This final chapter provides a discussion of the BFSS matrix model, which defines the matrix quantum mechanics, providing another basis for Matrix theory. The relation between M-theory and the IKKT model is briefly discussed.

We use the the supersymmetric harmonic oscillator to introduce supersymmetry, and to motivate how world-sheet supersymmetry leads to space-time fermions. After a short review of Poincaré Lie superalgebras, we introduce the RNS-model, both as an example of a supersymmetric field theory and as a stepping stone to type-II superstrings. We motivate the GSO-projection through modular invariance and show how the massless spectra as well as the D-branes of type-II string theories arise. Type-I and heterotic strings are briefly introduced, and we conclude with an overview of the dualities that relate the five modular invariant supersymmetric string theories to one another, and to eleven-dimensional M-theory.

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.

This article grew out of my February 2017 Scientific American column titled “Imagine No Universe,” eventually expanding into a feature-length cover story for Skeptic, necessarily an order of magnitude longer to nuance the many possible answers to the question. As readers will discover, far from a famine of scientific and philosophic answers to the question, we have a veritable feast to dine on. Which will turn out to be more or less supported by empirical research in both particle physics and cosmology remains to be seen – and it could be many decades to a century before we have a solid grasp of the problem – but do not succumb to the theists’ transcendental temptation to evoke God as an answer in the teeth of so many elegant explanations already on our plate.