Ultrarelativistic limit

In physics, a particle is called ultrarelativistic when its speed is very close to the speed of light , such that its total energy is almost completely due to its momentum (), and thus can be approximated by . This can result from holding the mass fixed and increasing p to very large values (the usual case); or by holding the energy E fixed and shrinking the mass m to negligible values. The latter is used to derive orbits of massless particles such as the photon from those of massive particles (cf. Kepler problem in general relativity).
See more at Wikipedia.org...