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What known physical processes convert matter into energy?

What known physical processes convert energy into matter?

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    $\begingroup$ Look up pair production and annihilation $\endgroup$ – bemjanim Jan 21 '20 at 7:43
  • $\begingroup$ @bemjanim: Are they the only two known processes? $\endgroup$ – Andrew Tomazos Jan 21 '20 at 7:48
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    $\begingroup$ It depends on whether you count photons as matter. Electrons can emit photons, e.g. when moving to a lower energy level in an atom. Leptons and baryons can only be produced/annihilated in particle-antiparticle pairs in order to conserve lepton and baryon numbers.The same goes for all charged particles, strange particles, etc. This leaves neutral bosons, i.e photon, higgs, which can be produced alone. $\endgroup$ – bemjanim Jan 21 '20 at 8:02
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Stereotypical examples include pair production and matter/antimatter annihilation.

Less stereotypical examples are processes that affect bound states as binding energy contributes to rest energy (aka mass). An example would be the mass defect in nuclear fusion or fission processes, though the same thing happens during other types of reactions (such as chemical ones), just less noticeably.

Finally, one should clarify that phrases like "converting matter to energy" are arguably a category error: Matter is a 'thing', whereas energy is a property of such things (or of a system thereof). Converting 'between' matter and energy means shuffling around excitations of massive as well as massless fields in ways that affect rest energy. That does not mean you can't use the phrase, just that it's a bit misleading, in particular if used in conjunction with terms like 'pure energy'.

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