摘要：

Descriptions of the most basic quantities like energy, momentum, andangular momentum are unexpectedly troublesome in gauge theories.This talk carefully explains these troubles and presents thesolutions for both Abelian and non-Abelian gauge theories. Thesesolutions lead to critical and testable implications for nucleonstructure and gravity, e.g., how the gluon contributes to thenucleon spin and momentum, and how a particle's spin generatesgravity.