Block time

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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"<ref>Physics does not consider the universe to be block-shaped, though it does consider time a true dimension, and does consider the universe to have a many-dimensional "shape".</ref>, in opposed to the common-sense view of the world as a three-dimensional space modulated by the passage of time.

Conventionally, time is divided into three distinct regions; the "past", the "present", and the "future". Using that representational model, the past is generally seen as being immutably fixed, and the future as undefined and nebulous. As time passes, the moment that was once the present becomes part of the past; and part of the future, in turn, 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 conventional model presents a number of difficult philosophical problems, and is difficult to reconcile with current accepted scientific theories. For example, special relativity has shown that the concept of simultaneity is not universal: observers in different frames of reference will have 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).

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. A universe in which random changes are possible may be indistinguishable from the many-worlds interpretation of quantum mechanics in which there are multiple 'block times'.

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.