In Duration and Simultaneity (1922), a critical examination of the philosophical implications of Einstein’s relativity theory, Bergson describes CN as “an admirable book,” “one of the most profound ever written on the philosophy of nature.” When it comes to interpreting relativity theory, Bergson’s own philosophical agenda bears striking resemblance to Whitehead’s. Yet, in many respects their thought seems to unfold in opposite directions on the crucial issue of simultaneity. In the three books where he explicitly deals with the philosophical implications of relativity theory (namely, PNK, CN and R), Whitehead aims to reconstruct the basic concepts of the physical world in order to bridge the gap between abstract mathematical constructs and the realm of lived experience. Although Bergson shares a similar concern for what is simply given in sense awareness, he is more inclined to draw demarcation lines than to explain why scientific theories give us such an effective grip on the real. Accordingly, his views on simultaneity stand in sharp contrast with Whitehead’s evolving characterization of the fact of temporal togetherness, from the early works on relativity theory to the doctrine of co-presence expounded in PR. Whitehead’s guiding intuition is that giving becoming its proper place within a philosophy of nature requires a reformulation of classical temporal issues in spatio-temporal terms—a task which requires in turn the construction of a positive philosophical theory of “space-time” and the spatial embedding of temporal perspectives, as opposed to a mere metaphysical rephrasing of the mathematical structure of physical theories.
His first step in this direction consists in assuming that time is primarily a “stratification of nature,” in other words, that time cannot be a self-subsistent entity, but only a character of process. Related to this claim is the idea, emphasized from 1915 onwards, that time (regardless of its ontological status as substantival or relational) is abstracted from a deeper unity of a spatio-temporal kind: “there can be no time apart from space, and no space apart from time” (CN 142). Absolute time is a “metaphysical monstrosity” (PNK 8); time (just like space) is “an abstraction from the passage of events” (CN 34; see R 21, 29). Hence, what is actually given for sense-awareness must be something more fundamental: a “slab” of nature whose extensive properties may well translate into space and time, but which is in itself neither spatial nor temporal. The concept of simultaneity is organically linked with this primitive experience of the passage of nature.
The second step (which Bergson never explicitly takes) consists in acknowledging the existence of infinitely many serial time orders describing the total creative advance of nature: “The various time-series each measure some aspect of the creative advance, and the whole bundle of them express all the properties of this advance” (CN 178). The problem of simultaneity arises between this second step and the first, in conjunction with the relativization of simultaneity relations effected by Einstein’s theory of relativity. If no meaning can be given to the idea of absolute simultaneity (a “now” valid for all places and perspectives), in what sense can distinct—if not disjoint—time-series be gathered in a “bundle?” How does the primitive experience of the whole of nature relate to this variety of temporal perspectives?
Whereas Bergson dismisses the simultaneity of distant events as an artificial construct in order to achieve a local resolution of the problem in strictly temporal terms, Whitehead’s aim is to root the multi-threaded time-systems in a natural concept of simultaneity which does not reduce to the local simultaneity of coincident events or co-present flows. On this basis alone can the relativity of simultaneity appear as something more than a mere artifact of our methods of time measurement, while still allowing the philosopher to recover the texture of the universe, if not a global view of it.
I maintain the old-fashioned belief in the fundamental character of simultaneity. But I adapt it to the novel outlook by the qualification that that meaning of simultaneity may be different in different individual experiences” (R 67).
The following is an overview of the philosophical issues that converge on the concept of simultaneity in Whitehead’s early period (up to SMW). It starts with a rough outline of some debates in contemporary metaphysics over the status of the “now,” and then proceeds to a comparative examination of various aspects of the simultaneity concept in Bergson’s and Whitehead’s writings on relativity. The concluding section focuses on some implications of the understanding of simultaneity for the resolution of the famous “twins paradox.”
1. Relativity and the Flow of Time
1.1. The “Flow” of Time and the “Advance” of Nature
Perhaps the most remarkable aspect of Whitehead’s posthumous contribution to the ongoing debate over the place of becoming in modern physics was to suggest a distinctly non-metaphysical formulation of problems, in line with what he sometimes described as “pan-physics,” or more simply, “philosophy of science.” This is apparent in the fact that his reflections on simultaneity—a pivotal concept in Einstein’s special theory of relativity—consistently avoid the logical puzzles of modality which arise as soon as one invokes the status of existence or actuality in relation to the idea of the present or the “now.” To see this more clearly, a reminder is in order.
“When is ‘now’?” This awkward question is often raised in connection with philosophical debates over the status of the present. Partisans of so-called “A-time” emphasize the need to make room for the passage of a transient “now,” which they acknowledge as a crucial dimension of lived experience as well as a basic feature of becoming. Partisans of “B-time,” satisfied with a merely serial conception of temporal relations (events being ordered in terms of before and after), insist on accounting for the special sense of the present (and the subsequent distinction of past, present and future) in purely indexical terms, thus dispensing with the elusive flowing “now.” According to the latter view, “now” refers to the time when I am using this very word, the time simultaneous with its utterance. But since this word can be used any time by anyone, there is really no privileged position attached to the present. Every instant on the timeline equally counts as “now,” just as every point of space is virtually “here,” for this is, indeed, a matter of perspective. There is no sense in distinguishing a particular temporal location as the present: every time is “now” relative to itself, every event in the history of the universe is “present” in its own right at the temporal location where it happens, and is eternally so. Hence, according to a notorious argument crafted by McTaggart, the A-series is bound to fall back unto the B-series as soon as it is relativized to a given perspective, as it should be. In short, “now relative to time t” eventually reduces to the tenseless relation “simultaneous with t,” and the various uses of that relation in conjunction with indexicals is all the B-theorist ever claimed there was to the passage of time.
Quite independently from any reference to physics, the B-series view of time has often lent support to a dubious metaphysics of the “block universe.” In conjunction with the standard interpretation of relativity theory (involving as it does the relativity of simultaneity and the denial of absolute time), it has been instrumental in conveying the sense that it is no longer possible to define real becoming as an infinity of layers of “the now” coming into existence successively, to quote a famous line by Gödel. Yet, such a picture of becoming is foreign to Whitehead’s conception of the “passage of nature,” as much as it is to Bergson’s concept of “duration.” Both agree that the so-called “Newtonian” view of absolute time, equably flowing from one instant to the next, cannot possibly be revived. There is more to the process conception of becoming than the inexorable advance of the “knife-edge” of the present, separating the ever-growing past from the open future. Accordingly, criticisms directed at the Augustinian description of the elusive “now” (whether one pictures it as an instantaneous present or a “specious” present) fall off the mark: they do not really address the problem of becoming, but merely a version of A-time, and a rather abstract one despite its phenomenological credentials. Besides, regardless of the place she is willing to concede to the “flow of time” in a general theory of nature, no serious B-theorist has ever wanted to deny the actual experience of the passage of time. We shall therefore take it for granted as a baseline for all further discussion.
1.1.2. Time, Becoming, Simultaneity
This rough sketch of some aspects of contemporary discussions on the nature of time may be helpful in appraising the exact significance of Whitehead’s doctrine of simultaneity and temporal perspectives. But it is not the ambition of this essay to challenge the flat, durationless picture of time drawn by the adepts of the B-series. In fact, this does not even seem necessary. After all, such a view expresses in its logical form the mathematical habit of spatializing time, and the resulting Parmenidean block picture of the universe makes it even more explicit by collapsing nature into the mathematical structures used to describe it. Extravagant as it may seem on metaphysical grounds, this move appears rather innocuous once we realize that most of the problems that await a systematic philosophy of nature are left untouched by the repudiation of the flowing present. The “block universe” picture offers nothing more than a quick metaphysical fix diverting us from (rather than solving or dissolving) more delicate issues.
For one thing, time may not be the problem at all. Whitehead himself carefully refrains from conflating the issue of becoming with that of time. This is perhaps one of his major points of disagreement with Bergson:
The process of nature can also be termed ‘the passage of nature.’ I definitely refrain at this stage from using the word ‘time,’ since the measurable time of science and of civilized life generally merely exhibits some aspects of the more fundamental fact of the passage of nature. I believe that in this doctrine I am in full accord with Bergson, though he uses here ‘time’ for the fundamental fact which I call the ‘passage of nature’ (CN 54).
The dispute is not merely verbal, for Bergson and Whitehead are really carving the experience of time along different lines. As we shall see, Whitehead’s reluctance to equate the passage of nature with time simpliciter is directly related to his decision to promote simultaneity as a key concept of his philosophy of nature.
This brings us to the second reason for not addressing the question of the “flow of time” head-on. The McTaggartian line of reasoning presented in the preceding section seems to presuppose all along that we agree on the meaning of the words “simultaneous with.” But this is far from obvious, especially if one refuses to grant independent reality to the successive times implied by our talk about temporal relations. If time is nothing substantial beyond the nexus of relations between events, if it is merely an aspect or dimension of the unfolding of nature, though a fundamental one, then we had better clarify what we mean by simultaneity, and how it relates to the primitive experience of things happening or unfolding “together.” The paradoxes surrounding the metaphysical quibbles over the nature of time do not have any direct bearing on these questions. They generally take the concept of simultaneity for granted.
Even Einstein’s definition of simultaneity, as Whitehead remarks, leaves unexplained why “[t]he same definition of simultaneity holds throughout the whole space of a consentient set in the Newtonian group” (PNK, 54). The idea of a spread of simultaneous events extending throughout the whole universe is granted from the start. The physicist does not need to interpret it: his only task, as far as simultaneity is concerned, is to unfold the relativistic consequences of certain measuring procedures, in accordance with well-grounded assumptions regarding fundamental facts of motion (such as the behavior of light in empty space). In this respect, one may say that the relativization of simultaneity leaves (the concept of) simultaneity itself untouched. A confirmation of this can be found in the fact that special relativity still enables us to attach a set of simultaneity relations and hence a global meaning of “space” (permanent as well as instantaneous) to any arbitrarily chosen inertial frame. This is not the case, of course, in general relativity, but relinquishing global simultaneity can hardly count as an elucidation of the natural concept of simultaneity.
But the trivialization of simultaneity is even more apparent in the usual metaphysical treatment of the problem of becoming. To put it bluntly, the now (in both A and B-series) is such a poor concept that it does not help us even to begin to understand what it means for things to happen together at once. Yet this is the fundamental fact of nature—not the time series itself which may well not be unique, certainly not its serial character which is, in any case, “derivative from the properties of durations” (CN 55). If change is something more than the successive happening of contradictory states of affairs, if it implies a genuine self-transformation of nature as a whole, it is not fair to jump from the idea that the “flow of time” is a metaphor (Bergson himself would readily concede that point) to the conclusion that change through time is an illusion. Metaphysicians have often shown much impatience on these matters: they have not paid enough attention to the nature of simultaneity, where the problem of time takes its roots.
Is simultaneity a relation or a system of relations holding between events or components of a single specious present? Is it a more primitive experience of togetherness? How should we go about describing such an experience and the way the more abstract relations find their way into it? Whitehead and Bergson both addressed these questions in connexion with the philosophical implications of the Einstein-Minkowski picture of the universe. In order to appreciate each of their contributions more fully, it is interesting to consider where they part ways, starting with their respective take on relativity theory as a whole.
1.2. Bergson’s and Whitehead’s Involvement with Relativity Theory
1.2.1. Bergson’s Critical Strategy
Bergson’s strategy is more defensive than constructive. Yet it is best characterized as a discussion of the philosophical interpretations of relativity theory rather than a criticism of its physical underpinnings. In order to defend Einstein against the superfluous and generally inconsistent metaphysics that have grown on the soil of relativistic ideas, he aims at the intuitive core of the theory behind artificial constructs and symbolisms. Because of his unconditional attachment to the idea of temporal becoming, he is reluctant to attach too much philosophical importance to the geometrization of spatio-temporal relations achieved by the theory of relativity. In fact, he hardly develops a personal account of space-time. “Space-time,” in his book, always refers to Minkowskian space-time, an ingenuous mathematical tool which unfortunately seems tailored for the kind of Parmenidean metaphysics that makes a point of “freezing” the passage of nature. According to Bergson, the formalization of the “absolute universe” (Minkowski’s words) where all events, past, present and future, are virtually represented as if they were spread out once and for all, epitomizes the “spatialization of time” which lies at the very heart of modern science ever since time was defined as a parameter for describing motion. Despite his repeated claims to be a full-blooded relativist (a relativist of a peculiar breed, to say the least), Bergson believes the philosophical importance of Einstein’s theory resides not in the four-dimensional framework per se, but in the way it offers a more complex picture of the articulation of time and space than the one implicit in Newtonian absolute, mathematical time. Central to his interpretation of Einstein’s results is the idea that, far from contradicting the unity of “real time,” relativity theory—in its special version at any rate—is more likely than any other to establish it on firm grounds and confer on it “special rigor and consistency” (Bergson 1999, 32). Hence, Bergson writes: “a philosophy of nature in which duration is considered real and even active can quite readily admit Minkowski’s and Einstein’s space-time,” provided that one pays sufficient attention to the way the time dimension “is no longer […] completely similar to the others.” Thus, while acknowledging the novelty and philosophical significance of the relativist framework, and even granting its puzzling metrical consequences on a purely empirical level (with the notable exception of the twins paradox, which he seemed to hold as a fancy thought experiment rather than a necessary implication of the theory), he never relinquished the idea that time could somehow be preserved from its articulation (rather than fusion) with space and continue to play a role in physics in the pure, unaltered form it takes in the experience of lived duration.
1.2.2. Whitehead’s Reconstructive Strategy
Whereas Bergson’s analyses most often aim at unraveling the constructs of the physicist in order to lay bare the points where scientific constructs connect with the reality of time, Whitehead is more disposed to engage on a reconstruction of the basic presuppositions of science. In the early “non-metaphysical” works under consideration, he operates from the bottom up—that is from the bare ground of experience to the systems of relations captured by the symbolisms of algebraic transformations and space-time diagrams, never losing touch with what is disclosed in “presentational immediacy.” In that sense, Whitehead’s contribution is typical of what he considered as the proper task of a “philosophy of science,” namely, to determine “the most general conceptions which apply to things observed by the sense” (R 4). This is nowhere clearer than in his treatment of simultaneity, a concept to which he literally gives its full extension.
2. Simultaneity between Time and Space
Remarkably, when compared to such philosophers as Husserl and even Bergson, Whitehead does not have much to say about the contexture of temporal consciousness per se. His emphasis is more on the way the “advance” or “passage of nature” expresses itself through the organization of temporal perspectives that necessarily comprise an element of spatial extension.
2.1. Simultaneity as a Spatio-temporal Concept
When dealing with simultaneity, one is indeed confronted with the problem of relating the experience of the present to the experience of space. Simultaneity, in ordinary language, is about attaching a moment to a place, a present duration to a stretch of space with variable extension, ranging from the neighborhood surrounding the perceiver’s body through a region as large as physical space itself. No doubt, speaking of space in such general terms already involves a certain degree of abstraction. Whitehead himself, as we shall see, insists on starting with what he calls a “duration” (not to be confused with Bergson’s own concept of duration!), by which he means all nature “now-present” for sense-awareness (PNK 68ff; CN 53ff). What role does “now” play, however, if not that of referring what is perceived to one and the same time (the time corresponding to an act of perception)? Here, one may well be under the impression of a circularity involved in the description of duration as the whole of nature simultaneous with an event (the perceiving event). Since simultaneity itself is defined as a factor in the present unity of such a duration, it would seem that we are indeed moving in a circle. This however is more likely to be an indication of the irreducible character of duration, than a logical flaw. The truth is that the presentational immediacy of nature as a whole is a primitive fact on which any concept of simultaneity (whether local or global) must ultimately rely. To that extent, simultaneity itself can be viewed as a positive and ultimate relation of events. The appearance of circularity is dispelled as soon as one realizes that in a fundamental sense, duration is no more spatial than temporal. We shall come back to this in Section 2.c, but it is important to emphasize the fact that the very use of the word “time” (as in “all nature at a time”) would be prejudicial at this stage: space and time as serial orders are in fact jointly derived from the spatio-temporal reality of durations.
The whole point of Whitehead’s doctrine of simultaneity is to show that relations exemplified in such statements as “Paul was taking a nap when the phone rang,” or “A star was born (in Andromeda) while Paul was taking a nap” (the examples are mine), although they rely on a certain number of conventions concerning the possibilities of coordination of distant happenings in terms of spatial and temporal systems of coordinates, are nevertheless in nature even before we decide what belongs to time and what belongs to space. Hence, the simultaneity relation holding between two distant events (defined by Einstein and Minkowski as discrete, infinitesimal happenings) is related to the problem of clock synchronization, but Whitehead does not believe it to be a primarily temporal matter, involving as it does the existence of reference frames where the observer can be considered at rest.
Besides, the Einsteinian definition of simultaneity is only one possibility among a wider array of simultaneity concepts. So is the instantaneous configuration of the whole universe considered from a particular perspective or reference frame: the relativistic “hyperplanes” of simultaneity slicing up a four-dimensional world have of course no exact counterpart in the context of spatio-temporal experience, and this explains why Whitehead describes “durations” as “slabs” with a certain temporal thickness, involving a definite lapse of time, as opposed to “moments” which refer to instantaneous spreads of the apparent world (“all nature at an instant,” CN 57). A moment is deduced by the method of extensive abstraction “as a limit which is a logical ideal of the exact precision inherent in nature” (R 7). The relations of interconnection within such moments define instantaneous three-dimensional spaces (R 69): they provide the basis for the idea of parallelism which, together with the admission of multiple time-systems, naturally leads to the geometrical ordering of space-time (affine structure). Another virtue of Whitehead’s method of “extensive abstraction” (besides its capacity to deduce limit-concepts from the facts of observation) consists in elucidating the fact that, in spite of their abstract character, such spatio-temporal concepts as parallel or intersecting “moments” nevertheless hinge on reality in a very effective way.
It has sometimes been held that Whitehead’s primary goal in framing the concept of duration was to complement the operationist and yet rather abstract Einsteinian definition of simultaneity with an intuitive definition involving a more primitive experience of co-presence. Yet, if Whitehead readily concedes that simultaneity is “immediate for sense-awareness” (CN 56), his chief concern is to exfoliate the mixed experience of “duration” (the only true “immediate data”) in order to come up with a working concept of simultaneity that may bridge the gap between the symbolical constructs of physics and the fabric of spatio-temporal experience. It is not enough to claim, along Bergsonian lines, the precedence of “lived” simultaneity over its more abstract definitions. If simultaneity is indeed inseparable from the experience of nature as given in sense-awareness, we shall see that its role, in this respect, is that of a limiting factor within the very experience of nature. This factor, one may add, can be located at different levels of experience. Its concept therefore appears rather flexible: it comes in various degrees of concreteness and determination, stretching from the obscure feeling of the interconnectedness of spatial relations beyond those experienced in the immediate vicinity of the perceiver, to more elaborate constructs involving the idea of the whole of nature extending through space. Bergson, on the other hand, refers simultaneity to the essentially local experience of the contiguity and continuity of durations, while insisting on the artificial character of the scientific construction of the simultaneity of distant, disconnected events. As a consequence, he ends up with two neatly separated concepts of simultaneity. This deserves closer inspection before we turn again to Whitehead.
2.2. Bergson’s Two Concepts of Simultaneity
2.2.1. Events and Flows
Simultaneity is a typical example of the kind of impure concepts Bergson denounces as the source of potential philosophical perplexity. It first presents itself to common sense as the corollary of a natural “belief” in the existence of universal time: a duration common to all things implies shared moments of coexistence. But this belief (or hypothesis) clearly mixes temporal intuitions and spatial representations, and the project of reconstructing temporal concepts from simultaneity relations is therefore likely to bring some confusion. One of the virtues of relativity is to have given sharp relief to this state of affairs, forcing the philosopher to spell out the assumptions underlying simultaneity. How does Bergson introduce the idea of simultaneity in this context? In order to see this, it is necessary to recall his general line of attack. According to Bergson, the only reason the measured, calculated, inferred time attributed to distant events is still called “time,” is that it can be harnessed to an immediate, intuitive experience of simultaneity and succession. The connexion with immediate facts of sense-awareness is complicated in this case by the fact that distant simultaneity involves a particular measuring procedure and a logic of its own (remember Einstein’s line about the relation between scientific concepts and sensations not being like that of the soup to the beef, but rather like that of the cloakroom check number to the coat). Nevertheless, the elaborate idea of distant time ultimately relies on “real time” which alone provides the “flesh” that is lacking from the algebraic formalization of spatio-temporal perspectives (Lorentz transformations).
Two points must be emphasized at this juncture:
(1) First, since it is attached to the individual experience of a percipient event, “real time” necessarily is a local concept. It is lived at a place, where the body is. Accordingly, real simultaneity does not extend beyond “here.” But the logic of this local approach to time is obscured if one understands “locality” in a narrow, spatial sense. Assigning spatial limits to perception itself would be a category mistake in this context. That simultaneity is a local matter does not mean that only what is “here” can be perceived (Bergson himself gives the example of the flight of a bird in the sky). The idea is that perception is always singular, and thus situated as the corollary of the activity of some consciousness. “Local,” as opposed to “global,” indicates a general orientation of thought more than it refers to any definite location in space. It means in fact that space is a problem, that it has no metaphysical immediacy and must itself be derived. Hence, Bergson’s local approach to the time of nature can be traced back to a fundamental assumption concerning the ideal, or at least derivative character of space. Bergson would like to account for spatial extension itself, in its various degrees, in terms of a fundamentally temporal reality. This is perhaps the crucial issue on which his philosophy and Whitehead’s ultimately diverge.
(2) The second point to be noted is that simultaneity is conceived as being primarily a matter of simultaneous flows, rather than simultaneous, “coincident” events in Einstein’s sense. The idea of flow is given a purely intensive characterization. Two flows are called “contemporaneous” when they are “equally one or two for my consciousness, the latter perceiving them together as a single flowing if it sees fit to engage in an undivided act of attention.” This primacy of flows is reflected in Bergson’s emphasis on the primarily temporal meaning of simultaneity.
2.2.2. Instants and the “Spatialization” of Time
Bergson’s defense of the unity of real time is first presented as a vindication of the common-sense view. Through the local connecting links provided by the overlapping fields of neighboring consciousnesses virtually scattered throughout the whole of nature, two centers of experience, however far apart, can be said to “have some part in common:” step by step, they can be “reunited in a single experience, unfolding in a single duration.” This idea of a unique, real time of nature reached through the continuous connexion of a virtually infinite bundle of durations, furnishes the backdrop for Bergson’s discussion of the relativistic conception of simultaneity. The whole point of his analysis is to show that without the simultaneity of immediately co-present flows embraced in a single act of perception we would not be able to confer any temporal sense on the concept of simultaneity at a distance in the first place. Global simultaneity only makes sense if one is ready to extend a local experience of simultaneity from place to place, in a continuous fashion.
However, the idea of global simultaneity reached by those means is the one shared by the common sense view: it is not easily reconciled with the idea of simultaneity at work in physical theory. Reconquering the unicity of time within relativistic physics therefore requires special effort. Bergson himself never doubted that temporal perspectives, together with reference frames as coordinating factors, would yield the shifting simultaneity relations described by the Lorentz transformations and captured in Minkowski’s space-time formalism. But his aim was to clarify what was presupposed in the very idea of physical simultaneity, even before the relativity of simultaneity is established. In order to achieve this, he had to show how the simultaneity of flows could be converted into the simultaneity of spatially separated events. The missing link was the idea of an instant of time. This idea is implicit in the idealized experience of instantaneous perceptions apprehended as simultaneous in one and the same mental act. But how do we get from flows to instantaneous events? Only the spatializing of flows through the experience of motion, suggesting a one-to-one correspondence between positions on a trajectory and instants of on a “timeline,” can account for the transition from the intensive characterization of the simultaneity of flows to the extensive concept of the simultaneity of instantaneous events. Only spatialized time allows for a “natural extension” of the local simultaneity of neighboring events to the global simultaneity between widely-separated events.
What about the relativity of simultaneity? Bergson’s analysis comes down to this: in so far as simultaneity is real, it is a purely local matter and the problem of the relativity of simultaneity does not even arise. But in so far as it is extended over space, it is conventional, a mere artifact of our measuring procedures, and its dislocation according to the relativistic formulae cannot possibly affect our understanding of real time, even if it is well confirmed by experimental data. This is only one side of the issue, however. For the “unreasonable effectiveness” of conventions, the fact that they give us such a strong grip on reality, would itself be a mystery if the simultaneity between distant instantaneous events, for all its artificial, conventional character, did not somehow draw upon the continuity of flowing durations. There is no other explanation for our persistent identification of distant times as “times,” rather than anything else.
3. Whitehead’s Theory of Simultaneity
3.1. Temporal Perspectives
3.1.1. Bergson and Whitehead on Perspectives
What is striking in Bergson’s analysis is the way it deliberately avoids conferring any reality on the extensive implications of the ordinary use of the word “simultaneity.” His only concession consists in imagining “a consciousness coextensive with the universe, capable of embracing the two events in a unique and instantaneous perception.” But this picture of a primarily local apprehension of simultaneity stretching over the universe as a whole is a mere phantasm, a theoretical fiction designed to expose the assumptions behind the physical concept of simultaneity. On closer examination, the whole operation seems to rely on the relativity of spatial separation, inferred from the homogeneity of space. But this amounts to short-circuiting space altogether. The idea of a single consciousness embracing the whole of nature is akin to some form of action-at-a-distance or telepathic signaling (an analogy favored by Bergson). It hardly qualifies as a proper acknowledgement of the extensive nature of simultaneity. As a consequence, the idea of temporal perspective cannot be taken too literally here. For perspectives to make sense at all in a Bergsonian context, they must be referred to the way the surrounding space of the body is gauged by the time it would take to interact with its different points. Bergson elaborates on this idea in the first chapter of Matter and Memory (1896). He describes a fundamental experience of distance preceding any form of measurement. This primitive sense of space is derived from the various degrees of temporal proximity or remoteness that structure the sphere of the acting body. Virtual actions and the anticipation of their future results are responsible for the layering of space according to distance relations. It is as if time (the time of the acting body) projected itself on space, as if space itself was a temporal perspective.
Things are quite different in a Whiteheadian context, because there is an original meaning of extension which precedes any differentiation between space and time. Accordingly, the perspective attached to a standpoint does not admit a fully temporal genesis. Time is a projection of space (unfolding space from a certain perspective) as much as space is a projection of time. More appropriately, time itself originally emerges as a perspective in conjunction with space: temporal and spatial perspectives come together. The reference to perspective is not merely metaphorical; it is not a fancy way of conveying the idea that we are situated in time and that consequently time is never given in full, as an object that could be surveyed along each of its dimensions. And yet it is quite different from Bergson’s idea of surrounding space organizing itself from various degrees of temporal depth. In actual experience there is no time without spatial depth. Time and space are derivative from a more fundamental extensive continuum of events that is neither temporal nor spatial, in such a way that when space and time jointly emerge from the spatio-temporal continuum, it is natural to see time extending immediately over space. Quite consistently, Whitehead favors the term “duration” rather than “moment” or even “time.” Thus understood, “duration” has a definitely non-Bergsonian ring. It points to the original perspectival nature of time itself. The two concepts should be kept distinct. Unsurprisingly, they allow for quite different accounts of simultaneity.
The meaning of Whiteheadian temporal perspectives is encapsulated in the concept of “cogredience,” which refers, through the basic idea of (relative) rest, to “the preservation of unbroken quality of standpoint within the duration” (CN 110; PNK 70 ff). Thus, temporal experience implies a spatio-temporal standpoint, which determines in turn a three-dimensional spatial background in the form of a particular sense of the distribution of “here” and “there,” in accordance with a particular experience of rest. Now, here is an interesting difference between our two philosophers. The local character of sense-awareness is of course conceded from the start: “perception is always ‘here,’ and a duration can only be posited as present for sense-awareness on condition that it affords one unbroken meaning of ‘here’ in its relation to the percipient event” (CN 110). However, the integration of durations in a prolonged present is not separable from the assumption that it makes sense to ask “what is now immediately happening in regions beyond the cognisance of our sense” (SMW 124). According to one of its meanings, “nature” includes the whole spatio-temporal system of perceived or unperceived facts which are connected with some local space-time region. Nature, one may say, is basically what extends beyond “here.” Thus, simultaneity is endowed from the outset with a trans-local meaning, it is immediately extended to the whole of nature, whereas Bergson, firmly rooting the experience of co-present flows in the synthetic operations of a determined consciousness, has no other choice but to deduce the extensive character of universal time (the time of nature as such) through the rather contrived operation of patching together neighboring durations with no clear spatial extent. One may perhaps venture to say that there is no pure experience of time from a Whiteheadian perspective. At least, such an experience is of no use for a philosophy of nature. Accordingly, there is no purely temporal sense of simultaneity. The extensive or spatial dimensions attached to a given time-system do not need to be explained, but merely articulated to those of alternative time-systems.
3.1.3. The Objective Reality of Perspectives
An essential point to be noted is that perspectives must not be viewed as mere sets of appearances attached tosubjective standpoints: they are real facts of nature, as much as they are real facts of awareness. A duration is locally given in sense-awareness with a definite region of discerned events and entities, but it is not itself local, nor does it reduce to a mere “point of view.” Whitehead’s use of the term “standpoint” should not confuse us here. His adamant refusal of the “bifurcation of nature” prevents time and space from collapsing into the subjective or psychological confines of some private experience: “Our experiences of the apparent world are nature itself” (R 62). Thus, standpoints participate in the interrelatedness of events. They are themselves significant of the matrix of spatio-temporal relations. They do not merely extend “beyond isolated cases subjected to the direct examination of individual perception” (R 64), but immediately spread over the whole of nature.
3.1.4. Alternative Time-systems
Once this is admitted, the idea of alternative “time-systems” being woven into the systematic unity of nature follows quite naturally. For the distribution of “here” and “there” varies with different distributions of “rest” and “motion” according to the relativity of motion (see CN 105ff). The set of events simultaneous with a percipient event can be broken into an infinity of distinct durations, depending on the perspective offered by various states of rest and motion. Yet, the percipient event is truly cogredient with only one of these durations, namely the one within which all its parts are unequivocally at rest, and which it is said to perceive (PNK 70-71; CN 111; R 8). Other events, endowed with different motions, will pick up different durations. Since this too is a fact of nature, Whitehead is committed to the multiplicity of “time-systems” as an expression of the creative advance of nature, rather than as a subjective affair:
This passage is not adequately expressed by any one time-system. The whole set of time-systems […] expresses the totality of those properties of the creative advance of nature which are capable of being rendered explicit in thought (PNK 81).
Nature, then, is woven from such perspectives, encroaching on each other. This fact can be expressed by saying that there is an infinity of different permanent spaces intersecting each other, or alternatively that there is an infinity of time-series, “alternative flows of time” (R 8) with their own relations of cogredience. This will prove essential for coming to terms with the paradoxes of relativistic time. But let us see more closely how the concept of simultaneity is introduced.
3.2. Framing the Simultaneity Concept
3.2.1. Two Roads to Simultaneity
At first sight, Whitehead seems to frame the problem of simultaneity on rather familiar grounds. Getting to grips with simultaneity implies somehow weaving together the “here-now” and the “there-now.” But he suggests that there are two ways to do this. The common-sense way would be to start with a notion of time-series along with some metrical device for evaluating periods of time. This already implies quite a number of elaborate ideas. As Whitehead writes in a rather condensed manner: “The concept of ‘period of time’ marks the disclosure in sense-awareness of entities in nature known merely by their temporal relations to discerned entities” (CN 51-52). An example would be the repeated coincidences between certain features of a periodical mechanism (such as a clock or a planetary motion) and salient phases of one’s own activity. The next step would imply comparing time periods thus defined (on a local basis) with other time periods (at remote places) in order to determine the simultaneity of distant events. This strategy is the one adopted by Einstein in the famous opening paragraphs of his 1905 article on the electrodynamics of moving bodies: there, using light signals, he devises a procedure for synchronizing clocks at rest or in relative motion. This procedure necessarily presupposes the kind of filtering of experience which takes place when certain aspects of events are emphasized at the expense of others. Just as the bare idea of “place” implies that certain entities are known only as spatially related to some other discerned entity, the bare idea of “time” or “period of time” implies that certain entities are known only as temporally related to some other discerned entity such as an object in my immediate surrounding, or the organic activity of my own body.
But this convenient two-stepped procedure by which temporal concepts are first extracted in order to be related to spatial concepts is not the only way to go. As a matter of fact, it is not difficult to see that it presupposes from the start a local meaning of simultaneity that needs to be spelled out. Einstein was very much aware of this, but he did not need to investigate further the philosophical significance of the immediate experience of simultaneity at a place. Whitehead, on the contrary, believes one should start there in order to exhibit the deeper unity of experience.
Even before time and space have been separated from each other as two “dimensions” of the relatedness of experience, there is a sense in which we discern “the character of a place through a period of time” (CN 52). This is what Whitehead means by an “event” (in CN at least), as distinct from Einstein-Minkowski’s instantaneous “point-events.” This is where simultaneity takes its roots, for in addition to the local awareness of actually discerned entities or events, there are, as we have just seen, all “the events which share the immediacy of the immediately present discerned events” (CN 52). In other words, there is a universe extending beyond ourselves, which is somehow immediately presented to the sense, even though not every part of it is distinctly perceived. This feature of experience provides a proto-concept of simultaneity:
The simultaneity of the whole of nature comprising the discerned events is the special relation of that background of nature to the percipient event, which is itself part of the whole. Such a complete whole of nature is called a ‘duration’ (PNK 68).
Such a relation does not involve any coincidence or correspondence between time periods or time series, for these are not yet defined and differentiated at this point. It does not even imply relations between determinate events. What is given is merely “all nature present for discernment” (CN 52), “the one event which is the totality of present nature” (CN 53), provided that one refrains from equating “the totality of present nature” with “nature at an instant.”
This primitive experience of the present already gives us a feeling of the “texture of time” (CN 53), as opposed to its structure or metrical properties. It is on this ground that simultaneity is introduced. Several points need to be stressed in this regard.
First, simultaneity is introduced as a concept rather than a direct component of sense-awareness: “The unity of this general present fact is expressed by the concept of simultaneity” (CN 53). In other words, simultaneity describes a general property of the passage of nature, rather than a distinct relation between events. This general property concerns the fact that, for all its variety, the passage of nature is, so to speak, of one piece.
Second, simultaneity plays the role of a limiting factor within duration. It defines “duration” by limiting the whole of nature to its “simultaneous occurrence” for sense-awareness. Hence, duration can be redescribed as “a certain whole of nature which is limited only by the property of being a simultaneity” (CN 53), while simultaneity appears as the factor that limits a duration to a spread of simultaneous occurrences. In other words, duration is “a concrete slab of nature limited by simultaneity which is an essential factor disclosed in sense-awareness” (CN 53). We have already mentioned the appearance of circularity involved in such statements, and the reason why this circularity need not bother us.
Third, the simultaneity relation acts as a fundamental connexion binding all events within a single duration which serves as the background of perception. It is an internal relation holding between the perceiver and its duration (see above the quote from PNK 68; SMW 124), rather than an external relation obtaining among any determinate pair or class of events within that duration. In this respect, the simultaneity relation can be viewed as constitutive of the experience of duration.
3.2.2. A Geometrical Analogy
One may be tempted to say that, considering its insistence upon the totality of nature, Whitehead’s theory of simultaneity is more concerned with the unity of space than the unity of time, which is Bergson’s major claim. But it is important to remember that the theory of simultaneity precedes the distinction of time and space as separate dimensions. Its role is to articulate these dimensions in a way that preserves the possibility of alternative meanings for space and time. Hence, the concept of simultaneity reduces neither to the idea of space “at a time,” nor to the idea of a relation between the moments of two time-series: it is in fact presupposed by them as the basis of more fundamental “space-time-systems,” just as durations are presupposed by the instants and points that limit or determine them. It is true, however, that on this point the strategies of Bergson and Whitehead seem to intersect at right angles. Bergson approaches simultaneity along the “vertical” axis, that of passage in its intensive sense (involving flows rather than overlapping “events”). Starting with the idea of duration as a “slab of nature,” Whitehead seems more concerned with giving its proper place to the “horizontal” axis, that of spatial extension. If one likes mathematical analogies, the Bergsonian concept of time would amount to a fibration of nature, whereas Whitehead’s would be a foliation or “stratification,” as he puts it himself. There is some truth to this picture. One of Whitehead’s guiding threads is that the passage of nature includes extension as an essential character. This aspect is systematically circumverted by Bergson, whose manoeuvre basically consists in drawing a sharp contrast between the abstract, extensive concept of simultaneity at work in physics and the concrete, lived sense of local simultaneity disclosed in the single sense-awareness of immediately co-present flows. The first concept, in his view, is a substitute for the very notion of space (a rather fragile substitute, considering that the relativity of simultaneity finally deprives the notion of instantaneous space from its remnant of objective meaning). The second concept he views as purely temporal. It corresponds to the sense of passage as “extending beyond nature.”
Whitehead, by contrast, stresses the importance, within immediate sense-awareness itself, of the correlated but opposite trend. Time does not merely pass: it extends over space, or if one prefers, it cuts across space at various angles. There lies the origin of our familiar serial concept of measurable time, for the passage of nature “is not measurable except as it occurs in nature in connexion with extension” (CN 55). More precisely, its involvement with space is what confers its uniformity on time, enabling alternative time-series to express the advance of nature through the metrical relations obtained from different perspectives.
Where the geometrical analogy falls short, however, is that Whitehead does not merely focus on the foliation of space-time in each single time-system, as opposed to the fibration of space as a whole: it is the “bundle” formed by the totality of time-series which actually expresses the measurable properties of the advance of nature. The concept of simultaneity is what makes such a bundle possible in the first place by laying bare the spatio-temporal unity constitutive of all durations.
Nature is a process. As in the case of everything directly exhibited in sense-awareness, there can be no explanation of this characteristic of nature. All that can be done is to use language which may speculatively demonstrate it, and also to express the relation of this factor in nature to other factors” (CN 54).
Accordingly, the language of simultaneity and duration is a speculative demonstration of one of the essential characters of nature as process. It expresses the two-fold character of nature’s “extension” (CN 185)—connectedness and passage, extending over and extending beyond. It exhibits the fact that nature holds itself together while “moving on,” that as an open totality it welcomes both immanence and transcendence, retaining within itself the passage of nature while simultaneously extending beyond itself through that passage. “Space” and “time” hardly convey this two-fold movement originating in the experience of duration. Yet, they both result from this more fundamental duality within unity which is so clearly captured by Whitehead’s concept of simultaneity.
3.2.3. Reservations about Einstein’s Definition of Simultaneity
It should be clear from the preceding remarks that what is remarkable in this concept is not so much its being directly given in perception (a property shared by Bergsonian simultaneity), as the fact that it springs from a notion of duration more primitive than time and space, which nevertheless encapsulates the extensive virtualities of nature (as opposed to Bergson’s local approach of flows). The first aspect, however, suffices to distinguish it sharply from simultaneity in Einstein’s sense, i.e. “instantaneousness.” Whitehead describes this idea of “all nature at an instant” as “a complex logical concept of a procedure in thought by which constructed logical entities are produced for the sake of the simple expression in thought of properties of nature” (CN 56-7). In this respect, the relativistic concept of simultaneity is not an ideal, limiting case of the Whiteheadian concept of simultaneity: it is a different concept altogether, distinct from it by nature. Whitehead was ready to concede this part of Bergson’s analysis, but he could not possibly consider this to be the last word on simultaneity.
His criticism of Einsteinian simultaneity, at any rate, does not limit itself to claiming an intuitive foundation for scientific constructs. It also implies a disavowal of the operationist climate surrounding the relativistic definition of simultaneity. The role of light signals (allegedly, the fastest available ones) is a case in point: it is difficult to see why the very meaning of simultaneity should be made to depend on them (PNK 53). As Whitehead wittily remarks:
There are blind people and dark cloudy nights, and neither blind people nor people in the dark are deficient in a sense of simultaneity. They know quite well what it means to bark their shins at the same instant (PNK 53).
Thus, there seems no justification for singling out light-rays (instead of, say, sound waves) for the definition of simultaneity. Two criticisms are in fact involved in this remark and others of a similar kind.
First, there is the feeling that Einstein is oscillating between two somewhat unrelated concepts of simultaneity: a natural, intuitive one, and a physical one based on an operational procedure. The physical concept is of course dependent on the intuitive one, for the definition using light signals presupposes the meaning of simultaneity at a point: “ultimately all physical measurement depends upon coincidence in time and place” (PNK 51). This point is also acknowledged by Bergson.
Second, the invariant velocity of light signals appears rather contingent as it stands: it should itself be derived from more fundamental structural properties of the spatio-temporal framework of relations in nature. On this account, Whitehead was proved right by later axiomatizations of relativity physics. It is now common knowledge that the important fact is not the particular behavior of light rays but the existence of a structural constant of space-time whose dimension is that of a finite velocity. This fact, in turn, accounts for the local division of space-time into distinct regions according to the possibilities of causal connexions between distant events (“conical order”).
Whitehead’s point, however, is a philosophical one. He has nothing against the operational criteria devised for clock synchronization, he simply refuses to derive the meaning of simultaneity itself from the procedures involved in the measurement of space and time. And as far as the relativity of simultaneity is concerned his point is rather simple: ascribing a particular meaning of simultaneity to each temporal perspectives according to the procedure recommended by Einstein may well imply a “dislocation” of simultaneity relations holding between distant events, but it in no way affects the concept of simultaneity itself, for any identification of simultaneous events ultimately relies on it in so far as these events are considered as parts of the same duration. In short, there may be different ways of assessing simultaneity relations between events, but there is only one road to simultaneity after all.
3.3. Whitehead’s Later Views on Simultaneity: A Brief Outline
The later evolution of the concept of simultaneity in Whitehead’s writings involves a shift from the mode of presentational immediacy to that of causal efficacy. In PR, every “actual occasion” (the analogue of the earlier concept of event) is a member of a causal and extended nexus of actual occasions which influence each other in some way. Some actual occasions, however, happen in causal independence of each other: those are called “contemporaries,” and the spatio-temporal region they define provides the background for simultaneity relations ascribed to durations or “unison[s] of becoming” (PR 192). In the Newtonian case, this region reduces to a unique set of simultaneous actual occasions, since there is no highest limit for causal transmission. The situation described by Whitehead is closer to the Einstein-Minkowski world with its “light cone” geometry defining regions of connectible and non-connectible events across space-time. Thus the characterization of simultaneity in PR develops into a causal theory of time, although an incomplete one, as some commentators have remarked. The main issue, in this context, is to show how exactly apparent presented durations hinge on the system of space-time relations described in the mode of causal efficacy. But the shift from “simultaneity” to “contemporaneity” can hardly be underestimated. It amounts to a genuine paradigm shift in Whitehead’s philosophy of time.
4. Conclusion: Philosophical Twins
The fecundity of Whitehead’s spatio-temporal view of simultaneity is perhaps best exemplified in the way it allows him to deal with the relativistic puzzles associated with the Einsteinian redefinition of temporal concepts. Langevin’s “twins paradox” is perhaps the most famous of all. It acts as a test for any philosophy which advocates a natural, objective meaning of simultaneity while acknowledging the philosophical significance of relativity theory. Contrary to Bergson, Whitehead not only passes the test, but manages to provide a very simple and illuminating reformulation of this temporal paradox. It can be found in the proceedings of a symposium entitled “The Problem of Simultaneity: Is There a Paradox in the Principle of Relativity in Regard to the Relation of Time Measured to Time Lived?” (Whitehead 1923). The solution (which by the way does not leave the domain of the special theory of relativity) consists in observing that the traveling twin, the one who eventually comes back younger than his brother, although he may be considered “at rest” throughout his journey in space, definitely undergoes some change as he turns around at mid-point in order to head back towards the Earth. The important fact is that in the process which the traveling twin undergoes there is a change in his relations of rest and motion, and a subsequent modification of his sense of simultaneity. This change, as Whitehead explains, is responsible for the obliteration of a whole portion of the Earth-twin’s accounted duration in terms of the traveler’s proper time.
A comparison with Bergson’s own handling of the problem in Duration and Simultaneity is instructive in many respects. The accounts disagree significantly on two points.
(1) First, whereas Bergson’s analysis focuses on an alleged symmetry between the points of view of each twin, leading him to the erroneous conclusion that the overall time dilatation effect is reciprocal, Whitehead establishes the existence of a deep asymmetry which the inequality in the total elapsed proper times tends to conceal. There is a gap in the view one of the twins takes on his brother’s overall duration. Adopting the traveler’s perspective, Whitehead draws our attention to this discontinuity in the experience of durations and simultaneities. Coexistence in space-time is no simple matter.
(2) Second, whereas Bergson insists on the necessity of retrieving the common temporal intuition of a local simultaneity of flows behind the artificial definition of simultaneity “slices” of distant events (as if it were possible to somehow plot the twins’ respective histories against each other at every moment, regardless of the spatial separation), Whitehead emphasizes the importance of locating the discontinuity in the actual separation of space-time frames that cannot be considered as mere conventions but count as genuine facts of nature. The dislocation of simultaneity may be an artifact of our measuring and accounting procedures, but simultaneity itself—that is, distant simultaneity—is as objective as spatial extension itself. So is its alteration from place to place, giving rise to alternative time-systems.
Although he does not elaborate on this point, the twins paradox illustrates the kind of confusion that arises from our continuous reliance on the ideas of absolute space and time. In particular, the source of our discomfort with the differential aging of the twins may be traced back to the absolute meaning we attach to the traveler’s “mid-point” at which the shift occurs. Where, indeed, does this U-turn take place? What spatial background are we taking for granted when we try to develop (as Bergson did) an intuition of our twins’ “contemporaneous” space-time trajectories? Similar questions would apply to the temporal aspect of the paradox. A little reflection on this particular case (and others related to the dislocation of simultaneity, such as the famous train thought-experiment devised by Einstein) may help shedding light on Whitehead’s rather elliptical observation that
in different circumstances of motion, space and time mean different things, the moments of one time-system are different from the moments of another time-system, the permanent points of one time-system are different from those of another time-system, so that the permanent space of one time-system is distinct from the permanent space of another time-system (R 8).
And more explicitly still:
The paradoxes of relativity arise from the fact that we have not noticed that when we change our time-system we change the meaning of time, the meaning of space and the meaning of points of space (conceived as permanent) (R 56).
 Bergson 1999, 43 and 47. Bergson’s tribute to Whitehead is an indirect response to CN 54 (see below).
 Gödel 1990, 262. Gödel’s worries have been revived in recent years by Hilary Putnam and others in defense of the “eternalist” view of space-time as an immutable four-dimensional reality. The argument goes somewhat like this: according to the relativity of simultaneity, for any future event E relative to observer A and space-like separated from her there will be a second observer B coincident with A (though in motion relative to A) such that E is present (hence real) for B. More troubling still: events causally connectible with A (lying in the future light-cone or “absolute future” of A) will be present (hence real) for some observer B who is present (hence real) for some observer C coincident with A (though in motion with respect to A). Thus, events in my absolute future must be considered real in the same sense as present events.
 For a more systematic overview of the issue becoming in relation to modern physics, see Savitt 2001.
 Bergson 1999, 44. This idea that the time dimension is not on an equal footing with the three spatial ones is not controversial and is assumed by most presentations of relativity theory. It is essential for understanding the difference between the Einstein-Minkowski four-dimensional framework and the ordinary four-dimensional representations of classical mechanics (d’Alembert and Lagrange already referred to time as a “fourth dimension”). The introduction of a limiting velocity for the propagation of causal influences as a structural feature of space-time induces a particular orientation and polarity on an otherwise homogeneous topological manifold (this is of course best expressed in the “conical order” defining the metric of relativistic “space-time,” which the usual Minkowski diagrams nicely capture).
 Whitehead coins the expression “pan-physics” for what he believes is “not even metaphysics.”
 See Herstein 2005, 132.
 Whitehead insists that “simultaneity must not be conceived as an irrelevant mental concept imposed upon nature” (CN 53).
 Einstein himself originally frames the problem as one of defining a common time for distant observers. Whitehead points to the underlying “Newtonian” assumption of the availability of an inertial frame.
 See Northrop 1951, 193ff.
 Bergson 1999, 32 ff.
 Things are of course more complex because Bergson’s original characterization of space as an ideal scheme receives qualifications in each of his subsequent books. Besides the ideal geometrical space (“espace”) of the Essay (1889), which inherits from the Kantian critique its formal or transcendental character, Matter and Memory (1896) makes room for a concrete, intuitive meaning of extension, as well as for a pragmatic space (“étendue”) related to the vital needs of the organism. In Creative Evolution (1907), Bergson grants a genuine metaphysical status to space and matter as the “inversion” of duration, bringing another twist to the discussion. The treatment of simultaneity in Duration and Simultaneity (1922) can therefore be viewed as the climax of an ongoing reflection on the meaning of spatial concepts in the context of a philosophy of duration.
 Bergson 1999, 35.
 Bergson 1999, 32.
 Bergson 1999, 38.
 “[T]he time-system actually observed is that one for which (roughly speaking) our body is at rest” (R 8). Alan White emphasizes the connection between cogredience and this idea of inertial rest frame as a key to the understanding of Whitehead’s discussion of relativity theory (White 1983). Inertial frames really exist in nature as kinetic perspectives, they pre-exist actual measurements and account for the homogeneity of spatio-temporal relations (CN 193; see PNK 31-32). Simultaneity requires a spatial spread of events in a given inertial frame as defined by a given duration. It arises from the experience of a real togetherness of events, but it is spatial extensiveness (the spatial relation of co-presence) that serves as the basis for simultaneity relations, and it always does so within a given inertial frame. This was already implicit in the quote from PNK given earlier in Section 1.a, with its reference to “the whole space of a consentient set in the Newtonian group.” Whitehead’s disagreement with Einstein over the status of special relativity within a generalized theory of relativity revolves around the issue of inertial frames: Einstein’s general relativity theory, prohibiting as it does the use of global simultaneity frames, cannot be reconciled with the given facts of experience as to simultaneity and spatial arrangement (SMW 122). As for Bergson, as one may have suspected, he is inclined to consider systems of reference as mere artificial constructs. In his view, they fulfill a coordinating function in the computation of spatio-temporal measurements but cannot count as real in the same sense as durations. On this account, he is surprisingly closer to Einstein than Whitehead.
 George Herbert Mead was very keen on this idea. See his 1927 essay entitled “The Objective Reality of Perspective,” republished as a supplement to Mead 1991; cf. Mead 1938, 114ff, 215ff. Mead’s commentaries on Whitehead’s philosophy of space-time provide subtle, if somewhat idiosyncratic, insights regarding simultaneity (see “Fragments on Whitehead” and “Fragments on Relativity” in Mead 1938, especially 573ff).
 Explaining in detail how Whitehead manages to derive the uniform Euclidean structure of spatial relations from the multiply intersecting time-series and their respective durations, is beyond the scope of this essay. Central to Whitehead’s deduction is the idea that “The extended space of one time-system is merely the expression of properties of other time-systems” (R 54). See Mead 1938, 530ff.
 When it comes to defining a relation on the basis of the concept of simultaneity, Whitehead does not say that events or entities themselves stand in simultaneity relations. He says that they can be “simultaneous with [a] duration” (CN 53), namely the whole of nature of which they are parts.
 The word “slab” indicates that durations of unlimited spatial extent have finite temporal thickness (PNK 111; CN 53, 56, 106-107). Only through the method of extensive abstraction do the ideal instantaneous spaces of physical theory find their proper place within the scheme of durations.
 The constitutive character of simultaneity with respect to duration is acknowledged in SMW, in the context of a new theory of the epochal character of time: “Thus an event in realising itself displays a pattern, and this pattern requires a definite duration determined by a definite meaning of simultaneity (SMW 124). This strongly suggests that without simultaneity durations would indeed be indefinite, and thus ineffective, as far as the realization of patterns in given events are concerned.
 See Palter 1960, 140-46. This new context for the understanding of co-presence was in fact already apparent in chapter two of The Principle of Relativity. Whereas CN defined co-presence as a dyadic relation holding between two events in one instantaneous space or “moment”(CN 177), it is defined in R as a relation between a given event and the whole four-dimensional region separating its causal past and its causal future (R 30). See Capek 1957.
 See Hammerschmidt 1947, 65-67. On difficulties related to the presentational immediacy of contemporary occasions (the fact, for example, that what is presented to us as now is already past), see Ferré 1986, 110 ff. On the non-transitivity of “unisons of becoming,” see Hurley 1986, 107-108, and the comments by Miller 1986, 115-17. These difficulties are easily dealt with if one adopts a local interpretation of temporal facts. For that reason, Niels Viggo Hansen considers that the issue of simultaneity is not fully expressed before PR. Only then does Whitehead give a genuinely causal and local account of time, thus making room for relations of co-presence freed from the ideal of global simultaneous presence (Hansen, 2004).
 A simple diagram suffices to show that there is a sudden shift in simultaneity relations occurring at the point where the traveling twin instantaneously accelerates by virtue of a mere change in direction. I have discussed elsewhere the details of this elegant solution (During 2006).
Works Cited and Further Readings
Bergson, Henri. 1991 . Matter and Memory, translated by N.M. Paul & W.S. Palmer (New York, Zone Books). [1959. Matière et Mémoire in Oeuvres edited by A. Robinet (Paris, Presses Universitaires de France)].
Bergson, Henri. 1999 . Duration and Simultaneity, translated by L. Jacobson (Manchester, Clinamen Press). [1972. Durée et Simultanéité, in Mélanges, edited by A. Robinet (Paris, Presses Universitaires de France)].
Čapek, Milič. 1957. “Note about Whitehead’s Definitions of Co-Presence,” Journal of Philosophy, 24, 1, 79-86.
Čapek, Milič. 1971. Bergson and Modern Physics (Dordrecht, D. Reidel).
During, Elie. 2006. “Philosophical twins? Bergson and Whitehead on Langevin’s Paradox and the Meaning of ‘Space-Time’,” in Les Principes de la connaissance naturelle d’Alfred North Whitehead—Alfred North Whitehead’s Principles of Natural Knowledge, edited by G. Durand and M. Weber (Frankfurt, Ontos Verlag).
Gödel, Kurt. 1990 . “A Remark about the Relationship between Relativity Theory and Idealistic Philosophy,” in Demonstratives, edited by P. Yourgrau (Oxford, Oxford University Press).
Hansen, Niels Viggo. 2004. “Spacetime and Becoming: Overcoming the Contradiction between Special Relativity and the Passage of Time,” in Physics and Whitehead: Quantum, Process, and Experience, edited by T. E. Eastman & H. Keeton (Albany, SUNY Press).
Herstein, Gary L. 2006. Whitehead and the Measurement Problem of Cosmology (Frankfurt, Ontos).
Mead, George Herbert. 1938. The Philosophy of the Act (Chicago, University of Chicago Press).
Mead, George Herbert. 2002 . The Philosophy of the Present (Amherst NY, Prometheus Books).
Northrop, Filmer. 1951. “Whitehead’s Philosophy of Science,” in The Philosophy of Alfred North Whitehead, edited byArthur Schilpp (La Salle IL, Open Court), 165-207.
Palter, Robert. 1960. Whitehead’s Philosophy of Science (Chicago, University of Chicago Press).
Savitt, Steven F. 2001. “Being and Becoming in Modern Physics,” in Zalta, Edward N. (ed.), The Stanford Encyclopaedia of Philosophy (http://plato.stanford.edu/archives/spr2002/entries/spacetime-bebecome/).
White, Alan Villard. 1983. “Whitehead, Special Relativity, and Simultaneity,” Process Studies, 13, 4, 275-85.
Whitehead, Alfred North. 1923. “The Problem of Simultaneity,” Relativity, Logic, and Mysticism (Aristotelian Society Supplementary Volume III), 34-41.
CIEPFC, École normale supérieure, Paris
How to Cite this Article
During, Elie, “Durations and Simultaneities: Temporal Perspectives and Relativistic Time in Whitehead and Bergson”, last modified 2008, The Whitehead Encyclopedia, Brian G. Henning and Joseph Petek (eds.), originally edited by Michel Weber and Will Desmond, URL = <http://encyclopedia.whiteheadresearch.org/entries/thematic/sciences/relativity-physics/durations-and-simultaneities/>.