Physics Phenomena Experiments

[Muriel Pelley]

Physicsand natural science in general, is a reasonable enterprisebased on valid experimental evidence, criticism, and rationaldiscussion. It provides us with knowledge of the physical world, andit is experiment that provides the evidence that grounds thisknowledge. Experiment plays many roles in science. One of itsimportant roles is to test theories and to provide the basis forscientific knowledge.[1] It can also call for a new theory, either by showing that an acceptedtheory is incorrect, or by exhibiting a new phenomenon that is in needof explanation. Experiment can provide hints toward the structure ormathematical form of a theory and it can provide evidence for theexistence of the entities involved in our theories. Finally, it mayalso have a life of its own, independent of theory. Scientists mayinvestigate a phenomenon just because it looks interesting. Suchexperiments may provide evidence for a future theory to explain.[Examples of these different roles will be presented below.] As weshall see below, a single experiment may play several of these rolesat once.

In what follows, the reader will find an epistemology of experiment, aset of strategies that provides reasonable belief in experimentalresults. Scientific knowledge can then be reasonably based on theseexperimental results.

Epistemology of experiment is a branch of philosophy of sciencefocusing on the diverse roles that experiment plays in science, itsvarious connections to theory, to the understanding and functions ofexperimental apparatus, and to the structure and culture of thescientific community in the laboratory setting. The epistemologicalanalysis of experiments ranges from highly abstract philosophicalarguments with only indirect connection to actual practice, toanalysis immersed in reflective case studies. For a long timeexperiments in physics have been the leading edge of experimentalscience, pioneering experimental techniques, methods and innovativesettings. This is why much of epistemology of experiment has focusedon physics.

The 17th century witnessed the first philosophical reflections on thenature of experimentation. This should not be surprising given thatexperiment was emerging as a central scientific tool at the time. Theaim of these reflections was to uncover why nature reveals its hiddenaspects to us when we force experimental methods upon it.

Some natural philosophers believed that scientific knowledge waslittle more than the proper application of observational andexperimental techniques on natural phenomena. Francis Bacon went sofar as to claim that it was possible to perform what he called acrucial experiment (experimentum crucis), an ideal experiment of sortsthat can determine alone which of two rival hypotheses is correct. Andeven some of the giants of modern science such as Newton subscribed tothe view that scientific theories are directly induced fromexperimental results and observations without the help of untestedhypotheses. It is little wonder, then, that many natural philosophersthought that experimental techniques and their proper applicationshould be a primary object of philosophical study of science.

This vigorous early debate in many ways anticipated the main points ofdisagreement in debates to come. Yet the philosophical interest inexperimentation almost completely lost its steam at the end of the19th century and did not recover until fairly late in the 20thcentury.

In fact, not only can one separate theory and observation, but theformer is considered justified only in light of its correspondencewith the latter. The theory of heat transfer is confirmed bypropositions originating in the kind of readings I perform on mymercury thermometer. Thus, observational propositions are simply aresult of an experiment or a set of observations a scientist performsin order to confirm or refute a theory.

Thomas Kuhn and Paul Feyerabend vigorously criticized this view. Theyargued that observations and experimental results are already part ofa theoretical framework and thus cannot confirm a theoryindependently. Nor there is a theory-neutral language for capturingobservations. Even a simple reading of a mercury thermometerinevitably depends on a theoretically-charged concept of temperature.In short, the evidence is always theory-laden.

In How Experiments End (1987), Peter Galison extended thediscussion of experiment to more complex situations. In his historiesof the measurements of the gyromagnetic ratio of the electron, thediscovery of the muon, and the discovery of weak neutral currents, heconsidered a series of experiments measuring a single quantity, a setof different experiments culminating in a discovery, and two high-energy physics experiments performed by large groups with complexexperimental apparatus.

Galison also discusses other aspects of the interaction betweenexperiment and theory. Theory may influence what is considered to be areal effect, demanding explanation, and what is considered background.In his discussion of the discovery of the muon, he argues that thecalculation of Oppenheimer and Carlson, which showed that showers wereto be expected in the passage of electrons through matter, left thepenetrating particles, later shown to be muons, as the unexplainedphenomenon. Prior to their work, physicists thought the showeringparticles were the problem, whereas the penetrating particles seemedto be understood.

There is also a danger that the design of an experiment may precludeobservation of a phenomenon. Galison points out that the originaldesign of one of the neutral current experiments, which included amuon trigger, would not have allowed the observation of neutralcurrents. In its original form the experiment was designed to observecharged currents, which produce a high energy muon. Neutral currentsdo not. Therefore, having a muon trigger precluded their observation.Only after the theoretical importance of the search for neutralcurrents was emphasized to the experimenters was the trigger changed.Changing the design did not, of course, guarantee that neutralcurrents would be observed.

Pickering has argued that the reasons for accepting results are thefuture utility of such results for both theoretical and experimentalpractice and the agreement of such results with the existing communitycommitments. In discussing the discovery of weak neutral currents,Pickering states,

Scientific communities tend to reject data that conflict with groupcommitments and, obversely, to adjust their experimental techniques totune in on phenomena consistent with those commitments. (1981, p.236)

The emphasis on future utility and existing commitments is clear.These two criteria do not necessarily agree. For example, there areepisodes in the history of science in which more opportunity forfuture work is provided by the overthrow of existing theory. (See, forexample, the history of the overthrow of parity conservation and of CPsymmetry discussed below and in Franklin 1986, Ch. 1, 3.)

Pickering offered a different view of experimental results in late1980s. In his view the material procedure (including the experimentalapparatus itself along with setting it up, running it, and monitoringits operation), the theoretical model of that apparatus, and thetheoretical model of the phenomena under investigation are all plasticresources that the investigator brings into relations of mutualsupport. (Pickering 1987; Pickering 1989). He says:

One might ask whether such mutual adjustment between theory andexperimental results can always be achieved? What happens when anexperimental result is produced by an apparatus on which several ofthe epistemological strategies, discussed earlier, have beensuccessfully applied, and the result is in disagreement with ourtheory of the phenomenon? Accepted theories can be refuted. Severalexamples will be presented below.

Another issue neglected by Pickering is the question of whether aparticular mutual adjustment of theory, of the apparatus or thephenomenon, and the experimental apparatus and evidence is justified.Pickering seems to believe that any such adjustment that providesstabilization, either for an individual or for the community, isacceptable. Others disagree. They note that experimenters sometimesexclude data and engage in selective analysis procedures in producingexperimental results. These practices are, at the very least,questionable as is the use of the results produced by such practicesin science. There are, in fact, procedures in the normal practice ofscience that provide safeguards against them. (For details seeFranklin, 2002, Section 1).

There is another point of disagreement between Pickering and Franklin.Pickering claims to be dealing with the practice of science, and yethe excludes certain practices from his discussions. One scientificpractice is the application of the epistemological strategies outlinedabove to argue for the correctness of an experimental results. Infact, one of the essential features of an experimental paper is thepresentation of such arguments. Writing such papers, a performativeact, is also a scientific practice and it would seem reasonable toexamine both the structure and content of those papers.

Another difference between Pickering and Franklin on contingencyconcerns the question of not whether an alternative is possible, butrather whether there are reasons why that alternative should bepursued. Pickering seems to identify can withought.

In the late 1970s there was a disagreement between the results oflow-energy experiments on atomic parity violation (the violation ofleft-right symmetry) performed at the University of Washington and atOxford University and the result of a high-energy experiment on thescattering of polarized electrons from deuterium (the SLAC E122experiment). The atomic-parity violation experiments failed to observethe parity-violating effects predicted by the Weinberg- Salam (W-S)unified theory of electroweak interactions, whereas the SLACexperiment observed the predicted effect. These early atomic physicsresults were quite uncertain in themselves and that uncertainty wasincreased by positive results obtained in similar experiments atBerkeley and Novosibirsk. At the time the theory had other evidentialsupport, but was not universally accepted. Pickering and Franklin arein agreement that the W-S theory was accepted on the basis of the SLACE122 result. They differ dramatically in their discussions of theexperiments. Their difference on contingency concerns a particulartheoretical alternative that was proposed at the time to explain thediscrepancy between the experimental results.

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