In particle physics, the invariant mass m 0 is equal to the mass in the rest frame of the particle, and can be calculated by the particle's energy E and its momentum p as measured in any frame, by the energy–momentum relation : m 0 2 c 2 = ( E c ) 2 − − ‖ p ‖ 2 or in natural units where c = 1, m 0 2 = E 2 − − ‖ p ‖ 2 .