time but, upon measurement, collapses suddenly to its original size.
The description of nature is essentially probabilistic, with the probability of an event related to the square of the amplitude of the wave function related to it. (The Born rule, after Max Born)
It is not possible to know the value of all the properties of the system at the same time; those properties that are not known with precision must be described by probabilities. (Heisenberg's uncertainty principle)
Matter exhibits a wave–particle duality. An experiment can show the particle-like properties of matter, or the wave-like properties; in some experiments both of these complementary viewpoints must be invoked to explain the results, according to the complementarity principle of Niels Bohr.
Measuring devices are essentially classical devices, and measure only classical properties such as position and momentum.
The quantum mechanical description of large systems will closely approximate the classical description. (The correspondence principle of Bohr and Heisenberg.)

Meaning of the wave function

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

The subjective view, that the wave function is merely a mathematical tool for calculating the probabilities in a specific experiment, is a similar approach to the Ensemble interpretation.

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 logical positivism and/or with some of Bohr's statements. Bohr emphasized that science is concerned with predictions of the outcomes of experiments, and that any additional propositions offered are not scientific but meta-physical. Bohr was heavily influenced by positivism. On the other hand, Bohr and Heisenberg were not in complete agreement, and they held different views at different times. Heisenberg in particular was prompted to move towards realism.[9]

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 Carl Friedrich von Weizsäcker, 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."[10]
Nature of collapse

All versions of the Copenhagen interpretation include at least a formal or methodological version of wave function collapse,[11] in which unobserved eigenvalues are removed from further consideration. (In other words, Copenhagenists have always made the assumption of collapse, even in the early days of quantum physics, in the way that adherents of the Many-worlds interpretation have not.) 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.

An adherent of the subjective view, that the wave function represents nothing but knowledge, would take an equally subjective view of "collapse".

Some argue that the concept of the collapse of a "real" wave function was introduced by Heisenberg and later developed by John Von Neumann in 1932.[12]
Acceptance among physicists

According to a poll at a Quantum Mechanics workshop in 1997,[13] the Copenhagen interpretation is the most widely-accepted specific interpretation of quantum mechanics, followed by the many-worlds interpretation.[14] 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.[15]

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

1. Schrödinger's Cat

This thought experiment highlights the implications that accepting uncertainty at the microscopic level has on macroscopic objects. A cat is put in a sealed box, with its life or death made dependent on the state of a subatomic particle. Thus a description of the cat during the course of the experiment—having been entangled with the state of a subatomic particle—becomes a "blur" of "living and dead cat." But this can't be accurate because it implies the cat is actually both dead and alive until the box is opened to check on it. But the cat, if he survives, will only remember being alive. Schrödinger resists "so naively accepting as valid a 'blurred model' for representing reality."[16] How can the cat be both alive and dead?

The Copenhagen Interpretation: The wave function reflects our knowledge of the system. The wave function \( (|\text{dead}\rangle + |\text{alive}\rangle)/\sqrt 2 \) means that, once the cat is observed, there is a 50% chance it will be dead, and 50% chance it will be alive.

2. Wigner's Friend

Wigner puts his friend in with the cat. The external observer believes the system is in the state \( (|\text{dead}\rangle + |\text{alive}\rangle)/\sqrt 2 \). His friend however is convinced that cat is alive, i.e. for him, the cat is in the state |\text{alive}\rangle. How can Wigner and his friend see different wave functions?

The Copenhagen Interpretation: Wigner's friend highlights the subjective nature of probability. Each observer (Wigner and his friend) has 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. Cf. Bayesian versus Frequentist interpretations 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?

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 Complementarity Principle).

The same experiment can in theory be performed with any physical system: electrons, protons, atoms, molecules, viruses, bacteria, cats, humans, elephants, planets, etc. In practice it has been performed for light, electrons, buckminsterfullerene,[17][18] and some atoms. Due to the smallness of Planck's constant it is practically impossible to realize experiments that directly reveal the wave nature of any system bigger than a few atoms but, in general, quantum mechanics considers all matter as possessing both particle and wave behaviors. The greater systems (like viruses, bacteria, cats, etc.) are considered as "classical" ones but only as an approximation, not exact.

4. EPR (Einstein–Podolsky–Rosen) 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 discomforting 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. But, according to Einstein's theory of special relativity, no information-bearing signal or entity can travel at or faster than the speed of light, which is finite. Thus, it seems as if the Copenhagen interpretation is inconsistent with special relativity.

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[19] and the Transactional interpretation[20][21] (TI) maintain that Copenhagen interpretation is fatally non-local.

The claim that EPR effects violate the principle that information cannot travel faster than the speed of light have been countered 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. However, it should be noted that is a somewhat spurious argument, in that speed of light limitations applies to all information, not to what can or can not be subsequently done with the information.

A further argument is that relativistic difficulties about establishing which measurement occurred first also undermine the idea that one observer is causing what the other is measuring. This is totally spurious, since no matter who measured first the other will measure the opposite spin despite the fact that (in theory) the other has a 50% 'probability' (50:50 chance) of measuring the same spin, unless data about the first spin measurement has somehow passed faster than light (of course TI gets around the light speed limit by having information travel backwards in time instead).


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.

Experimental tests of Bell's inequality using particles have supported the quantum mechanical prediction of entanglement.

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.[22]

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."[23] and "Do you really think the moon isn't there if you aren't looking at it?"[24] exemplify this. Bohr, in response, said "Einstein, don't tell God what to do".

Steven Weinberg in "Einstein's Mistakes", Physics Today, November 2005, page 31, said:

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.[25]

E. T. Jaynes,[26] from a Bayesian point of view, pointed out probability is a measure of human's information about the physical world. Quantum mechanics under Copenhagen Interpretation interpreted probability as a physical phenomenon, which is what Jaynes called a Mind Projection Fallacy. A similar view is adopted in Quantum Information Theories.
Further information: Interpretation of quantum mechanics

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". Although the Copenhagen interpretation is often confused with the idea that consciousness causes collapse, it defines an "observer" merely as that which collapses the wave function.[22]

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.

Many physicists have subscribed to the instrumentalist interpretation of quantum mechanics, a position often equated with eschewing all interpretation. It is summarized by the sentence "Shut up and calculate!". While this slogan is sometimes attributed to Paul Dirac[27] or Richard Feynman, it is in fact due to David Mermin.[28]
See also

Afshar experiment
Bohr–Einstein debates
Consciousness causes collapse
Fifth Solvay Conference
Interpretation of quantum mechanics
Philosophical interpretation of classical physics
Physical ontology
Popper's experiment
De Broglie–Bohm theory

Notes and references

^ Hermann Wimmel (1992). Quantum physics & observed reality: a critical interpretation of quantum mechanics. World Scientific. p. 2. ISBN 9789810210106. Retrieved 9 May 2011.
^ J. Mehra and H. Rechenberg, The historical development of quantum theory, Springer-Verlag, 2001, p. 271.
^ Howard, Don (2004). "Who invented the Copenhagen Interpretation? A study in mythology". Philosophy of Science: 669–682.
^ Bohm, David (1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden" Variables. I & II". Physical Review 85 (2): 166–193. Bibcode 1952PhRv...85..166B. doi:10.1103/PhysRev.85.166.
^ H. Kragh, Quantum generations: A History of Physics in the Twentieth Century, Princeton University Press, 1999, p. 210. ("the term 'Copenhagen interpretation' was not used in the 1930s but first entered the physicist’s vocabulary in 1955 when Heisenberg used it in criticizing certain unorthodox interpretations of quantum mechanics.")
^ Werner Heisenberg, Physics and Philosophy, Harper, 1958
^ In fact Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics. Bohr once distanced himself from what he considered to be Heisenberg's more subjective interpretation Stanford Encyclopedia of Philosophy
^ "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.'", Asher Peres (2002). "Popper's experiment and the Copenhagen interpretation". Stud. History Philos. Modern Physics 33 (23): 10078. arXiv:quant-ph/9910078. Bibcode
^ "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. This attitude changed drastically with his paper 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 Schrödinger’s wave mechanics 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.", from Claus Kiefer (2002). "On the interpretation of quantum theory - from Copenhagen to the present day". arXiv:quant-ph/0210152 [quant-ph].
^ John Cramer on the Copenhagen Interpretation
^ "To summarize, 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", Claus Kiefer (2002). "On the interpretation of quantum theory - from Copenhagen to the present day". arXiv:quant-ph/0210152 [quant-ph].
^ "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", Claus Kiefer (2002). "On the interpretation of quantum theory - from Copenhagen to the present day". arXiv:quant-ph/0210152 [quant-ph].
^ Max Tegmark (1998). "The Interpretation of Quantum Mechanics: Many Worlds or Many Words?". Fortsch.Phys. 46 (6–8): 855–862. arXiv:quant-ph/9709032. doi:10.1002/(SICI)1521-3978(199811)46:6/8<855::AID-PROP855>3.0.CO;2-Q.
^ The Many Worlds Interpretation of Quantum Mechanics
^ Gribbin, J. Q for Quantum
^ Erwin Schrödinger, in an article in the Proceedings of the American Philosophical Society, 124, 323-38.
^ Nairz, Olaf; Brezger, Björn; Arndt, Markus; Zeilinger, Anton (2001). "Diffraction of Complex Molecules by Structures Made of Light". Physical Review Letters 87 (16). arXiv:quant-ph/0110012. Bibcode 2001PhRvL..87p0401N. doi:10.1103/PhysRevLett.87.160401.
^ Brezger, Björn; Hackermüller, Lucia; Uttenthaler, Stefan; Petschinka, Julia; Arndt, Markus; Zeilinger, Anton (2002). "Matter-Wave Interferometer for Large Molecules". Physical Review Letters 88 (10): 100404. arXiv:quant-ph/0202158. Bibcode 2002PhRvL..88j0404B. doi:10.1103/PhysRevLett.88.100404. PMID 11909334.
^ Michael price on nonlocality in Many Worlds
^ Relativity and Causality in the Transactional Interpretation
^ Collapse and Nonlocality in the Transactional Interpretation
^ a b Werner Heisenberg, Physics and Philosophy, Harper, 1958, p. 137.
^ "God does not throw dice" quote
^ A. Pais, Einstein and the quantum theory, Reviews of Modern Physics 51, 863-914 (1979), p. 907.
^ '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.', Claus Kiefer (2002). "On the interpretation of quantum theory - from Copenhagen to the present day". arXiv:quant-ph/0210152 [quant-ph].
^ Jaynes, E. T. (1989). "Clearing up Mysteries--The Original Goal". Maximum Entropy and Bayesian Methods: 7.
^ N. David Mermin. "Could Feynman Have Said This?". Physics Today 57 (5).

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.
T. Schürmann, A Single Particle Uncertainty Relation, Acta Physica Polonica B39 (2008) 587. [1]

External links

Copenhagen Interpretation (Stanford Encyclopedia of Philosophy)
Physics FAQ section about Bell's inequality
The Copenhagen Interpretation of Quantum Mechanics
Preprint of Afshar Experiment
The Quantum Illusion

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