The Ragged Edge of Reality – a relativistic tensed ontology - A/Prof William Grey

A familiar objection to presentism (and so-called tensed ontologies) is that according to relativity theory there is no universally shared distant simultaneity. What is happening now (according to relativity) depends on the motion and distance (that is, the inertial frame) of an observer with respect to whom the event is being calibrated. This (it is argued) creates an intractable problem for presentists, who believe that the present (in the words of Arthur Prior) is "one facet of the great gulf that separates the real from the unreal". Without an objective, shared, present moment it seems that we cannot identify what is present with what is real. This challenges our intuitive understanding of the past, present and future. If time flows differently for different observers, then simultaneity – what is happening now – becomes subjective. That is, physics shows us that experience is misleading: the passage of time is an illusion – according to Einstein a "stubbornly persistent" illusion – but one which entails an eternalist metaphysic in which reality consists of a four-dimensional space-time manifold. I will reframe this venerable argument from a bihemispheric perspective and suggest an alternative way of understanding the dynamic unfolding process of reality which provides an objective past, present and future for everyone: a relativistic tensed ontology. This will involve undertaking a few thought experiments; in one of which we will revisit Paul Langevin's so-called 'twin' paradox.