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Matter-energy conversion is something that is often assumed for processes in calculations, but rarely justified.
Basic knowledge about how it works
Matter-energy conversion happens quite often in nature. For example any chemical reaction that requires energy to run will convert the energy required by the reaction into a very slight additional mass. For example, photosynthesis which takes
6 CO2 + 6 H2O + sunlight -> C6H12O6 + 6 O2
will convert the energy of the sunlight into a very very slight additional total mass of the product chemicals when compared to the total mass of reactant chemicals.
But what this article wants to focus on is mainly the option of creating new particles from energy.
Such a thing is possible through various processes, but one thing that should be known in general is that always a particle and its antiparticle is created. That has to be the case in order to conserve quantum numbers such as angular momentum and electrical charge. So for instance , if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1.
The other requirement is that energy has to be conserved. That is the important part most of the time, since it means that in order to create matter you need to have a certain amount of energy. That amount is equivalent to the rest mass of the particle, as expressed by the famous formula E = mc2 (E = Energy, m = mass). As one can see by this formula creating mass requires a very huge amount of energy.
In which cases should it be used?
Matter-energy conversion should only be used for a calculation if it is clearly stated that this is the progress used.
Why is that?
One of the most important reasons is that it simply produces unrealistic values in virtually all cases. The energy required to do so is so ridiculously high that it is almost never realistic by any means.
The other reason is that creating matter from nothing, be it now by creating objects out of nowhere, making things grow larger and heavier, or even energy beings taking physical form or regenerating injuries, is that it is almost never intended to work that way. Most of the time the process isn't thought through and simply exists for the plots sake. In fiction, where laws of physics are broken in various forms, the assumption that it has to have worked in this way doesn't hold and it may just be some other application of fictional science or just working, because it works, without any explanation considered.
One could further argue that depiction generally doesn't fit the actual phenomena (for example because of a lack of anti-particles being created and annihilated) and that, most of the time, characters or devices that perform the feat are officially aren't even mentioned to have anything similar to such abilities.
So all in all in such cases scientific analysis of such circumstances should not be done, as it leads to unrealistic results not depicting the actual power of the character/weapon.