Annihilation is accepted only if the energy and momentum ledgers both close. Here the input energy is 100 J plus 100 J, the output carries the same total, and photon momenta 20 and -20 kg m per s cancel to zero. Real reaction checks require measured particle energies, directions, detector resolution, missing-energy limits, and uncertainty, but this small ledger shows the conservation bookkeeping.
The reaction ledger must close energy and signed momentum together. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Input and output energies match
The input energy is 100 J plus 100 J, and the output is the same split.
100+100=200
Opposite photon momenta cancel
The photon momenta are 20 and -20 kg m per s, so the total is 0.