  A cornerstone of physics, Maxwell's theory of electromagnetism, apparently
contains a fatal flaw. The standard expressions for the electromagnetic field
energy and self-mass of an electron of finite extension do not obey Einstein's
famous equation, $E=mc^2$, but instead fulfill this relation with a factor 4/3
on the left-hand side. Many famous physicists have contributed to the debate of
this so-called 4/3-problem but without arriving at a complete solution. Here, a
comprehensive solution is presented. The problem is caused by an incorrect
treatment of rigid-body dynamics. Relativistic effects are important even at
low velocities and equivalence between electromagnetic field energy and
self-mass of the electron is restored when these effects are included properly.
In a description of the translational motion of a rigid body by point-particle
dynamics, its mechanical energy and momentum must be defined as a sum of the
energies and momenta of its parts for fixed time, not in the laboratory as in
the standard expressions but in the rest frame of the body, and for consistency
of the description, the energy and momentum of the associated field must be
defined in the same way.
