Studies in History and Philosophy of Modern Physics 41 (2010) 362–365

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Studies in History and Philosophyof Modern Physics

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Book Review

The Ashgate Companion to Philosophy of Physics, D. Rickles(Ed.), The Ashgate Companion to Philosophy of Physics,Ashgate, Aldershot (2008). 386 pp., Price: $124.95, £65,h 74.60, ISBN: 9780754655183

1. Introduction

The Ashgate Companion to Contemporary Philosophy ofPhysics (Ashgate, 2008)1 is a valuable reference tool for the areasof philosophy of physics it covers, namely the foundations ofquantum mechanics, statistical mechanics, quantum informationtheory and quantum gravity. The book is intended as anintroduction to the foundations of these subjects, pitched tobeginning graduate students in the field. In this capacity, the bookfills a gap in the literature, and promises to bring interestedstudents up to speed in these areas of study, focusing on recentdevelopments and open problems.

The book is organised into four substantive chapters, eachdevoted to a single topic and written by some of the finest youngresearchers in each field, respectively. As a result, the approachesvary considerably with each author, as does the level ofsophistication. Some chapters require one to already haveconsiderable fluency with the mathematical formalism of thetheory, while others are quite self-contained in their presentation.As such, the book as a whole might not be appropriate for self-directed study by graduate students early in their careers, giventhat one needs considerable familiarity with the structure ofseveral physical theories (especially quantum mechanics andrelativity) that goes beyond what is typically taught in under-graduate physics curricula. However, with some direction fromadvisors, or used in a seminar course, the book will be aninvaluable resource for students and even more advancedphilosophers looking to acquaint themselves with some of thepressing problems and issues in the sub-disciplines of philosophyof physics found in the companion. Conversely, the book is a littlemarred by several typesetting and editorial issues, ranging from alittle annoying (the labels for systems are often not subscripted onthe kets in Timpson’s chapter) to potentially misleading forsomeone not versed in the subject (among the more egregious:Rickles reports the Planck length as 1.62�1033 cm! (p. 270)).

The first, brief, chapter of the companion (authored by DeanRickles) outlines a particular view of what philosophy of physicsis all about, followed by a brief overview of the contents of eachsubstantive chapter. I take it that Rickles’ characterisation of whatphilosophy of physics is will strike many as being relativelyuncontroversial: to Rickles, the philosophy of physics endeavoursto find the ‘best’ interpretation of a given physical theory, i.e. topick out the best way(s) of describing what the world might be

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l references throughout are to page numbers in the Ashgate Companion.

like if the theory were true. A further task of the philosophy ofphysics is to reconcile such interpretations with our pre-theoreticconceptions of the world. Perhaps more controversially, Ricklessuggests that the philosophy of physics should attempt to movepast the seemingly stale debates of the past, and engage withcurrent areas of physical research such as those treated in thecompanion. As such, he suggests that the best avenues of researchare those that move away from the ‘classic’ problems of timeasymmetry in statistical mechanics, conventionalism and sub-stantivalism in spacetime physics, and more historically orientedinterpretational work. Whether one agrees with this orientationor not, the chapters Rickles assembles in the book offer anilluminating introduction to the cutting edge of both physical andfoundational research. In what follows, I provide a brief outlineand assessment of each of the four substantive chapters of thecompanion.

2. Quantum mechanics

Wallace’s chapter on the measurement problem provides aclear and useful review and taxonomy of the range of interpreta-tions of quantum mechanics that have been proposed andconsidered by philosophers. In terms of its breadth, the chapterprovides a thorough though brief summary of the main candi-dates for solutions to the measurement problem, and notes boththe technical and philosophical problems associated with each.Beyond its taxonomic role, Wallace provides a unifying frame-work for classifying interpretations, putting to work a generalisedframework of POVMs and decoherence rather than the morestandard presentation in terms of associating observables solelywith projectors.

Wallace draws heavily on his presentation of the measurementproblem in terms of POVMs to evaluate interpretations, so it isworthwhile to pause and assess its significance. The approachfavoured here eschews the eigenvalue–eigenstate link and theassociation of measurements with projectors. Rather, the moregeneral positive operator-valued-measurement framework ispreferred, where measurements correspond to a more generalclass of operations and the results of measurements are under-stood as localised wave-packets (which he calls the quantumalgorithm (p. 20)). Wallace justifies this presentation by notingthat it adheres more closely to the way in which QM is applied inpractice, and that the traditional eigenvalue–eigenstate link isactually denied by many of the interpretations he considers.

This justification seems less than conclusive. Is the methodaccording to which physicists actually apply QM a definitiveconsideration in presenting the measurement problem, i.e. shouldone justify the presentation of the problem by appealing to thepractices of physicists or by examining the structure of the theoryitself? Further, even if some interpretation or another denies theEE link (or some other principle in the statement of the traditionalmeasurement problem), is this not a useful way of classifying and

Book Review / Studies in History and Philosophy of Modern Physics 41 (2010) 362–365 363

organising interpretations, viz. by indentifying which principlesthat go into stating the problem they reject?

Wallace provides clear appraisals of the technical problemsfacing each interpretation. The framework he develops succeedsin giving a succinct and clear outline of each interpretation, andserves to facilitate their evaluation in a unified manner throughthe framework of decoherence. Given recent foci of attention inthe philosophy of physics, Wallace devotes a fair amount of spaceto the Everett interpretation, collapse theories (especially in thecontext of the ‘tails’ problem) and hidden variable theories (bothof the Bohmian and modal variety). These sections are exception-ally clear and provide an invaluable introduction to current areasof concentration in the foundations of QM, with an excellentcoverage of the literature and well-summarised discussion of thestate of play for each of these interpretive frameworks. Some heedis also paid to some interpretational programmes favoured byphysicists, such as consistent histories and operationalism. Afurther section introducing some issues in relativistic quantumfield theory ends the chapter, though it is rather short and doesnot delve too deeply into foundational issues.

But the treatment of conceptual issues is somewhat uneven.Wallace poses, as the main challenge, the interpretation of thequantum algorithm and an explanation of why and how it works,but little is done to clearly state the criteria according to which agiven interpretation is to be judged. To be sure, there are a myriadof arguments presented for and against each interpretation, andthese often draw on disparate issues in the philosophy of science.However, there is little attention paid to issues of locality,underdetermination, the interpretation of probability, realism orother typical bread and butter issues in the philosophy of physics;often they are mentioned only in passing. Rather, Wallace’sconceptual discussion of interpretations often hinges on issues offunctionalism, dualistic understandings of interpretations, andquestions of personal identity. Although there is surely some usein drawing on the literature of philosophy of mind or metaphysicsin the discussion of physics and these issues play central roles inthe Everett interpretation, most philosophers of physics (at anyrate, those that do not work on Everett) will find themselves inunfamiliar philosophical territory.

3. Statistical mechanics

The foundations of statistical mechanics (SM) have seen aboom in the past fifteen years or so since Sklar’s seminal bookPhysics and Chance, and Roman Frigg does an admirable job inreviewing some of the main conceptual problems facing thesubject, and bringing the reader up to speed with the recentdevelopments. This task is rendered especially difficult, as Friggnotes, by the lack of agreement between the different camps inthe field, foremost between those favouring the Boltzmann andGibbsian approaches. Insofar as these two approaches agree onvery little, including the correct framework for presenting SM,Frigg treats each tradition separately, carefully detailing themathematical and conceptual machinery each has proffered forformalising SM. Indeed, much of the chapter is devoted topainstakingly working through and introducing the respectiveformalisms, which is a precondition for fruitfully engaging inphilosophical debates in SM. These introductions are followed bydiscussions of the foundational problems facing each camp and abrief look at their respective proffered solutions. Given thesedisparate approaches, Frigg’s presentation provides a natural andflowing structure to a tangled subject, especially since theproblems faced and responses to these problems often differquite markedly between the Boltzmann and Gibbs schools.

Frigg begins with the Boltzmann framework, carefully definingthe notion of a macrostate and entropy in a full phase space, anddistinguishes between what he calls ‘‘macro’’ and ‘‘micro’’probabilities that assign probabilities to different macrostatesand microstates, respectively. The general framework of the fulls-space is carefully contrasted with Boltzmann’s 6-dimensionalm-space representation used in his famous combinatorial argu-ment, and two proposals to link these different representations ofSM systems are evaluated. The detail and care with which Friggpresents the relation between these two frameworks is itself avaluable contribution to the literature, and serves to clarify anoften confusing but crucial aspect of the Boltzmann approach.

In a like fashion, Frigg proceeds to enumerate a number ofproblems faced by the Boltzmann approach, such as providing adynamical account of the approach to equilibrium, how oneshould interpret the probabilities appearing in SM, the reversi-bility and recurrence objections, and issues concerning thereduction of thermodynamics to SM. Each of these problems isbriefly, though clearly presented, and the responses are groupedinto two sections. The first section discusses the ergodicprogramme as a way to provide a dynamical explanation of whythe conceptual apparatus of the Boltzmann approach succeeds indescribing the behaviour of SM systems and furthermore,provides a justification for assigning macroprobabilities to TDstates. A second cluster of problems is addressed through themodern Boltzmann approach favoured by Albert, Lebowitz,Goldstein and others. Here Frigg succinctly summarises themodern Boltzmann interpretation, and does an excellent job ofreviewing the numerous criticisms of this increasingly popularaccount of SM, focusing on the role that the low entropy initialstate of the universe (the past hypothesis) purportedly plays inunderwriting microprobabilities and in solving the reversibilityobjections.

In turning to the Gibbs approach, Frigg quickly moves throughsome of the standard problems facing the Gibbs approach,choosing to focus his attention on recent foundational work,specifically Malament and Zabell’s work on justifying Gibbs phaseaveraging and Vranas’ more recent extension of the programmewith his work on e-ergodicity. There is also a detailed discussionof the spin-echo experiments, and an excellent precis of Ridderbosand Redhead’s arguments against Gibbsian coarse-graining on thebasis of these experiments.

Khinchin’s programme, the thermodynamic limit and inter-ventionism are also briefly discussed, although little space isdevoted to them. As such, it is somewhat difficult to assess theirrespective foundational imports. However, a welcome section onJaynes’ epistemic approach receives considerable attention, andFrigg’s analysis is valuable, especially since its foundationalweaknesses are easily presented from the vantage of the standardGibbsian formalism. Frigg reviews some worries regarding theepistemic approach in an especially clear and perceptive way,although he seems to suggest that this approach is somehowworse off in its justification of equating the phase averages withexperimental outcomes. Here Frigg takes the case of rolling a fairdie: the expectation value for the outcome of the die is 3.5, but noparticular toss will yield this value. Naturally, this is a problem foranyone who looks to equate phase averages with outcomes, but itis not clear that Jaynes’ approach is any worse off in this respect toa standard Gibbsian reading (at least for finite ensembles). Thereason that this looks worse from in the Jaynesian case as opposedto the usual SM formulation is presumably that Jaynes believedthat the MEP approach was a formalism for dealing with generalproblems of statistical inference, and thus the SM formalism wasperfectly appropriate for characterising the outcomes of die rolls,whereas the ‘standard’ Gibbsian would hesitate to apply theformalism outside the usual scope of the theory. But as long as

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one limits the domain of concern to Jaynes’ ability to explicateand interpret the SM formalism as it is applied to thermodynamicsystems (which is the purpose of this chapter), it is unclear whyFrigg takes this apparent problem to be something unique orinterestingly different from the usual Gibbsian case (of course, aproposed solution to this problem might take a very differentform on the epistemic approach).

4. Quantum information theory

Among the book’s chapters, Timpson’s chapter on quantuminformation theory (QIT) has the least ground to cover in terms ofphilosophical literature, conceptual apparatus and mathematicalformalism. This combination, along with excellent clarity, makesthis the most accessible chapter in the book. After having read thechapter, a relative neophyte to QIT should have a good under-standing of the basic framework of the theory, and a grasp ofsome of the most common pitfalls that a naıve interpretation ofthe theory often engenders.

Timpson notes that among its presumed main sources ofinsight, QIT attempts to characterise what one can do withcharacteristically quantum systems over and above their classicalcounterparts, rather than directly interpreting the theoreticalframework itself. To this end, he reviews the details of severalinformation-theoretic and cryptographic protocols and results,such as superdense coding, key distribution, teleportation and theoperation of quantum computers. These algorithms have beentaken (primarily by the information theorists themselves) to pointto interesting foundational questions that were not evident in theusual formulations of QM or are more starkly posed from theinformational context. Typical of such claims are deep questionssuch as ‘where did the information go?’, or ‘how did the informa-tion get transferred’?

What follows is a philosophical analysis of the notion ofinformation, where Timpson attempts to deflate many of thesesupposed ‘deep foundational insights’ promulgated by quantuminformation theorists. Timpson identifies ‘information’ as anabstract noun, and argues that it is a form of category mistaketo treat information as if it were a concrete entity; that is, assomething that can be created, transferred, transmitted orreceived. He argues that this realisation obviates many of thebold foundational claims or puzzles suggested by QIT. AlthoughTimpson’s analysis is careful, its deflationary nature leaves oneasking what the foundational import of QIT might be, andTimpson does not leave many hints as to directions for furtherwork or unresolved problems.

Although this ground clearing comprises the vast majority ofthe chapter, this apparent shortcoming is remedied somewhat inthe final two sections of the chapter, where Timpson discusses thequestion of what explains the increased efficiency of quantumcomputers and how to interpret the Church–Turing thesis in aquantum context, and further discusses some interpretations ofQM that are directly inspired by QIT. Again, Timpson’s analysis issomewhat deflationary, and few positive conclusions are drawn.

The first issue he explores is whence the computationalspeedup of quantum computers comes. Timpson identifies themost common explanation as being an endorsement of the manyworlds interpretation, where the speedup is the result of acomputation taking place in multiple ‘worlds’ as opposed to theclassical case, where a computation takes place in only one‘world’. However, Timpson notes that this explanation is less thandefinitive, since it seems to treat a quantum computation as manyclassical computations, rather than just a single one. In this vein,Timpson also notes that there are physical implementations ofquantum algorithms that do not rely on parallel processing but on

cluster states, where there is no natural interpretation of thephysical computation in terms of the evolution of multipleworlds. Again, Timpson points to the fact that there exists noeasy answer to the question as they were posed, and nounambiguous account seems forthcoming.

In the final substantive section, Zeilinger’s foundationalprinciple, the Clifton–Bub–Halvorson theorem and QuantumBayesianism are assessed. Timpson takes each of these in turn,cogently presenting the gist of these programmes and theirrespective motivations. Again, in each case Timpson delivers anegative verdict, though the severity of the assessment variesbetween the programmes. He asserts that the FoundationalPrinciple is ‘‘wholly unsuccessful’’ (244), but merely appearsquite pessimistic as to the potential foundational value of the CBHand Quantum Bayesian approaches.

5. Quantum gravity

Of the four chapters, Rickles’ chapter exploring the state ofquantum gravity (QG) is both the longest and the mostchallenging. Unlike the previous chapters, where one finds(perhaps with the exception of SM) a more or less unifiedformalism for each theory and considerable agreement as to thenature of the theory, the foundations of QG remain a hodgepodgeof wildly different approaches still in their nascence, each withtheir own mathematical tools and proper foundational concerns.As a result, the breadth of mathematical sophistication requiredto follow Rickles’ description of different approaches to QG farexceeds those of the other chapters, though Rickles relies onworded descriptions of the mathematical formalism rather thanthe equations. This is surely for the best, as an understanding ofthe mathematics of the various approaches to QG requires afluency in various areas of mathematics that likely goes wellbeyond what is typical of beginning graduate students. Even so,the complexity and advanced nature of the subject make fordifficult reading, and considerable guidance from faculty will berequired to broach this chapter.

The chapter begins by providing an overview of the motiva-tions behind the search for a unified theory of quantummechanics and gravitational phenomena, and a prospectus as tohow philosophers of physics might fruitfully contribute to thisburgeoning area of physics research. Rickles divides the motiva-tions for a theory of QG into two sorts: the conceptual and theformal. Given the lack of empirical data requiring a synthesis ofthe quantum and the gravitational, one would like to see a carefulphilosophical analysis of why a theory of QG should be necessaryat all. However, Rickles largely eschews a philosophical evaluationof why a theory of QG is necessary, preferring to report on thevarious arguments proffered by physicists in support of QG.Specifically, he points to formal issues regarding non-renormali-sability of general relativity, and a few formal or mathematicalissues that suggest that a broader unificatory framework may bedesirable. On the more conceptual side, Rickles discusses Hawk-ing’s Black Hole Information Paradox as an argument for thenecessity of QG. Here, it would seem, a careful analysis of thecogency of this argument would be appropriate, but Ricklesmerely reports several responses from the physics community inanswering the question of ‘where did the information go?’.Questions of this kind, of course, were carefully identified as asort of category mistake in the previous chapter on QIT, and it isdisappointing to see Rickles co-opting the loose and misleadingtalk of information being a sort of concrete entity.

Indeed, this chapter, unlike the others, is not a review of thephilosophical literature concerning an area of physical research(mention of the philosophical literature on QG is often relegated

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to footnotes), but rather provides an overview of differentapproaches to QG, pointing out spots where Rickles thinksphilosophers might fruitfully contribute to ongoing debateswithin physics. Given the breadth of current research in quantumgravity, he is limited to sketching the broad outlines of thedifferent research programmes and identifying the apparentmathematical and conceptual problems each faces, though oneis left with the feeling that in order to grasp the subtleties of theseproblems, one must go well beyond the descriptions Ricklespresents and turn to the actual physical literature. As such, Ricklessees this chapter as ‘‘intended primarily as a catalyst for researchprojects by philosophers of physics’’ (262), rather than a review ofthe state of the philosophical literature on the topic.

Nonetheless, Rickles does provide relatively detailed andaccessible overviews of the major approaches to QG, especiallythe main canonical and covariant approaches to the subject (andto a lesser extent, Feynman quantization, causal set theory,twistors and other less mainstream approaches). Although theamount of mathematical knowledge required to grasp the detailsof, say, string theory or loop quantum gravity, can be quite vast,Rickles does an admirable job in describing these approaches andthe primary technical impediments they face as paths to aconsistent and successful theory of QG.

The problem of time receives special attention in the chapter,and is presented in a particularly clear form. Again, however, thediscussion focuses on physicists’ proposed solutions to theproblem, rather than the philosophical literature, even thoughhe cites it as the problem ‘‘that has received most attention fromphilosophers’’ (281). For instance, he notes (in a footnote) a paperby Curiel denying the apparent contradiction between QM and

GR, but simply counter-asserts that there is one! In a later section,Barbour’s ‘timeless’ view is emphasised and receives particularattention, along with those of Healey and Rovelli, and the discussionof Butterfield’s attempt to locate these views within moretraditional debates in the philosophy of time serves Rickles well.

6. Conclusion

Overall, the Ashgate companion will serve as an invaluableresource for graduate students looking to familiarise themselveswith the current state of play in foundational discussions of physics.Indeed, even philosophers of physics well past their post-graduatedays will find the concise and readable accounts found in the bookan important reference source. Although there may be some weakpoints in each chapter, each contribution assembles, summarisesand evaluates a vast range of contemporary literature into coherentand manageable chunks in an elegant and readable way. Naturally,there is a wealth of references in each chapter, and appropriately thebibliographies follow each one, rather than being found at the end ofthe book. Perhaps the greatest failing of the book may at once be itsgreatest strength: if it succeeds in its aim of spurring philosophers ofphysics to work and publish in these areas of current physicsresearch, it will have a short shelf life.

Daniel ParkerPhilosophy Department, Virginia Tech, USA

IHPST, Paris-1, France

E-mail address: parkerdn@vt.edu