Eternalism

Eternalism is the claim that the past and future exist. Under eternalism, the axis of time is analogous to the axes through space, and an object in the past or future exists equally to an object in the present but at a different point in space. The opposing view is Presentism, which states that only the present exists. Eternalism models the universe as an unchanging four-dimensional object, whereas presentism models the universe as a three-dimensional object that changes over time. Eternalism is also known as the Block-Universe theory.

Philosophical Considerations

Time is notorious for being philosophically puzzling and impossible to reduce to other concepts. Due to this impossibility, it is usually taken to be an ontological primitive, i.e., a fundamental feature of the universe. If one takes this position, the role of time in the equations of physics is not solely determined by the laws themselves but requires the additional postulate that the universe is a structure that evolves over time, rather than a larger structure in which time constitutes an internal axis. Conversely, eternalism takes a more uniform approach in which the difference between time and space is fully captured by their respective computational roles within the equations of physics.

Due to this difference, eternalism may be considered simpler and hence more plausible. A more extreme version of this argument is that the additional postulate of presentism disproves it outright since it cannot be formalized. In other words, the issue is not just that there is nothing in the equations of physics that grants time its unique ontological status, but that it is unclear what such a thing could even look like.

To illustrate this point, consider a much simpler case in which a system $S$ changes over time, and we wish to describe it through a mathematical model. The approach would typically be to use a function $f:\mathbb {R} \rightarrow S$ or $f:[t_{0},t_{1}]\rightarrow S$ that maps the real numbers $\mathbb {R}$ or a segment of the reals $[t_{0},t_{1}]$ to the system $S$, thereby determining its state at different points in time. In any such model, time necessarily becomes an internal axis.

Einstein's theory of General Relativity is sometimes considered a separate argument for eternalism since it does model the universe as a four-dimensional object in which time is an internal axis. As discussed above, this is likely true for any conceivable formalism; however, General Relativity has the further property that time passes at different rates for different objects due to the curvature of space-time. Thus, even if one takes a presentist point of view in which the universe is a structure that itself changes, General Relativity implies that the rate of change must vary across the spatial axes.

Relevance

As with many philosophical concepts, the significance of eternalism, if one accepts it as probable, is highly subjective. Depending on one's world view and aesthetic, it may fall anywhere between a life-changing revelation and a mildly stimulating philosophical curiosity.

On the one hand, eternalism substantially disagrees with presentism about what is, particularly the existence of the past. Consequently, a genuine belief in eternalism can greatly alleviate fear of death or frustration over having most of one's life behind oneself. (Conversely, regret over the past may be amplified since eternalism holds that moments of suffering are trapped in the block universe permanently.)

On the other hand, eternalism and presentism usually agree on what should be. In particular, while they disagree on the value of the past, they align on that of the future. Consequently, since our actions can only affect the future, the difference between presentism and eternalism usually affects all outcomes proportionately, rendering it irrelevant to decision-making.

That said, it is possible to construct scenarios that weigh the future against the past, even if these scenarios cannot be actionable (i.e., someone may decide which option they prefer but cannot affect which one is true). One such scenario is due to Derek Parfit's Reasons and Persons. In this thought experiment, you wake up in a hospital without memory, and the nurse tells you that you either have a very painful surgery behind you or a significantly less painful surgery ahead of you, but she's unsure which of the two it is. In this case, presentism suggests that you would rather have the more painful surgery behind you, whereas eternalism may prefer having the surgery in the future.

Consciousness and Phenomenal Time

If one accepts eternalism and also adopts a realist point of view on consciousness, it suggests that all moments of experience are a static part of the block universe. Each such moment may then correspond to a small four-dimensional slice, where the task of defining the boundaries of such slices is known as the Boundary Problem.

While this view provides a coherent ontology to analyze consciousness, it begs the question of how phenomenal time is implemented, i.e., why it seems to us like time moves forward (note that one could argue that this question has to be answered under presentism as well). QRI's answer to this question is a proposal called the Pseudo Time Arrow, which comes down to the idea that each moment of experience integrates an abbreviated history of previous moments that creates the illusion of a process. Conversely, there is no analogous integration of future states, which is why time appears to move forward only.