In thephilosophy of space and time,eternalism[1] is an approach to theontological nature oftime, which takes the view that all existence in time is equallyreal, as opposed topresentism or thegrowing block universe theory of time, in which at least the future is not the same as any other time.[2] Some forms of eternalism give time a similarontology to that ofspace, as adimension, with different times being as real as different places, andfuture events are "already there" in the same sense other places are already there, and that there is noobjective flow of time.[3]
It is sometimes referred to as the "block time" or "block universe" theory due to its description ofspace-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.
In classical philosophy, 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 at least partly undefined. 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. One view of this type,presentism, argues that only the present exists. The present does not travel forward through an environment of time, moving from a real point in the past and toward a real point in the future. Instead, it merely changes. The past and future do not exist and are only concepts used to describe the real, isolated, and changing present. This conventional model presents a number of difficult philosophical problems and may be difficult to reconcile with currently accepted scientific theories such as thetheory of relativity.[4]
It can be argued thatspecial relativity eliminates the concept of absolute simultaneity and a universal present: according to therelativity of simultaneity, observers in differentframes of reference can have different measurements 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 those of another. However, there are events that may be non-simultaneous in all frames of reference: when one event is within thelight cone of another—its causal past or causal future—then observers in all frames of reference show that one event preceded the other. The causal past and causal future are consistent within all frames of reference, but any other time is "elsewhere", and within it there is no present, past, or future. There is no physical basis for a set of events that represents the present.[5]
Many philosophers have argued that relativity implies eternalism.[6] Philosopher of science Dean Rickles says that, "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism."[7] Christian Wüthrich argues that supporters of presentism can salvage absolute simultaneity only if they reject eitherempiricism or relativity.[8]Dean Zimmerman and others argue fora single privileged frame whose judgments about length, time, and simultaneity are thetrue ones, even if there is no empirical way to distinguish this frame.[9][10]Hilary Putnam concluded in 1967 that it follows from special relativity that ″any future eventX is already real″ and eternalism is the only view compatible with special relativity.[11] The philosopher Mauro Dorato interprets Putnam's arguments differently and asserts that ″the opposition between presentism − only the presently existingevent exist − and eternalism − past present and future events are equally real − which issomewhat presupposed in Putnam 1967, is misguided.″[12]
Arguments for and against an independent flow of time have been raised since antiquity, represented byfatalism,reductionism, andPlatonism: Classical fatalism argues that everyproposition about the future exists, and it is either true or false, hence there is a set of every true proposition about the future, which means these propositions describe the future exactly as it is, and this future is true and unavoidable. Fatalism is challenged by positing that there are propositions that are neither true nor false, for example they may be indeterminate. Reductionism questions whether time can exist independently of the relation between events, and Platonism argues that time is absolute, and it exists independently of the events that occupy it.[4]
Earlier, pre-Socratic Greek philosopherParmenides of Elea had posited that existence is timeless and change is impossible (an idea popularized by his discipleZeno of Elea andhis paradoxes about motion).
The philosopher Katherin A. Rogers argued thatAnselm of Canterbury took an eternalist view of time,[13] although the philosopherBrian Leftow argued against this interpretation,[14] suggesting that Anselm instead advocated a type of presentism. Rogers responded to this paper, defending her original interpretation.[15] Rogers also discusses this issue in her bookAnselm on Freedom, using the term "four-dimensionalism" rather than "eternalism" for the view that "the present moment is not ontologically privileged", and commenting that "Boethius and Augustine do sometimes sound rather four-dimensionalist, but Anselm is apparently the first consistently and explicitly to embrace the position."[16] Taneli Kukkonen argues in theOxford Handbook of Medieval Philosophy that "what Augustine's and Anselm's mix of eternalist and presentist, tenseless and tensed language tells is that medieval philosophers saw no need to choose sides" the way modern philosophers do.[17]
Augustine of Hippo wrote thatGod isoutside of time—that time exists only within the created universe.Thomas Aquinas took the same view, and many theologians agree. On this view, God would perceive something like a block universe, while time might appear differently to the finite beings contained within it.[18]
One of the most famous arguments about the nature of time in modern philosophy is presented inThe Unreality of Time byJ. M. E. McTaggart.[19] It argues that time is an illusion. McTaggart argued that the description of events as existing in absolute time is self-contradictory, because the events have to have properties about being in the past and in the future, which are incompatible with each other. McTaggart viewed this as a contradiction in the concept of time itself, and concluded that reality is non-temporal. He called this concept theB-theory of time.[4]
Dirck Vorenkamp, a professor of religious studies, argued in his paper "B-Series Temporal Order in Dogen's Theory of Time"[20] that theZen Buddhist teacherDōgen presented views on time that contained all the main elements of McTaggart's B-series view of time (which denies any objective present), although he noted that some of Dōgen's reasoning also contained A-Series notions, which Vorenkamp argued may indicate some inconsistency in Dōgen's thinking.
Eternalism also encapsulates the theory of world lines, and the concept of linear reality that is - the individual perception of linear time.[21]
Some philosophers appeal to a specific theory that is "timeless" in a more radical sense than the rest of physics, the theory ofquantum gravity. This theory is used, for instance, inJulian Barbour's theory of timelessness.[22] On the other hand, George Ellis argues that time is absent in cosmological theories because of the details they leave out.[23]
Recently, Hrvoje Nikolić has argued that a block time model solves theblack hole information paradox.[24]
Philosophers such asJohn 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."[25] Similarly,Karl Popper argued in his discussion withAlbert Einstein against determinism and eternalism from a common-sense standpoint.[26]
A flow-of-time theory with a strictlydeterministic future, which nonetheless does not exist in the same sense as the present, would not satisfy common-sense intuitions about time. Some have argued that common-sense flow-of-time theories can be compatible with eternalism, for exampleJohn G. Cramer'stransactional interpretation. Kastner (2010) "proposed that in order to preserve the elegance and economy of the interpretation, it may be necessary to consider offer and confirmation waves as propagating in a "higher space" of possibilities.[27]
InTime Reborn,Lee Smolin argues that time is physically fundamental, in contrast to Einstein's view that time is an illusion.[clarification needed] Smolin hypothesizes that the laws of physics are not fixed, but rather evolve over time via a form of cosmological natural selection.[28][29] InThe Singular Universe and the Reality of Time, co-authored with philosopherRoberto Mangabeira Unger, Smolin goes into more detail on his views on the physical passage of time. In contrast to the orthodox block universe view, Smolin argues that what instead exists is a "thick present"[30][31] in which two events in the present can be causally related to each other. Marina Cortês and Lee Smolin also argue that certain classes of discrete dynamical systems demonstrate time asymmetry and irreversibility, which is inconsistent with the block universe interpretation of time.[32]
Avshalom Elitzur vehemently rejects the block universe interpretation of time. At the Time in Cosmology conference, held at thePerimeter Institute for Theoretical Physics in 2016, Elitzur said: "I'm sick and tired of this block universe, ... I don't think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it's not there."[33] Elitzur and Shahar Dolev argue that quantum mechanical experiments such as the Quantum Liar[34] and the evaporation of black holes[35] challenge the mainstream block universe model, and support the existence of an objective passage of time. Elitzur and Dolev believe that an objective passage of time and relativity can be reconciled, and that it would resolve many of the issues with the block universe and the conflict between relativity and quantum mechanics.[36] Additionally, Elitzur and Dolev believe that certain quantum mechanical experiments provide evidence of apparently inconsistent histories, and that spacetime itself may therefore be subject to change affecting entire histories.[37]
Some philosophers have made objections to eternalism based on the existence of the self and concepts such as Benj Hellie'svertiginous question. Vincent Conitzer argues that arguments in favor of the A-theory of time are more effective as arguments for the combined position of both A-theory being true and the "I" being metaphysically privileged from other perspectives.[38] Similar arguments have been made by Caspar Hare with the theories ofegocentric presentism andperspectival realism.
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