0le
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I've been reading a bit, and as I understand it, photons are "quantized" electric fields. They are one and the same. I've got some questions though:

- 1) So does a photon have no charge because it is just the field resulting from a charge? It just "carries" this disturbance to a new location. Is this the correct way to view it?

- 2) When we talk about (electric) polarization, this refers to the electric field vector pointing in a certain direction. Does this vector (generally speaking now) just refer to the direction of the electric fields I just mentioned or is that wave polarization? From reading some book, electric polarization is the average of "electric dipole moments". So (wave) unpolarized light would mean that the net average of the direction of the field is zero or what? Is it possible to link the classical picture of polarization to the quantized theory of photons and if so how? I guess I'm asking awhether electric polarization and wave polarization are linked and how it all links together with photons and electromagnetic waves.

Thanks
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dknt
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(Original post by djpailo)
I've been reading a bit, and as I understand it, photons are "quantized" electric fields. They are one and the same. I've got some questions though:

- 1) So does a photon have no charge because it is just the field resulting from a charge? It just "carries" this disturbance to a new location. Is this the correct way to view it?

- 2) When we talk about (electric) polarization, this refers to the electric field vector pointing in a certain direction. Does this vector (generally speaking now) just refer to the direction of the electric fields I just mentioned or is that wave polarization? From reading some book, electric polarization is the average of "electric dipole moments". So (wave) unpolarized light would mean that the net average of the direction of the field is zero or what? Is it possible to link the classical picture of polarization to the quantized theory of photons and if so how? I guess I'm asking awhether electric polarization and wave polarization are linked and how it all links together with photons and electromagnetic waves.

Thanks
Hey there,

1) A photon has no charge because the theory it follows (QED) says so! As you say, a photon (or light) is just oscillating electric fields. (In fact there are also oscillating magnetic fields. Maxwell's equations say you can't have one without the other.) A charged particle 'couples' to the electromagnetic field i.e. the photon couples to the charges of other particles, but it itself has no charge. It is the mediator of the EM force.

2) You're getting a little confused. We used polarization in 2 ways mainly. Firstly is the polarization of a dielectric material, to do with its electric dipoles and how they align.

Polarization can also refer to wave polarization, which is how the electric field oscillates. There are 2 main wave polarization states, linear and circular. You can the superpose these states to get elliptical polarization for example. Unpolarized light means the electric field is oscillating in lots of different directions, such that no single polarization state dominates.
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0le
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(Original post by dknt)
Hey there,

1) A photon has no charge because the theory it follows (QED) says so! As you say, a photon (or light) is just oscillating electric fields. (In fact there are also oscillating magnetic fields. Maxwell's equations say you can't have one without the other.) A charged particle 'couples' to the electromagnetic field i.e. the photon couples to the charges of other particles, but it itself has no charge. It is the mediator of the EM force.

2) You're getting a little confused. We used polarization in 2 ways mainly. Firstly is the polarization of a dielectric material, to do with its electric dipoles and how they align.

Polarization can also refer to wave polarization, which is how the electric field oscillates. There are 2 main wave polarization states, linear and circular. You can the superpose these states to get elliptical polarization for example. Unpolarized light means the electric field is oscillating in lots of different directions, such that no single polarization state dominates.
Many thanks for your reply. Can you expand on the polarization of a dielectric material? Does that refer to the distribution of electrons (dipoles) in a molecule?
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dknt
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(Original post by djpailo)
Many thanks for your reply. Can you expand on the polarization of a dielectric material? Does that refer to the distribution of electrons (dipoles) in a molecule?
Polarization of a material object occurs when the constituents of that substance align in some preferred direction associated with an electric field. If an electric field is applied to a material, two things typically happen:

- intrinsic electric dipoles (those that already exist in the material) will align with the electric field. They do this to minimize energy.

- atoms and molecules can be polarized, which themselves induce a dipole moment.

We define the polarization vector, P, of a material to be P = np, where n is the number density of atoms or molecules and p is the average dipole moment, due to the alignment of intrinsic dipoles and induced dipoles. You might like to know there is an analogous form for magnetic materials, called the magnetization.
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(Original post by djpailo)
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Just as an extension to 1), if the photon did have a charge it would be allowed to self couple. That is, couple itself to the EM field just like the gluon (mediator of the strong force) can couple to itself. This is not allowed by QED, which is a so-called abelian gauge theory (see group theory and commutativity). QCD on the other hand, which describes gluons, is non-abelian and the self-coupling of gluons is a direct consequence of this.
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