  Einstein claimed that the fundamental dynamical insight of special relativity
was the equivalence of mass and energy. I disagree. Not only are mass and
energy not equivalent (whatever exactly that means) but talk of such
equivalence obscures the real dynamical insight of special relativity, which
concerns the nature of 4-forces and interactions more generally. In this paper
I present and defend a new ontology of special relativistic particle dynamics
that makes this insight perspicuous and I explain how alleged cases of
mass-energy conversion can be accommodated within that ontology.
