I review the hypothesis that neither space nor quantum mechanics is fundamental, and both are emergent from a more fundamental description that is neither. This fundamental description is a completion of quantum mechanics based on relational hidden variables. Here, relational means that they give a fuller description, not of an individual particle but of a network of relations among particles. This completion of quantum mechanics does not live in space, rather space is an emergent description of an underlying network of relations. Since locality is, in this sense, emergent, locality can be disordered, and one of the effects of this is quantum nonlocality. This summarizes a line of thought that weaves through many of my papers on quantum foundations, from the early 1980s to the present.

In this article I review the reasons why gravity has proven much more difficult to quantize than the other forces. Primary among them is the existence of black holes, whose remarkable properties tell us that a theory of quantum gravity must have a mathematical structure that is quite different from the quantum field theories that describe the rest of particle physics. These observations motivated the introduction of the ‘holographic principle’, which argues that the fundamental degrees of freedom in a gravitational theory must live in a lower number of dimensions than the general relativity theory that it reduces to at low energies. The AdS/CFT correspondence gave the first sharp example of how this can be possible, and more recently several ‘toy models’ of this correspondence have been introduced that clearly illustrate not just how holography can be realized but also why it must be. This article gives an overview of these recent developments.

I claim that both Being and Becoming are incarnated in a cosmological model within the causal set approach to quantum gravity in which spacetime is fundamentally discrete. I argue that, in the model, Being is subjective whereas Becoming is objective.

Three central questions that face a future philosophy of quantum gravity: Does quantum gravity eliminate spacetime as fundamental structure? How does quantum gravity explain the appearance of spacetime? What are the broader implications of quantum gravity for metaphysical (and other) accounts of the world? In this essay I begin to lay out a conceptual scheme for (1) analyzing dualities as cases of theoretical equivalence and (2) assessing when cases of theoretical equivalence are also cases of physical equivalence. The scheme is applied to gauge/gravity dualities. I expound what I argue to be their contribution to questions about (3) the nature of spacetime in quantum gravity and (4) broader philosophical and physical discussions of spacetime. Iapply this scheme to questions (3) and (4) for gauge/gravity dualities. I argue that the things that are physically relevant are those that stand in a bijective correspondence under duality: the common core of the two models. I therefore conclude that most of the mathematical and physical structures that we are familiar with, in these models (the dimension of spacetime, tensor fields, Lie groups), are largely, though crucially never entirely, not part of that common core. Thus, the interpretation of dualities for theories of quantum gravity compels us to rethink the roles that spacetime, and many other tools in theoretical physics, play in theories of spacetime.

We argue for enlarging the traditional view of quantum gravity, based on ‘quantizing GR’, to include explicitly the nonspatiotemporal nature of the fundamental building blocks suggested by several modern quantum gravity approaches (and some semiclassical arguments) and to focus more on the issue of the emergence of continuum spacetime and geometry from their collective dynamics. We also discuss some recent developments in quantum gravity research, aiming at realizing these ideas, in the context of group field theory, random tensor models, simplicial quantum gravity, loop quantum gravity, and spin foam models.

After a brief introduction to issues that plague the realization of a theory of quantum gravity, I suggest that the main one concerns a quantization of the principle of relative simultaneity. This leads me to a distinction between time and space, to a further degree than that present in the canonical approach to general relativity. With this distinction, one can make sense of superpositions as interference between alternative paths in the relational configuration space of the entire universe. But the full use of relationalism brings us to a timeless picture of nature, as it does in the canonical approach (which culminates in the Wheeler–DeWitt equation). After a discussion of Parmenides and the Eleatics's rejection of time, I show that there is middle ground between their view of absolute timelessness and a view of physics taking place in timeless configuration space. In this middle ground, even though change does not fundamentally exist, the illusion of change can be recovered in a way not permitted by Parmenides. It is recovered through a particular density distribution over configuration space that gives rise to records. Incidentally, this distribution seems to have the potential to dissolve further aspects of the measurement problem that can still be argued to haunt the application of decoherence to many-worlds quantum mechanics. I end with a discussion indicating that the conflict between the conclusions of this paper and our view of the continuity of the self may still intuitively bother us. Nonetheless, those conclusions should be no more challenging to our intuition than Derek Parfit's thought experiments on the subject.

In this epilogue we place the theories of semiclassical and stochastic gravity in perspective, exploring their linkage with quantum gravity, defined as theories for the microscopic structures of spacetime, not necessarily and most likely not from quantizing general relativity. We distinguish two categorical approaches, ‘top-down’ (Planck energy) and ‘bottom-up’. The tasks of the ‘top-down’ approach, which include string theory and other proposed theories for the microstructures of spacetime, lie in explaining how the micro-constituents give rise to macroscopic structure. They are thus more appropriately called emergent gravity. warnings are issued not to blindly follow the dogma that quantizing general relativity naturally yields a microscopic structure of spacetime, or to accept, without checking the emergent mechanisms, the dictum that some micro-constituent is the theory that gives us everything. Stochastic gravity takes the more conservative ‘bottom-up’ approach. For the linkage with quantum gravity we mention (a) the kinetic theory approach, relying on the structure of a correlation hierarchy and the role played by noise and fluctuations, and (b) the effective theory approach, using large N techniques. The ingredients of both approaches have been developed in earlier chapters systematically. We end with a description of the advantages and limitations of stochastic gravity.