Thread: Anti-Matter

I was reading somewhere that nuclear fission releases only like .01% of the mass of U-235 as energy, and with fusion it was like .03% of the hydrogen getting released.

However, supposedly, if it were possible to generate a large quantity of anti-matter and use it to annihilate some matter, you'd get close to a 100% conversion. Am I understanding this right?

It gives Star Trek some more credibility, if that's correct. Apparently, however, anti-matter is so hard to create that it's not exactly seen as a viable solution to the world's energy problems, at least not with our technology.

Anti-matter is suppose to be the purest form of energy anywhere. The power generated between one particle of antimatter annihilating one particle of matter is astronomical and, I read, is enough to power a society for hundreds of years. If we can somehow harness that power it would be a gigantic leap forward in terms of production. Though as you say, there is a very very VERY limited amount available which only last for a few millionths of a second after a collision in one of our particle accelerators. If I remember correctly there is also only one particle of antimatter found in one one-millionth of the collisions or something as well.

Electron positron annihilation, at least in the low energy case, produces nearly 100% usable work. You could do the annihilation in a water bath, for instance, which would capture nearly all of the released gamma rays as heat which you could use to power a heat engine.

Higher energy annihilation because your positrons/electrons are faster or because you're using anti protons, anti hydrogen, etc., tend to produce neutrinos which pass through most matter harmlessly. Meaning extracting even a significant fraction of the energy as useful work is difficult or impossible.

Of course, antimatter is extremely rare in the universe, so it's unlikely we're going to find some vast cache of it somewhere. It's also dangerous to store, and requires a constant input of energy (to keep the magnetic containment active). Most likely, in the future a (coal, nuclear, fusion, wind, solar, etc.) power plant will produce it economically and it'll be used as fuel in places where mass is an issue, like a space ship (the space ship still needs reaction mass, though).

So, what are the properties of anti-matter?

Does a positron have the opposite charge of an electron? Is there any theory as to why, exactly, anti matter annihilates normal matter?

Yeah, a positron is exactly the same as an electron, except with the opposite charge.

As to why antimatter annihilates with matter at all, I admit I'm not sure. It probably has to do with quantum mechanics, which isn't my strong suit.

As to why antimatter annihilates with matter at all, I admit I'm not sure. It probably has to do with quantum mechanics, which isn't my strong suit.

This is because of two things: the opposite charges of the electron and positron, and their equal mass.

On colliding with each other, they turn completely into energy because the law of conservation of energy demands that the energy of their collision be conserved. Adding the two masses doesn't yield enough energy to create any neutral particles (I say neutral because the law of conservation of charge demands it), so they annihilate together to form the next best things: lots and lots of photons.

This is why the two annihilate each other. As for reactions between other kinds of antimatter, such as between neutrons and antineutrons, remember that they constitute of quarks. Apply the same explanation to individual quarks, and voila! You have your answer.

By the way, Numsgil, quantum mechanics doesn't really explain why they annihilate. The Dirac equation predicts them, but doesn't say anything about their modes of interaction.

By the way, Numsgil, quantum mechanics doesn't really explain why they annihilate. The Dirac equation predicts them, but doesn't say anything about their modes of interaction.

So you're saying the why isn't understood? Your explanation above that statement not withstanding?

No, Numsgil, I was responding to your statement as to that you probably QM predicted their annihilation. I was simply pointing out that QM didn't demand it, it was simple conservation laws.

But why wouldn't they just bounce off each other over and over like particles in a gas do? I mean clearly they don't, I just don't see the underlying why.

Liongold, what Numsgil is relating (I assume), is the problem of "why". If one keeps on asking why something happens, you eventually reach a point beyond which nobody knows. Laws and theories explain what will happen in certain circumstances and conversely what the circumstances need be for something to happen and then provide a way to quantify it all. The law/theory is in the form of mathematics, so conditions can be fed into them in order to make predictions. When these predictions pan out, the strength of the law/theory increases. These laws/theories then do not affect the elements in the experiments directly. What remains open is why it happens like it does. The laws/theories are like saying that stepping on the gas makes the car go forward.

This was not meant to be patronizing.

Thank you, Kalster.

But why wouldn't they just bounce off each other over and over like particles in a gas do? I mean clearly they don't, I just don't see the underlying why.

Well, Numsgil, particles in a gas bounce off thanks to electromagentic repulsion. However, if you force the two particles to come in contact, they will collide and create new particles. For example, pushing a proton and electron together gives us a neutron and a neutrino; the newly formed particle is a combination of the energies and charges of the original particles.

Anti-matter too follows the same laws; the reason they annihilate I have already given: because adding their charges together negates itself, and no particles of known mass can be formed by their collision (adding together 1/1840 the mass of a proton and 1/1840 of a proton'sm ass gives a ridiculously small number; no such mass particle exists.)

Consequently, the law of conservation of energy demands the production of new particles of energy, and hence they turn into photons.

I understand the conservation laws, but I'm saying there's another way they can be satisfied: by an elastic collision. Why can't electrons and positrons elastically collide? What physical law prevents this?

Originally Posted by BumFluff

The power generated between one particle of antimatter annihilating one particle of matter is astronomical and, I read, is enough to power a society for hundreds of years.

This is a gross exaggeration. With all due respect, one particle of antimatter wouldn't be enough to power a pocket light for a second.

Assuming average power use of 1kW per person (a value taken from the back of my head, could be off the mark by two orders of magnitude), a whole kilogram of antimatter (plus a kilogram of matter) would supply the needs of just under 60 million people for 100 years.

And there are a very big lot many gazillion particles (of any kind) in a kilogram.

Hope this helps,
L.

Originally Posted by Numsgil

I understand the conservation laws, but I'm saying there's another way they can be satisfied: by an elastic collision. Why can't electrons and positrons elastically collide? What physical law prevents this?

Since the two particles are oppositely charged, there's nothing to cause them to push away from each other. When you throw a ball at the wall, the atoms in the two never touch. The electrons around the atoms repel each other, causing the ball to bounce away.

Ah, I see. Is electromagnetism the only force at work during annihilation, or do the weak and strong forces come into play?

Ah, I see. Is electromagnetism the only force at work during annihilation, or do the weak and strong forces come into play?

Depends on the nature of the particle. If it interacts with the nuclear forces when interacting with another particle, then certainly they come into play. If it doesn't, they switch to another common force, such as electromagnetism.

However, don't be too sure about that. I'm not yet sure if this is correct.