  Traditional massive multiple-input multiple-output (MIMO) information theory
adopt non-physically consistent assumptions, including white-noised,
scalar-quantity, far-field, discretized, and monochromatic EM fields, which
mismatch the nature of the underlying electromagnetic (EM) fields supporting
the physical layer of wireless communication systems. To incorporate EM laws
into designing procedures of the physical layer, we first propose the novel
concept of EM physical layer, whose backbone theory is called EM information
theory (EIT). In this article, we systematically investigate the basic ideas
and main results of EIT. First, we review the fundamental analytical tools of
classical information theory and EM theory. Then, we introduce the modeling and
analysis methodologies of EIT, including continuous field modeling, degrees of
freedom, and mutual information analyses. Several EIT-inspired applications are
discussed to illustrate how EIT guides the design of practical wireless
systems. Finally, we point out the open problems of EIT, where further research
efforts are required for EIT to construct a unified interdisciplinary theory.
