Pair production: Has it been observed experimentally?

[LostConjugate]

Can you see where I'm going with all this? Bosons are not "made from" fermions, but we
can find reactions such that a collection of bosons can be converted into a collection of
fermions, etc, etc.

HTH.

I understand that charge and energy are conserved in the process. Though because you can convert energy into a massive particle and vice-ver now how can you say that a particle is "elementary". It may have discrete properties but everything can be broken down to energy.

It is the discrete states that are elementary and no longer the particles.

By the way do you learn about all this in QFT? That may be why I have not gotten to it yet, just started taking QFT only finished QM Core and GR.

 

[Drakkith]

I understand that charge and energy are conserved in the process. Though because you can convert energy into a massive particle and vice-ver now how can you say that a particle is "elementary". It may have discrete properties but everything can be broken down to energy.

It is the discrete states that are elementary and no longer the particles.

By the way do you learn about all this in QFT? That may be why I have not gotten to it yet, just started taking QFT only finished QM Core and GR.

Actually, you cannot break things down into energy. Energy does not exist by itself. Particles HAVE energy, but they are not made OF energy. For example, and Electron requires a certain amount of energy to be produced from a collision event. But when we investigate an electron, we have not found it to be anything other than an elementary particle. This is in contrast to a Proton where we have found it to be composed of 3 quarks.

Attempting to say things are made of energy is similar to saying they are made of charge or mass or spin. All of these are merely fundamental properties of the particle.

 

[Dale]

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

Here is a relevant wikipedia page. Down at the bottom are some references and links you may want to pursue.
https://www.physicsforums.com/newreply.php?do=newreply&p=3412501

 

[qsa]

(Perhaps my answer below should be in a separate thread, but I put it here for now...)

LostConjugate,

Have you come across the notion of the Wigner method for classification of elementary particles? I suspect the answer is "no", so here's the idea (very briefly):

All elementary particle types are classified according to the so-called "unitary irreducible representations of the Poincare group". Another way of saying this is that every type of elementary particle must correspond to one of the physically distinguishable ways that things can transform under rotations, boosts and translations. It turns out that these "ways" can be classified in terms of 2 invariants: the (relativistic) mass^2 and something known as W^2 -- where W is the Pauli-Lubanski vector, which can be regarded as a relativistic generalization of spin.

So (again cutting a longer story short) we can classify elementary particle types according to their mass, spin (and also their component of spin in an arbitrary direction). This classification is not exhaustive of course, because there's also the other so-called "intrinsic" properties such as electric charge, hypercharge, lepton number, baryon number, etc. The set of all these properties is thought of as the set of "quantum numbers" that make one type of particle different from another.

Additionally, we have ordinary energy, momentum, angular momentum, etc. For all these things to be useful in describing processes, we want their total amounts to be conserved during interactions. E.g., the total charge must be conserved, as must the total energy and momentum, the total lepton number, etc, etc.

So how is all this relevant to $\gamma + \gamma \leftrightarrow e^- + e^+$ ?

Well, basically the elementary particles are not made "from" anything (else they would not be "elementary"). However, we can describe any given particle instance by the set of quantum numbers outlined above and a few more -- which I'll illustrate by re-writing the equation above with some extra arguments:
$$\gamma(E_1, p_1, \sigma_1, \dots) + \gamma(E_2, p_2, \sigma_2, \dots) \leftrightarrow e^-(E_3, p_3, \sigma_3, \dots) + e^+(E_4, p_4, \sigma_4, \dots)$$
where E means energy, P means momentum, $\sigma$ means spin, and the
"..." means all the other quantum numbers.

For conservation of energy in the reaction, we must have
$$E_1 + E_2 = E_3 + E_4$$
and similarly for the other quantum numbers.

So is total electric charge conserved? Yes, because photons have charge=0 and the
sum of electric charge on the rhs is 1 + -1 = 0.

The only reactions that can occur are those that satisfy these conservation laws for
total quantum numbers.

Can you see where I'm going with all this? Bosons are not "made from" fermions, but we
can find reactions such that a collection of bosons can be converted into a collection of
fermions, etc, etc.

HTH.

you are describing standard textbook stuff ,fine. But there are tons of advanced "mainstream" theories that go beyond this standard picture. I hope you do agree , since the standard picture is lacking in more than one way.

 

[damianpaz]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

 

[Drakkith]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

It involves two photons and is very very rare.

 

[strangerep]

you are describing standard textbook stuff ,fine. But there are tons of advanced "mainstream" theories that go beyond this standard picture. I hope you do agree , since the standard picture is lacking in more than one way.

If the theories you have in mind are indeed "mainstream" (meaning having lots of publications in peer-reviewed journals, and having empirical evidence that supports them in a way that makes them experimentally distinguishable from "standard textbook stuff"), then there should be no problem mentioning them by name (and quoting references!) here on PF.

 

[strangerep]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

What Drakkith said. :-)

It requires very high energy (4th-order QED effect). Doing it in accelerator experiments was only achieved fairly recently, iirc.

Doing it by having a gamma ray interact with the Coulomb field of a nucleus is much easier. (Look up "Delbruck scattering".)
http://en.wikipedia.org/wiki/Delbruck_scattering

 

[qsa]

If the theories you have in mind are indeed "mainstream" (meaning having lots of publications in peer-reviewed journals, and having empirical evidence that supports them in a way that makes them experimentally distinguishable from "standard textbook stuff"), then there should be no problem mentioning them by name (and quoting references!) here on PF.

Do you consider String and LQG as "mainstream" or not.

 

[Dale]

Do you consider String and LQG as "mainstream" or not.

AFAIK, neither of those support your claim that fermions are made of bosons.

 

[qsa]

AFAIK, neither of those support your claim that fermions are made of bosons.

i was just saying that there are other ideas as to the nature of particles by people who are generally not considered crackpots. just bigger picture that's all.


like this one

http://arxiv.org/PS_cache/hep-th/pdf/0507/0507109v4.pdf

also similar idea appears in lqg

http://pirsa.org/11070005/
Spinfoam Fermions
Elena Magliaro
Abstract: A serious shortcoming of spinfoam loop gravity is the absence of matter. I present a minimal and surprisingly simple coupling of a chiral fermion field in the framework of spinfoam quantum gravity. This result resonates with similar ones in early canonical loop theory...
(6 July 2011)

also Torsten.

 

[strangerep]

Do you consider String and LQG as "mainstream" or not.

The lack of experimental results that could establish either of them as superior to SM+GR means they should be discussed in the BTSM forum.

 

[Dale]

i was just saying that there are other ideas as to the nature of particles by people who are generally not considered crackpots. just bigger picture that's all.

I don't think anyone was objecting to the bigger picture, just some specific assertions.

 

[qsa]

I don't think anyone was objecting to the bigger picture, just some specific assertions.

Although I do understand the sentiment, but most people come to PF because they are very smart and curious people. they can figure out equations and what not from textbooks, but largely they want to know more and deep. There is a misunderstanding, some guys like Lostconjugate test the deep water by making there own conjecture , it is a learning process and not a crackpottery. I gave the Lorentz theory (not mine) as an example of the many Ideas already tried and the ongoing asthetics. Even the QCD mass calculation has an element of the theory like was admited by its inventors Wilczek. Anyway, It was just an example.