Eternalists, such as McTaggart, argue that nothing which exists can be said to possess the characteristic of being in time because he argues time is not a true entity. Differences between past, present, and future are only an illusion in our minds as every moment is equally real. Thus this is akin to the image of a "block universe" in which every event already exists, but the passage of time itself is reduced to a logical fiction. This is similar to Parmenides as he regards change and the flow of time as only an illusion. He argues, “Whenever we perceive anything in time… we are perceiving it more or less as it really is not…”
In contrast, presentism argues that time classifications are not permanent but rather fluid. In this case, time appears to “flow”. This “A-series” corresponds to how we normally perceive events as unfolding temporally. We exist in the “now” and are being pulled through the flow of time down the river into the future, which remains an unfinished book. This is non-reductive in the sense that time is an independent entity.
Interestingly, in the book “Fabric of the Cosmos”, Brian Greene, a prominent string theorist, effectively argues that the B-series eternalist perspective most nearly accords with Einstein’s theory of relativity and physics conception of time.
He states, due to relativity,
“If you buy the notion that reality consists of all the things in your freeze-frame mental image right now, and if you agree that your now is no more valid than the now of someone located far away in space…then reality encompasses all of the events in spacetime… Events just are. They all exist. They eternally occupy their particular point in spacetime. There is no flow.” (Fabric of the Cosmos, p. 139)
Einstein stated, “For we convinced physicists, the distinction between past, present, and future, is only an illusion, however persistent.” (p. 139- For further reading see http://en.wikipedia.org/wiki/Rietdijk-Putnam_argument)
This is the ultimate eternalist perspective of time consisting of one loaf, equally real. The passage of time becomes once again illusion. But there is also more evidence: in physical equations, time is treated as symmetric. There is no difference between a glass falling or putting itself back together mathematically, nothing to distinguish past and future. This is known as time reversal symmetry. In physics, an arrow of time can only be distinguished by increasing entropy in a closed system from the statistically unlikely low entropy big bang. Over time, entropy and disorder tends to increase.
However, if all time is equally real, this has an important upshot: the universe then is fully deterministic and we have no free will. In the classical Newtonian machine universe, whose laws are immutably set in place, this would indeed be the case. Even in the quantum world where uncertainty has reality, “Schrodinger’s equation tells you how the wavefunction was or will be at any other moment you specify. This component of quantum physics is fully deterministic, just as in classical physics.” (Fabric of the Cosmos, p. 455) Greene notes, however, the missing piece is the controversial act of observation. Does observation cause the wavefunction to collapse? Or is science perhaps mistaken in believing that we can map mathematical laws onto such questions as free will? Perhaps the problem is trying to fit such conceptions as free will, value, subjectivity, and consciousness in the scientific framework in the first place and there are profound limitations on the human capacity to determine the true nature of reality and even ourselves. This is a subject for an entire book, or at least another post.
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