## why doesn't a magnet interfere with light?

Since light consists in part of magnetic fields, I was wondering why the magnetic field of a magnet never has any effects on the light passing through it. Is it because the magnet's field is static while the light is moving?

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 I could be wrong but I suspect a magnetic field would affect a beam of light if it were concentrated in a space similar in scale to the wavelength of the light.
 Recognitions: Gold Member I believe it is because photons are not charged particles and therefore do not react to the electromagnetic force. (Yet they are the force carrier for that force oddly enough) I believe the magnetic field is cancelled out by the electric field as it moves. This is slightly similar to an atom with equal numbers of protons and electrons. They balance each other out and therefore will not be attracted to or repulsed from a magnetic field as a whole.

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## why doesn't a magnet interfere with light?

 Quote by brainstorm Since light consists in part of magnetic fields, I was wondering why the magnetic field of a magnet never has any effects on the light passing through it. Is it because the magnet's field is static while the light is moving?

Magnets do interfere with light.

 Quote by wiki Faraday Effect In physics, the Faraday effect or Faraday rotation is a magneto-optical phenomenon, or an interaction between light and magnetic field in a medium.

 The classical theory of an electromagnetic wave goes something like this... A potential difference occurs between two points be it the two ends of a transmitting antenna or two energy states in an atom. As that potential voltage collapses it creates an expanding magnetic field. When the electrical potential is depleted the magnetic field is at it's maximum. With no more electrical potential to sustain it, it starts to collapse, in the process it induces a potential voltage, this voltage exists whether or not there are charged particles for it to affect. when the magnetic field has fully collapsed the potential voltage is at it's maximum and the cycle repeats. One consequence of this theory is that each magnetic field component will be oriented opposite from the previous magnetic field component. If an outside magnetic field is large compared to the wavelength of the light then it will distort each magnetic field component of the light just a little and it will affect every other cycle oppositely. If the magnetic field is concentrated enough to affect just one wave and to affect it significantly then I think that the effect would be noticeable.

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 Quote by OmCheeto Magnets do interfere with light.
Do they? I'd never heard that before. Where can i read more on this?

 Quote by OmCheeto Magnets do interfere with light.
Only in the presence of certain materials. Not in empty space!

Photons do not interact with each other. Electromagnetism (light, radio, and all those waves) is regulated by the superposition principle, which would not hold if waves interacted with each other. They can only sum with each other.

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 Quote by Dr Lots-o'watts Only in the presence of certain materials. Not in empty space! Photons do not interact with each other. Electromagnetism (light, radio, and all those waves) is regulated by the superposition principle, which would not hold if waves interacted with each other. They can only sum with each other.
What do you mean by "They can only sum with each other"?

 Quote by brainstorm Since light consists in part of magnetic fields, I was wondering why the magnetic field of a magnet never has any effects on the light passing through it. Is it because the magnet's field is static while the light is moving?
The theory of electromagnetism is about the electric and the magnetic field. It is about both, combining it. The Maxwell equations are describing both, a magnetic field is nothing but the field of moved charge. These equations are linear. Linearity means always that the superposition principle is applyiable. The equations are perfectly valid in the domain of special relativity, it was one of the reasons Einstein discovered the SRT. If you accept that light is nothing but an "electromagnetic effect" you will see that the superposition holds: There is no interaction. An interaction would mean an nonlinear effect which is given in masses having interaction but not in the issue you wrote about.

Best regards,
Jens

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 Quote by Drakkith What do you mean by "They can only sum with each other"?
superposition is a fundamental feature of linearity:

F(x) + F(y) = F(x+y)

if y and x are coupled to each other (i.e the waves interact) this needn't be true.

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 Quote by Dr Lots-o'watts Only in the presence of certain materials. Not in empty space! Photons do not interact with each other. Electromagnetism (light, radio, and all those waves) is regulated by the superposition principle, which would not hold if waves interacted with each other. They can only sum with each other.
Hmm.. I think I might have missed that part. Though it is possible that my wash machine of a brain took the Faraday Effect and the article I saw a few years ago and produced an erroneous conclusion.
 Polarized Space In 1997, Borge Nodland and John Ralston found in their analysis of astronomical polarization data that the universe had an optical axis: it was circularly birefringent! The universe appeared to behave in a similar way as a crystal with optical activity: it rotated the polarization direction of linearly polarized light! This cosmic "quartz crystal" had an optical axis parallel to the direction Aquila-Earth-Sextans. Is the universe birefringent? Is the universe rotating?
Ah ha! The following indicates that I was confused:

 Summary by Borge Nodland, University of Rochester Since the new rotation we find has such a systematic directional dependence, it is implausible that it is generated by cosmic ions and fields via some mechanism similar to the Faraday effect. One may therefore surmise that it is the vacuum itself that flaunts a form of electromagnetic birefringence, or anisotropy - similar to the birefringence exhibited by many crystals.
Never mind.

 Quote by Drakkith What do you mean by "They can only sum with each other"?
Two electromagnetic waves (or photons, which are physically the same thing) don't interact with each other.

All that happens if they get near is that they add up (summation). This is what fundamentally causes interference. But this isn't really an interaction. Both remain independent of each other. Once two photons have crossed, they forget they've met, as if the other never existed.

That's why a magnet won't affect a light beam. A light beam ('s field) technically adds to nearby magnet's field, but they don't affect each other. A particle in that region of space will feel the effects of both fields however.

 like sound waves they (light waves) just pass through each other. the net 'field' at any time is just the (linear) sum of the individual fields. in other words, the fields just 'superpose' on top of each other
 The thing to keep in mind, is that the vector potential has a greater physical significance than the magnetic field. Consider Feynman, vol 2, section 15-5, "the vector potential and quantum mechanics". The idea here is that if a beam of light passes through a region of space where the vector potential is non-zero (even if the magnetic field is zero), that the light beam will interact with the vector potential, and this will have a measurable effect, in the sense that the vector potential will influence the quantum phase of the light beam. The statement "the magnetic field of a magnet never has any effects on the light passing through it" is false. One very clear way to see this is to realize that photons do interact in QED through virtual particle production, this accounts for example, for the anomalous magnetic moment of the electron. The physical significance of the vector potential over the magnetic field can be seen from a more fundamental perspective. In order to quantize the electromagnetic field, it is customary to work in the Lorentz gauge and to consider photons as the harmonic frequencies of the vector potential.

 Quote by calhoun137 The statement "the magnetic field of a magnet never has any effects on the light passing through it" is false. One very clear way to see this is to realize that photons do interact in QED through virtual particle production, this accounts for example, for the anomalous magnetic moment of the electron.
Maybe I should have specified that I meant an empirically visible effect, such as image distortion, prism effect, etc. Are there any observable effects of the virtual particle production you're talking about?

 The poster in this old thread seems to imagine that a lens placed in front of a magnet interferes with the magnetic field.http://www.physicsforums.com/showthr...ghlight=magnet
 one way to see this would be to compute the cross section for photon photon scattering to some order in the fine structure constant. In other words, the effect is that two photons will have some QED scattering interaction. I'm not sure about macroscopic effects...
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