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.

It is widely believed that in quantum theories of gravity, the classical description of spacetime as a manifold is no longer viable as a fundamental concept. Instead, spacetime emerges as an approximation in appropriate regimes. In order to understand what is required to explain this emergence, it is necessary to have a good understanding of the classical structure of spacetime. I focus on the concept of spacetime as it appears in locally covariant quantum field theory (LCQFT), an axiomatic framework for describing quantum field theories in the presence of gravitational background fields. A key aspect of LCQFT is the way in which it formulates locality and general covariance, using the language of category theory. I argue that the use of category theory gives a precise and explicit statement of how spacetime acts as an organizing principle in a certain systems view of the world. Along the way I indicate how physical theories give rise to categories that act as a kind of model for modal logic, and how the categorical view of spacetime shifts the emphasis away from the manifold structure. The latter point suggests that the view of spacetime as an organizing principle may persist, perhaps in a generalized way, even in a quantum theory of gravity. I mention some new questions, which this shift in emphasis raises.

Loop quantum gravity has formalized a robust scheme in resolving classical singularities in a variety of symmetry-reduced models of gravity. In this essay, we demonstrate that the same quantum correction that is crucial for singularity resolution is also responsible for the phenomenon of signature change in these models, whereby one effectively transitions from a `fuzzy' Euclidean space to a Lorentzian space-time in deep quantum regimes. As long as one uses a quantization scheme that respects covariance, holonomy corrections from loop quantum gravity generically leads to nonsingular signature change, thereby giving an emergent notion of time in the theory. Robustness of this mechanism is established by comparison across a large class of midisuperspace models and allowing for diverse quantization ambiguities. Conceptual and mathematical consequences of such an underlying quantum-deformed spacetime are briefly discussed.

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 distinguish between two versions of the black hole information-loss paradox. The first arises from apparent failure of unitarity on the spacetime of a completely evaporating black hole, which appears to be non-globally hyperbolic; this is the most commonly discussed version of the paradox in the foundational and semipopular literature, and the case for calling it `paradoxical' is less than compelling. But the second arises from a clash between a fully statistical-mechanical interpretation of black hole evaporation and the quantum-field-theoretic description used in derivations of the Hawking effect. This version of the paradox arises long before a black hole completely evaporates, seems to be the version that has played a central role in quantum gravity, and is genuinely paradoxical. After explicating the paradox, I discuss the implications of more recent work on AdS/CFT duality and on the `Firewall paradox', and conclude that the paradox is if anything now sharper. The article is written at a (relatively) introductory level and does not assume advanced knowledge of quantum gravity.

String theory has not even come close to a complete formulation after half a century of intense research. On the other hand, a number of features of the theory suggest that the theory, once completed, may be a final theory. It is argued in this chapter that those two conspicuous characteristics of string physics are related to each other. What links them together is the fact that string theory has no dimensionless-free parameters at a fundamental level. The paper analyzes possible implications of this situation for the long-term prospects of theory building in fundamental physics.

We provide an analysis of the empirical consequences of the AdS/CFT duality with reference to the application of the duality in a fundamental theory, effective theory, and instrumental context. Analysis of the first two contexts is intended to serve as a guide to the potential empirical and ontological status of gauge/gravity dualities as descriptions of actual physics at the Planck scale. The third context is directly connected to the use of AdS/CFT to describe real quark-gluon plasmas. In the latter context, we find that neither of the two duals are confirmed by the empirical data.

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.

Quantum gravity is expected to require modifications of the notions of space and time. I discuss and clarify how this happens in loop quantum gravity.

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.

The inflationary scenario is not the only paradigm of early universe cosmology that is consistent with current observations. General criteria are presented that any successful early universe model must satisfy. Various ways, including inflation, are presented that satisfy these conditions. It will then be argued that if nature is described at a fundamental level by superstring theory, a cosmology without an initial spacetime singularity will emerge, and a structure formation scenario that does not include inflation may be realized.

Scientists and philosophers of science are most impressed by theories that make successful, novel predictions: that predict surprising facts in advance of their experimental or observational confirmation. There is a theory of cosmology that has repeatedly been successful in this privileged way, but it is not the standard, or 𝚲CDM, model. It is Mordehai Milgrom’s MOND theory (MOdified Newtonian Dynamics). Unlike the standard model, MOND does not postulate the existence of dark matter. Observations that are explained in the standard model by invoking dark matter are explained in MOND by postulating a change in the laws of gravity and motion.