Matter can block light, and thus appear opaque, by absorbing the light or by scattering it. Opaque materials with low absorptivity—milk, white paint, and some living tissue, for example—are generally well described as random arrangements of scattering particles. When light hits them, much of it is reflected, but some of it is transmitted as a diffuse blur with random speckles due to constructive and destructive interference. The thicker the material, the less light is transmitted.
Since the 1980s, theorists have used random matrix theory to study the scattering of both photons and electrons. 1 They found that for the transmission matrix of a randomly scattering material, each of the eigenvalues is either nearly zero or nearly one. That is, any wave entering a scattering material can be represented as a sum of modes, some of which are entirely transmitted and some of which are entirely reflected. Thicker samples have...