Block time

Block time is one way of approaching the problem of the nature of time. Its name is derived from its description of space-time as an unchanging four-dimensional "block", as opposed to a three-dimensional space that changes as it moves along a time axis.

In the conventional concept of how the passage of time operates, time is divided into three distinct regions; the "past", the "present", and the "future". The past is generally seen as being immutably fixed, and the future as undefined and nebulous. As time passes the current present becomes part of the past, and part of the future becomes the new present. In this way time is said to pass, with a distinct present moment "moving" forward into the future and leaving the past behind.

This model of time presents a number of difficult problems, both philosophically and in terms of current accepted scientific theories. For example, special relativity has shown that the concept of simultaneity is not universal, with different frames of reference having different perceptions of which events are in the future and which are in the past; there is no way to definitively identify a particular point in universal time as "the present". Furthermore, there is no fundamental reason why a particular "present" should be more valid than any other; observers at any point in time will always consider themselves to be in the present. Even the concept of "time passing" can be considered to be internally inconsistent, by asking "how fast does time pass?"

Block time overcomes these various difficulties by considering all points in time to be equally valid frames of reference, equally "real" if one prefers. It does not do away with the concept of past and future, but instead considers them as directions rather than as a state of being; whether some point in time is in the future or past is entirely dependent on which frame of reference you are using as a basis for observing it.

Since an observer at any given point in time can only remember events that are in the past relative to him, and not events that are in the future relative to him, the subjective illusion of the passage of time is maintained. The asymmetry of remembering past events but not future ones, as well as other irreversible events that progress in only one temporal direction (such as the increase in entropy) gives rise to the arrow of time. In reality, there is no passage of time; the ticking of a clock measures durations between events much as the marks on a measuring tape measures distances between places.

Block time has implications for the concept of free will, in that it proposes that future events are as immutably fixed and impossible to change as past events (see determinism). This will prove less troublesome if true that time behaves like another dimension of space. It's well-understood that there is no such thing as a ubiquitous present moment - a "now" - on, say, a galactic scale; similar arguments would show that the concept of block time is meaningful over only short "distances" of time. Also, the question about free will can be sidestepped by dropping the unwarranted assumption that each person's conscious experience is merely an epiphenomenon of matter and hence totally isolated from all others. As Jacques Barzun noted in From Dawn to Decadence: 500 Years of Western Cultural Life, the human will would in fact be "distributed among all the living."

Block time makes two assumptions, which are separable. One is that time is a full-fledged real dimension. The other is immutability. The latter is not a necessary consequence of the first. If random changes are possible, the result may be indistinguishable from the many-worlds interpretation of quantum mechanics.

Augustine of Hippo wrote that God is outside of time—that time exists only within the created universe. Many theologians (especially Catholics) agree. On this view, God would perceive something like block time, while time might appear differently to us finite beings.

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