An On-Shell Approach to Hawking radiation and Merger Physics
Recently, the amplitude community has shown that the interactions of
black holes can be described — in the weak field regime — purely in
terms of minimal building blocks known as on-shell amplitudes, without
ever needing to invoke a Lagrangian, Hamiltonian, or equations of motion.
However, very little is known about whether this approach can be made
systematic enough to address more complicated aspects of black holes.
Two extremes in this regard, which serve as tests of the on-shell
program, are the merger of binary black holes and the Hawking radiation
they emit.
In this talk, I will argue that both can be approached with an on-shell
mindset by considering a universal mass-changing three-point amplitude,
matched to physical and gauge-invariant quantities. Once the matching
conditions are specified, this simple building block can be iterated, as
in any other perturbative on-shell scheme in QFT, to compute higher-loop
corrections or processes with more external states.
As an application, I will show how to derive conservation laws and
memory waveforms in black hole mergers. I will then explain how to
compute both classical and quantum mass-changing effects arising from
the presence of event horizons. In the latter case, Hawking radiation
can be recovered from an on shell cut as a decay process, allowing us to
reproduce the known Hawking radiation for an isolated black hole in the
Unruh vacuum and to discuss its effects on binary scattering.