Is there any fundamental law of physics that says if you want to turn mc^2 energy into mass, that you have to create particle/antiparticle pairs together? Or could you, theoretically, create exclusively antimatter, and we just don't have a known method of doing so?
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?
Charge is conserved, so if your energy input is in the form of (say) a photon, you'll have to produce equal numbers of positive and negative charged particles.
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)
In the Forge of God universe there is a way to convert chunks of matter (like you and your spaceship) to antimatter, for instance it's a violation of the conservation rules that actually get conserved to turn hydrogen -> antihydrogen.
AFAIK a black hole could be a near perfect mass energy converter, and I think the evaporation rate (Hawking temperature) can be regulated by adjusting black hole spin.
There’s just the wee problem of getting or making a black hole and then grabbing it and controlling its spin.
There's the second problem that the temperature and power are directly related, so if you want the temperature low enough to not photo-ionise stuff much (6eV/70,000 K) then be power output is 114 microwatts.
> Charge is conserved, so if your energy input is in the form of (say) a photon, you'll have to produce equal numbers of positive and negative charged particles.
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"?
The neutrons and the antineutrons are made of multiple electrically charged particles whose electric charges sum to zero, exactly like the electric charges of all particles that compose a neutral atom or neutral anti-atom sum to zero.
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.
>The neutrons and the antineutrons are made of multiple electrically charged particles whose electric charges sum to zero, exactly like the electric charges of all particles that compose a neutral atom or neutral anti-atom sum to zero.
Does this mean neutrons are (or can be) polarized like a water molecule (but much weaker)?
Charge here is just one thing that has to be conserved. Baryon number is a property of quarks and must also be conserved, and this prevents a neutron turning into an anti neutron as that would have opposite baryon number.
There are others such as isospin, lepton number…
These types of conservation laws are what limits the possible particle interactions.
This is an open question, and an important one to understanding the early history of the universe. Essentially, as far as everything we have seen in the lab shows, and everything we know about the laws of physics says, certain quantities like charge must be conserved, so matter and antimatter are always generated together. However, this creates a glaring issue- where the Hell is all the antimatter? The universe is, as far as we know, made almost entirely of matter with very little antimatter, and so there must be some kind of asymmetry somewhere that we haven't found yet. Otherwise the universe would be 50/50 matter/antimatter, or would just be constantly forming and destroying matter and antimatter.
From Wikipedia: "The Standard Model of electroweak interactions has all the necessary ingredients for successful baryogenesis, although these interactions have never been observed[11] and may be insufficient to explain the total baryon number of the observed universe if the initial baryon number of the universe at the time of the Big Bang is zero." In other words, there is a process that can in in principle create more matter than antimatter, but it has not been observed experimentally yet.
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.
IANAP (physicist, just an Engineer) but any sort of reaction like this is probably pretty hard to control so you won’t get twice the energy. Just like with combustion engines, you’re probably looking at some significant percentage loss (~50%?) in efficiency from various things.
(I’m sure there are other things that would also hinder the 2x outlook you are asking about)
You have a solid intuition, in addition to the usual efficiency losses, a large amount (20%-50% would be usual, up to 80% in some cases) of the energy released during annihilation is in the form of... neutrinos. I don't foresee a realistic means of harnessing that energy any time soon, even where "soon" indicates quite a stretch of time.
Yeah, as the article here notes, the energy released is effectively an explosion, and harnessing that energy into something more useful is a substantial challenge. As is the concept of turning energy into specific (anti-)matter, which we have no idea how to do at the moment.
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.)
> 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.
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?
Project Orion style pusher plate nuclear pulse designs can handle turning big boom boom into space propulsion but as the parent said you are not 100% efficient.
One obstacle is momentum conservation: You can't just turn a single massless particle (a photon, say) into a massive one or vice versa because that would violate conservation of 4-momentum. The way out is to involve more than one massless particle in that interaction, e.g. convert two massless particles into one or more massive ones, or vice versa. (If a single massive particle is produced, the 3-momentum of the two massless particles cancels out in the center-of-mass frame, while their energy / p_0 adds up to the rest mass of the particle that's produced.)
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.
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?