Presentism and dynamic theories of time

     What is there? Since immemorial times philosophers have asked themselves this question, and still, there seems to be no answer or agreement.

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   Indeed, many theories have been proposed. The pre-Socratics thought of the arché or the fundamental being. Plato suggested the World of Ideas. Christianity and other religions have provided their views. Descartes revolutionised philosophy, along with other authors of the Enlightenment. Many new lines of thought were developed during the nineteenth and twentieth centuries. But, among this myriad of possibilities, a few proposals have survived the most: presentism and eternalism, two opposing views.

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    Presentism claims that only present things exist, that the present is all that exists. It must be clarified, however, that it is meant in a tenseless way [1]. Sider has defined it as follows:

Presentism is the doctrine that only the present is real... A presentist thinks that

everything is present; more generally, that, necessarily, it is always true that

everything is (then) present). (1999)

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     Its opposing views are eternalism, where past and future are as real as the present, and they all exist in an unprivileged way, and growing block theory argues that past and present are real, whereas the future is not. Presentism has also been defined as a dynamic theory of time, while eternalism is a static theory of time. The reason why is trivial: in presentism, events and things move or change from the future into the present and from the present into the past, whereas in the eternalist standpoint, events exist and are related to others as earlier than or later than, but this position never changes. There are, indeed, many metaphysical arguments both in favour and against each position. Nevertheless, the principal objections against presentism now come from the Special Theory of Relativity (STR, from now on).

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    Throughout this essay, the tension between presentism and STR is explained. Putman’s main argument in favour of STR is developed, along with some responses. The costs of each objection are exposed, and at the end of the essay, a modified version of presentism that agrees with STR is suggested.

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      Firstly, the origin of the conflict between presentism and STR should be clarified. In his paper “How relativity contradicts presentism”, Saunders introduces the topic as follows:

“If presentism is a thesis about ontology, and says that existence consists of a

three- dimensional spatial reality; if, in elaboration with this thesis, it opposes the tenseless view of time, and denies that talk of events as past, present or future is elliptical talk about the relation of events to our momentary selves: then it contradicts special relativity. It contradicts it in the sense that it implies that special relativity is badly deficient as a fundamental theory of the world.” (Saunders 2002)

    There are two main points where they clash. Firstly, presentism and STR hold opposing views regarding simultaneity. For the presentist, all that is present is simultaneous, whereas relativity rejects absolute simultaneity. Secondly, presentism assumes a privileged frame, the present itself, in contrast to STR, where all inertial frames are equally valid. Both points are highlighted in Putnam’s argument.

   

    In 1967, Hilary Putnam published a paper suggesting how presentism was contrary to special relativity. After showing that presentism was false, he concluded that the dilemma had then been solved and that, in his own words: “The problem of reality [...] is now solved. Moreover, it is solved by physics and not by philosophy” (Putnam 1967). He tried to show how relativity was incompatible with presentism and that the latter should be rejected. Despite his efforts, it seems it was not the end of the story. Stein’s response made it clear that his arguments were not strong enough, and Hinchliff, among others, suggested interesting alternatives. In what follows, Putnam’s argument is exposed. Later, the more common objections are explained, along with the one I find more convincing.

       

       Putman starts by posing what he calls “the man on the street’s view” (Putnam 1967): All (and only) things that exist now are real. Future things will be real, while past things were once real. And makes some assumptions:

1. I-now am real.

2. There are more observers, at least one more. And these observers could be in motion relative to me-now.

3. There are no privileged observers. If everything and only what is simultaneous to me-now is real, and you-now are simultaneous to me-now, then anything that is simultaneous to you-now will be simultaneous to me- now too. 

 

  Putnam states the third assumption about some relation R, instead of simultaneity straightforwardly. This relation must be, for absolute simultaneity, reflexive, symmetric and transitive. Where reflexive means that I am simultaneous with myself; symmetric implies that if you are simultaneous to me, then I am simultaneous to you; transitive denotes that something simultaneous to me will also be simultaneous to you, given that we are both simultaneous with each other. More technically, the relation R must obey the following scheme: Rxx; if Rxy, then Ryx and if Rxy and Ryz, then Rxz.

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      Such a relation would perfectly match Newtonian mechanics and understanding of time and space. And more importantly, this viewpoint is what presentism seems to suggest. Absolute simultaneity is granted since absolute space and absolute time are essential to classical mechanics. However, it is straightforward that absolute simultaneity cannot be in more disagreement with STR. What relativity denies is precisely such an absolute simultaneity through Minkowski space-time diagrams (see Figure 1 and 2).

       

       Therefore, in a relativistic scenario, assumption three must be restated to the form of simultaneity in the observer’s reference frame. Now, it can be seen how it adjusts to the no-privilege principle of STR. Some contradiction is, however, expected. Say event x is simultaneous to me-now and, as initially, me-now and you-now are also simultaneous, though moving at relativistic speeds relative to the other. According to STR, events that are simultaneous in my reference frame may, and some will, be in the past or future in your frame. Hence, event x is simultaneous to me-now (i.e., present in my reference frame), but at the same time, it is in the future of you-now. At the same time, due to the third assumption made above, if me-now and you-now are simultaneous and event x is simultaneous to me-now, then event x must also be simultaneous to you-now (i.e., if Sxy and Syz, then Sxz). We see an apparent contradiction. Event x is both present and future about you-now. Note that any variation of the case example still applies and for any number of observers too. Hence, it seems possible that everything is past, present and future for every observer. But this sounds familiar; it is what was earlier defined as eternalism. Thus, presentism is false. In Savitt’s own words: “[Presentism] cannot even be expressed in Minkowski spacetime” (Savitt 2000).

     

      Once Putnam’s argument has been analysed, let us now get engaged with its various objections. Firstly, objectors to Putnam’s second premise (i.e., that there is at least another observer) have proposed what is referred to as point presentism. The point presentist will argues that what is present is the here-now. Stein seems to hold a close view when he argues that “an event’s present is constituted by itself alone” (Stein 1968). Point presentism appears to restrict the notion of simultaneity. A spatial limitation is added to the temporal one. Moreover, it restricts simultaneity to oneself (as there can only be one observer here-now). It is for this that it has been accused of lonely and unrealistic. And I take this objection; in my view, we should aim for a broader notion of simultaneity rather than a limited one if we intend to fit it with STR.

 

    Additionally, point presentism implies some controversial and difficult-to-accept effects. In the lightcone diagram, events can be space-like separated (in the hypersurface of simultaneity), time-like separated (inside the lightcone, in both directions) and light-like separated (where events lie just on the surface of the lightcone). Consequently, as there is only one here-now but multiple here’s and now’s – those different events space-like and time-like separated – events that are never in the present (i.e., almost every event) may, however, be in the past (i.e., inside the lightcone), as the past is somehow broader or more open concerning the spatial dimension. Both objections are on their own, enough to disregard point presentism.

 

     Secondly, it has been denied that the present should be considered what is simultaneous. Cone presentism, proposed by Godfrey-Smith and adopted by Hinchliff, offers the view that in STR, what is real is what one sees, i.e., the past lightcone surface [2]. It, therefore, identifies the present with an invariant of STR, which is a good indicator that we are working in the right direction. It is not free of objections. Again, Savitt suggests that it brings about some consequences that “should make a red- blooded presentist squirm” (Savitt 2000). He is, indeed, justified, to some extent, in saying this. According to cone presentism, Savitt explains, the origin of the Cosmic Microwave Background Radiation, which occurred around 15 billion years ago, is as real and present as the physicist detecting it on the Earth. Despite the initial stupefaction, one can counterargue. I will follow Hinchliff’s line of thought. On the one hand, if one is to agree with STR, then one would have no problem in saying that there is no privileged inertial frame – i.e., no privileged present. And thus, there it should not be scandalous to say that the origin of the CMBR is as present as one’s observations from the Earth3. On the other hand, one could drop STR and be a presentist in its traditional meaning. In both cases, this reply falls short of tackling the issue.

     

     A second objection, again rosed by Savitt, confronts double-cone presentism. It seems unfair to define the present as the events that lie on the past lightcone surface only and not to include the future lightcone surface. But there are good reasons to deny double-cone presentism and still hold to cone presentism. The main one is the time asymmetry between the future and the past and the direction of the light signal. As Hinchliff argues, it is very different for an event to be in the past lightcone surface, from where light signals are emitted, from being in the future lightcone surface, where light signals are received. Furthermore, it involves “the asymmetric nature of causation itself

2 See Figure 3. Also for a clearer understanding of cone presen3sm.

3 Note that it is not being said that the origin of CMBR is simultaneous to current observa3ons made on the Earth. But rather that the space3me interval between both events is 0.

[...] on which to rest the distinction between cone and double-cone presentism.”

(Hinchliff 2000)

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    Despite the successful replies, cone presentism still has some battles to fight. The achronality principle presents an obstacle. For clarification, the achronality principle states that if an event is in the absolute past or the absolute future, this is incompatible with being present in any inertial frame. Once more, objectors to cone presentism should be more flexible with it. If one wants to adapt presentism to STR, one should be aware that STR states that “[...] space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality” (A. Lorentz, A. Einstein, H. Minkowski and H. Weyl 1923). Hence, presentism, in the form of cone presentism, should be allowed to refer not just to time but to spacetime. Consequently, what the achronality principle claims is valid for a temporal interval but not for a spacetime interval, which is invariant and, in this case, 0. The last of the objections against cone presentism appeals to the initial properties assigned to the relation of simultaneity: symmetry and transitivity. Cone presentism does not account for them. Event P’s past lightcone surface, where event Q lies, is very different to event Q’s past lightcone surface, where obviously event P does not lie (as it is in the future lightcone surface). Similarly, the past lightcone of event P is different and does not include the past lightcone of event Q (when Q is in P’s past lightcone), i.e., no transitivity.

     

     Last but not least, there are a few more ways to tackle Putnam’s arguments. What Brading calls presentism as an empirical hypothesis offers a dynamics-centred present extended in time and space, in contrast to a now or here-now present. The Neo-Lorentzian approach seems more appealing. Contrary to all of the above, it supports the idea of fitting STR into presentism and not the other way around. It challenges the No-Privileged-Observer Principle appealing to the Lorentz-Fitzgerald contraction in measuring instruments to account for the constant velocity of light. Thus, it is a combination between Newtonian principles and relativistic mechanics that correctly predicts what STR predicts. Still, no further evidence supports it and it has to face Ockham’s razor. Both standpoints have not been further developed in the essay as they offer no conclusive solutions – or at least not comparable to point presentism and cone presentism – and the scope of this essay is limited.

 

     In summary, Putnam has indeed shown that our classical conception of presentism cannot hold when faced with the Special Theory of Relativity. The old concepts involved in presentism: temporal simultaneity and its characteristic properties of reflexiveness, symmetry and transitivity, are in contradiction with the relativistic principles of no privileged observer and no absolute simultaneity. The most effective approach to the issue comes in hand with cone presentism, a variation of presentism that refers to spacetime rather than just to time. A variation that has, therefore, been able to adjust to the new relativistic demands. Other viewpoints, such as point presentism, presentism as an empirical hypothesis and the Neo-Lorentzian approach, seem somewhat contradictory, unconvincing, or lack support.