We first define energy conditions, which are gravitational analogs of the positivity of the energy in nongravitational theories. After defining the notion of singularity more precisely, we state (without proof) the singularity theorems of Hawking and Penrose, and a “counterexample,” which evades all of their assumptions. Then we define wormholes, traversable wormholes, and give as example the Morris–Thorne wormhole, with its embedding diagram.

After a discussion of X (Twitter) and the kinds of feelings, sentiments, and practices that it engenders in users, Chapter 7 explores what people do on screens: the social practices, thinking, and being that occur in postdigitality. Pursuing the mission of the book to seek the human in the machine, this chapter attends to how crescent voices act at screens, discovering the many learned and habituated digital literacy practices which allow screen users to perform their identities multimodally in multitudinous and diverse ways. Based on interviewee accounts, this chapter offers a model of postdigital practices which employs the concepts of fishbowls, antholes, rabbitholes, and wormholes, while also drawing on Charles Taylor’s social imaginaries.

This chapter (and the next one) covers some basic mathematics needed to describe four-dimensional curved spacetime geometry. Much of this is a generalization of the concepts introduced in Chapter 5 for flat spacetime. Coordinates are a systematic way of labeling the points of spacetime. The choice of coordinates is arbitrary as long as they supply a unique set of labels for each point in the region they cover, but for a particular problem, one coordinate system may be more useful than another. We then define the metric for a general geometry and explain common conventions. We show how to compute lengths of curves, areas, three-volumes, and four-volumes for a given metric. Concepts such as wormholes, extra dimensions, the Lorentz hyperboloid, and null spaces are introduced.