Casimir forces at the threshold of the Cherenkov effect

We study the Casimir-Lifshitz forces in a strongly nonreciprocal system: a waveguide filled with a medium moving at a relativistic velocity. In such a waveguide the waves propagate dominantly along a single direction that coincides with the direction of the velocity. Our theory shows that the Casimir forces acting on a piston in such a quasi-one-way waveguide vanish when the velocity approaches the Cherenkov threshold.