Classical particles with unusual properties

The paper deals with the possible existence of particles with spacelike momenta and superluminal velocity (tachyons), in contrast to bradyons (ordinary) particles with subluminal velocities and timelike momenta. If the existence of tachyons is indeed consistent with the theory of relativity and quantum theory, it seems possible to conceive the existence of other particles, in addition to tachyons and bradyons, in the framework of classical special-relativity mechanics. The particles discussed in this paper are characterized in that their four-momentum is orthogonal to the four-velocity. Some kinematic and dynamic properties of such particles are examined.