Leaving aside the multiverse hypothesis, M-theory and superstring theory as possible avenues for some ‘ultimate’ unification, let us now focus on the difficulties in producing a unified theory for this universe or cosmic epoch. If we accept events as our fundamental ontology, we have a basis for systematically interpreting both microscopic entities such as subatomic particles and macroscopic entities even at the largest cosmological scale in terms of events. This has been the upshot of our thesis thus far. Eliminating substances in favour of events, however, does not get us very far towards a unified theory. It is only the beginning, albeit a critically important one.

In this chapter, I examine the problem of time with regard to the unification of general relativity and quantum mechanics and advance in broad outline one direction that this unification might potentially follow, namely a version of C. D. Broad's growing block universe consistent with Whitehead's late metaphysics and relativistic quantum field theory. One of the great challenges of unification, as noted in Chapter 1, is to solve the problem of the very different conceptual roles that time plays in general relativity and quantum mechanics. Moreover, it is not at all clear how we are to reconcile the geometry of space-time within which each event has its own light cone and other obvious physical phenomena such as becoming and temporal succession. Whitehead certainly struggled with this problem when he moved from a purely four-dimensional system to one in which time plays a more fundamental role, that is a temporalisation of space rather than a spatialisation of time, and finally developed his solution by embracing a fundamental asymmetry of time in his metaphysics of process. Other philosophers and physicists have followed Whitehead's lead in formulating tentative solutions in which general relativity is modified in order to account for the creative advance of nature.

Aside from the problem of time, another major stumbling block to unifying general relativity with quantum mechanics is the failure to develop an overall picture of reality.