  We address and solve the long-standing gauge-invariance problem of the
nucleon spin structure. Explicitly gauge-invariant spin and orbital angular
momentum operators of quarks and gluons are obtained. This was previously
thought to be an impossible task, and opens a more promising avenue towards the
understanding of the nucleon spin. Our research also justifies the traditional
use of the canonical, gauge-dependent angular momentum operators of photons and
electrons in the multipole-radiation analysis and labeling of atomic states;
and sheds much light on the related energy-momentum problem in gauge theories,
especially in connection with the nucleon momentum.
