Pair Production

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

User avatar
Banksy
Posts: 54
Joined: Wed Mar 28, 2007 12:41 pm UTC
Location: The Frozen North
Contact:

Pair Production

Postby Banksy » Sun Jan 11, 2009 3:02 pm UTC

Now forgive me if this is all a bit garbled, but I had a general idea about pair production in particle physics.
Now Wikipedia and various other sources (teachers, textbooks etc) have led me to believe that when annihilation occurs, a particle and it's corresponding particle destroy each other, and subsequently release two or more photons.
Looks like this:
Spoiler:
Image

(kind of I made this on paint in like a minute).
Pair production, however, occurs whenever a photon (and here's where I get a little fuzzy) 'spontaneously' turns into a particle/antiparticle pair, provided it has sufficient energy and suchlike.
Now, as a photon technically has no anti-particle; it is it's own antiparticle, right?
Also; there is a theory somewhere, I'm sure, that states that as reaction work either way in space they must work in either direction in time (the proof Hawking used when he looked at black holes and reversed it for the big bang).
So surely it makes more sense that pair production is actually a photon-photon collision, as they are antiparticles and would annihilate each other, releasing particles/antiparticles as a result. This also makes kind of sense if you look at the Feynmann diagram(ish) for annihilation I drew above, and reverse the flow of time.

Long story short; is pair production a collision of photons rather than spontaneous decay?

Thanks :)
But if anyone were ever to discover the meaning of life, all of space and time would collapse in on itself, and in the place that remained a large sign would appear saying:

Level 2.

User avatar
parallax
Posts: 157
Joined: Wed Jan 31, 2007 5:06 pm UTC
Location: The Emergency Intelligence Incinerator

Re: Pair Production

Postby parallax » Sun Jan 11, 2009 5:37 pm UTC

http://en.wikipedia.org/wiki/Pair_production

According to wikipedia, pair production can't occur from a single photon, but it has more to do with conservation laws than antiparticles.
Cake and grief counseling will be available at the conclusion of the test.

User avatar
Banksy
Posts: 54
Joined: Wed Mar 28, 2007 12:41 pm UTC
Location: The Frozen North
Contact:

Re: Pair Production

Postby Banksy » Sun Jan 11, 2009 7:54 pm UTC

Oh yeah... didn't read that properly.
Misinterpreted it as being wrong... and came to the right conclusion to see that it was already on Wikipedia.
:)
Damn that dedicated squirrel!
But if anyone were ever to discover the meaning of life, all of space and time would collapse in on itself, and in the place that remained a large sign would appear saying:



Level 2.

Kyot
Posts: 7
Joined: Mon Jun 30, 2008 11:40 pm UTC

Re: Pair Production

Postby Kyot » Sun Jan 11, 2009 10:21 pm UTC

Ok, I looked it up in my Modern Physics text, and I'm pretty sure that pair production can occur with only one photon, but only within matter.

The basic idea here is that as mass is really just another form of energy and a photon is just a bundle of energy, a photon should be some sort of conversion between the two that is possible. The key question is whether all laws of conservation are obeyed, such as conservation of energy, momentum, charge, etc, because if so it should be possible for a photon to spontaneously change into matter. If it is physically possible, it stands to reason that we should observe it at some point.

In order to obey conservation of charge and all of the quantum numbers, the photon must produce two particles, such as an electron and positron, with opposite quantum numbers. It turns out that conservation of energy and momentum are obeyed only if the transformation takes place near an atomic nucleus. Were the photon to turn into a pair of particles in isolation, e.g. in the vacuum of space, the resulting energy and momentum would not be consistent with each other, making the transformation impossible. If the photon is near a nucleus however, the nucleus can absorb some of the momentum and everything balances.

If there is nothing around, I believe that it is possible for two photons to interact and produce a pair of particles, because in this case the equations can balance.

If two particles have opposite quantum numbers and collide, that is, if a particle and antiparticle come together, they can sum together to a photon with equivalent energy. If the transition occurs in space, two photons must be produced, traveling in opposite directions. If there is matter around, it is possible for a single photon to be produced.

User avatar
gwillums
Posts: 37
Joined: Mon Jun 30, 2008 9:38 pm UTC

Re: Pair Production

Postby gwillums » Mon Jan 12, 2009 5:18 am UTC

Note that the reason pair production is possible only in matter is that it is the result of the interaction between the photon and the very strong electric field near the nuclei. The field absorbs the required momentum (in general the positron and electron will be 180 deg. apart).

This explains why pair production is enhanced in heavy materials (lead, tungsten, etc...) as the electric field near these nuclei will be much stronger.

User avatar
ThinkerEmeritus
Posts: 416
Joined: Sat Jan 19, 2008 11:32 pm UTC

Re: Pair Production

Postby ThinkerEmeritus » Mon Jan 12, 2009 2:14 pm UTC

gwillums wrote:Note that the reason pair production is possible only in matter is that it is the result of the interaction between the photon and the very strong electric field near the nuclei. The field absorbs the required momentum (in general the positron and electron will be 180 deg. apart).


Good point, but it is the electron or the positron which feels the strong electric field. The photon is uncharged and relatively insensitive to the strength of electric fields.
"An expert is a person who has already made all possible mistakes." -- paraphrase of a statement by Niels Bohr
Seen on a bumper sticker: "My other vehicle is a Krebs cycle".

Certhas
Posts: 301
Joined: Thu Dec 13, 2007 11:16 pm UTC

Re: Pair Production

Postby Certhas » Mon Jan 12, 2009 5:29 pm UTC

The reason pair annihilation always creates two or more photons is the same one btw. you need to balance the books. Any electron positron pair has a center of momentum frame where it's total momentum is zero. A photon on its own doesn't though, so by conservation of momentum they can't decay into one photon but only into two.

Same thing the other way around, you need two to balance the books. But just as they decay incredibly quickly two photons on their own are incredibly unlikely to create an electron positron pair. It turns out that with a nucleus interacting it becomes much simpler. This is basically because with just the two photons you need to hit a very precise energy/momentum configuration, but if there is a nucleus around that can take an arbitrary part of the energy, so now you have a much broader set of allowed final configurations. (See Fermis golden rule, density of final states for a similar effect: http://en.wikipedia.org/wiki/Fermi%27s_golden_rule)
"I conclude that all is well," says Edipus, and that remark is sacred.
- Camus, The Myth of Sysiphus

Mental Health Break


Return to “Science”

Who is online

Users browsing this forum: No registered users and 11 guests