Dualities are a pervasive phenomenon in contemporary physics, in which two physical theories are empirically equivalent, yet prima facie make different ontological claims about the world (potentially very different claims—differing in e.g. the number and radius of dimensions of the universe). Dualities thus present a particular instantiation of the well-known notion of underdetermination of theory by evidence. Many different philosophical proposals have been made for how such putative underdetermination might be resolved—this continues to be a programme of active research.

A ‘duality’ is a formal mapping between the spaces of solutions of two empirically equivalent theories. In recent times, dualities have been found to be pervasive in string theory and quantum field theory. Naïvely interpreted, duality-related theories appear to make very different ontological claims about the world—differing in e.g. space-time structure, fundamental ontology, and mereological structure. In light of this, duality-related theories raise questions familiar from discussions of underdetermination in the philosophy of science: in the presence of dual theories, what (...) is one to say about the ontology of the world? In this paper, we undertake a comprehensive and non-technical survey of the landscape of possible ontological interpretations of duality-related theories. We provide a significantly enriched and clarified taxonomy of options—several of which are novel to the literature. (shrink)

The main aim of this paper is to make a remark about the relation between dualities between theories, as `duality' is understood in physics and equivalence of theories, as `equivalence' is understood in logic and philosophy. The remark is that in physics, two theories can be dual, and accordingly get called `the same theory', though we interpret them as disagreeing---so that they are certainly not equivalent, as `equivalent' is normally understood. So the remark is simple: but, I shall argue, worth (...) stressing---since often neglected. My argument for this is based on the account of duality developed by De Haro: which is illustrated here with several examples, from both elementary physics and string theory. Thus I argue that in some examples, including in string theory, two dual theories disagree in their claims about the world. I also spell out how this remark implies a limitation of proposals to understand theoretical equivalence as either logical equivalence or a weakening of it. (shrink)

Numerous approaches to a quantum theory of gravity posit fundamental ontologies that exclude spacetime, either partially or wholly. This situation raises deep questions about how such theories could relate to the empirical realm, since arguably only entities localized in spacetime can ever be observed. Are such entities even possible in a theory without fundamental spacetime? How might they be derived, formally speaking? Moreover, since by assumption the fundamental entities cannot be smaller than the derived and so cannot ‘compose’ them in (...) any ordinary sense, would a formal derivation actually show the physical reality of localized entities? We address these questions via a survey of a range of theories of quantum gravity, and generally sketch how they may be answered positively. (shrink)

This paper investigates the significance of T-duality in string theory: the indistinguisha- bility with respect to all observables, of models attributing radically different radii to space – larger than the observable universe, or far smaller than the Planck length, say. Two interpretational branch points are identified and discussed. First, whether duals are physically equivalent or not: by considering a duality of the familiar simple harmonic oscillator, I argue that they are. Unlike the oscillator, there are no measurements ‘outside’ string theory (...) that could distinguish the duals. Second, whether duals agree or disagree on the radius of ‘target space’, the space in which strings evolve according to string theory. I argue for the latter position, because the alternative leaves it unknown what the radius is. Since duals are physically equivalent yet disagree on the radius of target space, it follows that the radius is indeterminate between them. Using an analysis of Brandenberger and Vafa (1989), I explain why – even so – space is observed to have a determinate, large radius. The conclusion is that observed, ‘phenomenal’ space is not target space, since a space cannot have both a determinate and indeterminate radius: instead phenomenal space must be a higher-level phenomenon, not fundamental. (shrink)

There exists a common view that for theories related by a ‘duality’, dual models typically may be taken ab initio to represent the same physical state of affairs, i.e. to correspond to the same possible world. We question this view, by drawing a parallel with the distinction between ‘interpretational’ and ‘motivational’ approaches to symmetries.

In this paper, I will argue that metaphysicians ought to utilize quantum theories of gravity as incubators for a future metaphysics. In §1, I will argue why this ought to be done. In §2, I will present case studies from the history of science where physical theories have challenged both the dogmatic and speculative metaphysician. In §3, I will present two theories of QG and demonstrate the challenge they pose to certain aspects of our current metaphysics; in particular, how they (...) challenge our understanding of the abstract-concrete distinction. In this section I demonstrate how five different accounts of the distinction each fail to hold under the received interpretations of loop quantum gravity and string theory. The central goal of this paper is to encourage metaphysicians to look to physical theories, especially those involving cosmology such as string theory and loop quantum gravity, when doing metaphysics. (shrink)

We analyse the possibility that string-theoretic dualities present a genuine case of strong underdetermination of theory by evidence. Drawing on the parallel discussion of the hole argument, we assess the possible interpretations of dualities. We conclude that there exist at least two defensible interpretations on which dualities do not present a worrying case of underdetermination per se.

. Over the last three decades, physicists have engaged in, sometimes heated, debates about relative merits and prospects of string theory as a viable research program and even about its status as a science. The aim of this paper is to provide a deeper understanding of this controversy as a particular form of boundary discourse. Drawing on the sociological work of Thomas Gieryn and Lawrence Prelli, we bring to light the way in which protagonists appeal to, and rhetorically construct, different (...) views about the scientific method and the scientific ethos, in an effort to legitimize or delegitimize string theory. (shrink)

In this paper, I will argue that metaphysicians ought to utilize quantum theories of gravity as incubators for a future metaphysics. I will argue why this ought to be done and will present cases studies from the history of science where physical theories have challenged both the dogmatic and speculative metaphysician. I provide two theories of QG and demonstrate the challenge they pose to certain aspects of our current metaphysics; in particular, how they challenge our understanding of the abstract–concrete distinction. (...) I demonstrate how five different accounts of the distinction each fail to hold under the received interpretations of loop quantum gravity and string theory. The central goal of this paper is to encourage metaphysicians to look to physical theories, especially those involving cosmology such as string theory and loop quantum gravity, when doing metaphysics. (shrink)

In this paper, some conceptual issues are addressed in order to make sense of what string theory is supposed to tell us about spacetime. The dualities in string theory are used as a starting point for our argumentation. We explore the consequences of a standard view towards these dualities, namely that the dual descriptions represent the same physical situation. Given this view, one has to understand string theory in a manner such that what counts as physical spacetime is based only (...) on the shared physical content—or common core—of the dual descriptions. In general such a spatiotemporal picture does not have to agree with, or be similar to, any of the ones suggested by naïve readings of the dual descriptions. However, in certain regimes or limits, one or the other of the initial dual descriptions may give a good effective description of physical spacetime. (shrink)

In the absence of empirical confirmation, scientists may judge a theory's chances of being viable based on a wide range of arguments. The paper argues that such arguments can differ substantially with regard to their structural similarly to empirical confirmation. Arguments that resemble empirical confirmation in a number of crucial respects provide a better basis for reliable judgement and can, in a Bayesian sense, amount to significant \textit{non-empirical} confirmation. It is shown that three kinds of non-empirical confirmation that have been (...) specified in earlier work do satisfy those conditions. (shrink)

The PreambleIn the early seventies the newly discovered gauge symmetries had made it possible to give a unified description of three of the four basic forces in nature.Dawid [1]. However, it remained a mystery how the force of gravity was to be incorporated in the resulting standard model. In 1974 it was proposed by Joel Scherk and John Schwarz that string theory—which until then was regarded as a theory only of hadrons—was a theory that could unite the whole of microphysics. (...) The promise of full unification attracted many researchers to the field of string theory. A lot of work was done which expanded the mathematical framework and in 1984 Scherk and Schwarz finally succeeded in formulating the Lagrangian of the superstring: a quantised string that includes a matter field. With the mathematical formalism now firmly in place, the scientific community waited for experimental verification of the string hypothesis. And it waited.The Current DebateToday, more than 40 years after the earlies. (shrink)

In this chapter I consider what recent work on background independent physics can do for structuralism, and what structuralism can do for background independent physics. I focus on the problems of time and observables in gravitational physics. The ‘frozen’ character of the observables of general relativity is usually considered to constitute a serious problem for the theory. I argue that by invoking correlations between physical quantities we can provide a natural explanation of the appear- ance of time and change in (...) timeless structures. I argue that this response can resolve a problem with Max Tegmark’s ‘extreme structuralist’ position. I then consider what bearing the mathematical representation used has on the debate over the nature of structure in discussions of structural realism. I argue that it has both the resources to ground the notion of structure in physics and to answer the ‘no relations without relata’ objection. (shrink)

I examine the early history of quantum gravity and comment on its suitability as an episode that demands an integrated approach to history and philosophy of science.