In a world that has no room for an advancing now, time does not "flow." To philosophers, the perplexities and absurdities entailed by the notion of an advancing objective present or a flowing objective time are well-known. (Click here or here for discussions of a variety of philosophical issues pertaining to time.) To physicists, the unreality of a temporally unextended yet persistent and continually changing present was brought home by the discovery of the relativity of simultaneity. For any two events A, B there exist two reference frames F_A and F_B and a third event C such that C is simultaneous with A in F_A and simultaneous with B in F_B. This “simultaneity by proxy” of A with B implies that all parts of the spatiotemporal whole are coexistent and equally real. (It also follows from the relativity of simultaneity that this coexistence is not a simultaneous existence but must be regarded as an atemporal coexistence.)

Again, nothing in the physical world corresponds to the temporal asymmetry that is introduced by the notion that the future is determined by the past. The fundamental laws — the deterministic laws of classical physics as well as the statistical laws of quantum physics — are time-symmetric. They allow us to retrodict as well as to predict. They even allow us to assign probabilities to the possible outcomes of measurements on the basis of past and future outcomes, as the following article illustrates.

If we nevertheless project the exclusive reality of the experiential now into the physical world, as we do as persistently as inconsistently, we create the figment of an evolving instantaneous state. This is why we are in the habit of dividing physics into

And if we then combine the figment of an evolving instantaneous state with the figment of a causal arrow, we arrive at the well-known folk tale according to which causal influences reach from the (no longer existent) past to the (not yet existent) future through persistent "imprints" on the present. If the past and the future do not exist, the past can influence the future only through the mediation of something that persists. Causal influences reach from the past into the future by being "carried through time" by something that "stays in the present." This evolving instantaneous state includes not only all presently possessed properties but also traces of everything in the past that is causally relevant to the future. In classical physics this is how we come to conceive of "fields of force" that evolve in time (and therefore, in a relativistic world, according to the so-called principle of local action) and that mediate between the past and the future (and therefore, in a relativistic world, between local causes and their distant effects).

In quantum physics, this is how we come to seize on an algorithm that depends on the time of the measurement (to the possible outcomes of which it serves to assign probabilities), and to transmogrify this algorithm into an evolving instantaneous state.