 For example, at low energies the interaction of a photon with a nucleon is a very complicated calculation involving interactions between the photon and a sea of quarks and gluons, and often the calculation cannot be fully performed from first principles.Often in this context, form factors are also called "structure functions", since they can be used to describe the structure of the nucleon.However, the generic Lorentz-invariant form of the matrix element for the electromagnetic current interaction is known,
where formula_2 represents the photon momentum (equal in magnitude to "E"/"c", where "E" is the energy of the photon).