Likewise, \(\overline{\boldsymbol{x}} \cdot \overline{\boldsymbol{x}}\) is positive for a particle traveling slower than light, and negative for a particle traveling faster than light (which is of cou...Likewise, \(\overline{\boldsymbol{x}} \cdot \overline{\boldsymbol{x}}\) is positive for a particle traveling slower than light, and negative for a particle traveling faster than light (which is of course impossible, since such a particle would need to first reach the speed of light, which as we’ve seen can never be done).
