The Direction of Time

That things appear to be different across times when looking from earlier to later as opposed to from later to earlier is what it is for time to be anisotropic. But this alone doesn’t mean that time has a direction. Time has a direction if and only if there is an objective fact as to whether or not time goes from earlier to later or later to earlier. From this starting assumption, there are two core debates in the literature. The first is whether or not time has a direction. A-theorists and B-theorists hold that time has an objective direction, whereas C-theorists deny this. For those who think time does have a direction, then, there is the further disagreement about whether direction is an intrinsic or extrinsic feature of time. For the A-theorist, the directionality is intrinsic to time and is grounded in temporal passage, or objective becoming – i.e., time is directed from the past to the future because time passes as which moment is present changes. Some B-theorists likewise hold that direction is an intrinsic feature of time but that it is grounded in some B-theoretic feature, rather than temporal passage. On the other hand, other B-theorists make the stronger claim that the direction of time is reducible to B-theoretic features and is therefore extrinsic to time. The second core debate, then, is amongst those who hold that the direction of time is merely extrinsic. Put simply, the problem is how the temporal asymmetries that exist in the world can be accounted for, given that the laws of nature are time reversal invariant – i.e., hold even if the order of times were reversed. So called reductionist B-theorists have appealed to a few features of the universe in order to explain the directedness of time. They include the expansion of the universe, increase in entropy over time, causal order, and the fact that we deliberate about future events but not past events.

Early foundational texts include Reichenbach 1956, and Earman 1974. Craig 1999 argues for the direction of time on the A-theory, Price 1997 includes a range of interesting topics related to the physics of the direction of time and causation, and Zeh 1989 offers some options for grounding the direction of time. See also Albert 2000, Fernandes 2021, and Chen & Goldstein 2022.

Good introductions include Savitt 1996, Callender 1997, Ridderbos & Savitt 1997, and Mellor 2009.