Stupid Antimatter Question

[Darth Raptor]

Inspired by the fusion thread and an original sci-fi fic I'm working on. When we see antimatter in use in sci-fi like Star Trek, it's usually antihydrogen or isotopes of antihydrogen. Now, in fusion the reason such light elements are used is because it takes so little (relatively speaking) energy to fuse them. But in annihilation reactions the reason for the light elements is that they're less complex and easy to make (I think).

So, and here's the heart of the question: Assuming the energy and technology existed to do so, would it be worth it to manufacture more massive elements of antimatter? A gram of antiuranium would store vastly more energy than a gram of antihydrogen, as I understand it, or am I missing something?

[Darth Wong]

Pellets of anti-iron would certainly make it much easier to magnetically contain material at high densities. The annihilation reaction might be really inefficient though; I could foresee neutron/antineutron interactions which promptly cause photodisintegration of the nucleus and cause the other nucleons to go flying away too quickly to react with anything.

[Covenant]

Antihydrogen is just the easiest thing for them to wrap their head around because it doesn't get into anything more complex than just very basic antimatter pairs. Also, it's very efficent to blow up just a one-per-one thing.

Pellets of anti-iron would certainly make it much easier to magnetically contain material at high densities. The annihilation reaction might be really inefficient though; I could foresee neutron/antineutron interactions which promptly cause photodisintegration of the nucleus and cause the other nucleons to go flying away too quickly to react with anything.

Because of stuff like this you might need to use the antimatter in a bizzare fashion, such as converting it into a gas, or slicing it up with a laser, so as to get a greater degree of reaction but not to do so in an uncontrolled fashion. It also depends on how you're using the antimatter. Are you making an antimatter reactor that uses the antimatter decay to heat up another material, and creating power in a similar manner to a fission reactor? Or in the hypothetical future science thing, are you able to harvest it directly in some technofancy way?

In an ideal situation you're going to end up with a lot of two different things--neutrinos (which are next to worthless) and gamma rays, so you'll want to devise a method to best make use of that kind of reaction. Beyond that, yeah, fancier materials would work just fine. A laser could burn off some iron, which could them be magnetically sucked into the fancy device you've contrived for energy generation, and it's a bit easier than trying to create a stable blob of hydrogen or helium or something.

Anti-Uranium would be nice for the sake of density, but when dealing with antimatter you're going to want very stable elements since the actual chemical composition of the antibrick will have really no impact on the way it will create fuel. The only advantage could be if you want to create a material with an absolute truckload of neutrons on it in order to minimize the risk of it coming into contact with a proton, but somehow that doesn't seem ideal. Storing enriched anti-uranium in another brick of enriched plus-uranium is probably more complex and less intelligent than many of the other storage mechanisms! Kinda an interesting thought though. I wonder how many neutrons you'd need per proton before you slow the rate of annihilation down to the point where the brick is only incredibly hot, instead of popping and blowing itself to pieces.

[Darth Raptor]

Moving even more into the hypothetical: Efficiency isn't the paramount concern here, raw power output is. The civilization in question really only needs and uses annihilation reactions for propulsion in high performance spacecraft. Because of this, I wouldn't anticipate the gama radiation to be a problem.

Simply put, I need obscene amounts of energy that I can't get from any other source. If I can actually fly faster and further on a tank of antieka-radon than a tank of antihydrogen I'll use that. I don't really care how complex or implausible it is, so long as the benefits are actually there. Are super-heavy antielements desirable in the sense that they'll produce more energy when annihilated?

[Winston Blake]

If efficiency isn't a concern, then i see the main reason for heavier elements being storage. Since it's easier to store a slug of anti-metal than a tank of liquid anti-hydrogen (of the same mass), the mass and volume of equipment required for storage could very well let you carry more antimatter mass (ie. obscener [word?] amounts of energy) if you choose dense materials.

[Darth Wong]

One thing I find interesting is the assumption that the fuel would be carried inside the ship's fuselage. The most logical place to keep it would be floating outside the ship, dragged alongside it. Then the risk of a catastrophe is greatly minimized. If the ship is in real trouble you can just cast the chunk of antimatter adrift, make repairs, then come back to pick it up again.

[Darth Wong]

Of course, I'm assuming that we're talking about a non-military vessel here. A military vessel would presumably want to avoid keeping such a volatile object where enemies could shoot at it, even if there's a casing around it.

[Mad]

A gram of antiuranium would store vastly more energy than a gram of antihydrogen, as I understand it, or am I missing something?

They store the same energy, because a gram is a unit of mass. If you take e=mc^2, then plug in a gram of antihydrogen and a gram of antiuranium, you'll get the same result. (Though the actual conversion may cause other particles to appear depending on the particles that reacted, which would reduce the total gamma energy released.)

The decision for type of antimatter to be used is based more on ease of storage and effeciency of the reaction.

For storage, opposite subatomic particles cannot touch. An anti-proton cannot come into contact with a proton until the reaction, for example. So, basically, you'll want to keep your antimatter from having any contact with solid objects. So you'll need some kind of force field to keep the matter and antimatter from contacting. In a sci-fi setting, that could be magnetic containment, "energy" shields, or tractor beams. The material you choose needs to be stored and moved safely by whichever method your setting can use.

The second concern is reaction effeciency. When a particle and anti-particle collide, the mutual annihilation creates additional particles that fly out in different directions. These particles will impart their momentum to any other fuel in the area. As a result, the remaining matter and anti-matter could be pushed away from each other and never react, thus severely reducing effeciency.

So your reactor design and fuel will need to maximize the reaction rate. You could also process the fuel into a more useable form prior to transporting it to the reactor as well.

Of course, as has been mentioned, the gamma radiation still needs to be captured in order to be used.

A reasonably high efficiency is important because otherwise, what's the point of using the fuel? Higher effeciency means less fuel required for the same output, which means less fuel required for the ship, which means the ship is easier to push because it has less total mass.

[Darth Wong]

One thing most people don't realize is that at very high velocities, antimatter could pass right through matter and mostly fail to react (remember that matter is mostly empty space). Simply slamming a chunk of matter and a chunk of antimatter together would be incredibly inefficient as a result, and a reactor which collides subatomic particles would be most efficient. But then you'd need a way of converting your anti-iron into subatomic particles.

[Prozac the Robert]

I suspect that if you lobbed a bunch of heavy anti-ellements together, you would get all sort of shit produced, all of it woith different masses. This could make confining it problematic if you were using magnets.

[Darth Raptor]

Of course, I'm assuming that we're talking about a non-military vessel here. A military vessel would presumably want to avoid keeping such a volatile object where enemies could shoot at it, even if there's a casing around it.

Well, they're technically military vessels, but there hasn't been a war in thousands of years. This civilization's navy has been converted into what is essentially a merchant marine fleet.

Normally I wouldn't worry about the technical details, but I'd at least like some understanding of how the wanktech works for diologue and such. Maybe I'd better just go the SW route and say "the black box works because it does". Clarke's Law and all. Sorry if this has gravitated more towards FF or OSF fare.

On the plus side, I *really* want to take some physics classes now, because I feel stupid.

[Neko_Oni]

Here's a question. Is it possible to get a reaction between two particles, one matter, one anti-matter which are not opposites of each other i.e. anti-neutron + proton?

[Spin Echo]

Here's a question. Is it possible to get a reaction between two particles, one matter, one anti-matter which are not opposites of each other i.e. anti-neutron + proton?

It's possible to get a reaction between two particles that are not opposites of each other. For example, the experiment to first detect neutrinos used the reaction of an antineutrino with a proton, which gives a neutron and a positron.

Complete annihilation to pure energy, however, only occurs when the two particles are opposites.

[Winston Blake]

Here's a question. Is it possible to get a reaction between two particles, one matter, one anti-matter which are not opposites of each other i.e. anti-neutron + proton?

It's possible to get a reaction between two particles that are not opposites of each other. For example, the experiment to first detect neutrinos used the reaction of an antineutrino with a proton, which gives a neutron and a positron.

Complete annihilation to pure energy, however, only occurs when the two particles are opposites.

Then even then, anything bigger than an electron tends to have enough energy to result in free massive particles. I know that proton and neutron annihilations produce a mixed bag of charged and neutral pions, which soon end up as a spray of gamma rays/neutrinos/electrons/positrons. Certainly not the wonderful, clean, '100% efficient' fuel that antimatter gets characterised as.

[Dooey Jo]

Here's a question. Is it possible to get a reaction between two particles, one matter, one anti-matter which are not opposites of each other i.e. anti-neutron + proton?

It is, because it is not the anti-neutron and the proton that is reacting, but their constituent quarks. In a "best case" you will be left with 1 up-quark and 1 antidown-quark, but free quarks are not possible so they will form a meson. The annihilated quarks can form any number of particles of any type, as long as energy and momentum is preserved, and the total quantum number of the particles is 0. Of course, the quarks will probably not annihilate all the time, and instead they'll probably form mesons, which will decay quickly into other interesting things.