> As far as I know, there's no known physics which "forbids" directly converting matter to antimatter with substantially less energy than going via energy (no conservations violated, etc)
I think you'd have to put in at least as much energy as you get back out through annihilation of the antiparticle, which would be half the energy released (the other half coming from the destruction of the normal matter).
And actually, we already have a working energy source that works by annihilation of matter: nuclear energy. Any future antimatter-based power plant would have to compete for full-cycle efficiency against future nuclear (possibly even fusion) designs.
"I think you'd have to put in at least as much energy as you get back out through annihilation of the antiparticle, which would be half the energy released"
Yes, but if you put ten kilograms of matter in, and get back 10 kilograms of antimatter, the energy equations are balanced; 10 kilograms times c^2 on both sides. Some sort of hypothetical device which somehow reaches into matter and flips it to antimatter via some relatively cheap technique, such as perhaps rotating it in a rolled-up dimension in such a way that the result is antimatter (to give one example of plausible sounding English words that have no actual referent in the real world), does not violate conservation. It just doesn't seem to be possible, either.
And if you're going to get technical, nuclear power plants have no special claim on converting mass to energy. All power plants do that. Combustion products are ever so slightly less massive than their original components, corresponding to the energy released in the chemical reaction.
EDIT since I can't reply. Particle/antiparticle "annihilation" is the conversion of their mass to energy. This is the same outcome as with the converted mass in a nuclear reaction.
If your argument is that I misused the word "annihilation" because it's a technical term in physics referring strictly to the mass->energy conversion in a matter/antimatter reaction, then I will concede that semantic point to you.
Indeed that happens. But it's still not annihilation[1], it's just a conversion of relativistic mass (which comes from energy of nuclear bonds) into kinetic energy.
I think you'd have to put in at least as much energy as you get back out through annihilation of the antiparticle, which would be half the energy released (the other half coming from the destruction of the normal matter).
And actually, we already have a working energy source that works by annihilation of matter: nuclear energy. Any future antimatter-based power plant would have to compete for full-cycle efficiency against future nuclear (possibly even fusion) designs.