## 2021-2022

30 April 2022: **Mario Hubert (Caltech), “Is the Statistical Interpretation of Quantum Mechanics ψ-Ontic or ψ-Epistemic?**“

The ontological models framework distinguishes ψ-ontic from ψ-epistemic wave- functions. It is, in general, quite straightforward to categorize the wave-function of a certain quantum theory. Nevertheless, there has been a debate about the ontological status of the wave-function in the statistical interpretation of quantum mechanics: is it ψ-epistemic and incomplete or ψ-ontic and complete? I will argue that the wave- function in this interpretation is best regarded as ψ-ontic and incomplete. Furthermore, I will show that the probabilities in the statistical interpretation also point to the incompleteness of the theory if construed as hypothetical frequencies.

2 April 2022:

**Mahmoud Jalloh (USC), “The Π-Theorem as a Guide to Quantity Symmetries and the Argument Against Absolutism”**

In this paper a symmetry argument against quantity absolutism is amended. Rather than arguing against the fundamentality of intrinsic quantities on the basis of transformations of basic quantities, e.g. mass doubling, a class of symmetries defined by the Π-theorem is used. This theorem is a fundamental result of dimensional analysis and shows that all unit-invariant equations which adequately represent physical systems can be put into the form of a function of dimensionless quantities. Quantity transformations that leave those dimensionless quantities invariant are empirical and dynamical symmetries. The proposed symmetries of the original argument are open to counterexamples which show that they fail to be both dynamical and empirical symmetries. The amendment of the original argument requires consideration of the relationships between quantity dimensions, particularly the constraint of dimensional homogeneity on our physical equations. The discussion raises a pertinent issue: what is the modal status of the constants of nature which figure in the laws? Two positions, constant necessitism and constant contingentism, are introduced and their relationships to absolutism and comparativism undergo preliminary investigation. It is argued that the absolutist can only reject the amended symmetry argument by accepting constant necessitism, which has a costly outcome: unit transformations are no longer symmetries.

12 February 2022

**Porter Williams (USC), “The Aim and Structure of Cluster Decomposition”**

In the architecture of quantum field theory, one finds a handful of load-bearing locality or causality conditions. One of the most important is the cluster decomposition property: roughly speaking, a property intended to capture the fact that the outcome of experiments at Fermilab is independent of whatever might be happening in the accelerator tunnel at SLAC. Steven Weinberg went so far as to call it a foundational requirement of all experimental science. However, the satisfaction of cluster decomposition in quantum field theory is subtle: the mathematical statement of the principle is evidently incompatible with quantum entanglement. Nevertheless, I will ultimately conclude that something very much like Weinberg’s transcendental-ish claim is probably correct, but to get there will require disentangling the aim of the cluster decomposition property from its formal structure and elucidating a delicate relationship between the cluster decomposition property and the ubiquity of entanglement in quantum field theory.

11 December 2021

**Eddy Keming Chen (UC San Diego)**, “**The Wentaculus: Density Matrix Realism Meets the Arrow of Time**“

Two of the most difficult problems in the foundations of physics are (1) what gives rise to the arrow of time and (2) what the ontology of quantum mechanics is. They are difficult because the fundamental dynamical laws of physics do not pick out an arrow of time, and the quantum-mechanical wave function describes a high-dimensional reality that is dramatically different from the objects of our ordinary experiences. In this talk, I propose a unified solution by adopting a new theory of time’s arrow in a quantum universe—the Wentaculus [1-3]. Central to my solution are (i) Density Matrix Realism, the idea that the quantum state of the universe is objective but impure, and (ii) the Initial Projection Hypothesis, a new candidate law of nature that selects a unique initial quantum state. On the Wentaculus, the initial quantum state of the universe is sufficiently simple to be a law, and the arrow of time can be traced back to an exact boundary condition. As a bonus, we can use the theory to realize “strong determinism” as defined by Penrose [6] and remove the “fundamental nomic vagueness” of the Past Hypothesis as defined by Chen [4]. I end with some open problems for future research.

The presentation will be self-contained, but here are some optional background readings for those interested:

[1] Chen, E.K., Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State*The British Journal for the Philosophy of Science*, 2018

[2] Chen, E.K., Time’s Arrow in a Quantum Universe: On the Status of Statistical Mechanical Probabilities in Valia Allori (ed.), *Statistical Mechanics and Scientific Explanation: Determinism, Indeterminism and Laws of Nature*, World Scientific, 2020

[3] Chen, E.K., From Time Asymmetry to Quantum Entanglement: The Humean Unification*Noûs*, 2020

[4] Chen, E.K., Fundamental Nomic Vagueness*The Philosophical Review*, forthcoming

[5] Chen, E.K., The Past Hypothesis and the Nature of Physical Laws in Barry Loewer, Eric Winsberg, and Brad Weslake (eds.), *Time’s Arrows and the Probability Structure of the World*, Harvard University Press, forthcoming

[6] Penrose, R. *The Emperor’s New Mind: Concerning Computers, Minds and The Laws of Physics*, Oxford University Press, 1989, p.560 [Oxford Scholarship Online]

6 November 2021, 3pm, 777 Social Science Tower, UC Irvine

**Chip** **Sebens (Caltech), “The Fundamentality of Fields”**

There is debate as to whether quantum field theory is, at bottom, a quantum theory of fields or particles. One can take a field approach to the theory, using wave functionals over field configurations, or a particle approach, using wave functions over particle configurations. This article argues for a field approach, presenting three advantages over a particle approach: (1) photons cannot be treated as particles, (2) a classical field model of the electron is superior to a classical particle model (as regards both spin and self-interaction), and (3) field wave functionals can be used for interacting theories whereas particle wave functions cannot. The article also describes three tasks facing proponents of a field approach: (1) legitimize or excise the use of Grassmann numbers for fermionic field values and wave functional amplitudes, (2) describe how quantum fields give rise to particle-like behavior, and (3) explain the absence of electron self-repulsion in quantum electrodynamics.

Please read Chip’s paper prior to the talk.