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I know that small amounts of matter can be converted to energy via chemical and nuclear reactions, and that complete conversion is possible if matter meets anti-matter. Other types of conversion might proceed more exotically, e.g., in a black hole collision. But I have a more theoretical question; is there any reason in principle that there couldn't be some way to directly turn 100% of a normal piece of baryonic matter into pure energy, without the trouble of first making anti-matter? (And, ideally, without most of it coming in the form of gamma rays, though with a large enough reactor one could convert this to heat and hence useful energy.) Obviously we don't know any practical way to do this--we're not even at fusion yet--I'm just wondering if there's some fundamental barrier to it.

Part of my interest is speculation on the Fermi problem; if large civilizations even in distant galaxies used the most readily-available source of energy we are aware of (stars) via, say, dyson swarms, we would notice a spectrum change tipping us off to that, and haven't. But if they've discovered some way to just convert matter directly to energy, stellar energy might be passe for them, and they could be flying around their stars using relatively invisible energy sources.

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Some theories do allow baryon number violation, but they do conserve $B-L$, or the difference in baryon number and lepton number.

Under such a physics, proton decay is possible:

$$ p \rightarrow e^+ + \pi^0$$

followed by:

$$\pi^0 \rightarrow \gamma\gamma$$

In bulk hydrogen, the position would then undergo annihilation with the atomic electrons:

$$ e^+ + e^- \rightarrow \gamma\gamma$$

(Note, there these are the main decay channels, and are not meant to be comprehensive).

If this reaction used atoms (or isotopes) other than $^1H$, then you'd be left with neutrons, and they would also decay:

$$ n \rightarrow p + e^- + \bar\nu_e$$

leaving more products with which to make hydrogen.

So, yes, there is speculative physics (but not speculative to the point that large experiments haven't been conducted to look for it).

If the proton is unstable, then its lifetime is extremely long, and there is no standard physics that catalyzes the reaction, so engineering challenges remain.

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Conservation of baryon number might be a problem.

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  • $\begingroup$ Hello! Could you possibly further elaborate on why this might be a problem? Thanks! $\endgroup$
    – jng224
    Commented Oct 18, 2020 at 18:08
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    $\begingroup$ As it stands this is a comment, not an answer. $\endgroup$ Commented Oct 18, 2020 at 18:42
  • $\begingroup$ And my initial look into this suggests that while baryon number is /usually/ preserved, there appear to be exotic cases where it isn't (black hole decay, baryogenesis in the early universe). So unless there's some underlying reason why it can only happen in those exotic cases, it's possible that better understanding why it does happen there might be used to make it happen in less exotic cases, for a sufficient advanced civilization. $\endgroup$ Commented Oct 18, 2020 at 18:50
  • $\begingroup$ @StephenG I was initially going to make this a comment, but then I was reminded of the usual comment policy ("ask for more information or suggest improvements") and decided it would be more appropriate as an answer. $\endgroup$
    – Daniel
    Commented Oct 18, 2020 at 20:17
  • $\begingroup$ @Scott my (very shallow) understanding is that nothing in the Standard Model violates conservation of baryon number (EDIT: Looks more complicated, see here). It's possible that there's some beyond-standard-model physics that changes this, but almost by definition that means these processes are not well-understood enough for us to make predictions about how they could be exploited for power sources. $\endgroup$
    – Daniel
    Commented Oct 18, 2020 at 20:20

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