University of Arizona, Tucson, USA

An expert on instabilities jumps from optically bound plastic beads to the brain.

It's not often that reading scientific papers turns my mind to the melancholic work of great Russian writers, but a recent one did.

The paper reports observations of 'bistability' in a simple optical system. Bistable systems have two stable output states for the same input. In this case, the researchers had studied the behaviour of two plastic spheres, trapped side-by-side in a pair of counter propagating laser beams (N. K. Metzger et al. Phys. Rev. Lett. 96, 068102; 2006). They found that the beads could adopt two stable arrangements, differing in the beads' separation.

Bistability arises in optical systems that show nonlinear responses to changes in light intensity and include some kind of feedback process. Here, one bead feels the position of the other because each affects the light field around it, creating the necessary feedback.

The researchers modelled how the two stable states come about, combining equations that describe the propagation of the light with others that predict the forces on the beads. I was impressed by how many physical effects are taken into account in the model.

And this is what turned my thoughts away from the physics of my research to the literature of my homeland. It is believed that some Russian authors, including Leo Tolstoy, may have suffered from what is now known as a bipolar disorder, characterized by states of euphoria and depression.

I have wondered before whether bistability in optical systems might serve as a simple model to help understand the mechanisms that underlie bistability in the human brain. Papers such as this one put that challenge in perspective — modelling a system that involves just two beads is already nontrivial.