Religion in an Age of Science by Ian Barbour
Chapter 4: Physics and Metaphysics
I. Quantum Theory
2. Indeterminacy
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.
of a large group of radioactive atoms will have disintegrated, but we cannot predict when a particular atom will disintegrate; we can predict only the probability that it will disintegrate in a given time interval. The Heisenberg Uncertainty Principle states that the more accurately we determine the position of an electron, the less accurately we can
determine its momentum, and vice versa. A similar uncertainty relation connects other pairs of conjugate variables, such as energy and time.
Do these uncertainties represent the limitations of our knowledge or real indeterminancy and chance in the world? Three possible answers were given in the early years of quantum theory, and the debate among them continues today:
1. Uncertainty may be attributed to temporary human ignorance. Exact laws will eventually be discovered.
2. Uncertainty may be attributed to inherent experimental or conceptual limitations. The atom in itself is forever inaccessible to us.
3. Uncertainty may be attributed to indeterminacy in nature. There are alternative potentialities in the atomic world.
The three positions parallel the three epistemological positions of the preceding section. The first is classically realist (in epistemology) and deterministic (in metaphysics). The second is instrumentalist and agnostic about determinism; we can never know how the atom itself behaves between observations. The third, which I defend, is critically realist and indeterministic. Let us look at each of these interpretations.10 1. Uncertainty as Human Ignorance
Some of our uncertainties reflect our lack of knowledge about systems that conform to precise laws. Kinetic theory assumed that the motion of gas molecules is precisely determined but is too complicated to
calculate. The uncertainty was thought to be entirely subjective, representing incompleteness of information. A minority of physicists, including Einstein and Planck, have maintained that the uncertainties of quantum mechanics are similarly attributable to our present ignorance.
They believed that detailed subatomic mechanisms are rigidly causal and deterministic; someday the laws of these mechanisms will be found and exact prediction will be possible.
Einstein wrote, "The great initial success of quantum theory cannot convert me to believe in that fundamental game of dice. . . . I am
absolutely convinced that one will eventually arrive at a theory in which the objects connected by laws are not probabilities but conceived
facts."11 Einstein expressed his own faith in the order and predictability of the universe, which he thought would be marred by any element of chance. "God does not play dice," he said. As we saw, Einstein was a classical realist, holding that the concepts of classical physics must
"refer to things which claim real existence independent of perceiving subjects."
David Bohm has tried to preserve determinism and realism by
constructing a new formalism with hidden variables at a lower level.
The apparent randomness at the atomic level would arise from
variations in the concurrence of exact forces at the postulated subatomic level.12 So far his calculations yield no empirical conclusions differing from those of quantum mechanics, though Bohm hopes that in the future hidden variables may play a detectable role. Most scientists are dubious about such proposals. In the absence of any clear experimental evidence, the defense of determinism rests largely on philosophical grounds.
Unless someone can actually develop an alternative theory that can be tested, they say, we had better accept the probabilistic theories we have and give up our nostalgia for the certainties of the past.
2. Uncertainty as Experimental or Conceptual Limitations
Many physicists assert that uncertainty is not a product of temporary ignorance but a fundamental limitation permanently preventing exact knowledge of the atomic domain. The first version of this position, found in the early writings of Bohr and Heisenberg, claims that the difficulty is an experimental one; the uncertainty is introduced by the process of observation. Suppose that we want to observe an electron. To do so we must bombard it with a quantum of light, which disturbs the situation we were attempting to study. The disturbance of the system is unavoidable, since there must be at least a minimal interaction of the observer and the observed. Although this interpretation fits many
experiments, it appears unable to account for uncertainties when nothing is done to disturb the system -- for example, the unpredictability of the time at which a radioactive atom spontaneously disintegrates or the time at which an isolated atom makes a transition from an excited state.
The second version of the argument attributes uncertainty to our
inescapable conceptual limitations. By our choice of experimental situations we decide in which of our conceptual schemes (wave or particle, exact position or exact velocity) an electron will manifest itself to us. The structure of the atomic world is such that we must choose either causal descriptions (using probability functions that evolve deterministically) or spatiotemporal descriptions (using localized variables that are only statistically connected) -- but we cannot have both at once. This interpretation is agnostic as to whether the atom itself, which we can never know, is determinate or indeterminate (though a particular author expounding it may on other grounds favor one assumption or the other). As indicated above, many physicists since Bohr have been instrumentalists, though I have claimed that he himself was closer to critical realism.
3. Uncertainty as Indeterminacy in Nature
In his later writings, Heisenberg held that indeterminacy is an objective feature of nature and not a limitation of human knowledge.13 Such a viewpoint would accord with the critical realism I have advocated in which scientific theories are held to be representations of nature, albeit limited and imperfect ones. These limitations help to remind us that the denizens of the atomic realm are of a very different sort from the objects of everyday experience -- but this does not mean that they are less real.
Instead of assuming that an electron has a precise position and velocity that are unknown to us, we should conclude that it is not the sort of entity that always has such properties. Observing consists in extracting from the existing probability distribution one of the many possibilities it contains. The influence of the observer, in this view, does not consist in disturbing a previously precise though unknown value, but in forcing one of the many existing potentialities to be actualized. The observer’s activity becomes part of the history of the atomic event, but it is an objective history, and even the spontaneously disintegrating atom, left to itself, has its history.
If this interpretation is correct, indeterminacy characterizes the world.
Heisenberg calls this "the restoration of the concept of potentiality." In the Middle Ages the idea of potentiality referred to the tendency of an entity to develop in a particular way. Heisenberg does not accept the Aristotelian manner of describing a potentiality as a striving to attain a future purpose, but he does suggest that the probabilities of modern physics refer to tendencies in nature that include a range of possibilities.
The future is not simply unknown. It is "not decided." More than one
alternative is open and there is some opportunity for unpredictable novelty. Time involves a unique historicity and unrepeatability; the world would not repeat its course if it were restored to a former state, for at each point a different event from among the potentialities might be actualized. Potentiality and chance are objective and not merely subjective phenomena.
A more exotic version of objective indeterminacy is Hugh Everett’s many-universes interpretation. Everett proposed that every time a
quantum system can yield more than one possible outcome, the universe splits into many separate universes, in each of which one of these
possible outcomes occurs.14 We happen to be in the universe in which there occurs the outcome that we observe, and we have no access to the other universes in which duplicates of us observe other possibilities.
Since there are many atoms and many quantum events each second, the universe would have to divide into a mind-boggling proliferation of universes. Moreover, the theory seems to be in principle untestable, since we have no access to other universes containing the potentialities unrealized in ours. It seems much simpler to assume that potentialities not actualized in our universe are not actualized anywhere. Then we would have one universe which is objectively indeterminate.
In any case, adherents of the second and third of these basic positions -- which between them include by far the majority of contemporary
physicists -- agree in rejecting the determinism of Newtonian physics, even if they do not agree on their reasons for doing so.