Ultrarelativistic limit
（重定向自Ultrarelativistic）
In physics, a particle is called ultrarelativistic when its speed is very close to the speed of light c.
Max Planck showed that the expression for the relativistic energy of a particle with rest mass m and momentum p is given by
The energy of an ultrarelativistic particle is almost completely due to its momentum (pc ≫ mc), and thus can be approximated by E = pc. 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).