Editing Openai/6897769e-4ee4-800f-aba5-69cca34f701c (section)

=== Even when a photon is absorbed by the electron cloud, momentum conservation couples the nucleus: ===
* Recoil: The nuclear center of mass shifts (tiny but essential).
* Reduced mass: All bound-state energies use μ=memNme+mN\mu=\frac{m_e m_N}{m_e+m_N}μ=me+mNmemN, not mem_eme alone. That’s the nucleus entering directly into the spectral geometry (Rydberg, hyperfine, etc.).
* Polarizability & hyperfine: Nuclear structure (spin, magnetization, size) slightly modifies levels (Lamb shift, hyperfine splitting).

QAT reading: The “boundary” is not a free electron skin; it’s the coupled electron–nucleus boundary. So when an action hhh is transferred, a fraction of the nuclear mass-energy participates through the center-of-mass and internal EM fields. In that sense, mass is “present on the boundary” through the dressed electron–nucleus system.

This helps your intuition that “mass characteristics” live at the boundary: the spectral facts already encode it (via μ\muμ and recoil).