Presentism is the metaphysical view that “only presently existing things exist”1. While this view is coherent with Newton’s theory of spacetime, it faces some challenges when combined with the Theory of Special relativity (STR). In this essay, I will first articulate more specifically the thesis of presentism and show the main arguments of the opposite position known as eternalism. I will then outline the special theory and its consequences for the metaphysics of presentism. I will consider some of the problems that presentism faces in the relativistic context by considering Putnam’s “man on the street” argument.
From there I will address some of the solutions to these problems proposed by presentists. Ultimately I look to demonstrate that it is impossible to coherently accept both the physical theory of special relativity and the metaphysical theory of presentism; and that of these two, special relativity has far superior grounds for being accepted than the metaphysical view of presentism. Presentism privileges one instant of time which is the present. The main thesis can be formulated as “what exist is what exists now”.
This thesis should not be read as being in tensed language because if read in that way, it would be onsidered as trivially true (Saunders 2002). This distinction between tensed and tenseless time can be made clearer by introducing Mactaggart’s distinction between A and B series. A- series supporters think that events have tenses. Therefore, events should be considered depending on whether they are in the past, in the present or in the future. On the other hand, supporter of the B-series believe that events are tenseless. Events should be classified by the kind of temporal relations that they have with another event.
Such temporal relations can for example be: “being earlier than an event X” or “being later han an event X” (Hincliff, 2000). Presentism implies A-series while eternalism implies B-series. Eternalism in the opposite view to presentism which is based on the idea that there is nothing special about what exists now. According to eternalism there is not a specific privileged time, therefore, the present, past and future should be considered as equally real (Hinchliff, 2000). This distinction between presentism and eternalism can be made clearer by considering Callender’s example.
Consider a four-dimensional system formed of light bulbs which are on when the event exists and off when the event does not exist. According to presentism, only one light bulb will be on because only one present event exists. On the eternalist view, all the light bulbs are constantly on because all the events in time exist (Hinchliff, 2000). In the course of this essay, I will show that between the two metaphysical views of time, eternalism is the only one consistent with the Theory of Special Relativity. In Newtonian mechanics, time and simultaneity are considered absolute.
One point is present to another point if they share the same time coordinate. Therefore, what exists is absolutely simultaneous with some present event. Given this definition, resentism is compatible with Newtonian mechanics, the present simply consists in choosing a certain class of events which count as real. However, presentism is not compatible with the relativistic setting. STR rests on two postulates: The principle of Relativity (physical laws are the same in all inertial frames) and the Light Postulate (the speed of light is constant for all inertial observers).
Two significant consequences of these postulates are, first of all that time is not absolute (i. e. there are different ways in which one can fix the time axis) and secondly that simultaneity is relative. The latter asserts that whether two events are simultaneous is dependent on the choice of the inertial reference frame, different observers will judge different events to be simultaneous with them. Consider two observers A and B, A is stationary and B is moving at a constant velocity along the x-axis.
In a pre-relativistic setting, if we translate the values from A’s reference frame to B’s frame, we won’t need to change the temporal axis because both observers will agree on which events are simultaneous. Differently, in a relativistic setting the plane of simultaneity is not shared between the two bservers in the different reference frames so if we go from B’s moving frame to A’s stationary frame, the space and time axes of B’s frame get compressed with respect to the A’s axes. This length contraction and time dilation taking take place if the frame is moving at a speed near the speed of light.
Therefore, A’s and B’s reference frames are not identical and what is simultaneous for A is not simultaneous for B. This is the point where the conflict between presentism and STR arises. What events is the advocate of presentism choosing when they say that all that exists is what is simultaneous with me? Would they choose the events simultaneous to A or to B? Because there is no privileged way of dividing the world into instants, there is no sufficient justification for why a presentist would choose one set of events over another.
Hilary Putnam argues against presentism as he believes that presentism cannot be consistent in the special relativity framework. He does so by showing that the relations that the presentist takes to relate present points must either apply to all points or only relate each point to itself. His argument goes as follow: All (and only) things that exist now are real. I-now am real. At least one other observer is real, and it is possible for this other observer to be in motion relative to me.
If it is the case that all and only the things that stand in a certain relation R to me-now are real, and you-now are also real, then it is also the case that all and only the things that stand in the relation R to you-now are real (Putnam, 1967). The relation R is required to be equivalent (reflexive, symmetric and transitive), physical, independent of the choice of coordinate system, independent on anything accidental and definable in terms of fundamental physics. This relation R is the relation of simultaneity which shows from premise 1 that only the things simultaneous to me- now are real.
Premise 3 will then be consistent with premise 1 because the relation R of simultaneity is transitive (Putnam, 1967). In other words, if l-now am simultaneous with you-now and you-now are simultaneous with an event X then I-now am simultaneous with the event X. Therefore, everything which is simultaneous with l-now is real. The first premises entail the conclusion and the argument is therefore valid. In a relativistic setting, these requirements are not met. Two different moving bservers will consider different set of events as simultaneous to them.
In other words, there will be some events which are real but are not present at the same time. Consider more than just two moving observers. The third premise states that there is no privileged observers and so that there is a transitivity relation between different observers (if Rxy and Ryz then Rxz). This transitivity would easily hold in the Newtonian mechanics if I only consider my reference frame, because if in my coordinate system X is simultaneous with Y and Y is simultaneous with Z then X will be simultaneous with Z.
In special relativity, however, f an event X is simultaneous with an event Y in the coordinate system of X and if Y is simultaneous with an event Z in the coordinate system of Y then it does not mean that X is simultaneous with Z in the coordinate system of X. The event Z is a future event according to my coordinate system and therefore is already real. In order to make the argument valid and so make premise three consistent with the first premise (i. e. all things that exist now are real) one would have to accept the conclusion that future events are real.
This conclusion leads to an eternalist view of time and rejects presentism as it shows hat both the present and future events are real because they are simultaneous. Thus, Putnam’s argument shows that presentism is incompatible with special relativity and he support and eternalize view of reality. Hinchliff denies eternalism and aims to show that presentism and special relativity are compatible. In order to support presentism, one has to deny or reconsider the features of the R-relation between two events.
A first solution is “point presentism” in which the notion of the present is redefined. Present is no longer considered to be my simultaneity slices but is considered to be relativistic invariant nd is seen as the single point of “here-now”. In other words, the present events are the events which are simultaneous with one observer in a particular coordinate system at a time in the same location. One objection to this is that point presentism is lonely because nothing else exists apart from the “here-now”.
A more well defined solution for relativized presentism is “cone presentism”. On this view, the present, for an observer, are the events on the surface on the past light-cone of that observer. Cone presentism uses a relativistically invariant structure to define the present which means that two observers which are in he same point will both agree on what the light cone at that point is like. Cone presentism is consistent with our intuitions of the present on small scale because the things that a person considers as present lies in the past path of the light.
However, it is considered to be inconsistent with large scale intuitions of the present. This inconsistency is explained by Steve Savitt who affirms that because the surface of the past light cone is considered to be present then cone presentism is claiming that past events are present. This objection is best explained by referring to the cosmic microwave background radiation CMBR). CMBR formed about 15 billion years ago but they have been observed by astronomers in the present. Cone presentism suggests that the radiations are present now because they have been observed now.
But, how can Savit make judgements about how far in the past something happened if there’s no such thing as absolute time? Cone presentism has to be considered in a relativistic setting: statements about the time at which the big bang happened are frame dependent and therefore, if we consider the big bang a fact then we have to make a distinction between the reference frames. Furthermore, differently than hat Savit affirms, cone presentism does not assign the same time to the two different events of the physicist observing the radiation and the actual formation of the radiations.
Such time simultaneity is a feature of presentism. Differently than in presentism, in a relativistic scenario there is the simultaneity of spatiotemporal intervals. Therefore, the two events are defined simultaneous because their spatiotemporal interval is zero. In short, both “point presentism” and “cone presentism” avoid the contradiction that presentism encounters when taken together with the results of Special Relativity. Between the two, “cone presentism” is a stronger case as it provides a valid argument to combine presentism and special relativity.
There are a number of challenges that the theory of special relativity sets to presentism and all these challenges hinge on the consequence of the special theory that there is no relation of absolute simultaneity. Presentism is commonly accepted because is characterized by an intuition that is applicable in the scale of our human experience. At the scale we live at, we do not experience the relativistic effects therefore we approximate our experiences to the pre relativistic Newtonian theory. Nonetheless, at a larger scale presentism fails to capture the truth of reality as STR comes into play.
Even if we do not experience it directly, STR is one of the strongest confirmed physical theories and should be a starting point to build a metaphysical theory which is consistent with the physical one. So, it is better to give up the metaphysics of presentism and to adopt a four dimensional view rather than make attempts to preserve or refine the definition of the present, particularly as it leads to problems with relativizing existence. For these reasons | support the eternalist view as the most successful view as opposed to presentism.