The dressing in your salad might redefine science if you look carefully enough. Researchers in the Netherlands used a drop of olive oil to discover a new universal law of phase transitions. The research team, which also included Rodriguez’w PhD students Zou Geng and Kevin Peters, increased and decreased the distances between the mirrors at different speeds and noted how light transmitted through the cavity was affected. They saw that the direction in which the mirrors moved influenced how much light got through the cavity, finding that “the transmission of light through the cavity is non-linear.” This behavior of light, called hysteresis, is present in the phase transitions of boiling water or magnetic materials. What’s fascinating, this system had “memory” in how the oil made photons interact with themselves, as the group leader Said Rodriguez explained. “We created a system with memory by placing a drop of olive oil inside the cavity”, said Rodriguez. “The oil mediates effective photon-photon interactions, which we can see by measuring the transmission of laser light through this cavity.” However, in the cavity with olive oil, hysteresis is not always present, the researchers observed when they increased the speed at which the cavity opens and closes. Rodriguez: “In faster scans, we saw the hysteresis vanish as a function of the scanning speed. This happens at a universal rate, independent of parameters like light intensity or the strength of the non-linearity. The equations that describe how light behaves in our oil-filled cavity are similar to those describing collections of atoms, superconductors and even high energy physics. Therefore, the universal behavior we discovered is likely to be observed in such systems as well.” While it would be interesting to investigate the universal scaling behavior in other systems with memory, Rodriguez will keep his focus on oil-filled cavities. “Our system has a strong optical non-linearity at room temperature, which offers opportunities for potential applications,” he says. “We are currently investigating what happens when we couple two or more cavities. Because each system has memory, an array of cavities might eventually be useful as a computational tool, or maybe even in sensing applications.”