Four-dimensional physics in two dimensions. For the first time, physicists have built a two-dimensional experimental system that allows them to study the physical properties of materials theorized to exist only in four-dimensional space. An international team of researchers demonstrated that the behavior of particles of light can be made to match predictions about the four-dimensional version of the 'quantum Hall effect' -- a phenomenon at the root of three Nobel Prizes in physics -- in a two-dimensional array of 'waveguides.' Source 9y.