# Copenhagen interpretation

The Copenhagen interpretation is a loosely-knit informal collection of axioms or doctrines that attempt to express in everyday language the mathematical formalism of quantum mechanics. The interpretation was largely devised in the years 1925–1927 by Niels Bohr and Werner Heisenberg.

## Quotes

* The Copenhagen interpretation is a very ambiguous term. Some people use it just to mean the sort of practical quantum mechanics that you can do — like you can ride a bicycle without really knowing what you're doing. It's the rules for using quantum mechanics and the experience that we have in using it. […] Then there's another side to the Copenhagen interpretation, which is a philosophy of the whole thing. It tries to be very deep and tell you that these ambiguities, which you worry about, are somehow irreducible. It says that ambiguities are in the nature of things. We, the observers, are also part of nature. It's impossible for us to have any sharp conception of what is going on. because we, the observers, are involved. And so there is this philosophy, which was designed to reconcile people to the muddle; You shouldn't strive for clarity— that's naive.
** John Stewart Bell, in interview, Omni, May 1988, p. 86-88
* Bohr’s principle of complementarity – the heart of the Copenhagen philosophy – implies that quantum phenomena can only be described by pairs of partial, mutually exclusive, or ‘complementary’ perspectives. Though simultaneously inapplicable, both perspectives are necessary for the exhaustive description of phenomena. Bohr aspired to generalize complementarity into all fields of knowledge, maintaining that new epistemological insights are obtained by adjoining contrary, seemingly incompatible, viewpoints. [...] The value of Bohr’s philosophy for the advancement of physics is controversial. His followers consider complementarity a profound insight into the nature of the quantum realm. Others consider complementarity an illuminating but superfluous addendum to quantum theory. More severe is the opinion that Bohr’s philosophy is an obscure ‘web of words’ and mute on crucial foundational issues.
** Mara Beller, "Bohr, Niels (1885-1962)", Routledge Encyclopedia of Philosophy
* In recent years the debate on these ideas has reopened, and there are some who question what they call "the Copenhagen interpretation of quantum mechanics"—as if there existed more than one possible interpretation of the theory.
** Rudolf Peierls, Surprises in Theoretical Physics (1979), Ch. 1. General Quantum Mechanics
* [I]n the Copenhagen interpretation of quantum theory we can indeed proceed without mentioning ourselves as individuals, but we cannot disregard the fact that natural science is formed by men. Natural science does not simply describe and explain nature; it is part of the interplay between nature and ourselves; it describes nature as exposed to our nature of questioning. This was a possibility of which Descartes could not have thought, but it makes a sharp separation between the world and the I impossible. If one follows the great difficulty which even eminent scientists like Einstein had in understanding and accepting the Copenhagen interpretation... one can trace the roots... to the Cartesian partition....it will take a long time for it [this partition] to be replaced by a really different attitude toward the problem of reality.
** Werner Heisenberg, Physics and Philosophy: The Revolution in Modern Science (1958)
* Maxel, you know I love you and nothing can change that. But I do need to give you once a thorough head washing. So stand still. The impudence with which you assert time and again that the Copenhagen interpretation is practically universally accepted, assert it without reservation, even before an audience of the laity—who are completely at your mercy—it’s at the limit of the estimable […]. Have you no anxiety about the verdict of history? Are you so convinced that the human race will succumb before long to your own folly?
** Erwin Schrödinger, Letter to Max Born (October 10, 1960), quoted in Walter John Moore, A Life of Erwin Schrödinger (1994), p. 342
* ... I think, because of the split in the Copenhagen Interpretation between the thing we study and the observer himself, that it can't survive in a fundamental sense.
** Steven Weinberg, (July 3, 2012)"The Copenhagen Interpretation (Video clips from "The Copenhagen Interpretation" 2004 Arentoft Film)". YouTube. (quote at 0:12 of 8:32 in video) WorldCat entry for 2004 film
* As Bohr acknowledged, in the Copenhagen interpretation a measurement changes the state of a system in a way that cannot itself be described by quantum mechanics. […] In quantum mechanics the evolution of the state vector described by the time-dependent Schrödinger equation is deterministic. If the time-dependent Schrödinger equation described the measurement process, then whatever the details of the process, the end result would be some definite state, not a number of possibilities with different probabilities. This is clearly unsatisfactory. If quantum mechanics applies to everything, then it must apply to a physicist’s measurement apparatus, and to physicists themselves. On the other hand, if quantum mechanics does not apply to everything, then we need to know where to draw the boundary of its area of validity. Does it apply only to systems that are not too large? Does it apply if a measurement is made by some automatic apparatus, and no human reads the result?
** Steven Weinberg, Lectures on Quantum Mechanics (2012), Ch. 3 : General Principles of Quantum Mechanics
* It works, is useful to understand our experiments, and makes no unnecessary assumptions.
** Anton Zeilinger, quoted in Kate Becker, "Debating the Meaning of Quantum Mechanics" (2011)
* I have always felt bitter about the way how Bohr’s authority together with Pauli’s sarcasm killed any discussion about the fundamental problems of the quantum. [...] I expect that the Copenhagen interpretation will some time be called the greatest sophism in the history of science, but I would consider it a terrible injustice if—when some day a solution should be found—some people claim that ‘this is of course what Bohr always meant’, only because he was sufficiently vague.
** H. Dieter Zeh, letter to John Wheeler, 30 Oct 1980
* According to my attempts to understand them, reality is systematically denied in the Copenhagen interpretation in order to circumvent consistency problems (such as “Is the electron really a wave or a particle?”). If there is no reality, one does not need a consistent description!
** H. Dieter Zeh, referring to his attempts to understand Copenhagen interpretation proponents Nonlocality versus nonreality, FQXi (Foundational Questions in Physics & Cosmology) Blog (2008)