Eternalism is a philosophical approach to the ontological nature of time, which takes the view that all points in time are equally "real", as opposed to the presentist idea that only the present is real. Modern advocates often take inspiration from the way time is modeled as a dimension in the theory of relativity, giving time a similar ontology to that of space (although the basic idea dates back at least to McTaggart's B-Theory of time, first published in The Unreality of Time in 1908, only three years after the first paper on relativity). This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time. It is sometimes referred to as the "block time" or "block universe" theory due to its description of space-time as an unchanging four-dimensional "block", as opposed to the view of the world as a three-dimensional space modulated by the passage of time.
Problems with the flow of time
Conventionally, time is divided into three distinct regions; the "past", the "present", and the "future". Using that representational model, the past is generally seen as being immutably fixed, and the future as undefined and nebulous. As time passes, the moment that was once the present becomes part of the past; and part of the future, in turn, becomes the new present. In this way time is said to pass, with a distinct present moment "moving" forward into the future and leaving the past behind. This view of time is given the name presentism by philosophers.
This conventional model presents a number of difficult philosophical problems, and seems difficult to reconcile with currently accepted scientific theories such as the theory of relativity.
Simultaneity
Special relativity has shown that the concept of simultaneity is not universal: observers in different frames of reference can have different perceptions of whether a given pair of events happened at the same time or at different times, with there being no physical basis for preferring one frame's judgments over another's (though in a case where one event A happens in the past light cone of another event B, all frames will agree that A happened in the past of B). So, in special relativity there can be no physical basis for picking out a unique set of events that are all happening simultaneously in "the present".
Uniqueness of a present moment
There is no fundamental reason why a particular "present" should be more valid than any other; observers at any point in time will always consider themselves to be in the present. However, every moment of time has a "turn" at being a present moment in flow-of-time theories, so the situation ends up symmetrical. Although there is still an ontological distinction between past, future, and present that is not symmetrical.
Rate of flow
The concept of "time passing" can be considered to be internally inconsistent, by asking "how much time goes by in an hour?" However, the question could be no different from "how much space is contained in a meter?" — all measurements being equally arbitrary.
McTaggart's argument
In The Unreality of Time, J. M. E. McTaggart divided time into an A-series and a B-series, with the A-series describing events in absolute tensed terms (past, present, and future) and the B-series describing events in terms of untensed temporal relations (before and after). He went on to argue that the A-series was logically incoherent and should be discarded, and that the B-series was insufficient for a proper understanding of time. He endorsed the C-series instead, which is a fixed, changeless (and therefore timeless), non-directional ordering of events. While McTaggart's concluded that time is unreal, various philosophers and physicists have held that the remaining B-series is all that is needed for a complete theory of time, sometimes referred to as the B-Theory of time.
The Eternalist alternative
Eternalism addresses these various difficulties by considering all points in time to be equally valid frames of reference—or equally "real", if one prefers. It does not do away with the concept of past and future, but instead considers them directions rather than states of being; whether some point in time is in the future or past is entirely dependent on which frame of reference you are using as a basis for observing it.
Since an observer at any given point in time can only remember events that are in the past relative to him, and not events that are in the future relative to him, the subjective illusion of the passage of time is maintained. The asymmetry of remembering past events but not future ones, as well as other irreversible events that progress in only one temporal direction (such as the increase in entropy) gives rise to the arrow of time. In the view suggested by Eternalism, there is no passage of time; the ticking of a clock measures durations between events much as the marks on a measuring tape measures distances between places.
Eternalism may have implications for the concept of free will, in that it proposes that future events are as immutably fixed and impossible to change as past events (see determinism). However as the human subject, and any free will they have, is also 'present' throughout time, during their life, they may be exercising free will in the 'future' as it were.
Eternalism makes two assumptions, which are separable. One is that time is a full-fledged real dimension. The other is immutability. The latter is not a necessary consequence of the first. A universe in which random changes are possible may be indistinguishable from the many-worlds interpretation of quantum mechanics in which there are multiple "block times."
Augustine of Hippo wrote that God is outside of time—that time exists only within the created universe. Many theologians agree. On this view, God would perceive something like a block universe, while time might appear differently to us finite beings.
Philosophical objections
Philosophers such as John Lucas argue that "The Block universe gives a deeply inadequate view of time. It fails to account for the passage of time, the pre-eminence of the present, the directedness of time and the difference between the future and the past"
The comment summarizes the main objections. In more detail, they are:
Subjective sense of flow
Whilst the idea that there is some objective sense in which time is flowing can be denied, the fact that conscious beings feel as though it is in some sense flowing cannot. However, if the flow of time didn't have an objective existence, then it is argued conscious beings would simultaneously experience all moments in their lives. A response is that since the brain presumably perceives time through information processing of external stimuli, not by extrasensory perception, and obeys the laws of causality, it is hard to see how the flow of time, whether it exists or not, could make any subjective difference: all conscious beings are built to perceive time as a chain of events, whether or not it occurs as such.
Apparent differences between past, present and future
Many of our common-sense attitudes treat the past, present and future differently.
1.
We apparently fear death because we believe that we will no longer exist after we die. But if Eternalism is correct, death is just one of our temporal borders, and should be no more worrisome than birth.
2.
You are about to go to the dentist, or you have already been. Commonsense says you should prefer to have been. But if Eternalism is correct, it shouldn't matter which situation you're in.
3.
When some unpleasant experience is behind us, we feel glad that it is over. But if the Eternalism is correct, there is no such property as being over or no longer happening now—it continues to exist timelessly.
Status of conscious observers
Eternalists often appeal to the idea that the flow of time is a subjective illusion. However, Eternalism takes its inspiration from physics and needs to give a physical account of observers. One could, for instance, portray conscious observers as moving through the block universe, in some physically inexplicable way, in order to account for the subjective sense of a flow of time. But there is no need to do so to explain the subjective flow of time. Their opponents claim that the time-flow itself, as an objective phenomenon, is physically inexplicable, and that physics is simply misrepresenting time in treating it as a dimension.
Determinism and indeterminism
Previously, it was noted that people tend to have very different attitudes towards the past and the future. This might be explained by an underlying attitude that the future is not fixed, but can be changed, and is therefore worth worrying about.* If that is correct, the flow of time is perhaps less important to our intuitions than an open, undetermined, future. In other words, a flow-of-time theory with a strictly determined future (which nonetheless does not exist at the present) would not satisfy common-sense intuitions about time. If indeterminism can be removed from flow-of-time theories, can it be added to Eternalist theories? Surprisingly, the answer is a qualified "yes" in the form of multiverse theories, where multiple alternate futures exist in a fixed framework, but individual observers have no way of knowing which alternative, or "branch" they will end up in.
[* Another, perhaps more compelling, explanation for this difference of attitudes would be that it really is worse to be having a bad experience than to have had one. A bad experience in the future is seen as one that has not been had, but will be. Unfortunately for Eternalism, though, it appears to lack the conceptual apparatus needed to explain 'having an experience', as distinct from having had, or being about to have, that experience. This is why most people find Eternalism strongly counter-intuitive.]
Relation to physics
Eternalism takes its inspiration from physics, especially the Rietdijk-Putnam argument, in which the relativity of simultaneity is used to show that each point in the universe can have a different set of events that are in its present moment. According to Presentism this is impossible because there is only one present moment that is instantaneous and encompasses the entire universe.
Some philosophers also appeal to a specific theory which is "timeless" in a more radical sense than the rest of physics, the theory of quantum gravity. This theory is used, for instance, in Julian Barbour's theory of timelessness. On the other hand, George Ellis argues that time is absent in cosmological theories because of the details they leave out.
Relation to Eastern body of thought
In Buddhism, a special term Dharmadhatu is translated as 'total field of events and meanings' or 'field of all events and meanings.' Here the 'Block Universe' seems to be encompassing not only every possible event in the physical universe but also having a psychological component.
In fiction
Eternalism is a major theme in Kurt Vonnegut’s novel, Slaughterhouse-Five. The Tralfamadorians, an alien species in the novel, have a four-dimensional sight and can therefore see all points in time simultaneously. They explain that since all moments exist simultaneously, everyone is always alive. The hero, Billy Pilgrim, lives his life out of sequence, which, among other things, means that his point of death occurs at a random point in his life rather than at the end of it.
Eternalism also appears in the comic book series Watchmen by Alan Moore. In one chapter, Dr. Manhattan explains how he perceives time. Since past, present, and future events all occur at the "same time" for him, he speaks about them all in the present tense. For example, he says "Forty years ago, cogs rain on Brooklyn" referring to an event in his youth when his father throws old watch parts out a window. His last line of the series is "Nothing ends, Adrian. Nothing ever ends."