# I have an unlimited supply of antimatter -- how do I actually make my spaceship go?

I'm making a book series set in the medium and far future of humanity, at various stages. Some elements are based in harder science than others. Throughout the timeline, Dyson swarms serve as the VIP energy sources, and antimatter serves as the ultimate battery material.

My initial goal with this question is to see how much I should rely on annihilation products. A rocket or interstellar engine that uses this would be 100% efficient in its conversion of mass to energy, but I've found it difficult to find a theoretical engine (and its specific impulse) that mainly uses antimatter and isn't lumped into a catalyzed fusion engine. In my mind, fusion fuel is the limiting factor, not the antimatter. And in this case, I want the opposite (because there's a lot of antimatter produced).

For reference:

(antimatter represented with $$\overline{M}$$, matter with M, all in kg)

I assume that the energy captured by a DS, built from Mercury, would give me 3.8x10^24 Joules per second, somewhere around 1% the energy output from Sol.

Given E=mc^2, if I were to dedicate all the energy from the swarm, I would be pumping 10^9 kg of $$\overline{M}$$ per second.

Obviously that isn't the most realistic, even for fiction, so let's say only 5% of the energy being output is used to create antimatter batteries, which I'm thinking could be $$\overline{M}$$-Iron for easy magnetic containment (although I'm not sure how much easier that would be to produce). Maybe the act of transferring energy throughout the swarm and production sites isn't optimized or constantly has some flaw that needs fixing, so it never runs past 10% true capacity.

Multiply $$\overline{M}$$/s by that, round down just to be safe, and we get a more likely 5*10^6 $$\overline{M}$$/s

That's still a lot, no? Especially when it's effectively doubled by the addition of M. And I'd like to greedily exploit this energy density of $$\overline{M}$$ and M, but alas, a method to extract a satisfactory amount of work from $$\overline{M}$$+M annihilation has alluded me, so here I am. And I don't want to have humanity go all over the solar system or beyond looking for deuterium and helium-3. That's boring!

Edit: The first few replies seem to be confused on what my question is. I'm asking if there's any theoretical way of specifically using the products of annihilation for thrust.

• Is there a worldbuilding question here? It appears you're asking if your understanding of current scientific knowledge concerning antimatter is correct (something that might be more easily done on Physics). Humanity is only just scratching the surface of what antimatter is and what it can do, so asking if it can support your imaginary world is a bit like asking if a cartoon character can shout. The only viable answer is "yes" because use of antimatter as an energy source is more technobabble than science today. Are you only verifying your understanding of antimatter?
– JBH
Mar 13, 2023 at 22:41
• Are you asking: what is a plausible anitmatter engine? Such as injecting x g/s of antimatter into a 'combustion' chamber along with y Kg of matter? Mar 13, 2023 at 23:54
• Have you checked Wikipedia? en.wikipedia.org/wiki/Antimatter_rocket Mar 14, 2023 at 2:51
• @RobertRapplean I suppose the plasma core is the closest thing I have to an answer (I should've put that in the question desc). Just asking people who might know more than the wiki page, sorry if this was a silly question. Mar 14, 2023 at 3:18
• It's not a silly question, it's just one that can be readily answered based on a little bit of googling. Wikipedia actually does an pretty good job of identifying the reality and theory behind hard science fiction technologies, and it gives you links to where you can learn more. I know as much about quantum physics as anyone here, and I'd just be embellishing on them. Our lack of useable antimatter makes actual experimentation impossible, so your fiction is as good as mine. Mar 14, 2023 at 5:52

## Antimatter Reactions

Mixing matter and antimatter gets you some energy, some charged particles, and some neutral particles. The energy is in the form of high-energy gamma rays which radiate with spherical symmetry and aren't easily reflected/collimated, so aren't great for thrusting directly; the charged particles are pions and are easy to thrust off of electromagnetically; and the neutral particles are neutral pions which quickly decay into more gamma rays.

There are some ideas for using the annihilation gammas for thrust, like Winterberg's photon rocket which uses electron-positron annihilation and special plasma shenanigans to create a collimated gamma ray exhaust. Basically, riding a gamma ray laser.

Most approaches redirect the charged pions for thrust, which make up about 66% of the annihilation energy. The "energy" here is in the form of kinetic energy of the particles which travel at >90%c. The remaining 30% is wasted as useless gammas, unless otherwise used for thrust by somehow collimating or reflecting them.
(In my worldbuilding universe, builders use metric engineering to collect and down-scatter the gammas into more useful energies.)

All the above assumes proton-antiproton annihilation, from something like antihydrogen snow (perhaps coated in a thin layer of antilithium to alleviate sublimation and keep the vapor pressure down) and ordinary cryogenic hydrogen. Heavier nuclei have the benefit of a high antimatter-mass-to-storage ratio, especially using materials with magnetic properties like iron or YBCO. However, the drawback is that you'll have to contend with major neutron (and antineutron) radiation, and fast nuclear fragments, all of which are hard to shield against. The heavier the anti-element, the more neutrons you'll have to deal with. The neutrons, pions, and gammas will all work to transmute engine components into radioactive isotopes. Antineutrons will annihilate with ordinary ship matter (protons and neutrons) too, creating more of the same. On average, they'll travel through ~1 meter of solid/liquid material before interacting with something.

With heavy magnetic antimatter, your storage mass (the mass of containment machinery) goes waaay down, but your shielding goes waaay up (as if it wasn't huge enough with just proton-antiproton). This may actually be preferable when your storage mass is i.e. a large multiple of your antimatter mass. The technological penalties of storing antihydrogen are for you, the author, to decide.

## Antimatter Engine

If you want to go the heavy route, something like anti-buckyballs might be useful. They're diamagnetic so you could levitate them with a magnet. They're solid up to really high temperatures so you don't have to worry about cryogenics. A hypothetical design might use a laser to ablate a bunch off a larger "dust ball" and electrostatically transport them to a reaction point in the engine, annihilating with an ordinary matter stream, e.g. hydrogen, and pushing off the charged pions/particles with a magnetic nozzle. Particles may range from anti- protons, deuterons, tritons, alphas, etc. You might also get fragments of anti- beryllium, lithium, and boron. It might be more efficient to use the products to heat a little more normal reaction mass. The magnetic pressure from the reaction can work backwards to generate onboard electricity for the starship, perhaps powering some kind of Bussard magnetic scoop to augment the normal matter propellant supply.

The nozzle structure would be large and massive with lots of narrow-angles and a bladed appearance, like a wedge bent into a circle (and many of those stacked into a cone), to help deflect neutrons and gammas by shallow-angle effects and to create the smallest possible cross-section facing the antimatter reaction while still containing all the machinery for the magnetic nozzle (electrostatic nozzles might also work), like heat exchangers for the superconducting ring cryogenics (assuming no RTSCs). Somewhat like the image below:

Source: NASA

The nozzle shielding may largely be heavy tungsten, and the machinery might use supplemental radiators extending outwards to help drive some of the intense heat away. The shielding will fluoresce in all wavelengths visible, thermal, and x-ray, and will ideally operate at the edge of what it's capable of handling, temperatures just shy of vaporizing.

## Hydrogen Funnel

also called a Bussard Ramjet. Larry Niven invented it. So you know it's good. Your spaceship travels by filling itself up with hydrogen and opening the back door.

Then someone with an antimatter gun crosses the beams with someone with a matter gun. This makes a big boom. All the hydrogens go flying around with blinding speed. Some of the hydrogens hit the front wall of the spaceship. This makes the spaceship faster. Some of the hydrogens go flying out the back door. Due to conservation of momentum, this makes the spaceship faster.

But what do you do when you run out of Hydrogen? Why simply gather more. There are hydrogen atoms in space. Not many, but once you are going a million miles a second, there are a million miles a second worth of not many's to suck up and shovel into your antimatter reactor. The spaceship gathers interstellar hydrogen atoms in its funnel and poos them out to go fast.

Toot toot.

And if we scoop up anything other than boring old hydrogen atoms -- then even better. Maybe some ancient alien artifacts, or an exciting and contagious new lifeform.

If we are super lucky, we might have a sexy alien babe fall down out funnel, leading to hilarious antics. Imagine the adventures we will have, respecting her as a clearly not human but obviously sentient individual being, and going to great effort, no doubt, to return her safely to her home planet.

• Why isn't it called the Niven ramjet? Also you failed to mention Bussard and how he invented the megastructure known as the Niven Ring. Mar 14, 2023 at 21:10

Antimatter-annihilation gamma ray laser relativistic photon rocket. Or, the Winterblaster.

The 'Winterblaster', proposed by theoretical physicist F. Winterberg, is a type of photon rocket powered purely by antimatter-matter reactions. It is effectively a 'torch-ship', both in the literal and abstract sense. Point towards your desired direction, and off you go.

The usual problem with antimatter being your energy source is that although it is the most energy-dense fuel available, the reaction products of its annihilation with normal matter counterparts are for the most part entirely unusable - unable to be directed effectively to create thrust.

However, Winterberg proposes an ingenious method to direct gamma emissions preferentially along an axis, creating high energy photon exhaust. With simple anti-hydrogen and positrons, along with their matter counterparts (hydrogen and electrons), you can achieve some pretty crazy outputs.

How it works:

The electrons and positrons are directed into counter-propagating relativistic beams, which produce an extremely powerful 'pinching' magnetic field which heavily compresses the remaining anti-hydrogen/hydrogen along a line. They promptly annihilate, and due to their compression along a singular linear axis, preferentially emit gammas in that direction.

[![H0w 1t wurkz][1]][1]

Both the extreme high energy photons and the resulting plasma against the magnetic field work to propel the ship to relativistic speeds.

Some things to consider.

We're not totally sure whether the laser comes out... both ends? So you could end up getting high energy gammas UP your backside instead of OUT. The question of whether antiproton-proton annihilation can even be forced to annihilate with only photon production and no other products is also up in the air.

However, isn't this thing just cool?