  We formulate Wilson loop observables as products of eikonal Wilson lines
given in terms of on-shell scattering amplitudes. Using these, we derive the
Dirac-Schwinger-Zwanziger quantization condition and its gravitational
(Taub-NUT) double copy, where we find a relativistic generalisation of the
usual non-relativistic gravitational quantization condition. We also compute
the relativistic Wilson loop for an anyon-anyon system, obtaining a similar
relativistic generalisation of the Aharonov-Bohm phase for gravitational
anyons.
