My main area of research since graduate school, I have written on a variety of topics concerning general relativistic spacetime, the relationship between general relativity (and standard model cosmology) and quantum gravity research, and quantum gravity phenomenology. I have also written on the early history of expanding universe cosmology. Below, articles are organized by most recent.
[Philosophy of Physics (open access)]
Physicists and philosophers are increasingly prone to regarding our current physical theories as providing ‘effective descriptions’ of real-world systems. In the context of quantum gravity research, this fuels a common view that the classical spacetime theory of general relativity provides effective descriptions where it is successfully applied. That common view of general relativity, in turn, encourages an ‘effective’ understanding of spacetime emergence. But descriptions of spacetime in general relativity irreducibly include global physical content, which is not effective. Recognizing this fact reigns in the interpretive scope of the common view of general relativity and specifically undermines our thinking about space-time emergence effectively.
[Philosophy of Science, preprint here: http://philsci-archive.pitt.edu/22187/]
Recent work on the status of astrophysical modeling in the wake of quantum gravity indicates that a ‘fauxrizon’ (portmanteau of ‘faux horizon’), such as is relevant to understanding astrophysical black holes according to the fuzzball proposal within string theory, might ultimately solve the familiar black hole evaporation paradox. I clarify, with general upshots for the foundations of quantum gravity research, some of what this suggestion would amount to: identification of intertheoretic constraints on global spacetime structure in (observer-relative) semiclassical models of fuzzballs.
[Foundations of Physics, preprint available here: http://philsci-archive.pitt.edu/22367/2206.07783v4]
The Big Bang singularity in standard model cosmology suggests a program of study in ‘early universe' quantum gravity phenomenology. Inflation is usually thought to undermine this program's prospects by means of a dynamical diluting argument, but such a view has recently been disputed within inflationary cosmology, in the form of a ‘trans-Planckian censorship' conjecture. Meanwhile, trans-Planckian censorship has been used outside of inflationary cosmology to motivate alternative early universe scenarios that are tightly linked to ongoing theorizing in quantum gravity. Against the resulting trend toward early universe quantum gravity phenomenology within and without inflation, Ijjas and Steindhardt suggest a further alternative: a ‘generalized cosmic censorship' principle. I contrast the generalized cosmic censorship principle with the logic of its namesake, the cosmic censorship conjectures. I also remark on foundational concerns in the effective field theory approach to cosmology beyond the standard model, which would be based on that principle.
[SHPS, preprint here: http://philsci-archive.pitt.edu/21936/]
I discuss empty space as it appears in the physical foundations of relativistic field theories, as well as the popular idea that there is some freedom available for physicists to pick between two inequivalent spacetime representations of empty space, moving forward: de Sitter spacetime or its ‘elliptic' cousin. Against the view that there exists a correspondence between the two representations (what I take to be the standard view), I argue that the freedom named is rather a freedom to postpone justification for having picked either one spacetime setting over the other, in the course of ongoing research.
[with Nick Huggett and Niels Linnemann, Monograph in Cambridge Elements in the Foundations of Contemporary Physics series, earlier preprint here: http://philsci-archive.pitt.edu/20623/]
It has long been thought that observing distinctive traces of quantum gravity in a laboratory setting is effectively impossible, since gravity is so much weaker than all the other familiar forces in particle physics. But the quantum gravity phenomenology community today seeks to do the (effectively) impossible, using a challenging novel class of `tabletop' Gravitationally Induced Entanglement (GIE) experiments, surveyed here. The hypothesized outcomes of the GIE experiments are claimed by some (but disputed by others) to provide a `witness' of the underlying quantum nature of gravity in the non-relativistic limit, using superpositions of Planck-mass bodies. We inspect what sort of achievement it would possibly be to perform GIE experiments, as proposed, ultimately arguing that the positive claim of witness is equivocal. Despite various sweeping arguments to the contrary in the vicinity of quantum information theory or given low-energy quantum gravity, whether or not one can claim to witness the quantum nature of the gravitational field in these experiments decisively depends on which out of two legitimate modelling paradigms one finds oneself in. However, by situating GIE experiments in a tradition of existing experiments aimed at making gravity interestingly quantum in the laboratory, we argue that, independently of witnessing or paradigms, there are powerful reasons to perform the experiments, and that their successful undertaking would indeed be a major advance in physics.
[with Siska De Baerdemaeker, Comment in Nature Astronomy, earlier preprint here: https://philsci-archive.pitt.edu/23989/]
What is the link between the discovery of the relativistic expanding Universe and British imperialism? A public panel debate in the early days of relativistic cosmology shows how fundamental scientific research, whether there are obvious political stakeholders (like biosecurity and climate) or not, runs real-time risks of being repurposed for political ends.
[many co-authored, peer-reviewed paper in Galaxies special issue on the ngEHT (open access)]
This white paper outlines the plans of the History Philosophy Culture Working Group of the Next Generation Event Horizon Telescope Collaboration.
[Foundations of Physics, preprint here: https://philsci-archive.pitt.edu/20551/]
I critically discuss a controversial ‘trans-Planckian censorship’ conjecture, which has recently been introduced to researchers working at the intersection of fundamental physics and cosmology. My focus explicitly avoids any appeals to contingent research within string theory (the sociological origins of the conjecture) or regarding the more general (quantum) gravitational ‘swampland’. Rather, I concern myself with the conjecture’s foundations in our current, well-trodden physics of quantized fields, spacetime, and (classical) gravity. In doing so, I locate what exactly within trans-Planckian censorship amounts to a departure from current physics—identifying what is, ultimately, so conjectural about the conjecture.
[with Siska De Baerdemaeker, HOPOS, preprint here: https://philsci-archive.pitt.edu/19250/]
In September 1931, a panel discussion was convened at Central Hall Westminster on the subject of the ‘evolution of the universe’ at the centenary meeting of the British Association for the Advancement of Science. Center stage was what to do about the evolving universe being younger than the stars, evidently a paradox in the relativistic study of the evolving universe at the time. Here, we discuss two diametrically opposed reactions to the paradox, which were each broadcast at the meeting by Lemaître and de Sitter, respectively. As we argue, that both could be projected to the public as viable reflects an unsettled question at the foundations of the then-nascent discipline: What is the role for considerations of scale in relativistic cosmology?
[SHPS, preprint here: https://philsci-archive.pitt.edu/19652/]
I provide some philosophical groundwork for the recently proposed ‘trans-Planckian censorship’ conjecture in theoretical physics. In particular, I argue that structure formation in early universe cosmology is, at least as we typically understand it, autonomous with regards to quantum gravity, the high energy physics that governs the Planck regime in our universe. Trans-Planckian censorship is then seen as a means of rendering this autonomy an empirical constraint within ongoing quantum gravity research.
[BJPS, preprint here: https://philsci-archive.pitt.edu/16410/]
The ‘cosmological constant problem’ (CCP) has historically been understood as describing a conflict between cosmological observations in the framework of general relativity (GR) and theoretical predictions from quantum field theory (QFT), which a future theory of quantum gravity ought to resolve. I argue that this view of the CCP is best understood in terms of a bet about future physics made on the basis of particular interpretational choices in GR and QFT, respectively. Crucially, each of these choices must be taken as itself grounded in the sucesses of the respective theory for this bet to be justified.
[Philosophy of Science, preprint here: https://philsci-archive.pitt.edu/16016/]
The “Cosmological Constant Problem” (CCP) is widely considered a crisis in contemporary theoretical physics. Unfortunately, the search for its resolution is hampered by open disagreement about what is, strictly, the problem. This disagreement stems from the observation that the CCP is not a problem within any of our current theories, and nearly all of the details of those future theories for which the CCP could be made a problem are up for grabs. Given this state of affairs, I discuss how one ought to make sense of the role of the CCP in physics and generalize some lessons from it.
[with Samuel C. Fletcher, JB Manchak, and James Owen Weatherall, SHPMP, preprint here: https://philsci-archive.pitt.edu/14248/]
We consider various curious features of general relativity, and relativistic field theory, in two spacetime dimensions. In particular, we discuss: the vanishing of the Einstein tensor; the failure of an initial-value formulation for vacuum spacetimes; the status of singularity theorems; the non-existence of a Newtonian limit; the status of the cosmological constant; and the character of matter fields, including perfect fluids and electromagnetic fields. We conclude with a discussion of what constrains our understanding of physics in different dimensions.