Einstein’s theory of relativity appears to drain time of its most familiar feature: its flow. If spacetime is a four-dimensional block, as many physicists argue, then there is no privileged present moment. But over the last two decades, a small group of philosophers has been assembling a radical alternative picture, which aims to save time’s flow. In this article, Italian philosopher and logician Samuele Iaquinto introduces Fragmentalism, a view on which spacetime is not a single, coherent whole, but rather a patchwork of incompatible perspectives, or fragments, which disagree about which events are past, present and future. These fragments are not mere appearances of an underlying timeless block. They are fundamental. Time, on this view, can flow, but it must do so in a plurality of ways, each relative to a perspective.

It is tempting to think of reality as centered on the present moment, the very instant at which you are reading these words. If asked to describe reality as it really is, you would hardly begin by portraying the Roman Empire as a still-living political order. That is now just a collection of ruins and records. Reality, it seems, is nothing more than how things are from the privileged standpoint of the present moment.

In contemporary philosophy, this idea is voiced by the dynamic theory of time, according to which temporal passage—the continual change in what is present—is an objective, mind-independent feature of the universe. There are several ways of spelling out this idea. One is presentism, which holds that only present entities exist. On this view, the flow of time is a continual process in which present objects come into being and cease to exist. Neither past nor future entities exist. A more permissive dynamic theory is the growing block view, according to which reality comprises not only present entities but also past ones, such as dinosaurs or Julius Caesar. These past entities are just as concrete as we are, even though they are no longer present. On this view, the present is the edge of a four-dimensional block of past events, which grows as new objects continually come into existence. Finally, the moving spotlight view extends reality even further by admitting future entities alongside past and present ones, such as the first child to be born in the year 2100. While all moments of time exist, only one is illuminated by the spotlight of the present, which moves along the temporal dimension, lighting up one moment at a time.

___

Philosophers have traditionally assumed that if the present is absolute, it must be unique: there can be only one privileged temporal perspective, locating the present at a single region of reality. But what if this assumption is mistaken?

___

But how does the idea of an absolute present fare in the context of modern physics? Not very well, it would seem. To see why, consider the classic train thought-experiment, often used to illustrate the relativity of simultaneity. An observer stands in the middle of a wagon moving at constant velocity. They hold two lamps, one aimed at a sensor at the back of the wagon and the other at a sensor at the front (see Figure 1).

Figure 1

If the observer switches the lamps on simultaneously, the light from each lamp reaches its target at the same time. From the observer’s perspective inside the wagon, the two detection events—call them A (at the rear) and B (at the front)—are simultaneous. Now consider these events from the perspective of an observer standing on the platform as the wagon passes by (Figure 2).