[ADNAN NASIR]

Adnannasir asked the Naked Scientists:
   
In pair production one photon breaks into two particles - i.e electron and positron.

In pair annihilation, why are 2 photons generated by collision of e & e?

If such happens then there must be 2 photons moving opposite should combine to give 2 anti particles.

Does this voilate the law of conservation of mass?

What do you think?

[syhprum]

Is it possible for two Electrons to collide does the Coulomb force not keep them apart ?

[Bored chemist]

Define "collide"; two billiard balls collide and bounce off each other because of the same electric repulsion that stops two electrons "hitting" each other. Do the billiard balls collide or do they just get close enough that their electrons repel one another?

Anyway,
if a photon is converted into particles they have opposite charges so the collision isn't
e and e
It's
e+ and e-
that collide to give two photons.
Since each photon only has an energy equivalent to the mass of one electron it can't undergo pair production- there's nothing with a mass of half an electron that it could form a pair of.

[syhprum]

Other particles such as Protons and anti Protons which are composite entities can collide and dissolve into a Quark Gluon soup but electrons not being composite bodies can presumably only have elastic collisions.

[Ron Hughes]

Not true syph. Proton anti-proton annihilations at rest will produce two photons with a combined a energy of 1.88 GeV not any quark soup that I'm aware.

[syhprum]

I had in mind Proton Anti Proton collisions when they had been accelerated to high energy by the LHC or similar accelerator.

[Ron Hughes]

The fact that annihilation's produce only two photons and no particles puts a question mark on the validity of the standard model in my mind.

[Vern]

I haven't seen that inhalations produce only two photons. Seems that I remember that even -e +e collisions don't always simply produce two photons. There's always the two photons, but there are usually more, depending upon the energy of collision.

In fact, a very large number of things can come out of high-energy -e +e collisions, including hadrons.

[JP]

Indeed.  What you get out of any collision depends on a number of factors that take the form of conservation laws.  Energy has to be conserved, momentum has to be conserved, charge (sometimes) has to be conserved, etc.^* 

When you collide an electron and positron at very low energies, the conservation of energy doesn't allow much to be created aside from photons.  At higher energies, the input energy of the particles is enough to create heavier particles (remember that the mass of a particle is related to its energy): http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation#High_energy_case

^*The exact rules that a collision has to satisfy depends on the forces involved.  Conservation of energy and other conservation laws are always satisfied, but some depend on the interaction: http://en.wikipedia.org/wiki/Conservation_laws

Edit:  Regarding the question about conservation of mass.  Mass isn't conserved.  Energy is the quantity that gets conserved, and it is in this case, since the photons have energy.  The electron and positron have energy due to their mass and also due to their speed (the part from their mass if they're not moving is the famous E=mc²).

[syhprum]

No one has mentioned Neutrinos yet they play an important part in these interactions, also the original question was what happens when Electrons collide (not Electrons and Positrons) which I believe can only have elastic collisions

[JP]

I'm fairly sure that electrons can interact via the weak force too, so if you give them enough energy, you'll get something other than an elastic collision. Although getting them to collide head-on rather than scattering elastically (since they repel) is probably an insanely difficult feat of engineering.

[syhprum]

I believe that contemplating colliding Electrons takes us to the centre of white dwarf stars where they are forced into Protons to make Neutrons.

[lightarrow]

charge (sometimes) has to be conserved, etc.^* 

why sometimes and not always?

[lightarrow]

In pair production one photon breaks into two particles - i.e electron and positron.

That's not complete: the photon need to collide with another massive particle, for example a nucleus, to generate a couple e⁺ e^-, otherwise energy and momentum couldn't be conserved simultaneously. When a couple annihilate, it generates a couple of photons exactly for this reason.

[lightarrow]

Since each photon only has an energy equivalent to the mass of one electron it can't undergo pair production- there's nothing with a mass of half an electron that it could form a pair of.

A photon can have any energy you want, photons in the visible range have energy around 1 eV, an electron's mass is 511*10³ eV/c²...

[JP]

charge (sometimes) has to be conserved, etc.^* 

why sometimes and not always?

Oops.  I mis-typed there.  I was thinking of CP symmetry, which is more complicated.  Charge is always conserved, as far as I know.

[lunar11]

Hi ADNAN NASIR.
I know exactly what you mean.
Electron + positron give 2 gamma ray photons moving in opposite directions to conserve momentum.
But, according to books, only 1 gamma ray photon changes to give an electron + positron.
Electron has rest mass = 0.51MeV
So, for pair production, the single photon must have energy = 1.02MeV or greater.

I would like to further ask; is it possible for 2 photons each of energy 0.51MeV to collide and form the electron + positron.

Lunar

[lightarrow]

Hi ADNAN NASIR.
I know exactly what you mean.
Electron + positron give 2 gamma ray photons moving in opposite directions to conserve momentum.
But, according to books, only 1 gamma ray photon changes to give an electron + positron.
Electron has rest mass = 0.51MeV
So, for pair production, the single photon must have energy = 1.02MeV or greater.

I would like to further ask; is it possible for 2 photons each of energy 0.51MeV to collide and form the electron + positron.

The problem is that photons don't collide.
Actually, a non-zero probability of interaction does exist, but it's extremely low and energies have to be extremely high; you can always forget it.

[Ron Hughes]

Stanford linear accelerator 1991, two beams of radiation(photons) were collided. Four photons of the colliding beams formed an electron positron pair.

[Bored chemist]

Since each photon only has an energy equivalent to the mass of one electron it can't undergo pair production- there's nothing with a mass of half an electron that it could form a pair of.

A photon can have any energy you want, photons in the visible range have energy around 1 eV, an electron's mass is 511*10³ eV/c²...

In general photons can have any energy you want.
In the case of photons produced by pair production from electron + positron annihilation they have an energy of 511KeV. Unless the particles had a lot of energy to begin with, the photon energies produced will not be big enough to produce a particle by pair production beccause there isn't a particle with a mass of about 255 KeV.

BTW, does anyone know why SLAC would be colliding massless changeless particles like photons?