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Our most basic, fundamental assumptions are often the ones that get scrutinised the least. This very much holds true for the primary ontological and conceptual distinction that underlies much of physics, philosophy of physics, and metaphysics: the idea that all entities and structures in our universe are to be categorised and conceptualized as either space (or, in modern physics, spacetime) or matter, never both, never neither. Everything must be either the “container” or the “contained”. Although this strict conceptual dichotomy did make a lot of sense in the context of our pre-20th-century worldview, the editors suspect that it is no longer tenable, and even a hindrance to further progress. 

Although  makes an intriguing case that the distinction between space(time) and matter already became blurred over a century ago, with the introduction of the aether and field theories, most members of the select group of scholars who are sympathetic to some breakdown of a strict spacetime–matter distinction (occurring already before the quantum gravity regime) associate the trouble with General Relativity (GR). For instance,  argues that the metric field in GR, typically presented as the representative of spacetime, also exhibits most paradigmatic properties of matter—it is dynamical, it can act and be acted upon, and (according to Rovelli) it also carries energy. Whether the metric indeed carries gravitational energy is rather controversial, due to its pseudotensorial nature; it is for this reason that it is not the received view that already in GR the spacetime–matter dichotomy dissolves. (An interesting case-study in this regard is the spin-2 gravity alternative to GR, which more obviously fits the matter mold.) If not in GR, perhaps in Quantum Field Theory? In that context, one might wish to define matter as a particle. It is however somewhat controversial in which sense we can still truly speak about particles in the context of operator-valued fields. Even if we can, this literature has never to our knowledge explicitly been connected with criteria for being matter, perhaps because the matter (and spacetime) concepts are often supposed to be too obvious to have to be spelled out. 

A particularly fruitful context to consider when analysing the spacetime–matter distinction is modern astronomy and cosmology. Prior work by one of us (Martens and Lehmkuhl , ) has analysed superfluid dark matter theory. It turns out that this theory can be understood either as a matter theory, or as a modification of gravity/spacetime.  considers whether f(R) gravity, a minimal extension of GR that may account for inflation or dark energy, makes it a mere conventional matter whether the novel ingredient of that theory is classified as spacetime or matter. Two of us (together with Alex Fleuren) have an advanced draft on whether spacetime and matter classification becomes conventional or even disappears altogether in the broader class of scalar-tensor theories (which also includes various mainstream inflation and dark energy models). Dark energy is a particularly interesting case study, as 1) it brings in a third category, i.e. a constant of nature, if one considers the simplest model for dark energy, i.e. a cosmological constant, and 2) the accelerated expansion of our universe that is the main reason for invoking dark energy is explained in terms of so-called energy conditions, which in this case can be interpreted either as a matter or spatiotemporal condition and therefore explanation. Another prime case study in astronomy, besides dark matter, is a black hole: is a black hole a form of matter (as the astronomers would have it), or spacetime (as the vacuum black hole solutions of the relativists would have it), or both, or neither? (Two of us have an advanced draft on this case study.)

Although the core of the collected volume will be dedicated to case studies from established physics, such as those above, it is intriguing to already look ahead for inspiration at the regime of quantum gravity. In various theories of quantum gravity, spacetime and/or matter either disappear at the fundamental—instead emerging from some other fundamental substratum—or lose many of their paradigmatic features. Particularly interesting here are supersymmetric theories. Supersymmetry transformations are purely internal transformations between kinds of matter (i.e. fermions and bosons) that nonetheless lead to spatiotemporal translations, living on a superspace that merges regular spacetime and internal space. If one takes symmetries to be mere redescriptions of the same underlying physics, one may wish to conclude that there is no objective distinction between matter and spacetime.

The contexts and case studies above put pressure on the outdated Newtonian idea that the space(time) and matter concepts can and should be strictly distinguished. A systematic interdisciplinary analysis of the extent to which this dichotomy breaks down will have important ramifications for both physicists and philosophers. The main upshot for physicists is a broader outlook on model building. There is a clear disconnect between treating a black hole as the end result of throwing together lots of massive stuff, or as a vacuum solution to General Relativity. In the context of dark matter phenomenology, one community only considers matter-solutions to this problem, and a separate community only modified-spacetime-solutions. The analysis in this collected volume is intended to provide a framework to navigate the single space of theories, rather than a separate space of spacetime theories and space of matter theories. 

The upshots for philosophers of science and metaphysicians relate to various main philosophical debates that depend on or relate to the spacetime–matter dichotomy. Here are some examples:

• Substantivalism vs relationalism: this three-centuries-old debate concerns the question whether spacetime exists independently from matter (as Newton thought), or whether spacetime’s existence is parasitic upon matter (and its spatiotemporal relations) (as Leibniz thought). The container metaphor is often used in this context: space(time) as a container, with matter being contained in spacetime. The only thing that these two positions do agree upon is that there is a strict conceptual and metaphysical dichotomy between matter and space(time)—everything in our universe fits into exactly one of those mutually exclusive categories. Indeed, without this assumption the disagreement between substantivalism and relationalism cannot even be formulated! If something (e.g. dark energy, black hole) is both spacetime and matter, it is incoherent to ask whether that thing is contained in itself.
• Conventionalism: As mentioned above, an interesting way in which the spacetime–matter distinction might crumble is if it becomes a matter of mere convention rather than an objective fact whether an entity is to be categorised as spacetime or matter (or a constant of nature). This thus constitutes an aspect of conventionalism that is conceptually prior to traditional discussions of conventionality. That literature, originating with Poincaré, focuses on geometric conventionalism: assuming that an entity is objectively spacetime, is its specific geometric structure an objective fact or merely a convention?
• Scientific realism:  A breakdown of the dichotomy between spacetime and matter provides a fresh challenge for scientific realism, in particular with regards to its semantic dimension and perspectival variants of realism. If we are uncertain, with regards to the major constituents of our universe, as to how to answer even the primary, most basic, ontological question—are they matter, or are they spacetime?—then it is far from clear what it would mean to be a scientific realist about such metaphysically elusive entities. Can we be a realist about some theoretical, unobservable term if we have (barely) any idea of what this term is, ontologically speaking ()? 

All in all, the edited volume will consider arguments in favour and against relinquishing a strict conceptual distinction between spacetime and matter, analyse the various ways in which such a breakdown could occur, and investigate the consequences of such breakdowns for model building in physics, astronomy and cosmology, and for various philosophical debates. Far from being an unwelcome babel, a conceptual undoing, giving up the spacetime-matter distinction will provide guidance as to which traditional debates become moot and which novel avenues open up.

The book will follow a three-part structure. Part 1 focuses on (problems with) possible definitions of matter, part 2 on (problems with) possible definitions of spacetime, and the larger part 3 focuses on the distinction between them, and connections with model building in physics and various philosophical debates. The chapters below are suggestions; proposals for alternative chapters are welcome. Confirmed invited authors are listed below. Multiple chapters on similar topics are possible.

0.    Introduction, by Niels Martens, Sanne Vergouwen & Antonio Ferreiro

PART 1: MATTER

1.    Defining matter as being a particle

2.    Idealised matter, by Shelly Shi (UC San Diego) [she may write Ch.10 instead]

3.    Mass and energy in field theories

PART 2: SPACETIME

4.    Types of spacetime definitions

For instance: necessary & sufficient definitions vs. cluster concepts vs. Wittgensteinian family resemblances vs. metaphors, etc.

5.    The container metaphor

6.    (Beyond) spacetime in GR, by Dennis Lehmkuhl (ľ�ϸ���Ӱ�� of Bonn & Oxford ľ�ϸ���Ӱ��)

7.    GR & gauge

PART 3: SPACETIME-MATTER DISTINCTION

8.    Philosophy of language/ concepts & metaphors/ pragmatism, by Tushar Menon (MIT)

9.    Black holes

10.  Gravitational waves, by Shelly Shi (UC San Diego) [she may write Ch.2 instead]

11.   Inflation

12.   Dark matter (and modified gravity)

13.   Dark energy (and modified gravity theories) 

14.   Spin-2 gravity (vs GR)

15.   Matter and spacetime between supersymmetry and supergravity, by Enrico Cinti (ľ�ϸ���Ӱ�� of Geneva) & Marco Sanchioni (Sophia Institute)

16.   Conventionality of the spacetime-matter dichotomy

17.   Scientific realism/ metaphysics of the spacetime-matter distinction