  In a strong magnetic field, ultra-relativistic electrons or positrons undergo
spin flip transitions as they radiate, preferentially spin polarizing in one
direction -- the Sokolov-Ternov effect. Here we show that this effect could
occur very rapidly (in less than 10 fs) in high intensity ($I\gtrsim10^{23}$
W/cm$^{2}$) laser-matter interactions, resulting in a high degree of electron
spin polarization (70%-90%).
