Two beams of light, red and blue, enter a tiny ring made of silicon nitride. The ring is smaller than the thickness of a human hair. At the input, both beams are equally bright. But inside the ring, one of them suddenly starts to outshine the other, like two equally matched people in a debate where one unexpectedly gains the upper hand.

This is the first time such spontaneous color separation has been seen in a micro-ring. Light changes the material it passes through: the bright beam slightly stretches the crystal lattice, which then begins to transmit different colors differently. A feedback loop emerges: one color gets stronger, the other weaker. The threshold at which this begins is just 19 milliwatts — less than what a laser pointer needs.

This behavior of light resembles the workings of a neuron: a weak signal doesn't pass, a strong one does, and in between there's a smooth transition. The scientists showed that the ring can produce three different responses to an input signal, like three different personalities. This means that such rings could be assembled into entire computing circuits where light flows instead of electricity.

The light in the ring chooses on its own which color to favor. This whimsy can be tamed and made to work for us, like a river that chooses its own course, and we build a mill on it.