Nick Huggett

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.

There has been considerable recent philosophical debate over the implications of many particle quantum mechanics for the metaphysics of individuality (cf. Huggett [1997]). In this paper I look at things from a rather different perspective: by investigating the significance of permutation symmetry. I consider how various philosophical positions link up to the physical postulate of the indistinguishability of permuted states-permutation invariance-and how this postulate is used to explain quantum statistics. I offer an explanation of the statistics that relies on the neglected parallel between permutations and covariant spatial transformation. And I explore the parallel, showing that a further kind of symmetry explains why permutations are invariant when spatial symmetries are not.

This paper develops and defends three related forms of relationism about spacetime against attacks by contemporary substantivalists. It clarifies Newton's globes argument to show that it does not bear on relations that fail to determine geodesic motions, since the inertial effects on which Newton relies are not simply correlated with affine structure, but must be understood in dynamical terms. It develops remarks by Sklar and van Fraassen into relational versions of Newtonian mechanics, and argues that Earman does not show them to trivialize the debate.

Why does time pass and space does not? Are there just three dimensions? What is a quantum particle? Nick Huggett shows that philosophy -- armed with a power to analyze fundamental concepts and their relationship to the human experience -- has much to say about these profound questions about the universe. In Everywhere and Everywhen, Huggett charts a journey that peers into some of the oldest questions about the world, through some of the newest, such as: What shape is space? Does it have an edge? What is the difference between past and future? What is time in relativity? Is time travel possible? Are there other universes? Huggett shows that answers to these profound questions are not just reserved for physics, and that philosophy can not only address but help advance our view of our deepest questions about the universe, space, and time, and their implications for humanity. His lively, accessible introduction to these topics is suitable for a general reader with no previous exposure to these profound and exciting questions.

Almost everything that we know about Zeno of Elea is to be found in the opening pages of Plato's Parmenides. There we learn that Zeno was nearly 40 years old when Socrates was a young man, say 20. Since Socrates was born in 469 BC we can estimate a birth date for Zeno around 490 BC. Beyond this, really all we know is that he was close to Parmenides (Plato reports the gossip that they were lovers when Zeno was young), and that he wrote a book of paradoxes defending Parmenides' philosophy. Sadly this book has not survived, and what we know of his arguments is second-hand, principally through Aristotle and his commentators (here I have drawn particularly on Simplicius, who, though writing a thousand years after Zeno, apparently possessed at least some of his book). There were apparently 40 ‘paradoxes of plurality’, attempting to show that ontological pluralism — a belief in the existence of many things rather than only one — leads to absurd conclusions; of these paradoxes only two definitely survive, though a third argument can probably be attributed to Zeno. Aristotle speaks of a further four arguments against motion (and by extension change generally), all of which he gives and attempts to refute. In addition Aristotle attributes two other paradoxes to Zeno. Sadly again, almost none of these paradoxes are quoted in Zeno's original words by their various commentators, but in paraphrase.

This is the table of contents and first chapter of Physics Meets Philosophy at the Planck Scale (Cambridge University Press, 2001), edited by Craig Callender and Nick Huggett. The chapter discusses the question of why there should be a theory of quantum gravity. We tackle arguments that purport to show that the gravitational field *must* be quantized. We then introduce various programs in quantum gravity and discuss areas where quantum gravity and philosophy seem to have something to say to each other.

Much apprehension has been expressed by philosophers about the method of renormalisation in quantum field theory, as it apparently requires illegitimate procedure of infinite cancellation. This has lead to various speculations, in particular in Teller (1989). We examine Teller's discussion of perturbative renormalisation of quantum fields, and show why it is inadequate. To really approach the matter one needs to understand the ideas and results of the renormalisation group, so we give a simple but comprehensive account of this topic. With this in hand, we explain how renormalisation can and should be understood. One thing that is revealed is that apparently very successful theories such as quantum electro-dynamics cannot be universally true; resolving the tension between success and falsity leads to a picture in which any theory may be viewed as irreducibly phenomenological. We explain how, and argue that the support for this view is tenuous at best.

This paper considers the implications for the relational-substantival debate of observations of parity nonconservation in weak interactions, a much neglected topic. It is argued that 'geometric proofs' of absolute space, first proposed by Kant (1768), fail, but that parity violating laws allow 'mechanical proofs', like Newton's laws. Parity violating laws are explained and arguments analogous to those of Newton's Scholium are constructed to show that they require absolute spacetime structure--namely, an orientation--as Newtonian mechanics requires affine structure. Finally, it is considered how standard relationist responses to Newton's argument might respond to the new challenge of parity nonconservation

We explicate recent results that shed light on the obscure and troubling problem of renormalization in Quantum Field Theory (QFT). We review how divergent predictions arise in perturbative QFT, and how they are renormalized into finite quantities. Commentators have worried that there is no foundation for renormalization, and hence that QFTs are not logically coherent. We dispute this by describing the physics behind liquid diffusion, in which exactly analogous divergences are found and renormalized. But now we are looking at a problem that is physically and formally well-defined, proving that the problems of renormalization, by themselves, cannot refute QFT

The quantum gravity program seeks a theory that handles quantum matter fields and gravity consistently. But is such a theory really required and must it involve quantizing the gravitational field? We give reasons for a positive answer to the first question, but dispute a widespread contention that it is inconsistent for the gravitational field to be classical while matter is quantum. In particular, we show how a popular argument (Eppley and Hannah 1997) falls short of a no-go theorem, and discuss possible counterexamples. Important issues in the foundations of physics are shown to bear crucially on all these considerations

This work first explicates the philosophy of classical and quantum fields and particles. I am interested in determining how science can have a metaphysical dimension, and then with the claim that the quantum revolution has an important metaphysical component. I argue that the metaphysical implications of a theory are properties of its models, as classical mechanics determines properties of atomic diversity and temporal continuity with its representations of distinct, continuous trajectories. ;It is often suggested that classical statistical physics requires that many particle states be represented so that permuting properties leads to distinct states; this implies that individuals can be reidentified across possible worlds in a non-qualitative way. I show there is no evidence for this conclusion, an important result, for it is claimed that quantum particles are not individuals. This claim is based on the misconception about classical statistics, but also on a conflation of notions of identity; I show that, while transworld identity is incompatible with quantum mechanics, other classical notions may be consistently ascribed. I also give a field-particle distinction that applies usefully in both quantum and classical domains. In the former the distinction helps defeat claims of underdetermined by data, in the latter it helps provide a minimal field metaphysics. ;Next I tackle renormalisation: I show how divergences occur in approximate, perturbative calculations, and demonstrate how finite, empirically verified, answers are obtained. These techniques seem to show that the predictions are not logical consequences of the exact theory. I use the techniques of the renormalisation group to establish that perturbative renormalised quantum field theory does indeed approximate the consequences of field theory. ;Finally, I discuss the idea that renormalisation proves that there can be no quantum theory of everything, only a patchwork of effective theories. The preceding chapter shows that renormalisation demonstrates only that the picture is consistent, and this is insufficient to show that physics must be phenomenological

In this paper we contrast the idea of a field as a system with an infinite number of degrees of freedom with a recent alternative proposed by Paul Teller in Teller (1990). We show that, although our characterisation lacks the immediate appeal of Teller's, it has more success producing agreement with intuitive categorisations than his does. We go on to extend the distinction to Quantum Mechanics, explaining the important role that it plays there. Finally, we take some time to investigate the way in which strings are to be considered fields, and the important differences with scalar fields. Overall, we aim to show that many types of systems may be viewed as fields, and to point out significant distinctions amongst them, thereby expanding our understanding of what it is to fall in this category.

This paper explains the phenomenon of `entanglement exchange' within the Bohmian approach to quantum mechanics. After explaining Bohmian mechanics and entanglement exchange, in which pairs of particles become entangled without ever interacting causally in the usual, unitary sense, our aim is to use this example, to illustrate how the `pilot wave' mediates non-local correlations. The discussion thus gives a useful new way to think about entanglement exchange, and clarifies the structure of Bohmian mechanics

This item is a chapter from a book in progress, entitled "True Motion". Leibniz’s mechanics was, as we shall see, a theory of elastic collisions, not formulated like Huygens’ in terms of rules explicitly covering every possible combination of relative masses and velocities, but in terms of three conservation principles, including (effectively) the conservation of momentum and kinetic energy. That is, he proposed what we now call (ironically enough) ‘Newtonian’ (or ‘classical’) elastic collision theory. While such a theory is, for instance, vital to the foundations of the kinetic theory of gases, it is not applicable to systems – like gravitational systems – in which fields of force are present. Thus, Leibniz’s mechanical principles never led to developments of the order of Newton’s in the Principia (additionally, he hamstrung their application by embedding them in a baroque philosophical system).

I criticize a certain view of the 'quanta' of quantum mechanics that sees them as fundamentally non-atomistic and fundamentally significant for our understanding of quantum fields. In particular, I have in mind work by Redhead and Teller (1991, 1992 and Teller 1990). I prove that classical particles do not have the rather strong flavour of identity often associated with them; permuting positions and momenta does not produce distinct states. I show that even the label free excitation formalism is compatible with a mild form of atomism. Finally, I summarise some of the principle objections to an 'oscillator' interpretation of quantum fields.

This paper investigates the mathematical representation of time in physics. In existing theories time is represented by the real numbers, hence their formal proper- ties represent properties of time: these are surveyed. The central question of the paper is whether the existing representation of time is adequate, or whether it can or should be supplemented: especially, do we need a physics incorporating some kind of ‘dynamical passage’ of time? The paper argues that the existing mathematical framework is resistant to such changes, and might have to be rejected by anyone seeking a physics of passage. Then it rebuts two common arguments for incorporating passage into physics, especially the claim that it is an element of experience. Finally the paper investigates whether, as has been claimed, ‘causal set theory’ provides a physics of passage.