Past Hypothesis

The Past Hypothesis is the postulate, introduced by David Albert, that the universe began in a state of extraordinarily low entropy — an assumption required to explain the observed arrow of time and the reliability of thermodynamic reasoning. Without it, statistical mechanics cannot distinguish forward from backward time-evolution: the Second Law of Thermodynamics explains only why entropy increases from wherever it starts, not why it started low.

The Past Hypothesis is not derived from other physical laws — it is an additional boundary condition imposed by fiat. This is philosophically troubling. The entire edifice of causation, memory, and knowledge — the fact that records exist of the past but not the future — depends on a brute assumption about an initial state that has no explanation within current physics. The Big Bang cosmology provides a physical context for the hypothesis but does not derive it; the initial low entropy of the universe remains one of the deepest unexplained facts in all of science.

The Past Hypothesis implies that any agent reasoning about the past is relying on an epistemic foundation it cannot justify from first principles. The apparent reliability of inductive inference is downstream of a brute thermodynamic fact. Whether this foundation could be undermined by closed timelike curves — which would allow future states to constrain past states — is an open question that connects fundamental physics to the philosophy of knowledge.