A fully degenerate, adiabatic gas with this property is modeled by the polytrope n equals three. The energy density of a universe of particles with this property scales with scale factor to the negative fourth. Degenerate electrons with this property have pressure proportional to density to the four-thirds, which occurs near the Chandrasekhar limit. For the first 50,000 years after the Big Bang, the universe was dominated by particles with this property. Hot dark matter consists of particles that had this property in the early universe, like neutrinos. An alternate name for astrophysical jets indicates that they have this property, corresponding to a Lorentz factor larger than one. (10[1])For (10[1])10 points, name this property of (-5[1])particles that cannot be described with classical mechanics, since their velocity is near the speed of light. ■END■ (10[1]0[1])