Hello, By far, the easiest explanation of E=mc2, Einstein’s famous mass-energy equation, means that mass is a form of energy and that, mass can be turned into energy (heat, light, other particles, etc) and vice versa. It also seems easiest to say that photons are “pure energy” and are “massless”, based on E = pc. However, I ran across this on Wikipedia: Mass–energy equivalence: Efficiency. Although mass cannot be converted to energy, in some reactions matter particles (which contain a form of rest energy) can be destroyed and converted to other types of energy that are more usable and obvious as forms of energy—such as light and energy of motion (heat, etc.). However, the total amount of energy and mass does not change in such a transformation. Even when particles are not destroyed, a certain fraction of the ill-defined "matter" in ordinary objects can be destroyed, and its associated energy liberated and made available as the more dramatic energies of light and heat, even though no identifiable real particles are destroyed, and even though (again) the total energy is unchanged (as also the total mass). This doesn’t seem correct to me. If the wiki statement above is correct, I would like to understand why. As a counter-example, it seems to me that if two photons (energy) come together and produce a matter-antimatter pair (give that all requirements are met), it seems to me that: before the reaction you had two photons (invariant rest massless) and after the reaction you had two particles with real invariant rest mass, a matter and antimatter particle. The invariant-rest-mass-less photons are gone; in their place are two real particles that have real invariant mass. Obviously, I have no problem accepting that energy is conserved, but how can invariant rest mass be conserved if there isn’t any invariant rest mass associated with the two photons or annhilation? Of course, if someone mentions the “mass of a photon”?, it certainly cannot be the same “mass” as the invariant rest mass of a proton, electrons, pion, etc. If you can have a mass of a photon, why is there no such term in E2 = mc2 + pc? I do feel its splitting hairs to say: “Energy isn’t really created because mass is already a form of energy.” As one article I found said. Then I would just say, “Well, then, what I mean is that the photon mass can be converted into the invariant rest mass of particles and vice versa.” Also, it seems, that in the article below, the idea that matter and energy can be transformed from one to another via E=mc2] might be conceptually sound. I do not know how experts on this website feel about the validity of the below proposal, but I would be interested to hear what they say. The article below is one-way so far: photons to mass. Of course, if that is true then wouldn't there be a concept to go from mass to photons also? http://phys.org/news/2014-05-scientists-year-quest.html Anyway, I am confused.