Fission and fusion can convert mass to energy, what is the mechanism for converting energy to mass?

Question

Has it been observed? Is it just theoretical? Is it one of those simple-but-profound things?

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Answer 1

First off, matter is simply a _form_ of energy, and _mass_ is a property of energy. All energy has mass, by E = mc², and all mass is energy. This goes so far as it being possible to create a black hole from light alone, called a _Kugelblitz_ (German for “ball lightning”, but not the same).

Or from a slightly different perspective, anything that changes the energy of an object also changes the mass. The underlying reaction can be nuclear, chemical, (de-)compression, falling down the stairs, … . Energy is technically never converted into mass; but you can convert it to matter, so lets talk about that:

The most purest form of what you ask for is the creation of antimatter (and along with it, equal amounts of matter). You effectively put enough energy into a small space and it can form (anti)matter. This has been achieved in several labs, CERN likely being the best known one.

Positrons, that is, anti-electrons, are sometimes even created this way by nature on our planet, by extreme lightning or cosmic rays hitting the atmosphere. This is not to be confused with “beta^^+ decay”: an atom shooting out a positron; this if anything releases energy, not uses it to create more matter.

You can also fuse or split atoms in cases where it is not energetically favourable, meaning that it will use energy instead of releasing some in total. With fission, his happens all the time in nuclear reactors whenever a neutron is absorbed in a way not intended for the reaction (that is, most of them). We can also do it on purpose.

Fusion is generally a bit tricky as you need quite extreme conditions to fuse atoms to begin with, and doing it with quite heavy ones is even harder. But this is for example what we do when we create new elements by shooting very heavy nuclei (gold, lead, and the like) into a piece made from another heavy element. In nature, it happens inside collapsing dying stars and colliding neutron stars, the extremely energetic explosions we call _Supernovae_. So there is a lot of energy (as compression, heat, light, …) around to work with.