Colloquium : Homochirality: Symmetry breaking in systems driven far from equilibrium

In 1848, Louis Pasteur discovered that certain molecules can rotate the polarization of light either clockwise or anticlockwise, defining their chirality. Some biological molecules, such as amino acids and sugars, are homochiral, that is, they rotate their polarization in only one direction. Although this observation seems to be directly related with the existence of life on Earth, it is still poorly understood partially because of its complexity and emergent nature. Recent experiments, discussed in this Colloquium, have provided new evidence for the nonlinear, nonequilibrium, nature of this important phenomenon. Subsequent to the discovery of chirality of organic molecules by Pasteur, living organisms have been found to utilize biomolecules of only one handedness. The origin of this homochirality in life still remains unknown. It is believed that homochirality is attained in two stages: the initial creation of a chirality bias and its subsequent amplification to pure chirality. In the last two decades, two novel experiments have established the second stage in different fields: Soai and co-workers achieved the amplification of enantiomeric excess in the production of chiral organic molecules, and Viedma obtained homochirality in the solution growth of sodium chlorate crystals. These experiments are explained by a theory with a nonlinear evolution equation for the chiral order parameter; nonlinear processes in reactions or in crystal growth induce enantiomeric excess amplification, and the recycling of achiral elements ensures homochirality. Recycling drives the system to a state far from equilibrium with a free energy higher than that of the equilibrium state.