Recent foundational work in QTD reveals a `falling apart' of various classical thermodynamic concepts in this new domain, akin to the situation in relativistic thermodynamics discussed by \citet{Chua2023}. This paper focuses on one such cluster of conceptual issues for the thermodynamic work. I first diagnose a widely-discussed no-go theorem due to Perarnau-Llobet et al (2017), which asserts that no 'universal' thermodynamic work can be defined in QTD, given some intuitive and widely accepted desiderata. I argue that, contrary to the consilient classical context, these desiderata encode physically distinct scenarios in quantum mechanics for reasons to do with the quantum measurement problem. Because different procedures which were consilient in defining work in the classical regime no longer agree in the quantum regime, the classical work concept falls apart in the quantum regime. I discuss how we can attempt to restore consilience in this regard by appealing to work defined in terms of quantum forces over trajectories, but argue that this breaks consilience in other aspects: the bump in the rug is merely moved, not removed. This raises questions about the goals of quantum thermodynamics, and when 'universality' is universal enough for the purposes of generalizing thermodynamics to the quantum domain.
There is a longstanding view in the philosophy of physics that the variant quantities of a physical theory are defective in some ways compared to invariant quantities. Many have attributed this defect to the thought that variant quantities are not objective, unmeasurable, or even unreal. We argue against this view by scrutinizing examples in physics, in which variant quantities are not only measurable, but crucial for capturing the physical content of many theories. Furthermore, we diagnose why many physical laws are variant under simple transformations, e.g., spatial translations or boosts: these equations track relational quantities that encode information about how physical systems behave relative to particular observers or instruments, e.g. instruments at rest relative to a system, or systems in thermodynamic equilibrium relative to an observer. We end by responding to the objection that the search for invariance was crucial to the development of special and general relativity.
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.
Relativistic Locality from Electromagnetism to Quantum Field Theory
Joint work with Chip Sebens.
Invited contribution to an OUP collected volume Many-Worlds Interpretations and Locality, edited by Alyssa Ney, preprint available here: https://arxiv.org/abs/2412.11532.
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 contracting 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), examining two different ways of assigning states (reduced density matrices) to regions of space. Our preferred method begins from field wave functionals and judges quantum field theory to be local. Another method begins from particle wave functions (states in Fock space) and leads to either non-locality or an inability to assign states to regions, depending on the choice of creation operators. We take this analysis of Everettian (no collapse) quantum field theory to show that the many-worlds interpretation of quantum physics is local at the fundamental level. 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.
This paper brings together two questions. On the one hand, there is a question of how approximations relate to idealizations. On the other hand, there is a question of whether de-idealization is needed for justified use of --- for `checking' --- idealizations. We propose a generalized account of asymptotic reasoning which answers both questions. On this view, idealizations and approximations are both steps in the general process of asymptotic reasoning, which we understand broadly in terms of the search for stable convergence between models. Furthermore, while stable convergence is often cashed out in the literature in terms of formal approximation schemes and Galilean de-idealizations which `adds back' all the relevant details and brings us back to the ``full representation", we show that such an understanding of stable convergence is itself idealized. We propose three ways of de-idealizing de-idealization, in terms of intra-model, inter-model, and measurement de-idealizations. This highlights ways in which idealizations can be `checked' without appealing to Galilean de-idealizations, which in turn provides us with an understanding of stable convergence in line with scientific practice in physics.
Physical Coherence and Time's Emergence
Under review, draft available upon request!
It is often said that time vanishes in quantum gravity. One general approach to quantum gravity accepts this fundamental timelessness but seeks to derive time's emergence at a non-fundamental level. To better assess such approaches, I develop the criterion of physical coherence and situate it in context by applying it to two programs for time's emergence, drawing from recent works by Chua and Callender (2021) and Chua (forthcoming): semiclassical time and thermal time. Unlike some recent arguments for the metaphysical incoherence of time's emergence, which rule out all claims of time’s emergence `from on high' once we’ve fixed a definition of metaphysical emergence, my criterion of physical coherence leaves open the possibility that some programs in quantum gravity may succeed on their own terms in providing a physically coherent derivation of time from no-time. This sets a challenge for proponents of time's emergence to clarify the conceptual foundations of their program, while at the same time acting as a litmus test for a program's success.