Yeah ok because KE depends on velocity and velocity is relative to something, I get that but i'm still confused about reference frames.

If you have 2 spaceships travelling in different directions, spaceship A and spaceship B, you could consider them both to be moving apart or you could consider one to be stationary and the other to be moving. If each spaceship has one of these detectors, and you considered spaceship A to be stationary, they would just get the energy E from converting the mass to energy. But relative to them spaceship B's matter/anti-matter has alot more energy because of their velocity, so they would predict spaceship B to get more energy out of their matter than they did from theirs, because they would get E + KE, wouldn't they?

"they would predict spaceship B to get more energy out of their matter". This phrase is meaningless if you don't know which is the relative velocity between A and B. A detects energy coming from B, but A

cannot establish how much mass (invariant mass) was associated to that energy, if you don't know the relative velocity. Example: if A and B are stationary to each other and B converts 1 kg of uranium into photons sent to A, then A will receive 1*c^2 = 9*10

^{16} Joules of energy in the form of photons, and A will be able to deduce that 9*10

^{16}/(3*10

^{8})

^{2} = 1 kg of matter has been converted into energy from B. But if A and B are moving relative to each-other and you

don't know which is the relative velocity, then, from the photon's energy that A receives from B (let's say it's still 9*10

^{16} Joules), it's

impossible to establish how much it was the invariant mass converted into photons inside the spaceship B.

If, instead, A knows which is the relative velocity between them, he can compute how much mass B has converted into energy: let's say A receives E joules and the relative velocity is V, approaching; then the energy E

_{0} which B has converted into photons is:

E

_{0} = E(1-V/c)/(1+V/c)

and the mass he has converted is E

_{0}/c

^{2}This without knowing if A is stationary and B moving or the other way around or both moving, it doesn't matter, you only need to know the relative velocity V. Clearly, if the velocity is of receding, you only have to change the sign of V.

Example: A receives 9*10

^{16} Joules of energy as photons, and he knows that the relative velocity of approaching between them is 0.5c. So:

E

_{0} = E(1-0.5)/(1+0.5) = E/3 = 3*10

^{16} Joules

So B has converted 3*10

^{16}/(3*10

^{8})

^{2} = 3*10

^{16}/9*10

^{16} = 0.333... kg into energy.

I don't know if this helps you, in case ask again.