Electromagnetic field theory. Q and A

steenkh said:
* BUMP *

It is a pity that Roger Coghill never showed up here. He is actually very silent for the time being. Maybe he is too busy pedling his magic water or the magnetic therapies at some important conference?
Click to expand...
Sooner or later, most people tire of shooting themselves in the foot.

Hans
 
New EM Questions, non woo

New question:

I understand that electromagnetic force is mediated via photon exchange ONLY. Does this mean that photons are being exchanged between the magnet stuck to my refrigerator and the steel material the refrigerator is made of? I understand that there's no net flow of photons as no energy is being consumed by that magnet just stuck there, but are photons "really" being exchanged?

Same goes for my butt and my clothes -- I'm sitting in a chair and it is the electrons surrounding the atoms making up my butt's skin that is repelling the electons of the atoms in my clothes. Are they exchanging photons?

If so to either question, what wavelength photons would they be (if that could be determined, maybe that depends on the objects' temperatures)?
 
garys_2K question goes to what I have been trying to understand also.

We often here that photons mediate the electromagnetic force, but what does that mean?

I have been thinking about a static electric charge. There is a field around it and the field is advancing through space at the speed of light. At the photon level what is happening? Is there a theoretical quantum nature to this field?
 
I don't think the fields themselves use photons, but any type of interchange of energy or force from the fields are thought to be (I think). I realize photons are involved in EM radiation (radio, light, x-rays, etc.) but how are they (or, are they) involved when I pick up a paper clip with a magnet?
 
garys_2k said:
I don't think the fields themselves use photons, but any type of interchange of energy or force from the fields are thought to be (I think). I realize photons are involved in EM radiation (radio, light, x-rays, etc.) but how are they (or, are they) involved when I pick up a paper clip with a magnet?
Click to expand...
When you are talking about quantum electrodynamics, all electromagnetic forces involve the exchange of photons. So yes, photons are involved when you pick up a paperclip with a magnet.

How does this work? Well, magnetic forces involve currents, that is, moving charges. This includes spinning charged particles, which is what you've got in a magnet. Now, when you have balanced electrical charges (equal numbers of + and - particles) the electric charge balances out, but if some of them are moving, the magnetic forces won't balance, because in some other frame of reference the charges will appear to be unbalanced (this is like a contraction in special relativity, which is part of QED). So "from a certain point of view," as Obi-Wan Kenobi would say, magnetism works just like electrostatic forces.

Now how do photons mediate electrostatic forces? Every charged particle couples to photons. If you have two charged particles near each other, there is some probability that they will both couple to the same photon - appearing at one particle and disappearing at the other particle. Photons have energy and momentum proportional to their frequency, so by conservation of energy and momentum, if particles exchange a photon, in general the momentum of the two particles will change by equal and opposite amounts. This change in momentum, averaged over many exchanged photons, will be percieved as a force acting on the particle.

Classical electric and magnetic fields, in this view, are just a representation of the average amount of force due to exchanged photons on a test particle.
 
Re: New EM Questions, non woo

garys_2k said:
New question:

I understand that electromagnetic force is mediated via photon exchange ONLY. Does this mean that photons are being exchanged between the magnet stuck to my refrigerator and the steel material the refrigerator is made of? I understand that there's no net flow of photons as no energy is being consumed by that magnet just stuck there, but are photons "really" being exchanged?

Same goes for my butt and my clothes -- I'm sitting in a chair and it is the electrons surrounding the atoms making up my butt's skin that is repelling the electons of the atoms in my clothes. Are they exchanging photons?

If so to either question, what wavelength photons would they be (if that could be determined, maybe that depends on the objects' temperatures)?
Click to expand...
Going a bit out on a limb here, I have to say that a static field cannot be mediated by photons in the normal sense as a flow of photons, because there is no energy enxcange. Any change in the direction and/or intensity of the field is, of course, mediated by photons, the energy, and thus wavelenght, of which depends on the speed and strenght of the change.

That said, in quantum physics, everything is per devinition regarded as particles, and if quantum physicists deem a field to consist of photons, then so be it. But I am out of bounds here.

Hans
 
Well, the only way to measure a classic field is to stick a test charge in it and see if it moves. So you've got a particle that can exchange photons with whatever charge is causing the field in the first place.

Changes in a field are just changes in the probability of interacting with a photon of some energy at a given point, because a source charge moved or something.
 
Zombified said:
Well, the only way to measure a classic field is to stick a test charge in it and see if it moves. So you've got a particle that can exchange photons with whatever charge is causing the field in the first place.

Changes in a field are just changes in the probability of interacting with a photon of some energy at a given point, because a source charge moved or something.
Click to expand...
Yes, good point! In principle we cannot measure the field except by excanging photons with it, thus, for all practical purposes, it consists of photons.

Hans
 
OK, so a brick sitting on the ground (held there by the mutual repulsion of the electron shells of the atoms on the surface of the brick and the dirt) may not be "exchanging" photons while it's sitting there, but they may when you put your foot on the brick and press it down harder, right? Same as when you let the additional force off.

I think this is getting through, thanks for the patient explanations!
 
Since magnetism is seen as just a manifestation of the electrical force, I tried to simplify the situation by just asking about the electrical force.

Assuming the electrical force is mediated by photons the mediation has to be going on whether the charge is moving or not because the charge can't know whether some place out there its field will run into another charge. It is also not clear to me that the charge can even know if it is moving. Since what absolute marker would you use to determine what it is moving relative to.

I am not clear on whether an electron could sense that it is being accelerated. Does anybody have some thoughts on this?

Consider two electrons, one that has existed for a long time and one that has just sprung into existence as the result of nuclear decay type event. Will the new electron immediately be accelerated by the existence of the field from the preexisting electron, or will the new electron not be accelerated by the existing field until its own field has had time to reach the older electrons?

For my own edification I offer this explanation of the reason the magnetic force is considered a manifestation of the electric force:

Consider two parallel wires with equal currents flowing through them in the same direction. The free electrons in each wire will see the free electrons in the other wire as the same distance apart since on average the electrons in each wire are traveling at the same speed. However due to a relativistic contraction the electrons in each wire will sense the positive charges in the opposite wire as being closer together. Therefore the electrons (and the wire they are in) will be attracted to the opposite wire.

Zombified,
The questions above may make it seem like I didn't read your post. I did actually read it and I am trying to understand it a little better and I am still thinking about it.
Thanks, Dave
 
If, in my example above, the electron that came into existence as the result of nuclear decay was attracted to the already existing electron instantaneously, it appears that momentum would not be conserved. So I am leaning to the idea that for the electron to be repeled by the other electron the electric field from each electron must have had time to reach the opposite electron.
 
Well, even simple field theory is at the limit of my understanding, but I believe the new particle would be affected immediately, because even without that charge, there's always the possibility to find a photon at a certain location in the vicinity of the source charge. Of course, any test device imaginable would introduce a charged particle for a photon to interact with.

Also, even if you have only a single charged particle, it can spit out photons and reabsorb them, and there can also be pair-production of virtual electron-positron pairs that appear and vanish, and those can interact with the photon field. All of these things, strange as they may sound, result in higher-order corrections to the forces you'd calculate from a simple one-photon exchange. One-photon exchanges are your first order term, two particle exchanges are second-order terms, etc, as in a power-series approximation.

Virtual particles do not necessarily conserve energy & momentum "in flight," but the total energy and momentum do have to add up before and after the interaction. This is why they're called "virtual" - real particles are the ones that emerge from interactions, are detected, and must conserve energy and momentum.

It's possible I'm not quite explaining this correctly - as I said, I'm at the edges of my understanding...
 
Davefoc:

"Consider two electrons, one that has existed for a long time and one that has just sprung into existence as the result of nuclear decay type event."

At the risk of asking an incredibly stupid question, can you give an example of a "nuclear decay type event" that brings an electron into existence?
 
davefoc said:
It has been postulated that protons might decay into neutrons and electrons very rarely. I think experiments to find this have not been successful.
Click to expand...
This was the original purpose of the Kamiokande experiment, which ended up producing initial indications of neutrino oscillation which were confirmed by SuperKamiokande. The search for proton decay was also repeated in SuperK, but there were no observed events in that case either. This puts a pretty significant lower bound on the half-life of the proton.
But Beta radiation/decay is a real observed phenomena.

http://en.wikipedia.org/wiki/Beta_particle

beta particles are electrons or positrons
Click to expand...
Yes, electrons from beta decay are quite common. You can also get electron-positron pairs by colliding all sorts of particles with enough energy.
 
davefoc said:
Since magnetism is seen as just a manifestation of the electrical force, I tried to simplify the situation by just asking about the electrical force.

Assuming the electrical force is mediated by photons the mediation has to be going on whether the charge is moving or not because the charge can't know whether some place out there its field will run into another charge. It is also not clear to me that the charge can even know if it is moving. Since what absolute marker would you use to determine what it is moving relative to.

I am not clear on whether an electron could sense that it is being accelerated. Does anybody have some thoughts on this?

Consider two electrons, one that has existed for a long time and one that has just sprung into existence as the result of nuclear decay type event. Will the new electron immediately be accelerated by the existence of the field from the preexisting electron, or will the new electron not be accelerated by the existing field until its own field has had time to reach the older electrons?

For my own edification I offer this explanation of the reason the magnetic force is considered a manifestation of the electric force:

Consider two parallel wires with equal currents flowing through them in the same direction. The free electrons in each wire will see the free electrons in the other wire as the same distance apart since on average the electrons in each wire are traveling at the same speed. However due to a relativistic contraction the electrons in each wire will sense the positive charges in the opposite wire as being closer together. Therefore the electrons (and the wire they are in) will be attracted to the opposite wire.

Zombified,
The questions above may make it seem like I didn't read your post. I did actually read it and I am trying to understand it a little better and I am still thinking about it.
Thanks, Dave
Click to expand...

As I mentioned before, the main problem in this field is that there are several different models in use, and each of them is more or less valid under particular circumstances, but no one model is ideal under ALL circumstances. The reason this is a problem is because when one tries to get a mental picture of something "physical", there are various contradictory or irreconcilable elements between each of the models and this leads to further confusion rather than clarification.

To get a better idea, you really need to stand back from the specific models and look at the more general elements. For example, we know that there is a "field" of some description surrounding an electric charge. The field concept originated with Faraday as far as I recall. We know that the field is apparently a "field of charge", we know that charge is quantized and we also know that the field contains energy. So putting these elements together we have to account for how energy can be stored in apparently empty space, and how it can occur in discrete units or quanta. The obvious answer is to propose some sort of discrete particle (which covers the quantization) and which is somehow "suspended" in free space around the charge, but which does not apparently propagate (otherwise the charge would run down), which hold electromagnetic energy and which can not be independently isolated from the field as a whole. The answer is the "virtual photon". The photon holds electromagnetic energy. But it cannot be a "real" photon because real photons propagate and can be isolated. So we propose a "virtual photon". And in those terms the field is reduced to a seething mass of virtual photons which pop in and out of existence (or are emitted and reabsorbed by the charge). When we introduce another charge into the vicinity and there is energetic interaction between the charges, we can account for that by proposing that virtual photons (contributed by each separate charge) interfere with each other and become real photons which then mediate the interaction.

As I have mentioned before, you could think of a virtual photon as being analogous to an EM wave in which the E and H field are out of phase, so that a virtual photon is reactive (as opposed to resistive) - in other words it has an imaginary impedance (in complex numbers). A real photon is "resistive" and has a real impedance because the E and H fields are in phase. Of course it seems ridiculous to define an E and H field in terms of photons and then define photons in terms of E and H fields! :) But again these are only models. In reality we have no idea what charge actually IS, or what an E or B field is either. Anyway, it is possible to imagine that the interaction between purely reactive elements could lead to a resistive element, i.e. that virtual photon interactions lead to real photons. A virtual photon cannot propagate (E and H need to be in phase to propagate) - at least not very far - I don't want to complicate the issue with evanescent waves! This is rather like saying in electrical terms that purely reactive elements do not actually transfer real power. So when the charges are isolated the virtual photons remain virtual, but when they are in the vicinity of the other, their virtual photons interfere resulting in some real photons which then carry the energies (or transfer power) associated with the specific interaction.

So to go back to the question in hand, the photonic model is most useful when we are trying to define INTERACTIONS between charges. But if we are talking about a static field in which there are no apparent interactions (i.e. the field around an isolated charge) the idea of photons (both real and virtual) is rather redundant - and in fact is rather an inappropriate model. The reason why the QED interactive model is generally preferred is because it gives more accurate answers to real problems over a wider range of energies than the classical model does. But that does not mean that the classical model is totally wrong or always inappropriate. In the case of a static charge and a static field, then the classical model is actually superior to the QED model. And, more importantly, QM as a whole recognises the EM field (note EM, not isolated classical E and B) as a fundamental quantum entity.

Therefore the idea of an isolated charge shooting out photons (real or virtual) is inappropriate. Once you realise that the model doesn't work that way, then you avoid the problems associated with trying to figure out how one charge "knows" about another. Just imagine a quantum EM field already being in existence so there is no discrepancy. If another charge appears as the result of a radioactive decay, it does not simply appear instantaneously out of nothing. The process of beta decay takes a finite time and there are usually other charged particles coupled to the decaying element which are already coupled to the external field. This isn't really an answer as such, it's just to point out that the situation is complex and we need to take every aspect into account - it's easy to forget important aspects in thought experiments. The point being that the idea of two totally isolated charges suddenly popping into existence WITHOUT being previously coupled or entangled in some quantum sense via an EM field is wrong. It's really just a question of something changing form or altering the nature of something which is already coupled. The QED approximation involving photon exchanges is just a convenient tool for calculation of the interaction more than anything else.

Similar arguments apply to relativistic models of the magnetic force. We know in essence that what we call classical electric and magnetic fields are apparently connected by motion. We also know that we can see (static) electric fields without being able to detect any magnetic field, hence it is natural to think of the electric field as being more fundamental and the magnetic field as a motion of an electric field. However, in reality we have no way of determining (at present) whether one is more fundamental than the other. And QM sidesteps the problem by defining a fundamental composite "EM field". For example, Maxwell's equations assume that electric charge monopoles exist and that magnetic monopoles don't. This makes the equations asymmetric, which does tend to give the impression that maybe there is something wrong with them - many people (myself included) feel that they ought to be symmetrical. But in order to make them symmetrical you either need to add magnetic monopoles (which have never been observed in practice) or subtract the idea of charge (electric monopoles). And if you do either you get interesting results. Dirac added magnetic monopoles in his quantum theories and managed to derive fundamental proofs of quantization and even to deduce the existence of antimatter. Others, (Harmuth for example) have tried to remove electric monopoles and have developed QED type solutions that avoid the problems of rampant infinities and remove the need for renormalization in QM calculations. I rather like the latter approach (no electric charge monopoles) but that of course then introduces the problem of how one explains observed charge monopoles - you can get round it by assuming that what we observe as an isolated electric charge is some kind of complex motion of a more fundamental magnetic field element - which brings me back to my original point about it being impossible to determine which is more fundamental.

To use an analogy, electric fields are rather analogous to the energy associated with a mass. A static electric field could be thought of as being analogous (in form not dimension) to "potential energy" (in fact, if you think of a potential energy operating over a length it would be dimensionally correct). A changing electric field is an electric field in motion, and hence analogously to a mass in motion, is similar to "kinetic energy". But if you have a mass in motion, it not only has kinetic energy it also has momentum - and the momentum is analogous to the magnetic field (dimensionally a momentum operating over a length). The direct analogues of energy and momentum would be voltage and magnetic vector potential.

The reason I mention this is because I sometimes find it useful to think of electric fields as being similar (in form) to the energy associated with a mass and magnetic fields as being similar in form to the momentum associated with a moving mass. And one of the ways in which this is useful is that it highlights the idea that a magnetic field is something independent and real, because in relativistic models there is a tendency to dismiss magnetic fields as just being a relativistic effect of motion - but with an implication that they are somehow not as "real" as electric fields.

Please bear in mind that I'm not trying to be rigorous, this is just a simple overview of some concepts that I think will help.

Anyway, the overall point is that you need to be extremely careful with how you use these models, and in particular, to remember at all times that they ARE only models. In practice, as Zombified already mentioned, one can only define fields etc., in terms of their interactions with other things. That doesn't imply that they don't exist in the absence of interaction, but that in reality it's easy to over extrapolate the interactive models into isolated ones. The fact is, we don't actually know what lies underneath everything else.

Oh, and before I forget, on the basis of what I said above, your relativistic model is probably as good as any other.
 
You must log in or register to reply here.

Back
Top Bottom