In theory, could you use mc^2 energy to create a mass m of antimatter, combine it with m matter (which is rather more readily available), get 2mc^2 energy back out, and repeat, effectively consuming matter to make energy?
Baryon and lepton number are almost conserved, which would require you to produce (or consume) equal numbers of particles and antiparticles, unless you can figure out a way to make nonconservation happen outside of a black hole or whatever.
(Feeding matter to a black hole and using the Hawking radiation as an energy source would probably do what you describe, but there are practical difficulties)
There’s just the wee problem of getting or making a black hole and then grabbing it and controlling its spin.
Charge alone doesn't seem like it'd be a fundamental limitation here, considering that (for instance) antineutrons exist.
> Baryon and lepton number are almost conserved
Is the "almost" here something other than the Hawking radiation "one half falls in a black hole and the other half doesn't"?
There are others such as isospin, lepton number…
These types of conservation laws are what limits the possible particle interactions.
Therefore there is no path to generate antineutrons in which you do not have pairs of electrically charged particles with opposite charges that are generated or annihilated.
The annihilation and pair generation reactions are electromagnetic interactions between electrically charged particles and antiparticles with opposite electric charges, which are otherwise identical, in order to satisfy all conservation laws.
Only the neutrinos do not have electric charge and about them it has not been proved beyond reasonable doubt that the antineutrinos are different from the neutrinos (in other ways except opposite spin). Neutrinos do not participate in annihilation and pair generation reactions.
Neutrinos may appear and disappear in similar reactions that are mediated by weak interactions (i.e. by the heavy bosons), like the inter-conversions between protons and neutrons (actually between u and d quarks), but in these weak-force based reactions the energy that is produced is much less than in annihilation reactions and frequently much of it is lost by being carried away by neutrinos. It is possible to make electric generators with beta-radioactive isotopes, where this kind of reactions happen, but the only advantage of those is the extremely long lifetime, because otherwise the power density and energy density are low.
So when talking about using antimatter for energy storage purposes, that refers exclusively to generating electrically-charged particle-antiparticle pairs, separating and storing the antiparticles, then annihilating the antiparticles with their corresponding particles, resulting in extremely intense gamma radiation, which carries the reaction energy.
When the annihilation is done inside matter, which includes annihilation between nucleons (where there may be multiple annihilation events between the component quarks) the gamma photons interact with the surrounding matter or sub-nucleon components, producing a cascade of various accelerated particles, including many new particle-antiparticle pairs, which will cause later other annihilations. So from a single initial annihilation a great number of accelerated particles and gamma photons may result, but the first stage is always the generation of a pair of gamma photons.
Does this mean neutrons are (or can be) polarized like a water molecule (but much weaker)?
https://en.m.wikipedia.org/wiki/Neutron_electric_dipole_mome...
I think other than this, there are no known ways to even create more antimatter than matter in a process. But it is believed that more processes must exist, in order to explain the predominance of matter over antimatter in our universe.
(I’m sure there are other things that would also hinder the 2x outlook you are asking about)
This is more a theoretical question of whether any law of physics makes this impossible (e.g. you can't create unpaired particles), or whether this is theoretically possible but it's difficult to get enough efficiency to make it net positive.
(We currently haven't even gotten fusion to be reliably net positive; practicality is as always a set of concerns all its own.)
It's theoretically impossible, with the slight problem that there seems to be more matter than antimatter in the universe today and nobody really knows why.
Either the theory is wrong (and it being a conservation rule, then by Noether's theorem there's an equivalent symmetry* you'd have to violate if the conservation doesn't hold), or the initial value that's getting conserved wasn't ever zero.
* the wikipedia page says this is specific to continuous symmetry; but integers aren't continuous, so has this been generalised, or is it just assumed?
Which interactions exactly are possible depends on the particles & forces involved, and further conservation laws for quantum numbers (e.g. charge) that the force obeys.
TL;DR Turning a single massless particle into a single massive one is not possible, you always need at least two.
(What conservation law would it violate?)