# Projects in Progress

Fundamental Locality from Electromagnetism to Quantum Field Theory (Joint work with Chip Sebens)

Electromagnetism is the paradigm case of a theory that satisfies relativistic locality. This can be proven by demonstrating that, once the theory’s laws are imposed, the state of a region fixes what happens in a certain portion of the future: the converging light-cone with that region as its base. The Klein-Gordon and Dirac equations meet the same standard. We show that this standard can also be applied to quantum field theory (without collapse) by examining two different ways of assigning states (reduced density matrices) to regions of space. One method begins from field wave functionals and the other from particle wave functions (states in Fock space). We take this kind of analysis of the fundamental dynamics of Everettian (no collapse) quantum field theory to demonstrate that the many-worlds interpretation of quantum physics is local. We argue that this fundamental locality is compatible with either local or global (non-local) accounts of the non-fundamental branching of worlds, countering an objection that has been raised to the Sebens-Carroll derivation of the Born Rule from self-locating uncertainty.

Decoherence, Branching, and the Born Rule for a Mixed-State Everettian Multiverse (joint work with Eddy Keming Chen)

(under review, preprint available here)

In Everettian quantum mechanics, justifications for the Born rule appeal to self-locating uncertainty or decision theory. Such justifications have focused exclusively on a pure-state Everettian multiverse, represented by a wave function. Recent works in quantum foundations suggest that it is viable to consider a mixed-state Everettian multiverse, represented by a (mixed-state) density matrix. Here, we develop the conceptual foundations for decoherence and branching in a mixed-state multiverse, and extend the standard Everettian justifications for the Born rule to this setting. This extended framework provides a unification of 'classical' and 'quantum' probabilities, and additional theoretical benefits, for the Everettian picture.

## Putting Pressure Under Pressure: On the Status of the Classical Pressure in Relativity (draft available upon request!)

Much of the century-old debate surrounding the status of thermodynamics in relativity has centered on the search for a suitably relativistic temperature; recent work by Chua (2023) has suggested that the classical temperature concept – consilient as it is in classical settings – ‘falls apart’ in relativity. However, these discussions have still tended to assume an unproblematic Lorentz transformation for – specifically, the Lorentz invariance of – the pressure concept. Here I argue that, just like the classical temperature, the classical concept of pressure breaks down in relativistic settings. This situation suggests a new thermodynamic limit – a ‘rest frame limit’ – without which an unambiguous thermodynamic description of systems does not emerge. I end by briefly discussing how thermodynamics, in requiring preferred frames, bears on the idea of so-called symmetry-to-reality inferences.

## Do Black Holes Evaporate? (under review)

Since Hawking first predicted that black holes lose mass and `evaporate' via Hawking radiation, the phenomenon has become a linchpin of black hole research. However, the derivation of black hole evaporation requires some global notion of mass-energy conservation, which in turn rests on assuming that the spacetime in question has certain idealized properties; the ubiquitous ones are stationarity and asymptotic flatness. By examining these two idealizations and how they cannot be appropriately `de-idealized' in describing actual physical systems, I argue that we lack justification at present for concluding that actual black holes evaporate.

## The Diaulos of Asymptotic Reasoning and De-Idealization (Joint work with Yichen Luo)

It is doubtless that scientific inquiry inextricably involves the use of idealizations: for ease of calculation and representation, we assume the absence of friction, the perfect sphericity of cows, or the impeccable rationality of human beings. Idealizations are, strictly speaking, false. Yet they play a crucial role in many of our best sciences in representing all sorts of phenomena, from coffee cups to black holes. Much ink has thus been spilled over how to justify them. A predominant view focuses on justifying these idealizations via de-idealization procedures. A more recent cluster of views, due to Potochnik (2017) and Knuttila & Morgan (2019), pushes back by arguing that these idealizations stand on their own and need no explicit de-idealization, and that de-idealization is too demanding, respectively. I’ll argue that this disagreement hinges on a strong philosopher’s construct of de-idealization, an idea that itself needs to be de-idealized. I explicate two weaker senses in which idealizations can be de-idealized.