Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))

The authors claim to have found a “proper”, “gauge-invariant” definition of a charged-particle’s momentum in gauge theory, which is more “superior” than the textbook version. I show that their result arises from a misunderstanding of gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is not physical.