energy see
The two differences of form 



H
−
E


{\displaystyle H-E}

 seen in the above equation have a straightforward physical interpretation. Since 



H


{\displaystyle H}

 and 



E


{\displaystyle E}

 are the energies of the arbitrary body in the moving and stationary frames, 




H

0


−

E

0




{\displaystyle H_{0}-E_{0}}

 and 




H

1


−

E

1




{\displaystyle H_{1}-E_{1}}

 represents the kinetic energies of the bodies before and after the emission of light (except for an additive constant that fixes the zero point of energy and is conventionally set to zero). Hence,
