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Q & A: Magnetic field is made of photons

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Most recent answer: 07/26/2017
Q:
My question is--What is a magnetic field made of? I’ve read a lot of things about magnets and the fields they generate, and even that electrons themselves have magnetic fields around them, but I haven’t as of yet come across anything saying what the field itself is made of. It is matter so it has to be made of something. Is there a name for these "particles"? Or are they simply electrons themselves?
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- Douglas (age 35)
Louisiana USA
A:
Hi Douglas,

The electromagnetic interaction is mediated by the constant exchange of photons from one charged object to another. The magnetic field is really just a classical approximation to the photon-exchange picture. In a moving reference frame, a magnetic field appears instead as a combination of a magnetic field and an electric field, so electric and magnetic fields are made of the same "stuff" (photons).

Some electromagnetic interactions involve "real" photons with definite frequencies, energies, and momenta. Electrostatic and magnetic fields involve the exchange of "virtual" photons instead. Very close to an electron is a dense cloud of virtual photons which are constantly being emitted and re-absorbed by the electron. Some of these photons split into electron-positron pairs (or pairs of even heavier stuff), which recombine into photons which are re-absorbed by the original electron. These virtual particle loops screen the charge of the electron so that far away from an electron it appears as if it has less charge than close by.

Normally we wouldn't call any of these fields "matter", but it is true that the electric and magnetic fields which surround a charged object like an electron do store energy, and therefore have a rest mass, via E=mc^2 (in a reference frame in which the electron has no momentum).

Tom
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(published on 10/22/2007)

Follow-Up #1: picturing magnetism

Q:
Would it be possible to answer the question clearly using more elementary terms for elementary particles? Or, perhaps a diagram of particles as the cascade in various energy states and produce reciprocal forces or actions on other matter?
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- JS BAIRD (age 67)
Davao City,Philippines
A:

It sounds like what you're asking for is a Feynman diagram to represent electromagnetic interactions. You can get that with a discussion on Wikipedia:


Meanwhile, I'll take the opportunity to somewhat modify Tom's presentation. We routinely say things like "virtual photons ... are constantly being emitted and re-absorbed by the electron" but that isn't really what we mean.  Two particles that are interacting electromagnetically are indeed surrounded by a virtual photon cloud. However, in familiar cases (e.g. a hydrogen atom) that cloud is completely unchanging in time. Nothing at all is going on. The words about things fluctuating around are a rough way to convey one of the peculiar properties of quantum fields. The electric and magnetic fields have not only average values but also ranges of possible values around the average. That's what's so different from classical fields.  (It's just like the positions of quantum particles, which have ranges around the average position, unlike classical particles.) You can convey an image of that range by pretending that the fields are jumping around between the different possible values, just like you can pretend that a particle  is jumping around among the different positions in its cloud. However, the fields (including their spread of values) no more need to be jumping around than do the particles in space. In a nice stable atomic state, for example, nothing changes in time. The static range of possibilities turns into an actual range of outcomes only when the system interacts in particular ways with the bigger world outside.

Mike W.

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(published on 04/27/2011)

Follow-Up #2: back to Maxwell's mechanical fields

Q:
I really want to know what the magnetic field is made up of? Please do not use photons in your answer as we all know magnetic fields are not composed of photons. Douglas's question was never answered. Douglas was not asking about the electromagnetic field which is most closely associated with the photon. The answers provided were incomplete and not relavant to Douglas's question. What is the magnetic field made up of? Here is what modern day science actually knows about Magnetic fields. The honest answer is we do not know what a magnetic field is. What we do know is that a Magnet field is generated by the motion of electrostatic charges within the the magnet itself. The electric charges being electrons. The electrons move in a coherent and synchronized fashion which causes a strong magnetic field to be projected out from the magnet. What we do not know is what that field is made up of. Some people used to say that it was made up of magnetic monopoles. Magnetic monopoles have never been discovered so there is a good chance, a very good chance that theory is incorrect. It is my opinion that a magnetic field is not made up of any particle field at all. Douglas, think of a magnetic field as being a direct deformation of physical space. All pure fields must work this way. They must be mechanical deformations of space. You can think of space being a low density, high tension solid elastic. The magnetic field is a mechanical deformation of space itself. I Wish you would print this question and answer but we both know you won't.
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- Mark (age 58)
Florida
A:

When Maxwell first came up with his famous equations for electromagnetism, he tried to make a mechanical model with little gears and wheels and things. It was quickly dropped because it added nothing but complication to the field equations. 

You have various verbal assertions about what a field "really" is. You say it's not made out of monopoles, but I've never heard of anyone suggesting that it is. You say it has nothing to do with photons, i.e. quantum mechanics. Your picture sounds like it wouldn't fit with special relativity.  

The combination of special relativity and quantum mechanics allows calculations of things that can be measured. For example, it gives a prediction for the electron gyromagnetic ratio. Experimentally, the value is   2.00231930462, with a little uncertainty in the last decimal place. "The QED prediction agrees with the experimentally measured value to more than 10 significant figures..."   

What value does your model give?

Mike W.

p.s. I can't resist giving the first two lines of a ~ 100 page interview that the Chemical Heritage Foundation conducted with a scientist (my father) who had been working with magnets for ~ 90 years. "As a kid I discovered that I could make the pins in a box stand up, and by moving a magnet around, I could make them march. I had no idea what a magnetic field  was and I suspect I have no idea still what a magnetic field is, except for some of the things it does."

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(published on 09/04/2013)

Follow-Up #3: electric fields as virtual photon clouds

Q:
It seems like the virtual photon cloud only exists very close to a static charge. How do we reconcile this with (a) the fact that electric field lines extend forever and (b) the fact that the virtual photons are exchanged at distances larger than the virtual photon cloud?I'm imagining a virtual photon cloud surrounding an isolated electron change. Suddenly, a proton enters the vicinity of the electron. Before the proton arrives on the scene, the virtual photon cloud is tightly packed around the electron. (The virtual photon cloud represents a static quantum state, but this doesn't mean the virtual photons aren't moving--similar to the electron cloud around a hydrogen atom.) Once the proton enters the vicinity of the electron, does the dense virtual photon cloud change shape? For example, does the virtual photon cloud "stretch out" to reach the proton, representing the increased presence of virtual photons along the path where they are being exchanged between the two charged particles?
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- JD (age 29)
Louisville, Kentucky, USA
A:

The classical field expression tells you how the virtual photon cloud spreads out. So those virtual photons are no more or less concentrated on the charge than are the classical fields. The shape changes when two charges are present are just those given by adding up the classical vector fields. So yes, there is a particularly strong field between the charges but far away the fields from them tend to cancel.

Mike W.

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(published on 04/11/2015)

Follow-Up #4: photons and magnetic fields

Q:
Magnetic Fields cannot be explained by simply saying they are made up of 'photons'. What up date is their on their physical make up?
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- Robert Ponce (age 69)
Port Hueneme
A:

There is one sense in which you're right that you shouldn't think of magnetic fields as being made of photons. If you have any specific numbers of photons of each type, the expected magnetic field is zero. To get anything like a classical well-defined magnetic field you need a big spread of possible photon numbers.* That's a lot different than thinking each photon contributes some field.

On the other hand, maybe you're thinking there's some other ingredient, besides photons. There isn't.

Mike W.

*If you look at, e.g., equation 21 in this paper (https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf) you'll find the expression for the magnetic field in terms of the photon creation and annihilation operators.

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(published on 12/13/2015)

Follow-Up #5: photons and magnetism

Q:
Mike W said: "If you have any specific numbers of photons of each type, the expected magnetic field is zero. To get anything like a classical well-defined magnetic field you need a big spread of possible photon numbers."I was hoping he could expand on that in a manner that might be intelligible to those not familiar with the math of quantum mechanics. This topic is confusing to me because I think I have a basic grasp of "normal" photons (e.g., visible light), but such photons have defined energies and wavelengths, and they don't travel in a closed loop like a magnetic field appears to. The "magnetic" photon appears to be different than the "normal" photon, and I would like to understand why, if that is possible without understanding the math.Thank you!
- James (age 30)
A:

I won't succeed in explaining this, but can clear up a few points.

The photons that contribute to magnet fields are no different than the ones that contribute to electrical fields. The particular pattern of which phases are present for different numbers of photons determines what classical fields are present.

As for the relation of photon numbers to classical fields, I can suggest an analogy that you could study that might be easier to picture. Look at the wave functions that represent states of a simple harmonic oscillator (mass on a spring). (https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator) Ones with definite energy are just like photon states with definite numbers and hence definite energies. The SHO single-energy states are always equally distributed about the midpoint, with no average displacement and no average velocity. By exact analogy, the definite-number photon states have no average magnetic field or electric field. To get an SHO to swing around classically, you need to make states where interference between parts with different energies causes cancelations in some parts and enhancements in others. As the wave changes in time, the positions of those low and high density points swing back and forth, meaning changing positions and velocities. The analog for photons is changing fields.

Mike W.


(published on 01/16/2017)

Follow-Up #6: ingredients of magnetism?

Q:
Mike W said "On the other hand, maybe you're thinking there's some other ingredient, besides photons. There isn't"Boy! That's a bold statement. Surely, you mean "We do not know of any other ingredient" - like we don't know anything about Dark Energy, and precious little about Dark Matter. I wonder what he would have said before the postulation of photons.Having read all of the discussion, it seems to me that the only true answer is "We don't know"How many atoms/cc are there in inter-galactic space? Not many, I'd warrant, so you would expect that there would be fewer of these vitual photons popping in and out of electrons in an environment where there are fewer electrons to pop in and out of - but what would be the attraction between two magnets in that environment? It would be the same, would it not?It certainly is puzzling that if two strong magnets were taken into the most rarefied region of space, they would still exert a powerful force on each other (attractive or repulsive), even though they may be held motionless relative to one another. There is clearly something between them.Somebody said above that a magnetic field can't exist on its own - it is a component of the electro-magnetic field, and I'm interested in that because I believe that this field must be present when the magnets are not. I think it is likely that whatever constitutes this field also constitutes whatever it is in which light creates a wave - and light, we are told, is an electromagnetic wave, the same as a high-frequency radio wave. To me (a complete layman), it seems nonsense to say that light is a wave traveling through nothing. Other waves are all waves in something - air, water, rope, etc. Light travels through space at a very accurately determined speed. Surely, if the constitution of space was a bit different, light would travel a bit slower, or a bit faster - so this field must be amazingly constant throughout space - and independent of the density of atoms. It must certainly be a very efficient transport medium, because light can travel for billions of years and still get to us without dissipating more than by the square of the distance. So what is it traveling through? Is there any clue in that the electrical and magnetic components of a radio transmission are at right-angles? (I refer here to angle of polarisation). I have seen explanations of how light travels through glass or water (where its speed is different from in space), but that depends on atoms being excited by the light and emitting a proton in response - amazingly always in exactly the same direction. This explanation explains diffraction - however, the same mechanism can't be applied to empty space.Is there any answer that can be seized upon by the layman, or is it one of those things like sqrt(-1), that can only be understood in the realms of mathematics? Even there, is there a firm understanding of what it is, rather than what its properties are?
- Mike Collins (age 71)
Gwynedd, Wales
A:

You're right that in general we don't know all the ingredients of the world. We probably don't even know the basic form of the theory, how spacetime emerges from some deeper forms, etc. Nevertheless, within the context of what we do know, there is no special mystery to magnetism. In fact, magnetism is part of the electroweak theory, which is the best-known theory we have of anything. It predicts, for example, the magnetism of an electron to better than one part in one hundred billion. 

The waves you mention are all describable as a type of behavior of underlying media- water, air, etc. At a deeper level, however, the ingredients of the universe (photons, quarks, neutrinos, gluons,...) are currently only describable as pure quantum waves in and of themselves. Photons are as basic as any of the other ingredients we have.

Maybe someday some deeper ingredients will be found and all of our currently fundamental particle fields will be seen as emerging from the behavior of that deeper theory. Will the deeper theory then turn out to emerge from a still deeper one? Will that pattern go on forever or reach a deepest level? We don't know. So long as there are dangling ends (inconsistency between general relativity and quantum field theory, dark energy and dark matter,...) we know we aren't at the bottom of the stack. If some point is reached with no basic dangling ends, then maybe we will be at the deepest level.

Mike W.


(published on 06/15/2017)

Follow-Up #7: are magnetic fields necessary?

Q:
I would like to ask a seemingly daft question. What is the experimental evidence for the traditional idea of magnetic fields? We need to remember that the idea of a rotating perpendicular flux was based on an ignorance of magnetism - orbiting and spinning electrons were unknown 150 years ago. I have asked in vain for evidence of this flux. It seems to be just a guess which turned into a belief.Suppose magnets had been unknown at the time. Experiments with electricity would then have led to a simple law: like currents attract and opposite currents repel. This basic law then explains magnetism, such as the alignment of iron filings around a magnet.Using Ockham�s principle, the complication of a circulating perpendicular flux seems unjustified. (So instead of using Biot and Savart�s law to predict perpendicular flux density, a dot product of the current vectors can be included to predict force). Hence magnetic forces just act along the straight lines between moving charges. This is the same simple principle that works for electrostatic forces between stationary charges. We need not assume the universe uses two completely different force mechanisms. Motion just modifies electrostatic forces.Magnetic fields are defined as continuous. So the field emanating from the north end of a bar magnet loops round the outside of the magnet to the south pole and returns through the magnet�s body back to the north pole. Now imagine a magnet made of a very viscous material that allows a free-moving north pole to drift within it. This internal north pole would be repelled by the magnet�s south pole (?) and leave again through its north pole. We are all taught this stuff, but it doesn�t make sense to me. We should not view magnets as perpetual motion machines. Forces begin and end at points: they do not keep going round in circles. Lines of flux show how magnetic compasses are deflected, but nothing is circulating except charges.The notion of circular fields perhaps arose when rings of iron filings were seen around a conducting wire, but it was a very odd idea. The circular magnetic field at any point is defined as a vector that is perpendicular to the magnetic force it produces. However, if a vector represents something that demonstrably exists, e.g. a physical force or a wind velocity, its perpendicular components are zero, i.e. it has no effect in a perpendicular direction. So we can say that a circulating magnetic field having its greatest effect in a perpendicular direction does not exist.A magnetic field is no more mysterious than an electric one.
- Andrew (age 67)
Shropshire, England
A:

You're right that the forces between classical currents with fairly static arrangements can be expressed directly in terms of the currents and the displacements between them. It's not as simple as electrostatics because the direction of the displacement and the directions of the currents enter in a slightly more complicated. way. But still, for that case the use of magnetic field descriptions is just an optional mathematical convenience. Once you start talking about accelerating classical charges, however, the full electromagnetic field equations become involved. It would be extremely inconvenient to describe electromagnetic waves (e.g. radio waves and light) in a notation based on charges and currents, because those waves propagate freely far from any charges and currents.

Once you wish to include quantum spins, descriptions that leave out magnetic fields or even more abstract entities (vector potentials,..) become useless. Practical magnetic materials all involve such spins. 

Mike W.


(published on 07/26/2017)

Follow-up on this answer.