The energy–momentum relation E 2 = p 2 c 2 + m 0 2 c 4 in natural units (with c = 1 ) E 2 = p 2 + m 0 2 is a Pythagorean equation. When a relatively high energy is applied to a particle with relatively low rest mass, it can be approximated as E ≃ ≃ p in high-energy physics such that an applied energy with expressed in the unit eV conveniently results in a numerically approximately equivalent change of momentum when expressed with the unit eV/ c.