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.

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.