Magnetic field is made of photons?

[taylaron]

i've been researching photons and i discovered on
http://http://van.physics.uiuc.edu/qa/listing.php?id=414"
and
http://en.wikipedia.org/wiki/Photon"

both resources state that all mangetic fields are made of photons. soooooo, this brings up a lot of questions... first, why (when I am in the presence of a mangetic field) don't i see light? (because light is mad of photons)
I am sure if i get a simple understanding of this concept, it will probably rule out most of my questions, so i'll wait for a reply to continue.SO WHAT ARE PHOTONS MADE OF? -obviously they're charged (hence mangetic fields)

 

[Severian]

Since it doesn't look like the lost replies that were made to this question will be coming back, I will answer it again.

Magnetic fields are indeed made of photons. The reason that you don't see them is simply that your eyes are not sensitive enough. Electric fields are also made of photons, but you don't see tham either. You also don't see infrared or ultra-violet radiation, because your eyes are only sensitive to very specific photon energies.

 

[Burnsys]

And photons are fundamental units of energy so they are not made of anything else.

 

[Meir Achuz]

"both resources state that all mangetic fields are made of photons"

I couldn't get to the first website. The Wikipedia was confusing as usual, but I didn't see that it said "all magnetic fields are made of photons".
A static magnetic field is not made of photons. EM waves are quantized in packets called photons.

 

[taylaron]

thank you,
i have a better understanding of photons now.
so photons arent really particles (no mass) since they're energy. energy that apparently takes form of light, and simply energy.
so photons are basically energy. there are more than one type of photon (light, mangetics. ... etc...)

but if these photons that make up (most) mangetic fields, are in a frequency/energy frequency -such as frequencys of light-
what frequency are these in? there if they're not gamma, UV, x-ray, IF, visible light.
what are they? do we know??

 

[quinn]

the force exchange from a magnetic field to something else is via photons,

 

[Severian]

No tayleron, photons are particles (see the photoelectrc effect), energy is not light (although photons (light) have energy), and photons are more than just energy (they have other quantum numbers). Finally the photons in light and the photons in magnetism are the same thing - just with different energies.

 

[FunkyDwarf]

so photons are the force carriers for the electric and magnetic fields in the standard model? i thought it was something else?

 

[Severian]

so photons are the force carriers for the electric and magnetic fields in the standard model?

Yes...

 

[taylaron]

thanks severian
i get the basic idea

 

[petm1]

Does this mean that we can think of a magnetic and/or a electric field as a standing wave of photons, much like a river?

 

[petm1]

Could you think of a magnetic field as being length contracted, because the photons are in a closed loop, and time contracted because they appears as a standing wave having no relative movement, instead of an expanding wave with relative movement. Keeping in mind they always travel at c relative.

 

[taylaron]

that makes sence

 

[Demystifier]

Photons, as they are usualy defined, are associated with electromagnetic fields in the absence of sources (charged currents). So a better defined question could be: Do Maxwell equations without the sources admit purely electric and purely magnetic fields? If the answer is no, then there is no photon state that corresponds to a purely electric or purely magnetic field.

 

[Jacquesl]

If Photons are light waves and you’re able you reflect them with a mirror, and some are saying magnetic waves are virtual photons so can you reflect magnetic waves.

And how could you reflect a magnetic wave “flux”, is it possible, a mirror will probably not work.

 

[DaveC426913]

Now *I'm* confused. Electric fields and magnetic fields are made of photons? (What an embarrassing gap in my knowledge...) What frequency are they?

 

[marlon]

i've been researching photons and i discovered on
http://http://van.physics.uiuc.edu/qa/listing.php?id=414"
and
http://en.wikipedia.org/wiki/Photon"

both resources state that all mangetic fields are made of photons. soooooo, this brings up a lot of questions...

OK, this is going to be a long one...

First question, you say you have been researching photons. What exactly does that mean ? Also, when doing research you should NOT be using public libraries like Wikipedia.

Secondly, i am sure you know that photons are defined as pieces of energy. Hence, the basis in which the "live" is the energy base. Photons are NOT point particles but energy-particles. It is very important to realize that. Knowing this, and knowing how E/M field are defined, you should already see that photons do NOT make up E/M fields.

Thirdly, the theory that describes photons and their relations to E/M fields is QED. In QED we learn that photons are the force carriers of the EM interaction. This means that photons are the particles that mediate the electrical and magnetic interactions. In QED, you can also study (very important to know btw) how photons arise due to the vibration of quantum fields. Think of a mattress on which you jump in one place. The surface of the mattress vibrates and due to this vibration, there is energy coming free from the mattress (ie the vibrational energy).

If you replace YOU by AN ELECTRON and the mattress by the quantumfield and you take into account that energy is the same as mass (E=mc^2) you can see how the vibration of a quantumfield can mimic a certain particle with mass m and momentum p.

This is how photons arise in QED. Ofcourse photons do not have restmass m but still the principle of vibrating fields versus particles remains the same. How QED deals with the zero restmass of photons is a more complicated story. More specifically, the quantumfield in the photon case is the 4-vector potential A.

One can actually rewrite the Maxwell equations with this A potential in such a way that they respect all necessary symmetries that need to be respcted (i know, this may sound a bit vaque but i don't know what level you are on so let me know). From this A potential one can also calculate the E and B fields.


Finally, photons are NOT the constituent particles of EM fields, photons describe the interactions between electric charges (which exhibit an electric field) or/and magnetic dipoles, quadrupoles (which have an associated magnetic field) etc etc ...

In QED photons arise thanks to the vibrating A field (think of it as the A mattress ). In other words, photons arise thanks to the quantisation of the waves in the A field. This A field is closely related to the E and B fields, so being a bit less accurate, you could also say that photons correspond to the quantisized energy of vibrating E and B fields.

greets
marlon

 

[Severian]

Now *I'm* confused. Electric fields and magnetic fields are made of photons? (What an embarrassing gap in my knowledge...) What frequency are they?

Any frequency you like. Any field of photons is an electric field or magnetic field. Which of the two it is is dependent on your frame of reference, since a Lorentz boost will transform one into the other.

I have no idea what Marlon is smoking...

Think of it this way. A charged particle moves in an electric or magnetic field. Where does it get the momentum from? It gets it from absorbing a (perhaps virtual) photon.

 

[marlon]

I have no idea what Marlon is smoking...

Marlon is smoking QED

Think of it this way. A charged particle moves in an electric or magnetic field. Where does it get the momentum from? It gets it from absorbing a (perhaps virtual) photon.

Oohh, so electrons are absorbing photons now ? I would suggest you study which processes allow for this to happen in high energy physics. You might be surprised.

When describing charged particles in fields (Maxwell, Lorentz force) we don't use photons. All is classical field theory here.

Indeed, in QED, charged particles interact through the exchange of virtual photons. But again, such photons do NOT MAKE UP and E/B field. In their original definition E and B are classical fields NOT quantumfields.

Also, as i told in my previous post. In field theory, particles arise through vibrations of quantumfields.
Photons are the quanta of the electromagnetic field (expressed by the A potential). You may also
think of it as the energy contained in the electric and magnetic fields which make up the
light wave. This duality is a basic part of quantum theory.

Clearly, one cannot just say...ohhh we have particles that make up a field...This is NOT WHAT QED teaches us.


marlon

 

[Jacquesl]

Guy’s I previously asked this question but it’s in the first page and nobody probably going to turn one page back.

Will I be able to reflect magnetic waves from a permanent magnet just like you should reflect light with a mirror?
Because some say magnetic waves are virtual photons or something and light are just photons.

And to my understanding the word photo means something that you can see with your eyes and virtual also has to do with the word photo?? So I don’t know if magnetic wave are virtual photons and a normal light bulb is also a photon generator because photons are light. Does someone have a clear answer for this confused understanding from me or maybe some other people to

 

[Severian]

Marlon is smoking QED

OK, very well, and QED is a quantum theory (that's what the 'Q' stands for).

When describing charged particles in fields (Maxwell, Lorentz force) we don't use photons. All is classical field theory here.

So if you are smoking QED, why are you now referring to classical fields? There are really no such thing as classical fields - classical physics is just a limit of quantum physics.

Indeed, in QED, charged particles interact through the exchange of virtual photons. But again, such photons do NOT MAKE UP and E/B field. In their original definition E and B are classical fields NOT quantumfields.

I am not sure what you mean by 'original definition', but the definition of E and B in QED (that is what we are talking about, isn't it?) are the components of the field strength tensor $$F^{\mu \nu}$$. And the field strength tensor can be written directly in terms of the vector potential $$A^{\mu}$$, which (as you pointed out yourself earlier) is the photon field. Therefore photons do make up the E/B fields.

Oohh, so electrons are absorbing photons now ? I would suggest you study which processes allow for this to happen in high energy physics. You might be surprised.

That would only reinforce my point. The only interaction present in QED is the three-point vertex involving one photon and 2 electrons. So the only way that you can transfer momentum to an electron is by absorbing or emitting a photon. Period.

 

[DaveC426913]

Any frequency you like. Any field of photons is an electric field or magnetic field.

I'm looking at it the other way around.

That means if I have a magnetic field around a magnet, it's made of photons, thus emitting EM radiation.

 

[Hans de Vries]

I'm looking at it the other way around.

That means if I have a magnetic field around a magnet, it's made of photons, thus emitting EM radiation.

Well, Static EM fields are of course associated with zero frequency photons
in QED with zero momentum transfer. A nice example is the magnetic anomaly
of the electron in a static magnetic field:

The calculation is made as if the electron interacts with a photon representing
the magnetic field. The magnetic anomaly represents the radiative correction
for this vertex. The first order (Schwinger) result is:

$$1\ +\ \frac{\alpha}{2\pi}\ \frac{\Theta}{\sinh{\Theta}}$$

Where $\Theta$ relates to the angle of deflection of the electron absorbing the
photon. Now only in the limit case of the interaction with a zero frequency
photon we get the correct expression for the first order term we measure
for the magnetic anomaly of an electron in a static magnetic field:

$$1\ +\ \frac{\alpha}{2\pi}, \qquad \mbox{for: }\ \ \Theta=0$$

(See Itzykson & Zuber, 7-1-3, Vertex Function, eqn: (7-58), page 341)Regards, Hans

P.S. The exact expression for theta is: $p \cdot p\' \ =\ m^2\ \cosh{\Theta}$

 

[marlon]

I am not sure what you mean by 'original definition',

The E abd B fields were first defined in "classical physics" and properly described by Maxwell. That is what i meant.

but the definition of E and B in QED (that is what we are talking about, isn't it?) are the components of the field strength tensor $$F^{\mu \nu}$$. And the field strength tensor can be written directly in terms of the vector potential $$A^{\mu}$$, which (as you pointed out yourself earlier) is the photon field. Therefore photons do make up the E/B fields.

I disagree. In QED photons arise due to fluctuations of those fields, so the fields "are already there". Thus, photons are not the particles that make up such fields nor generate such fields. Ofcourse, i am NOT saying they aren't closely related because THEY ARE (through the quantization of the A vector which is indeed directly related to E and B). But i object against the "constituent particles" terminology because this picture does not correspond to what QED teaches us.

I think there is a fundamental difference between :

1) how photons arise in QED
2) how E and B fields are defined (or introduced) in QED

Indeed, naively, E and B fields "are just there" in the field strength tensor, as you have pointed out. E and B are concepts that we already knew from classical physics. Beware, this does NOT imply they behave in the same way in QED. What is ofcourse special is the quantisation of these fields (described by A) which gives rise to the photons.

That would only reinforce my point. The only interaction present in QED is the three-point vertex involving one photon and 2 electrons. So the only way that you can transfer momentum to an electron is by absorbing or emitting a photon. Period.

Ofcourse but this is not what i was talking about. I wanted to object against the fact that you said this :

Think of it this way. A charged particle moves in an electric or magnetic field. Where does it get the momentum from? It gets it from absorbing a (perhaps virtual) photon.

What i didn't get is :

1) why perhaps a virtual photon ?
2) how does this prove that photons "make up electric fields"

In QFT (thus also in QED), force is introduced by the "vibrational mattress picture". Even A Zee uses this very nice analogy in his first chapters of "QFT in a Nutshell". My point, the A field (and thus the E and B fields) are quantisized. This quantisation of their vibrations generates the photons. look at the chronology here, THAT IS MY POINT.


Greets
marlon

 

[Flux]

More of an outside question but does the Earth's quantum field have anything to do with gravity as a vacuum effect?
A large mass object could create a warp in space this way but I was wondering what the relationship was with EM fields in studies?

 

[Severian]

The E abd B fields were first defined in "classical physics" and properly described by Maxwell. That is what i meant.

I am still not sure what you are getting at. If you want to talk about classical EM fields, then clearly they are not made of photons since there are no photons (classically, since there is no second quantization). But I thought we were talking about QED...?

I disagree. In QED photons arise due to fluctuations of those fields, so the fields "are already there". Thus, photons are not the particles that make up such fields nor generate such fields. Ofcourse, i am NOT saying they aren't closely related because THEY ARE (through the quantization of the A vector which is indeed directly related to E and B).

What exactly are you objecting to? Are you objecting to using the term 'photon field' to describe $$A^\mu$$? Normally one thinks (or at least I think) of a photon as being the quanta created by the appropriate creation operator. Since $$A^\mu$$ is written (defined!) in terms of these creation (and annihilation) operators, I think it is very appropriate to call $$A^\mu$$ the 'photon field' and to say that it is made up of photons (how else would one define 'made up of'?). Similarly, since E and B are written in terms of A, they are written in terms of creation and annihilation operators, so are also 'made up of' photons.

Beware, this does NOT imply they behave in the same way in QED.

Obviously, since one description is classical and the other is quantum. But the quantum description should match the classical one in the classical limit.

Ofcourse but this is not what i was talking about. I wanted to object against the fact that you said this :

"Think of it this way. A charged particle moves in an electric or magnetic field. Where does it get the momentum from? It gets it from absorbing a (perhaps virtual) photon."

What i didn't get is :

1) why perhaps a virtual photon ?
[\quote]

In reality, all interacting photons are virtual, since $$p^2=0$$ is one point on a line, so has zero probability. (Taling particles on-shell is always an approximation.)

2) how does this prove that photons "make up electric fields"

So, in your view, in QFT, how does one transfer energy or momentum to an electron moving in an electromagnetic field?

My point, the A field (and thus the E and B fields) are quantisized. This quantisation of their vibrations generates the photons. look at the chronology here, THAT IS MY POINT.

Isn't this supporting my statement that the electromagnetic field is made of photons? If it is the (second) quantization of E and B (via A) give rise to photons, then the quantized version must be made up of photons (or at least photon creation and annihilation operators).

 

[marlon]

What exactly are you objecting to? Are you objecting to using the term 'photon field' to describe $$A^\mu$$? Normally one thinks (or at least I think) of a photon as being the quanta created by the appropriate creation operator. Since $$A^\mu$$ is written (defined!) in terms of these creation (and annihilation) operators, I think it is very appropriate to call $$A^\mu$$ the 'photon field' and to say that it is made up of photons (how else would one define 'made up of'?). Similarly, since E and B are written in terms of A, they are written in terms of creation and annihilation operators, so are also 'made up of' photons.

Ok, you described the formalism correctly so to be clear, no dicsussion on that. Indeed, i understand your point better now thanks to your post above. The annihilations and creation operators "generate" the photons and we could say that, in more easy terms, this corresponds to the vibrating mattress analogy i was using.

What you say about the E and B fields being written interms of such annihilation and creation operators is also very correct. The quantum state of the electromagnetic field is written in terms of separate photon states (a certain number of photons per state). This quantum state is a Fock state, a tensor product of the states for each electromagnetic mode. Yeah, looking at it like this, you could say that photons make up the E/B fields.

But my point is this : the mattress must be vibating, or the operators must be working "onto something". Photons that manifest themselves in interactions first need to be created by applying the a/c operators onto the A Fock quantumstate.

So, you convinced me to change my perspective on this but i would not entirely change it to "photons make up the E and B fields". I just think it is important to make the remark that the photons in the A quantum state are not the photons that are involved in actual physical interactions. Photons first need to be created from such a state. You see my point ?

But again, i have to agree with what you say although the fact we need to be careful that we make the distinction i just mentioned.

In reality, all interacting photons are virtual, since $$p^2=0$$ is one point on a line, so has zero probability. (Taling particles on-shell is always an approximation.)

Agreed

So, in your view, in QFT, how does one transfer energy or momentum to an electron moving in an electromagnetic field?

Like you said, through the emission/absorption of virtual photons. I agree on that, but what i don't get is how does this prove that photons make up the A field ?

Isn't this supporting my statement that the electromagnetic field is made of photons? If it is the (second) quantization of E and B (via A) give rise to photons, then the quantized version must be made up of photons (or at least photon creation and annihilation operators).

Ok, i agree here but again "being made up of photon creation or annihilation operators" is not equal to "actually containing photons" IMO. I just look at it like this : photon = photon creation operator + EM Fock state

But , then again this is a semantics thing. You made a good point with which i agree.

Greets
marlon

 

[Severian]

Yes, it seems that we are getting confused by semantics. I often think this is one of the biggest sources of confusion in quantum mechanical systems, sinply because our language wasn't built to deal with it. One has to be very careful with what one actually means by phrases like 'made up of'.

 

[interested_learner]

It seems to me that fields are just a mathematical model for what is really going on. From QED it seems that the actual energy transfers occur via photons of various frequencies, but the fields as abstractions really aren't made of anything.

Sometimes it is best just to leave the abstraction as it is and not try to force it into some box.

 

[marlon]

More of an outside question but does the Earth's quantum field have anything to do with gravity as a vacuum effect?
A large mass object could create a warp in space this way but I was wondering what the relationship was with EM fields in studies?

The Earth's quantum field ? What is that ?

Gravity as a vacuum effect ? What do you mean here ?

A large mass object creates a warp in space ? Blame it on English being my third language but what is a warp in space time ?

marlon