The second law of learning from the past light cone
Tomi Koivisto
Abstract
In the parallel-frame formulation of Einstein gravity the frame field determines the edge modes and gives an observer’s clock and energy. The entropy of a causal diamond is the boost charge at its edge, inferred from a null cut. It obeys an exact nonlinear second law for any radiation and matter obeying the null energy condition, and for a black hole formed by null dust equals the apparent-horizon entropy to all orders. Its time reverse, inferred from the past light cone, decreases as observations accumulate and bounds the generalized entropy compatible with the data.