A particle's mass is always either equal to or smaller than the time-component of its four-momentum.

Question

Quizplus · Accepted Answer

The time-component of a particle's four-momentum is given by $E/c$, where $E$ is the energy of the particle and $c$ is the speed of light. Since the energy of a particle includes its rest mass energy ($mc^2$) and its kinetic energy (which is always positive), the time-component of the four-momentum will always be greater than or equal to the rest mass of the particle. 
Note that $p_{t}=m / \sqrt{1-\left|\vec{v}^{2}\right|}$.

A Particle's Mass Is Always Either Equal to or Smaller