Original Source
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Complementarity and the Nature of Empirical Knowledge
While Bohr's failure to convert Einstein to his point of view was a
significant disappointment to him personally, the nature of their disagreement
could be put to profitable issues, as both men spoke the language of physics.
However, the failure of complementarity to draw any appreciable audience among
philosophers was for Bohr a cause of bewilderment and frustration. This was
because Bohr and the philosophers hardly spoke the same language. Bohr's
statement in his last interview: "I think that it would be reasonable to say
that no man who is called a philosopher really understands what is meant by
the complementarity description" really says it all.
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It would perhaps be helpful to consider in general the central issue in
philosophy over which communications between Bohr and the "philosophers" broke
down. This issue divides those who hold scientific theories are attempts to
describe the phenomena we experience as the empirical evidence of the behavior
of the objects behind these phenomena and those who don't. The former hold the
view called "realism" while their opponents are called "anti-realists".
However, it is hardly the case that all defenders of realism in science defend
the same form of realism. Indeed, the issues that separated Bohr and Einstein
were between two alternative forms of realism. Therefore, it is convenient to
distinguish "classical realism" from that of "complementarity realism".
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The classical realist holds that the success of the theories in classical
physics legitimizes the claim that phenomena are a consequence of the objects
having properties corresponding to the terms used to characterize physical
systems in classical mechanics. From the classical realist point of view, the
concept of the state of the system defined in terms of classical parameters
may be regarded as referring to properties possessed by an independently real
object, which causes the phenomena that confirm theoretical representations of
the system. Bohr's complementarity realism denies this claim of an
independently real object, but offers the possibility of an alternative form
of knowledge of the domain lying behind the phenomena.
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Just as there are various forms of realism, so are there various forms of
anti-realism. The dominant one during Bohr's life was that of
"instrumentalism", the view that theoretical terms serve only as constructs
enabling predictions concerning phenomena observed in specific circumstances.
Another form of anti-realism was known as "phenomenalism", the assertion that
the only reality with any content is that of phenomena, and therefore
statements about a reality lying behind the phenomena are meaningless. Both
these views have been incorrectly imputed to Bohr.
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Bohr did not see the issue between himself and the philosophers as an
ontological question about the nature of physical reality. It was important
for Bohr to discover whether or not complementarity provided an "objective"
description of phenomena. In his last interview, Bohr complained that the
philosophers "did not see that it [complementarity] was an objective
description, and it was the only possible description. So therefore the
relationship between scientists and philosophers is a very curious one".
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Bohr saw the misunderstanding between himself and the philosophers as an
epistemological issue of the requirements for an "objective description".
Complementarity, as a conceptual framework for describing nature, is meant to
be a rational generalization of the framework of classical mechanics. Since
classical mechanics is not an epistemology per se, by analogy neither is
complementarity. Nevertheless, the advent and subsequent dominance of
mechanism had repercussions in epistemology, and the success of classical
mechanics was considered a major datum for the traditions of rationalism and
empiricism, as well as Kant's critical philosophy. Therefore it is natural to
ask, if complementarity is a general framework by which we are able to better
understand the description of nature in natural science, what does it tell us
about the nature of scientific knowledge?
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The Relationship Between Complementarity and Epistemology
Classical mechanism provides a directive to look for theories and descriptions
of a certain kind, which can be accessed as adequate or acceptable
descriptions. Mechanism provides an "ideal" for the description of nature by
stating which accounts of phenomena can be considered as acceptable claims of
scientific knowledge. This classical ideal gives a standard against which
prospective theoretical descriptions can be measured. Mechanism not only
provides a framework of concepts such that they have specific meaning in the
description of nature, but it also serves
a directive function orientating the progress of science towards a projected
goal of describing an observed phenomenon in accord with its ideal of
description.
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The development of quantum mechanics was conditioned by the ideal of classical
physics. Since these standards were, which Einstein said, not to be regarded
as changeable, quantum theory was necessarily incomplete. These standards
imply that a completely objective description is one that determines
properties possessed by an independent reality by representing the object as a
physical system isolated from any other object. If these properties are
represented by classical parameters, then it follows that the inability of
quantum theory to represent the object of its descriptions in well-defined
classical states means that it does not fulfill what the classical framework
considers acceptable scientific method.
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Bohr developed his complementarity to change the standards for this "ideal"
description so that quantum theory would be evaluated as presenting an
acceptable, consistent and complete description of atomic systems. Therefore,
the switch from mechanism to complementarity implies a change of the standards
of what defines acceptable science. Complementarity changes the significance
of the terms of description of objectivity from what Einstein demanded.
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Bohr's epistemological lesson begins with the recognition that empirical
knowledge must ultimately be be based upon experiences which are the property
of a perceiving subject, but in scientific knowledge we proceed from this
subjective starting point to an objective account of experienced phenomena.
For Bohr, "objectivity" is a property of the descriptions of phenomena which
science provides. However, the common perception of classical mechanics as a
program for eliminating purposive descriptions of natural phenomena is a
consequence that such descriptions were the result of importing subjective
categories into the description. They provide terms for an "objective
description" because they refer to what is directly observable to any normal
experiencing subject and can be communicated by unambiguous measurements
expressed in the language of mathematics.
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Classically, it is possible to hold that unambiguous descriptions which these
terms made possible referred to a reality behind the experienced phenomena. To
a classical realist, the "objectivity" of mechanical descriptions implied much
more than that they were unambiguously communicable. Objectivity could be
grounded in a reality behind the phenomena because the terms defining the
state of the system could be consistently held to refer to the properties of
an independent reality. If one assumed some sort of causal link between the
mental and the physical, it was possible to consider the properties of the
observed phenomena as the mechanical causal effects of properties possessed by
an independent reality interacting with the perceiving subject's sense organs.
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Subjectivity and the Description of Experience
Bohr's complementarity was vitally concerned with how to reconcile the
empiricist conviction that, ultimately, individual subjective experiences are
the foundation of all knowledge of nature with the professed goal of
objectively describing those experiences. Classical physics had an answer, but
it was based on a presupposition that the quantum postulate required
discarding. Therefore, a reanalysis of the concept of objectivity and a
radical redefinition of this criterion of scientific knowledge form the
philosophical heart of complementarity.
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Bohr was very much aware that the requirement of objectivity imposed on the
scientific description of nature was in paradoxical contrast to the subjective
status on which such scientific description was based. He writes: "Yet
occasionally just this 'objectivity' of physical observations becomes
particularly suited to emphasize the subjective character of all experience."
[1]
Here Bohr clearly regards a "physical observation" as an objectively given
datum. However, to count as an observation, the experienced phenomena of that
interaction must be described unambiguously. The basis for this observation is
the experimenter's experience of the phenomena. Since that experience is an
everyday experience, it must be described by the usual terms for describing
everyday objects. In this way, "objectivity" of "physical observations"
emphasizes the "subjective character of all experience".
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Unfortunately, this use of "subjectivity" by Bohr was reckless, as he came to
realize from the persistent attempts of critics to interpret complementarity
subjectively. Thus, in later years, every essay contains a disclaimer of any
subjective intentions, for example:
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...the decisive point is that in neither case [i.e., in neither quantum
physics nor in relativity] does the approximate widening of our conceptual
framework imply any appeal to an observing subject, which would hinder
unambiguous communication of
experience. [2]
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Nevertheless, the classical tendency to regard experience as subjective leads
to reading complementarity as endorsing a subjective foundations for science.
Bohr was very aware of this temptation:
In view of the influence of the mechanical conception of nature on
philosophical thinking, it is understandable that one has sometimes seen in
the notion of complementarity a reference to the subjective observer,
incompatible with the objectivity of scientific description. Of course in
every field of experience we must retain a sharp distinction between the
observer and the content of the observations, but we must realize that the
discovery of the quantum of action has thrown new light on the very foundation
of the description of nature and revealed hitherto un-noticed presuppositions
to the rational use of the concepts on which the communication of experience
rests. In quantum physics, as we have seen, an account of the functioning of
the measuring instruments is indispensable to the definition of the phenomena
and we must, so to speak, distinguish between subject and object in such a way
that each single case secures the unambiguous application of the elementary
physical concepts used in that
description. [3]
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Bohr believed this shifting of the distinction between subject and object also
caused problems in psychology and biology, as noted in part 6 of this review.
In the essay from which the above paragraph was taken he continues by pointing
out:
While in the mechanical conception of nature the subject - object distinction
was fixed, room is provided for a wider description through the recognition
that the consequent use of our concepts requires different placings of such a
separation. [4]
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These comments indicate that Bohr believed the task of securing objectivity of
empirical knowledge is presumed to refer to the description of experience, or
in other words, what happens after the subject has the experience rather than
how experience originates. Bohr avoids grounding objectivity in either the
objects contribution or the subjects contribution to the formation of
experience. His view was that objectivity is secured by understanding the
proper use of descriptive concepts and so it is a matter of how we describe an
experienced phenomena rather than how such experience originates. Whether
consciousness is regarded as reducible to a physical process or as a mental
presentation of ideas is irrelevant to complementarity. What concerned Bohr
was not the interplay of distinct ontological orders, but the necessity of
describing experienced phenomena unambiguously.
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Observation, for Bohr, did not involve any mysterious mental/physical
interactions. In order to describe an observation in physics, both systems
which interact to produce the observation must be capable of being described
as purely physical systems. Bohr is emphatic in his claim that there is no
need for new observational language in quantum physics. Since classical
descriptive concepts have unambiguous empirical meaning, quantum experimental
language presents no need to be described in any fashion other than that of
classical physics.
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When speaking of "observing" an atomic system, if Bohr's use of "observing" is
confused with his use of "experiencing" it may be philosophically misleading.
An observer "experiences" the phenomena that confirms the theory. Some of
these phenomena may be described as observations of atomic systems. Classical
descriptive terms have unambiguous reference only for describing phenomenal
objects, and since "atomic system" cannot refer to a phenomenal object,
classical descriptive terms cannot be used to describe atomic systems.
Therefore, if any description of the atomic system is at all possible, such a
description cannot be communicated unambiguously.
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Bohr's complementarity dramatically alters the classical description of
nature. "Observer" and "object" become categories of description having
precise meaning only in the context of a particular description of an
experiential event.
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The Ideal of Objectivity
Epistemology addresses the fundamental question of how to formulate a
description of experience to meet the criterion of objectivity. Classically, a
scientific description was held to be "objective" because it was believed that
it determined properties possessed by the object as it exists independently
apart from any observational determination of those properties. The
methodology of science was built around the goal of developing the means for
determining such properties and at the same time eliminating any element that
arises from the role of the observing system in acquiring that empirical
knowledge necessary to confirm scientific theory.
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Bohr's argument was that this understanding of objectivity of a scientific
description of nature must be revised. For Bohr, the objectivity of scientific
description is grounded in the use of concepts used for describing the
experience. These concepts secure that objectivity not by playing any role in
the origin or formation of experience, but by guaranteeing that communications
expressed in terms of such concepts are unambiguous. Bohr writes:
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Every scientist, however, is constantly confronted with the problem of
objective description of experience, by which we mean unambiguous
communication. Our basic tool is, of course, plain language. ...we shall not
be concerned here with the origin of such language, but with its scope in
scientific communication, and especially with the problem of how objectivity
may be retained during the growth of experience beyond the event of daily
life.
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The main point to realize is that all knowledge presents itself within a
conceptual framework adapted to account for previous experience and that any
such frame may prove too narrow to
comprehend new experiences... [5]
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As Bohr understood natural science, the objectivity of a description cannot be
secured by separating the "objective" properties possessed by an independent
reality from the "subjective" properties which exist only in relation to an
experiencing subject. In complementarity, the definition of "objectivity"
requires the scientists goal to be development of a conceptual scheme or
framework for describing the phenomena in a way which can be communicated
unambiguously.
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Once the quantum postulate is accepted, it is necessary to realize that any
description of a phenomena is a description of a physical interaction in which
the distinction between observed object and observing system is made for the
purpose of unambiguously describing the interaction as an observation of a
specific phenomenal object. To secure unambiguous communication of the result
of this observation, it is necessary that the description includes a precise
specification of the whole physical situation in which the observation occurs.
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Bohr's epistemological lesson teaches that on the presuppositions of the
classical framework, descriptions expressed by those classical mechanical
concepts can be considered unambiguous. But the exploration of the atomic
system dictates a need to adopt a new presupposition about these phenomena to
that expressed by the quantum postulate. This change then renders descriptions
expressed in classical terms ambiguous, consequently a new, more general
framework is needed to restore unambiguity.
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Because it is necessary to describe the observation as a causal process, which
must be done if that observation is to be interpreted as an observation of an
object system, it is also necessary to apply classical dynamical conservation
principles. And to do this, there must be a theoretical representation of the
system isolated from the interaction. But in quantum formalism it is
impossible to derive a classical mechanical state of the system that would
enable us to "picture" the object apart from the observation. Therefore it
must be recognized that the theoretical representation of the atomic system is
an abstraction, not a picture of a concrete object.
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The generalization of the classical mechanical framework which Bohr advocated
does much more than eliminate the ambiguities that result from quantum
formalism. Ultimately, in proposing to revise the basis for objective
scientific descriptions, complementarity alters the very concept of physical
reality. In Bohr's complementarity, the distinction between the subject that
experiences and the phenomena that is experienced is made within the
description of the phenomena as an observational interaction, whereas in
classical mechanics the distinction is made between the experiencer and the
experienced. Bohr writes:
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The notion of complementarity does in no way involve a departure from our
notion as detached observers
of nature, but must be regarded as the logical
expression of our situation as regard objective description in this field of
experience. The recognition that the interaction between the measuring tools
and the physical system under investigation constitutes an integral part of
the quantum phenomena has not only revealed an unsuspected limitation of the
mechanical conception of nature, as characterized by the attribution of
separate properties in physical systems, but has forced us, in the ordering
of experience, to pay proper attention to
the conditions of observation. [6]
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Bohr's claim was challenged by his student and friendly critic, Wolfgang
Pauli. Upon reading Bohr's first draft of the essay from which the above quote
is taken, Pauli replied to Bohr in his style which was overladen with sarcasm
and irony, for which he become infamous for:
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Under your great influence it was indeed getting more and more difficult for
me to find something on which I have a different opinion than you. To a
certain extent I am therefore glad, that eventually I found something: the
definition and the use of the expression "detached observer". ...According
to my own point of view, the degree of this "detachment" is gradually
lessened in our theoretical explanation of nature and I am expecting further
steps in this direction. ...it seems quite appropriate to call the
conceptual description of nature in classical physics, which Einstein so
emphatically wishes to retain, "the ideal of the detached observer". To put
it drastically the observer has according to this ideal to disappear
entirely in a discrete manner as a hidden spectator, never as actor, nature
being left alone in a predetermined course of events, independent of the way
in which the phenomena are observed. "Like the moon has a definite position"
Einstein said to me last winter, "whether or not we look at the moon, the
same must also hold for atomic objects, as there are no sharp distinctions
between these and macroscopic objects. Observation cannot create an element
of reality like a position, there must be something contained in the
complete description which corresponds to the possibility of observing a
position, already before the observation has been made." ...I considered the
impredictable [sic] change of the state by a single observation in spite of
the objective character of the result of every observation and
notwithstanding the statistical laws for the frequencies of repeated
observation under equal conditions - to be an abandonment of the idea of the
isolation (detachment) of the observer from the course of physical events
outside himself. [7]
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It is apparent that, even though Pauli worked with Bohr on his Como paper and
always regarded himself as an advocate of complementarity, both he and
Einstein confused the distinction made in the description of phenomena as an
interaction between the observing system and the atomic object with which it
interacts with the distinction between the subject that experiences the whole
phenomena and the phenomena the subject experiences. Bohr did not realize the
phrase "detached observer" confused the issue by tending to identify the
experiencing subject, the "observer" in classical physics, with the "observing
system", which, according to Bohr's view, is treated in the description of the
phenomena as a physical system partially causing the phenomena given to the
subject. Bohr replied to Pauli like this:
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As always, you touch on a very central point. A phrase like "detached
observer" ... used in connection with the phrase: "objective description"
... had to me a very definite meaning. In all unambiguous account it is
indeed a primary demand that the separation between the observing subject
and the objective content of communication [i.e., the phenomena to be
described] is clearly defined and agreed upon. ... this condition is
indispensable in all scientific knowledge. ... It appears that what we have
really learned in physics is how to eliminate subjective elements in the
account of experience, and it is rather this recognition which in turn
offers guidance as regards objective description in other fields of science.
To my mind this situation is well described by the phrase "detached
observer". [8]
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Bohr insisted that the description of a phenomena must include a description
of the whole interaction between observing system and observed object, all of
which forms part of the whole phenomena to be described by an adequate theory.
Pauli was certainly misunderstanding Bohr's intention in the phrase "detached
observer". While there need not be a separation between observer and observed
on a physical level, and indeed there cannot be if there is to be an
observation at all, by requiring a description of a "detached observer",
objectivity demands that subjective elements in the accounting of experience
are eliminated. Thus, the "observer", in this sense, is detached from the
observation.
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For further reading, see Humberto R. Maturana, Reality; The Search for
Objectivity or the Quest for a Compelling Argument.
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Complementarity and Kantian Epistemology
When Bohr refused to define how experience originates, he turned his back on
the whole representational tradition of epistemology. Although there is a
superficial resemblance between complementarity and Kantian epistemology, the
apparent similarities are created by equating such terms as "experience" and
"objectivity", which change in meaning in the shift from Kantian epistemology
to complementarity.
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A true Kantian would never argue, as Bohr does, that a physical description
would demand a change in the proper use of concepts which give experience its
form. If Bohr were a Kantian, he would argue, as he does, that the classical
concepts are indispensable, but then his claim that the quantum postulate
demonstrates that the classical framework is no longer tenable would be a
complete non sequitur. Bohr's rejection of the Kantian outlook follows from
the fact that as he understood Kant, the critical philosophy was designed to
show that classical concepts were independent of the content of experience and
thus could not be shown to be inadequate by any empirical discovery.
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Nevertheless, Bohr's use of classical concepts in the description of
phenomenal objects has a certain Kantian appearance. But that appearance is
deceptive, which we see when it is recalled that Bohr's complementarity has
nothing to do with how experienced phenomena arises, as Kantian philosophy
does, but have only to do with communicating a description of a phenomena as
an objective datum already given by experience. For further reading, see
Distinguishing the Observer; An Attempt at Interpreting Maturana
by Ernst von Glasersfeld and
Ontology of Observing; The Biological Foundations of Self
Consciousness and the Physical Domain of Existence
by Humberto R. Maturana.
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This ends Part 7 of this review. Thanks for reading!
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Other Links
Perceptions of Quality
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Footnotes
[1] Bohr, Atomic Theory and the Description of Nature
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[2] Bohr, Quantum Physics and Philosophy
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[3] Bohr, Unity Of Knowledge
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[4] Ibid.
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[5] Ibid.
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[6] Ibid.
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[7] Letter from Wolfgang Pauli to Niels Bohr
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[8] Letter from Niels Bohr to Wolfgang Pauli
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The Framework of Complementarity
Part 1 - Overview Early Years Bohr Formulates Complementarity
Part 2 - Argument for Complementarity
Part 3 - Comments on Complementarity
Part 4 - Complementarity and the Uncertainty Principle
Part 5 - Refinement of Complementarity
Part 6 - Extension of Complementarity
Part 7 - The Nature of Empirical Knowledge
Part 8 - Complementarity and the Metaphysics of Quality