observer (quantum physics)
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interpretations of quantum mechanics An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraord ...
posit a central role for an observer of a quantum phenomenon. The quantum mechanical observer is tied to the issue of
observer effect Observer effect, observer bias, observation bias, etc. may refer to a number of concepts, some of them closely related: General experimental biases * Hawthorne effect, a form of reactivity in which subjects modify an aspect of their behavior, in ...
, where a measurement necessarily requires interacting with the physical object being measured, affecting its properties through the interaction. The term "observable" has gained a technical meaning, denoting a
Hermitian operator In mathematics, a self-adjoint operator on an infinite-dimensional complex vector space ''V'' with inner product \langle\cdot,\cdot\rangle (equivalently, a Hermitian operator in the finite-dimensional case) is a linear map ''A'' (from ''V'' to it ...
that represents a measurement. The prominence of seemingly subjective or
anthropocentric Anthropocentrism (; ) is the belief that human beings are the central or most important entity in the universe. The term can be used interchangeably with humanocentrism, and some refer to the concept as human supremacy or human exceptionalism. ...
ideas like "observer" in the early development of the theory has been a continuing source of disquiet and
philosophical Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...
dispute. A number of new-age religious or philosophical views give the observer a more special role, or place constraints on who or what can be an observer. There is no credible peer-reviewed research that backs such claims. As an example of such claims,
Fritjof Capra Fritjof Capra (born February 1, 1939) is an Austrian-born American physicist, systems theorist and deep ecologist. In 1995, he became a founding director of the Center for Ecoliteracy in Berkeley, California. He is on the faculty of Schumacher ...
declared, "The crucial feature of atomic physics is that the human observer is not only necessary to observe the properties of an object, but is necessary even to define these properties." The
Copenhagen interpretation The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics, principally attributed to Niels Bohr and Werner Heisenberg. It is one of the oldest of numerous proposed interpretations of quantum mechanics, as featu ...
, which is the most widely accepted
interpretation of quantum mechanics An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraord ...
among physicists, posits that an "observer" or a "measurement" is merely a physical process. One of the founders of the Copenhagen interpretation,
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent series ...
, wrote:
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.
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
, also a founder of the Copenhagen interpretation, wrote:
all unambiguous information concerning atomic objects is derived from the permanent marks such as a spot on a photographic plate, caused by the impact of an electron left on the bodies which define the experimental conditions. Far from involving any special intricacy, the irreversible amplification effects on which the recording of the presence of atomic objects rests rather remind us of the essential irreversibility inherent in the very concept of observation. The description of atomic phenomena has in these respects a perfectly objective character, in the sense that no explicit reference is made to any individual observer and that therefore, with proper regard to relativistic exigencies, no ambiguity is involved in the communication of information.
Likewise,
Asher Peres Asher Peres ( he, אשר פרס; January 30, 1934 – January 1, 2005) was an Israeli physicist. He is well known for his work relating quantum mechanics and information theory. He helped to develop the Peres–Horodecki criterion for quantum en ...
stated that "observers" in quantum physics are
similar to the ubiquitous "observers" who send and receive light signals in
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
. Obviously, this terminology does not imply the actual presence of human beings. These fictitious physicists may as well be inanimate automata that can perform all the required tasks, if suitably programmed.
Critics of the special role of the observer also point out that observers can themselves be observed, leading to paradoxes such as that of
Wigner's friend Wigner's friend is a thought experiment in theoretical quantum physics, first conceived by the physicist Eugene Wigner in 1961, Reprinted in and further developed by David Deutsch in 1985. The scenario involves an indirect observation of a Measure ...
; and that it is not clear how much consciousness is required. As John Bell inquired, "Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer—with a PhD?"
John Stewart Bell John Stewart Bell FRS (28 July 1928 – 1 October 1990) was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden-variable theories. In 2022, the Nobel Prize in Phy ...
, 1981, "Quantum Mechanics for Cosmologists". In C. J. Isham, R. Penrose and D.W. Sciama (eds.), ''Quantum Gravity 2: A Second Oxford Symposium''. Oxford: Clarendon Press, p. 611.


See also

*
Observer (physics) General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. G ...
*
Quantum foundations Quantum foundations is a discipline of science that seeks to understand the most counter-intuitive aspects of quantum theory, reformulate it and even propose new generalizations thereof. Contrary to other physical theories, such as general relat ...


References

Concepts in physics Quantum mechanics {{quantum-stub