B What is ¨unpolarized¨ light exactly?

[Khashishi]

It seems like almost every confusion in quantum mechanics boils down into a question about the interpretation of a collapse. We don't really have an answer. We can't pin down a time when the collapse occurs. It seems to occur when the system interacts with a macroscopic environment. So, if you pass a photon into a polarizer and then into a detector, you either measure a count or you don't. You can't say a photon becomes polarized. Reality isn't so simple. We can talk about what we finally measure. What happens in between is kind of fuzzy.

 

[A. Neumaier]

I prefer to avoid using the word photon; then things are much clearer. A beam of light can be unpolarized, partially polarized, or fully polarized, and there is no doubt about what this means. It becomes polarized when passed through a polarization fielter. When a beam falls upon a detector, the detector responds with random clicks whose rate is proportional o the intensity of the incident beam.

Populating the beam of light with photons doesn't add any explanatory value but creats a lot of confusion and puzzlement. Ockham's razor suggests that one better does not do this.

 

[N88]

I prefer to avoid using the word photon; then things are much clearer. A beam of light can be unpolarized, partially polarized, or fully polarized, and there is no doubt about what this means. It becomes polarized when passed through a polarization fielter. When a beam falls upon a detector, the detector responds with random clicks whose rate is proportional o the intensity of the incident beam.

Populating the beam of light with photons doesn't add any explanatory value but creats a lot of confusion and puzzlement. Ockham's razor suggests that one better does not do this.

I understand the need for caution when discussing "photons". But can we clarify this need for caution by moving the discussion to PROTONS?

In his article "Einstein-Podolsky-Rosen experiments", John Bell ('Speakable and unspeakable …', 2004: p.82) writes: "… each particle [PROTON], considered separately, IS UNpolarised here." The italicised IS is Bell's emphasis, yet earlier, on p.81 we have: "… filters that pass only particles [PROTONS] of a given polarization".

How do you represent and clarify these "entangled" matters?

 

[StevieTNZ]

1. Is this wording correct?
2. Is it not meaningless to talk of an UNpolarized photon?
3. Am I correct in thinking that #2 is the point that Prof. Neumaier makes?

An unpolarized photon is defined as per the email I quoted.
A photon which has no polarization, well, I'm not sure the technical term for that (if there is one)

 

[stevendaryl]

1. Is this wording correct?
2. Is it not meaningless to talk of an UNpolarized photon?
3. Am I correct in thinking that #2 is the point that Prof. Neumaier makes?

If a photon is entangled with something else (another photon, for instance), then there is a sense in which it really doesn't have a polarization until measured, as opposed to having an unknown polarization. At least, that's one possible conclusion from Bell's analysis of the EPR experiment.

 

[A. Neumaier]

particles [PROTONS] of a given polarization

Protons have spin, not polarization.

Talking of ''each proton'' in a proton beam as if each were an individual object creates the same sort of problems as talking of photons in a beam of light Talk of one proton makes operational sense only if one can point to this one proton.

 

[N88]

Protons have spin, not polarization.

Talking of ''each proton'' in a proton beam as if each were an individual object creates the same sort of problems as talking of photons in a beam of light Talk of one proton makes operational sense only if one can point to this one proton.

Thank you. I was citing Bell and seeking to understand his terminology in the context of a single particle (considered separately, photon or proton) being UNpolarized.

That is: Under Bell's influence, I was believing that there were such things as UNpolarized photons and protons. I would now be happy to be influenced by your view on this confusing (to me) terminology.

 

[A. Neumaier]

Thank you. I was citing Bell and seeking to understand his terminology in the context of a single particle (considered separately, photon or proton) being UNpolarized.

That is: Under Bell's influence, I was believing that there were such things as UNpolarized photons and protons. I would now be happy to be influenced by your view on this confusing (to me) terminology.

There are unpolarized beams of light (defined by a coherence matrix that is a multiple of the identity), and there are proton beams whose spin coherence matrix is a multiple of the identity. Figuratively, this is interpreted as that the photons or protons in the beam are unpolarized, meaning that their density matrix is half the identity.

 

[N88]

There are unpolarized beams of light (defined by a coherence matrix that is a multiple of the identity), and there are proton beams whose spin coherence matrix is a multiple of the identity. Figuratively, this is interpreted as that the photons or protons in the beam are unpolarized, meaning that their density matrix is half the identity.

Ah, yes! FIGURATIVELY! 'Gold, in the figurative language of the people, was “the tears wept by the sun.”'

 

[A. Neumaier]

Ah, yes! FIGURATIVELY!

I consider all photons with which a beam of light can be populated as figurative talk only.

In quantum field theory, a beam of stationary laser light is an electromagnetic radiation field described by a coherent state. In this state, the expectation value of the photon number operator is given by the beam intensity. It is a finite number. A photodetector at the other end of the laser beam clicks with a rate proportional to the intensity. Each click is figurately interpreted as a photon arriving. If one waits long enough one can extract from the beam as many photons as one likes - in spite of the fact that the expected number of photons can be small (even less than one). Thus the concept of photon number in quantum field theory has very little to do with the concept of photons defined by detector clicks. The only case where the two match is when one uses quantum field theory to calculate a scattering event with a well-defined photon content.

In quantum mechanics, one describes single scattering events in the same manner as in QFT (though usually in simplified models). However, sequences of events are never treated according to a formal prescription but only figuratively by applying the single event formalism in a heuristic way to sequential processes. This is done by
replacing the QFT description of a beam by an imagined sequence of photons created by the source and traveling along the beam to the detector where they produce a click. This is an intuitive picture (hence figurative) that in simple situations gives a quantitatively correct description of the experimental result. This picture is then generalized to entangled photons etc. in a way that allows one to have some intuition about how these behave. This intuition is only of very limited validity, hence it is accompanied by riddles that make everything look very weird.

Thus photons as flying objects causing detector clicks are purely figurative talk. The correct information content can be found only in a quantum field treatment and in approximate formal treatments derived from it - in practice via Lindblad equations.