Researchers propose a new way to understand thermal radiation and dissipation from large objects, including black holes, by treating those objects as particles in scattering experiments. In their on-shell framework, an equilibrium object is represented by a particle that satisfies the usual energy–momentum relation (“on-shell”), while non-equilibrium processes are transition amplitudes between such particle states. A key finding is that states with spin remain essential even for objects that are not macroscopically rotating, and consistency with macroscopic symmetries forces a single universal coupling that controls all spinning states.

To make this concrete the authors model a macroscopic object as a one‑particle state with a large degeneracy of internal microstates. That degeneracy is encoded in a spectral density ρ(µ,s) = C(µ,s) e^{S(µ,s)}, where S is identified with the microcanonical entropy. They classify the relevant three‑point amplitudes that describe the absorption or emission of a massless boson (a scalar, photon, or graviton) by unequal‑mass particles, and they represent spinning states using coherent spin states so that a classical spin vector emerges in the usual limit.

The central mechanism is what they call spin universality. Requiring that microscopic spinning states respect the macroscopic spherical symmetry of the object forces all these spinning states to share one universal coupling. Because the same coupling appears in both absorption and emission amplitudes, the absorption and emission probabilities are controlled by the same on‑shell data. From this follows local detailed balance — a local version of the familiar relation between forward and reverse transition rates in statistical physics — directly from the S‑matrix (the object that encodes scattering probabilities). Applied to black holes, the framework reproduces a thermal emission spectrum and relates the Hawking temperature to a condition of maximal absorption that is still consistent with unitary time evolution.