Aims and Motivations

Modern physics rests on two firm theoretical pillars, namely, quantum theory and general relativity. Both theoretical frameworks enjoy huge empirical success and have wide technological applications.  It is then plausible to assume that each of these theories captures at least some relevant features of reality. The main aims of the research group in philosophy of physics is (i) to investigate which type of ontic commitments towards the physical world are best supported by quantum and general relativistic physics, and (ii) to consider to what extent these commitments should be modified when trying to merge quantum theory and general relativity into a theory of quantum gravity.

Coming up with a consistent ontology of the world based on fundamental physics is anything but a simple task. For example, one of the main obstacles in finding a clear metaphysical picture of the quantum realm is represented by the so-called measurement problem. This problem has many facets but, in a nutshell, it can be stated as a simple question: Why does any quantum measurement always select just one definite outcome out of the many possible outcomes encoded in the probability distribution defined by the squared amplitude of the quantum state? Standard quantum theory is unable to answer this question because it always describes the dynamical evolution of a quantum state as being linear and unitary. The implications of this problem go far beyond mere empirical testing once we accept that measurement interactions enjoy no privileged status at all among physical interactions. In this case, the issue quickly escalates to a problem concerning the physical world itself: How can the stable and determinate features of the world, such as for example macroscopic material structures, be accounted for by a theory that just talks about the linear and unitary evolution of quantum states? The group’s attitude towards this problem is to endorse the claim according to which, if quantum mechanics has to be a theory that describes how the world behaves (instead of being a mere recipe for generating probabilistic predictions for measurement outcomes), it has to be supplemented by an interpretation or, less neutrally speaking, with a clear ontology able to overcome the measurement problem.

Also general relativistic physics is not immune to metaphysical issues. One of these issues concerns the metaphysical status of spacetime itself. Indeed, general relativity is compatible with many possible metaphysical stances, e.g. spacetime being a substance capable of existing on its own, or being just an intricate web of relations ontologically parasitic upon the existence of matter. Another issue regards the so-called problem of time. Simply speaking, this problem amounts to the fact that, when the dynamics of the theory is cast in Hamiltonian form, the time-like degrees of freedom seem to become pure gauge, thus endangering the physical meaning of anything related to temporal becoming and change. A third major conceptual problem concerns the type of dependence relation underpinning the talk of mutual interaction between spacetime and matter commonly used to address the physical meaning of the Einstein field equations. For example, in what sense does the geometry of spacetime determine the motion of material bodies? Standard metaphysics is in trouble with this question since no garden-variety dependence relation (e.g. causation, metaphysical grounding) seems to fully capture the physical meaning of such a “determination”. As part of the strategy to deal with all these problems, the group pays attention to alternative approaches to gravitational physics, in particular the shape dynamics framework originally proposed by Julian Barbour and Bruno Bertotti.

Clearly, the above mentioned metaphysical issues add up when trying to merge together quantum theory and general relativity in a single theory of quantum gravity. The quest for a theory of quantum gravity is a huge and still ongoing theoretical enterprise, which has so far generated a galaxy of approaches with different aims and ambitions. Theoretical programs such as the one that goes under the umbrella term string theory have the goal of constructing a grand unified theory of all physical interactions. Slightly more modest programs, instead, just aim at finding a quantum description of the general relativistic gravitational field. Some remarkable examples of this second family of approaches are loop quantum gravity and causal dynamical triangulations. There are also “eliminative” approaches that seek to show how gravity is an emergent phenomenon stemming from the quantum-level description of reality: Erik Verlinde’s emergent gravity and Daniele Oriti’s geometrogenesys are two examples of this attitude. Against the background of this varied theoretical landscape, one of the research priorities of the group is to inquire into the metaphysical status of spacetime in the main approaches to quantum gravity, with a special focus on investigating the kind of ontological characterizations of the “emergence” of spacetime from an underlying non-spatiotemporal regime that are most viable and helpful in developing a full-fledged theory.