Antimatter - could it ever be utilized as poss. energy source?

Vern

I think there must be something else involved, otherwise we would never see photons with more energy than that required to produce an electron-positron pair. Not to mention that if something else were not involved it would falsify a pet theory of mine

Keep on chuggin !!

Vern

Nenad

the photon creates the electron/positron pair, not the other way around. When the photon has enough energy (usualy gamma rays > 1.1MeV) Then an electron/positron pair is formed, and almost instantaineously anniahlated.

Nenad

Yes there is something else involved. Pair production depends on temperatude. If the temperature is <[tex] 10^{9} K[/tex], then no pair production occurs. If the temp is between [tex] 6*10^{9} K\ to\ 10^{13} K [/tex] then an electron/positron pair is produced, and if temp is greater than the upper limit, a proton and a antiproton is produced.

Entropy

Photons have temperture?

russ_watters

Not exactly, but photons have energy and temperature is a measure of energy. Thus, you can tell the temperature of an object by the color of the light it throws off.

Tom Mattson

I think I get your drift, but the way this is worded can be confusing. When you say "the temperature", I think people could take this to mean "the ambient temperature", which is not right. The determining factor as to whether pair production is "go-or-no-go" is the photon energy. If it is at least as large as 2mc² (where m is the mass of the particle in question) in the lab frame, then you can produce a pair, no matter what the ambient temperature is.

Nenad

I mean surrounding temp, not temp of a photon.

Tom Mattson

Oh yes, and as to the original question, consider the following numerical data.

From a single e⁺e^- annihilation, you get about 1 MeV of energy (from the masses of the two particles). Compare that with the 200 MeV you get for the fission of every ²³⁵U nucleus. Then once you consider that the uranium is available by the kilogram, and positrons are so difficult to store en masse, and you've got quite an engineering problem on your hands. In that respect, the situation would have something in common with fusion power generation: We understand the physics just fine, but we can't get the technology to cooperate with what we know.

Tom Mattson

In that case, what you wrote is not correct. Pairs can be produced in deep space, where the average temperature is about 3 Kelvin.

Nenad

heres a link, scroll down to pair production. Youll see what I mean: http://instruct1.cit.cornell.edu/cou...o101/lec32.htm

Tom Mattson

OK, I see. The link is discussing the early universe in which there is a "sea" of photons. In an environment in which the universe is awash in radiation, the energy of the radiation field determines the ambient temperature. So in that case, there is a correletion between temperature and pair production. But in a way this hides the real quantity that determines the thresholds for pair production, and that quantity is photon energy.

Today, the universe is awash in weak radiation (with a temp of about 3K, as I said). As is known from QFT and from experiment, pair production is a quantum phenomenon, and as such it only takes a single photon to produce a pair in the Coulomb field of a heavy nucleus. It is the energy of this single photon that determines the thresholds for pair production, and it does not matter what the surrounding temperature is.

Nenad

Im just stationg what the site says. I trust Cornell, theyre a good university. But you do have a point.

Tom Mattson

To be sure, Cornell is trustworthy. But that site is meant to be a layperson's introduction to the early universe, not a tutorial on particle physics.

alexkerhead

But wouldn't the anti-matter react negatively to the matter?

ArmoSkater87

Im not sure what you mean by "react negatively" but matter and antimatter annihilate each other when they touch, in other words they get converted completely into energy. So, we WANT them to "react" with each other, because the energy relased from it is a very good prepelant for spaceships if we had a sufficient quantity of antimatter. If we eventually make antimatter propeled spaceships, we could reach to almost the speed of light.

I was wondering...how come its possible for ONE photon to get converted into an electron-positron pair, while when the electron-positron pair annihilate, they are ALWAYS converted into TWO gamma rays with equal energy in opposite direction, in order to conserve energy and momentum. Doesnt the fact that one photon can be converted into the pair in a sense violate conservation of momentum?

Tom Mattson

Yes it does, and that's why the photon does not create a pair all by itself. Pair production from a single photon is done in the presence of a Coulomb field, often that due to a heavy nucleus. The nucleus recoils after the production, and so is able to conserve both energy and momentum in the process.

alexkerhead

Thanks for the clear up...
I see now..
I was really under the close minded impression that when they reacted, they would disappear..lol

I understand, thanks for clearing it up..

But the idea has been around for decades..
If we could develope an efficient way of space travel, anti-matter could be found, but would be hard to collect and manage..