In the world of chemistry, one minus one almost always equals zero.

Almost always? Yes. In 1848, Louis Pasteur showed that molecules that are mirror images of each other had exactly opposite rotations of light. When these "left-handed" and "right-handed" molecules are mixed together in solution, however, they cancel the effects of the other, and no rotation of light is observed. Thus, "one minus one equals zero."

But now our understanding of mirror-image molecules and their optical activity is a little more nuanced. 

Northwestern University's Kenneth R. Poeppelmeier and his research team have demonstrated that a mixture of mirror-image molecules crystallized in the solid state can be optically active. The scientists first designed and made the materials and then measured their optical properties.

"In our case, one minus one does not always equal zero," said first author Romain Gautier of CNRS. "This discovery will change scientists' understanding of these molecules, and new applications could emerge from this observation."

The property of rotating light, which has been known for more than two centuries to exist in many molecules, already has many applications in medicine, electronics, lasers and display devices.

Although this phenomenon has been predicted for a long time, no one -- until now -- had created such a racemic mixture (a combination of equal amounts of mirror-image molecules) and measured the optical activity.

"How do you deliberately create these materials?" Poeppelmeier said. "That's what excites me as a chemist." He and Gautier painstakingly designed the material, using one of four possible solid-state arrangements known to exhibit circular dichroism (the ability to absorb differently the "rotated" light).

Published in Nature Materials.