has itself the properties of a vector; these quantities are referred to an instantaneous condition of the body (e.g. at the time
=
).
This 4-vector may also be expressed in terms of the mass
, and the velocity of the body, considered as a material particle. To form this expression, we note first, that
is an invariant which refers to an infinitely short portion of the four-dimensional line which represents the motion of the material particle. The physical significance of the invariant