Excitations through the medium — speed c Both ride the BEC quasiparticle dispersion modon metric ripple speed = c = 3 × 10⁸ m/s Photon Gravitational wave Oscillation of the lattice — speed c_T Elastic wobble of the vortex scaffolding speed = c_T ≈ 9 km/s ≈ 3 × 10⁻⁵ c Displaced vortices (Tkachenko wave) Undisturbed lattice