[Dumb_Blonde (OP)]

I read photons can collide and create fermions, but then reading two-photon physics it said photons cannot couple directly to each other, since they have no mass, can someone explain to me what I'm not getting?¿

[evan_au]

You need a large amount of energy to create fermions, so you are talking about photons in the gamma-ray range.

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

[lightarrow]

I read photons can collide and create fermions, but then reading two-photon physics it said photons cannot couple directly to each other, since they have no mass, can someone explain to me what I'm not getting?¿

Photons are bosons so they can traverse each other without collide.
Photon collisions are theorized at very high energy but never measured, for what I know.

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lightarrow

[Pmb]

I read photons can collide and create fermions, but then reading two-photon physics it said photons cannot couple directly to each other, since they have no mass, can someone explain to me what I'm not getting?¿

What do you mean by "photons cannot couple directly to each other"? What is it you mean when you speak of particles coupling to each other?

[JP]

I read photons can collide and create fermions, but then reading two-photon physics it said photons cannot couple directly to each other, since they have no mass, can someone explain to me what I'm not getting?¿

I think you mis-read it.  They can't couple to each other because they have no charge.  Photons only interact with things that have charges, such as electrons.  So to get two photons to influence each other you need an intermediary, such as an electron or positron.

[Pmb]

I think you mis-read it.  They can't couple to each other ...

What does "couple to each other" mean? Is this just another way to say "interact"?

[JP]

Basically yes, assuming "couple" in the OP means the coupling of the standard model, which describes the interaction between particles/fields (remember in field theory a particle and field are closely related concepts).


Wikpedia has a nice diagram showing which particles/fields couple to each other here:

http://en.wikipedia.org/wiki/File:Elementary_particle_interactions.svg

[Atomic-S]

I think you mis-read it.  They can't couple to each other because they have no charge.  Photons only interact with things that have charges, such as electrons.  So to get two photons to influence each other you need an intermediary, such as an electron or positron.

Nevertheless, an electron and positron can couple to each other and turn into photons without the presence of an intermediary -- it appears. However, that creates a problem: If that is so, then by running the equations backwards, it should be possible for photons to interact and turn into an electron and positron without an intermediary. Obviously, something does not add up here.

[lightarrow]

I think you mis-read it.  They can't couple to each other because they have no charge.  Photons only interact with things that have charges, such as electrons.  So to get two photons to influence each other you need an intermediary, such as an electron or positron.

Nevertheless, an electron and positron can couple to each other and turn into photons without the presence of an intermediary -- it appears. However, that creates a problem: If that is so, then by running the equations backwards, it should be possible for photons to interact and turn into an electron and positron without an intermediary. Obviously, something does not add up here.

Eh, no!   []  You can't make photons into an e^- e⁺ pair if their total energy is less than 1022 keV and if they don't hit other particles (such as nuclei) to keep momentum conservation.

[JP]

It's not a problem, Atomic-S.  Both electrons and positrons are charged and can interact with the electromagnetic field directly (no need for intermediaries).  But as lightarrow points out, being able to directly interact is only one requirement.  The products of the interaction must also satisfy conservation of energy, momentum, charge, etc., which restricts what you can make.    I believe you can make two photons into an e+ e- pair, as you can satisfy all the conservation requirements in that case provided the total photon energy is high enough.  You cannot make a single photon into an e+ e- pair, since you cannot satisfy both conservation of energy and conservation of momentum in that case.

[Pmb]

Eh, no!   []  You can't make photons into an e^- e⁺ pair if their total energy is less than 1022 keV and if they don't hit other particles (such as nuclei) to keep momentum conservation.

I don't understand the purpose of this comment. His comment didn't indicate otherwise, so why bother mentioning it? When information is input out of the blue of no apparent reason it only serves to confuse.

[evan_au]

Photons have:

• Momentum: hc/λ (where h is the Planck constant and c is the velocity=speed of light)
  
• Energy: hc/λ       (where h is the Planck constant and c is the speed = speed of light)
  
• Charge: 0

An Electron/Positron has:

• Momentum: mv        (where v is the velocity, and 0<=|v|<c)
  
• Energy: mv²+0.511 MeV/c²
  
• Electric charge: -1e (electron) or +1e (positron), which cancel to 0

As I read it, it is easy to balance the charge, since it balances to 0.

During Pair Production (photon pair -> electron/positron pair), it is not possible to simultaneously balance both momentum and energy, without a third particle like an atomic nucleus to carry away the excess.

However, during annihilation (electron/positron pair -> photons), in the absence of a nucleus, it is possible to form three photons with a total energy of 1.022MeV; the third photon balances the energy and momentum.

The cause of this apparent asymmetry in the production rates seems to be that in our part of the universe, it is more likely that two gamma ray photons will hit a dense accumulation of atoms (ie a solid) than it is that three gamma rays will meet in a perfect vacuum...

[lightarrow]

Eh, no!   []  You can't make photons into an e^- e⁺ pair if their total energy is less than 1022 keV and if they don't hit other particles (such as nuclei) to keep momentum conservation.

I don't understand the purpose of this comment. His comment didn't indicate otherwise, so why bother mentioning it? When information is input out of the blue of no apparent reason it only serves to confuse.

And I don't understand the purpose of your comment. Atomic-s, essentially, made a question: if a pair e^- e⁺ generates photons without intermediary, why can't those photons, reversing the equations, generate that couple of particles, that is e^- e⁺?
I showed that the reverse reaction doesn't happen, so, clearly it's not possible to reverse the equations...

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lightarrow

[Bored chemist]

The reverse reaction does happen.
Atomic S is pretty much correct.
OK, you need photons with enough energy, but that's not an issue.
If you annihilate an electron and positron you get two photons with energies that are big enough.
http://en.wikipedia.org/wiki/Matter_creation

[Pmb]

I showed that the reverse reaction doesn't happen, so, clearly it's not possible to reverse the equations...

No. You did no such thing. You made an error in that post. This is what you said

You can't make photons into an e- e+ pair if their total energy is less than 1022 keV and if they don't hit other particles (such as nuclei) to keep momentum conservation.

That is quite wrong. The process of making an electron-positron pair from two photons is possible and is referred to as photon pair production or pair creation. See this as an example at http://en.wikipedia.org/wiki/Pair_production

See also An Introduction to Quantum Field Theory by George Sterman, page 217.

You implied that an electron can't collide head on with a positron and in the process produce two photons moving in opposite directions with the same energy because momentum won't be conserved. That's clearly wrong. You made the mistake of thinking that there had to be another particle present to act like a catalyst in order to take up some  momentum in order for momentum to be conserved. It seems that you're confusing this with pair production use in a single photon to create an electron-positron pair. In such a case you'd be right. The appropriate analogy would then be for an electron to annihilate a positron and produce a single photon.

You're quite wrong. A photon can collide with another photon and produce an electron-positron pair. This is referred to as "Matter Creation" in Wikipedia and is listed under http://en.wikipedia.org/wiki/Matter_creation

See also - http://web.pdx.edu/~egertonr/ph311-12/pair-p&a.htm

The example given by the OP, namely photon pair production, is described by the equation; photon + photon -> electron + positron which is quite possible and is called photon pair production.

[lightarrow]

The reverse reaction does happen.
Atomic S is pretty much correct.
OK, you need photons with enough energy, but that's not an issue.
If you annihilate an electron and positron you get two photons with energies that are big enough.
http://en.wikipedia.org/wiki/Matter_creation

We (almost) all know the annihilation e⁺ + e^- --> gamma photons
to actually happen in nature or in laboratories. Have you ever heard of something similar to be measured for the opposite reaction? It has the same probability to happen?

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lightarrow

[lightarrow]

The example given by the OP, namely photon pair production, is described by the equation; photon + photon -> electron + positron which is quite possible and is called photon pair production.

In physics a lot of things are possible, even that all air molecules in your room will suddenly concentrate to a little region making you breathless  []. Often what counts is the probability that processes occur.

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lightarrow

[Atomic-S]

Regarding probability, charged particles can find each other easily; uncharged not so well. Maybe that is why e⁺ + e^- => photons is seen more frequently than the reverse process.

[Atomic-S]

Concerning which, it would seem therefore that the process can be looked upon thermodynamically, as follows:  In a container in which photons of high enough energy exist in thermal equibrium, there is a certain probability that they will form pairs of fermions, and also a certain probability that the fermions will annihilate to form photons, so that there must be a position of thermodynamic equilibrium in which the two rates balance, and that is characterized by a specific ratio of the number of photons to the number of other particles. This is presumably a function of temperature and pressure, otherwise describable as mean photon energy and photon density.