MODELING IN QUANTUM FIELD THEORY
2.1. The Instrumentarium
A Metaphysics for Experiments 179
and the engine's fate was uncertain. Only as a umfied whole with all the components working properly did it make its way into the practical world. According to Latour a 'black box' exists when many elements are brought together to act as one.
This step having been taken the reconstitution the formerly material thing, the apparatus, as text seems to be a natural step. As Latour says, 'no distinction has been made [by him] between what is called a "scientific fact" [a proposition] and what is called a "technical object" or
"artifact'" (Latour 1987, p. 131).
There is only one difference between colleagues as allies and machines as allies in settling scientific controversies, according to Latour. It is easier to see that the gathered resources are made to act as one unified whole in the case of machines and pieces of hardware than in the case of colleagues and communities and runs of learned journals.
Latour has attended to the importance of practical skills, in such matters as 'making the equipment work.' However, these appear as bargaining encounters in social competition for community hegemony. Such skills are defined in a quite complex way with respect to the material nature of experimental equipment and the tasks of technicians in using it to display what it is expected to display, and sometimes does not. Failures are not always due to incompetent manipulations but to the intransigence of Nature which, as integrated with this apparatus, is other than the scientific community thought it to be. In Latour's treatment, it is hard to see that any distinction between these/grids of failures can be made.
No doubt we get the idea of what Nature is like from experiments. However, it is not that idea that is integrated with a bit of itself in a laboratory apparatus. Despite attention to the real world of the laboratory, Latour (and others) do not clearly distinguish the role of people talking and writing and so producing 'science', in the sense of a community discourse from the role of people making and manipulating material things and using them. The former are manipulating an idea or representation of Nature, the latter are manipulating Nature. It is evident that Latour does not treat these as different. He is led by the illuminating power of his metaphors, especially 'ally', to assume an identity. Nevertheless, the fact that a metaphor 'sticks' does not justify taking the similarity of source and subject that the metaphor makes visible to be an identity.
180 Section Five: Introduction
would reveal a common essence. What follows is a preliminary effort at a taxonomy setting up some generic categories, based on an analysis of laboratory equipment world relationships.
The broadest distinction, which gives us distinct families of laboratory equipment, is between instruments some relevant states of which are causally related to some feature of the world in a reliable way, and apparatus that is not so related because it is serving as a working model of some part of the world. Causal relations relevant to apparatus are within the model system. Causal relations relevant to instruments link the equipment to the world.
Among the most important genera of apparatus are working, bench-top models of natural processes and the material systems in which they occur. Allied to these are computer generated models of natural systems.
However, there is another genus of apparatus of great interest in contemporary physics.
Apparatus, as conceived by Niels Bohr, does not model the production of naturally occurring phenomena as a discharge tube might. It creates phenomena that do not occur in nature in the absence of the apparatus. I will offer a detailed analysis of Bohrian apparatus below.
2. 2. Apparatus as Models of the Systems in the World
2 . 2 . 1. Material Models as Domesticated Versions of Natural Systems
An apparatus of this genus is a domesticated and simplified version of a material set-up, which has two main features of interest to us.
a. It is found in the wild, that is occurs in Nature in the absence of human beings and their constructions and interventions. For example, a model of a fen'o-concrete structure would not fall into this genus, however useful it might prove in architecture.
b. The feral set-up is such that certain phenomena can be perceived, seen, heard, tasted and so on.
The apparatus is a material model of the naturally occurring material set-up. I shall use the metaphor of'domestication' to explore the relation between apparatus as model and that of which it is a model. The history of experimental science offers us a rich catalogue of apparatus as domesticated material systems.
Let me illustrate this genus of models with some examples.
Example i: Theodoric of Friborg set up a rack of water-filled spherical flasks as an apparatus to study the formation and geometry of the rainbow. It is not too fanciful to think of his rack as a domesticated version of the curtain of raindrops implicated in the coming to be of a rainbow. The drops replace each other sufficiently quickly in the falling shower that they can be considered as if they were a fixed array. If certain conditions on manipulability of the domestic version are met, for example finding a moveable light source to simulate the sun, the whole set-up makes possible an experimental laboratory study of the rainbow. The rack of water filled flasks is a curtain of spherical drops, which is like the curtain of spherical drops in the naturally occurring shower of rain.
Example ii: A drosophila colony is a domesticated version of an orchard replete with a breeding population of fi~it flies, which display variation by selection. If certain conditions on manipulability of the laboratory colony are met, it makes possible the experimental, laboratory study of inheritance.
Example iii. An Atwood's machine is a domesticated version of a cliff down which a stone falls or a Leaning Tower from which objects can be dropped. The machine consists of a graduated vertical column with various moveable attachments, allowing for the releasing of standard weights from different heights, and the determination of the locations of the falling masses at different
A Metaphysics for Experiments
181times.
Example iv. A tokomac is a domesticated version of a star. A powerful magnetic field confines hydrogen atoms in small volume, fusion to helium being ignited by an external energy source. The process so set going is a domesticated version of stellar fusion.
A useful image with which to explore the metaphor of apparatus as domesticated versions of feral originals could be farmyard creatures. A cow is a domesticated version of the auroc, which was found in the wild and which did give milk. However, cows are more tractable than aurocs.
One notes that life on the farm is simpler than in the wild. There is abundant fodder, and there are no predators. Such a life is lived with more regular and less extreme mental and bodily states than life in the wild. Hunger and fear are rare in the farmyard. Not only is a cow spared the anxieties of feral living but it has been bred to be docile. Furthermore, unlike its feral relatives, it is therefore easily manipulable, for example, it will stand patiently to be milked.
Domesticated versions of material set-ups and processes that occur in feral form in Nature, versions that we know as experimental apparatus and procedures, are, relative to their feral ancestors, simpler, more regular and more manipulable. The drosophila colony in the laboratory is a simpler bio-system, with more regular life patterns and is more manipulable than the swarms of flies in the apple orchard.
Things can happen in the domesticated model world that do not happen in the region of nature that is the source of the model. For example strange variants of the insect appear and can even be maintained as living examples of mutations that would either not occur at all in the wild or be immediately eliminated.
2. 2. 2. Back Inference from 'Domesticated' Models to the World
Domestication permits strong back inference to the wild, since the same kind of material systems and phenomena occur in the wild and in domestication. An apparatus, of this sort, is a piece of Nature in the laboratory. Of course, the richness of back inference will depend on how relations of similarity and difference are weighted by the interests of the researcher in performing the experimental manipulations.
There is no ontological disparity between apparatus and the natural set-up. The choice of apparatus and procedure guarantees this identity, since the apparatus is a version of the naturally occurring phenomenon and the material set-up in which it occurs. Theodoric's apparatus brings a rainbow to be by refraction and internal reflection in spherical volumes of water, just as the curtain of raindrops does. So, whatever can be learned about the paths of rays of light in the laboratory can be back inferred to the wild, to Nature.
A weight falls in an Atwood's machine just as a stone falls from a cliff or an iron ball falls from the Tower of Pisa. What we study in the laboratory by the use of this apparatus is not the effect of a causal relation between a state of Nature and the corresponding state of an apparatus.
It is a simplified version of the phenomenon itself
Galileo's inclined plane experiment is, in some respects, an intermediate ease. In order to use the results of times and distance of descent as a test of his hypothesis of free fall, he could not treat the ball rolling down the plane as just a domesticated version of free fall. He had to perform a mental operation on his results, effectively resolving the inclined motion into a vertical and a horizontal component. The back inference required this intermediate step.
2.2. 3. Apparatus-world Complexes and the Production of Phenomena
The analysis offered above does not seem to fit the case of the Wilson Cloud Chamber, the Stern-Gerlach apparatus and many other well-known pieces of laboratory equipment. At first
182 Section Five: Introduction
sight, it might seem that these belong in the causal family. We might be tempted to say that the lines of droplets in the cloud chamber are the effects of the ionization produced by the passage of electrons. Yet, are there electrons as moving particles in the absence of this kind of apparatus?
Are there electrons as interfering wave fronts in the absence of double slits and photosensitive screens? There is molecular motion in the absence of thermometers and the roughly global earth exists in the absence of cartographic surveys.
An apparatus is a piece of junk until it is integrated into a unitary entity by fusion with Nature. A retort exhibited in a museum is not an apparatus. Let us call the apparatus/world complexes which scientists, engineers, gardeners and cooks bring into being Bohrian artifacts.
Properly manipulated they bring into existence phenomena that do not exist as such in the wild, that is in nature. In general, there is no material structure in nature like the apparatus. Ice cream does not occur in nature, only in kitchens with refrigerators.
In the famous Bohr-Einstein debate around the EPR paradox, it is possible to see the outlines of Bohr's account of experimental physics. While Einstein is insisting that for every distinct symbol in a theoretical discourse there must be a corresponding state in the world (logical atomism under another name). Bohr (1958) is concerned with the concrete apparatus, and its relation to the world, as part o f the world An apparatus is not something transcendent to the world, outside it, interacting causally with Nature. That is the role of the instrument. The apparatus and the neighbouring part of the world in which it is embedded constitute one thing.
Bohr realized that the seeming ontological paradoxes of subatomic physics could be resolved by taking a right view of experimental apparatus. It is possible to see, for example, how Nature can yield both particles and waves, by treating particle phenomena and wave phenomena as products of the running of different apparatus world complexes. Particles and waves are phenomena that occur in such complexes. They do not occur in nature.
Bohr's philosophy of experimentation was misconstrued as some kind of positivism. It was never Bohr's intention to argue, as Math had argued that science was the study of the properties of apparatus. In an experiment, what was 'run' was not just the apparatus. Nor was Bohr advocating a straight-forward realist interpretation. Physicists could not treat the apparatus in this class of experiments as a transparent window through which to see the world as it would have existed had the apparatus never been constructed and switched on. Science was the study of the apparatus/world complexes. Neither component could be detached from the reality which produced phenomena.
The laboratory is full of equipment, apparatus, drawn from the local instrumentarium. People are setting it up and making it work, and so bringing phenomena into being. In some cases, the apparatus is a materially independent entity, with all relevant causal processes entirely internal to it, for instance the experimental drosophila colony. Its relation to the world is analogy. In Bohrian experiments, the apparatus is indissolubly melded with the world. In that case, the phenomena are properties of the apparatus/world complex. It is materially part of the world. The question of whether there are natural set-ups like the Bohrian complexes we have constructed, such as tokomacs, is germane to this class of apparatus as it is to the simpler, materially independent models, such as Theodoric's flasks. The flasks contain spherical masses of water.
That is what rain-drops are.
Therefore, we have two matters to examine. There is the issue of the nature of the apparatus as a constructed material object in relation to a naturally occurring material system. We must also examine the nature of the phenomena created by using it. These may be states of the apparatus that conic into being as effects of causal processes in the world. On the other hand, they may be phenomena that are brought into being by running the apparatus as a model of some material
A Metaphysics for Experiments 183
system. If the apparatus is a model of something in the world, we can ask what is the relation between the phenomena we produce in the apparatus and those which occur naturally. It is well to bear in mind that apparatus has properties before it is switched on, heated up or otherwise manipulated.
In thinking through the meaning of the products of experimental activity it is important to keep in mind that the phenomena generated by experimenting with Bohr-type apparatus are properties of a complex unity, the apparatus/world entity. Bohr was driven to this insight by the duality of types of quantum phenomena, but the point is quite general. In this chapter, I shall use Humphrey Davy's isolation of sodium in the metallic state by electrolysis as my prime exemplar of Bohrian experimentation. As far as I know, there is no set-up similar to Davy's equipment anywhere in the universe. Free metallic sodium exists only by virtue of the apparatus/world complex Davy built. Humphrey Davy used electrolysis on molten common salt in a crucible to bring metallic sodium to light. Think of how much is presupposed in describing this experiment as the 'discovery of sodium' or as the 'extraction of sodium.' There is no metallic sodium in the universe to the best of my belief Sodium-as-a metal is a Bohrian phenomenon.
This experiment contrasts sharply with Faraday's use of a tube of rarefied gas to study discharge phenomena. A similar set-up to the apparatus existed 'in nature', in the electron wind in the rarefied upper atmosphere. Therefore, we can understand the glow in the laboratory tube as an analogue, in a domesticated version of the upper atmosphere of the aurora borealis.
Just as the cow and the auroc can serve as a metaphor for the relation of apparatus to the world, so the homely image of a loaf of bread can serve as a metaphor for the Bohrian apparatus/world complex. A loaf is brought into existence from wheat and other ingredients by the use of material structures that do not exist in the wild, such as flour mills and ovens. Loaves do not appear spontaneously in nature.
At the Cern Laboratories a huge apparatus/world complex brings certain tracks into existence, which simplifying, we could imagine are recorded in photosensitive plates. One such set of tracks was lauded as the 'discovery of the W particle'. There are probably no free W particles in the universe now. They are exchange particles, intermediate vector bosons, postulated in quantum field theory. They are wrenched into momentary isolation at Cern. The pattern of reasoning that lies behind the 'discovery of the W particle' seems to have been something like this: photons can be studied in the propagation of light, and they also play a role as exchange or virtual particles in quantum field theory. So we have the idea of the free version of the virtual exchange particle. The W particle was introduced to physics as the virtual exchange particle for a certain class of interactions. By parity of reasoning there should be a free W particle, analogous to the free photon.
Is there an analogy between the discovery of the W particle and the isolation of metallic sodium? I think that few chemists would interpret the Na atom as a virtual constituent of common salt. So, Davy's experiment brought metallic sodium to fight by aggregating enough pre-existing Na atoms. Reflecting on the possible analogy of this ease to that of the W particle we can see that what would be at issue of the analogy were to be taken seriously is the ontological status of virtual particles in quantum field theory. This is a deep issue which cannot be gone into in this discussion, except to point out that the virtual W particle is a representation of just one of the exchange modes possible in any relevant particle interaction. It is hard to make a case for a pre- existing particle in any exchange process (Brown and Hart6 1990).
184 Section Five: Introduction
2. 2. 4. Back inference from Phenomena to Nature
Back inference from phenomena created in Bohrian artifacts is problematic, since there is an ontological question to be solved. What is the standing of the apparatus/world complex in relation to the world, to Nature?
The general form of this question seems to be whether it is legitimate to analyze the situation in Aristotelian terms (cf. Wallace 1996), that is in the principle: An actual phenomenon produced in an apparatus is the manifestation of a potentiality in the world. This would allow a back inference that would simply ascribe a natural propensity or potentiality for whatever occupies some region of Nature in contact or casual connection with the apparatus to appear as the experimental phenomenon. This sounds as if we could say 'an apparatus makes actual in the laboratory that which is potential in nature'. This still treats apparatus as a kind of'window on the world'.
It ignores the contribution of the apparatus to the form and qualities of the phenomenon.
Reflecting on this issue takes us deeper into the Bohr interpretation. The Bohrian phenomena are neither properties of the apparatus nor properties of the world that are elicited by the apparatus.
They are properties of a novel kind of entity, an indissoluble union of apparatus and world, the apparatus/world complex.
This makes the question 'In what form does metallic sodium exist before the electrolysis begins?' illegitimate. Nature, in conjunction with Davy's apparatus, affords metallic sodium, just as Nature, in conjunction with Wilson's apparatus, affords tracks, and thereby affords electrons as particles. By parity of reasoning, the question 'in what form do electrons as particles exist before the cloud chamber is activated?' is equally illegitimate.
To follow this line of analysis further would take us into the metaphysics of powers, dispositions and affordances, the neo-Aristotelian metaphysics of physics implicit in the writings of Nancy Cartwright (1989) and explicit in the recent work of William Wallace (1996). I will return to this issue in the final section.
2. 3. Instruments in causal relation to the world
The distinction between apparatus and instruments is vital to an understanding of how knowledge is acquired in laboratories, and what sort of knowledge it is. In many discussions of the nature of experiments it is simply assumed that the state of an instrument is an effect of an independently existing state of the world. In the ideal experiment the producing of the effect in equipment, the instrument, does not change the state of the world of which the state of the instrument is an effect. Sometimes the thermometer requires so much heat to expand the mercury that the liquid being studied cools down substantially. Skilled experimenters know how to compensate for these exceptions Usually the pressure in the ear tires is not significantly reduced by the amount needed to activate the tire gauge.
2. 3. 1. Kinds of Instruments
There are two main genera of 'causal' instruments. They can be differentiated by the use of an extended version of the old but useful distinction between primary and secondary qualities.
Ideas of primary qualities are those, which, as apprehended by a sensitive organism, resemble the state of the material entity that caused the experience For example, 'shape' is a property of a material thing whether or not it is being observed. It is experienced by an observer as a shape, according to some rule of projection.
Ideas of secondary qualities are those conscious states of an organism that do not resemble