Understanding Photon: Models from Classical Physics to Quantum Field Theory

[malawi_glenn]

But the classical description should explain light of ANY intensity, even a very weak intensity. I do not remember that in classical physics we had some restriction, like ... "classical physics of light is valid only when intensity is not too weak"

Yes it should, but in order to get low intensity with classical light (many photons), low frequency EM rad must be used.

 

[jtbell]

I am not shure that these two statements correspond to each other...

The first statement refers to what light really is. The second statement refers to the mathematical models that we use for calculating the results of measurements and experiments.

Whatever light really is, it is not a classical electromagnetic wave, because that description makes incorrect predictions at low intensities. Light may or may not really be photons, but we don't know for sure. All we know for sure is that the quantum description of light makes correct predictions so far as we have been able to test it.

 

[malawi_glenn]

The first statement refers to what light really is. The second statement refers to the mathematical models that we use for calculating the results of measurements and experiments.

Whatever light really is, it is not a classical electromagnetic wave, because that description makes incorrect predictions at low intensities. Light may or may not really be photons, but we don't know for sure. All we know for sure is that the quantum description of light makes correct predictions so far as we have been able to test it.

That is true, light is light, as I have said in some threads this month, but not many is buying that explanation. Same holds for electrons (as an example), we can have many models of them, some are good, some are bad; but the nature is always bigger than our descripions of it.

 

[Bright]

Light may or may not really be photons, but we don't know for sure. All we know for sure is that the quantum description of light makes correct predictions so far as we have been able to test it.

...but the nature is always bigger than our descripions of it.

Yes, it is!

What we can discuss are only MODELS of photon, but not ultimate reality behind the phenomena we may observe in optical experiments!

 

[ZapperZ]

Yes, it is!

What we can discuss are only MODELS of photon, but not ultimate reality behind the phenomena we may observe in optical experiments!

This is a fallacy.

Point to me something in which you can claim to know the "ultimate reality" and I'll show you a model. EVERYTHING that we know of today is based on some theoretical model. That is how we understand the physics of our world. Maxwell equations are "models", and in fact, they are phenomenological models!

So why you are picking only on "photons", I haven't a clue.

Zz.

 

[Mentz114]

What we can discuss are only MODELS of photon, but not ultimate reality behind the phenomena we may observe in optical experiments!

So why don't you do or read about experiments instead of hand-waving ? I think you just enjoy a good argument, which doesn't leave you much time for real physics, or studying the literature.

 

[rohanprabhu]

This is a fallacy.

Point to me something in which you can claim to know the "ultimate reality" and I'll show you a model. EVERYTHING that we know of today is based on some theoretical model. That is how we understand the physics of our world. Maxwell equations are "models", and in fact, they are phenomenological models!

So why you are picking only on "photons", I haven't a clue.

Zz.

totally. +1

i always think like.. you know.. the way we explain things is based on some other phenomena. Classical mechanics used 'obvious observations'. Like it was when a force is applied on a body, it moves.. there was no explanation as to why it moves.. you have to make that basic assumption.. so.. it's like no matter how deep we go, there shall always be this last level of abstraction that we will never be able to explain..

 

[Cthugha]

4. Heisenberg model
I think that is similar to the Schrödinger model, except there is NO FREE photons. Because one of ideas of Heisenberg was to make a theory that describes ONLY what can be measured. We cannot measure photonic field without destroying it.

Sorry, that I go back to the very beginning of the discussion now, but your last assumption is not true. Although most usual measurements are indeed destructive, there are also so called QND (quantum nondemolition) measurements, which do not change the number of photons. Usually these use the optical Kerr effect or atoms in Rydberg states to measure the photon number.

Of course this procedure does change the state of the photon field, if it consists of a superposition of states, but it is truly nondestructive as soon as a photon number state is measured. Repeated measurements will give the same result. No photons are destroyed.

 

[Bright]

This is a fallacy.

Point to me something in which you can claim to know the "ultimate reality" and I'll show you a model. EVERYTHING that we know of today is based on some theoretical model.

With great pleasure

Let us start with some preliminary (probably not perfect) definitions.

Let "ultimate reality" be something SO PRECISE, that it is impossible make it better.
Let 'model' be some approximation of the "ultimate reality", something that we may improve or something that may be in principle improved and done better.

Now, consider Pythagorean theorem on plane (not in curved space) a^2 + b^2 = c^2
As soon as we proved this theorem IN OUR HEAD, WE GET ULTIMATE REALITY.
Note: I did what you asked me to do. I pointed you to something in which you can claim to know the "ultimate reality".
Note: In all real models that we can use to prove Pythagorean theorem, we may have very good precision, but not ABSOLUTE precision. In correct theoretical prove we have ABSOLUTE PRECISION.

So, I did what you asked me to do.

So why you are picking only on "photons", I haven't a clue.

Because photons seems to me easier...

 

[Bright]

Sorry, that I go back to the very beginning of the discussion now, but your last assumption is not true. Although most usual measurements are indeed destructive, there are also so called QND (quantum nondemolition) measurements, which do not change the number of photons. Usually these use the optical Kerr effect or atoms in Rydberg states to measure the photon number.

Of course this procedure does change the state of the photon field, if it consists of a superposition of states, but it is truly nondestructive as soon as a photon number state is measured. Repeated measurements will give the same result. No photons are destroyed.

Thank you so much for very interesting comment. I heard about QND (quantum nondemolition) measurements. But it was probably some modification of the experiment you described. In another modification all photons are coherent (exactly the same) and when one make measurement, he destroyed ONE photon, but (N-1) remain in the same state. So, destroying one photons is the COST of knowing state of remaining (N-1) photons.

Actually, the original statement, you commented, was about some restrictions of QFT, about only ONE photon between measurements... so, I think ONE photon is not enough to produce Kerr effect and make QND

 

[ZapperZ]

With great pleasure

Let us start with some preliminary (probably not perfect) definitions.

Let "ultimate reality" be something SO PRECISE, that it is impossible make it better.
Let 'model' be some approximation of the "ultimate reality", something that we may improve or something that may be in principle improved and done better.

Now, consider Pythagorean theorem on plane (not in curved space) a^2 + b^2 = c^2
As soon as we proved this theorem IN OUR HEAD, WE GET ULTIMATE REALITY.
Note: I did what you asked me to do. I pointed you to something in which you can claim to know the "ultimate reality".
Note: In all real models that we can use to prove Pythagorean theorem, we may have very good precision, but not ABSOLUTE precision. In correct theoretical prove we have ABSOLUTE PRECISION.

So, I did what you asked me to do.Because photons seems to me easier...

Er... you seem to be confusing physics with mathematics. So try again.

If you can't come up with something, I'll give you an example. Forget photons. Tell me that the 3 Newton's Laws of Motion are not "models". Don't tell me you find "photons" easier than Newton's Laws.

Zz.

 

[Bright]

Er... you seem to be confusing physics with mathematics.

I am not confusing physics with mathematics... you did not tell me give an example from physics. Why do you think that physics is the only possible way to study nature?

Tell me that the 3 Newton's Laws of Motion are not "models".

Sorry, I cannot tell you that...

 

[Cthugha]

Thank you so much for very interesting comment. I heard about QND (quantum nondemolition) measurements. But it was probably some modification of the experiment you described. In another modification all photons are coherent (exactly the same) and when one make measurement, he destroyed ONE photon, but (N-1) remain in the same state. So, destroying one photons is the COST of knowing state of remaining (N-1) photons.

This formalism you use is a bit strange. If the photon field was in a coherent state before the measurement there even was no strictly defined photon number before, so I am not sure, whether you talk about a coherent state or photons coming from the same coherence volume (which are indistinguishable in terms of QED).

However, I am also not sure, what kind of experiment you are actually talking about. The usage of N and (N-1) always makes me think of second order intensity correlation measurements, but this does not seem to be what you are talking about.

Actually, the original statement, you commented, was about some restrictions of QFT, about only ONE photon between measurements... so, I think ONE photon is not enough to produce Kerr effect and make QND

Oh, if you do clever measurements, one photon is enough. My favourite paper about QND is:

Progressive field-state collapse and quantum non-demolition photon counting
Nature 448, 889-893 (23 August 2007)

 

[ZapperZ]

I am not confusing physics with mathematics... you did not tell me give an example from physics.

Last time I checked, we are in the physics sub-forums in here and we are talking about physics issues. Whatever made you think that this is about mathematics?

Why do you think that physics is the only possible way to study nature?

Because it is. Why do you think "mathematics" is nature? Can you derive using nothing more than mathematical principle at ANY of the physics principles? Try driving the conservation of momentum from purely mathematical axioms.

Sorry, I cannot tell you that...

Then I have proven my point that all of physics are based on theoretical/model description. Why you picked on photons to argue about "models" is baffling.

Zz.

 

[Bright]

However, I am also not sure, what kind of experiment you are actually talking about.

There are more than 2000 published papers on non-demolition measurements...

 

[Bright]

Why do you think that physics is the only possible way to study nature?

Because it is.

Thank you so much for your brief and absolutely precise answer. Now I know that Mathematics, Biology, Chemistry, which are sub-forums of this forum, ARE NOT WAYS TO STUDY NATURE, Only Physics are the way to study nature.

Thanks again.

 

[Cthugha]

Let "ultimate reality" be something SO PRECISE, that it is impossible make it better.
Let 'model' be some approximation of the "ultimate reality", something that we may improve or something that may be in principle improved and done better.

Now, consider Pythagorean theorem on plane (not in curved space) a^2 + b^2 = c^2
As soon as we proved this theorem IN OUR HEAD, WE GET ULTIMATE REALITY.
Note: I did what you asked me to do. I pointed you to something in which you can claim to know the "ultimate reality".
Note: In all real models that we can use to prove Pythagorean theorem, we may have very good precision, but not ABSOLUTE precision. In correct theoretical prove we have ABSOLUTE PRECISION.

Hilbert would have liked to hear that.

So how do you prove this theorem in your head? You take some math, which has already been verified and derive the theorem.

Ok, so how did you prove the math you needed to verify the theorem? You took some other verified theorem and derived the math.

And so on...and on...until you get to pretty basic stuff. Going back one step further, you arrive at the axioms of your axiomatic system. These are just true "by definition", but you are not able to verify them. Therefore I would not call anything, which is just derived from defined axioms absolute reality as there is no unique choice of "right" axioms.

Just as some predictions of theories in physics are just true in the framework of special relativity or qm, some mathematical theorems are just true, if you choose the matching set of axioms...not very absolute.

There are more than 2000 published papers on non-demolition measurements...

Right, so I assume you do not know exactly which special kind of QND measurement you meant before. No problem. This was getting slightly off topic anyway.

 

[Bright]

Therefore I would not call anything, which is just derived from defined axioms absolute reality as there is no unique choice of "right" axioms.

Foundations of mathematics is a very hot area of research in the last years... and you pointed at very important issue "unique choice of "right" axioms"... Great!
Now, look again at my post you just cited "(not in curved space) a^2 + b^2 = c^2"
So, if the space is flat, the choice of right axioms, which are necessary to prove Pythagorean Theorem, becomes unique.