Spacetime: Engineering and Metaphysics

Syne Salem


This thesis will deal with two topics, both clearly related, but also clearly distinct. The first three chapters deal with what we have chosen to call 'spacetime engineering.' It involves the systematic testing of basic perturbations of the spacetime metric, followed by observations of the way these perturbations affect the geodesics, the paths of extremal length. Part of this is visualization — general relativity is a highly complex theory based on four-dimensional spacetime structures. Another part is trying to determine what those visualizations mean. The last part is applying them to create exotic custom spacetimes — a sort of tradition for general relativists. We don't really get into the applications here; that is left for future work. We do begin to build the opto-mechanical analogy with the way light rays bend as they pass through spacetime steps.

The second half deals with metaphysics of time, primarily the debate between the positions known as presentism and eternalism. Chapter 4 introduces the debate and builds up the compatible positions from a primarily metaphysics perspective. Chapter 5 introduces what is considered to be the best argument against presentism and in favor of eternalism: the argument from special relativity. In the course of the chapter we introduce spacetime models as they are typically built in the philosophy of physics (simplified, of course). Chapter 6 examines five arguments one can make to try to lessen the impact of special relativity on presentism. Of those arguments the first three fail, and the fourth inconclusively softens the blow. The fifth, however, genuinely opens the door to a metaphysics of time that could be both presentist and compatible with quantum gravity.

The motivation that drives each of these projects is the unifying factor — that is, the desire to better understand what our most up-to-date physical theories mean for us. General relativity is highly counterintuitive, and physics will probably only get more complicated in the future. Hopefully a reader of this thesis will be able to come away with both a better grasp of the formal theory, its empirical implications, and its metaphysical ones.