What Distinguishes Photons from Gluons Despite Their Similar Properties?

[taylordnz]

photons and gluons have the same mass, charge, width and GeV?

so what tells them apart?

 

[1100f]

The main things in common between photons and gluons are that they are both massless (rest mass = 0), they have both spin 1 and are both carrier (or mediator) of interractions.

The main differences are that the photons mediate the electromagnetic interraction while the gluons mediate the strong interraction. One major difference is that although the photon mediates the electromagnetic interraction, its electric charge is zero, so that there is no electromagnetic interraction between 2 photons. However, gluons mediate the strong interraction and the have a "strong" charge (called color). So that gluons interract among themselves.

 

[quartodeciman]

1100f said: there is no electromagnetic interraction between 2 photons

But, two photons in the gamma range of frequencies, having sufficient total energy and getting close to each other near a massive nucleus, change into an electron and a positron. The sufficiency of total energy means enough total energy to produce the total energy of the two charged particles. Any excess energy becomes the total kinetic energy of the particles. This happens in nuclear experiments all the time. In a bubble-chamber photograph, in the vicinity of a magnetic field, this pair shows up as a pair of back-to-back spirals coming from the point where the charged particles were produced.

This reaction is evidently done by the electromagnetic force. The production of electron-positron pairs figures into quantum electrodynamic calculations. In this case, the two particles don't separate, but just turn right back into gamma ray photons again.

 

[1100f]

Originally posted by quartodeciman

But, two photons in the gamma range of frequencies, having sufficient total energy and getting close to each other near a massive nucleus, change into an electron and a positron. The sufficiency of total energy means enough total energy to produce the total energy of the two charged particles. Any excess energy becomes the total kinetic energy of the particles. This happens in nuclear experiments all the time. In a bubble-chamber photograph, in the vicinity of a magnetic field, this pair shows up as a pair of back-to-back spirals coming from the point where the charged particles were produced.

This reaction is evidently done by the electromagnetic force. The production of electron-positron pairs figures into quantum electrodynamic calculations. In this case, the two particles don't separate, but just turn right back into gamma ray photons again. [/B]

What you describe here is not a direct interraction between two photons but an interraction between photons by intermediate states.
When I said that there is no interraction between photons, I meant that you do not have any vertex that includes just photons. And if you look at the lowest order at which an interraction between two photons occur, the Feynman diagram will look as follows: (I tried to draw the diagram but I didn't succeed, so I will describe the diagram) Draw a square and at each corner of the sq2uare, draw a photon. Each line in the square represents an electron (positron).We see that we have at the lowest order, 4 vertices. So that the cross section of the photon-photon scattering will be proportional to e^2 at the fourth power. So that this process will be highly suppressed.

On the contrary, for gluons, you have vertices containing only gluons (3 or 4). So we see that gluons have a direct interractions among them.

 

[quartodeciman]

Originally posted by 1100f
On the contrary, for gluons, you have vertices containing only gluons (3 or 4). So we see that gluons have a direct interractions among them.

Would that be glueball interactions, for example? It is hard to find Feynman diagrams for gluon-gluon interactions outside of (maybe) some hairy research reports. There is certainly nothing like that with photons.

 

[mormonator_rm]

A glueball will act a lot like a meson. The vertices with multiple gluons are not glueballs, unless two or more of the gluons are co-confined before leaving the box in the Feynman diagram.

For photons, you will not see bound states of photons with other photons, but there are positronium bound states that can exist. That's the closest you can get to glueballs in a QED analogy.

 

[quartodeciman]

Positronium - I forgot about that. Thanks.

 

[motai]

Positronium... *jogs memory* isn't that the state in which an electron orbits a positron for a very short amount of time?

 

[mormonator_rm]

Correct. A very, very short period of time.

 

[Nereid]

Originally posted by mormonator_rm
Correct. A very, very short period of time.

Except in the far, far distant future, where electrons and positrons are all that's left (all black holes have evaporated, all protons have decayed), and they orbit each other at a distance of approx 15 billion light-years

Of course, we may learn something about dark matter or dark energy that renders this somewhat bleak picture invalid.

 

[arivero]

Originally posted by quartodeciman
Would that be glueball interactions, for example? It is hard to find Feynman diagrams for gluon-gluon interactions outside of (maybe) some hairy research reports.

Tarrach's book on practical QCD surely has some.

 

[mormonator_rm]

Originally posted by quartodeciman
It is hard to find Feynman diagrams for gluon-gluon interactions outside of (maybe) some hairy research reports.

There is a really good, but old, book called "Introduction to Quarks and Partons" that has quite a few diagrams you won't find in many other places. Feynman diagrams for gluon-gluon interactions can indeed get really hairy, i.e. multi-gluon vertices and such.

 

[1100f]

Originally posted by quartodeciman
It is hard to find Feynman diagrams for gluon-gluon interactions outside of (maybe) some hairy research reports.

In any book on QFT, go to the QCD section, after the Lagrangian is written, the Feynman rules are given, including the Feynman rules for gluon-gluon vertex.