# The energy of an em radiation

apratim.ankur
the energy of em radiation (or photon) is proportional to the frequency of the radiation.
the em radiation (or photon) is composed of oscillating Electric and Magnetic fields.
as such this energy must be stored in the oscillating electric and magnetic fields constituting the radiation.
therefore the energy of the em radiation (or photon) must be related to the Magnitude of Electric and Magnetic fields associated with it...(i.e. the Amplitude of the em field constituting it)...(because greater the magnitude of electromagnetic field , greater would be the energy stored in it and vice-versa).
What is inappropriate here?

Mentor
the energy of em radiation (or photon) is proportional to the frequency of the radiation.
the em radiation (or photon) is composed of oscillating Electric and Magnetic fields.
as such this energy must be stored in the oscillating electric and magnetic fields constituting the radiation.
therefore the energy of the em radiation (or photon) must be related to the Magnitude of Electric and Magnetic fields associated with it...(i.e. the Amplitude of the em field constituting it)...(because greater the magnitude of electromagnetic field , greater would be the energy stored in it and vice-versa).
What is inappropriate here?

You are mixing concepts from a single photon in with general EM radiation (consisting of many photons).

For a single photon, you are correct that E = hf (f = frequency). So given that energy, you can calculate the magnitude of the associated electric and magnetic fields for that single photon.

soothsayer
I think you are confusing the energy of a single photon (which is only a function of frequency) to the energy contained in electromagnetic radiation, which is made of a bunch of photons. Also remember that photons don't have an electric and magnetic fields to contribute to their energies (they are electrically neutral) but virtual photons create electric and magnetic fields.

apratim.ankur

ok, for a single photon the energy E = hv .
Given that energy I calculate the magnitude of the electric and magnetic fields associated with that single photon.
I get a definite answer. Doesn't this mean that its energy is related to the magnitude of em fields associated with it (as knowledge of the former allows me to calculate the latter or vice-versa)?
sorry if it appears stupid, but I am confused...

apratim.ankur

ok..but isn't the energy associated with a single photon of electromagnetic nature? and if that is so why isn't it related to the amplitude of the em fields associated to it?

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Mentor

For a single photon, you are correct that E = hf (f = frequency). So given that energy, you can calculate the magnitude of the associated electric and magnetic fields for that single photon.

ok, for a single photon the energy E = hv .
Given that energy I calculate the magnitude of the electric and magnetic fields associated with that single photon.

Classically, the E and B fields at a given point are associated with an energy density (joules / m3) ##u## at that point:

$$u = \frac{\epsilon_0}{2} E^2 + \frac{1}{2 \mu_0} B^2$$

Therefore, in order to associate magnitudes of E and B fields to a photon, you need to associate a volume with the photon. But photons are quanta of energy, and volume does not enter into their fundamental description. In general, you can't think of a photon as a small localized object, as a sort of "fuzzy ball," as far as I know. The "spatial extent" of a photon depends on the method that you use to produce it, that is, it's basically a result of the production apparatus and not a fundamental statement about photons in general.

soothsayer

ok..but isn't the energy associated with a single photon of electromagnetic nature? and if that is so why isn't it related to the amplitude of the em fields associated to it?

No, the energy associated with a single photon is not of an electromagnetic nature. That is to say, photons are not made of tiny quantum electromagnetic fields, but rather, photons are the building blocks of electromagnetic fields.

Given that energy I calculate the magnitude of the electric and magnetic fields associated with that single photon.

This is also incorrect thinking, for the same reason as above. It does not make sense to talk about the "magnitude of the electric and magnetic fields associated with a single photon." Photons are fundamental particles, they are neither made of E/M fields nor can a lone photon create an E/M field.

Dickfore

Classically, the E and B fields at a given point are associated with an energy density (joules / m3) ##u## at that point:

$$u = \frac{\epsilon_0}{2} E^2 + \frac{1}{2 \mu_0} B^2$$

Therefore, in order to associate magnitudes of E and B fields to a photon, you need to associate a volume with the photon. But photons are quanta of energy, and volume does not enter into their fundamental description. In general, you can't think of a photon as a small localized object, as a sort of "fuzzy ball," as far as I know. The "spatial extent" of a photon depends on the method that you use to produce it, that is, it's basically a result of the production apparatus and not a fundamental statement about photons in general.

But you may certainly find the average density of photons (number of photons per unit volume) corresponding to a monochromatic em wave.

apratim.ankur

No, the energy associated with a single photon is not of an electromagnetic nature. That is to say, photons are not made of tiny quantum electromagnetic fields, but rather, photons are the building blocks of electromagnetic fields.

This is also incorrect thinking, for the same reason as above. It does not make sense to talk about the "magnitude of the electric and magnetic fields associated with a single photon." Photons are fundamental particles, they are neither made of E/M fields nor can a lone photon create an E/M field.

But then as photon is what we may conceive as the fundamental building block of em energy, can't we consider it as the smallest bit/quanta/lump of the same em energy? Wouldn't it make some sense then (to talk about the corresponding em field)?

soothsayer

But you may certainly find the average density of photons (number of photons per unit volume) corresponding to a monochromatic em wave.

This is definitely true.

apratim.ankur said:
But then as photon is what we may conceive as the fundamental building block of em energy, can't we consider it as the smallest bit/quanta/lump of the same em energy? Wouldn't it make some sense then (to talk about the corresponding em field)?

Ehh, a single particle alone does not a field make. It's sort of a semantic argument; you're trying too hard to apply CLASSICAL E/M principles and equations to a totally QUANTUM situation. As Dickfore mentioned above, the amplitude of an E/M field is related to the density of photons in the field, and obviously, it makes no sense to talk about the density of photons in a photon. You cannot look at a single grain of sand and ask what the size of its beach is.

cmos

But then as photon is what we may conceive as the fundamental building block of em energy, can't we consider it as the smallest bit/quanta/lump of the same em energy? Wouldn't it make some sense then (to talk about the corresponding em field)?

I have to disagree with most of what soothsayer has posted above as being wrong. The above quote is correct.

Photons are the quantization of the EM field -- end of story. Going back to the middle of the story, jtbell indicated above a way to calculate the energy density of the EM field. Then given a certain volume in space, one can directly calculate the (mean) number of photons in that volume. The assumption is that a continuous EM wave propagates throughout that volume. A more useful metric would be to determine the intensity (J/s/m^2 = W/m^2) of an EM wave that passes through a surface. Then you could determine the number of photons that pass through the surface per unit time.

The important concept to note here is that as you increase the energy/power of the EM wave, it's the number of photons the increases. Why? Because of E=hf. More energy, more photons.

soothsayer

I have to disagree with most of what soothsayer has posted above as being wrong. The above quote is correct.

Photons are the quantization of the EM field -- end of story. Going back to the middle of the story, jtbell indicated above a way to calculate the energy density of the EM field. Then given a certain volume in space, one can directly calculate the (mean) number of photons in that volume. The assumption is that a continuous EM wave propagates throughout that volume. A more useful metric would be to determine the intensity (J/s/m^2 = W/m^2) of an EM wave that passes through a surface. Then you could determine the number of photons that pass through the surface per unit time.

The important concept to note here is that as you increase the energy/power of the EM wave, it's the number of photons the increases. Why? Because of E=hf. More energy, more photons.

How is that different from what I told him?

cmos

Ehh, a single particle alone does not a field make. It's sort of a semantic argument; you're trying too hard to apply CLASSICAL E/M principles and equations to a totally QUANTUM situation. As Dickfore mentioned above, the amplitude of an E/M field is related to the density of photons in the field, and obviously, it makes no sense to talk about the density of photons in a photon. You cannot look at a single grain of sand and ask what the size of its beach is.

My last post seems to have been posted at the same time as the above quote. The quote clears up some of what I disagreed about with soothsayer.

True that "it makes no sense to talk about the density of photons in a photon," however, the OP essentially asked about the density of photons in the (classical) field. As you said, this is a valid inquiry. But, going further, a single photon is (in the language of QFT) a excitation of the EM field. As such, you cannot say that it has nothing to do with electric and magnetic fields since it IS an EM field.

PhilDSP

How is that different from what I told him?

No, the energy associated with a single photon is not of an electromagnetic nature.

I was going to question why you think that and what are your references. As far as I know, the only reason to believe a photon is not 100% EM energy is that there needs to be something to hold the energy focalized so that it doesn't dissipate or spread in space as waves tend to do.

Gold Member

The "spatial extent" of a photon depends on the method that you use to produce it, that is, it's basically a result of the production apparatus and not a fundamental statement about photons in general.

This is a difficult area because that statement suggests that not all photons with the same energy are 'identical'. I don't see that this can be true. I would rather say that the 'effective extent' of a photon relates only at the period of interaction with the source or receiver and is more to do with what goes on at each end than with any idea of 'size' for the photon.
If you think in terms of a resonant system absorbing or emitting a quantum of energy, the time it would take to change its energy state would relate, in a classical way, to the Q factor (God knows what that means in the context of QM) of the resonant system. I would tend to think that QM systems which could undergo the same energy change would not be likely to have this same 'Q factor' so one type of system could produce a photon and this same photon could expect to be absorbed by a totally different type of system.

soothsayer

I was not trying to say that photons are completely unrelated from EM radiation, what I was trying to get across is that you cannot determine the energy of a photon via classical computation for energy in an electromagnetic field, as the OP wanted to do. You can't say: "Well, for a single photon, the amplitude of its electric field is this and the amplitude of its magnetic field is this, so it's energy should be this: derived from the strength of its E and B fields."

It was inaccurate of me to say that the energy of a photon was not associated with electromagnetic energy, but what I was trying to say is that the energy of a photon is not determined by the strength of some quantum EM field that is contained in it, it's indeed determined purely by frequency of light.

Gold Member
It would be a meaningful thing to relate the density of a stream of photons to the energy density of an EM 'beam' - which would give you the values of the fields. But that would really be putting it 'the other way round' and very different from assigning an actual set of fields to an individual photon. For a start, you couldn't give the photon an actual 'area' because that is undefinable so you couldn't, consequently, give it a value of energy flux or field values. It can be regarded as extending over the whole of the wave front and over a range of possible positions on any 'ray path' you could assume.
It's just not a good idea to keep trying to impose 'old fashioned' values on photons. It may give an illusion of better understanding but it doesn't actually get you anywhere. They are 'photons' and photons are like photons - nothing else.

soothsayer
It would be a meaningful thing to relate the density of a stream of photons to the energy density of an EM 'beam' - which would give you the values of the fields. But that would really be putting it 'the other way round' and very different from assigning an actual set of fields to an individual photon...

...It's just not a good idea to keep trying to impose 'old fashioned' values on photons. It may give an illusion of better understanding but it doesn't actually get you anywhere. They are 'photons' and photons are like photons - nothing else.

Thank you, that was exactly my point but put a little bit more succinctly.

PhilDSP
Interesting point about the beam. That seems like a simple experiment to perform and control. Have there been experiments generating meaningful data where some sort of side-by-side arrangement and density of photons is involved? I suppose a wide beam laser experiment would qualify.

Gold Member
Interesting point about the beam. That seems like a simple experiment to perform and control. Have there been experiments generating meaningful data where some sort of side-by-side arrangement and density of photons is involved? I suppose a wide beam laser experiment would qualify.

I don't quite see what sort of "experiment" you think would tell you anything about this. It is surely well enough established what the photon energy is at a given frequency. You only need to divide the total energy flux by hf to find how many photons are involved. But that would tell you nothing (it would be meaningless) or imply anything about the 'fields' associated with each photon. You might as well ask "How many houses are there in a brick?" The simple Mathematical operation of taking the reciprocal of "how many bricks are there in a house?" would give you a numerical answer but what would it mean? - One brick is 1/2000 of a front wall plus 1/2000 of a back wall plus 1/1000 of some foundations etc. etc. But it isn't, is it? A brick is just a brick and it is just a contribution to the part of the house where you happen to find it.
If you are trying to build a picture of a photon as a 'little squiggle' of fields, somewhere in space, you are onto a loser. That is nothing like any of the valid models of the photon that we use these days.

cmos
You only need to divide the total energy flux by hf to find how many photons are involved. But that would tell you nothing (it would be meaningless) or imply anything about the 'fields' associated with each photon.
...
If you are trying to build a picture of a photon as a 'little squiggle' of fields, somewhere in space, you are onto a loser. That is nothing like any of the valid models of the photon that we use these days.

I disagree with this. By quantizing the EM field, what you are left with is a quantum of the field, i.e. the field of one photon. Under canonical quantization, the electric and magnetic fields (if you want to be pedantic, the electric field and vector potential) are elevated to the status of Hermitian operators. If you were to quantize the field in a cubic box of side L, then the "magnitude" of the electric-field operator is

$$e = \sqrt{\frac{\hbar\omega}{2\epsilon_0 L^3}} .$$

I was under the impression that the common interpretation of e is that it is the electric field of one photon. Indeed, this make sense in terms of basic intuition. For example, if a single photon were radiated into completely empty space, then L would be so large that at any arbitrary location we would most likely never be able to detect the photon since it's field would be too weak to interact with our instruments (intuitively, send out a single photon into the whole of the universe and chances are you'll never find it). On the other hand, an extremely confined photon would exhibit such a large-valued field that it would more than certainly interact with whatever matter surrounds it.

Gold Member
I was under the impression that the common interpretation of e is that it is the electric field of one photon. Indeed, this make sense in terms of basic intuition. For example, if a single photon were radiated into completely empty space, then L would be so large that at any arbitrary location we would most likely never be able to detect the photon since it's field would be too weak to interact with our instruments (intuitively, send out a single photon into the whole of the universe and chances are you'll never find it). On the other hand, an extremely confined photon would exhibit such a large-valued field that it would more than certainly interact with whatever matter surrounds it.

You need to ask yourself, if it's all down to "common intuition", why was QM such a revolutionary idea and why has it taken so long to sort it out? It just can't be 'that obvious' or the great minds wouldn't have been struggling so long.
I am not sure that I have ever read this "common interpretation" in anywhere of substance. ("e" usually stands for the electronic Charge and charge is not Field). If you consider the definition of what a Field is, then it is a set of values (scalar or vector) over a region of space; it is defined at a point. Which point would you choose to apply your definition of the field of a photon? In the case of the two slits experiment, would you apply it to each slit or just one. And what about a diffraction grating? Suddenly you would need to have a field at every slot in the grating. In fact, there is no reasonable way to describe the photon in terms of a field distribution in space because the same photon would hava a different field distribution, according to the situation it's in. We can have no knowledge about the 'position' of a photon in space, nor any notion of its 'size'.

cmos
You need to ask yourself, if it's all down to "common intuition", why was QM such a revolutionary idea and why has it taken so long to sort it out? It just can't be 'that obvious' or the great minds wouldn't have been struggling so long

It took 1300 years from the fall of Rome for Newton to appear and formalize modern physics. It then took ~230 years to formalize particle-based quantum mechanics. Within 20-30 years from that, quantum field theory was formalized. In the grand scheme of things, quantum theory didn't really take that long.

I didn't say that the result was obvious but merely that the results relate to some sort of basic intuition if you make the "correct" interpretation.

I am not sure that I have ever read this "common interpretation" in anywhere of substance. ("e" usually stands for the electronic Charge and charge is not Field).

Clearly I used the variable e to stand for the "magnitude" of the electric-field operator of quantum mechanics. It does not stand for charge; this is obvious. Give me better LaTeX commands and next time I'll make a script capital "E" for you.

If you've never even quantized the EM field before, then Google "quantization of the electromagnetic field." The 2nd edition of Sakurai does it, most/all books on quantum optics do it, even some books on quantum field theory do it.

If you consider the definition of what a Field is, then it is a set of values (scalar or vector) over a region of space; it is defined at a point. Which point would you choose to apply your definition of the field of a photon? In the case of the two slits experiment, would you apply it to each slit or just one. And what about a diffraction grating? Suddenly you would need to have a field at every slot in the grating. In fact, there is no reasonable way to describe the photon in terms of a field distribution in space because the same photon would hava a different field distribution, according to the situation it's in. We can have no knowledge about the 'position' of a photon in space, nor any notion of its 'size'.

As with any quantum-mechanical operator, the operator yields statistics/amplitudes related to measurements only after you make projections on quantum states (i.e. calculating matrix elements). True, the field at a specific point is not well characterized in this scheme, but the field associated with the photon is still defined. Also note that I was very careful to say that the result I quoted applies for a field quantized "in a cubic box of side L." This works well for cavity fields (cavity QED is a hot research topic) and for forming statistics of a photodetector; however, other quantization schemes may exist.

Gold Member
I was being a bit dumb about the "e" thing!
I also take your point about the 'bound photon' situation but this is only relevant when it is actually bound - which is not the case when you consider a photon on its way from A to B, which is the situation when you have a flux of energy.
My argument against quantising a photon field still holds when you can't specify where and when you are measuring it, I think.

soothsayer
Can someone clear something up for me? It seems like there are two types of photons to deal with here. We have electromagnetic radiation: alternating E and B fields, which are made up of force-carrying virtual photons, are they not? But these are not the photons that we see when the radiation reaches us, are they? What's the difference? Seems like there would be force carrying and non force carrying photons in EM radiation.

cmos
I was being a bit dumb about the "e" thing!
I also take your point about the 'bound photon' situation but this is only relevant when it is actually bound - which is not the case when you consider a photon on its way from A to B, which is the situation when you have a flux of energy.
My argument against quantising a photon field still holds when you can't specify where and when you are measuring it, I think.

Ah, I think you are arguing against (or assuming the OP's inquiry is) something along the lines of, "what happens when we find a field/photon somewhere in space and then quantize it?" This, to the best of my knowledge, is not how quantum mechanics is/can be done. Rather, you take the general properties of a classical field, irrespective of whether you have a field in front of your or not, and then quantize it. Only once you've reach that point, can you even begin to look for photons.

In more mathematical language, you first form (Hermitian) field operators -- these represent the quantization of the field. At that point, you use the operators on photon states to give you measurable amplitudes -- this represents your search for actual photons (remember, the quantum states are always there, whether or not they are occupied is the question).

cmos
Can someone clear something up for me? It seems like there are two types of photons to deal with here. We have electromagnetic radiation: alternating E and B fields, which are made up of force-carrying virtual photons, are they not? But these are not the photons that we see when the radiation reaches us, are they? What's the difference? Seems like there would be force carrying and non force carrying photons in EM radiation.

All photons (real or virtual) carry energy and momentum. Their interaction with matter imparts energy and momentum to matter. The distinction of real/virtual is in the dynamics of how the photons interact with matter. A good rule of thumb is this: if it is "easy" to explicitly detect the photons (e.g. being blinded by the sun), then they are real; if it is "difficult" to explicitly detect the photons (e.g. Coulomb scattering of two electrons), then they are virtual.

Gold Member
Ah, I think you are arguing against (or assuming the OP's inquiry is) something along the lines of, "what happens when we find a field/photon somewhere in space and then quantize it?" This, to the best of my knowledge, is not how quantum mechanics is/can be done. Rather, you take the general properties of a classical field, irrespective of whether you have a field in front of your or not, and then quantize it. Only once you've reach that point, can you even begin to look for photons.

In more mathematical language, you first form (Hermitian) field operators -- these represent the quantization of the field. At that point, you use the operators on photon states to give you measurable amplitudes -- this represents your search for actual photons (remember, the quantum states are always there, whether or not they are occupied is the question).

We can predict or infer the field at a point in space but I wouldn't say that the same thing goes for the photon. We can only 'predict' the presence of a photon somewhere in space on a probability basis but when we 'find' a photon somewhere then it is nowhere else (we have resolved the uncertainty).
I find it much easier to say that the photon only 'exists' when the field is produced or detected. That means that it doesn't have to be anywhere /anywhen between its emission or its detection so discussing its (conventional) properties in between is pretty meaningless. I feel very strongly that people should be strongly discouraged from using the very concrete models of the photon that abound in this and other forums. Most of those models disregard the quality and 'authority' of the vast amount of work that has been done on QM in favour of 'intuitive' and backward-looking ideas. I may be batting on about this, over-much but the little bullets or squiggles models are difficult to discourage.

btw - re your statement about 'intuition', earlier. As far as I can see, 'intuition' is only the process of taking enough of what you already know to use that to advance your knowledge and understanding. And that is something which frequently leads to wrong conclusions. Models of the Universe (Flat Earth / Geocentric / Heliocentric etc) are all perfectly intuitive but, with hindsight they appear 'misguided'. So the 'intuition argument' can't ever be a very strong one. As for the relatively short timescale for QM to be worked out - I could suggest that there were at least as many 'informed-man-hours' of effort involved as in any of the other, earlier, revolutions, despite the long transition times involved.

I understand that and I think it actually agrees with what I have been saying.

cmos
But the photon IS a quantum of the electromagnetic field; they are one and the same. To predict or infer the (classical) field is to predict or infer quanta of the field.

Also, the "squiggle models" are precisely how we do calculations...

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Darwin123
the energy of em radiation (or photon) is proportional to the frequency of the radiation.
...
What is inappropriate here?
The energy of em radiation is not equal to a photon. You start out by assuming it is. That is like equating the strength of a crowd to the strength of an individual. They are not the same thing.
EM radiation consists of many photons. The total energy is the product of the number of photons and the energy per photon.
If the frequency of the radiation increases slightly, but the number of photons decreases by a large amount, then the total energy of the em radiation can decrease. The total energy of the em radiation is still proportional to the product.

Gold Member
But the photon IS a quantum of the electromagnetic field; they are one and the same. To predict or infer the (classical) field is to predict or infer quanta of the field.

Also, the "squiggle models" are precisely how we do calculations...

Just because the electromagnetic field can be characterised as consisting of quanta, this in no way implies that the quanta (of energy?) are actually 'like' that field. I refer you to my analogy with bricks and houses. I guess you would acknowledge the fact that photons are not 'anywhere' until they are actually detected. However, at the same time as this is true, the Fields with which they are associated are very predictable with only the uncertainty associated with any classical measurement. A statistical distribution doesn't describe individuals which are part of the data.

The 'squiggle' model in the Feynman diagram is purely symbolic, I think you'll find. I am pretty sure that a reference to Feynman's actual statement to that effect was in a fairly recent thread here - but I couldn't put my finger on it, I'm afraid. To assign any more significance to it would be like saying an H field is 'H-shaped' or a Force has an arrow on it because that's the way we often draw vectors. I would be interested in your personal mental picture of the photon that you are using for this discussion. I think it sounds far too 'concrete' to be part of QM. Does it have a wavelength, an extent etc.?

Darwin123
Can someone clear something up for me? It seems like there are two types of photons to deal with here. We have electromagnetic radiation: alternating E and B fields, which are made up of force-carrying virtual photons, are they not? But these are not the photons that we see when the radiation reaches us, are they? What's the difference? Seems like there would be force carrying and non force carrying photons in EM radiation.
The photons that we see a referred to as "real photons". The force mediators in static fields are referred to as "virtual photons". Photons carry force, whether they are real or virtual. Real photons persist until they collide with another particle.
Real photons are generated by an electric current or the magnet moments that oscillate. Real photons travel a much greater distance than virtual photons.
Virtual photons are generated by an electric charge or magnetic moment whether or not it oscillates. Virtual photons don't travel very far from the electric charge or magnetic moment. They disappear at very short distances from the electric charge or moment. The distance at which they disappear is determined by the uncertainty principle.
The words "real" and "virtual" do not confer any judgement on the metaphysical existence of these photons. They refer to the mathematical treatment of the corresponding physical properties. Whenever a physicist talks about "photons" without specifying "real" or virtual, by default he is talking about "real" photons. Most of the discussions about photons in optics deals with real photons.
Here are two extreme physical limits that may clarify things a little. Energy is transferred large distances from an oscillating electric charge by real photons. Energy is transferred by quasistatic electric and magnetic fields for short distances by virtual photons.

soothsayer
Virtual photons don't travel very far from the electric charge or magnetic moment. They disappear at very short distances from the electric charge or moment. The distance at which they disappear is determined by the uncertainty principle.

That's not true; Electromagnetism would not have an infinite range if the virtual photons mediating it had a finite range. I was under the impression that the range of the virtual photon should be infinite since it is massless, thus it circumvents the uncertainty principle because the photon has no reference frame or Δt of which to speak of.

Darwin123
That's not true; Electromagnetism would not have an infinite range if the virtual photons mediating it had a finite range. I was under the impression that the range of the virtual photon should be infinite since it is massless, thus it circumvents the uncertainty principle because the photon has no reference frame or Δt of which to speak of.
The photon is not truly "massless". It has a zero rest mass. However, it can have a large relativistic mass. The energy of a photon is never zero. The photon always has a finite energy in any inertial frame. The uncertainty relationship holds to energy and time, not rest mass and time.
Photons have a zero rest mass. This does not mean that they have no energy. So your reasoning doesn't really work on photons.
When we talk about "range", we are talking about the distance a photon can travel from the electric charge that generates it. Therefore, the discussion is simplest in the inertial frame where the electric charge isn't moving, or in which it is moving slowly. As far as an observer traveling with the electric charge is concerned, the photon has energy.
There is more than one way to relate "virtual" with "forbidden polarization". Some physicists like to express the equations in covariant form, consistent with special relativity. They divide photons into four polarization states. Horizontal and vertical are the two polarization states for "real" photons. "Longitudinal" and "time-like" are additional polarization states for "virtual photons."
So the SR picture looks like this. A photon is characterized by a polarization four vector that can point in any of the four dimensions of space time. It can in the direction of the propagation vector, in the direction of the time dimension, horizontal to the propagation vector, or vertical to the propagation vector. A virtual photon can have any of these four polarizations. However, "real" photons have a polarization vector that is either horizontal or vertical.
Here is another quote. This is from the following book:
"Advanced Quantum Mechanics" by J. J. Sakurai (Pearson Education, 2008) page 268.
"We have argued that a virtual photon can be visualized as having four states of polarization. One the other hand, we know that a real photon, or a free photon, has only two states of polarization...If one wishes, one may say that the photon always has four states of polarization regardless of whether it is virtual or real, but that whenever it is real, the timelike photon and the longitudinal photon always give rise to contributions which are equal in magnitude but opposite in sign."

I find it somewhat satisfying to think about the four dimensional generalization of polarization. The four polarization states correspond to the four dimensions in space-time. However, real photons are characterized by the transverse polarization states (horizontal, vertical). The other two polarization states (longitudinal, timelike) are purely virtual.

At least I can make a classical analog out of this. The mathematics in quantum mechanics corresponds somewhat to the mathematics in classical electrodynamics. So when my brain wilts from the quantum mechanics, I have a classical picture to fall back on.

Darwin123

ok, for a single photon the energy E = hv .

I get a definite answer. Doesn't this mean that its energy is related to the magnitude of em fields associated with it (as knowledge of the former allows me to calculate the latter or vice-versa)?
sorry if it appears stupid, but I am confused...
The number of photons is proportional to the square of the magnitude of the EM fields.