  It is commonly assumed that a charged particle does not accelerate linearly
along a spatially uniform magnetic field. We show that this is no longer the
case if the interaction of the particle with the quantum vacuum is chiral, in
which case parity and time-reversal symmetries are simultaneously broken. In
particular, this is the situation of an electroweak interacting particle in the
presence of a uniform magnetic field. We demonstrate first that, in a spatially
uniform and adiabatically time-varying magnetic field, a proton coupled to the
leptonic vacuum acquires a kinetic momentum antiparallel to the magnetic field,
whereas virtual leptons gain an equivalent Casimir momentum in the opposite
direction. Remarkably, leptons remain virtual throughout the process, which
means that the proton acceleration is not caused by the recoil associated to
the emission of any actual particle. The kinetic energy of the proton is part
of its electroweak self-energy, which is provided by the source of magnetic
field. In addition we find that, in a constant and uniform magnetic field, the
adiabatic spin-relaxation of a single proton is accompanied by its acceleration
along the magnetic field. We estimate that, at the end of the spin-polarization
process, the proton reaches a velocity of the order of $\mu$m/s. The latter
finding may lie within the scope of experimental observations.
