  The authors) Whenever light is slowed down, for any cause, two different
formulas give its momentum. For dielectrics, the coexistence of those momenta
was the heart of the century-old Abraham-Minkowski dilemma, recently resolved.
We demonstrate that this framework extends to momentum exchange in
wave-particle interaction; in particular to Langmuir waves for Landau damping
and to vacuum waveguides of electron tubes (metallic slow-wave structures).
Focussing on the latter, we show that the dilemma resolution is not limited to
discriminating between kinematic and canonical momenta but also involves a
non-negligible momentum flux from Maxwell's electromagnetic stress. The
existence of two momenta in materials, plasmas, and waveguides, for which light
velocity modification has entirely different origin, points to the universality
of the Abraham-Minkowski dilemma.
