The argument from the relativity of simultaneity

The "argument from the relativity of simultaneity": does special relativity favour a B-theoretic, eternalist view of space and time ? David Hirst, 2008

1. Questions of time At the risk of oversimplifying a dynamic and highly complex field of investigation, it is reasonable to say that four1, closely-related questions have provoked much of recent debate in the philosophy of time: I.

Are other times are as real as other places, or are only the present and the things of the present real?

II.

Is there an objective "flow of time"? Do the properties of 'being past', 'being present' and 'being future' represent some fundamental feature or characteristic of the world, or can they be reduced to an atemporal sequence of 'earlier than – later than' relations?

III.

Is tense an irreducible characteristic of language, or can the truth conditions of tensed utterances be given by atemporal propositions?

IV.

Do material entities persist through time by being wholly present at each instant of their existence, or are they extended in both space and time?

While certain terminological confusions persist, and there are variations of each view, much of contemporary discussion has crystallised around the following pairs of opposed views: 1.1 The ontology of space and time: presentism vs. eternalism2 Presentism is the view that only the present, and present entities, are real; the past and the things of the past no longer exist, and what reality the future has it has – at most – in potentia. Generally speaking, presentists hold that there is an absolute present, common to all observers, and that there is an objective flow of time from the future to the past (the absolute future becomes the 1

There is a fifth area: the debate between those who hold that spacetime is itself a substance, independent of the entities it contains („substantivalism‟), and those who hold that spacetime is no more than the relations between material entities („relativism‟). While special relativity tends to favour the latter position, certain commentators argue that spacetime relativism is incompatible with general relativity ; however, and given that the debate is beyond the scope of the present discussion, I shall set the subject aside for later comment.. 2 Noted four-dimensionalists (eternalists) include Russell, Smart, Quine, Lewis and, more recently, Yuri Balashov, Craig Callender, Kathryn Hawley and Ted Sider. Among more recent three-dimensionalists (presentists) are Sally Haslanger, Brian Lombard, David Mellors, Trenton Merricks, Michael Rea, Peter van Inwagen and Dean Zimmerman.

absolute present, and the absolute present becomes the absolute past). While not all presentists are three-dimensionalists, many (perhaps the majority) hold that there is a real distinction of kind between time and space; on this view, the three-dimensional universe progresses through time by being entirely present at each successive instant (see 'endurance' below). Presentists usually hold that their view is preferable to eternalism as it conforms more closely to our phenomenal experience and to our commonsense intuitions concerning the nature and the passage of time3. Eternalism is the opposed view: past, present, and future are at best local distinctions, and have neither absolute scope nor objective reality. Many contemporary eternalists hold spacetime to be four-dimensional: other places and other times are other regions of spacetime, and spacetime is an atemporal four-dimensional 'block' in which no time is ontologically privileged - indeed, it has become almost customary for both its admirers and its detractors to introduce the eternalist view by saying that it holds "other times to be as real as other places". Notwithstanding, and apart from accepting that past, present, and future entities are all equally "real", the eternalist need make no assumptions about the way material entities occupy spacetime (and more specifically, about how they occupy the temporal dimension – see 'endurance' and 'perdurance' below). Many eternalists argue that, as spacetime is an atemporal four-dimensional block in which there is no absolute distinction between past, present, and future, there is no objective flow of time. Thus, on these eternalist views, there is no 'absolute becoming' (in the sense that events generate or bring into existence other events); events are situated in time much as objects can be situated in a three-dimensional space, and the interval between successive events closely resembles the spatial separation between distinct material objects. There is a third „intermediate‟ view, often called the „growing block theory’. On this view, the present and the past are real, while the future is indeterminate. However, and over and above certain specific objections4, the growing block is also subject to many of the criticisms levelled against both presentism and eternalism, and has fallen out of favour over the last few years. It has few, if any, contemporary supporters5. 1.2 The order of events and the flow of time : the "A-series" and the "B-series"

3

While I shall set the question aside in the present article, we can ask whether presentism‟s supposed “compatibility with phenomenal experience” is indeed borne out by the classical presentist model of time. 4 Cf. Merricks, 2006. 5 The view was notably defended by Broad; more recent proponents include Michael Tooley and Peter Forrest.

The terminology of the debate about the ordering of events in time is derived from McTaggart‟s (1908) article “The Unreality of Time”: the A-series is “that series of positions which runs from the far past through the near past to the present, and then from the present through the near future to the far future, or conversely” while the B-series is the "series of positions which runs from earlier to later, or conversely". A-theorists hold that the monadic „A-properties‟ of being past, being present, and being future are fundamental and cannot be reduced to the „B-relations‟ of being earlier than, being later than, and being simultaneous with. B-theorists generally hold that A-properties can be reduced to B-relations. While the eternalist-presentist opposition is a debate about ontology (is the domain of existential quantification restricted to present entities, or does it include future and past entities?), the Aseries/B-series opposition is a metaphysical debate concerning the objective reality of the passage of time. The A-theorist holds that there is an objective flow of time from the future to the present and into the past – indeed, the A-series can be seen as describing this flow. An event is future, becomes present, and then enters the past: it is successively future, present, and past6. 'Being past', 'being present' and 'being future' are properties of events and are thus not relative to anything other than the ever-moving 'now'. For the B-theorist, B-relations are permanent: if an event e is earlier than some other event f, then e is always earlier than f; the pairwise ordering of events is atemporally fixed, and static. While a large majority of presentists are A-theorists and a large number of eternalists are Btheorists, there is no direct requirement that the one position should entail the other. B-theoretical presentism is uninteresting, as it makes the trivial reduction of 'being past' to 'before now', of 'being future" to 'after now', and of 'being present' to 'simultaneous with now' – and these just are the A-properties of 'being past' etc. A-theoretical eternalism is interesting, as – according to certain of its defenders7 - it allows for an ontologically four-dimensional universe with a metaphysically-privileged flow of time. 1.3 The status of verbal tense : serious tensing vs. detensing The A-series/B-series opposition is frequently associated with the debate between "serious tensers", who 'take tense seriously' and who believe that statements or utterances are "irreducibly tensed", and "detensers", who believe that the truth conditions of tensed utterances can be given 6

McTaggart famously overlooked the sequential nature of A-properties when he identified as a 'contradiction in our perception of time' that all events "exemplify all three of the properties of the A-series" (being past, being present, and being future). All physical events exemplify these properties, but neither simultaneously nor with respect to each and every other event. 7 For example, Craig, 2000a and 2000b.

by atemporal or untensed propositions indexed to a time8. Thus, the serious tenser holds that atemporal propositions are derived from tensed utterances (and that the tensed form is therefore fundamental), while the detenser holds that the tensed form is a local formulation that depends semantically on an atemporal proposition (the atemporal form thus being fundamental). A "tensed sentence or utterance" is one in which the verb form implies a particular vantage point in time; this vantage point is usually labelled with the adverb "now" - the 'time of speaking'. Indeed, when teaching or presenting verbal tense, we usually give the following timeline as a basic representation:

Events occurring, or states of affairs obtaining, before 'now' are reported by use of a past form; those occurring or obtaining simultaneously with or around 'now' are reported by a present form; and those supposedly occurring or obtaining after 'now' by a future form. The prevalence of such timelines suggests that figure 1 is probably a reasonable approximation of a certain aspect of our "naïve understanding of time"; more specifically, it seems a fair approximation of the way events or states of affairs are held to 'occupy' time - and this is borne out by the parallel use of timelines in the representation of history (incidentally, similar timelines have been used in philosophical accounts of events in time – cf. Thomson, 1971). The timeline in figure 1 differs from those generally used in the representation of historical sequences as it projects from the present into the future; in figure 1 the distinction between the „known‟ past and present and the „unknown‟ future is represented by the transition from a solid to a dotted line (this distinction is given using various schema; nonetheless, most if not all schema make a common distinction between the „known‟ past and present and the „unknown‟ or „projected‟ future). While we should be wary of drawing any too-hasty parallels, it‟s hard not to notice the similarity between figure 1 and the “growing block theory” – the view that the past and present „exist‟ or „are real‟ while the future is at best a set of as-yet unrealised eventualities.

8

There are two classic versions of the detensing view – the „token-reflexive theory‟ and the „date theory‟. On the token-reflexive theory, a token utterance is true iff there are B-theoretical relations obtaining between the token utterance and the event or state of affairs mentioned by or designated by the token. On the date theory, an utterance is true iff there are B-theoretical relations obtaining between the time at which the utterance is produced and the (time of the) event mentioned in the token.. For a fuller discussion, see Torre, 2007.

The parallels between verbal tense and both the A- and the B-series are easy enough to see. The distinction between past, present, and future is clear and - as the dotted line in figure 1. indicates – 'now' appears to proceed through time as the future becomes the past; in these respects, verbal tense resembles an A-series. On the other hand, past, present, or future events or states of affairs are described relative to some other event (that of 'speaking now'); this can be understood as expressing a B-relation obtaining at the time of speaking. Past events are positioned 'earlier than' the time of speaking, present events are 'simultaneous with' the time of speaking, and future events are 'later than' the time of speaking. Although A-theorists are sometimes described as "taking tense seriously", we should be careful to distinguish between the debate opposing Atheorists to B-theorists, and that opposing serious tensers and detensers9. As I remark above, the opposition between A-theorists and B-theorists is a metaphysical debate about the objective reality of the flow of time from future to past; the opposition between serious tensers and detensers is a semantic debate over - as Sally Haslanger10 remarks – "what is the case about, or what is true or false about what exists". For the serious tenser, "to say that something exists, or walks, or is red, is to say something about how it is in the present; and statements about what is, was, or will be the case, cannot be reduced to statements in a “timeless” idiom. The possibility or impossibility of such reduction may depend on whether „now‟ is irreducibly indexical". 1.3.1 Adverbial modification – the irreducibility of 'now'? Tensed sentences are frequently qualified by adverbs such as "now", "yesterday", or "tomorrow", or by adverbial phrases such as "in the 20th century" or "ten years ago". Certain of these "timewhen" adverbials are deictics (indexical or 'token-reflexive' expressions) and depend on the speaker's "temporal perspective"; others relate the event or state of affairs to a given semiobjective timescale (we could say that they date the event). If we take two examples: a. Julius Caesar was assassinated 2,052 years ago b. Julius Caesar was assassinated on March 15th 44 BC. (b) is true for all times after March 15th 44 BC, while (a) has only been true since March 15th 2008 and will only remain true until March 15th 2009. (b) is dated – fixed relative to one particular point in space and time - while (a) is indexical. Just as expressions containing "I",

9

Cf. the long discussion of the distinction in the first part of Zimmerman, 2005. Haslanger, 2003.

10

"here", or "this" are only true when spoken in certain circumstances, (a) is only true when spoken during a certain period of time. Similarly, an 'occasion sentence' is only true when spoken at a particular 'point' in time – to take an example, "it's raining" is only true iff it is raining in the vicinity of the speaker at the time of speaking – iff it is raining in the speaker's "here-and-now". Much of the opposition between serious tensers and detensers can be traced to a debate over the irreducibility of "the speaker's here-and-now" – though it is more usual to frame the debate in terms of the irreducibility of 'now'. It's not really clear whether the debate has metaphysical import in itself, or whether its metaphysical scope is subordinate to certain other assumptions about time – and this is borne out by attitudes towards 'now'. The four-dimensionalist eternalist will consider that talking of the speaker's 'here-and-now' better captures the nature of existence in space and time than does talking solely of the speaker's 'now'; the three-dimensionalist presentist will argue the contrary – that there is a fundamental and motivated difference between 'here' and 'now', and that 'now' indicates a particular ontological status. Similarly, the B-theorist will point to invariable 'earlier than/later than' relations obtaining between any particular 'now' and the events in its local past and future, while the A-theorist will insist that the relations are fundamentally variable – what is future becomes 'now', and what is 'now' becomes past. The original, reductionist version of detensing11, which required that types of tensed sentence should be translated into an atemporal idiom, has been largely superseded by the more modest requirement that we be able to give tenselessly the truth conditions of a tensed sentence-token. This is simple enough in those cases where the sentence token is 'adverbially dated' (as in the case of sentence (b) above), and no more difficult where the index can be resolved to a particular period (as is the case with sentence (a), which can be resolved to "is true between March 15th 2008 and March 15th 2009"). However, the detenser must assume that any 'now'-based index can in principle be reduced to a place and a time. If we remember, we gave the truth conditions of the sentence token "it's raining" as follows: c. "It's raining" is true iff it is effectively raining in the vicinity of the speaker at the time of speaking12 When faced with such a sentence token, the detenser can only realise his project if he can identify the speaker and the time of speaking; he takes it that, for any true instance of the token "it's raining", the associated sentence 11

Cf. Quine, 1960. For the sake of economy, I take only those situations where the speaker is commenting on local conditions, and overlook the possibility that she is reporting (for example) information received from a distant location. 12

d. "It's raining" is true iff it is effectively raining in the vicinity of S at t can in principle be satisfied by an individual and a particular time. If the speaker or time of speaking are unknown, he might argue that we cannot determine the truth-value of the utterance at all (as might be the case if one overheard an unknown voice saying "it's raining" on a bad telephone connection – one would know, more or less, at what time it was raining, but not where the speaker was). Past and future tensed sentence tokens can similarly be satisfied in principle by a "time of speaking"; likewise, if the time of speaking is unknown, we cannot (in certain cases) determine their truth value. The serious tenser, on the other hand, holds that there is something about 'now' that cannot reduced to "a particular, dated location in time and space": either absolutely or locally, there is for him some quality of 'now' that escapes the 'at t - earlier than t – later than t' B-relations of the detenser. The absolutist about 'now' is likely to be either a presentist or a growing block-theorist; as such, his view stands or falls with presentism or the growing block. However, a more 'localised' view is available: that of temporal perspective13. We can ask whether, while accepting the four-dimensional picture of time, we might perhaps allow that some point, or series of points, in time is metaphysically privileged – to all intents and purposes, the 'moving present'. A strong version of this, which would seem to call for an absolute and universal 'present', has been called the 'moving spotlight' view14; such a view poses the obvious question of why one particular instant in spacetime should thus be privileged (and is therefore open to the objections to both the presentist and the growing block views). A weaker view – but one which is perhaps more defensible - is that it is metaphysically significant for the speaker – from the speaker's unique perspective in time (and space), past and future events are literally 'not present' in the sense of 'not being in the speaker's immediate vicinity at the time of speaking'. This is perhaps no more than an accommodation of the 'empirical fact' of our intuitive experience of time: we would certainly seem to be faced with the fait accompli of at least a subjective and intuitive sense of the passage of time. As far as the serious tensing-detensing debate is concerned, it would seem that there are two cases in which the argument from the relativity of simultaneity might have an impact: firstly, in militating for or against the metaphysical assumptions which underlie a given position (presentism or eternalism; A-properties or B-relations); and secondly, by militating for or against

13 14

Cf. Zimmerman, 2005; also Sider, 2001, pp 17-18. See Sider, 2001, pp. 17-25 for a full discussion.

some locally-determined 'privileged instant of speaking' relative to which a 'locally irreducible' A-series of events is apparent to the speaker. 1.3 Persistence through time: endurance vs. perdurance Presentism and eternalism are views about the status of entities in time; the A-theory and the Btheory are views about the passage of time; and serious tensing and detensing are views about what is true about entities in time. Endurance and perdurance are views about how entities persist through time. Endurantists hold that material entities have no temporal extent; that they exist in their entirety at any one instant; and that they persist through time by being wholly present – by having 'all their parts' - at each instant of their existence. Thus, on endurance, three-dimensional objects have spatial parts, but no temporal parts. Perdurantists hold that an entity persists through time by having temporal parts at different times; most also hold that entities are extended in time much as they are extended in space - Ted Sider (2001) remarks that, while eternalism is the view that spacetime is ontologically fourdimensional insofar as other times are as real as other places, perdurantism is the view that material objects are mereologically four-dimensional, insofar as they have temporal as well as spatial parts. More recently, a distinction has arisen within the camp of those who favour perdurance between „worm‟ theorists and „stage‟ theorists. The „worm‟ theory corresponds to the traditional perdurantist view; on this view, physical entities are four-dimensional wholes (spatiotemporal „worms‟) occupying determinate regions of spacetime. For stage theorists (notably, Ted Sider or Katherine Hawley), parts or stages of four-dimensional entities are themselves the fundamental entities of the perdurantist ontology; parts or stages at other times are "counterparts" of a stage at a given time. As with the question of tense, there is no general consensus to whether arguments drawn from special relativity have any impact on the question of persistence itself 15; nevertheless, such arguments may well militate against assumptions underlying a preference for one account or the other. 15

For example, Balashov, 2002, argues that relativity favours the 'worm' over the 'stage' view, while Gibson and Pooley, 2006, argue that we can perfectly well defend the notion of 'relativistic endurance'.