Hierarchies in matter

A single neuron in isolation cannot be said to possess memory, feelings, or consciousness. However, group many neurons together and the type of advanced information processing that takes place in the human brain suddenly appears. This suggests that perhaps each level in the micro-macro hierarchy can only be understood with a different logic. Might such a hierarchy also exist in ordinary materials that appear on first glance to be more straightforward and less complicated than neurons?

Professor Yoshinori Tokura of the Graduate School of Engineering (also Center Director of the Riken Center for Emergent Matter Science) typically begins his undergraduate lectures in solid-state physics—the science of the characteristics of matter—in the following manner: “What gives things their color? Why are metals silvery and leaves green? The phenomenon of color is entirely due to the movement of electrons.” What does he mean by this?

“Due to the movement of electrons” doesn’t necessarily mean that individual electrons are responsible for the luster of a metal or the color of an object. The real significance is rather that it is the interactions between electrons that are important.

It may be tempting to think that a material’s macroscopic characteristics—for example, its mechanical, electrical, magnetic, optical or other physical properties—can be understood by reducing it down to the microscopic state, focusing on aspects such as the arrangement of atoms, bonding between atoms, and the behavior of electrons around atoms. However, when emergent phenomena arise for a collection of individual parts, in which the characteristics of the whole surpass the sum of the parts, it is no longer reasonable to adopt this type of reductionist approach of trying to understand macroscopic properties by recourse to the microscopic level.