Mongrel Gravity

Abstract

It was recognized almost from the original formulation of general relativity that the theory was incomplete because it dealt only with classical, rather than quantum, matter. What must be done in order to complete the theory has been a subject of considerable debate over the last century, and here I just mention a few of the various options that have been suggested for a quantum theory of gravity. The aim of what follows is twofold. First, I address worries about the consistency and physical plausibility of hybrid theories of gravity-theories involving a classical gravitational field and quantum matter fields. Such worries are shown to be unfounded. These hybrid theories-mongrel gravity-in fact comprise the only current, actual theories of gravity that incorporate quantum matter, and they also offer legitimate promise as tools for discovering the full theory of gravity. So my second aim is to highlight these theories as providing an interesting example of scientific revolution in action. I begin to try to draw some philosophical lessons from mongrel gravity theories, but more importantly I try to convince philosophers of physics that they should pay more attention to them.QFT in curved space-time, however, is not a unification. It is a mongrel, and as such deserves to be put down.-M. J. Duff

Quantum Gravity at Low Energies

“…On sufficiently long timescales, no unitarily-evolving quantum state will remain macroscopically definite. 19 The Page-Geilker experiment has received some pushback in the philosophy literature, notably by Mattingly (2005Mattingly ( , 2009). Mattingly's objection is that Page and Geilker assume the Everett interpretation of quantum mechanics (they do indeed mention it explicitly in their paper) and that their experiment does not have any implications for semiclassical gravity without this assumption.…”

Defining a crisis: the roles of principles in the search for a theory of quantum gravity

“…The physical position of each point on the world sheet is given by N numbers specifying the coordinates of the point, and these N quantities form N fields on the world sheet. Now, instead of thinking of a 27 See, e.g., Huggett & Callender (2001); Mattingly (2005Mattingly ( , 2006Mattingly ( , 2009; Wüthrich (2005). 28 GR treated in the framework of QFT is apparently perturbatively non-renormalisable, breaking down at energies approaching the Planck scale, and so cannot represent a theory of QG, if QG is required to describe physics at this scale.…”

Statistical consistency of quantum-classical hybrids

“…While this view is quite extended among the scientific community it is by no means universally accepted, due to the conceptual problems involved in the quantum description [39][40][41][42]. Also, the technical and conceptual difficulties encountered in quantum gravity have prompted a certain discussion regarding the necessity or not of treating gravity quantum mechanically [43][44][45][46][47][48][49][50][51][52][53][54].…”