Religion in an Age of Science by Ian Barbour
Chapter 4: Physics and Metaphysics
I. Quantum Theory
1. Complementarity
Niels Bohr defended the use of wave and particle models and other pairs of sharply contrasting sets of concepts. His discussion of what he called the Complementarity Principle included several themes. Bohr
emphasized that we must always talk about an atomic system in relation to an experimental arrangement; we can never talk about it in isolation, in itself. We must consider the interaction between subject and object in every experiment. No sharp line can be drawn between the process of observation and what is observed. We are actors and not merely
spectators, and we choose the experimental tools we will employ. Bohr held that it is the interactive process of observation, not the mind or consciousness of the observer, that must be taken into account.
Another theme in Bohr’s writing is the conceptual limitation of human understanding. The human as knower, rather than as experimenter, is the center of attention here. Bohr shares Kant’s skepticism about the
possibility of knowing the world in itself. If we try, as it were, to force nature into certain conceptual molds, we preclude the full use of other molds. Thus we must choose between complete causal or
spatiotemporal descriptions, between wave or particle models, between accurate knowledge of position or momentum. The more one set of concepts is used, the less the complementary set can be applied simultaneously. This reciprocal limitation occurs because the atomic world cannot be described in terms of the concepts of classical physics and observable phenomena.5
How, then, are the concepts of quantum physics related to the real world? Three differing views of the status of theoretical entities in science yield differing interpretations of quantum theory.
1. Classical Realism. Newton and almost all physicists through the nineteenth century said that theories are descriptions of nature as it is in itself apart from the observer. Space, time, mass, and other "primary qualities" are properties of all real objects. Conceptual models are replicas of the world that enable us to visualize the unobservable
structure of the world in familiar classical terms. Einstein continued this tradition, insisting that a full description of an atomic system requires specifying the classical spatiotemporal variables that define its state objectively and unambiguously. He held that since quantum theory does not do this, it is incomplete and will eventually be superseded by a theory that fulfills classical expectations.
2. Instrumentalism. Here theories are said to be convenient human constructs, calculating devices for correlating observations and making predictions. They are also practical tools for achieving technical control.
They are to be judged by their usefulness in fulfilling these goals, not by their correspondence to reality (which is inaccessible to us). Models are imaginative fictions used temporarily to construct theories, after which they can be discarded; they are not literal representations of the world.
We cannot say anything about the atom between our observations, though we can use the quantum equations to make predictions about observable phenomena.
It is often assumed that Bohr must have been an instrumentalist because he rejected classical realism in his protracted debate with Einstein. He
did indeed say that classical concepts cannot be used unambiguously to describe independently existing atomic systems. Classical concepts can be used only to describe observable phenomena in particular
experimental situations. We cannot visualize the world as it is in itself apart from our interaction with it. Bohr did agree with much of the instrumentalist critique of classical realism, but he did not endorse instrumentalism specifically, and more careful analysis suggests that he adopted a third alternative.
3. Critical Realism. Critical realists view theories as partial
representations of limited aspects of the world as it interacts with us.
Theories allow us to correlate diverse aspects of the world manifest in differing experimental situations. To the critical realist, models are abstract and selective but indispensable attempts to imagine the
structures of the world that give rise to these interactions. The goal of science, in this view, is understanding, not control. The corroboration of predictions is one test for valid understanding, but prediction is not in itself a goal of science.
A good case can be made that Bohr adopted a form of critical realism, though his writings were not always clear. In the debate with Einstein, he was not denying the reality of electrons or atoms, only that they are the sort of things that admit of precise classical space-time descriptions.
He did not accept Mach’s phenomenalism, which questioned the reality of atoms. Summarizing the dispute, Henry Folse says, "He discarded the classical framework and kept a realistic understanding of the scientific description of nature. What he rejects is not realism, but the classical version of it."4
Bohr presupposed the reality of the atomic system that is interacting with the observing system. In contrast to subjectivist interpretations of quantum theory, which take observation to be a mental-physical
interaction, Bohr talks about physical interactions between instrumental and atomic systems in a total experimental situation. Moreover, wave and particle or momentum and position or other complementary
descriptions apply to the same object, even though the concepts are not unambiguously applicable to it. They represent different manifestations of the same atomic system. Folse writes:
Bohr argues that such representations are ‘abstractions’ which serve an indispensable role in allowing a phenomenon to be described as an interaction between observing systems and
atomic systems, but which cannot picture the properties of an independent reality. . . . We can describe such a reality in terms of its power to produce the various different interactions
described by the theory as providing complementary evidence about the same object.5
Bohr did not accept the classical realist view of the world as consisting of entities with determinate classical properties, but he still held that there is a real world, which in interacting has the power to produce observable phenomena. Folse concludes his book on Bohr with this summary:
The ontology implied by this interpretation of Bohr’s message characterizes physical objects through their powers to appear in different phenomenal manifestations rather than through
determinate properties corresponding to those of phenomenal objects as was held in the classical framework. The case is then made that, within the framework of complementarity, it is possible to preserve a realistic understanding of science and accept the completeness of quantum theory only by revising our understanding 0f the nature of an independent physical reality and how we can have knowledge of it.6
In short, we do have to abandon the sharp separation of the observer and the observed that was assumed in classical physics. In quantum theory, the observer is always a participant. We will find a similar
epistemological lesson in relativity. In complementarity, the use of one model limits the use of other models. Models are symbolic
representations of aspects of interactive reality that cannot be uniquely visualized in terms of analogies with everyday experience; they are only very indirectly related to either the atomic world or the observable phenomena. But we do not have to accept an instrumentalism that
makes theories and models useful intellectual and practical tools that tell us nothing about the world.
Bohr himself proposed that the idea of complementarity could be extended to other phenomena susceptible to analysis by two kinds of models: mechanistic and organic models in biology, behavioristic and introspective models in psychology, models of free will and
determinism in philosophy, or of divine justice and divine love in theology. Some authors go further and speak of the complementarity of science and religion. Thus C. A. Coulson, after explaining the wave-
particle duality and Bohr’s generalization of it, calls science and religion
"complementary accounts of one reality."7
I am dubious about such extended usage of the term. I would set down several conditions for applying the concept of complementarity.8
1. Models should be called complementary only if they refer to the same entity and are of the same logical type. Wave and particle are models of a single entity (for example, an electron) in a single situation (for
example, a two-slit experiment); they are on the same logical level and had previously been employed in the same discipline. These conditions do not apply to science and religion." They do not refer to the same entity. They arise typically in differing situations and serve differing functions in human life.9 For these reasons I speak of science and
religion as alternative languages and restrict the term complementary to models of the same logical type within a given language, such as
personal and impersonal models of God (chap. 2).
2. One should make clear that the use of the term outside physics is analogical and not inferential. There must be independent evidence of the value of two alternative models or sets of constructs in the other field. It cannot be assumed that methods found useful in physics will be fruitful in other disciplines.
3. Complementarity provides no justification for an uncritical acceptance of dichotomies. It cannot be used to avoid dealing with inconsistencies or to veto the search for unity. The paradoxical element in the wave-particle duality should not be overemphasized. We do not say that an electron is both a wave and a particle, but only that it exhibits wavelike and particlelike behavior; moreover we do have a unified mathematical formalism, which provides for at least
probabilistic predictions. We cannot rule out the search for new unifying models, even though previous attempts have not yielded any theories in better agreement with the data than quantum theory. Coherence remains an important ideal in all reflective inquiry, even if it is qualified by acknowledgment of the limitations of human language and thought.