1. Introduction

The above four quotations exemplify the substantial conflict between supporters and critics of the string theoretical research program in assessing the latter's status and success. String theory was first proposed as a universal description of all fundamental physical forces in 1974 and gained momentum in 1984 when some basic theoretical problems were solved. From that time onward, the theory has assumed a central role in fundamental physics despite the fact that, up to this day, it is not supported by any direct empirical evidence and remains a highly incomplete theory. Soon after the theoretical breakthrough of 1984, some degree of skepticism developed among physicists in other fields with respect to the string theorists’ high level of trust in their theory. This skepticism grew with time as string theory remained empirically unconfirmed and theoretically incomplete, whereas its exponents showed no sign of abandoning their strong conviction that they were making a fundamental step toward a better understanding of nature. In recent years, criticism of string physics has been presented in an increasing number of articles and books, which made the conflict about the status of string physics clearly visible to a wider public.

The present article is an attempt to define the character of the described conflict. A better understanding of the conflict's conceptual and philosophical foundation may tell something new about scientific progress and could also contribute to a more fertile debate between the two sides involved.

2. The Nature of the Dispute

Critics of string theory claim that the string theorists’ trust in their theory is not tenable on the basis of generally acknowledged scientific criteria of theory assessment. String theorists retort that the convincing quality of string theory, being based on the theory's specific structural characteristics, reveals itself to the string theory expert only, which implies that most of the critics are just not competent to evaluate the situation. The critics, in turn, do not feel impressed by this argument, since, according to their own understanding, they make a general point about the character of the scientific process, which should remain unaffected by any specific analysis of the theory's technical details.

The dispute can be construed as a discussion that fails to be productive because of a paradigmatic rift between the two disputants: each side bases its arguments on a different set of fundamental preconceptions. The paradigmatic rift does not occur at the level of specific theoretical conceptions, however, but at the more fundamental level of defining the notion of viable scientific argumentation. The situation may be characterized in terms of the concept of the scientific paradigm. Scientists in a scientific field usually share or believe that they share a common understanding with regard to the kinds of argumentation and activity that can be called scientifically legitimate. For the present analysis, it can remain undecided to what extent this common understanding—henceforth called the scientific paradigm—has a universal core that applies to all science and to what extent it differs among various scientific disciplines. It shall suffice to assume that a characteristic scientific paradigm has emerged in modern physics and is taken to be a reliable guide for scientific activity by physicists today. As shall be argued in the following, the critics of string theory attack the theory on the basis of the classical scientific paradigm prevalent in physics, while string theorists have in certain respects moved away from that paradigm. The fact that the paradigmatic rift between the two sides is not as such addressed in the discussion prevents mutual understanding.

Before we have a closer look at the arguments put forward by exponents and critics of string theory, it should be emphasized that there is far-reaching agreement between both sides with respect to the problems string theory faces. Two of these problems have already been mentioned above. First, there is no empirical evidence for the existence of strings, and it cannot be expected that any such evidence will be found in the foreseeable future. While some scenarios in string physics suggest that direct effects of strings could become observable by the next generation of experiments, the majority of models imply a characteristic energy scale of string physics that lies far beyond the grasp of any experiment imaginable today.Footnote ¹ Second, string theory up to now remains a highly incomplete theory. The theory's structure is not yet understood sufficiently well for providing specific quantitative predictions or for offering a clear strategy on how to achieve such predictions. Once again, there are no indications that this situation will improve significantly in the foreseeable future.

A third problem has become more visible in recent years. While the fundamental structure of string theory is understood to be determined uniquely on the basis of very general presumptions, the theory seems to have a huge number of possible ground states. The selection of the ground state “we are living in” determines the values of all those parameters that define our observable world. Since this selection constitutes the outcome of a quantum process, its prediction must remain of a statistical nature, just like the outcome of some individual microphysical process. This implies a considerable reduction of string theory's predictive power. The present incomplete understanding of string theory does not allow a clear assessment as to what extent string theory retains predictive power under the stated conditions. Although speculations that string physics might end up predictively empty seem hardly tenable on the basis of current physical understanding, the unclear situation naturally adds to the impression that string physics is unsatisfactorily detached from empirical confirmation.

All described problems are acknowledged fully by string physicists as well as by critics of string theory. The differences between the two sides lie in the conclusions drawn.Footnote ²

3. The Critics’ Perspective

The string critics’ case shall be presented largely on the basis of Lee Smolin's (Reference Smolin2006) book and Roger Penrose's (Reference Penrose2004) remarks on the topic in The Road to Reality. Similar arguments can be found, for example, in Woit Reference Woit2006. By and large, the sketched arguments are representational of the considerations that have led many physicists who do not work in string physics or particle physics model building toward adopting a skeptical assessment of the current status of string physics.

Penrose and Smolin base their assessments on their canonical understanding of the scientific process. It is a core principle of science that scientific theories must face continuous empirical testing in order to avoid going astray. As formulated suggestively in Penrose's quotation above, the steady sequence of theoretical alternatives that open up in the course of a research program's evolution makes it seem highly implausible that the scientific community could consistently make the right theoretical choices in the absence of empirical guidance. If empirical testing remains absent for a long period of time, chances seem high that scientists will find themselves—to use Penrose's picture—lost in a wrong part of town. Therefore, in order to be conducive to scientific progress, scientific theories are expected to fulfill a certain pattern of evolution. A theory is expected to reach a largely complete theoretical state within a reasonable period of time. This state should allow an assessment of the theory's internal consistency and provide quantitative predictions of empirical data. Once the theory has reached that state, one may expect that it can be empirically tested within a limited time frame in order to help scientists decide whether further work along the lines suggested by the theory makes sense or, in case the theory were empirically false, would be a waste of time. Clearly—and sometimes explicitly emphasized by its proponents—this understanding of the role a theory has to play within the scientific process is strongly influenced by Popperian falsificationism.

Looking at string theory's present condition 34 years after it was first proposed as a fundamental theory of all interactions, we see it has achieved neither of the goals described in the last paragraph. Now, even in the eyes of most of its critics, this does not mean that the theory should be fully abandoned. It has happened before that theories did not reach the stage of maturity and empirical testability for a long time and were shelved until, after some period of general experimental or theoretical progress, they turned out to have the capacity of contributing to scientific progress after all. The critics’ point is, however, that one cannot know whether or not such late success will occur before a theory has found empirical confirmation. A theory that has not reached theoretical completion and empirical confirmation therefore cannot be called successful by classical scientific standards. Given the large number of physicists who have worked on string theory with high intensity during the past 25 years, the theory may actually be called surprisingly unsuccessful by those standards. Thus, its critics take string theory as a scientific speculation that may deserve a certain degree of attention because of its interesting theoretical properties but that is unfit to play the role of a pivotal, let alone dominating, conceptual focal point of an entire scientific discipline. Still, this is exactly what string theory has been doing for a quarter of a century by now.

Smolin and Penrose criticize string theorists for ignoring the canonical rationale for theory assessment that was presented above and for developing an unwarranted degree of trust in their theory's validity. According to Smolin, string theorists systematically overestimate their theory's “performance” by creating their own criteria of success that are tailored to be met by string physics. Examples of this strategy would be the straightforward interpretation of mathematical progress as physical progress without empirical backing or the string theorists’ frequent allusions to structural beauty (see, e.g., the quotation of Michael Green in the introduction). Smolin argues that such “soft” criteria create arbitrary mirages of genuine scientific success. Their application in his eyes impedes the field's ability to carry out an objective assessment of its progress and moves the field away from legitimate scientific reasoning with respect to theory appraisal.Footnote ³ The resulting overestimation of the theory's status, according to Smolin, disturbs a healthy scientific process since it binds to string physics too many resources whose allocation to other parts of physics could produce more significant results.

The question of how a considerable share of the most excellent physicists—after all, string physics today represents the largest field of research in theoretical physics—can jointly commit such a serious methodological blunder is answered by Smolin at a sociological level by deploying the concept of groupthink. This phenomenon allegedly tends to occur in professional groups with high status, strong internal competition, and intense internal interaction. Under such circumstances, the members of a group may be forced into the unreflected adoption of the group's standard positions by a mix of intellectual group pressure, admiration for the group's leading figures, and the understanding that fundamental dissent would harm carrier perspectives. An all too positive and uncritical self-assessment of the group is the natural consequence.

4. The String Theorists’ Perspective

String theorists, as stated above, do acknowledge the problems their theory faces but believe they have strong theoretical reasons for placing trust in the theory's viability despite these problems. They often consider the disregard for such reasons shown by many non–string physicists to be a blatant violation of the scientific expert principle: given a well-established scientific field, one would expect that nonexperts base their opinion about the status of theories in that field largely on the assessments of those who know the theories best. In the case of string physics, quite to the contrary, most critics from other fields follow their own general scientific intuition, bolstered by the statements of a couple of prominent opponents of string theory who have expertise in the field but have never been very influential there. Exponents of string theory tend to locate the reasons for this unusual phenomenon in a lack of openness and flexibility with respect to the acceptance of new physical ideas on the side of the string critics.

The theoretical reasons for believing in the physical viability of string theory have been reconstructed and discussed in detail in Dawid Reference Dawid2006 and shall be sketched briefly along those lines in the following. One can distinguish two different types of argument. On the one hand, a number of arguments are of an external nature—that is, they derive trust in string theory either from the scientific dynamics that has led to the theory's development and guides its evolution or from the way the theory fits into the existing context of physical problems. Three main arguments of that type can be isolated. The “argument of no choice” asserts that, based on very general physical presumptions, string theory seems to be the only possible solution to the problem of a coherent physical description of all known natural forces.Footnote ⁴ A second, meta-inductive argument derives trust in string theory from the fact that theoretical predictions based on consistency arguments of the very kind that drives the development of string theory have remarkably often turned out to be empirically viable during the past evolution of particle physics. Finally, a third argument emphasizes the surprising explanatory coherence of string theory and points out that a considerable number of interconnections between different theoretical problems have emerged in the context of string theory although the theory was not devised to deliver them. Such surprising interconnections are taken to play a similar role for theory corroboration at a theoretical level while the successful prediction of phenomena a theory was not devised to describe plays at an empirical level.

On the other hand, besides these external arguments, there exist arguments of a theory-internal nature that are based on characteristics of string theory's structure itself. These arguments are related to string theory's property of structural uniqueness—that is, the lack of free parameters and model choice at a fundamental level—and to the final theory claims that arise in various forms.Footnote ⁵ While the external arguments can be understood as stronger realizations of types of argument that, to a lesser extent, had been deployed in theoretical physics before, the internal arguments are genuinely string theoretical. By relying on the assumption of string theory's validity, they are circular if taken on their own but can strengthen the overall case in combination with external arguments.

All internal as well as external arguments, it is claimed in Dawid Reference Dawid2006, can be understood in terms of a strengthening of the notion of limitations to scientific underdetermination. “Scientific underdetermination” denotes the underdetermination of scientific theory building by the currently available data. The assumption that scientific underdetermination is prevalent at each stage of the scientific process constitutes a pivotal element of the canonical understanding of scientific progress and provides one crucial reason for the scientist's lack of trust in scientific theories that are empirically unconfirmed.Footnote ⁶ By reducing the strength of the principle of scientific underdetermination at various levels, string theory thus changes the balance between empirical and theoretical methods of theory appraisal. Purely theoretical methods assume a status that seemed unacceptable in other scientific contexts.

5. A Meta-Paradigmatic Shift

The described meta-paradigmatic shift induced by the evolution of string theory, unlike the theory's conceptual novelty itself, is not emphasized by string physicists. A number of reasons may be responsible for this fact. First, being physicists rather than philosophers, string theorists naturally focus on their theory's direct physical import and consider the functionality of the scientific process a precondition that is more or less taken for granted. Second, the meta-paradigmatic changes can be understood to be gradual shifts once they are measured against the scientific praxis rather than against principles of scientific legitimacy (a little more on that will be said later on), which makes those changes less noticeable to the working scientist. And third, conceding a deviation from canonical scientific praxis would invite a level of criticism string physicists have no interest to incur.

The critics of string theory, however, develop their arguments without acknowledging the possibility that a shift of the scientific paradigm might constitute a scientifically legitimate development under some circumstances. Therefore, they discuss string physics strictly on the basis of the canonical scientific paradigm and interpret each mismatch between the two straightforwardly in terms of string theory's failure to meet scientific standards.

Both sides thus appear to agree in disregarding the meta-paradigmatic aspect of their discussion. In doing so, however, they actually lead the discussion on the basis of incompatible sets of hidden preconceptions and therefore must miss each other's point. Seen from either perspective, the respective opponent's position does not have legitimacy based on the preconceptions taken to provide the valid framework for the entire debate. The recourse to mutual imputations of personal insufficiencies follows as a natural consequence. Claims of scientific hubris thus are summoned against claims of insufficient intellectual acuteness.

Framing the controversy in terms of a shift of the scientific paradigm (see Kuhn Reference Kuhn1962) allows acknowledgment of the reasonability of both positions on the basis of their respective preconceptions. Largely unintentionally, string physicists have been led toward a novel conception of scientific theory appraisal by their scientific research, which they had carried out in accordance with all standards of scientific reasoning. The scientific process itself has thus led beyond the canonical limits of scientific reasoning. The success of the new understanding of theory evaluation was clearly accelerated by the fact that the stronger role of purely theoretical argumentation it suggested came at hand in view of the lack of available empirical support for string theory: string theorists had to rely just on theoretical reasoning as long as empirical evidence was missing. The lack of empirical support was not the primary source of the described shift of the scientific paradigm, though. Physicists in other fields who have not experienced the scientific dynamics that instigated the described shift, as well as some physicists who do have an inside perspective on string physics but may feel a particularly strong attachment to scientific tradition, nevertheless understand that shift as a purely defensive ad hoc measure instigated by the long empirical drought. Therefore, they see no sound scientific reason to follow it. Within the framework of their traditional and well-tested scientific paradigm, they find ample justification for repudiating the string theorists’ assessments of string theory's status.

6. The Viability of the String Theorists’ Perspective

Is the string theorists’ move a legitimate one? Clearly, a shift of the scientific paradigm that is induced by the dynamic evolution of a research field must be considered a legitimate option in principle. No paradigm of scientific reasoning has been installed as a God-given law before the commencement of scientific research. Rather, such paradigms have emerged based on the successes and the failures of scientific reasoning witnessed by scientists in the past. Novel scientific input thus must be expected to alter the scientific paradigm in the future. Whether an emerging shift of the scientific paradigm actually constitutes an improvement over the prior situation, however, can be very difficult to decide.

The formulation of the dispute between string physicists and their critics in terms of the notion of the scientific paradigm has been chosen for a specific reason. To a much higher extent than in many other examples of scientific theory change, the situation is characterized by fundamental difficulties in deciding between the two positions on an ‘objective’ basis, that is, without anticipating the outcome by employing the preconceptions related to one or the other position. From the critic's perspective, it is quite plausible to argue that a modification of the scientific paradigm in times of crisis is counterproductive, as it carries the risk of overlooking a solution that would satisfy the criteria set up by the old paradigm. From the new paradigm's perspective, sticking to the old criteria too long inhibits scientific progress by sticking to a misguided chimera of the static nature of scientific principles. Since the notion of the new theory's success depends on adopting the new paradigm, the claims of success based on the new paradigm cannot easily convince the old paradigm's supporters. All one can do in this case is assess the internal coherence and attractiveness of the old and new positions on their own terms and compare the two internal assessments on a more general—and therefore necessarily more vague—argumentative basis.

As has been shown above, the powerful position of theoretical criteria of theory evaluation that is implied by the new scientific paradigm is rooted in nontrivial characteristics of string physics itself. It remains to be shown that these criteria do not contain an automatism of theoretical self-confirmation but are capable of providing the foundations for a critical scientific discipline. Indeed, this condition seems to be fulfilled though the processes of theoretical confirmation and refutation are more complex, indirect, and time consuming than empirical testing tends to be in more traditional scientific contexts. A number of scenarios could significantly reduce or even nullify the trust in string theory on the basis of purely theoretical evaluation criteria. For example, it could turn out that another theoretical concept can solve the conceptual problems string theory was developed to solve in a similar or better way. It could also happen that an improved theoretical understanding of string physics reveals theoretical weaknesses that change the theoretical assessment of the theory's chances of being physically viable. An interruption of theoretical progress over a long period of time would raise doubts about whether a more complete theoretical understanding of string physics is attainable at all. In addition, trust in string physics would be influenced by new empirical data that do not represent a test of string physics but have implications for the status of theoretical theory evaluation in the field. The latter's status would grow if experiments confirmed predictions whose viability had been considered likely on the basis of purely theoretical reasoning. (The discovery of supersymmetry would be an example in case—though, being predicted by string theory, it could also count as indirect empirical evidence for the theory itself.) The status of theoretical theory evaluation would seriously suffer, however, if predictions that are strongly supported theoretically were empirically refuted.

When the old (classical) scientific paradigm is considered, the most powerful meta-inductive argument in its favor appears to be its consistent success in the past. While this point seems undisputable as a viable first approximation, closer examination leads to an important qualification: in fact, the classical paradigm of scientific theory assessment was never fully supported by the scientific praxis. The notion of the exclusive role of empirical data for theory confirmation significantly relies on the empiricist preconceptions about science that were deployed for offering a clear-cut distinction between science and nonscience. While there is no doubt, of course, that science indeed distinguishes itself by its strict reliance on empirical data, philosophers of science during the first two-thirds of the twentieth century and many scientists until today tended to maximize the distinction's sharpness by ignoring those aspects of the scientific process that contradict the simple empiricist picture. The importance of theoretical aspects of theory assessment is emphasized, for example, in Laudan Reference Laudan1977. Laudan points out that, besides empirical arguments, many theoretical considerations enter the evaluation of a scientific theory (simplicity, elegance, universality, or coherence with general worldview would be some examples). What is even more significant, when empirical and theoretical arguments are at variance, the empirical arguments do not always win. According to Laudan, the notion that theoretical arguments play a strictly secondary role in theory assessment and serve as mere preliminary guidelines before empirical tests have decided the case on solid grounds is too simple and in many cases is contradicted by scientific praxis. Theoretical theory assessment rather constitutes an independent and substantial part of theory assessment that must be taken into consideration in a full appraisal of the scientific process.

The roots of the specific theoretical strategy of assessing limitations to scientific underdetermination—which, according to the present article, assumes a pivotal role in the context of string physics—also reach back a long time, and the strategy's value increases in connection with the growing importance of the theoretical realm throughout the twentieth century. Without going into detail, I note that the general scientific acceptance of specific unobservable scientific objects in the early twentieth century or the confident prediction of new particles based on the particle physics standard model may be taken as significant stages in a process that leads toward increasing trust in theoretical conceptions based on the assessment that, under some circumstances, an equally convincing theoretical structuring of the available data with substantially different prognostics is unlikely to exist.

Seen in this light, arguments from string physics do not open up an entirely new way of assessing scientific theories but merely strengthen a strand of theory assessment that has always been part of scientific reasoning. In fact, contemporary string physics does not seem to mark the endpoint of this development. A look at the current situation in fundamental physics shows that the focus on purely theoretical argumentation is considerably stronger in some other fields than in string physics proper. In cosmology, to name the most striking example, the multiverse scenario of eternal inflation has assumed an influential position during recent years. This scenario posits the existence of other (sub)universes that, according to present understanding, are unobservable in principle and whose existence can only be inferred on theoretical grounds. If so, an assessment of limitations to scientific underdetermination would be the only possible basis for supporting the theory's viability.Footnote ⁷

Of course, the sharp rise in the importance of theoretical criteria of theory assessment in recent decades and the arguments that can be found in support of the reasonability of that development do not necessarily imply that these criteria will be able to defend their status in the future. The decision of that question, however, may itself be expected to be played out on the basis of the new paradigm of theory assessment.