The role of heuristic appraisal in conflicting assessments of string theory

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Highlights

  • We provide a philosophical analysis of conflicting assessments of string theory.

  • Physicists differ in their heuristic appraisal of string theory and its potential.

  • Disagreement revolves around the status of solved problems and explanatory coherence.

  • We analyse different conceptions of theoretical progress in quantum gravity research.

Abstract

Over the last three decades, string theory has emerged as one of the leading hopes for a consistent theory of quantum gravity that unifies particle physics with general relativity. Despite the fact that string theory has been a thriving research program for the better part of three decades, it has been subjected to extensive criticism from a number of prominent physicists. The aim of this paper is to obtain a clearer picture of where the conflict lies in competing assessments of string theory, through a close reading of the argumentative strategies employed by protagonists on both sides. Although it has become commonplace to construe this debate as stemming from different attitudes to the absence of testable predictions, we argue that this presents an overly simplified view of the controversy, which ignores the critical role of heuristic appraisal. While string theorists and their defenders see the theoretical achievements of the string theory program as providing strong indication that it is ‘on the right track’, critics have challenged such claims, by calling into question the status of certain ‘solved problems’ and its purported ‘explanatory coherence’. The debates over string theory are therefore particularly instructive from a philosophical point of view, not only because they offer important insights into the nature of heuristic appraisal and theoretical progress, but also because they raise deep questions about what constitutes a solved problem and an explanation in fundamental physics.

Introduction

Over the last three decades, string theory (broadly construed here to include its conjectured successor M-theory) has emerged as one of the leading hopes for resolving one of the fundamental problems, if not the fundamental problem, of high-energy physics—the formulation of a consistent theory of quantum gravity. String theory attempts to solve the problem by treating elementary particles as quantum vibrational states of extended one-dimensional objects called strings, with the aim of bringing the general theory of relativity within a unified theoretical framework that encompasses the Standard Model of elementary particle physics. In effect, it attempts to solve two problems at once. In their landmark textbook, Michael Green, John Schwarz and Edward Witten explain that the guiding principle of string theory “has always been that in the course of learning how to make a consistent theory of quantum gravity one might learn how gravity must be unified with other forces” (Green, Schwarz, & Witten, 1987, p. 14). While the underlying ontology of string theory is still far from clear, many leading theorists remain convinced it is the only viable candidate for a quantum theory of gravity that unites the fundamental particles and forces in nature.

Despite the fact that string theory has been a thriving research program for the better part of three decades, having gone through two self-proclaimed ‘revolutions’, it has been the subject of considerable controversy. Physicists have engaged in, sometimes heated, debates about its relative merits and prospects as a viable research program, and even its status as a science. In his widely read The Elegant Universe, Brian Greene describes string theory “as a work in progress whose partial completion has already revealed astonishing insights into the nature of space, time, and matter” (Greene, 1999, p. 18). While acknowledging that many problems remain unresolved, Greene declares that “superstring theory, and its evolution into M-theory” has provided physicists with “a cogent framework for merging quantum mechanics, general relativity, and the strong, weak and electromagnetic forces” (Greene, 1999, p. 386). In 1998 John Schwarz would go further, in declaring that in his view string theory had already uncovered “the unique mathematical structure that consistently combines quantum mechanics and general relativity” (Schwarz, 1998, p. 2).

Yet in striking contrast to this assessment, critics have painted a picture of string theory as languishing in a state of crisis, having stagnated, floundering amidst mounting anomalies and a lack of empirical support. Criticisms of string theory date back to the 1980s, when such leading figures as Richard Feynmann and Sheldon Glashow voiced their concerns (Davies and Brown, 1988, Ginsparg and Glashow, 1986, Glashow, 1986). But by 2006 the controversy reached a climax, culminating in the publication of two popular books, The Trouble with Physics by Smolin (2008) and Not Even Wrong by Woit (2007). Both books have attracted widespread criticism from string theorists—Mike Duff, a leading string theorist, described Smolin׳s book as a “venomous attack on string theory” (Smolin Duff & Cartwright, 2007). In stark contrast to the “inspirational narratives of progress and triumph” that emerged in the wake of the second superstring revolution, Woit argued that it has become increasingly clear that “superstring theory has failed as a viable idea about unification” and that “any further progress” in the quest for a unified theory “will require physicists to abandon the now ossified ideology of supersymmetry and superstring theory that has dominated the last two decades” (Woit, 2007, pp. 6, 216, 267). Not all critics go so far as to write string theory off as an unmitigated failure. In his more charitable moments, Smolin acknowledges that string theory “succeeds at enough things so that it is reasonable to hope that parts of it, or perhaps something like it might comprise some future theory” of quantum gravity, and to this extent, it is “certainly among the directions that deserve more investigation”. Yet he maintains, “there is compelling evidence that something has gone wrong” (Smolin, 2008, p. 198).

An obvious question arises here. Why have physicists (and more recently philosophers) drawn such divergent and conflicting assessments of string theory? Undoubtedly, one of the major issues is that string theory has failed to make experimentally testable predictions. Many leading experimental and theoretical physicists, such as Lawrence Krauss, Martinus Veltmann and Burton Richter, have echoed the concerns of Smolin and Woit, in arguing that the continuing absence of testable predictions constitutes a serious concern, and even casts doubt on its scientific legitimacy. Indeed such concerns about string theory had already begun to emerge in the 1980s in the work of Sheldon Glashow and Richard Feynman. Historians and philosophers of science, such as Galison (1995) and more recently Dawid, 2006, Dawid, 2009, Dawid, 2013a, Dawid, 2013b, have argued that the development of string theory has brought with it a “meta-paradigmatic shift” in the evaluation of physical theories. In Dawid׳s view, the debates over string theory can be understood as “disputes between defenders of the traditional paradigm of theory assessment” based on experimental confirmation “and adherents of a newly emerging one” (Dawid, 2013a, p. 82). Yet while the lack of experimental support for string theory has been the focus of much of the popular debate and has attracted much of the recent philosophical interest in string theory, it has not been sufficiently appreciated that the more recent criticisms of string theory go considerably beyond this aspect.

While many of the criticisms levelled at string theory have been summarily dismissed by string theorists as fundamentally misguided, ignorant of actual developments in the field, and ideologically motivated attacks by disgruntled outsiders, the philosophical questions raised concerning the how we should assess string theory are worthy of more careful consideration. The aim of this paper is to obtain a clearer picture of where the conflict lies in competing assessments of string theory. Here we propose to simply ignore the often-made criticisms that there have been no testable predictions. While it is true that many points of disagreement between critics and defenders of string theory rest on the lack of empirical support, here we focus on the most recent wave of criticisms which have sought to undermine the claim that string theory is ‘on the right track’ on purely theoretical grounds. These claims raise interesting epistemological questions that have become the subject of much discussion within philosophy of science since the 1970s. The debates over string theory are therefore particularly instructive from a philosophical point of view, precisely because they offer important insights into the nature of epistemic and heuristic appraisal, and judgments concerning the non-empirical dimensions of scientific progress.

What emerges from a closer analysis of the debates over string theory is the central role that heuristic appraisal plays in conflicting evaluations of string theory. This has been largely overlooked because of a preoccupation with lack of empirical testability and with post-Kuhnian philosophical accounts of scientific progress. It is crucial to note that while critics agree that string theory research has opened up new areas of research in areas of physics and mathematics, they express varying degrees of skepticism about that claim that such developments bring us closer to the goal of a unified theory of quantum gravity and particle physics. String theorists, on the other hand, have argued that the impressive achievements of the string theory program provide strong indications that string theory is ‘on the right track’. While string theorists point to the theory׳s track record of problem-solving success and its deep explanatory coherence in defense of this claim, critics maintain that these successes do not, upon careful reflection, provide reasons to be optimistic that string theory is ‘on the right track’, and indeed one can equally point to indications that something has gone amiss. Thus while the debate appears on the surface to be about justification and epistemic appraisal, much of the disagreement actually concerns heuristic appraisal (an assessment of the promise and future potential of the theoretical program).

Here we identify three examples of this kind of competing assessments of string theory.

  • 1.

    The first difficulty concerns the ambiguity surrounding the heuristic significance of solved problems. While string theorists typically point to the successful solution of outstanding problems, such as a finite quantum theory of gravity, critics have argued that these only constitute partial successes, or at best, provide hints to a more complete solution. In general, they maintain, many of these problems remain unsolved. Yet, string theorists and their defenders claim that these partial successes provide the strongest indication that string theory is on the right track.

  • 2.

    String theorists typically point to the problem-solving effectiveness of string theory as evidence that it is progressing. However, critics argue that these developments betray a deeper sense in which the program is stagnating, or worse still, degenerating. Here critics highlight the way in which solutions to problems have only served to exacerbate the conceptual difficulties confronting us by generating further problems. String theory, they claim, now appears to be dominated by problems of its own making. This situation, critics suggest, calls into question whether ‘problem-solving’ is actually indicative of teleological progress.

  • 3.

    The third point concerns the explanatory power of string theory. String theorists argue that a series of new and unexpected interconnections have come to light. These unexpected theoretical connections, which have arisen in course of the historical development of string theory, have made possible explanations of certain phenomenal or conceptual features of the Standard Model of particle physics and modern cosmology. Yet, critics point out that while string theory can certainly explain certain ‘generic features’ of our world, such as the existence of fermions, bosons and gravitons, it cannot derive or explain the specific features of the Standard Model. The question then boils down to whether the worlds described by string theory provide clues to a deeper understanding of our world. This turn out to be a very complex question, as we shall, see, given that there is at present, no agreement, even among string theorists as to what we should take as the explanatory aim of string theory.

Before addressing each of these points of conflict in turn, it is worth exploring the way in which different conceptions of scientific progress have been employed by physicists and philosophers in the debates over string theory.

Section snippets

Are we on the right track?

While string theory has undergone considerable transformation over the past three decades, it is well documented that it has remained exclusively a theoretical endeavour. There have been no experimentally testable predictions, nor has experiment has played a guiding role in its theoretical development (Shifman, 2012). Indeed, there remains widespread agreement that there is at present “no realistic possibility for a definitive confirmation or falsification of a unique prediction from it by a

The heuristic significance of ‘solved problems’

As we noted earlier, much of the debate over progress in string theory, focuses on the ‘solved problem’ as the principal ‘unit of achievement’. Yet, as Thomas Nickles noted more than thirty years ago, any problem-solving model of scientific progress must address “what a problem is and what an ‘account’ of problems should do” (1981, p. 85). While in traditional problem-solving models of scientific progress, the “solved problem is the principal unit of achievement”, in “heuristic appraisal”,

When solving problems generates further problems

In the 1970s philosophers of science began to move away from the static view of scientific theories to a more dynamic model, according to which theories are continually modified over time. A good theory, as Ernan McMullin defines it, is “one which [has] successfully guided research over an extended period of time”. Of course much hangs on how we define ‘success’, but it is clear that in evaluating theories, we must refer to “the historical record” (McMullin, 1976, pp. 401–402). The construction

What does string theory explain?

One of the strongest reasons for confidence in string theory is the “glimpses” of its “explanatory power” evident in the various structural characteristics of the theory (Greene, 1999, p. 19). While there is still no direct empirical evidence for supersymmetry, it remains extremely attractive to string theorists for theoretical reasons. As noted earlier, only supersymmetric versions of string theories avoid the tachyon problem and include fermions. Supersymmetric quantum field theories, which

Conclusions

While there have been a number of attempts in recent years to experimentally confirm versions of supersymmetry at the Large Hadron Collider, or to find evidence for string theory in cosmological observations, the development of string theory over the past four decades has remained an exclusively theoretical endeavour. For this reason, philosophical reflections on string theory have typically focused on the lack of testable predictions. Yet the debates over string theory go well beyond the lack

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