The Asymmetric Nature of Time

My PhD dissertation is mainly related to the metaphysics of time and the philosophy of science. It is devoted to the study of a basic intuition we have regarding the nature of time: the future is open, whereas the past is fixed. There are many ways in which this intuition may manifest itself. But, as a first approach, it suffices to think of some future and past events. For example, whereas it seems unsettled whether there will be a fourth world war, it is settled that there was a first world war. In that sense, whereas nothing (in the present or the past) a priori predetermines that a large-scale armed conflict will blow up in the future (it is ours to prevent such a disaster!), there is nothing we can do to prevent WWI from having taken place in the past. Likewise, whereas it seems that behaving in an environmentally responsible manner may prevent some animal species from extinction, there is nothing we can do to bring back the dodo birds. It is however worth noting that, although the question of human abilities (what we can or cannot do) may inform us on the asymmetry in openness between the future and the past, it would be a mistake to reduce this asymmetry to purely anthropocentric considerations. In my PhD dissertation, I also consider senses in which the future and the past may respectively be said ‘open’ and ‘fixed’ in a world without humans (or before humanity emerged).

For many practical purposes, it seems very hard to discard the intuition that the future is open while the past is fixed. Whereas we do not deliberate about the past, our beliefs about opportunities, possibilities, alternatives, and so on, are all future-oriented. However, the main models of the temporal structure of the world do not reflect any asymmetry between the future and the past. According to presentism and eternalism, the future and the past are ontologically on a par. Eternalists hold that both the future and the past exist, while presentists hold that neither the future nor the past exists. In other words, the two main competing models of the temporal structure of the world do not ontologically distinguish the future from the past (either both of them exist or none of them exists). Therefore, neither eternalism nor presentism seems able to account for our basic intuition regarding the nature of time. Of course, one might claim that the asymmetry between the open future and the fixed past does not need to be grounded in the temporal structure of the world, but merely in some local phenomena (e.g. entropy, irreversibility). Nonetheless, in my PhD dissertation, I provide reasons to think that these local phenomena are not as important as some philosophers would have us believe for temporal asymmetries. For example, the increase of entropy – which arises from the collective behavior of many microscopic entities – at best postpones the problem: if there is no directedness in fundamental physics, where does the thermodynamic asymmetry in time come from?

This project might be criticized for taking too seriously the intuitive asymmetry between the open future and the fixed past. After all, some arguments taken from science, especially from contemporary physics, have been put forward to show that the asymmetry is at best a non-fundamental phenomenon, at worst an illusion. For example, the ‘block universe’ view of time, which is inferred from the results of SR, does not reflect any asymmetry. It regards reality as a block-like four-dimensional ensemble, lacking a moving present, wherein all times and events are equally real. In other words, the spatio-temporal model favored by contemporary physics does not reflect any difference between space and time that somehow accounts for the fact that whereas there is no here-there space-asymmetry, there should be a past-future time asymmetry. Likewise, the fundamental laws of physics, which are time-reversal invariant, do not underpin any asymmetry regarding the nature of time. In that sense, for every physically allowable sequence of events, the inverse sequence of time-reversed events is also physically allowable. For example, if one watches a film that shows a ball rolling, the fundamental laws of physics cannot tell whether the film is being projected correctly or in reverse. Therefore, accounting for the asymmetry between the open future and the fixed past as a fundamental phenomenon seems to require developing an alternative model of the temporal structure of the world to that favored by contemporary physics, namely a spatio-temporal model that has the intrinsic resources to ground the asymmetry. This model turns out to be a specific version of C. D. Broad’s growing block theory of time (GBT).

The final step of the PhD dissertation is the reconciliation of this alternative model (GBT) with contemporary physics. Indeed, although physics cannot settle the debate about the nature of time, it crucially informs and frames the debate. It therefore seems that no metaphysical contribution to the question of the nature of physical time can be provided without observing the main imperatives of our best physical theories. In that respect, considerable efforts have to be devoted to the understanding of the main postulates and consequences of both the Special and General theory of relativity, but also to the nascent theories of quantum gravity (which aim to unify the General theory of relativity with the principles of quantum mechanics). The main purpose of this final step is to show that GBT is expressible in a relativistic spacetime setting and can, thereby, offer some physical relevance to the intuitive asymmetry between the open future and the fixed past. In this perspective, the causal set approach of quantum gravity (CST) is a matter of great interest, since it allows for the ‘coming-into-existence’ of events through a discrete stochastic process and might, therefore, underwrite a specific version of GBT. At the end of the day, the time of human experience might turn out to be a more faithful reflection of the time of science than what most philosophers believe.