Caltech
Philosophy of Physics Reading Group '23-'24

Eugene Y. S. Chua (Caltech):
The Time in Thermal Time (draft here)
(11 Oct 2023)

Quantizing gravity in the Hamiltonian approach leads to the ‘problem of time,’ rendering the world fundamentally timeless. One proposed solution is the ’thermal time hypothesis,’ which defines time in terms of states representing systems in thermal equilibrium. On this view, time emerges thermodynamically, with time being defined by systems in thermal equilibrium, even in a fundamentally timeless context. Here, I raise the worry that the thermal time hypothesis requires dynamics – and hence time – to get off the ground, thereby running into worries of circularity.

Bixin Guo (Pittsburgh HPS):
Next Best Thing —What Can Quantum Mechanics Tell Us About the Fundamental Ontology of the World? (draft here)
(25 Oct 2023)

Many discussions in the metaphysics and philosophy of physics literature aim to use physics as a guide to elucidate what theworld really, fundamentally is like. However,we don’t yet have a confirmed fundamental theory of physics—what’s the next best thing we can possibly say about the fundamental that is properly informed by our best theories of physics? This paper offers a starting point to address this question. It focuses on the literature on the ontology of quantum mechanics, where the problem is especially salient: Many proposals aim at drawing the fundamental ontology of the world from quantum mechanics, even though they often focus on a nonfundamental theory such as nonrelativistic quantum particle mechanics. I argue that quantum mechanics can plausibly be informative about the fundamental if it is taken as a general framework theory, which covers a range of specific concrete theories, including nonrelativistic quantum particle mechanics, the Standard Model of particle physics, and string theory. I use Wavefunction Realism as an example to demonstrate what kind of ontological lessons about the world at the fundamental level the quantum framework may teach us.

Sebastián Murgueitio Ramírez (Purdue):
Relativity, Symmetries, and the Harmonies of Guitars 
(draft here)
(8 Nov 2023)

In this paper, I argue that there is an unnoticed ambiguity in standard presentations of the so-called Galilean principle of relativity, roughly, the principle that says the laws of mechanical systems are the same in all inertial systems. In particular, I argue that standard presentations fail to distinguish between what ultimately are two inequivalent principles, which I call “External Galilean Relativity Principle” (EGRP) and “Internal Galilean Relativity Principle” (IGRP). IGRP concerns the invariance of the laws of mechanical systems as seen from the perspective of a co-moving inertial frame. The paper is structured into three main parts. The first is mostly historical and shows that when physicists define and explain the Galilean principle of relativity, they sometimes refer to IGRP, some other times to EGRP, and sometimes to both. In the second part, I prove that IGRP and EGRP are not just two different versions of the very same principle but actually two inequivalent principles altogether. A direct and surprising implication of this result is that the laws of some (properly isolated) mechanical systems are not the same in all inertial systems. In the third part, I show how the distinction between IGRP and EGRP offers new insights into recent debates on the philosophy of symmetries.

Niels Linnemann (Geneva): 
Constructive Axiomatics in Spacetime Physics Part II: Constructive Axiomatics in Context (joint work with Emily Adlam and James Read)
(draft here)
(29 Nov 2023)

The Ehlers-Pirani-Schild (EPS) constructive axiomatisation of general relativity, published in 1972, purports to build up the kinematical structure of that theory from only axioms which have indubitable empirical content. It is, therefore, of profound significance both to the epistemology and to the metaphysics of spacetime theories. In this article, we set the EPS approach in its proper context, by (a) discussing the history of constructive approaches to spacetime theories in the lead-up to EPS; (b) addressing some of the major concerns raised against EPS; (c) considering how EPS compares with ‘chronometric’ approaches to affording the metric field of general  relativity its operational significance; (d) distinguishing quite generally between different kinds of constructive approach, and fitting EPS into this classification; (e) discussing how constructivism bears on a number of other issues in the foundations of physics; and (f) assessing the merits of constructivism qua local foundationalist project. There are two companion papers, in which we provide a pedagogical walkthrough to the EPS axiomatisation (Part I), and discuss/develop versions of EPS with quantum mechanical inputs (Part III).

Chip Sebens (Caltech):
How do Laws Produce the Future?
(draft here)
(6 Dec 2023)

The view that the laws of nature produce later states of the universe from earlier ones (prominently defended by Maudlin) faces difficult questions as to how the laws produce the future and whether that production is compatible with special relativity.  This article grapples with those questions, arguing that the concerns can be overcome through a close analysis of the laws of classical mechanics and electromagnetism.  The view that laws produce the future seems to require that the laws of nature take a certain form, fitting what Adlam has called “the time evolution paradigm.”  Making that paradigm precise, we might demand that there be temporally local dynamical laws that take properties of the present and the arbitrarily-short past as input, returning as output changes in such properties into the arbitrarily-short future.  In classical mechanics, Newton's second law can be fit into this form if we follow a proposal from Easwaran and understand the acceleration that appears in the law to capture how velocity (taken to be a property of the present and the arbitrarily-short past) changes into the arbitrarily-short future.  The dynamical laws of electromagnetism can be fit into this form as well, though because electromagnetism is a special relativistic theory we might require that the laws meet a higher standard: linking past light-cone to future light-cone.  With some work, the laws governing the evolution of the vector and scalar potentials, as well as the evolution of charged matter, for electromagnetism in the Lorenz gauge can be put in a form that meets this higher standard.

Francisco Pipa (Kansas):
Beyond the Wigner's friend dilemma:
A new indeterminacy-based quantum theory (draft here)
(10 Jan 2024)

I propose a novel local, non-relational single-world, non-collapse, non-superdeterministic/non-retrocausal (interpretation of) quantum theory called Environmental Determinacy-based or EnD Quantum Theory (EnDQT). In contrast to some quantum theories, EnDQT is not in tension with relativity and provides a local causal explanation of Bell correlations. Additionally, unlike collapse theories, in principle, arbitrary systems can be placed in a superposition for an arbitrary amount of time. Furthermore, it provides a series of novel empirical posits that may distinguish it from other quantum theories. According to EnDQT, some systems acquire determinate values at some point in time, and the capacity to give rise to determinate values through interactions propagates to other systems in spacetime via local interactions. This process can be represented via certain networks. When there is isolation from the rest of the systems that belong to these networks, such as inside the friend’s isolated lab in the extended Wigner’s friend scenarios, indeterminate values non-relationally arise inside.

Nick Huggett (U Illinois Chicago)
Finding Time for Wheeler-DeWitt Cosmology (w/ Karim Thebault,
draft here)
(24 Jan 2024)

We conduct a case study analysis of a proposal for the emergence of time based upon the approximate derivation of three grades of temporal structure within an explicit quantum cosmological model which obeys a Wheeler-DeWitt type equation without an extrinsic time parameter. Our main focus will be issues regarding the consistency of the approximations and derivations in question. Our conclusion is that the model provides a self-consistent account of the emergence of chronordinal, chronometric and chronodirected structure. Residual concerns relate to explanatory rather than consistency considerations.

Postponed!

TBA!

Joshua Eisenthal (Caltech, Einstein Papers Project)
Back to the Problem of Space, draft here
(21 Feb 2024)

Following the proliferation of non-Euclidean geometries in the nineteenth century, the “problem of space” emerged as the problem of demarcating which mathematical geometries were candidate physical geometries. By around 1900, a consensus formed around the following purported solution. The possibility of measuring spatial magnitudes depends on the possibility of moving rigid bodies (such as rulers and compasses) without changing their dimensions. As only the constant curvature geometries could represent this kind of rigid transport, only these geometries were candidate physical geometries — or so the argument went. However, it was only after the development of general relativity in 1915 that the physical significance of transport along affine geodesics (“straightest” lines) was understood. When an object is not affected by external forces and moves inertially it moves along an affine geodesic, but if this takes place in a curved space, those geodesics do not stay a fixed distance apart. Thus an extended object will experience elastic tension when it moves in a curved space, even when there are no forces acting on it. In this talk I will explore what impact this insight might have had for the nineteenth century problem of space. In particular, I will explore the consequences for the two main positions in the philosophy of geometry in this period: geometrical empiricism (the view that experiments determine which geometry is “true”) and geometrical conventionalism (the view that we ourselves must decide, based on simplicity and convenience, which geometry is best to use).

Christopher Dorst (Florida):
Superfluous Ideological Superstructure (or: What Humeans Should Say about Quantum Mechanics, and what that Says about Humeanism), draft here
(6 Mar 2024)

This paper examines two perennial problems with quantum mechanics, namely the measurement problem and what I’ll call the “ontology problem,” from the perspective of Humeanism about laws of nature. The main argument of the paper is that Humeanism, properly developed, has the resources to dissolve both of these problems. However, once it has done so, we’ll need to reevaluate what Humeanism has become in the process. Superficially, then, this is a paper about quantum mechanics, but really it’s about Humeanism.

Katie Robertson (Stirling):
TBA!
(10 Apr 2024)

TBA!

Travis McKenna (Pittsburgh):
TBA!
(24 Apr 2024)

TBA!

Shelly Yiran Shi (UCSD):
TBA!
(8 May 2024)

TBA!

David Glick (UC Davis)
The principle of least action and teleological explanation in physics (preprint here)
(5 Jun 2024)

The Principle of Least Action (PLA) has often been cited as a counterexample to the dominant mode of causal explanation in physics. In particular, PLA seems to involve an appeal to final causes or some other teleological ideology. However, Ben-Menahem (2018) argues that such implications no longer apply given that PLA can be recovered as limiting case from quantum theory. In this paper, I argue that the metaphysical implications of PLA-based explanations are not undermined by its status as derivative. However, I contend that PLA functions as a diachronic constraint that licenses explanations by constraint (Lange, 2016). PLA-based explanations, on this account, are non-causal but also differ from familiar cases of teleological explanations in several respects.