The special-relativistic relation connecting energy with mass, $E^2 - (m c^2)^2= (pc)^2$. May be used to provide accounting constraints in energy and momentum, both conserved in total, even in reactions where $m$ is not.
The special-relativistic relation connecting energy with mass, $E^2 - (m c^2)^2= (pc)^2$. May be used to provide accounting constraints in energy and momentum, both conserved in total, even in reactions where $m$ is not.