"T"1 relaxation involves redistributing the populations of the nuclear spin states in order to reach the thermal equilibrium distribution. By definition, this is not energy conserving. Moreover, spontaneous emission is negligibly slow at NMR frequencies. Hence truly isolated nuclear spins would show negligible rates of "T"1 relaxation. However, a variety of "relaxation mechanisms" allow nuclear spins to exchange energy with their surroundings, the "lattice", allowing the spin populations to equilibrate. The fact that "T"1 relaxation involves an interaction with the surroundings is the origin of the alternative description, "spin-lattice relaxation".