These motors can be viewed as examples of flashing ratchets , where switching between two different energy surfaces -here the ground state and the excited state-drives directional motion. In the absence of light, the motor would go to equilibrium on the ground state PES, and its conformational probabilities would follow the Boltzmann distribution. Consequently, the stable form would be at much higher concentration than the metastable form, and the principle of detailed balance would mean that no net rotation occurs. In the presence of a constant light source, photo-excitations break detailed balance and create a non-Boltzmann steady-state distribution, the so-called photostationary state. Unlike in equilibrium, the steady state concentration of the metastable form can match or surpass the stable form . Thus, the motor molecules can transduce light energy by letting photoisomerizations pump them to the metastable form, and releasing their stored energies during Stable and metastable forms are described in Figure ).