"The point at issue between the two theories [A and B theory] is whether 'time' really is, in some deep ontological sense, differentiated into past, present and future. ...Reichenbach and Whitrow propose that there is indeed such a type of event and this is the 'becoming', or 'coming into being' of factual states-of-affairs in the physical world. Reichenbach ...claimed that 'becoming' is in fact made manifest through the Uncertainty Principle of Heisenberg: "The concept of becoming, he wrote, acquires a meaning in physics: The present, which separates the future from the past, is the moment when that which was undetermined becomes determined, and 'becoming' means the same as 'becoming determined' ". Whitrow expressed ..."The past is the determined, the present is the moment of 'becoming' when events become determined, and the future is as-yet undetermined. Although neither Reichenbach nor Whitrow developed their thesis at any length, the general purport of what they meant is clear: there is a basic chance element in nature, at least at the micro-level, and the moment of 'becoming', which they identify with 'the present', is marked by a transition from what is merely possible to what is factual. However... this important attempt to provide a physical basis for the A-theory is by no means immune from criticism."