Scientific Realism in Real ScienceAuthor(s): Roger JonesSource: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association,Vol. 1988, Volume Two: Symposia and Invited Papers (1988), pp. 167-178Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/192881 .

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Scientific Realism in Real Science1

Roger Jones

University of Tennessee

1. Introduction

Scientific realism is a doctrine about the relationship of our ideas on the nature of things to the nature of things itself. Part of the doctrine is that there is a nature of things itself. With regard to the rest, Jarrett Leplin has said, "Like the Equal Rights Movement, scientific realism is a majority position whose advocates are so divided as to appear a minority" (Leplin 1984, p. 1). Still, it can be said that what realists would like is that the account of the nature of things provided by science be true and that those things really exist.

Characterized in this way, realism would seem to be a majority position indeed. As Ernst Mach has said of the doctrine,

It has arisen in the process of immeasurable time without the intentional assistance of man. It is a product of nature, and is preserved by nature. Everything that phi- losophy has accomplished...is, as compared with it, but an insignificant and ephemeral product of art. The fact is, every thinker, every philosopher, the moment he is forced to abandon his one-sided intellectual occupation..., immedi- ately returns [to realism]. (Mach 1959, p. 37, quoted in Fine 1986, p. 134)

Pre-analytically, we are all realists. We would all like to be realists. But analytically, in the course of their one-sided intellectual occupations, philosophers have offered a number of objections to realism.2 These range from highly abstract and general objec- tions, to objections based on analysis of specific historical circumstances in science.

Another academic community troubled by issues related to scientific realism is that of contemporary physicists. Their difficulties certainly have nothing to do with the general and sweeping objections of philosophers. But neither do such difficulties seem to have been examined in philosophers' analyses of historical cases. So what I want to do here is to provide three illustrations of just what these difficulties with realism are for contempo- rary physicists. These illustrations come from the basic pedagogical tradition in classical mechanics and from the interpretive traditions of non-relativistic quantum mechanics and of general relativity. I think philosophers ought to be aware of these real-science difficul- ties with scientific realism, and I will provide some advice about how they ought to respond at the end of the paper.

PSA 1988, Volume 2, pp. 167-178 Copyright ? 1989 by the Philosophy of Science Association

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2. Realism

I have said that beyond a commitment to a "nature of things itself," advocates of real- ism are severely divided. But I have also said that they share the general hope that the scientific enterprise has the capacity to provide accounts of this nature-of-things-itself that are true, accounts involving just the things that are there themselves.

In what is more or less the "classical" realist position, this hope is elevated to a belief. Indeed, such classical realists are willing to go out on a limb and claim that theories in the "mature" areas of science should already be judged as "approximately true," and that more recent theories in these areas are closer to the truth than older theories. They see the more recent theories encompassing the older ones as limiting cases, and accounting for such success as they had. These claims are all closely linked to the claim that the lan- guage of entities and processes - both "observational" and "theoretical" ones - in terms of which these theories characterize the-nature-of-things-itself genuinely refers. That is, there are entities and processes that are part of the nature-of-things-itself that cor- respond to the ontologies of these theories.

The way in which this reference is fixed, and thus the nature of this correspondence are topics of intense current debate even among the classical realists who follow the position this far. All I want to point out however, is what a hearty and confident doctrine this "classical" realism is. It envisions mature science as populating the world with a clearly defined and described set of objects, properties, and processes, and progressing by steady refinement of the descriptions and consequent clarification of the referential tax- onomy to a full-blown correspondence with the natural order. It is surely a grand ideal. Let me illustrate now how it fares in real contexts.

3. Classical Mechanics

In the beginning of an undergraduate education in physics comes Newtonian dynamics. That is, physics is presented as beginning with Newton's three laws, and they are typically introduced as simply formal generalizations of directly observable particle behavior. (See, e.g., Symon 1960, Ch. 1.) Thus a "particle" is introduced as the unit of matter, a gritty bit whose size, shape, and internal structure are regarded as negligible. Particles have posi- tions at various times, and quantitative measures of positions and times lead to (continu- ous) functional relationships between them. Properties of this kind of functional relation are the velocity and acceleration of a particle at each time. The properties of mass and force are initially introduced quite operationally, as simply other aspects of the functional behavior of particles in space over time. Mass (or the ratio of two masses) is asserted as a constant of proportionality in the relative accelerations of two particles involved in an interaction. Force is introduced as simply the name of the mass-acceleration product, automatically identical, though with opposite sign, for pairs of interacting particles. However operationally they are introduced, however, these two properties of force and mass are soon identified with particle-nature. Force in particular is able to be "exerted" on other particles, and is asserted to be the source of their accelerations.

An early application of this Newtonian dynamical apparatus is to planetary motion. In the style in which Newton himself asserted it, it is asserted that each body/particle in the universe exerts at each instant on each other body/particle an attractive force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. In accord with the previously established dynamical properties of particles, each body reacts to this force (as to any force) by accelerating in the direc- tion of the force in amount directly proportional to the magnitude of the force and inversely proportional to the particle's mass.

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Somewhere toward the end of a good first-year course in physics, it is pointed out that this approach to gravitational interaction is only convenient so long as bodies can be treat- ed as massive particles, i.e., so long as unique centers of gravity can be defined for the interacting bodies. For extended bodies in general, a different approach must be taken. In this approach the gravitational interaction is described by means of a new kind of (general- ly un-analyzed) entity- a field in space, the gravitational potential. This potential varies from point to point, and a massive object placed at a point in space experiences a force in the direction in which the potential gradient is greatest, in magnitude proportional to the potential gradient. This last statement, plus a statement characterizing the variation of the potential from point to infinitesimally nearby point, provides a mathematical route to deriving the law of universal gravitation in precisely the form of the original approach. Introduced to gravitational potentials late in the first-year course, a young physicist-to-be is generally trained to work fluently with them in a second course in mechanics.

Newton's force law and the laws of planetary motion can be derived from an approach more general yet, that based on "minimum principles." In this context, such a principle may be taken to assert that if a massive particle is to proceed from one point to another in some fixed time, then of all possible fixed paths between these two points, that path is physically realized on which some quantity associated with the motion is a minimum. Practically speaking, the generality of the approach stems from the fact that the path need not be described in the familiar coordinates of Euclidean space. The method of charac- terization of the motion of a particle in terms of more "generalized" coordinates opens up to dynamical scrutiny the behavior of systems of many particles coupled by various inter- actions, systems virtually untreatable using previous approaches. One may, for instance, speak of the "configuration" of a many-particle system as characterized by a single gen- eralized coordinate location, and characterize the evolution of the system wholly in terms of a trajectory in "configuration space." But again, for simple systems, results mathemati- cally identical to those derivable from the approaches above emerge. This approach to mechanics, the Lagrangian and Hamiltonian approach, is the substance of the first-year graduate course in mechanics still traditional in most physics departments.

Finally, in light of the most recent reformulation of the laws of planetary motion, one may consider the space of classical Newtonian theory to be curved by the presence of matter. In this most modern theory of "Newtonian gravitation," the gravitational poten- tial field of the approach above is absorbed into the structure of space itself, though time remains an autonomous parameter (thus distinguishing this theory from relativity theory). There is no "gravitational force" in this approach. Massive bodies move in "straight lines" in the curved space (unless some non-gravitational, e.g., electromagnetic, interac- tion intervenes). This approach to particle interactions, particularly gravitational ones, would be introduced as part of a good upper-level graduate course in general relativity.

These then are the stages in the development of competence in classical mechanics, as reflected in the treatment of planetary motion. At each stage the new approach is intro- duced as a generalization of the old, as capable of handling a class of problem inaccessi- ble, or not conveniently accessible, to the old. The power, breadth, and elegance of the new treatment is extolled; but it is never suggested that any "new physics" has been intro- duced. As one widely-used textbook describes the Lagrangian and Hamiltonian approach,

They are not new physical theories, for they may be derived from Newton's laws, but they are different ways of expressing the same physical theory. They use more advanced mathematical concepts, they are in some respects more elegant than Newton's formulation, and they are in some cases more powerful in that they allow the solutions of some problems whose solution based directly on Newton's laws would be very difficult. The more ways we know how to formulate a physi- cal theory, the better chance we have of learning how to modify it to fit new kinds of phenomena as they are discovered. (Symon 1960, p. 3)

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The point is, all the approaches to planetary motion described above, as viewed from the standpoint of the education of a young physicist, are somehow on a par, are "different ways of expressing the same physical theory." It is important that they all be part of the mathematico-conceptual repertoire of a young physicist, the better for him or her to be able to handle "new kinds of phenomena as they are discovered."3 In an image provided by the physicist Richard Feynman to characterize his attitude toward such a multiplicity of approaches to a particular fundamental law, "It is like a bridge with lots of members, and it is over-connected: if pieces have dropped out you can reconnect it another way" (Feynman 1965, p. 47; his discussion of Newton's law appears on pp. 50-53).

Actually, though this over-connected bridge serves the young physicist best as one of both mathematical and conceptual structure, the mathematical structure is considerably more over-connected than the conceptual. Though they are rarely analyzed in standard textbook treatments, the explanatory accounts provided of planetary motion in the approaches above are in fact radically different. In the first approach, for instance, the approach attributed to Newton, the law of universal gravitation is usually taken to describe the properties of a fundamental gravitational force which has about it a renowned kind of dual non-locality: the gravitational force is associated with the instantaneous positions and masses of massive bodies in empty space, bodies which respond to this force instantaneous- ly and as they move about instantaneously change it.4 The second approach described above does characterize the gravitational interaction in purely local terms. But it does so by means of the introduction of a new physical entity into the explanatory picture - the poten- tial field. This field is eliminated if physical space - heretofore treated (implicitly) as a flat, Euclidean space - is assumed to be curved by the presence of matter, as in the fourth account. But then a kind of causal efficacy is associated with the structure of space itself. Finally, the approach in terms of a minimum principle seems to have no connotations of causality at all. The instantaneous motion of a massive body in this approach is determined by a property associated only with a complete path between two points in space.

Of course these sketches of the explanatory structures associated with the diverse mathematical approaches to classical mechanics ignore a whole wealth of difficulties. It might be argued that none of the explanatory frameworks will bear detailed scrutiny, that none provides the kind of thorough causal account which may be considered an important part of any fully satisfactory explanation. Certainly Newton was never satisfied with his understanding of the nature of the gravitational force. (See Note 4.) Associating the approach in terms of a minimum principle with a causal account is difficult even on the face of it. (See, e.g, Yourgrau and Mandelstam 1968, Ch. 14.) The field concept itself has its own problems (again, for a recent reference, see Nersessian 1984), as does an account of the causal efficacy of spatial structure (see Friedman 1983, pp. 67ff).

So what is "the account of planetary motion provided by classical physics?"5 I hope it is clear that in the mind of a young physicist, there is likely to be no univocal, canonical account of the above description. All the approaches described above lie within the vast- ly over-connected structure of concepts labelled "classical physics," even "Newtonian dynamics." They in some sense "save the same phenomena," but with very different explanatory frameworks, very different ontological commitments. Even if a young physi- cist is a non-critical realist, he or she will have trouble when asked to articulate the funda- mental (theoretical) furniture of the Newtonian universe. He or she doesn't know, in some canonical sense, what to be a realist about.

4. Quantum Mechanics

In classical mechanics the alternative explanatory frameworks (with their alternative ontologies) were associated with different mathematical approaches. But even taking the most straightforward approach to quantum mechanics - ignoring the stepping stones of the original Bohr-Sommerfeld theory, Schrodinger's wave mechanics, and Heisenberg's

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matrix mechanics; ignoring contemporary reformulations such as those due to Feynman and Schwinger - considering only the non-relativistic theory in its standard formulation, the same kinds of difficulties for applied realism are present: there is a variety of interpre- tations of the mathematical formalism of the theory and no single interpretation has really survived detailed scrutiny.

The existence of such a multiplicity of interpretations for this formal apparatus of quantum mechanics is so well known that I will merely point to it here.6 Physicists readi- ly characterize themselves as holding a version of the Copenhagen interpretation, or the statistical interpretation, sometimes even as accepting an approach to the theory in terms of quantum logic. More recently, many physicists express themselves particularly inter- ested in the so-called "many worlds" interpretation due to Hugh Everett (Everett 1957, reprinted in DeWitt and Graham 1973).

It is important to underscore three aspects of this interpretive multiplicity. First is the disparateness of these various interpretations. The worlds that they picture are utterly dif- ferent. Their ontological commitments are different; their imputations of causal structure are different; their focus on the mathematical structure is different. In some cases it is difficult even to translate from the role of a certain element of the mathematical structure in one interpretation to that in another. A second aspect of this interpretive multiplicity is the failure of any interpretation to provide an "explanatorily satisfactory" link between the mathematical formalism and the world of laboratory experience. The unsatisfactory elements themselves vary among the interpretations, but there are difficulties with every one. This certainly is part of the reason for the persistence of multiplicity here, for the failure of any one interpretation to emerge as "standard." Finally, it is important to appre- ciate the vividness of these views for their adherents, and the fervor with which they identify the ontology of the theory thereby. This point can be fully appreciated only by observing physicists in their moments of speculative candor - in late Friday afternoon conversations with graduate students, in dinner-table controversies during conferences. I can but heartily recommend these moments.

But then what is "the account of microworld behavior provided by quantum physics?" The interpretive state of quantum physics - even in terms of this single theoretical framework - simply does not allow a univocal, canonical such account to be identified. The general approach of one interpretation may suit a physicist more than the general approach of others, and he or she may spend some time adapting it to issues that he or she thinks particularly important and developing arguments as to why its lacunae are not dev- astating for its coherence. But every physicist will admit that such allegiance is to some degree a matter of taste. No physicist is unaware of competing interpretations, and none expects there to be decisive evidence, or argument, for one against the others. Physicists don't know what deep explanatory structure of the microworld to be realists about.

5. General Relativity

Mathematically, the moder, standard approach to relativity theory begins with the pos- tulation of a four-dimensional, differentiable manifold, generally called "space-time." Associated with this manifold, purely on the basis of its mathematical structure, is a host of derived structures, most notably tensor fields of various sorts. The "space-time frame- work," as an interpretive stance, postulates various of these fields as "physically distin- guished," that is, as in some sense physically interpretable. What this amounts to generally is that one may identify various of these fields as having the attributes, or some of the attributes, by which we have traditionally recognized certain physical concepts. One may identify, for instance, fields taken to characterize the spatio-temporal history of material particles, their mass and charge distribution, their energy. More particularly, the general theory of relativity postulates a certain tensor field as "the metric field" of space-time, and postulates certain relationships among this field and other physically distinguished fields.

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In textbook presentations, this metric tensor field is almost universally regarded as ontologically fundamental. (See, e.g., Misner, Thome, and Wheeler (hereafter MTW) 1973, Chapter 13.) It is part of the essential nature of space-time, its basic geometricality, one of whose aspects is curvature. This curvature is linked by the field equations of the theory to the density of matter and energy, expressed in a tensor field usually character- ized as the "stress-energy field." There is some ontological ambiguity even at this stage of typical textbook presentations, however. Does the stress-energy field share the funda- mental status of the metric field? Or should this status be reserved for the fields from which it is computed - of which it is in some sense composed - for example the mass and charge fields just mentioned? What about the fields in terms of which it is locally measured by variously moving observers?

These kinds of interpretive issues - the status of composed versus composing fields, that of locally measurable properties of space-time and the intrinsic structures they arise as manifestations of - are not my particular concern here however. What I want to concentrate on here are difficulties which arise when one considers particular solutions of the field equa- tions of the general theory, as one must in order to apply the theory in the concrete world.

One of the most discussed solutions of the field equations is that due to Karl Schwarzschild, interpreted as describing an isolated, spherically symmetric, massive body. All applications of the general theory to solar system phenomena usually begin with the Schwarzschild solution, so it is very important. To focus on an example, consid- er measurements of the deflection of starlight in passing close to the sun.7 In these mea- surements apparent stellar positions are compared in the presence and absence of the sun. The Schwarzschild solution provides light and particle trajectories in the neighborhood of the massive body, and these trajectories can be compared to the results of measuring these positions.0 More importantly for the ontological prospector, however, the solution provides the very notions of "isolated," "spherically symmetric," and "massive," as well as other important components of our intuitive conceptual vocabulary, e.g., the notions of global space and time.

The Schwarzschild solution is extremely unusual in this respect, for it is a static solu- tion. From the four-dimensional space-time structure of such a solution global three- dimensional and one-dimensional sub-structures can be projected. But this possibility is not uniquely a property of a static solution. What is unique is that one can interpret the four-dimensional structure in this case to have been factored into a global three-dimen- sional "space" and a one-dimensional "time": the spatial distances between the points of the three-dimensional space can consistently be interpreted as the space-time distances between points judged, in terms of the one-dimensional time, to have simultaneous posi- tions (Earman 1970, p. 260).

Another special feature of the Schwarzschild solution is its "asymptotically flat" char- acter: the curvature of space-time vanishes as one approaches "infinity" purely spatially, and along the trajectories of particles and light rays. It is only for such a metric structure that the notion of an "isolated body" makes sense at all, and only for certain kinds of such structure that a property such as "mass" can be ascribed to the isolated body (Geroch 1977; MTW 1976, Ch. 19; Wald 1984, Ch. 11).

Thus, the availability of this whole classical vocabulary for the starlight deflection experiments hinges on the status of the Schwarzschild solution as a description appropri- ate for phenomena within the solar system. At the present time, there seems to be general consensus among working astrophysicists that it is quantitatively adequate for the analy- sis of starlight deflection data. (See, e.g., Shapiro 1980, p. 122.)9 But the qualitative divergences of the idealization from what is known independently about solar system behavior are obvious. After all, the sun is neither static, nor spherically symmetric, nor isolated. It is rotating, oblate, and accompanied by some planets of considerable mass.

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The problem in all this for the ontological prospector is that a progression to a less idealized description that takes the form of giving up the Schwarzschild solution also requires re-evaluation of the status of the traditional conceptual vocabulary in this case the description in terms of space, time, and mass. Even treating the sun as a rotating body, for instance, obviously a less idealized approach than assuming it not to be so, and one accomplished analytically by employing the Kerr solution to the field equations (MTW 1976, Ch. 33), involves interpretive problems with "space" and "time": the deli- cate relationship between distances in the three-dimensional subspace and space-time dis- tances between points judged simultaneous in terms of the one-dimensional subspace that permitted us to identify global "space" and "time" is lost.

Such problems with idealized treatments of complex systems are not unique to the applications of relativity theory. How does the typical physicist respond? One familiar response is to argue that, locally, the failure of the mathematical conditions of definability would be operationally negligible, that the gravitational field of the sun by itself is suffi- ciently weak and its rotation sufficiently slow that this local region might well encompass the solar system, and that the concepts of space and time are on these grounds applicable in these circumstances. While possible for the starlight deflection experiments, such a line cannot be taken even on experimental grounds in the analysis of planetary motion. Relativistic effects due to the sun's rotation play a distinct role in the interpretation of measurements of Mercury's orbit (Shapiro 1980, p. 127). In any case, the conditions for global definability of space and time clearly do fail analytically when the rotation of the sun is taken into account. Their existence thus becomes approximate, subject to detailed analysis of these particular circumstances of application of the Kerr solution.

One would expect a similar problem with the notion of mass were the sun and Jupiter, say, to be treated analytically as a genuine two-body system. No general solution corre- sponding to such a system is known. But certainly the picture of "two interacting mas- sive bodies" would be compromised in a fully relativistic treatment, and more so in the case of an orbiting, close binary star system (such as the binary pulsar, Shapiro 1980, p. 127; see also Ehlers 1980, p. 287).10

What I hope to have illustrated in this extended example is that the ontological com- mitments of the general theory are far from straightforward. One might say of space and time that they "exist only locally," in several senses. "Total mass," even of something like the sun, treated in the Schwarzschild solution, may "exist only at infinity." (Specifying the location of, i.e., the conditions for the definability of, infinity itself is an interesting problem, one that has been largely solved, albeit with different strategies and results for approaches to it along spacelike and lightlike trajectories. See Geroch 1977.) In fact specifying the conditions under which properties such as "total mass of a system" may be defined in general, that is, the conditions on a solution of the field equations such that some derived structure with some of the attributes by which we recognize "mass" can be identified, is an ongoing research problem among relativity theorists.11

Let me recapitulate the points of this section to make the implications for ontological commitment concise.

General relativity postulates a four-dimensional manifold, ordinarily called space-time; it postulates a physical field, called the metric field, on this manifold. This field is coupled to other physical fields on the manifold by Einstein's field equations, which relate the cur- vature at each point of space-time to the density of matter and energy there. To apply the theory to the concrete world one must assume a solution of the field equations. But a solu- tion describing all details of the known cosmos is unknown and almost surely unknow- able, and all known solutions are enormous idealizations. For some solutions structures may be defined with attributes we have associated with such concepts as space, time, mass, energy, charge, momentum. Sometimes these structures may be defined convenient-

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ly; other times the definitions are extremely cumbersome; often they are not available. All such structures are tied together by the elegant and powerful mathematics of the theory as aspects of "the gravitational field," interwoven with the omnipresent metric tensor field of space-time with its dual geometrical and material significance. One will not, in this sort of theoretical and interpretive framework, I think, ever settle on an intuitively appealing canonical set of entities and their properties to characterize the relativistic universe. A contemporary relativity physicist, no matter how pre-analytically inclined to realism, will not be able to determine what to be a realist about.

6. Ontology in Contemporary Physics

I have tried in these three examples to capture at least the spirit of the ontological scene in contemporary physics. The examples have come from the pedagogical tradition in clas- sical mechanics to which all physicists are exposed, and from two of the major theoretical frameworks which dominate contemporary research. The focus in the examples has been chosen in an attempt to make them broadly representative of one locus of the sources of ontological ambiguity in contemporary physics. Thus, the example of classical mechanics features a variety of interpretive frameworks each associated with a distinct mathematical formulation. In quantum mechanics, the focus is on the variety of interpretive frameworks arising from a single mathematical formulation. And in general relativity, the ontological ambiguity is presented as arising from within a single interpretive framework associated with a single mathematical formulation, as a result of the inevitable idealizations necessary to apply the theory in concrete contexts. The focus in each case seems to me appropriate to the way in which the area of physics is understood by most contemporary physicists.

But there are additional aspects of the ontological scene that need to be mentioned even to begin to fill out the account. In the first place, the interpretive situation in nearly all areas of contemporary fundamental physics is a good deal more complex than I have indicated. There are always alternative mathematical formulations for the fundamental equations in any of these areas of physics. There is always a multiplicity of interpreta- tions, in each inevitably some problems of articulation, for any mathematical formulation of any fundamental equations. There is always a host of theoretical and physical ideal- izations variously appropriate to applying any fundamental equations in concrete circum- stances. Moreover, the panorama of formulations, interpretations, and idealizations is always changing: new ones are being introduced and in some cases proving fertile; the potential of others is being exhausted and their role in active research is declining. So the characterizations of the areas of contemporary physics I have provided give only glimpses of what is a much more elaborate and constantly changing environment of onto- logical ambiguity within the community of physicists.

But there is yet another element of structure in this ontological environment. I have focussed on what physicists themselves would say when asked to provide an explanatori- ly satisfactory account of some class of phenomena. The additional element of structure comes from the fact that physicists think and talk about a theory in different ways depending on what they are doing with it. Physicists write textbooks about theories and teach the theories out of textbooks at basic and advanced levels; they engage in medium- scale theoretical speculation about theories; they articulate them theoretically, seeking solutions to fundamental equations; they do calculations seeking precise values for in- principle measurable quantities; and they do calculations aimed at a particular measure- ment of a particular quantity associated with a particular system by means of a particular piece of apparatus. It has been my experience that physicists' ontological focus (at least) depends strongly on which of these activities they are engaged in. Thus, a relativity physicist engaged with the theory in high and mid-level theoretical articulation- either in teaching or research - may speak exclusively in terms of relatively abstract manifold structure, at best of metric and stress-energy fields. Whereas in contemplating light tra- jectories in the solar system, the talk may be all of distances, velocities, and the mass of

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the system. Unchallenged, physicists do invoke these various ontologies in a realist way; so this observation adds only quantitative richness to the picture of ontological ambiguity I have developed so far. But sometimes even the nature of physicists' ontological com- mitment seems to vary with their focus on the theory.

In the first place, a physicist talking like a realist while engaged with a theory in one way, or concentrating on one level in its articulation and elaboration, may take a less than realist posture with respect to ontological commitments particularly associated with anoth- er level. Then again, it has been my experience that as one passes down the ladder of activities mentioned above, from fundamentally theoretical to more applied ones, one is more apt to hear talk of a more instrumentalist flavor. That is, physicists may talk like realists when engaged in theoretical speculation and articulation within the theory - seek- ing new solutions to fundamental equations, proving theorems about the general properties of solutions, elaborating the interpretations of solutions and such. They may talk like real- ists when they begin producing "numbers" from the theory. But somewhere down the road toward calculations aimed at particular measurement situations, a certain instrumentalist tendency often creeps in. Such instrumentalist hedging is most obvious when physicists are made self-conscious about the large number of assumptions associated with such cal- culations - the drastic idealizations necessary to generate solutions to fundamental equa- tions, the vagaries of the search for data to instantiate the solutions, the equally drastic simplifying assumptions required to isolate a physical system likely to provide measurable comparisons with the resulting predictions, even the theories of instrumentation required actually to generate numbers from the system and provide measures of goodness of fit.12 Still, such instrumentalist caution is by no means systematic, and one sometimes hears the most incautious ontological invocation at this most applied level.

Thus, the ontological scene in significant areas of contemporary physics seems to me to have an enormous amount of structure. There is the structure arising from the constant- ly shifting panorama of formulations, interpretations, and idealizations. Then there is the additional structure arising from the different kinds of activities which physicists engage in, the different levels of articulation, elaboration, and application of theories in which they are involved. And there is the structure arising from the shifting character of their ontological commitment as they move among these activities.

7. Advice to Philosophers

The lesson in all this for philosophers seems to me this: it is extremely difficult, if not impossible, to read off a clear, and clearly described ontology for much of contemporary fundamental physics from listening, however carefully, to what physicists say about it. In fact, the more carefully you listen, and the more physicists you listen to, the more jum- bled it all becomes.

Thus I think it is extremely difficult, if not impossible, to use contemporary funda- mental physics - after all the usual paradigm of "mature" science - to support the clas- sical realist position I mentioned at the beginning. This position, remember, envisions mature science as populating the world with a clearly defined and described set of objects, properties, and processes, and progressing by a steady refinement of descriptions and consequent clarification of the referential taxonomy. One simply does not find in fundamental physics today the requisite unambiguous populations and clear descriptions. But it's not just realism that fails to draw support from listening carefully to contempo- rary physicists. I doubt that any global "ism," any large-scale attempt to provide an abstract account of what science aims and arrives at, can find consistent support in the shifting sands of ontological commitment I have described here.

It does seem to me, though, that there is yet good work to do for hopeful realists among philosophers, certainly in science generally, even in fundamental physics. This

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work involves going in with an open mind to see what it is that actually gets established in this kind of science - firmly, stably, and robustly established - whether entity, law, effect, phenomenon, or whatever. And it involves paying close attention to how it gets established, on what basis scientists become convinced that something is established. I am convinced that there is much more rationality and continuity in science than some critics have claimed. But putting one's finger on the nature of that continuity, particularly in fundamental physics, is not easy.

In any case, one can point to recent examples of this good work in fundamental physics. Nancy Cartwright (1983) has certainly done some of it; Ian Hacking (1983) has as well. And more recently, Peter Galison has provided an unprecedentedly clear picture of how things get established in high energy physics, in his book, How Experiments End (Galison 1987). I am very enthusiastic about this good work, because it seems to me that in such work lies our best hope for getting a firmer grip on what can be established firmly, stably, and robustly established- in philosophy of science.

Notes

1The research on which this paper is based was partially supported by National Science Foundation Grant SES-86-18758. I would like to thank Paul Teller, Arthur Fine, and Ernan McMullin for comments on an earlier version.

2Jarret Leplin 1984, p. 3 provides a nice summary of philosophical arguments against realism.

3This is a good deal more than a promissory note to a young physicist. Each of these mathematico-conceptual approaches - save perhaps the action-at-a-distance approach - has been very important in the formulation of more contemporary theories. The field-the- oretical approach is vital in the formulation of electrodynamics and later field theories; the Hamiltonian- Lagrangian approach is central to the formulation of quantum theory, and a formulation of relativity theory as well; the approach in terms of curved space is the heart of the general relativistic framework. These approaches continue to co-exist in the mind of a mature physicist who contemplates an overview of contemporary theories.

4As I point out below, Newton was never satisfied with his understanding of the nature of the gravitational force. But he did steadfastly reject the view that particles act at a distance across empty space. For a recent reference, see McMullin 1978.

5An "account" in a mature science is generally judged to consist of an explanatory model, the relationships between whose fundamental explanatory concepts are expressed in terms of an underlying mathematical theory, which makes possible quantitatively pre- cise and qualitatively novel predictions.

6See, for instance, Jammer 1974. More recently, Pagels (1984) presents an engaging vision of this multiplicity in his chapter on "The Reality Market Place."

7These measurements constitute one of the three "classical" tests of relativity theory, along with measurements of the precession of the perihelion of Mercury's orbit and the red-shift of radiation emitted in a gravitational field. For a philosophical analysis of their status as "tests" of the theory see Glymour 1980.

8An interesting philosophical treatment of this experimental circumstance is presented in Laymon 1984.

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9In fact, actual deflection experiments are analyzed under more idealized assumptions yet. These "weak-field, slow-motion" assumptions are manifested in a treatment of the metric structure of space-time as merely a perturbation of a flat, Minskowskian metric, a "post-Newtonian correction" to a flat, empty space (MTW 1976, Ch. 39). A full analysis of this experimental application would have to investigate possible differences in onto- logical commitment between this treatment and the Schwarzschild treatment.

10On the general problem of a description in general relativity of isolated, interacting bodies, see the contributions by Dixon and D'Eath in Ehlers 1979. A brief general dis- cussion of the relationship between properties defined "exactly" and those associated with approximative methods appears on p. 165.

11For recent references, see the papers in Flaherty 1984. The following question and answer, from the paper by R. Penrose in this volume, gives some of the flavor of the cur- rent discussion:

Q: What properties of "mass" are you trying to capture in your formulation of quasi-local mass? A: I honestly don't have a complete answer to that question. We want agreement with the weak field limit. We want agreement on I(+) (future light-like infinity) and i(o) (spatial infinity). We would like positivity. Physical intuition is important, but it must be malleable, allowing certain principles to be dropped, if necessary. (Penrose 1984, p. 30)

12See Laymon 1984 for a bit more detail about these assumptions in the stellar deflec- tion experiments mentioned in section 5.

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