Dr Caspar Jacobs

Julia de Lacy Mann Fellow, Early Career Researcher in the Philosophy of Physics

I am a Junior Research Fellow at Merton College with a specialisation in the philosophy of physics. Before that, I completed my DPhil at Magdalen College, University of Oxford, with a dissertation on the interpretation of symmetries in physics. I then spent a year at the University of Pittsburgh to continue this line of research. My current research at Merton focuses on the use of dimensions, units and constants in physics.


I am a philosopher of physics interested in the interpretation of symmetries, the metaphysics of quantities and the use of dimensions, units and constants in physics, amongst other topics. I have also published papers on the philosophy of quantum field theory, and the work of the Early Modern women philosopher Émilie du Châtelet. I am currently working on two related projects on the intersection of metaphysics and philosophy of physics.

The first project, which builds on my doctoral research, focuses on one of the most distinctive features of contemporary theories in physics: their symmetries. In my dissertation I I defended a view called ‘sophistication’, which aims to solve various puzzles posed by the occurrence of symmetries. This view relies on a picture of the world known as ‘structuralism’, which says that the fundamental entities of physics are constituted by their relations to each other, i.e. their structure. The aim of my proposed research is to better understand this view. This question is in itself interesting metaphysics, but is chiefly of relevance to contemporary issues in physics, in which symmetries and structure play an increasingly important role.

The second project concerns the role of units, dimensions and constants in contemporary physics. Units—the numerical scales used in measurement of quantities such as mass or charge—are the means by which scientific theories make empirical contact with the world, in both experiments and applications. The dimension of a quantity expresses how that quantity relates to others, while a system of units assigns real numbers to quantity-magnitudes: for example, the dimensions of velocity are [length]/[time], but possible units include m/s, mph, etc. In this, units are analogous to coordinates, which assign real numbers to space-time points: they are ways of mathematically mapping the world. But despite their importance to the practice of science, units have received relatively little philosophical attention; especially in comparison to the literature on coordinates, which is vast and well-known. This stands in contrast to the historical literature on this topic in the first half of the 20th century, in which the status of both dimensions and units is hotly debated.

In my research project, I aim to resolve the following tension. On the one hand, it is often said that a choice of units is arbitrary. It does not matter, for instance, whether one measures mass in kg or lb. On the other hand, it does seem to be the case that certain systems of units are preferable. This is the case for length, as the dinches scale illustrates, but also for quantities such as temperature which have an absolute zero: the Kelvin scale is preferable to °C or °F. We also see this tension in contemporary physics, where ‘natural units’ are used in which all fundamental constants are set to 1. It is sometimes claimed that the typical energy scale of these units (the ‘Planck scale’) will reveal novel physical phenomena. This, in turn, seems to contradict the idea that units are conventional. This leads to the main question: in virtue of what are certain systems of units privileged in physics? This question is highly relevant to philosophy of physics as the answer would allow us to distinguish merely conventional from truly representational structures.

  1. Invariance, Intrinsicality, and Perspicuity, Synthese (2022). In which I argue that perspicuous interpretations of a theory's invariant content should be intrinsic in the sense of Field (1980).
  2. Invariance or Equivalence: a Tale of Two Principles, Synthese (2021). In which I argue that Leibniz Equivalence and the Invariance Principle can come apart when we reject the link between mathematical values and physical magnitudes.
  3. The Coalescence Approach to Inequivalent Representation: Pre-QM∞ Parallels, British Journal for the Philosophy of Science (2021). An extension and defence of Laura Ruetsche's 'Coalescence Approach' in the context of pre-QFT theories.
  4. Du Châtelet: Idealist about extension, bodies and space, Studies in the History and Philosophy of Science Part A (2020). A reconstruction and analysis of Du Châtelet's views of extension and space.