This really isn't a homework question per se, but I really don't want to post in the big boys' fora. I am learning about basic modern physics at school, as the title suggests, but I am very confused on one matter. Take the tritium nucleus as an example. If tritium nucleons are separate from each other, but still within 10-15m of each other, the particles have potential energy for the nuclear force they are experiencing. When they come together, this potential energy is released entirely as a photon. However, this newly formed nucleus also has lost mass when compared to the sum of the individual nucleons. This mass has magically vanished and is proportional to the energy released in the photon. Basically, if you view the magical mass as energy (which it is, at this level), conservation of energy has been broken. There is twice as much energy in the individual state (as mass and nuclear potential energy) as there is in the combined state (just a photon). I don't suppose anyone can explain this? I feel like I'm missing something obvious here.