Introduction

Space is not like time. For one thing, there is no intrinsic direction – no metaphysical grain – to any dimension of space. Instead, every spatial dimension is perfectly symmetrical. For another thing, space does not exhibit any movement or flow; unlike time, there is no dynamic aspect to the dimensions of space. And for a third thing, space is ontologically indiscriminate. It doesn't matter whether you're located right here or over there in space, since all spatial locations are equally real. Space just sits there: a gigantic, three-dimensional continuum, static and homogeneous, with nothing but their contents to distinguish one spatial region from another.

Time is different. Time has pizzazz. For starters, there is a distinctive direction to time, sometimes called time's arrow, which points always toward the future. Also, time is dynamic: each moment of time approaches inexorably from the future, enjoys its brief heyday in the spotlight of the present, and then ever after recedes serenely into the shadowy realm of the past. Not only that, but your temporal location does matter, ontologically speaking: for the past is the domain of has-beens, and the future is a land of mere potential. Only the present is truly real.

Or so it might seem. But many have thought otherwise. A large number of philosophers and scientists, especially in the last hundred years, have defended the view that time, despite appearances to the contrary, is one of four more or less similar dimensions of the universe.