alisa bokulich- rethinking thought experiments

8/3/2019 Alisa Bokulich- Rethinking Thought Experiments

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Rethinking Thought

Experiments

Alisa BokulichBoston University

An examination of two thought experiments in contemporary physics reveals

that the same thought experiment can be reanalyzed from the perspective ofdifferent and incompatible theories. This fact undermines those accounts ofthought experiments that claim their justicatory power comes from their

ability to reveal the laws of nature. While thought experiments do play agenuine evaluative role in science, they do so by testing the nonempirical vir-tues of a theory, such as consistency and explanatory power. I conclude that,while their interpretation presupposes a whole set of background theories and

putative laws, thought experiments nonetheless can evolve and be retooled fordifferent theories and ends.

1. Introduction

A thought experiment can be understood as a hypothetical or

counterfactual scenario from which inferences are drawn.

1

Historically,thought experiments have played a central role in the articulation andevaluation of scientic theories. One of the earliest discussions of the use

I would like to thank Harvey Brown for stimulating discussions about special relativityand ether theories. I am also grateful to Jim Cushing, Don Howard, and anonymous refer-ees for helpful comments on an earlier draft of this paper. A portion of the research for thispaper was made possible by the generous support of the National Science Foundation.

1. There is some controversy over how thought experiments should be dened. JohnNorton (1996) argues that thought experiments are essentially nothing but arguments fromhypothetical states of affairs. The shortcomings of this approach have been adequately ad-dressed by Michael Bishop (1999) and Tamar Szab Gendler (1998) and will not be dis-cussed here. Nancy Nersessian (1993), by contrast, has argued that thought experimentsshould be understood as narratives. My aim here is not to enter this debate concerning thedenition of thought experiments, but rather to clarify some misunderstandings about

their function.

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Perspectives on Science 2001, vol. 9, no. 32002 by The Massachusetts Institute of Technology


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of thought experiments, or Gedankenexperimente, in physics is due to ErnstMach (1897). In his book Knowledge and Error, he writes, besides physicalexperiments there are others that are extensively used at a higher intellec-tual level, namely thought experiments. . . . Our ideas are more readily tohand than physical facts: thought experiments cost less, as it were. It isthus small wonder that thought experiment often precedes and preparesphysical experiments (Mach [1926] 1976, p. 136). For Mach, there is acontinuum between thought experiments and ordinary experimentsboth in the sense that they use a similar methodology2 and in the sensethat many thought experiments nd a future realization in the laboratory.3

While a discussion of Machs views on the continuity between thought ex-periments and physical experiments is outside the scope of this paper, thechallenge for any adequate account of thought experiments is to deter-

mine in which respects this continuity view can be maintained, and inwhich respects it breaks down. I argue below that, on one hand, there arecertain respects in which thought experiments are more like ordinary ex-periments than has been previously admitted, while on the other hand,when it comes to the function of thought experiments, there is an impor-tant respect in which this continuity view breaks down. The assumptionthat thought experiments have the same function in the evaluation of the-ories as ordinary experiments do has led to difculties in several currentaccounts of thought experiments.

In what follows, I examine two thought experiments in physics andshow how, in both cases, the same thought experiment can be rethoughtfrom the perspective of differentand even incompatibletheories.While there are no necessary conditions for what is to count as the same

thought experiment, one can argue for a lenient construal of the identityof two thought experiments by appealing to features such as a resemblanceof the central narratives and a continuity through historical connection.4

While my discussion is focused specically on the role of thought experi-ments in contemporary physics, the conclusions drawn here do have im-plications for understanding the function of thought experiments in sci-ence more generally.

286 Rethinking Thought Experiments

2. Mach ([1926] 1976, p. 139).3. A famous contemporary example of this is Alain Aspect et al.s 1982 realization of

the Einstein-Podolsky-Rosen thought experiment. The EPR thought experiment will bediscussed in Section 4.

4. Sorenesen has similarly argued for a leniency in the standards for what is to count

as an instance of the same thought experiment (Sorensen 1992a, p. 163). It shouldbe noted that this is not a difculty unique to thought experiments; similar problemsplague attempts to characterize what is to count as two instances of the same physical ex-periment.


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As I shall argue, the fact that the same thought experiment can bereanalyzed from the perspective of two incompatible theories has a num-ber of important implications for understanding the nature and functionof thought experiments. First, just as Duhem showed in the case of ordi-nary physical experiments, the interpretation of a thought experimentpresupposes a whole set of background theories and laws. For this reason,crucial experiments are no more possible in thought experiments thanthey are in physical experiments.

Second, the ability to rethink a thought experiment from the perspec-tive of two incompatible theories challenges two recent accounts of howthought experiments function. More specically, both James RobertBrown (1991) and Roy Sorensen (1992b) argue that the knowledge wegain through some thought experiments is knowledge about the laws of

nature, which can then be used in testing the empirical adequacy of ourtheories. The difculties in explaining where this knowledge comes fromlead Brown, on the one hand, to give an a priori platonic account of our ac-cess to the laws of nature,5 while, on the other hand, they lead Sorensen toclaim that our access to the laws of nature can be given a biological expla-nation in terms of natural selection.6 Both of these accounts share a mis-conception about the function of thought experiments in physics and failto give an adequate account of the knowledge we gain from them.

Third, the following analysis suggests an important respect in whichthe function of thought experiments differs from that of ordinary physicalexperiments: the evaluative function of thought experiments is not totest the empirical adequacy of our theories, but rather to test their non-empirical virtuessuch as consistency and explanatory power. Finally, I

shall argue that a more careful look at the history and development of twothought experiments in physics reveals that, contrary to Ian Hackingsclaim, thought experiments can have a life of their own (Hacking 1993,p. 307).

2. Duhem, Crucial Experiments, and Expriences Fictives

Thought experiments are often presented in the form of reductio ad absur-dum arguments. The strategy is to create a scenario in which the theoryone is arguing against is shown to imply a contradiction or absurdity. Afamous example of this is Galileos thought experiment on falling bodiesused to undermine the Aristotelian theory of motion.7 On the rst day of

P er s pe ct i ve s o n S c ie nc e 287

5. Brown (1991, p. 155).

6. Sorensen (1992b, p. 15).7. Tamar Gendler (1998) has provided a detailed examination of this particular thoughtexperiment and used it to argue against the view that thought experiments can be reducedto, or eliminated in favor of, pure arguments.


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choice, according to Duhem, is that it is never an isolated hypothesis thatfaces an experiment, rather it is a whole group of interlocking hypotheses.A natural question, then, is whether these insights about crucial physicalexperiments apply to thought experiments as well.

Duhem, like Mach, is among the earliest philosophers to discuss themethodological role that thought experiments play in physics. UnlikeMach, however, Duhems assessment of thought experiments is not favor-able. Duhem refers to thought experiments as ctitious experiments(expriences ctives) and sees their use in physics as illegitimate. I would ar-gue that Duhems distrust of thought experiments can be understood asconnected to his critique of crucial experiments. For Duhem, the inter-pretation of the slightest experiment in physics presupposes the use of awhole set of theories, and . . . the very description of this experiment re-

quires a great many abstract symbolic expressions whose meaning and cor-respondence with the facts are indicated only by theories (Duhem [1914]1954, p. 204). Although he does not explicitly draw the connection be-tween crucial experiments and thought experiments, it is arguably thisholistic aspect to ordinary experiments that Duhem believed to be miss-ing from thought experiments. This interpretation of Duhem gains sup-port when one notes that his critique of thought experiments occurs in themiddle of the chapter in which he argues that crucial experiments arenot possible. It is also the case that in the paragraph which immediatelyprecedes his discussion of thought experiments, Duhem reiterates thefundamental difference that he sees between the methods of physics andgeometry.

Apart from concerns about whether thought experiments exhibit ho-

lism, Duhem presents an even more difcult challenge to the legitimacyof the use of thought experiments in science. He writes,

To invoke such a ctitious experiment is to offer an experiment tobe done for an experiment done; this is justifying a principle not bymeans of facts observed but by means of facts whose existence ispredicted, and this prediction has no other foundation than the be-lief in the principle supported by the alleged experiment. Such amethod of demonstration implicates him who trusts it in a viciouscircle (Duhem [1914] 1954, p. 202).

While Duhem is right to point out that thought experiments cannot pro-vide any new empirical foundation for a theory or principle, he is wrong inconcluding that they have no legitimate role to play in the evaluation andjustication of scientic theories. As the following two examples show,thought experiments do play a legitimate role in evaluating the non-empiricalvirtues of a theory.



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3. The Rockets and Thread Thought Experiment: Einstein vs. Lorentz

The following thought experiment appeared in a 1959 American Journal ofPhysics article (Dewan and Beran 1959, pp. 517518). Imagine two iden-tically constructed rockets, B and C, both initially at rest in an inertialframe, S. The two rockets are arranged one behind the other, 100 metersapart in S and are connected by a thin piece of thread just long enough toconnect the two rockets (as in Figure 1). Now imagine that both rocketsre up their engines simultaneously in this frame and gently accelerate torelativistic velocities. Once they reach four-fths the speed of light rela-tive to S, they simultaneously stop accelerating, and are now moving witha uniform velocity. According to an observer at rest in S, the two rocketshave been moving in tandem and are still 100 meters apart. The questionnow is whether or not the thread will break.

By carefully analyzing the situation in accordance with the special the-ory of relativity, it can be shown that, according to an observer in S, thethread must break because it is Lorentz-contracted (to a length of 60 me-ters) and so can no longer span the full 100 meters between the rockets.The mistaken belief that Lorentz contraction is simply an artifact of amathematical transformation, and not a real effect, might lead one toworry that the thread breaking will lead to some inconsistency when weconsider the same situation from the point of view of an observer on rocketA, at rest in the rockets nal inertial frame S moving with a uniform ve-locity of four-fths the speed of light relative to S. From the point of viewof an observer on rocket A at rest in S , the initial separation of the rock-ets is only 60 meters. Rather than seeing the rockets re their engines si-multaneously, however, the observer on rocket A will see rocket B acceler-

ate and come to rest in S rst, followed by rocket C accelerating andcoming to rest at a later time.

Because the two rockets do not accelerate and come to rest simulta-neously according to an observer in S , the distance between these rocketshas grown from 60 meters to 166.67 meters. In the S rest frame thethread is, of course, 100 meters and so must break since it cannot stretchthe 166.67 meters between the rockets.

The intent of E. Dewan and M. Beran in rst introducing this thoughtexperiment was to show that Lorentz contraction can cause measurablestresses on moving bodies.9 This conclusion is counterintuitive because,according to special relativity, Lorentz contraction is a frame-dependent


9. There is an interesting pre-history to this thought experiment that involves an ex-change between Paul Ehrenfest and Einstein on this question of whether Lorentz contrac-tion can produce stress effects. See Document 44 in Stachel (1989). I thank Don Howardfor bringing this reference to my attention.


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phenomenon, and hence, is not thought to lead to any observable effects,such as a thread breaking.10 Almost twenty years later, John S. Bell re-tooled this very thought experiment to show that the same conclusioncould be reached by means of a very different sort of analysis. Rather thanusing the special theory of relativity to analyze this thought experiment,Bell uses Lorentzs ether theory.

Lorentzs ether theory was, at least in 1905, an observationally equiva-lent rival theory to special relativity.11 According to Lorentzs theory, allmotion is relative to a stationary ether frame. The ether was thought to bethe medium through which electromagnetic forces were propagated. Lo-

rentz presents the following argument:We assume that molecular forces are also transmitted through theether, like the electric and magnetic forces. . . . If they are so trans-mitted, the translation will very probably affect the action betweentwo molecules or atoms in a manner resembling the attraction orrepulsion between charged particles. Now, since the form and di-mensions of a solid body are ultimately conditioned by the inten-sity of molecular actions, there cannot fail to be a change of dimen-sions as well (Lorentz [1895] 1952, p. 6).


10. The counterintuitiveness of this conclusion is evidenced by John Bells account ofhow the majority of physicists in the theory division at CERN, when presented with thisthought experiment, initially gave the incorrect answer ([1976] 1993, p. 68).

11. Elie Zahar (1973) has argued that not only was Lorentzs theory empirically ade-quate but it was also part of a non-ad hoc research program. Today it remains an open ques-tion whether or not it is possible to construct a Lorentzian ether theory that is in all re-spects empirically equivalent to special relativity.

Figure 1. The initial situation before rockets B and C re, as viewed in the iner-tial frame S.


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From the molecular force hypothesis described above, Lorentz is able to de-

rive the now well-known expression for Lorentz contraction, 1 2 2 v c/ ,where v is the velocity of the object and c is the speed of light (Zahar1973, pp. 114115).

Lorentz originally used the above argument as part of an explanationfor how his ether theory could account for the results of the Michelson-Morley experiment (which many took to be a crucial experiment resultingin the refutation of the stationary ether theory). As Bell shows, however,the same argument can also be used to explain the results of the rockets

and thread thought experiment. Bell models the atoms making up thethread in terms of nuclei with circular electron orbits. He then shows thatas the nuclei begin to move relative to the stationary ether, the initiallycircular orbits will deform into ellipses, contracting in the direction ofmotion by the usual Lorentz factor (Bell [1976] 1993, p. 70). As the at-oms and molecules contract, so too will the thread. If the thread is notstrong enough to overcome the inertia of the rockets and draw them closertogether as it contracts, then the thread will break.

The rockets and thread thought experiment can be analyzed not onlyfrom the perspective of Einsteins special theory of relativity, but also fromthe perspective of Lorentzs ether theory. Although both theories agree onwhat happens (i.e., the thread breaks), they differ greatly when it comes toexplaining how and why that event occurs. Bell draws two sorts of lessons

from this thought experiment. The rst lesson is a pedagogical one. In hisdiscussion of why theoretical physicists often draw the wrong conclusion


Figure 2. A Minkowski space-time diagram of the relativistic account of therockets and thread thought experiment.


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(that, according to special relativity, the thread will not break) he notesthat those who are familiar with the work of Lorentz are more likely to seethat the thread will indeed break in this thought experiment. The reasonis that Lorentzs ether theory, though by no means obvious, is more in ac-cord with our classical views about space and time.

In addition to noting the pedagogical advantage that teaching Lorentzstheory might offer by being more in accord with our classical views, Bellalso draws a stronger epistemological lesson from his Lorentzian analysisof this thought experiment. In his comparison of Lorentzs approach toEinsteins he writes,

Lorentz, on the other hand, preferred the view that there is indeed astate ofrealrest, dened by the aether, even though the laws ofphysics conspire to prevent us identifying it experimentally. The

facts of physics do not oblige us to accept one philosophy ratherthan the other. . . . [T]he laws of physics in any one reference frameaccount for all physical phenomena, including the observations ofmoving observers (Bell [1976] 1993, p. 77).

Whether one could consistently develop a Lorentzian space time theoryusing a single preferred rest frame is an issue that is still debated, andnot one that I wish to address here. Instead, the important conclusions todraw from Bells analysis are rst, that thought experiments are no morebound to any one particular theory than ordinary physical experimentsare, and second, they can underdetermine theory choice in the same waytoo.

4. The EPR Thought Experiment: Copenhagen vs. BohmThe Einstein-Podolsky-Rosen (EPR) Gedankenexperimentis an example of athought experiment that has evolved and been modied over time. Theoriginal 1935 thought experiment can be described as follows. Considertwo particles, 1 and 2, that interact and then become spatially separated.With the help of Schrdingers equation one can calculate the state of thecombined system (1 and 2) at some later time, which will be a superposi-tion of various possible states for these two particles. If one decides tomeasure the position, for example, of particle 1, then the result of thismeasurement plus the information about the combined system allows oneto determine a denite value for the position of particle 2 (i.e., withouthaving to make a measurement on that particle). EPR then invoke the fol-lowing criterion: If, without in any way disturbing a system, we can pre-dict with certainty (i.e., with probability equal to unity) the value of aphysical quantity, then there exists an element of physical reality corre-



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sponding to this physical quantity (Einstein et al. [1935] 1983, p. 138).Thus, EPR conclude that particle 2 must really have a denite position,since it is assumed that the decision to make a measurement of the posi-tion of particle 1 in no way could affect the state of particle 2 which is spa-tially separated from it (this is called the locality or separation principle 12).The paradoxical aspect of this thought experiment arises when EPR pointout that one could just as well have chosen to measure the momentum ofparticle 1. In this case, one could use the result of the momentum mea-surement on 1, plus the information about the state of the combined sys-tem, to determine a denite value for the momentum of particle 2. Fromthis EPR conclude that particle 2 must simultaneously have a denite po-sition and a denite momentum (recall that by the locality assumptionnothing we do to particle 1 can affect particle 2).

The stated intention of EPR is to use this thought experiment to showthat quantum mechanics is incomplete. If quantum mechanics is com-plete, then a particle cannot simultaneously have a denite position and adenite momentum; the accuracy to which complementary observables,such as position and momentum, can be dened is limited by Heisen-bergs uncertainty principle. But, according to the EPR thought experi-ment, a particle can simultaneously have a denite position and a denitemomentum.13 Thus, they conclude, quantum mechanics is incomplete.

With evidence from Einsteins letters, Arthur Fine (1986) has shownthat it was likely Podolsky who wrote up the EPR paper and that Einsteinwas unhappy with the way the paper came out, feeling that the essentialpoint he wanted to make was obscured. Fine provides a convincing argu-ment that Einstein saw this thought experiment, not as showing that

quantum mechanics was incomplete, but rather as showing that one couldnot maintain both that quantum mechanics is complete and that states ofspatially separated objects are independent from each other.14 This illus-trates the fact that even the coauthors of a thought experiment can dis-


12. Fine (1986, p. 36).13. Speaking more precisely, it is possible to assign two different wavefunctions to

the same reality and these wavefunctions can be eigenstates of noncommuting operators.According to the rules of the standard interpretation of quantum mechanics, if a systemis in an eigenstate of some observable then that observable has a denite value for thatsystem.

14. Don Howard (1985) has argued for a similar interpretation of Einsteins views onthe EPR paper. Howard shows that in 1936 Einstein reformulated the EPR Gedan-kenexperimentin such a way to make the tension between completeness and the separation

principle more explicit. Specically, Einstein showed that the thought experiment doesnot depend crucially on the reality criterion or noncommuting operators, only on the factthat two different wavefunctions can be ascribed to the same reality. The relevant Einsteinreferences can be found in Howard (1985).


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agree about what it is precisely that the thought experiment is supposedto show.

In 1951, David Bohm retooled the EPR Gedankenexperiment into theform in which it is more generally known today. He writes, we havemodied the experiment somewhat, but the form is conceptually equiva-lent to that suggested by them, and considerably easier to treat mathemat-ically (Bohm 1951, p. 614). Rather than considering position and mo-mentum measurements, he considers spin measurements on two atomsthat are the result of the disintegration of an initial spin-zero molecule.These atoms will thus have equal and opposite spin, irrespective of the di-rection along which the spin is measured (see Figure 3). Since the opera-tors associated with spins in any two directions not on the same axis donot commute, we are presented with a situation similar to the original

EPR Gedankenexperiment.The simpler conceptual and mathematical form of Bohms version ofthe thought experiment played a critical role in facilitating Bells con-struction of his famous inequality. Very briey, this inequality is derivablefrom (i.e., a logical consequence of) a condition that Bell called locality,which is essentially equivalent to Einsteins separation principle (Bell[1971] 1993, p. 36).15 This inequality, however, is incompatible with thepredictions of quantum mechanics and with well-established empiricaldata. The experimental violation of Bells inequality is typically taken toimply that Bells locality condition must be given up. Fine, however,makes the following point:

Arguments by Bell and others suggest that separation alone may beincompatible with the quantum theory. . . . Should that be correct,then the dilemma of EPR could be resolved by abandoning separa-tion. I do not believe that the Bell arguments are in fact strongenough to force the issue this way, but even if they are, the questionof completeness would remain. For it is possible that both separationand completeness turn out to be false (Fine 1986, pp. 3839).

The great value of the EPR Gedankenexperimentis that it reveals an incon-sistency between a certain set of theoretical assumptions. It is by no meansa crucial experiment, however, that forces the resolution of this contradic-tion one way or the other.


15. More information on Bells inequality can be found in the Cushing and McMullin(1989) anthology. Subsequent work has shown that what is here called locality is in fact the

conjunction of two distinct conditions (Jarrett 1984). Howard (1985) has argued thatshortly after the 1935 EPR paper, Einstein himself became aware of essentially this dis-tinction. Unfortunately, a discussion of these issues is outside the scope of this paper and Imust refer the interested reader to the excellent collection of articles referenced above.


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In 1952 Bohm made a radical reversal, not only of his interpretation ofthe EPR thought experiment, but also of his entire interpretation of quan-tum mechanics. His new resolution of the EPR dilemma is to abandon

both separation and completeness. In the paper in which Bohm developsthe causal interpretation of quantum mechanics, he writes,

In the usual interpretation of quantum theory, there is no . . . con-ceptual model showing in detail how the second particle, which isnot in any way supposed to interact with the rst particle, is never-theless able to obtain either an uncontrollable disturbance of its po-sition or an uncontrollable disturbance of its momentum dependingon what kind of measurement the observer decided to carry out onthe rst particle. . . . In our suggested new interpretation of thequantum theory, however, we can describe this [EPR] experimentin terms of a . . . precisely denable conceptual model (Bohm[1952] 1983, p. 389).

Very briey described, Bohms causal interpretation begins with the stan-dard Schrdinger equation and rewrites it in a form resembling a classicalequation of motion (the Hamilton-Jacobi equation) containing the usualclassical potential plus a new quantum potential term. 16 This way ofwriting the fundamental equations of quantum mechanics suggests a newway of interpreting this formalism. While the standard interpretation ofthe formalism of quantum mechanics takes it to describe a fundamentallyindeterministic world, in which particles cannot have denite trajectories,Bohm showed that one could also consistently interpret the formalism ofquantum mechanics as describing a fundamentally deterministic worldwhere particles do always follow denite trajectories.

According to Bohms causal interpretation, the correlations in the EPRthought experiment can be explained in terms of a direct disturbance. On


Figure 3. Bohms version of the EPR Gedankenexperiment.

16. For further information about Bohms causal interpretation see, for example, Cush-ing (1994).


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this causal view, the wavefunction is interpreted as a real eld that encodesinformation about the entire two-particle and two-apparatus system.When the experimenter changes the settings on the apparatus to measurea certain observable of particle 1, this immediately produces a change inthe overall wavefunction and alters the quantum potential. This change inthe quantum potential can be said to generate a quantum force thatacts instantaneously on the particles. Thus, the measurement made onparticle 1 instantaneously disturbs particle 2 via this nonlocal quantumpotential.17

As in the rockets and thread thought experiment described in the lastsection, the EPR Gedankenexperimentcan be reanalyzed from the perspec-tive of different, and even incompatible, theories. While one might havethought that the EPR Gedankenexperiment could function as a crucial ex-

periment to decide between standard quantum mechanics and hiddenvariable theories, it turns out to be explainable equally well by each ofthese rivals.

The EPR Gedankenexperiment has been discussed in the literature onthought experiments before. Allen Janis, for example, has used the EPRGedankenexperimentas an example of one of the ways in which thought ex-periments can fail. With regard to EPR, he writes, the thought experi-ment failed to provide a clear basis for concluding that quantum mechan-ics is incomplete. Since this goal was the motivation for the thoughtexperiment, however, the thought experiment failed(Janis 1991, p. 116).The difculty with saying that thought experiments can fail in this way isthat it makes sense only with respect to the intentions of the proponent ofthought experiment. As we have seen, however, different people have put

forward the EPR Gedankenexperimentwith the intention of showing differ-ent things. For example, if Fines reading of Einstein is correct, then Janisshould have concluded that the EPR did not failit succeeded in show-ing that one could not maintain both locality and completeness (i.e., thatone or the other, or both had to be given up).

A more careful reading of the history of this thought experiment re-veals that many discussions of the EPR Gedankenexperiment are over-simplied and consequently have led to a mistaken understanding ofthought experiments. An important lesson to take away from this discus-sion is that to say that a thought experiment succeeds or fails makes senseonly in reference to the intentions of the proponent of the thought experi-ment. As we have seen, however, there can be disagreement over what it isthat a thought experiment shows. Moreover, we should not say that only


17. Because these nonlocal disturbances are uncontrollable and cannot be used to send asignal, there is arguably no conict with the rst principle of relativity.


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the intentions of the original author(s) of the thought experiment are tocount; thought experiments can be rethought and retooled for new pur-poses.

5. Thought Experiments and the Laws of Nature

In thinking through a thought experiment, it is difcult not to believethat one is learning something new. Kuhn, for example, asks How, then,relying exclusively upon familiar data, can a thought experiment lead tonew knowledge or to a new understanding of nature? (Kuhn [1964]1977, p. 241). Two sorts of answers to this question have been given re-cently in the literature on thought experiments. Both Brown (1991) andSorensen (1992b) argue that thought experiments function by revealingthe laws of nature. While Brown argues that thought experiments give

us a priori insights into the laws of nature, Sorensen argues that thoughtexperiments harness physical intuitions shaped by laws through naturalselection. Both accounts, however, misconceive the function of thoughtexperiments and fail to give a satisfactory account of the knowledge thatwe gain from them.

Brown argues for the existence of a special class of thought experimentswhich he calls platonic. He explains,

A platonic thought experimentis a single thought experiment whichdestroys an old or existing theory and simultaneously generates anew one; it is a priori in that it is not based on new empirical evi-dence nor is it merely logically derived from old data; and it is anadvance in that the resulting theory is better than the predecessor

theory (Brown 1991, p. 77).The function of a platonic thought experiment is that of a crucial experi-ment, designed to decide unambiguously in favor of one theory andagainst another. I suspect that it was this sort of interpretation of thoughtexperiments that led Duhem to be wary of them. Browns discussion ofplatonic thought experiments assumes that there is a direct path fromsuch thought experiments to the relevant laws of nature.

Although he sees these thought experiments functioning as crucial ex-periments revealing the laws of nature, he is not an empiricist. Rather, heargues that [platonic] thought experiments give us (fallible) a priori be-liefs of how the physical world works. With the minds eye, we can see thelaws of nature (Brown 1991, p. 155). Brown looks for support for thisview in the interpretation of laws as necessary relations among independ-ently existing universals. The difculty, however, with dening laws ofnature this way is that it divorces this notion from the sort of things that



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scientists typically refer to as laws of nature.18 While a full discussion ofthis notion of law is outside the scope of this paper, I want to argue thatBrowns account of laws fails to illuminate our understanding of howthought experiments, like the EPR Gedankenexperiment, work.

Brown views the EPR Gedankenexperiment as an example of platonicthought experiment that destroyed the Copenhagen interpretation andestablished hidden variables in its place (Brown 1991, p. 77). Brown, ofcourse, notes that historically this was not the outcome of this thought ex-periment. Rather than seeing cases like this as posing a problem for his a

priori platonic account, he appends the term fallibilist to his position. Tosimply say that the EPR Gedankenexperimentwas a failed case of seeing thelaws of nature, still greatly misrepresents this thought experiment.19

Brown furthermore wants to claim that not only are these laws the cause

of the knowledge we gain in the thought experiment, but that they arealso somehow the cause of the correlations involved in the EPR Gedanken-experiment. Brown writes,

Distant correlations are caused by the laws of nature. . . . A law ofnature is an independently existing entity. . . . It is this very sameentity, the abstract law, which plays a role in our knowledge ofwhat is going on at the distant wing of an EPR-type experiment(Brown 1991, p. 152).

Unfortunately, Brown does not go on to explain exactly what this particu-lar law of nature is, nor what are the universals between which it is sup-posed to be a necessary relation. Further problems arise when one tries tomake sense of what it might mean to attribute causal powers to a law. In

the case of Bohms causal interpretation of the EPR Gedankenexperiment, forexample, there is no need to postulate a law as the cause of these correla-tions, since the correlations are said to be caused by a real physical quan-tum potential, or force.

In the case of the Copenhagen interpretation, the laws at work in thisthought experiment are fundamentally indeterministic, whereas in thecase of the causal interpretation the laws at work in this same thought ex-periment are fundamentally deterministic. The fact that the same thoughtexperiment can be rethought from the perspective of different and incom-patible theories makes it less plausible that thought experiments work byallowing us to see the laws of nature.


18. Although David Armstrong, who is a proponent of this view of laws, admits this is

the case, he does not view it as a shortcoming (Armstrong 1983, pp. 138139).19. I offer an alternative account of the knowledge we gain through thought experi-ments in Section 6.


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Sorensen offers an alternative account of the role that laws play inthought experiments. He writes,

The laws of nature have led us to develop rough and ready intu-itions of physical possibility which are then exploited by thoughtexperimenters to reveal some of the very laws responsible for thoseintuitions. The good news is that natural selection ensures a degreeof reliability for the intuitions. The bad news is that the evolution-ary account seems to limit the range of reliable thought experimentto highly practical and concrete contexts (Sorensen 1992b, p. 15).

On this account, thought experiments are said to reveal the laws of natureby harnessing our physical, or modal, intuitions. In order to explain thesource of these modal intuitions, and why they should generally give us

reliable insight into the laws, Sorensen appeals to evolutionary theory. Al-though he admits that this biological guarantee does not logically entailreliable belief formation, he nonetheless thinks that it can explain our suc-cesses in using thought experiments (Sorensen 1992b, p. 35).

A fundamental difculty for Sorensens account is to explain why one ofthe most successful areas in which thought experiments are used is theo-retical physics. The description of the world given by contemporary phys-ics is very far removed from our ordinary physical intuitions. In these con-texts, it is particularly implausible that any appeal to the sort of physicalintuitions that might have evolved evolutionarily would offer us any in-sight into how such thought experiments function.

Although Sorensen admits that the pessimism his evolutionary accountimplies regarding the more theoretical and abstract thought experiments

may not be entirely founded, he gives no clear explanation for how thisfact is to be reconciled with his view. In my view, the reason that thoughtexperiments can be successfully carried out in physics is that the formal-ism and mathematical structure of the theory play a central heuristic rolein carrying our reasoning further than our common-sense physical intu-itions could. This is precisely why we are able to evaluate thought experi-ments in theories like special relativity even though the relativistic ac-count of distance and simultaneity is very far removed from our everydayunderstanding of these concepts. It is simply a mistake to describe the sortof knowledge involved in these thought experiments as intuitions.

Brown and Sorensen share a common misconception that the immedi-ate function of thought experiments is to reveal the laws of nature. It isbecause they see this as the function of thought experiments that theythen must introduce an a priori or evolutionary account to explain howthis might work. Thought experiments, however, are not pre-theoreticalentities that provide a pure empirical or a priori basis from which to dis-



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cover laws. In the spirit of Duhem we might put the point the followingway: the interpretation of a thought experiment presupposes the use of awhole set of theories and (for thought experiments in physics) the very de-scription of a thought experiment requires a great many abstract symbolicexpressions whose meaning and correspondence with the facts are indi-cated only by theories.20

6. What Do Thought Experiments Teach Us?

If thought experiments do not work by allowing us to see the laws of na-ture, then the question of what it is that we are learning, when we performa thought experiment, remains. The simple answer is that thought experi-ments work by drawing out the physical implications of our theories. In athought experiment we begin with a theory or set of assumed laws and

then use the thought experiment to uncover certain consequences fromthese laws that might otherwise remain hidden. For example, in the rock-ets and thread thought experiment, one might understand the special the-ory of relativity, and know the laws that this theory postulates, but stillnot be aware that they imply the existence of relativistic stress effects. Thethought experiment makes these consequences explicit.

While drawing out the implications of our theories is an importantpart of science, thought experiments are not limited to this function.Thought experiments do play a role in evaluating, accepting, and reject-ing theories. It is a mistake, however, to see the role of thought experi-ments as testing the empirical adequacy of a theory. On this point, Duhemwas right to charge those who attempt to do so with implicating them-selves in a vicious circle. The key point to recognize, however, is that

empirical adequacy is not the only criterion used in the evaluation of theo-ries.

In 1964 Kuhn provided the beginning of an answer to the question ofhow thought experiments teach us something new when he pointed outthat thought experiments reveal contradictions between nature and thescientists conceptual apparatus. However, it is only after Kuhns 1973 ar-ticle Objectivity, Value Judgement, and Theory Choice that a fullerKuhnian explanation of the function of thought experiments in theoryevaluation could be made clear.21 In this article, Kuhn points out that the-ory choice is not simply a matter of determining whether the theory isempirically adequate, but that it also depends crucially on othernonempirical criteria. These criteria can be described as internal consis-


20. Recall the quotation from Duhem (Duhem [1914] 1954, p. 204) given in Section 2of the present work.21. Kuhn himself did not, in later writings, return to the question of how thought ex-

periments function.


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tency, external coherence with other theories, simplicity, and explanatorypower. Philosophers such as Ernan McMullin (1988) and Paul Churchland(1985) have argued that these nonempirical criteria can be just as impor-tant as, and in some cases more important than, empirical criteria in eval-uating a theory. This realization, that the testing and evaluating of theo-ries involves not just empirical criteria, suggests a legitimate andsubstantial way in which thought experiments can be used in theory eval-uation.

In addition to bringing out the physical implications of our theories, acentral function of thought experiments is to test and evaluate the internalconsistency, external coherence, simplicity, and explanatory power of ourtheories. In special relativity, thought experiments are typically used toshow that apparent contradictions in the theory can in fact be consistently

accounted for. The special relativistic version of the rockets and threadthought experiment can be understood along similar lines. One mightthink that, because Lorentz contraction is a frame dependent phenome-non, an effect like the thread breaking would lead to an inconsistencywhen considered from another frame. If one could not account for thethread breaking from the perspective of the nal rest frame, then thiswould suggest an internal inconsistency in the theory. By carefully work-ing through the thought experiment, however, one can show that there isno such inconsistency.

The rethinking of the rockets and thread thought experiment from theperspective of Lorentzs ether theory illustrates the great explanatorypower that this theory has for accounting for the result. According to Bell,the ability of this theory to provide a concrete, visualizable model for the

Lorentz contraction of the thread gives Lorentzs theory an explanatory ad-vantage over special relativity.

In a similar way, the novel insights given by the EPR Gedanken-experimentcan be seen in terms of its function in testing the nonempiricalvirtues of quantum theory. Quantum mechanics is one of the most empiri-cally successful theories in history, and both the standard and causal inter-pretations share this empirical adequacy. The remarkable result of this1935 thought experiment was to show that two fundamental assumptionsof standard quantum theory, namely locality (the separation principle)and completeness, are inconsistent. When this thought experiment wasanalyzed in accordance with the causal interpretation, however, its pur-pose was both to demonstrate the explanatory power of this theory and toaddress concerns about its external coherence with relativity theory. Oncewe see that a central function of thought experiments is to test the non-empirical virtues of theories, it is not surprising that Bohm makes the fol-



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develop, change, and yet retain a certain long-term developmentwhich makes us talk about repeating and replicating experi-ments. . . . I think of experiments as having a life: maturing, evolv-ing, adapting, being not only recycled but also, quite literally be-ing retooled. But thought experiments are rather xed, largelyimmutable. . . . what they think is what was once thought(Hacking 1993, p. 307).

By life of its own Hacking does not mean that the interpretation or de-scription of an experiment can be made independently of all theories.23

Rather, he means that experiments have the ability to evolve and beadapted to different theories and ends.

The closer analyses of the rockets and thread thought experiment inSection 2 and the EPR Gedankenexperimentin Section 3 reveal that, by the

denition given above, these thought experiments do indeed have a life oftheir own. The rockets and thread thought experiment, which was rst in-troduced by Dewan and Beran to show an overlooked consequence of thespecial theory of relativity, was readapted by Bell to show that the sameresult could be explained much more naturally in terms of Lorentzs ethertheory. Similarly, the 1935 EPR Gedankenexperimentevolved, at the handsof Bohm in 1951, into the simpler form it is usually known by today. In1952, Bohm made an even more radical retooling of this thought experi-ment by reanalyzing it in terms of the causal interpretation of quantummechanics.

Far from being xed and immutable, what is being thought at eachstage of these thought experiments is signicantly different from what

had been thought before. In the one case we saw how thoughts about theclassical notions of distance and simultaneity breaking down gave way tothoughts about molecular forces transmitted by an ether. In the other case,thoughts about particles that do not possess well dened positions andmomenta prior to measurement gave way to thoughts about particles thatdo always have well dened positions and momenta being guided by aquantum potential. In short, these examples show that thought experi-ments can have a life of their own.

The impression that thought experiments do not have a life of theirown can be understood as a result of the oversimplied and ahistorical waythat they are typically presented. Thought experiments are often used aspedagogical and rhetorical devices. In these contexts, the complexities and


23. Hacking (1983) presents a much more comprehensive set of arguments for the in-

dependence of physical experiments from theory. Obviously not all of these points can becarried over to thought experiments. My point here is much more limitednamely, toshow that this particular argument of Hackings for a fundamental difference betweenphysical and thought experiments does not hold.


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historical evolution of the thought experiment are omitted. It would be amistake, however, to conclude that rhetoric and pedagogy are their onlyfunction; thought experiments have a more substantial role to play inscientic practice. They are important, not only for drawing out the phys-ical implications of our theories, but also for testing their internal consis-tency, external coherence, simplicity, and explanatory power. In this way,thought experiments, just like ordinary physical experiments, can teach ussomething new.

References

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Armstrong, D. 1983. What is a Law of Nature? Cambridge: CambridgeUniversity Press.Bell, J.S. [1971] 1993. Introduction to the Hidden-Variable Question.

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Nersessian, N. 1993. In the Theoreticians Laboratory: Thought Experi-menting as Mental Modeling. Pp. 291301 in PSA 1992. Volume 2.Edited by D. Hull, M. Forbes, and K. Okruhlik. East Lansing: Philoso-phy of Science Association.

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alisa bokulich- rethinking thought experiments

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