It is difficult enough to describe what light is, but Xiao-Gang Wen of Massachusetts Institute of Technology in Cambridge has undertaken to explain why it is.Proceeding from the failure of Landau's theory of phase transitions to explain such phenomena as the fractional quantum Hall effect and superconductivity, Wen has developed the concept of quantum orders -- descriptions of ground-state wave functions where quantum effects are important. In the Jan. 7 issue of Physical Review Letters, he illustrated that three-dimensional spin models of the vacuum lead to quantum orders that generate gauge fluctuations that behave like light.Moreover, according to Wen's model, perturbations around the mean-field state do not lead to symmetry breaking in the quantum order, and the existence of massless gauge bosons such as light thus is preserved.