energy
Next, consider the same system observed from frame S′ that is moving along the x-axis at speed 



v


{\displaystyle v}

 relative to frame S. In this frame, light from the forwards and reverse pulses will be relativistically Doppler-shifted. Let 




H

0




{\displaystyle H_{0}}

 be the energy of the body measured in reference frame S′ before emission of the two pulses and 




H

1




{\displaystyle H_{1}}

 after their emission. We obtain the following relationships:
