what does quantum theory tell us about the world?

WHAT DOES QUANTUM THEORY TELL US ABOUT THE WORLD?Author(s): Henry J. FolseSource: Soundings: An Interdisciplinary Journal, Vol. 72, No. 1 (Spring 1989), pp. 179-205Published by: Penn State University PressStable URL: http://www.jstor.org/stable/41178473 .

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WHAT DOES QUANTUM THEORY TELL US ABOUT THE WORLD?

Henry J. Fohe

"рнЕ question posed by my title seems a straightforward and reasonable request.1 One obvious reason why it seems a

natural question is that for the two hundred year reign of classical physics there was widespread belief that the explanations furnished by its theories warranted some set of beliefs about the nature of physical reality. Philosophers disputed exactly what specific beliefs were warranted, but the consensus clearly fa- vored the view that successful scientific theories provide a springboard to the "philosophy of nature."

Of course the glory of the classical reign belongs to an earlier day. Virtually no one any longer doubts that the theories of current physics are so utterly transformed from their classical predecessors that the world-view of classical physics, whatever it was, lies today in shattered fragments. In particular, quantum physics has not only recast the conceptual basis of physics; it also has remolded chemistry and biology in its own image. The range of its triumphs from cosmology to genetics makes it a virtually unrivaled achievement of the human intellect. More- over, the changes wrought by the quantum revolution have not been confined to the theoretical aspect of natural science. Be- yond anyone's wildest dreams, it has revolutionized the prac- tice of science, the technology it makes possible, and the character of human culture. Thus it seems natural to expect that today's philosophers would have set about rebuilding a successor world-view based on this awesomely successful theory of contemporary physicists.

Henry J. Folse, Jr., is Professor of Philosophy at Loyola University, New Orle- ans. His publications include The Philosophy of NieL· Bohr: The Framework of Complementanty (1985).

Soundings 72.1 (Spring 1989). ISSN 0038-1861



180 SOUNDINGS Henry J. Fohe

But this expectation is hardly fulfilled in any convincing way. Although we might have thought that reflection on what we know of the behavior of the atomic domain would have profound implications transcending the bounds of physics, what in fact we have is a tightly restricted, quite technical, semi- private discussion among a small group of professional specialists in philosophy of physics. Above this we have a rather free- floating epiphenomenal body of "pop" literature purporting to show some relation between the teachings of quantum physics and those of Eastern mysticism or some other ingredient of the California blend. In contrast to the classical case, where we had a multitude of rival cosmologies jostling for the title of "the world-view of classical physics," in the contemporary scene we are rather more impressed by the absence of any philosophically developed contender that could be regarded as the world-view of contemporary physics.

Thus the dismantling of classical physics has presented the classical story of competing cosmologies with a new turn. Rather than ask which view science supports, we are forced to the more radical question of whether there can be any coherent vision of physical nature which is (perhaps) presupposed and/or supported by the description of nature in contemporary physics. Why is this the case? What is it about quantum theory which seems to block any attempt to extract a philosophy of nature from the theories of fundamental physics?

Answering that question will take us a long way towards answering the first question posed by my title, for unless we understand why no new philosophy of nature has rushed in to fill the vacuum left by the collapse of the classical view, we are not in a good position to understand what quantum theory does tell us about the world and why the older resistance to develop- ing a contemporary philosophy of nature is just now beginning to fade.

Realism and the Philosophy of Nature

One reason often given for why our title question cannot be easily answered is that quantum theory can be clearly expressed only in an abstract mathematical language, a "formalism," which requires years of advanced study of higher mathematics. That stalwart customer of the bookshops, the ed-



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ucated layperson, could be expected to know the mathematics required for Newtonian dynamics, but cannot be presumed to have familiarity with vectors in a multi-dimensional Hubert space. The most likely source of this kind of put-off is the physicist, who may defend the common attitude that quantum mysteries are no mystery because quantum theory speaks the beautifully consistent language of the "mathematical formalism," a language inscrutable to all but the initiates.

The educated layperson may indeed feel rebuffed by this im- putation of eternal ignorance, but no matter how unfair that may be, this put-off will not work against another class: the specialists in philosophy of physics who make it their business to speak on the philosophical lesson of quantum theory. Although not huge, the class of persons with professional train- ing in both physics and philosophy is large enough to establish that even when one does know the mathematical formalism - even when one does "speak the language" - quantum theory becomes no less mysterious and its implications for a philosophy of nature no more obvious. Indeed, the evidence from this class should suggest just the reverse of the common physicists' claim: the better one understands the formalism, the deeper the strangeness of the quantum world becomes.2

But there is an even better reason the educated layperson need not feel rebuffed. The philosopher-physicists mentioned above are not the first generation to be concerned with what quantum theory tells us about the world; they are preceded by the founders of the theory themselves. No two persons contributed more to the development of quantum physics than Al- bert Einstein and Niels Bohr, and no two could have differed more in their attitudes towards the theory they helped to build. Yet in spite of their momentous collision of outlooks, both agreed in this: understanding the problems in the interpretation of quantum mechanics does not require comprehension of an abstract mathematical formalism but can be gained from a direct purely qualitative understanding of the way the theory permits describing phenomena. In fact the several accounts both men gave of their differences never rely on a knowledge of the mathematical formalism by which the phenomena are described. Quantum theory is strange in the sense that if it tells us anything at all about the world, it tells us things which



182 SOUNDINGS Henry J. FoUe

sharply clash with our classical preconceptions. No matter how beautifully consistent the mathematical formalism may be, the strangeness of the quantum world does not go away. So whether or not we understand the abstract mathematics of the formalism, the question what does quantum theory tell us about the world remains a natural request.

If the difficulty of answering this question does not lie in the technical nature of the theory, then why has the attempt been so frustrated? We will do better to look for reasons in history rather than in the mathematics. Quantum theory grew up in a world wary of metaphysics. Not only did Ernst Mach's phe- nomenalism present an ideal of physics without theoretical entities "behind the phenomena," but also logical positivism, whose rise was almost coeval with the completion of the quantum revolution, could dispel all quandaries concerning physical reality by consigning them to the wasteland of metaphysical nonsense. This historical fact has contributed strongly to the common belief that the physicist who accepts the "orthodox" interpretation of quantum theory has washed his hands of philosophy of nature and does his research with blithe unconcern for the nature of reality.3 Any defender of such an anti-metaphysical outlook will likely point out that our title commits the fallacy of complex question, assuming we have already answered in the affirmative the prior question, "Does quantum theory tell us anything about the world?"

This philosopher's put-off poses a much more serious obstacle to anyone seeking a philosophy of nature in modern physics. Thus we must be diverted from our title question into a brief analysis of what philosophers today call the "realism vs. anti- realism" debate. In recent discussion of this issue a distinction between two distinct sets of questions has emerged.4 One set is essentially metaphysical, concerned with the existence or reality of entities and structures postulated by theories which are widely accepted because of their empirical success and fertility. Questions of this sort have often arisen in the historical advance of science. In philosophy, however, a different set of epistemological questions has arisen, concerned with the truth of the statements made by successful theories about the "furniture of the universe." It is worthwhile noting that in the concrete historical debates in science in which "existence" questions have



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arisen, they have been settled by the development of science. However, like most of the perennial questions in philosophy, the epistemological problems posed by realism have never had a definitive solution.

Scientists are typically unconcerned about the very issues which cause most consternation among the philosophers. For the practical scientist, the philosopher's concern with epistemological doubts about realism are strange neuroses that testify to how lost philosophers can become. To be sure, in science there has been deep concern with ontological questions concerning whether postulated theoretical constructs referred to real, existing entities or were only convenient fictions. Debates such as these are particularly pertinent to the philosophy of nature. In- deed they emphasize the continuum between the "metaphysical" task of framing a conception of the physical world within which science can ask its questions and the "scientific" task of em- ploying that vision to devise a theory which is empirically testa- ble. Thus from the point of view of the scientist, the great realism debates in the history of science are likely to be seen as scientific debates which were settled in the further progress of science. In Galileo's dispute with the Church at least history (if not philosophy) proclaims the verdict that Galileo was right. In the late nineteenth-century dispute over the reality of atoms, Machian skepticism has been laid to rest.

How have scientists managed to settle such ontological questions? One obvious answer is, not by solving philosophical problems in epistemology. The problem is not to construct an epistemological defense of our warrant for believing that the theories we accept truly describe some aspect of nature. In- stead, it is the metaphysical task of reconstructing the scientists' world-view in whatever way is necessary to accommodate or exclude the entities and structures postulated by newly accepted or rejected theories. Consequently, it demands to be dealt with from a perspective that accepts at least the possibility of constructing a philosophy of nature. For this reason the op- ponent to the scientific realist cannot be a Humean skeptic, much less a thoroughgoing solipsist. If the scientific realist is called upon to vanquish these dragons, he had best retire from the fray.




What establishes the victory for realism in these scientific contexts is the assumption of the realism of common sense, not the epistemological victory over skepticism. Indeed, it is by roughly the same method that common sense realism establishes its warrant for believing in the reality of the objects of everyday experience that scientific realism establishes its warrant for believing in the reality of microsystems or of distant galaxies. I say it is only roughly, not exactly the same method, because what is called "observable" in the scientific context differs from common sense. No other single term is likely to be so monstrously equivocated in the exchange between the philosopher and the particle physicist.

Common sense realism convinces us of the reality of the objects perceived with the senses; it allows us to discriminate, al- beit imperfectly and without ultimate Cartesian certainty, between dream life and waking, between the illusions of ap- pearance and what is really there. To the extent that it is a rational framework within which to interpret the experience of the perceiving subject, it assumes (though it does not explain why) there is a causal connection between the sense organs (which are themselves physical objects) and the objects of whose reality they convince us.

In a parallel fashion, scientific realism provides a framework for discriminating between that which really exists and that which is an artifact of our way of describing phenomena. An acceptable theory about electrons licenses the belief in electrons in the same way that an acceptable theory about billiard balls legitimizes the beliefs of the common sense realist about the behavior of perceived billiard balls. For the scientific realist, electrons have the same reality that tables and chairs have for the common-sense realist. And the justification for both is essentially of the same kind, but differs enormously with respect to the nature of the empirical input. For the common sense realist, it is bodily sense organs; for the scientific realist, it is the extension of those senses by often awesomely complex physical "detectors." Thus in science the criterion of ob- servability lies not with our sense organs, but with their "ampli- fication" by detectors which allow us to determine the properties of postulated entities in the same way that vision allows us to observe the position, say, of billiard balls. Just as



Quantum Theory 185

with common sense realism, there is a presumed causal link between these detectors and the entities whose properties they are designed to determine. However, in order to justify the claim that such detectors in effect amplify what is observable, now it becomes the burden of the scientific theory to explain that causal link. But the point is that the ontological models- the interpretations of the theory - make possible the interpretation of the phenomenal behavior of these detectors in terms of the interactions between components of these detectors and the behavior of the postulated entities of the microdomain. In the same way the common-sense story about the interactions between physical bodies surrounding us and our sense organs convinces us of the things we see, touch, and otherwise bump into.

In the conduct of everyday life common-sense realism provides a rational framework within which to interpret experience; so in the conduct of science it is rational to accept that framework which postulates the existence of the entities that according to accepted theory cause observable phenomena. Of course the presupposition of such an approach is that there are some entities and that these entities stand in some constitutive relation to the phenomena we cite as evidence for the theory which is the source of our beliefs about them. Nevertheless, in either context, inferences to what does or does not exist are inevitably fallible. We stand by common sense and regard it as rational to do so even though occasionally illusions do trick us; there might be that rare experience where we are not quite sure, did we really do that, or did we just dream it? Thus in science there are those episodes where we were led to believe, through faulty causal theories, to accept the reality of entities which, when the accompanying theory was rejected, we decided did not exist after all. But if we are occasionally taken in by illusions, that does not make solipsists of us one and all. Thus, in the probing advance of science, if our realism requires that we continuously reconstruct our world-view to accommodate the evidence of an ever-growing range of phenomena, we need not reject the presumption that scientific theories when successful do inform us, no matter how imperfectly, of entities and structures causing that which is present in sensory experience. From such a point of view it can hardly be thought surprising




that the sort of growth of knowledge which science has achieved in this century should require that we alter our vision of the natural world.

The sort of scientific realism outlined in the previous paragraphs is the view that has established the victory for a realist interpretation of the ontological questions which have confronted scientific theorizing. It is the sort of realism which makes science significant to philosophy of nature, for it holds that through science we learn the contours of the world which causes our experience. Of course the philosophers' epistemological doubts about realism are not likely to be silenced by the scientific decision to count as real those entities which the framework of a well supported theory posits. As the motive for scientific realism is the quest for knowledge about the reality producing the phenomena we experience, so the motive for anti-realism is to show that metaphysics is not significant to science, that science can be done without it. The realist's bold belief is precisely what makes science interesting to the specu- lative mind; for the anti-realist, it is unjustifiable folly. Ulti- mately the difference may be one's philosophical temperament.

Thus at least this much seems to characterize the epistemological question which torments philosophers: realists and anti-realists agree that realists believe "something more" than do anti-realists. Anti-realists typically believe that in believing this something more, the realist makes an unjustifiable, perhaps even meaningless, claim. Furthermore, the anti-realist is likely to think that there is something bad about believing this something more which the realist craves. At the very least it is the false conceit of wisdom, which it is the duty of every philosopher to stamp out, and at the worst, it is injurious to the progress of science and possibly even human well-being. The realist regards the anti-realist as a spoil-sport, because for him everything that makes the philosophical quest significant lies in the "something more." He sees the anti-realist as metaphysically anemic.

Thus we find a way to relate the philosophers ' question about

realism to the scientists ' concerns about the systems described by quantum physics. If a neutral observer were to follow the discussion between realists and their opponents when it comes to quantum theory, and then is asked to whom to award the



Quantum Theory 187

palm, one reasonable reply would be to say, "No decision can be made until we first know what is this 'something more' that realists want me to believe and anti-realists find so unaccept- able?" Answering this question is the job of a contemporary philosophy of nature, and it is precisely here that the realist interpretation of quantum theory finds itself most embarrassed.

The Status of Reality in the Classical Framework

In order to understand the realist's embarrassment when asked for an account of nature at the atomic level, it is best to consider how this sort of question was answered in the classical framework. The realist thirst for a reality behind the phenomena could be reasonably satisfied during the dominance of classical physics because the classical theory could be interpreted by a model constructed of entities possessing properties "corresponding" to the parameters defining the system's classical mechanical state as it exists isolated from observation. Most importantly these same properties of the model could be regarded as corresponding to properties of real entities that cause the observable phenomena classical theory is assumed to explain. Thus the realist interpretation of classical physics yielded a kind of conceptual space-time "model" allowing the realist to form a more or less concrete picture of what reality behind the phenomena is presumed to be like.

This classical account of how we know the nature of reality behind the phenomena relies on the presupposition that knowledge requires the "truth" of theoretical statements to re- side in a correspondence between at least some terms in these statements and the properties of independently existing entities. This correspondence account of truth implied that the resulting "spectator theory of knowledge" stipulates the objective knowledge must describe reality in terms of the properties actually possessed by an independent reality. (To be sure, even in a classical account, observation involves an interaction between observing and observed systems, thus what is recorded in an observation is strictly speaking a relation between the inter- actors, as even the Ancients well understood. But insofar as this interaction involves systems that can be defined as existing in separate mechanical states, such relations entirely supervene on the possessed properties of the relata, and thus can be "re-




duced" to them.) According to this outlook, the "objectivity" essential to scientific knowledge is guaranteed by the fact that classical mechanics makes it possible to provide a description of the object which eliminates any reference to the observer as a physical system interacting with the object to produce the "observations" on which that description is based. Thus the "subject" is "detached" from the object by treating the "observer" {qua physical system) as mechanically isolated from the "observed" object. In this way the observer is treated as a "ghost spectator" and any physical effect of observation is eliminated from the account in order that the description can be considered as referring to a physical world existing apart from observation.

Classically such a description is defined by the mechanical state of the isolated physical system which evolves over time, thus permit- ting a sort of "moving picture" of reality as it exists independently of any observation of it. The "state function" which defines the classical mechanical state is well defined for every system at all times and it changes continuously over time, thus providing a theoretical justification for upholding the identity of the same system through the course of its changing state. The ideal of description which came to be regarded as paradigmatic in the classical framework was based upon this definition of the mechanical state of a physical system. Thus, the manifold properties through which we describe the objects of ordinary human experience could be explained as arising from the structure and interaction of substantial entities possessing only the properties which corresponded to the parameters that defined their mechanical state. These so-called "primary properties" were the only ones for which there was postulated to be a direct correspondence between the properties of the objects of perceived experience and the properties of objects as they exist independently of human perception. Thus they alone were regarded as metaphysically "fundamental" in the sense that they alone characterized completely an objective order of nature.

Since observation ultimately rests on the determination of spatial loci at temporal instants, and the primary properties were those necessary to construct a space-time picture of the world and its evolution through time, this direct correspondence between the perceived primary properties and the fun-



Quantum Theory 189

damental properties of an independent reality allowed observation to be regarded as providing empirical warrant for the classical picture of a reality behind the phenomena. Thus the spatio-temporal picture of the system as isolated was natu- rally assumed to be the conceptual photograph of the independently real, not in all of its colorful secondariness, to be sure, but a "black and white" photograph stripped of the subjective secondary qualities found only in the perceived phenomena.

Throughout its evolution classical physics came to recognize the existence of two distinct sorts of substantial "bodies" as interacting with each other in this space-time picture: those which are defined as existing at discrete loci in space - identified with the presence of "material particles" in that space - and those which are defined as existing continuously through a region of space - identified with the presence of "fields." The parameters which defined the spatial position and momentum of the particle at that point in space as a function of time could be interpreted as corresponding to properties possessed by the discrete corpuscle of matter. The parameters which define the propagation of wave disturbances through the field as a function of time (with perhaps a bit more difficulty) could be conceived to be possessed by some substantial stuff, the ether. Any physical system could be regarded as "composed" of any number of such "bodies" treated as interacting components, all relations between which were regarded as "internal" to the system. Although the whole physical universe, considered as a single system of interacting components, is the only system which is free from the influence of any "external" system, to a good enough approximation it is generally possible to treat at least simple phenomena as the result of the behavior of rela- tively isolated systems. As long as a physical system remains isolated, the evolution of its mechanical state is precisely defined with exact values for all mechanical state parameters by the continuous differential equations of classical mechanistic physics.

Thus it is logically possible to "interpret" classical physics by a "mechanical model" of reality expressed in terms of the spatio-temporal careers of material particles and fields. However, right from the first use of such models, there was serious skepticism concerning whether they were merely conceptual con-



1 90 SOUNDINGS Henry J. Fohe

structions allowing the prediction of observable phenomena or whether they could be regarded as expressing true descriptions of an objective physical reality. Realists who affirmed that they should be interpreted as at least approximately true descriptions asserted that there existed a one-to-one correspondence between the properties of the model and the properties possessed by an independently existing physical reality composed of real material particles and fields. In this way physics could be advanced as evidence for metaphysics by providing a set of constraints within which a philosophy of nature based on classical physics must operate.

Berkeley understood that there was something fishy about a picture of what the universe would look like when no one is looking at it. The knowledge given by the mechanical pictures could be understood as a construction of thought allowing predictions of empirical import. But it was not to be understood as à picture of the spatio-temporal careers of real material entities which really exist with properties corresponding to the properties of phenomenal objects, for the very idea of "matter" as that which possessed such properties was incoherent. Thus the anti-realism that Galileo had endured censure to combat returned as an option for interpreting the classical framework.

Today anti-realists who deny that such models justify any claims to the truth about nature generally deny that "truth" (in the realists' traditional correspondence sense) has anything to do with why we accept a scientific theory. They are contented with the instrumental value of a theory in successfully predicting observable results. The more conservative anti-realists admit that theoretical statements may be true but deny that we can ever know this and in any event, they claim, truth has nothing to do with why scientists accept a theory. Thus the widespread acceptance of a theory does not license any inference to metaphysical conclusions about the nature of physical reality. The more radical anti-realists take the line that the very notion of a reality existing independently of human experience can be given no intelligible meaning and, therefore, the question of whether or not theoretical statements are true descriptions of this reality is a metaphysical pseudo-problem. Thus with respect to classical physics realists are in favor of a correspondence between models and the nature of reality. Anti-realists accept the



(Quantum Theory 191

view of "truth" as correspondence, but deny that science aims at "truth," at least as long as that correspondence is to reach beyond the properties of phenomena out to the properties of an objective reality existing independently of observation. They remain scornful of the metaphysical pretensions of the realists.

What both interpretations hold in common, and what has perhaps given the debate its intractable character, is com- mitment to a correspondence notion of truth and a spectator theory of knowledge. These philosophical commitments have problems which have long been recognized; but partially because they did present a plausible way to understand knowledge, truth, and reality in the classical framework, and partially from lack of any better alternative, their acceptance is often taken as a matter of course. It is one of the virtues of the quantum revolution that these dubious commitments have been shown to be inadequate and that the need for a better understanding of the relation of knowledge to truth and reality has been made more pressing.

The Status of Reality in the Quantum Description

In order to understand why the quantum revolution creates problems for the classical route connecting scientific theory and the philosophy of nature, we need to understand why a philosophy of nature based on a spatio-temporal model interpreting the classical formalism becomes impossible when we accept the completeness of the quantum theoretical description. We have seen that classically the objectivity of the knowledge claims of physics was justified by the detachment of the observer from the observed. This detachment is represented in the theory by the mechanical isolation of the physical system which is described as the "observed object" from that which serves as the "observing system."5 But this justification fol- lowed from the belief that even though observation requires that the observing system interacts with the observed object, because the object possesses the properties corresponding to the parameters which define its mechanical state, each system could be characterized as existing in its own mechanical state throughout the interaction. This theoretical distinction between separate states could be regarded as corresponding to reality because it could be



192 SOUNDINGS Henry I Folse

represented in the model by picturing a space-time separation between the observed and observing systems. This classical belief finds its theoretical expression in what has come to be known as the "principle of separability," according to which any two systems existing in discrete places in space and time must, by definition, be capable of being assigned separate mechanical states, even if interacting with other systems.6

The first shot of the quantum revolution kills this classical presupposition, the "quantum postulate" stipulates that at the atomic level the interaction between two systems is such that a system which prior to the interaction was in a well-defined state changes its state discontinuously in a way that can be predicted only probabilistically. This discontinuity results in the explicit denial of separability, for it means that after an interaction the formalism can define a state only for the whole system considered as composed of the two interacting "subsystems."

The definition of "continuous" relevant in the quantum postulate is not continuity in spatial trajectory over time, much less continuity in time itself. The discontinuity which enters physics with the quantum revolution "atomizes" neither space nor time nor energy. What changes in how the state of the system is formally represented as evolving through time results from the quantization of the parameter which forms the dimensions of Planck's discovery: "the quantum of action" "Action" may be regarded as referring to volumes of "phase space," the multi- dimensional space created by mapping onto co-ordinate axes those parameters which define the state of the system. Classi- cally the state of the system could be represented as a determined trajectory through a continuous phase space; the quantum revolution gives this formal space a granular structure. The state of the system at any instant would then be a specific locus in this phase space. However, in the quantum theoretical representation of the change of state in a system which is interacting with another system, the trajectory in phase space which represents the temporal evolution of the system is "discontinuous." Nevertheless, if one considers interactions between systems in the macrodomain, these quantum disconti- nuities become negligible relative to the large scale transfers of action between systems, hence making the "classical ideal" virtually attainable. Thus within the limits of measurable preci-



Quantum Theory 193

sion, the "special case" of interactions between systems in the macrodomain can be represented by the "smooth" continuous phase space of the classical framework.

Therefore, if we presuppose that the quantum postulate presents a true account of what happens in an interaction in the microdomain, then since the interacting systems change state discontinuously in a way which is not determined by the state parameters prior to the interaction, after the interaction it is impossible to define their separate mechanical states. Hence, the quantum analog to the classical state function of two systems in interaction is defined only for the whole of the interaction, not for the separate component interacting systems. In other words, the quantum formalism permits defining only a single physical state for the two systems after their interaction. The fact that classically we were able to distinguish separate states for the components of an interaction is due only to the special case assumption that at this level the interaction between systems is so enormous compared to that at the atomic level.

Because the separate states of the interacting systems are not defined in the quantum formalism, it becomes impossible to determine theoretically the precise value of the state parameters of either of the formerly interacting systems at any time after their interaction. Nevertheless, the formalism can be con- firmed only if observations can in fact determine such state parameters. Consequently, if the microsystem is made to interact with an observing system or "detector," the resulting interaction will be interpreted as one which measures a particular value of that parameter. A formalism which could make no predictions whatsoever of the result of such an observation would of course become empirically untestable and scientifi- cally worthless. But this is not the case with the quantum formalism, because what it does permit predicting is the probability of an observation to determine a state parameter on any system having a specific outcome. Thus in the formalism the state of a system after it has interacted with another system is defined as in a "superposition" of different possible states, each yielding a certain probability of observing a particular state parameter with a particular value. In this way the confirmation of the formalism consists not in the successful prediction of the outcome




of single observations, but in the successful prediction of the probability distribution of observations made on a large collection of "identically prepared" systems. In other words, prior to observation the formalism defines a "quantum state function" which characterizes the system not by any precise values of observable parameters but rather as having the probability of being observed with this or that value. However, when it is observed, the system is found to have one precise value out of these possibilities.

What does this quantum state function refer to? Two possibilities appear to be open. First, it could be that the interaction puts each system in a definite state with precise values of the observable state parameters. However, because the change of state in the interaction does not follow any deterministic laws involving the parameters which now define the state of the system, it is impossible to say which of these possible states any particular system is in. Different identically prepared systems which interact will, as a result of this interaction, find themselves in different determinate states, but we can predict only a probability distribution for a collection of such systems. Ac- cording to such an interpretation the indeterminacy inherent in the quantum state description expresses an "epistemic" bar- rier: we are unable to know what state the system is in, because of the discontinuous change of state caused by the interaction necessary to determine its state by observation.

On this possibility we could continue to hold - perhaps for quite independent reasons - that the system actually is in a state with determinate values of the state parameters. If we take this option, then we are able to retain the model-based philosophy of nature which supported the realist interpretation of classical physics by allowing us to picture the system as in a particular state at all times. However, if we do so, then we surrender the claim that the quantum state function as defined by the current formalism represents an objective feature of reality; it now would be seen to characterize instead only what we know about reality. Therefore, a defender of this interpretation is not likely to rest content with the current formalism but will regard it as "incomplete." Such a conclusion would make it rational to search for a better way to characterize the microsystem in terms of hypothesized "hidden variables" which would



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make it possible to define what is presumed to be its objectively possessed mechanical state.

The alternative option would be to accept the completeness of the description permitted by the current formalism and regard the system as not existing in a state with determinate values of the state parameters apart from its interaction with the observing instruments. In this case the discontinuous change in the state function which takes place when we observe the system {i.e., when we represent the system as interacting with another system) is not a reflection of the change in our knowledge merely, but represents some kind of real event taking place at the microlevel.

Since the different outcomes of an observational interaction are represented in the quantum state function by the "superposition" of different waves to form a "wave packet," in an observation where one of these outcomes takes place, the "wave packet" is said to "collapse" or be "reduced." But this discontinuous change of state should not be understood as simply a "jump" from one mechanical state to another. It is not a kind of change of state that can be represented as a change of mechanical state picturable by any classical model. It is instead the selection of the one outcome out of a range of possibilities which is the result actually recorded.

Consequently, in the discontinuous change of state which takes place in an interaction, the realist who accepts the completeness of the quantum description and a correspondence theory of truth confronts the infamous problem of the "reduction of the wave-packet." Thus we discover that if the quantum theoretical formalism is taken to provide all that can be known about nature at the microlevel, it not only precludes a classical model interpretation, but also mightily resists any alternative interpretation based on correspondence lines. Is this "reduction of the wave packet" a non-mechanical physical event taking place in observing interactions? Does this discontinuous change of state refer to something that happens in reality, or is it an artifact of the theoretical formalism? Reflection on the strangeness of this discontinuous change of state has led one philosopher after another away from realism into anti-realism.

However, we could escape anti-realism by another exit: we might consider giving up the belief that the current quantum



196 SOUNDINGS Henry I Fohe

formalism defines the state in a way which contains all that can be known about the microsystem, and we might hold out the hope for hidden variables which will allow a complete description. Of course taking this option is equivalent to refusing to base a philosophy of nature on what is undoubtedly the best theory we have of the behavior of the microdomain. If we take this route, we are not rejecting a philosophy of nature as impossible, but deferring it until physics produces a more complete theory. In this way we could preserve realism as an interpretation of science in general, but we could avoid having to come to grips with the thorny task of formulating a new philosophy of nature based on quantum theory. Although this may seem an attractive alternative, and indeed it has had a dis- tinguished history, it now seems to be an option which has been eliminated by recent experimental findings. Thus in what may well be regarded as the last battle of the quantum revolution, experimentally produced phenomena reveal microsystems behaving in a way which dictates that the realist must either confront the necessity for radical reconstruction in the philosophy of nature or surrender unconditionally to anti-realism.

Philosophy of Nature After the Quantum Revolution

Ironically it was the quest for the possibility of just those sorts of hidden variables which would allow retaining a mechanical model-based philosophy of nature that led to these experiments. The original version was a thought- experiment designed by Einstein and presented in a famous paper in con- junction with Podolsky and Rosen.7 The authors claimed that one could show that the parameters which define the mechanical state of an unobserved system are an "element of reality," or in other words that the system does exist in a particular well- defined state even though all the quantum formalism can provide is the probability distribution for a superposition of different states. Thus it was urged that the quantum formalism was incomplete.

Twenty-nine years after Einstein proposed his thought-experiment, by reflecting on a descendant of the original version, John Bell showed it was possible to design a physical situation in which Einstein's seemingly reasonable assumptions would lead to predictions different from those of quantum mechan-



(Quantum Theory 197

ics.8 In the last several years the consensus has emerged that experimental tests have produced phenomena which are in- compatible with these assumptions but perfectly well predicted by the quantum formalism.

This result has been widely accepted as dealing the death blow to all hidden-variable theories of a sort which would allow resuscitation of the model-based realism of classical physics. This conclusion can be seen most easily by considering the par- adoxical situation that results from attempting to read the space-time picture used for interpreting the experiments as a representation of the properties of an independent reality. The experimental arrangements which produce what are now called "Bell phenomena" are described by a picture of two "particles" which interact at a central point and then move in a straight line in opposite directions away from each other towards detectors which measure some state defining parameter of each. Crucial to Einstein's reasoning had been the assumption that mechanically isolated systems must exist in separate mechanical states. After their initial interaction the spatial separation between the particles assures their mechanical isolation from each other. According to the principle of separability this means that the state of each system cannot be affected by what happens to the other. Nevertheless, as a consequence of their initial interaction, only the state of the whole system of two particle subsystems is well-defined by the quantum formalism. Of course all that can be predicted from this state is a probability distribution for the outcome of measurements determining the parameter of either paniculate "subsystem."

Is this lack of determination the consequence of a limitation of the formalism or does it represent some objective feature of the state of the quantum mechanical objects? The particular genius of the experiment is that it arranges the situation such that the two different alternatives of this disjunction have different observable consequences. The parameters which are measured are found to be correlated in a way such that the sta- tistical distribution of the outcomes rules out the possibility of either having had a definite value of that parameter at their mo- ment of interaction. Thus experiment decides the question between the hope for hidden variables versus the completeness of the quantum description dramatically in favor of the latter.



198 SOUNDINGS Henry J. Folse

The inescapable conclusion is that the one outcome out of the set of possible outcomes which is in fact observed in the measurement is made actual only in the interaction in which the detector measures the relevant parameter. Nevertheless the fact that the outcomes of the observations made on the two subsystems are strictly correlated seems to imply that the two are somehow connected. In other words, it would appear that a measurement on one subsystem somehow determines the outcome of a measurement on the other. Since the systems are mechanically isolated after their interaction, how could the measured value of one property - which becomes determined only when it is observed - affect the measured value of the correlated property of the other subsystem? If we assume that the two systems cannot be characterized by definite values of the relevant properties before they were measured, then, the observed correlation between their measured values could not have been determined at the time when they were interacting before the measurement. Furthermore, the detectors can be designed such that the particular parameter which the detector measures is only chosen after the particles have separated. Thus it seems that if the observed outcomes of measurements made on the two separated subsystems are somehow connected, that connection can take place only after they have separated and are mechanically isolated from each other. What sort of connection could this be? It would seem that each particle on interacting with its detector instantaneously "knows" the fate of its brother's interaction with its detector, even though the two can be separated by more than twenty meters.

How, if at all, can such a phenomenon be explained? The obvious manner of postulating an undetected communication between the systems runs afoul of relativistic limits in positing what would have to be a superluminal signal between the component subsystems. However, it has now been shown that the nature of the correlations is such that it does not allow sending information superluminally.9 Thus, quantum mechanics is not a heresy from the relativistic point of view.

Instead, the consensus today seems to favor speaking of the "wholeness" of the state of the single system of the interacting pair of subsystems.10 This would mean that quantum theory forces abandoning the classical separability principle which as-



(Quantum Theory 199

sumed that spatially separated systems exist in separate states which can causally affect each other only through spatial contact or communication through some medium which can prop- agate no faster than light. However, this denial of separability is tantamount to admitting that the classical spatio-temporal models of the states of physical systems do not in fact correspond to the properties objectively possessed by real objects existing in mechanical isolation from observing instruments.

If a defense of realism is conceived to be equivalent to constructing a space-time model with properties corresponding to the state parameters, then it seems the wholeness of the quantum state precludes a realist interpretation of the theory. If explaining the correlations is tied to presenting a mechanical picture of how one subsystem affects the other, then the denial of separability seems to exclude explaining the correlations. However, realism need not be so constrained; we may accept that the explanation provided by the formalism as it now exists is the only explanation we are going to get. Accordingly, we must conclude that the reason for the correlations between the subsystems lies in the fact that the two do not exist in separate states, but share a single indivisible whole state.

Thus a quantum-era philosophy of nature cannot presuppose that the microsystems which compose material objects are the seats οι possessed properties that in some way correspond to those they are "observed" to have when interacting with the systems which are interpreted as measuring or determining their state parameters. Because the Bell phenomena indicate that we can regard the state parameters as having determinate values only in the interactions in which the values of these parameters are determined, these parameters cannot correspond to properties possessed by the object system apart from interaction. Instead they may be conceived as referring to relations which cannot be reduced to properties separately possessed by observed and observing systems in the interaction. Because the state parameters refer to such irreducible relations, the observational interaction has a "wholeness" which prevents attributing separate states to the parts of the interaction.

Of course we can still describe the outcomes of these observations in the spatio-temporal terms of the classical framework, and from such observations we can construct pictures of "parti-




cles" and "waves" traversing empty space just as though these unobserved systems did possess the properties necessary for being particles or waves. But because we cannot regard the microsystem as possessing such properties, these pictures cannot be interpreted as the snapshots of a ghost spectator representing what reality looks like when no one is looking. Nature at the microlevel consists not of entities possessing properties corresponding to any observable properties of the objects of ordinary human experience. Instead nature itself (not merely our knowledge of it) is characterized in terms of the probability of outcomes of observational interactions. As a consequence of this fact, the reduction of the wave packet can be understood as representing a transition from objective potentiality to actuality.

Over thirty years ago Heisenberg suggested the term "poten- tia" to characterize quantum mechanical objects as exhibiting such an objective potential for the actualization of observational outcomes. 1 1 Moreover, Bohr was fond of speaking of the "wholeness" of interactions:

A new epoch in physical science was inaugurated ... by Plank's discovery of the elementary quantum of action, which revealed a feature of wholeness inherent in atomic processes, going far beyond the ancient idea of the limited divisibility of matter. Indeed, it became clear that the pictorial descriptions of classical physical theories represents an idealization valid only for phenomena in the analysis of which all actions involved are sufficiently large to permit neglect of the quantum.12

However, although such ideas have been in the literature for a considerable time, and were possibly in the minds of some of the founders of the theory, the common mistake that the orthodox quantum theorist is an anti-realist and the hope held out for hidden variables have led philosophers not to take them so seriously. Thus philosophers have not attempted to develop a detailed systematic philosophy of nature along such lines.

Whatever reconstruction in the philosophy of nature will be required to bring it into conformity with contemporary physics, it must reflect a revision in our epistemological beliefs about how we acquire knowledge of nature. Since the quantum revolution makes it impossible to formulate a mechanical model of the microsystem, a realist who accepts the completeness of the quantum description must abandon the "spectator"



(Quantum Theory 201

epistemology which led to the space-time picture being regarded as the ideal of objective knowledge. Instead, as a result of the quantum revolution, the realist must now replace the correspondence theory with an account of truth in which "truth" resides in the successfully predictive description of the observer's interaction with the observed rather than in the spatio-temporal picture or representation of an independently existing, but inaccessible, order of nature. Clearly if we accept the quantum postulate as a fundamental fact of nature, the spectator theory of knowledge is seen to rest on a false presupposition, an "idealization" which was good enough for pur- poses of describing everyday objects, but is not strictly true. Thus "objectivity" cannot mean what it was thought to mean in the classical framework.

Nevertheless, scientific knowledge still requires that we preserve "objectivity" in the sense of presenting an account in which a clear distinction is made between the physical system which is considered the observing system and the microsystem which is the object of observation. Without such a distinction, no interaction can be interpreted as an observation in which the value of some parameter is measured, and so no unambigu- ous information about the objects of such observation could be communicated. But we can learn about microsystems in quantum physics, because we can continue to interpret "observations" of microsystems in terms of the classical concepts of particle and wave. Those phenomena which are described as "observations" of microentities can be theoretically interpreted in terms of space-time pictures of the careers of particles and wave disturbances in a field. Nevertheless, these conceptual pictures cannot be the basis for a realistic philosophy of nature because the fact that the property which is measured exists only as a relation implies that whether the microsystem will be observed in a way that requires characterizing it as a particle or as a wave will depend only partially on the state of the system; it will also depend partially on what sort of system the microen- tity interacts with to create the phenomenon of the observational interaction. Therefore, the "pictures" we present in interpreting an observational interaction cannot be regarded as corresponding to the properties of an objective reality. Thus wave and particle pictures are "abstractions" that do not refer




to any real state of affairs. The epistemological lesson of the quantum revolution teaches that we know nature not by "picturing" it, as was classically presupposed, but rather by interacting with it.

The knowledge of atomic systems which we acquire by ac- cepting the quantum theoretical description is not that of a detached spectator but rather is that which enables us to say that within any particular observational interaction there is an objective probability that a specific sort of phenomenon will occur. Hence the quantum revolution entails the overthrow of the Cartesian spectator account, because what we've learned is at least this: the classical representation of reality in terms of the careers of spatio-temporally defined substances which possess sets of time-dependent properties is inconsistent with contemporary physics, because it rested on the idealization that one could observe physical systems as though interacting with them does not affect their states. This was justified on its view because "observation" in the Cartesian framework refers to an event in the cognitive domain, i.e. , the human "mind," and thus even though the careers of spatio-temporal substances are the "cause" of this observation, it cannot be described as -à physical interaction. The Cartesian ideal which pictures what the universe would look like even if no one was there to look at it is the viewpoint of a ghost spectator who pilots without physical effort his corporeal submarine through a space-time sea.

In a quantum-era philosophy of nature this dualist approach to observation must be thrown out. The empirical starting point of science is the description of a phenomenon through a very specialized set of concepts in which that phenomenon is described as an observation of a neutrino or a quasar. Thus the description of observation as interaction which was exactly what had to be left out in the classical account now becomes exactly what it is that is described in the quantum description of microphysical reality.

For this reason the realistic interpretation of quantum physics requires not only that we discard the spectator account of knowledge, but it also denies the presupposition that "truth" refers to a property of statements and exists in virtue of a reference relationship between terms in these statements and the properties of an independent reality to which these terms cor-



Quantum Theory 203

respond. Now we learn that in physics we characterize an independent reality not by attributing properties to some substantial entity which is imagined to possess those properties apart from our interaction. Instead we characterize it through the phenomena which occur in our interactions with it. Truth, then, is not a property of statements but a property of the whole theoretical structure which allows us to predict those sorts of phenomena, and such a theoretical structure has that "truth" in virtue of its power to predict successfully precisely those phenomena in which we are said to observe these objects.

The collapse of the hope for a hidden- variables theory which would preserve the separability of the states of mechanically isolated systems, and the dispelling of the myth that quantum physics was conceived in an anti-realist spirit now make it necessary to take seriously a philosophy of nature which represents real microentities as the seats of objective potentials for interaction. Such a philosophy of nature will no longer characterize as ontologically fundamental those primary properties which characterized the classical body. Material objects are not vast collections of tiny extended bodies, Democritean atoms or Car- tesian rei extensae. In breaking the presumed link between the primary properties of the classical mechanistic framework and those properties which are conceived to be ontologically fundamental, the quantum revolution point towards a philosophy of nature which "atomizes" not bits of matter, but elementary processes of interaction.

NOTES

1 . In fact a question such as this has been put to me many times by friends when they learn that I have written on the philosophical implications of quantum theory.

2. I am thinking here of the writings of such philosophers as Abner Shimony, Arthur Fine, Bas vanFrassen, Nancy Cartwright, Paul Teller, Michael Redhead, and others. I do not want to suggest that these philosophers are just in a state of wandering perplexity, quite the contrary, but one cannot read them without feeling that they are deeply aware of just how strange the quantum world appears to behave.

3. The "orthodox" interpretation is generally associated with the view defended by Niels Bohr and those who claimed him as their philosophical mentor. The phrase "Copenhagen Interpretation" is commonly used to refer to this supposed point of view. Actually Bohr never used or ap- proved of this label and his outlook has had almost as many interpreta-



204 SOUNDINGS Henry I Fohe

tions as it has had defenders and critics. Nevertheless, it is certainly not the case that Bohr defended a "facile positivism" which banned "reality" from physics, as I have argued in The Philosophy of NieL· Bohr: The Frame- work of Complementary (Amsterdam: North Holland Physics Publishing, 1985); "Niels Bohr, Complementarity, and Realism" in A. Fine and P. Machamer, eds., PSA 1986: Proceedings of the Biennial Meeting of the Philoso- phy of Science Association, Vol. I (East Lansing, Michigan: Philosophy of Sci- ence Association, 1986) 96-104; "Complementarity and Scientific Realism" in P. Weingartner and G. Dorn, eds., Foundations of Physics (Vi- enna: Holder-Pichler-Tempsky, 1986) 93-101; "Realism and the Quan- tum Revolution" in Abstracts of the 8th International Congress of Logic, Methodology, and Philosophy of Science, Vol. 4, Part I (Moscow: Institute of Philosophy of the Academy of Sciences of the USSR, 1987) 199-200; and "Niels Bohr's Concept of Reality" in P. Lahti and P. Mittelstaedt, eds., Proceedings of the Symposium on the Foundations of Modern Physics: The Copenha- gen Interpretation 60 Years After the Como Lecture- Joensuu, Finland, August 6-8, 1987 (Singapore: World Scientific Publishing, forthcoming). Other recent work on Bohr and the origins of the quantum revolution have also supported this reading of the "orthodox" position; cf. John Honner, The Description of Nature: NieL· Bohr and the Philosophy of Quantum Physics (Ox- ford: Clarendon Press, 1987), and Dugald Murdoch, NieL· Bohrs Philoso- phy of Physics (Cambridge: Cambridge University Press, 1987).

4. An umbrella term like "the realism vs. anti-realism debate" suggests two sides disputing a single issue. In fact nothing could be further from the case; there are today so many issues and positions no one has bothered to keep count. Nevertheless, in at least some discussions there has been an effort to distinguish between ontological and epistemological questions. Cf for example Ian Hacking, Representing and Intervening (Cam- bridge: The University Press, 1983); Nancy Cartwright, How the Laws of Physics Lie (Oxford: Clarendon Press, 1983); Brian Ellis, "What Science Aims to Do," in Images of Science, in Paul M. Churchland and Clifford A. Hooker, eds., (Chicago: University of Chicago Press, 1985) 48-74.

^ »ι» il «·· · « « * * 9 « · · 9 9 é * 9 · · · ď* 5. 1 he distinction between the observed and the observer is ol course inherent in the very concept of an "observation." In epistemology it is usually made in terms of "subject" and "object," and in fact one does find this terminology sometimes in discussions of observation in the atomic domain. However, it tends to encourage a serious misunderstanding, for the world "subject" in philosophy is often taken to mean the subjective consciousness of the human observer. When such a reading is given to the term in quantum physics, the "observational interaction" between the observing and observed systems which the quantum formalism symbolizes by a discontinuous change of state becomes a Cartesian style "interaction" between "minds" and "bodies." This is certainly not what is represented by the formalism of quantum physics, for it treats both observing system and observed objects as physical systems. In the case of human observers it is the interaction between the object and the sense organs qua physical objects with which we are concerned. How the physical change of state of the sense organ "causes" an awareness in the consciousness of the "observer" is no more a matter to be explained by quantum physics than it was in the case of classical physics. Thus the interaction to which the description of observation refers is a purely physical event, not the epistemic event of a consciousness coming to know the



Quantum Theory 205

results of this physical interaction. This single misunderstanding has re- sulted in much nonsense written about quantum physics.

6. This principle figures centrally in recent discussions of the debate between Einstein and Bohr and its subsequent denouement in the Bell phenomena; cf. Section 4 below and the sources cited there.

7. A. Einstein, B. Podolsky, and N. Rosen, "Can Quantum Mechanical De- scription of Physical Reality Be Considered Complete?" Physical Review 47 (1935): 777-80. Bohr's reply appears under the exact same title in Physical Review 38: 696-702. The subsequent literature on the debate is quite huge. Bohr reviewed the debate in his "Discussion with Einstein on Epistemological Problems in Atomic Physics," and Einstein in his "Auto- biographical Notes," both in Paul A. Schilpp, ed., Albert Einstein: Philoso- pher-Scientist (LaSalle, 111.: Open Court, 1949). Recent analyses which shed new light on Einstein's views in the paper are given by Don How- ard, "Einstein on Locality and Separability," Studies in History and Philoso- phy of Science 16 (1985): 171-201; Arthur Fine, The Shaky Game: Einstein, Realism and the (Quantum Theory (Chicago: The University of Chicago Press, 1986); and Max Jammer, "The EPR Problem in Its Historical Set- ting," Symposium on the Foundations of Modern Physics (New Jersey: World Scientific, 1985) 142-46.

8. J.S. Bell, "On the Einstein-Podolsky-Rosen Paradox," Physics I: 195-200. Abner Shimony who helped design the experiments explains their philosophical significance in "Metaphysical Problems in the Foundations of Quantum Mechanics," International Philosophical Quarterly 18 (1978): 3-17; and "The Reality of the Quantum World," Scientific Amencan 250 (January 1988): 46-53. Other excellent analyses for the non-physicist are given by Bernard d'Espagnat, "The Quantum Theory and Reality," Scientific Ameri- can 241 (1979): 158-80; and David Mermin, "Quantum Mysteries for Anyone," The Journal of Philosophy 78 (1981): 397-408; and "Is the Moon there When Nobody Looks?" Physics Today 38 (1985): 38-47.

9. Abner Shimony, "Controllable and Uncontrollable Non-Locality," in S. Kamefuchi, et al., eds., Proceedings of the International Symposium: Foundations of Quantum Mechanics in the Light of New Technology (Tokyo: Physical Society of Japan, 1983) 225-30.

1Ü. 1 base this judgment on the papers presented at the recent conference on "Philosophical Lessons from Quantum Theory," at The University of Notre Dame, October 1-3, 1987. These papers are collected in the forthcoming volume, Philosophical Consequences of Quantum Theory, ed. by J. Cushing and E. McMullin (Notre Dame: University of Notre Dame Press, 1989); cf especially the papers by Abner Shimony, Paul Teller, Don Howard, and Linda Wessels, as well as the current author.

1 1 . Werner Heisenberg, Physics and Philosophy (New York: Harper and Row, 1958) 147-66.

12. Niels Bohr, Essays 1958-1962 on Atomic Physics and Human Knowledge (New York: Random House, 1963) 2.



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