"The present is the time that is associated with the events perceived directly and in the first time,[1] not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is a period of time between the past and the future, and can vary in meaning from being an instant to a day or longer. It is also known as the here-now. In radiocarbon dating, the "Present" is defined as AD 1950.

It is sometimes represented as a hyperplane in space-time,[2] typically called now, although modern physics demonstrates that such a hyperplane can not be defined uniquely for observers in relative motion. The present may also be viewed as a duration (see specious present)."

"The status of the doctrine of the open future in the context of Minkowski spacetime will be examined by discussing the notion of ontological separation of space-time. This notion we define as follows:

Minkowski space-time is ontologically separated with respect to an event e if, assuming that e is here and now, every other event e' is either:
(a) in the past or in the present—which is taken to mean that e' is realized, or
(b) in the future—which is taken to mean that e' is not realized.
Minkowski space-time is ontologically separated if it is ontologically separated with respect to every event e.

In the opinion of many authors the STR provides a strong argument against the ontological separation of space-time. The heart of the argument is the claim that the STR implies the relativization of the ontological separation of spacetime to a reference frame, but the relativized ontological separation implies an unacceptable frame-dependent concept of existence.

To conclude that the STR implies that the notion of 'ontological separation of space-time' becomes relativized to a frame of reference, consider the following situation. Let O1 and O2 be two observers in uniform relative motion. Suppose O1 and O2 agree that event e1 occurs 'here and now'—take e1 to be their meeting (see Figure 1). Let e2 be an event space-like separated from e1 — say, the birth of a star—and simultaneous with e1 for O1. According to the STR, e2 is not simultaneous with e1 for O2. Assume that O1 and O2 are in such relative motion that e1 is earlier than e2 for O2 (see Figure 1). Then, only relative to O1, is the star birth occurring 'now', and for the two observers spacetime is differently ontologically separated with respect to ei (see Section 2.2). In general, any change of the velocity of O2 will result in a different separation of space-time with respect to e1 for O2. Since there are no preferred frames of reference, the separation of space-time depends on a frame of reference. Relative to O1 event e2 is realized, ontologically fixed and definite. Relative to O2 the star birth is an event not (yet) realized. Furthermore, reality itself will be different depending on the reference frame! Relative to observer O1 the star exists 'now', while relative to observer O2 the star does not (yet) exist."