1. The problem statement, all variables and given/known data one is powering a photonic rocket with photons. what is the speed it reaches when its mass is half its starting mass. and per say you would start stoping then ... how would that look like (equations and sentences) 2. Relevant equations invariance of the 4D vector of momentum (W/c)^2 - p^2=invariant 3. The attempt at a solution i know the problem is solvable using the law that energy is constant and that momentum is comstant but i was wondering how would one do this using the above equation ... i d prolly need to consider the momentum of the light the rocket emitted, yes?