Black hole thermodynamics is built on horizons. The first law — the relationship between mass, area, and surface gravity — is formulated on an event horizon or an apparent horizon, a codimension-one surface that separates the trapped region from the rest of spacetime. This works beautifully for stationary black holes. For dynamic ones — evaporating, accreting, merging — the horizon is either defined teleologically (the event horizon requires knowledge of the entire future) or ambiguously (apparent horizons are foliation-dependent).
Torres (arXiv:2603.11422) detaches the first law from the horizon entirely. Instead of working on a codimension-one horizon worldtube, the first law is formulated on individual marginally trapped surfaces — codimension-two closed surfaces where outgoing light rays have zero expansion. These surfaces exist at an instant. They do not require a preferred slicing of spacetime. They do not require knowledge of the future. They are intrinsically quasi-local objects, defined by the geometry of a single surface.
The construction assigns each marginally trapped surface an energy (Hawking energy), a temperature (invariant effective surface gravity), and derives a balance law: the change in energy splits into a heat term and a work term. For spherically symmetric spacetimes, the formalism reproduces the standard results exactly. For Kerr, it extends to non-spherical trapped surfaces where horizon-based methods become cumbersome or non-unique.
The structural point is that the thermodynamic content was never in the horizon. It was in the surface. The horizon was a convenient organizational device — a tube assembled from many surfaces — but the thermodynamic law holds for each surface individually. The codimension-one structure is redundant. Everything the first law says about mass, entropy, and temperature is already present at codimension two. The horizon added global structure (causal isolation, the teleological definition of an event horizon) that the thermodynamics does not use.