# What IS happening with photon frequency?

But there's something bothering me- what IS oscillating in the photon? Nobody makes it clear. Is it the energy of the photon? But that would mean that the photon has less energy when the wave is at the middle, and more energy when the wave is at the peak, and doesn't that disobey the law of conservation of energy? Also, what is making whatever is oscillating in a photon go up and down? In water waves, gravity pulls the waves downwards, but what pulls whatever is a photon "down"
I'm sorry if my analogies aren't applicable here, but I've never really been taught this, aside from glancing mentions in class. I also don't mind if I need to learn new math to understand this, if anyone could point me in a direction, that would be great.

Delta2
Homework Helper
Gold Member
No the energy of photon does not oscillate, it remains constant. What is oscillating is the electromagnetic field(this oscillation of the field is what makes the wave) that "corresponds" to the photon. I myself havent understand what exactly is this "correspondance". Official answer from quantum field theory is that photon is the quanta of electromagnetic field. In order to fully understand what is quanta you have to go through the mathematics of the quantization of the electromagnetic field.

The electromagnetic field oscillates when charged particles like electron, are accelerated.

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atyy
A quantum state with a definite number of photons is a Fock state.

A quantum state that resembles a classical electromagnetic wave is a coherent state. A coherent state does not have a definite number of photons.

Delta2
Homework Helper
Gold Member
A quantum state with a definite number of photons is a Fock state.

A quantum state that resembles a classical electromagnetic wave is a coherent state. A coherent state does not have a definite number of photons.

I havent my self read alot on quantum field theory. Are you saying that a classical EM wave does not have a definite number of photons? what gives the wave property to the photon if not the em wave?

Cthugha
You can consider a classical em wave as the large photon number limit of the quantum description of a coherent state. The photon number distribution of a coherent state is Poissonian which means that the variance is the same as the mean photon number. So you get a standard deviation of $$\sqrt{n}$$ and therefore a relative standard deviation that vanishes as the photon number goes to infinity.
However, strictly speaking from a qm point of view, such waves do not have a fixed photon number.

The wave property comes directly from the em field. You just have problems getting a one-to-one correspondence between the field coming from some emitter and a certain photon. If you have detected a photon somewhere and several fields overlap at that point and you cannot distinguish them, things get more complicated. This is basically an issue of coherence.

f95toli
Gold Member
Just to add to what Cthugha has already written. There are several "types" (states) of light: Thermal (what you get from e.g. a blackbody), coherent (laser) and a Fock states (also known as a number state), only the latter has a definite number of photons (but no definite phase).
Note that this is not only a "mathematical" issue, a good example is a recent paper in Nature where they described an experiment where they created a Bose-Einstein condensate of photons. This was only possible because the managed to create a thermalized number state (i.e. the photons behaved as a gas with a Maxwellian distribution); only systems with a fixed number of particles can be in a BEC.

Cthugha
only systems with a fixed number of particles can be in a BEC.

Well, the photon number in the Weitz experiment is more steady-state than fixed as you have a constant loss rate leaking through the cavity mirrors which is balanced by the pump, but essentially that is correct. However, I still wonder how one should call condensates out of equilibrium. As Littlewood and others have pointed out, it is not strictly necessary to have thermal equilibrium to see condensation effects that are still well distinguishable from common lasing or such stuff.

My dear members nhmllr is new here and new to quantum mechanics too By debating at high note you all are are doing nothing but derailling the thread.
Dear nhmllr as you are new to quantum mechanics I would ask you to first be apt with classical mechanics and you should understand various types of statistical distribution.You should have to be fluent in calculus and all 1 to 3 dimensional geometries.
For a quick course youm may refer "Advance Engineering Mathematics" by Erwin Kreszig.After that you should start with the development of quantum quantum mechanics.
"The conceptual development of quantum mechanis" by M. Jammer is a good read.Or if you are fast then you may read "Molecular Quantum Mechanics" by Peter W. Atkins.For detail study you might also refer Planck's original formulation of quantum mechanics.Feynmann lectures are also nice if you like.

I think I've solved your problem here.

Thank you :-)

(PS. myself started studying quantum mechanics few weeks ago and the above is my personal experience)

But there's something bothering me- what IS oscillating in the photon? Nobody makes it clear
Because no one knows the answer; the question itself could be meaningless. There isn't a "model" of a photon in the same way there is, e.g., a model for an atom. What is called "photon" is not an object as we usually, with our everyday experience, imagine it. "Photon" means only "quantized excitation of electromagnetic field"(*), so nothing implies it have to be, for example, a corpuscle and (even less) that it should have an internal structure.
We only know that it moves at c, that it can be associated to the frequency f of the em wave since the photon's energy E is: E = h*f, and just a little more than this, nothing else.

(*) it means that the em field's energy is not continuous but discrete: if the lower energy is zero, the greater ones must be E, 2E, 3E, ecc., cannot be values in between those. That discrete value "E" is called "photon".

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A. Neumaier
"Photon" means only "quantized excitation of electromagnetic field"

But this fully answers the question of #1 - see post #2.

Lightarrow you've done nothing but simplified post #2 but it was helpful as it was lucid.Thanks!
Simply photon or quanta are concepts, concepts which are beyond our normal 5 senses.

So as I'm understanding it from these posts, photons aren't necessarily the correct way of looking at the world, or a wrong way. They're a model that's helpful to think about and applicable, so there's no internal "number" inside a particle moving up and down, but then why call it a wave!?

What I took away from this video, is not how to measure the speed of light, because I don't really care about that right now. What I took away is that SOMETHING is changing in a wave-like pattern. My question is, WHAT is changing? WHAT is making the chocolate melt better in one place, and worse in another?

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Delta2
Homework Helper
Gold Member
Ok so :
1) Photon is NOT a short pulse of EM radiation neither a small segment of a classical EM wave
2) Photon is NOT particle whose trajectory is shaped probabilistically by the EM wave (as conceived by Einstein and others according to wikipedia)

1 and 2 were the only satisfying physical understandings of the photon i could make so far , however

3) Photons are the quanta of the EM field, that is the result of a mathematical operator acting on an element of the Fock space which element represent a state of the EM field.

If i am not wrong with 3 ( or i am wrong and photon is something else) then i see little physical meaning in what a photon is. That is photon is just a small amount of energy added(when emitted) or subtracted(when absorbed) to/from the EM field.

A. Neumaier
Ok so :
1) Photon is NOT a short pulse of EM radiation neither a small segment of a classical EM wave
2) Photon is NOT particle whose trajectory is shaped probabilistically by the EM wave (as conceived by Einstein and others according to wikipedia)

1 and 2 were the only satisfying physical understandings of the photon i could make so far , however

3) Photons are the quanta of the EM field, that is the result of a mathematical operator acting on an element of the Fock space which element represent a state of the EM field.

If i am not wrong with 3 ( or i am wrong and photon is something else) then i see little physical meaning in what a photon is. That is photon is just a small amount of energy added(when emitted) or subtracted(when absorbed) to/from the EM field.

The notion of a photon is used in two quite different ways:

a) as a localized wave packet in the e/m field carrying the energy E=h*nu.
This is the notion relevant for ''photons on demand'' for quantum information experiments.

b) as irreducible excitations of the e/m field.
This is the way it appears in QFT textbooks.

In many cases (namely if you make statistics over many photons), the difference hardly matters. But when considering single photons, mixing up the two uses produces (very common) confusion.

I tried to clarify the issues in two talks whose slides are at
http://www.mat.univie.ac.at/~neum/ms/lightslides.pdf
http://www.mat.univie.ac.at/~neum/ms/optslides.pdf

A. Neumaier
What I took away is that SOMETHING is changing in a wave-like pattern. My question is, WHAT is changing? WHAT is making the chocolate melt better in one place, and worse in another?

The stuff that is oscillating is precisely what physicists call a field.

For photons, it is the electromagnetic field, describes by quantum field theory.

For chocolate, it is the mass field, described by hydromechanics. (Chocolate is a classical, non-Newtonian fluid.)

The stuff that is oscillating is precisely what physicists call a field.

For photons, it is the electromagnetic field, describes by quantum field theory.

For chocolate, it is the mass field, described by hydromechanics. (Chocolate is a classical, non-Newtonian fluid.)

It may be that Ketchup or corn starch + water are more familiar in fluid forms. (after all, who but cooks melts chocolate anymore? Sad...)

I'd just ask: within the "mass" field (higgs, whatever), we call chocolate, you also have the weak, strong, and EM components as well, which form the whole we call chocolate? Of those, I'm under the impression that we tend to have our tactile experiences mediated by EM as opposed to the strong or weak nuclear forces. Obviously mass plays a huge role it that as well, in terms of how we feel weight and elements of consistency, but in terms of what we "touch"... it's the EM field first and foremost, right?

zonde
Gold Member
But there's something bothering me- what IS oscillating in the photon?
Classical model of light is oscillation between electric field and magnetic field.
This model at least seems quite useful when explaining polarization of light with wire-grid polarizer (http://en.wikipedia.org/wiki/Polarizer#Absorptive_polarizers")

As field is not something that is contained in certain volume it seems pointless to ask what is oscillating in the photon.
I would say that it is more useful to think of photon as periodically repeating pattern in the field that maintains certain parameters over time. With such a model it's obvious that photon can't be strictly described as particle but still particle description can be more fitting than wave description over longer periods.

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A. Neumaier
It may be that Ketchup or corn starch + water are more familiar in fluid forms. (after all, who but cooks melts chocolate anymore? Sad...)

I'd just ask: within the "mass" field (higgs, whatever), we call chocolate, you also have the weak, strong, and EM components as well, which form the whole we call chocolate? Of those, I'm under the impression that we tend to have our tactile experiences mediated by EM as opposed to the strong or weak nuclear forces. Obviously mass plays a huge role it that as well, in terms of how we feel weight and elements of consistency, but in terms of what we "touch"... it's the EM field first and foremost, right?

No. The electromagnetic field is essentially gone in macroscopic matter (except for magnets, capacitors, lightnings, etc.).
What we touch is an effective field whose extension is created by the electrons, and whose mass is created by the quarks.
In macroscopic neutral matter, the effective fields are one field for each participating chemical substance, a stress field, a momentum field, and an energy field. and their conjugate thermodynamic fields.

No. The electromagnetic field is essentially gone in macroscopic matter (except for magnets, capacitors, lightnings, etc.).
What we touch is an effective field whose extension is created by the electrons, and whose mass is created by the quarks.
In macroscopic neutral matter, the effective fields are one field for each participating chemical substance, a stress field, a momentum field, and an energy field. and their conjugate thermodynamic fields.

OK, that does make sense, thanks!

The notion of a photon is used in two quite different ways:

a) as a localized wave packet in the e/m field carrying the energy E=h*nu.
This is the notion relevant for ''photons on demand'' for quantum information experiments.
Isn't "localized" a bit too strong statement, in the case, e.g., of a Mach–Zehnder interferometer experiment?

Isn't "localized" a bit too strong statement, in the case, e.g., of a Mach–Zehnder interferometer experiment?

I'm genuinely ignorant of this... what do you mean?

A. Neumaier
Isn't "localized" a bit too strong statement, in the case, e.g., of a Mach–Zehnder interferometer experiment?

Well, thinking in intuitive pictures about quantum field theory phenomena soon reaches its limits; so one cannot make statements that hold under all circumstances. A free photon can have the shape of an arbitrary solution of Maxwell's equation in vacuum. But only very special solutions are controllable and hence useful for experiments or applications.

Upon production in a laser, photons are more or less localized (not precisely, this is impossible, as photons cannot have an exact position, due to the lack of a unique position operator with commuting coordinates); often only in the transversal direction of the beam - then you don't know where it is in the beam, except probabilistically.

For photons on demand (that you can program to transmit information) you need to know when and where you transmit the photon, so it must be well-localized.

Of course, a slit or a half-silvered mirror delocalizes a photon, and only a measurement (or decoherence along the way) relocalizes it. This enables interference effects.
In these cases, the photon stops being particle-like and behaves just like an arbitrary excitation of the e/m field, i.e., like a wave.

The particle picture of light is good only in the approximation where geometric optics is applicable. This has been known for almost 200 years now.

The paradoxes and the alleged queerness of quantum theory both have their origin in misguided attempts to insist on a particle picture where it cannot be justified.

For more details see the slides I had mentioned.

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Ah, got it. A. Neumaier: You're very good!

Well, thinking in intuitive pictures about quantum field theory phenomena soon reaches its limits; so one cannot make statements that hold under all circumstances. A free photon can have the shape of an arbitrary solution of Maxwell's equation in vacuum. But only very special solutions are controllable and hence useful for experiments or applications.

Upon production in a laser, photons are more or less localized (not precisely, this is impossible, as photons cannot have an exact position, due to the lack of a unique position operator with commuting coordinates); often only in the transversal direction of the beam - then you don't know where it is in the beam, except probabilistically.

For photons on demand (that you can program to transmit information) you need to know when and where you transmit the photon, so it must be well-localized.

Of course, a slit or a half-silvered mirror delocalizes a photon, and only a measurement (or decoherence along the way) relocalizes it. This enables interference effects.
In these cases, the photon stops being particle-like and behaves just like an arbitrary excitation of the e/m field, i.e., like a wave.

The particle picture of light is good only in the approximation where geometric optics is applicable. This has been known for almost 200 years now.

The paradoxes and the alleged queerness of quantum theory both have their origin in misguided attempts to insist on a particle picture where it cannot be justified.

For more details see the slides I has mentioned.

Guys GUYS! This is all well an good, but I still really don't understand WHAT is oscillating and why the chocolate behaves the way it does in the microwave!

GUYS nhmllr is right.Your discussions are taking him nowhere. nhmllr is here to learn and understand quantum mechanics and we are here to help him.
And nhmllr to you would say the same what Dr. Feynmann said to me (not literally 'said' as I read) that the concept of quanta of energy is clear to an expert is next to what a novice has.It is absurd to both of them and is out of bounds for equally both of them.But not if you give enough time to it(I mean years of research).
(Sir A. Neumaier please check the bellow and infom me incase I go wrong)
nhmllr Dr. Planck gave his quantum theory of light to explain why it behaves like it is and not according to classical mechanics.
What happend there was that the results as obtained from Sir Rayleigh's formula (look up 'Rayleigh-Jeans Law' in Wikipedia) for black body behaviour were untenable.It suggested that, regardless of temprature, there should be an infinite energy density at shorter wavelengths which is absurd as this can't really happen thus Sir Ehrenfest rightly call it the "Ultraviolet Catastrophe".
At this juncture what Sir Plack suggested was close to propsing that an oscillation of the electromagnetic field of frequency could be excitded only in steps of energy of magnitude hf, where h is the new fundamental constant now known as Planck's constant.
Therefore, according to this quantization of energy, that supposition that energy can be transferred only in discrete amounts, the oscillator can have energies hf, 2hf,.... and not like 0.5hf or so.
Quantum theory is thus characterised by dicreteness of in energies and the need for a minimum excitation effectively switches off oscillators of very high frequency,and hence eliminates the ultraviolet catastrophe.
So when your choclate is kept in a microwave oven the microwave radiation are absorbed by the molecules of the choclate only in the form of discrete excitations in the dipole moment of molecule resulting in thermal randomness due to which the weak intermolecular bonds weaken and the choclate begins to melt.REMEMBER the energy is not spread out in the whole body like a wave would do but is present in dicrete forms thus even if you reduce the intensity the excitation would be there of same energy but for a lesser part of the population.
Here oscillations can roughly be termed as electromagnetic pulses.

A. Neumaier
(Sir A. Neumaier please check the bellow and infom me incase I go wrong)

What you write seems to me far to unclear to have meaning.

Moreover, the fact that elsewhere on PF you like to make noneinsical remarks isn't inviting enough to take your contributions seriously.

If you want to have a serious discussion, ask short and clear questions, and make assertions only when you really know something is true, and not when you hardly understand the basics of the topic.

A. Neumaier
Guys GUYS! This is all well an good, but I still really don't understand WHAT is oscillating and why the chocolate behaves the way it does in the microwave!

If the answer mentioning the appropriate buzzwords doesn't satisfy you,
you need to dig deeper and try to understand what is the electromagnetic field (which oscillates)?, and what is a non-Newtonian fluid? You can read the basics about that in wikipedia or many other sources, and then come back with specific questions where you are stuck.

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A. Neumaier
Ah, got it. A. Neumaier: You're very good!

Since you seem to like my style of explaining things, have a look at my theoretical physics FAQ at
http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html
where I explain lots of other things about quantum mechanics and related topics that came up in earlier discussions! Or at my book
Classical and Quantum Mechanics via Lie algebras
http://lanl.arxiv.org/abs/0810.1019
where I build up quantum mechanics from scratch, in a somewhat unconventional way.

Since you seem to like my style of explaining things, have a look at my theoretical physics FAQ at
http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html
where I explain lots of other things about quantum mechanics and related topics that came up in earlier discussions! Or at my book
Classical and Quantum Mechanics via Lie algebras
http://lanl.arxiv.org/abs/0810.1019
where I build up quantum mechanics from scratch, in a somewhat unconventional way.

Thanks, I do like your style, and I'll give those a read.