In fact, the EM stress-energy tensor is not sufficient to define EM momentum correctly, because the way to construct EM tensors is not unique. According to Minkowski and Abraham, a general EM tensor could be defined as formula_45, where formula_46 is the Abraham tensor, formula_47 is the Minkowski tensor, and formula_48 is an arbitrary constant. Thus there are infinite EM tensors within the Maxwell-equation frame; formula_49 for formula_50, formula_51 for formula_52, and formula_53 in free space. From this one can see that the EM tensor is not sufficient to correctly define the momentum of light in a medium.