  It was shown that in the small Wigner group there is a one-parameter subgroup
of the Lorentz transformations, which leave unchanged not only the momentum of
the fermion with spin h/2, but also its spin characteristics. This is the group
of symmetry transformations of the moving fermion. These transformations
consist of rotations around the projection axis of the spin and boosts
coordinated with them. In other words, there are infinitely many frames of
reference in which the fermion has the same momentum and the same spin
projection onto the selected axis, although these frames move relative to each
other and have been rotated relative to each other.

Quantum Physics;High Energy Physics - Theory