Hints towards the emergent nature of gravity
Introduction
Gravity is notoriously harder to quantize than other interactions. It is well-known that combining general relativity (GR) with quantum mechanics poses several difficulties which are standardly traced back to the perturbative non-renormalizability of general relativity. Proponents of quantum gravity (QG) approaches proper (such as loop quantum gravity, asymptotic safety and causal dynamical triangulation) nevertheless expect that gravity is after all non-perturbatively renormalizable, and thereby possibly fundamental. In contrast, according to other approaches – like string theory1 and emergent gravity accounts by Sakharov (1968), Jacobson (1995), Padmanabhan (2010), Verlinde (2011), and Hu (2009) – the gravitational field cannot be fundamental but rather emerges from underlying microstructure. These approaches can be subsumed under the term “emergent gravity” (EG).
The idea that gravity and spacetime originate from some underlying microscopic reality in which they do not exist is a widely held belief in, for example, the string theory community.2 The possibility that gravitational degrees are rooted in non-gravitational degrees of freedom is often supported by reference to the AdS/CFT correspondence.3 The AdS/CFT correspondence, however, only provides a straightforward4 gateway to a theory of emergent gravity under the controversial assumption that the CFT side of the correspondence is more fundamental than the AdS side. Now, it is precisely the philosopher of physics’ task to reflect on such current trends in quantum gravity research. In our view a particularly pressing question in this field is why it is widely believed that gravity is emergent – and what is meant by the term “emergent gravity” in the first place.
As a matter of fact, it is surprising that the current philosophical literature5 on EG mainly focuses on notions of emergence or possible implications for QG rather than appraising general arguments for the emergent nature of GR in the first place! We intend to fill this gap in the literature by reconstructing and assessing arguments for the high-levelness of gravity, which are often only implicitly contained in the relevant physics literature. These arguments do not assume any specific QG model, but rather consist of pointing out features of gravity which are (at least typically) characteristic of systems with underlying microstructure. In other words, certain features – ranging from universal coupling to black hole thermodynamics – allow for mounting arguments from analogy to the effect that the metric field and its dynamics are emergent. Arguably, these gravitational features even remain “mysteries” in a fundamental take on the metric and appear to be in need of explanation, which however can be easily achieved in an emergent gravity framework.
The matter is worth exploring in particular in light of the interesting consequences a high-level view of GR could have for the conception of spacetime: it serves promoting a physical view of the metric as one field among others, as opposed to a conventional (geometric) spacetime view as advocated by Friedman (2014) and Maudlin (2012). Hence emergent gravity is very much in line with the particle physics view on GR (the “spin-2 approach”) advocated by Feynman (2003) and Weinberg (1979), and with the dynamical approach to relativity by Brown (2005). In any case, if one agrees with the call to philosophers by Huggett and Wüthrich (2013) to engage with conceptual problems in the development of theories of QG, an assessment of the indications for the emergent nature of gravity will count as a sensible, if not necessary, contribution.
This paper is organised as follows. We start out in section 2 by clarifying our notions of emergence, microscopic structure and high-levelness. Ultimately, we provide a definition of emergent gravity which we take to capture best what is meant by emergent gravity in the physics literature. In section 3, we work through individual features of gravity which can at least naively be taken as suggestive of emergence. In section 4 we end by assessing the general structure of the arguments put forward.
Section snippets
Definition of emergent gravity
Emergent gravity is roughly taken to be the view that gravity arises due to the “collective action of the dynamics of more fundamental non-gravitational degrees of freedom” (Huggett and Wüthrich (2013)). The aim of this section is to sharpen this rough characterization of emergent gravity by providing answers to the following core questions: (i) “emergence of what?”, (ii) “emergence from what?” and (iii) “what kind of emergence?”. The first question asks what properties of gravitational physics
Hints towards an emergent nature
In this section we will construct and criticize arguments to the effect of an emergent gravity paradigm. Naively speaking, there are two strategies for arguing that gravity is emergent: (1) to look for indications in our best theories of gravity (GR, its higher derivative extensions or semi-classical gravity) for an emergent paradigm; and (2) to directly present a model for the supposed microstructure from which gravity emerges.
The first strategy is to look for analogies between (X)
Conclusion: from hints to arguments
We have assessed four different hints for the emergent nature of gravity: (1) universal coupling, (2) non-renormalizability, (3) black hole thermodynamics, and (4) the holographic principle. Whether you take any of these hints to be strong indications in favour of an emergent paradigm or not, depends to a good degree on (not fully uncontroversial) background assumptions. If you are committed to the geometric view, universal coupling is just a consequence of the fact that gravity relates to the
Acknowledgments
We are grateful to Harvey Brown, Karen Crowther, James Read, Jeroen van Dongen, Sebastian de Haro, Niels Martens, Kian Salimkhani, Erik Verlinde, Christian Wüthrich and two anonymous referees for useful comments on the paper and/or general discussions on emergent gravity. We would also like to thank the History and Philosophy of Gravity group at the University of Amsterdam, and the audiences of the EPSA conference in Exeter and the “Thinking about Space and Time” conference in Bern for useful
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