General Relativity as a Hybrid theory: The Genesis of Einstein's work on the problem of motion

In this paper I describe the genesis of Einstein's early work on the problem of motion in general relativity (GR): the question of whether the motion of matter subject to gravity can be derived directly from the Einstein field equations. In addressing this question, Einstein himself always preferred the vacuum approach to the problem: the attempt to derive geodesic motion of matter from the vacuum Einstein equations. The paper first investigates why Einstein was so skeptical of the energy-momentum tensor and its role in GR. Drawing on hitherto unknown correspondence between Einstein and George Yuri Rainich, I then show step by step how his work on the vacuum approach came about, and how his quest for a unified field theory informed his interpretation of GR. I show that Einstein saw GR as a hybrid theory from very early on: fundamental and correct as far as gravity was concerned but phenomenological and effective in how it accounted for matter. As a result, Einstein saw energy-momentum tensors and singularities in GR as placeholders for a theory of matter not yet delivered. The reason he preferred singularities was that he hoped that their mathematical treatment would give a hint as to the sought after theory of matter, a theory that would do justice to quantum features of matter

Mass-Energy-Momentum: Only there because of Spacetime?

I describe how relativistic field theory generalises the paradigm property of material systems, the possession of mass, to the requirement that they have a mass-energy-momentum density tensor associated with them. I argue that the latter does not represent an intrinsic property of matter. For it will become evident that its definition depends on the metric field in a variety of ways. Accordingly, since the metric field represents the geometry of spacetime itself, the properties of mass, stress, energy and momentum should not be seen as intrinsic properties of matter, but as relational properties that material systems have only in virtue of their relation to spacetime structure

Literal versus Careful Interpretations of Scientific Theories: The Vacuum Approach to the Problem of Motion in General Relativity

The problem of motion in general relativity is about how exactly the gravitational field equations, the Einstein equations, are related to the equations of motion of material bodies subject to gravitational fields. This article compares two approaches to derive the geodesic motion of (test) matter from the field equations: the ‘T approach’ and the ‘vacuum approach’. The latter approach has been dismissed by philosophers of physics because it apparently represents material bodies by singularities. I argue that a careful interpretation of the approach shows that it does not depend on introducing singularities at all and that it holds at least as much promise as the T approach

Literal vs. careful interpretations of scientific theories: the vacuum approach to the problem of motion in general relativity

The problem of motion in general relativity is about how exactly the gravitational field equations, the Einstein equations, are related to the equations of motion of material bodies subject to gravitational fields. This paper compares two approaches to derive the geodesic motion of (test) matter from the field equations: `the T approach' and `the vacuum approach'. The latter approach has been dismissed by philosophers of physics because it apparently represents material bodies by singularities. I shall argue that a careful interpretation of the approach shows that it does not depend on introducing singularities at all, and that it holds at least as much promise as the T approach. I conclude with some general lessons about careful vs. literal interpretations of scientific theories

Review of \u3ci\u3e Damselflies of Alberta: Flying Neon Toothpicks in the Grass\u3c/i\u3e by John Acorn

This book is small enough to fit into a field pack or be rolled up and put in your pocket, but don\u27t be misled by the size: it contains many compelling topics in addition to identification and species accounts. Of special interest is the chapter on the history of damselfly study in Alberta. Settlement of many areas of the Canadian and American Plains occurred about a hundred years ago, and the pattern was similar in many places-avid amateurs or professional biologists began the study of local faunas, and the stories of their personal lives and experiences in those pioneer years enrich the study of plant and animal groups for us today. Tales told by John Acorn about past and present damselfly enthusiasts in Alberta provide a context that makes the insects all the more interesting

The Equivalence Principle(s)

I discuss the relationship between different versions of the equivalence principle in general relativity, among them Einstein's equivalence principle, the weak equivalence principle, and the strong equivalence principle. I show that Einstein's version of the equivalence principle is intimately linked to his idea that in GR gravity and inertia are unified to a single field, quite like the electric and magnetic field had been unified in special relativistic electrodynamics. At the same time, what is now often called the strong equivalence principle, related to the local validity of special relativity, can also be found in Einstein's writings, albeit by a different name and clearly separated from what he calls the equivalence principle. I discuss both the development of Einstein's thoughts on the different versions of the equivalence principle, their relationship to the relativity principle, as well as later reflections and variants proposed

Review of \u3ci\u3e Damselflies of Alberta: Flying Neon Toothpicks in the Grass\u3c/i\u3e by John Acorn

This book is small enough to fit into a field pack or be rolled up and put in your pocket, but don\u27t be misled by the size: it contains many compelling topics in addition to identification and species accounts. Of special interest is the chapter on the history of damselfly study in Alberta. Settlement of many areas of the Canadian and American Plains occurred about a hundred years ago, and the pattern was similar in many places-avid amateurs or professional biologists began the study of local faunas, and the stories of their personal lives and experiences in those pioneer years enrich the study of plant and animal groups for us today. Tales told by John Acorn about past and present damselfly enthusiasts in Alberta provide a context that makes the insects all the more interesting

Dark Matter = Modified Gravity? Scrutinising the spacetime–matter distinction through the modified gravity/ dark matter lens

This paper scrutinises the tenability of a strict conceptual distinction between space(time) and matter via the lens of the debate between modified gravity and dark matter. In particular, we consider Berezhiani and Khoury's novel 'superfluid dark matter theory' (SFDM) as a case study. Two families of criteria for being matter and being spacetime, respectively, are extracted from the literature. Evaluation of the new scalar field postulated by SFDM according to these criteria reveals that it is as much (dark) matter as anything could possibly be, but alsoꟷbelow the critical temperature for superfluidityꟷas much (of a modifcation of) spacetime as anything could possibly be. A sequel paper examines possible interpretations of SFDM in light of this result, as well as the consequences for our understanding of (the importance of) the modified gravity/ dark matter distinction and the broader spacetime–matter distinction