Most of us believe that only the present exists. Events in the future will exist, and events in the past did exist, but neither future nor past events exist now. This view is called presentism and treats time and space in very different ways. The different parts of space all coexist in a present moment, but only one part of time exists; namely, the present.
Presentism is compatible with either the existence of persistent objects moving through time or with an event ontology. A presentist merely insists that “only the present exists now” and is indifferent to what the present consists of, that is, whether it is persistent objects or events.
Many interpreters of relativity have asserted that the theory proves that presentism is false. Instead, the past and future coexist with the present, and are just as real as the present. This is strange and perhaps even frightening. It means that past wars are still being fought, and that every step of our future lives is already happening in some sense. In debates over relativity theory, such a world is called the block universe, because the entire four-dimensional universe, including the past and the future, seems to be like a giant block of ice: all events in the past, present and future coexist and are frozen in their locations in space and time.
For our purposes, we will assume that either presentism or the block universe view must be true. That is, other combinations (like an existent present and past, but nonexistent future) will be ignored.
Metaphors are often used to help us mentally picture a block universe. It has been compared to a loaf of bread or bologna. The present moment is a slice across the middle of the loaf; the future and the past lie on either side. Of course, the loaf is only a three- dimensional object, and the block universe is four-dimensional. Thus slices of the block universe would each be a three-dimensional world at an instant: just like the world we see around us now. The series of such three-dimensional “slices” – past, present and future – together make up the whole four-dimensional block.
Since each slice of the block universe is a complete world-at-an- instant, it contains all objects that exist at the time. This is, again, like cinema film: each still photograph in the film is a picture of a scene at an instant, and the sequence of all the stills makes up the entire film. If the still photographs were cut apart and bundled together in a pile, we would have yet another image of the block universe.
If it is true that we live in a block universe, then there are no objects persisting through time. That is, a block universe implies an event ontology. This is because there is no real motion or change in a block universe. True motion occurs when a body now in one place occupies another place in the future: that is, when one and the same body moves from one location to another as, for example, when someone walks across a room. This could not happen in a block universe, where future events already exist. In a block universe, future events have an existence that is just as real and full-blooded as present events.
Advocates of the block universe also claim that the movements and changes we see around us are all a kind of illusion. The star of a film may occur in every still on the reel, and may appear to be moving when the film is shown, but actually does not move at all: each still is fixed. Similarly, the slices in a block universe are each slightly different. If we believe that someone is walking across the room, there is actually a series of slices each with a walker in slightly different positions. In each slice, the walker is standing stock still like a sentry.
Thus both the event ontology view and the block universe view assert that motion is just a series of fixed events, like a sequence of still photographs. In the block universe all the events in the series exist at once: from the past, to the present and into the future.
When discussing the block universe or event ontologies, philoso- phers sometime find it awkward to use expressions like “the past exists now” or “the future has already happened”. The reason is that verbs like “to exist” include a reference to the passage of time, that is, they are past, present or future tense. Thus, to say “the past exists” seems like a contradiction because the verb is in the present tense. To avoid this, philosophers tend to talk about tenseless existence, that is, a way of existing that does not imply a flow from the past into the future but is instead eternally static. Thus they say that, in a block universe, the past and the future “exist tenselessly”, and mean that “exist” here is not to be understood as a verb in the ordinary present tense.
A final distinction that is important for understanding the block universe view is that between Laplacian determinism and fatalism:
• Laplacian determinism: the view that conditions at the present moment together with physical laws determine all future events.
That is, laws ensure that the future can happen in only one way.
• Fatalism: the view that all future events are fixed, but not necessarily by physical laws. That is, the future can happen in only one way, but there may be no regular or law-like patterns in future events. Perhaps God or fortune has decreed that a series of miracles or physically uncaused events come about.
The block universe view is fatalistic. In a block universe, there can be only one future because it is already there, and in some sense has already happened. But the block universe view does not depend on the existence of laws, or any regularities between slices. Laplacian determinism may be true in a block universe, or may not be.
It was believed that classical physics before Einstein provided evidence for the truth of Laplacian determinism, but many now believe that twentieth-century physics disproved this view and showed that there is true randomness in microscopic events. The defeat of Laplacian determinism, however, would not count against the block universe view. (Thus the block universe view is compatible with probabilistic interpretations of quantum theory.)