# Photon never have charge¿

1. Jun 2, 2009

### Nikaro

Why photon exist without rest mass and why it can never have a charge. Plz reply.

2. Jun 3, 2009

### malawi_glenn

well physics tend not to answer all "why" questions, we can only move them to a different level.

i) the photon moves at the speed of light, only massless (i.e. 0 rest mass) can do so

ii) The gauge symmetry group of electromagnetism is U(1), which is abelian, thus the photon has not charge.

Then you can ask why it moves at the speed of light and why EM has U(1) gauge symmetry group and so on.

3. Jun 3, 2009

### nicksauce

There are however "photons" with charge and mass, the W- and W+ bosons. (Roughly speaking of course)

4. Jun 3, 2009

### Nikaro

Please explain it simple i can't under stand above reply

5. Jun 3, 2009

### malawi_glenn

and I don't understand what part of them you did not grasp or at what level you currently are at. What is "simple" is relative...

6. Jun 3, 2009

### Bob_for_short

Photon is a wave, it travels. In vacuum it travels fast (v=c), in a transparent medium (glass, water) it travels slower, with v<c, so there is an accompanying reference frame where it looks as a standing wave. Despite that it still have energy ћω. You can assign some mass to it, if you like.

Photon carries energy-momentum. It can transfer part of them to another particle or system. Photons strongly interacts with charged particles: roughly speaking, photon is an electric wave. Photon practically does not interact with another photon in vacuum. It is due to the principle of superposition of electric fields, if you like. Electric fields make only sense when they appear in the particle equation motion as external forces.

Bob_for_short.

Last edited: Jun 3, 2009
7. Jun 3, 2009

### prysdieheer

I was wondering the same thing.
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In Love
Prys die Heer!

8. Jun 3, 2009

9. Jun 7, 2009

### Nikaro

Thanks everybody.

10. Jun 7, 2009

### RedX

Keeping with the theme of particle physics, the question should really be why other particles have mass (and why they have the value of the mass that they have), rather than why doesn't the photon have mass. 0 is easy to understand.

Mass is related to the spontaneous symmetry breaking of a scalar Higgs. The photon gains no mass as it is the gauge field corresponding to a direction in field space where symmetry is not broken from the electroweak phase transition. For more information there is plenty of material on the web about the Higgs mechanism.

11. Jun 8, 2009

### Count Iblis

The photon could still be massive if it couples to another scalar and gains mass that way:

http://arxiv.org/abs/hep-ph/0306245

If the photon has a small mass that is generated by such a Higgs mechanism, then it turns out that the very sharp upper bounds on the photon mass that depend on the vector potential of the galactic magnetic field, are not valid.

12. Jun 8, 2009

### RedX

So I assume this other scalar would develop a vacuum expectation value that will be a different value than usual one, i.e., the one whose ground state is U(1) charge invariant and gives mass to the Standard Model. But if this happens, then don't you lose conservation of charge, since your theory will lose U(1) charge invariance?

13. Jun 8, 2009

### samalkhaiat

14. Jun 9, 2009

### Count Iblis

Actually, the article assumes that the ordinary Higgs mechanism would generate the photon mass. Anyway, you still have a conserved current, even in Proca theory, as the article explains.

15. Jun 10, 2009

### darkwood

The Photon is an electro-magnetic wave and thus should be viewed as pure energy to understand it better, this will give insight into the no mass question, different particles are in simple a combination of a set mass ratio to their energy at ground state. The absorbsion of a photon in a particle will raise its energy state and then subsequently releases the exact energy out as a duplicate photon, this happens almost instantaneously but a fragment of time is used thus the speed of light through air liquid and solids becomes increasingly slower.

16. Jun 10, 2009

### malawi_glenn

ohh Define pure energy please

17. Jun 11, 2009

### Sidnv

Are photons made of quarks?

18. Jun 11, 2009

### malawi_glenn

no they are made of air

19. Jun 11, 2009

### nicksauce

Quarks have mass. Photons don't have mass.
Quarks have electric charge. Photons don't have electric charge.
Quarks interact strongly. Photons don't interact strongly.
Quarks interact weakly. Photons don't interact weakly.

So no, photons are not made of quarks.

20. Jun 12, 2009

### Sidnv

Thanks.
So photons aren't really particles in the traditional sense are they?. Their particle nature is just a reflection of the quantization of their energy.