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.
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