What Gives Time its Direction or “Arrow”?
a. What Needs to be Explained The arrow of time is what distinguishes events ordered by the happens-before relation from those ordered by its converse, the happens-after relation. Time’s arrow is evident in the process of mixing cool cream into hot coffee. You soon get lukewarm coffee, but you never notice the reverse–lukewarm coffee separating into a cool part and a hot part. Such is the way this irreversible thermodynamic process goes. Time’s arrow is also evident when you prick a balloon. The air inside the balloon rushes out; it never stays in the balloon as it was before the pricking. So, the pricking starts an irreversible process. The arrow of a physical process is the way it normally goes, the way it normally unfolds through time. If a process goes only one-way, we call it an “irreversible process.” (Strictly speaking, a reversible process is one that is reversed by an infinitesimal change of its surrounding conditions, but we can overlook this fine point because of the genera
