Charge invariance refers to the fixed value of the electric charge of a particle regardless of its motion. Like mass, total spin and magnetic moment, particle's charge quantum number remains unchanged between two reference frames in relative motion. For example, an electron has a specific charge e, total spin
\tfrac{\hbar}{2}
\tfrac{\hbar}{2}
The origin of charge invariance, and all relativistic invariants, is presently unclear. There may be some hints proposed by string/M-theory. It is possible the concept of charge invariance may provide a key to unlocking the mystery of unification in physics – the single theory of gravity, electromagnetism, the strong, and weak nuclear forces.
j\mu=\left(c\rho,\vec{j}\right)
\partial\muj\mu=0