Reversibility energy, veshestva

hmm, you might like reaction with the absorption of energy? these, of course, are called endothermic.
for example, N2 + O2 = 2NO — Q (specifically: 181 kJ)

What do You think, and where did the radioactive substance that disintegrate with the transformation of mass into energy?
Despite the fact that initially we had only atomic hydrogen.
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I.e. Yes, these reactions occur in the interiors of stars constantly, and thanks to them we have a whole set of the periodic table.

Energy is a kind of measure; interactions, thermal motion of the particles, etc., is a scalar value. So the energy is not convertible to substance. It is not the radiation (perhaps you are confusing one with the other).

In quantum physics there is the concept of "virtual particles", which can appear in the vacuum in pairs and exist for a short time. If there is a pair electron-positron, at certain intensity, the electric field can "take away" this pair, and as a result, under gatsbies electric field energy formed 2 separate particles of matter.

when protons collide at the LHC (for example) gives rise to a shower of various particles whose total mass can be much greater than the mass of the original protons. Where does the "extra" weight? :)

Conventional nuclear fission is not an option? I attach energy, get a larger total mass. Annihilation (particle-antiparticle) as far as I remember is also reversible.

Called "synthesis".