Why is light not affected by magnetic fields?

[logy]

Moderation note: the first 5 post of this thread were moved from [Q] Light and magnets , because they are more of an in-depth discussion of physics than a simple question & answer

in my humble oppinion:

light is not affected by magneting or static electrical fields because it has no mass and it's speed is constant !
magnets and static electricity creat a force field which can accelerate objects that have mass and gain gain or loose kinetic energy and momentum, like say other magnets or an electrically chared charged piece of metal, but light itself travels at the speed of light, had no "rest mass" which means no mass for the purpose of this discussion and therefore will travel in a streight line unless one of the following happens.
1.it hits a reflecting or refracting surface
2. it enters a gravitational field, in which case, the curve in space itself will make it "curve".

simply put, since light had no mass and cannot be accelerated (as acceleration shanges speed), it will not react to a force.

[Moontanman]

Originally Posted by logy

in my hombel oppinion:

light is not affected by magneting or static electrical fields because it has no mass and it's speed is constant !
magnets and static electricity creat a force field which can accelerate objects that have mass and gain gain or loose kinetic energy and momentum, like say other magnets or an electrically chared charged piece of metal, but light itself travels at the speed of light, had no "rest mass" which means no mass for the purpose of this discussion and therefore will travel in a streight line unless one of the following happens.
1.it hits a reflecting or refracting surface
2. it enters a gravitational field, in which case, the curve in space itself will make it "curve".

simply put, since light had no mass and cannot be accelerated (as acceleration shanges speed), it will not react to a force.

While what you say may have some bearing on the process the fact that Photons have no charge is the main reason magnetic fields to not affect them. Massive particles with charge are affected as are less massive particles with charge. I know of no massless particles with charge but if they had charge a magnetic field would affect them. Particles with no charge are not affected no matter how massive they are.

[logy]

Originally Posted by Moontanman

While what you say may have some bearing on the process the fact that Photons have no charge is the main reason magnetic fields to not affect them. Massive particles with charge are affected as are less massive particles with charge. I know of no massless particles with charge but if they had charge a magnetic field would affect them. Particles with no charge are not affected no matter how massive they are.

the question asked was how come EM waves are not affected by EM force fields, and yes, EM waves don't have a constant charge, but they do have an alternating electical/magnetic field and as a result some of the time they do exist as a static electrical field or as a magnetic field, so why are they not affected by by a magnet? the answer IMHO lies in the fact that they are only fields and the interaction between two charged particles relies on they mass, distance and charge according to the formula
force = (KxQ1xQ2)/R^2
acceleration = (force x distance)/mass

where K is the electric constant
Q1 and Q2 are the charge of the particles in columbs
R is the distance in meters between the 2 particles

if there where massless partiles with a constant charge, the math will not work as the acceleration is inversly proportional to the mass and therefore any force would accelerate such a particle to infinite speed (division by zero since the mass is zero) the natural conclusion is that massless particles with a 'charge' cannot be acclerated. my argument is that massless particles cannot have their speed affected by force fields due to the division by zero problem.

in my understanding, it is the fact that force will accelerate massive particles, but has no effect on EM waves as they are an alrernating static/magnetic force fields with no mass. the concept of charge, is simply saying that a massive particle has a constant EM force field. if i where to create a system that has mass and has an alternating electrical/magnetic field it will be affected by a magnet, accelerating, decelerating or changing direction depending on it's configuration.

am i wrong?

[Moontanman]

Originally Posted by logy

the question asked was how come EM waves are not affected by EM force fields, and yes, EM waves don't have a constant charge, but they do have an alternating electical/magnetic field and as a result some of the time they do exist as a static electrical field or as a magnetic field, so why are they not affected by by a magnet? the answer IMHO lies in the fact that they are only fields and the interaction between two charged particles relies on they mass, distance and charge according to the formula
force = (KxQ1xQ2)/R^2
acceleration = (force x distance)/mass

where K is the electric constant
Q1 and Q2 are the charge of the particles in columbs
R is the distance in meters between the 2 particles

if there where massless partiles with a constant charge, the math will not work as the acceleration is inversly proportional to the mass and therefore any force would accelerate such a particle to infinite speed (division by zero since the mass is zero) the natural conclusion is that massless particles with a 'charge' cannot be acclerated. my argument is that massless particles cannot have their speed affected by force fields due to the division by zero problem.

in my understanding, it is the fact that force will accelerate massive particles, but has no effect on EM waves as they are an alrernating static/magnetic force fields with no mass. the concept of charge, is simply saying that a massive particle has a constant EM force field. if i where to create a system that has mass and has an alternating electrical/magnetic field it will be affected by a magnet, accelerating, decelerating or changing direction depending on it's configuration.

am i wrong?

You are far beyond my level of expertise, the original question was is light affected by a magnetic field, it is not, i honest do not know why except that photons are not charged. If indeed massless particles were charged I would expect them to be affected but as far as I know there are no massless charged particles. Your take is interesting but will require someone who is above me on the food chain.

[CraigD]

Originally Posted by logy

the question asked was how come EM waves are not affected by EM force fields, and yes, EM waves don't have a constant charge, but they do have an alternating electical/magnetic field and as a result some of the time they do exist as a static electrical field or as a magnetic field, so why are they not affected by by a magnet?

I think I’m wandering onto semantically difficult ground, but I’d explain this by noting that magnetic fields aren’t affected by magnetic fields, rather changes in the position and velocity of charged particles result in differences in their associated magnetic fields. Magnetic field are really just descriptions of how a hypothetical, arbitrarily low mass and change test body’s momentum would change at points within them.

Originally Posted by logy

the answer IMHO lies in the fact that they are only fields and the interaction between two charged particles relies on they mass, distance and charge according to the formula
force = (KxQ1xQ2)/R^2
acceleration = (force x distance)/mass
…

I think what you’re saying here is that particles with zero invariant (“rest”) mass () can’t be meaningfully considered to be subject to force or acceleration.

A problem with this limitation is that such particles (eg: photons) behave in many ways as if they have non-zero masses when their speed is, as it always is, the speed of light in vacuum (c). They appear to be subject to gravity, and to transfer momentum (mass x velocity) when they collide with other particles.

Though formal purists cringe at it, an easy solution to this problem is to consider photons and other such particles to have “relativistic” (dialated) mass , where is the photon’s frequency, and is Planck’s constant.

One of the reasons purists cringe at this is that it implies some arithmetic absurdities. For example, the usual equation for mass dilation is . being defined for a body with speed implies a nonsensical .

However, if you’re careful not to use it inappropriately, the concept of photons with non-zero relativistic masses is useful and convenient.

Justification and objections to this approach can be found in many texts and websites, including the wikipedia articles “mass in special relativity” and “photon”.

[GAHD]

Light *IS* affected by magnetic feilds: they cause polarization of the waves. Faraday noted this effect and it is used quite extensively in solid state polorization controllers.

minor semantics, but there IS interaction between the two.

[CraigD]

Originally Posted by GAHD

Light *IS* affected by magnetic feilds: they cause polarization of the waves. Faraday noted this effect and it is used quite extensively in solid state polorization controllers.

minor semantics, but there IS interaction between the two.

Just the sort of semantic we’re here to discuss!

As far as I know – and I could well know wrongly - the Faraday effect can only occur when light passes through a non-vacuum medium – that is, a volume with non-zero rest mass particles in it. I don’t believe a beam of polarized light has its polarization rotated if it passes through a magnetic field in vacuum.

So it’s not an example photons interacting with other photons – which I understand is forbidden – but magnetic interaction photons interacting with fermions (usually electrons), which interact with EM radiation photons. In practical terms, true vacuums don’t exist enough to worry about – interstellar space, for example, has at least 2000 electrons/m, so magneto-optical effects like the Faraday effect happen effectively everywhere - but for understanding fundamental interactions, the distinction is, I think, critical.

I don’t know how to describe light polarization effects – Faraday or others – in terms of photon-fermion interactions. If someone can, I’d greatly appreciate it – it’s puzzled me for a long time.

[logy]

Originally Posted by CraigD

Just the sort of semantic we’re here to discuss!

As far as I know – and I could well know wrongly - the Faraday effect can only occur when light passes through a non-vacuum medium – that is, a volume with non-zero rest mass particles in it. I don’t believe a beam of polarized light has its polarization rotated if it passes through a magnetic field in vacuum.

So it’s not an example photons interacting with other photons – which I understand is forbidden – but magnetic interaction photons interacting with fermions (usually electrons), which interact with EM radiation photons. In practical terms, true vacuums don’t exist enough to worry about – interstellar space, for example, has at least 2000 electrons/m, so magneto-optical effects like the Faraday effect happen effectively everywhere - but for understanding fundamental interactions, the distinction is, I think, critical.

I don’t know how to describe light polarization effects – Faraday or others – in terms of photon-fermion interactions. If someone can, I’d greatly appreciate it – it’s puzzled me for a long time.

thank you craig, i think it is important to note that the farady effect is a result of the magnetic field acting on an electron, and not on a photon.

i don't fully understand the way fermoins work but i do know that they all have half integer spin so if an electron absorbs a photon, then rotates around their its axis due to the spin of the fermion, and then re-emit a photon, that photon's plane of polatization will move in a clock wize or anty clock wize direction.

what 'spin' really is, is something that i am still learning about but it can be considered to be an actual spin of the electron according to wiki.
"spins obey the same mathematical laws as do quantized angular momenta."
...i can't post links yet , it's the wiki page titled "Spin_(physics)"

the above mentioned page from wiki about the farady effect has a good diagram that shows how it works. it's important to remmember that the vector of a magnetic field point to the north pole, while it's spin is around the south-north axis !

i hope this will shed some light on the topic. i think that in order to properly understand the whole magnet thing i will have no choice but to study maxwell's equations... hmm

[Little Bang]

If there have been experiments that confirm magnetic fields have no effect what so ever on light please point them out. Since EMR has both electric and magnetic components how do we know that a magnet field doesn't cause something like rotation of the wave?

[logy]

Originally Posted by Little Bang

If there have been experiments that confirm magnetic fields have no effect what so ever on light please point them out. Since EMR has both electric and magnetic components how do we know that a magnet field doesn't cause something like rotation of the wave?

light is an alternating electromagnetic field, and when you have 2 light waves in the same spot you get constructive or destructive interference. if light was affected by electromagnetic fields, it would show very clearly in such experiments. the rules of physics are that fields generate forces and forces move objects, but fields do not move fields, they just interfere with fields that are similar. usually fields are generated by objects (particles with mass) like magnets and electrons and then, if you move the object, you move the field that is generated by that object, but light has no (rest)mass and cannot be accelerated.

the experiments you are talking about have been done in mechanics, and it has been discovered that moving an object with rest mass requires a force, which a massless object cannot be moved as such, it will always go in a straight line and have the same polar plane. one exception to that is gravity, since according to general relativity, space itself is curved and therefore a "straight line" is not really straight when it goes through such a curve.