According to the Boltzmann distribution, as the number of systems having energy E out of N 0 at temperature T is N 0 e − − E / k T (where k is the Boltzmann constant), the lower energy level, associated with spin ℏ ℏ / 2 , is more populated than the other. In the presence of a rotating magnetic field more protons flip from S z = + ℏ ℏ / 2 to S z = − − ℏ ℏ / 2 than the other way, causing absorption of microwave or radio-wave radiation (from the rotating field). When the field is withdrawn, protons tend to re-equilibrate along the Boltzmann distribution, so some of them transition from higher energy levels to lower ones, emitting microwave or radio-wave radiation at specific frequencies.