## 2015-2016

11 June 2016, 3pm, LPS seminar room

**Dennis Lehmkuhl (Caltech), on the Problem of Motion**

The problem of motion of general relativity is about how exactly the gravitational field equations, the Einstein equations, are related to the equations of motion of material bodies subject to gravitational fields. This paper compares two approaches to derive the geodesic motion of (test) matter from the field equations: `the T approach’ and `the vacuum approach’. The latter approach has been dismissed by philosophers of physics because of it apparently representing material bodies by singularities. I shall argue that a careful interpretation of the approach shows that it does not depend on introducing singularities at all, and that it holds at least as much promise as the T approach. I finish with some general lessons about careful vs. literal interpretations of scientific theories.

Please read Dennis’s manuscript before the meeting.

7 May 2016, 3pm, LPS seminar room

**Bihui Li (USC), “Solutions in Constructive Field Theory”**

Constructive field theory aims to rigorously construct concrete solutions to Lagrangians used in particle physics, where the solutions satisfy some relevant set of axioms. I examine the nature of solutions in constructive field theory and their relationship to both axiomatic and Lagrangian quantum field theory (QFT). I argue that Lagrangian QFT provides conditions for what counts as a successful constructive solution, and that it provides other important information that guides constructive field theorists to solutions. Solutions matter because they describe the behavior of systems in QFT, and thus what QFT says the world is like. Constructive field theory, in incorporating ingredients from both axiomatic and Lagrangian QFT, clarifies existing disputes about which parts of QFT are relevant to philosophers and the role of rigor in these disputes.

Please read Bihui’s manuscript before the meeting.

16 April 2016, 3pm, LPS seminar room

**Julia Bursten (SFSU) on Multiscale Modeling**

Winsberg’s “handshaking” account of inter-model relations is a well-known theory of multiscale modeling in physical systems. Winsberg argues that relations among the component models in a multiscale modeling system are not related mereologically, but rather by empirically determined algorithms. I argue that while the handshaking account does demonstrate the existence of non-mereological relationships among component models, Winsberg does not attend to the different ways in which handshaking algorithms are developed. By overlooking the distinct strategies employed in different handshake models, Winsberg’s account fails to capture the central feature of effective multiscale modeling practices, namely, how the dominant behaviors of the modeled systems vary across the dierent scales, and how this variation constrains the ways modelers can combine component models. Using Winsberg’s example of nanoscale crack propagation, I distinguish two modes of handshaking and show how the different modes arise from the scale-dependent physics involved in each component model.

Please read Julia’s draft manuscript before the talk.

5 March 2016, 3pm, LPS seminar room

**Matt Leifer (Chapman), “Does time-symmetry in quantum theory imply retrocausality?”**

In [1], Huw Price argued that, under certain assumptions about the underlying ontology, an interpretation of quantum theory that is both realist and time-symmetric must be retrocausal, i.e. it must involve influences that travel backwards in time. Price’s argument is based on an analysis of a photon travelling between two polarizing beam-splitters. One of his assumptions is that the usual forward-evolving polarization vector of the photon is a beable, i.e. part of the ontology. He argues, on the basis of this and his other assumptions,

that a backward-evolving polarization vector must also be a beable.

The assumption that the forward evolving polarization vector is a beable is an assumption of the reality of the quantum state. But one of the reasons for exploring retrocausal interpretations of quantum theory is that they offer the potential for evading the unpleasant conclusions of no-go theorems, such as Bell’s theorem and, in particular, recent proofs of the reality of the quantum state [2]. In this talk, I will show how Price’s argument can, in fact, be generalized so that it does not assume the reality of the quantum state. The relationship between Price’s argument and ours bears a strong parallel to the relationship between the EPR argument and Bell’s theorem. I also reformulate the common assumptions of Price’s and our arguments to make them more generally applicable and to pin down the notion of time-symmetry involved more precisely. The notion of time-symmetry used in the argument is stronger than the notion of time-symmetry usually used in physics, but bears a family resemblance to conditions used in other no-go theorems, such as parameter independence in Bell’s theorem, and Spekkens’ notion of contextuality. All of these can be derived from the assumption that, if a theory has a symmetry in its operational predictions, then that symmetry ought to hold at the ontological level as well.

This talk is based on joint work with Matt Pusey.

[1] H. Price. Does time-symmetry imply retrocausality? How the quantum world says “maybe”. Stud. Hist. Phil. Mod. Phys., 43(2):75–83, 2012. arXiv:1002.0906

[2] For a review see M. Leifer. Is the quantum state real? An extended review of psi-ontology theorems. Quanta, 3:67-155, 2014. arXiv:1409.1570

13 February 2016, 3pm, LPS seminar room

**Richard Dawid (Stockholm) on String Dualities and Empirical Equivalence**

String dualities constitute a specific form of empirical equivalence in physics. One may argue that, after a century when empirical equivalence was primarily of interest to philosophy of science, the rise of duality in string physics marks the first time that empirical equivalence takes centre stage in physics itself. The paper will make the case, however, that the philosophical repercussions of string dualities are in fact directly opposed to the way the significance of empirical equivalence was understood throughout most of the 20th century in philosophy of science as well as physics. Comparing the canonical perspective on empirical equivalence with the role played by duality today provides an interesting indicator of the way string physics has altered the physicists’ perspective on physical theory building.

Richard expects to distribute a paper a few days before the meeting.

16 January 2016, 3pm, LPS seminar room

**Patricia Palacios (Munich) on the role of approximation in a reductive model for phase transitions**

Abstract: Phase transitions, roughly understood as sudden changes in the phenomenological properties of a system, have recently motivated a debate about reduction and emergence in the physical sciences. In this debate there are two main positions: i) Phase transitions are paradigmatic cases of emergent or irreducible behavior (Lebowitz 1999, Batterman 2000, 2002, Bangu 2011); ii) phase transitions represent a successful case of Nagelian reduction (Butterfield 2011, Menon and Callender 2011, Norton 2012). This leads one to conceive of the discussion in the following terms: Phase transitions are either non-reductive phenomena or reductive phenomena satisfying the Nagelian model of reduction. In this paper I will suggest that this dichotomy is misleading. In fact, there are good reasons for considering phase transitions as a case of reduction that does not satisfy the Nagelian model of reduction, either in its strict or more liberal versions.

21 November 2015, 3pm, LPS seminar room

**John Byron Manchak (UCI) on Epistemic Holes in Spacetime**

Abstract: A number of models of general relativity seem to contain “holes” which are thought to be “physically unreasonable”. One seeks a condition to rule out these models. We examine a number of possibilities already in use. We then introduce a new condition: epistemic hole-freeness. Epistemic hole-freeness is not just a new condition — it is new in kind. In particular, it does not presuppose a distinction between spacetimes which are “physically reasonable” and those which are not.

Please read JB’s manuscript before the meeting.

24 October 2015, 3pm, LPS seminar room

**Chip Sebens (CalTech & UCSD) on Constructing and Constraining Wave Functions for Identical Quantum Particles**

Abstract: I address the problem of explaining the symmetry dichotomy within two interpretations of quantum mechanics which clarify the connection between particles and the wave function by including particles following definite trajectories in addition to, or in lieu of, the wave function: Bohmian mechanics and Newtonian quantum mechanics. I present and examine what I take to be the most illuminating explanation of the symmetry dichotomy given in the context of Bohmian mechanics (Bacciagaluppi, 2003). If the Bohmian guidance equation is formulated as to not rely on a correspondence between particles and arguments of the wave function, then the symmetry dichotomy must be postulated for the particle velocities to be well-defined. Still, the symmetry dichotomy remains an additional postulate. I propose an explanation of the symmetry dichotomy in Newtonian quantum mechanics which parallels Bacciagaluppi’s, but is simpler and stronger. In Newtonian quantum mechanics the wave function is not part of the fundamental ontology, but simply a convenient way of describing the positions and velocities of particles. Because a world in Newtonian quantum mechanics is equally well represented by multiple points in configuration space, it turns out that any wave function constructed to describe these worlds will be either symmetric or antisymmetric. The symmetry dichotomy need not be postulated. I show that both of these explanations can be given in reduced configuration space, though doing so is only necessary if one wants to defend exotic approaches to these interpretations which take configuration space as fundamental.

The talk will be based on Chip’s draft manuscript. This project further examines the theory proposed in Quantum Mechanics as Classical Physics (arXiv) (blog post).