Does a controversy still exist ?

[George Jones]

Same happened to me, in fact. I learned about the existence of Haag's theorem on s.p.r. What's funny is that if you look up "Haag" in Weinberg's account on QFT, it isn't even mentioned!

"Haag's theorem" is in the index of Haag's booK! It was on s.p.r that I too first learned of its importance.

I just had a brief look through Weinberg for mention of asymptotic series - nothing caught my eye.

Regards,
George

 

[McQueen]

I don't understand what you're getting at here? What do you mean by "altering the polarity of the first spatially separated photon"?

Sorry it should have been polarization , not "polarity". Surely , performing an experiment which could prove conclusively that many basic precepts of QM such as the collapse of the wave function and FTL interactions is more important than sending a few thousand dollars by an encryption process.
http://www.newscientist.com/article.ns?id=dn4914 . Think of it using this analogy , I have two objects , one black and one white , which I put in two boxes. The boxes are mixed up so that no-one knows which is which and one is sent to a spatially separated location at A while the other is sent to B. If A gets the white object then B must get the black object and vice versa. Now introduce two more oppositely coloured objects say Red and blue. And while sending the object to A assume that it is intercepted and the whi8te object taken out and replaced with the Blue object , will the box at B now have a Red object ?

 

[vanesch]

"Haag's theorem" is in the index of Haag's booK! It was on s.p.r that I too first learned of its importance.

Yeah... I never managed to read it. Have it on my shelf allright, but never got down to reading it.

 

[George Jones]

Yeah... I never managed to read it. Have it on my shelf allright, but never got down to reading it.

I didn't claim that I'd read it, only that I'd read part of the index! :-)

Regards,
George

 

[selfAdjoint]

Haag actually is pretty light in his presentation of "Haag's Theorem" in his book. Kind of s**t kicking, if you catch my drift.

 

[Igor_S]

McQueen,
I assure you, there is no problem with QM, what so ever. I think Hurkyl gave you a nice explanation concerning your question. I would like to add that al these "measurement problems" are all just coming from people who are interpreting the result and formalism of QM in the WRONG way.
QM works, Einstein was wrong, "point final"
regards
marlon

There is a nice book by R. Feynman "QED: The strange theory of light and matter" where he explains things in his simple way (its a popular book). I read only a part of it and it seemed ok. He even pulls out some number, comparing theoretical and experimental results and they match up to something like 10 digits or so.

Maybe the person who wrote 1st post could learn more from this book then from other stuff mentioned here. :) I just got that impression.

 

[Sherlock]

Using the same equipment and altering the polarization of the first spatially separated photon should always result in an opposite polarization of the other photon.

It depends on how the entangled photons are produced. In most of the Bell tests for example, paired photons at A and B are identically polarized. That is, with the polarizers aligned, if A registers a detection, then so does B, and vice versa.

In the case of opposite polarization, then with polarizers aligned, if A registers a detection then B doesn't register a detection.

The test you're talking about has been done many times, and it doesn't reveal whether A and B are, or aren't, interacting FTL.

QM doesn't assign a definite polarization prior to measurement. It does assign a definite combined angular momentum prior to measurement.

Surely , performing an experiment which could prove conclusively that many basic precepts of QM such as the collapse of the wave function and FTL interactions is more important than sending a few thousand dollars by an encryption process.

"Collapse of the wave function" has to do with changes in the wave function due to the quantum object's interaction with a measuring apparatus. There's no 'test' required to prove this, it just has to do with the way qm works.

"Collapse of the wave function" doesn't imply FTL interactions. If it was possible with existing technology to prove conclusively that FTL interactions either do or don't exist, then it seems like a good bet that it would have been done.

Think of it using this analogy , I have two objects , one black and one white , which I put in two boxes. The boxes are mixed up so that no-one knows which is which and one is sent to a spatially separated location at A while the other is sent to B. If A gets the white object then B must get the black object and vice versa. Now introduce two more oppositely coloured objects say Red and blue. And while sending the object to A assume that it is intercepted and the whi8te object taken out and replaced with the Blue object , will the box at B now have a Red object ?

A probability function is meaningful only insofar as it refers to a set of identically prepared and executed experiments. The probability function that applies to quantum correlations between A and B is the mean value of psi*psi, averaged over many experiments. The mean value for B is the same after a measurement at A as it is before a measurement at A, and vice versa.

 

[McQueen]

"Collapse of the wave function" has to do with changes in the wave function due to the quantum object's interaction with a measuring apparatus. There's no 'test' required to prove this, it just has to do with the way qm works.

Right , but just to make sure that we don't lose sight of the original reason and meaning of the EPR experiment , here is a quote from Einstein. "One can escape from the conclusion [that QM is incomplete] only byt assuming that measuring S1 (telepathically) changes the real situation at S2 or by denying independent real situations as such to things which are spatially separated from each other. Both alternatives appear to me unacceptable." Again "...on one supposition we should , in my opinion , absolutely hold fast; the real factual situation of the system S2 ( the particle in Area B) is independent of what is done with the system S2 ( the particle in area A) which is spatially separated from the former. "

 

[McQueen]

A probability function is meaningful only insofar as it refers to a set of identically prepared and executed experiments. The probability function that applies to quantum correlations between A and B is the mean value of psi*psi, averaged over many experiments. The mean value for B is the same after a measurement at A as it is before a measurement at A, and vice versa.

Can a commercially marketed encryption system , which is used for the transfer of money be based on such a system of averages , I think not , at least not above that necessary for corrective purposes.For the same reason it should be easy to carry out and confirm , the experiment I had referred to earlier.

 

[McQueen]

...

"Consider the curious incident of the dog in the night-time," Sherlock Holmes said to Watson in "Silver Blaze"
"The dog did nothing in the night-time."
"That was the curious incident,"remarked Holmes.

 

[Sherlock]

... just to make sure that we don't lose sight of the original reason and meaning of the EPR experiment , here is a quote from Einstein. "One can escape from the conclusion [that QM is incomplete] only by assuming that measuring S1 (telepathically) changes the real situation at S2 or by denying independent real situations as such to things which are spatially separated from each other. Both alternatives appear to me unacceptable."

If we want to talk about the physical reality of quantum phenomena as being something other than instrumental phenomena, then the conclusion that qm is an incomplete description of physical reality is inescapable. But as far as quantum theory is concerned the physical reality is the instrumental phenomena. Whatever might be happening independent of that can't be described in realistic terms. That is, a one to one mapping between events in the microworld and the mathematical entities of qm is impossible according to the principles of the theory. A visualizable geometrical representation of the microworld that qm deals with had to be sacrificed in order for the theory to be consistent internally and wrt experimental results. That's the tradeoff. And there's no way around it as far as anybody knows.

Again "...on one supposition we should , in my opinion , absolutely hold fast; the real factual situation of the system S2 ( the particle in Area B) is independent of what is done with the system S1 ( the particle in area A) which is spatially separated from the former."

This supposition seems correct so far, because there's nothing in the qm formulation of the situations that Einstein is talking about which necessitates the conclusion that the "particle in area A" is affecting the "particle in area B" in any physical sense which would require a transmission between A and B during the joint measurement interval. A and B can be linearly combined in a wave function and are statistically related because of certain isomorphisms in the experimental situation.

Can a commercially marketed encryption system , which is used for the transfer of money be based on such a system of averages , I think not , at least not above that necessary for corrective purposes. For the same reason it should be easy to carry out and confirm , the experiment I had referred to earlier.

The potential for greater security using quantum encryption, and also the main stumbling block to its practical implementation, is the sensitivity of quantum entangled systems to environmental (external) influences.
As for your experiment. Afaik, it's been done many times and there's no way to tell if A and B are physically affecting each other during the joint measurement interval. Non-locality (FTL transmission) remains a possibility, but isn't a necessary conclusion -- and it isn't what quantum encryption depends on, afaik.
Concerning the question posed in the title of this thread, a controversy still exists wrt the qualitative nature of light.
Because qm is a probabilistic accounting of quantitative experimental results, it can't, at least in its present form, resolve this controversy.

"Consider the curious incident of the dog in the night-time," Sherlock Holmes said to Watson in "Silver Blaze"
"The dog did nothing in the night-time."
"That was the curious incident,"remarked Holmes.

All I know about Sherlock Holmes is that he was a detective whose deductive analyses were often correct because he knew a lot of stuff. He knew lots of facts. I don't know lots of facts yet. I chose the nickname Sherlock because I aspire to the sort of investigative skills that his legacy represents.
Unfortunately, I don't know exactly what your citing above is supposed to be an analogy of.

 

[McQueen]

Sherlock while I appreciate that you have taken a lot of trouble to answer my questions and doubts and have furthermore done a good job of it , I also get the feeling that many of the pertinent points I had raised have been glossed over in your replies or simply ignored. I hope you don’t mind my making this criticism and will try to justify my statements during the course of this reply. Take first your statement that FTL is at the most a marginal issue viz-a-viz quantum mechanics.: In actual fact FTL turns out to be very much a central issue as far as QM is concerned. Take for example the phenomenon of Quantum Tunneling which is inevitably raised whenever QM is discussed. FTL is implicit in the QM explanation of quantum tunneling. I clearly remember a reference to a group of scientists who had claimed to have transmitted a Mozart composition using FTL at 4.7 c ! If I remember rightly the subject was discussed in Physics Forums and eventually these claims were proved wrong. (I am unable to find the exact post) . Nevertheless , the fact remains that FTL is central to Quantum tunneling. Again , take your insistence that EPR has nothing whatsoever to do with QM .

"Collapse of the wave function" doesn't imply FTL interactions. If it was possible with existing technology to prove conclusively that FTL interactions either do or don't exist, then it seems like a good bet that it would have been done.

Yet many people feel that the EPR is a decisive issue as to whether Quantum Mechanics , at least as regards the superposition of states , is viable or not. Which is precisely the point I had repeatedly made in this thread. Also your claim that the Quantum Encryption system used in the transfer of money , posted at New Scientist and to which I had given a link in one of my earlier replies , would result in an anomalous result , seems to me quite incredible

The potential for greater security using quantum encryption, and also the main stumbling block to its practical implementation, is the sensitivity of quantum entangled systems to environmental (external) influences. As for your experiment. Afaik, it's been done many times and there's no way to tell if A and B are physically affecting each other during the joint measurement interval.

Consider the fact that this is an encryption system , where even the slightest mistake made could result in catastrophic misunderstandings. For instance take a three letter word such as cat if any of the three letters are changed the whole meaning would be drastically changed. As for instance , hat , mat , fat , cot ,etc., etc., . Yet this is a system which is now being manufactured and sold commercially , you or I could just go to a shop and buy one. Given that this is so it should be possible to conduct the experiment by changing the polarization of one of the spatially separated photons and determining if the other photon also undergoes the relevant change. Lastly the quote from “Silver Blaze” referred to just this obtuseness which is demonstrated by (a) either not conducting the experiment or (b) obfuscating the results if the experiment has been conducted. Hence the question of why the dog did not bark. i.e., why hasn’t the experiment been carried out.

 

[McQueen]

Sherlock while I appreciate that you have taken a lot of trouble to answer my questions and doubts and have furthermore done a good job of it , I also get the feeling that many of the pertinent points I had raised have been glossed over in your replies or simply ignored. I hope you don’t mind my making this criticism and will try to justify my statements during the course of this reply. Take first your statement that FTL is at the most a marginal issue viz-a-viz quantum mechanics.:

"Collapse of the wave function" doesn't imply FTL interactions. If it was possible with existing technology to prove conclusively that FTL interactions either do or don't exist, then it seems like a good bet that it would have been done. "

In actual fact FTL turns out to be very much a central issue as far as QM is concerned. Take for example the phenomenon of Quantum Tunneling which is inevitably raised whenever QM is discussed. FTL is implicit in the QM explanation of quantum tunneling. I clearly remember a reference to a group of scientists who had claimed to have transmitted a Mozart composition using FTL at 4.7 c ! If I remember rightly the subject was discussed in Physics Forums and eventually these claims were proved wrong. (I am unable to find the exact post) . Nevertheless , the fact remains that FTL is central to Quantum tunneling. Again , take your insistence that EPR has nothing whatsoever to do with QM .

This supposition seems correct so far, because there's nothing in the qm formulation of the situations that Einstein is talking about which necessitates the conclusion that the "particle in area A" is affecting the "particle in area B" in any physical sense which would require a transmission between A and B during the joint measurement interval. A and B can be linearly combined in a wave function and are statistically related because of certain isomorphisms in the experimental situation.

Yet many people feel that the EPR is a decisive issue as to whether Quantum Mechanics , at least as regards the superposition of states , is viable or not. Which is precisely the point I had repeatedly made in this thread. Also your claim that the Quantum Encryption system used in the transfer of money , posted at New Scientist and to which I had given a link in one of my earlier replies , would result in an anomalous result , seems to me quite incredible

The potential for greater security using quantum encryption, and also the main stumbling block to its practical implementation, is the sensitivity of quantum entangled systems to environmental (external) influences. As for your experiment. Afaik, it's been done many times and there's no way to tell if A and B are physically affecting each other during the joint measurement interval.

Consider the fact that this is an encryption system , where even the slightest mistake made could result in catastrophic misunderstandings. For instance take a three letter word such as cat if any of the three letters are changed the whole meaning would be drastically changed. As for instance , hat , mat , fat , cot ,etc., etc., . Yet this is a system which is now being manufactured and sold commercially , you or I could just go to a shop and buy one. Given that this is so it should be possible to conduct the experiment by changing the polarization of one of the spatially separated photons and determining if the other photon also undergoes the relevant change. Lastly the quote from “Silver Blaze” referred to just this obtuseness which is demonstrated by (a) either not conducting the experiment or (b) obfuscating the results if the experiment has been conducted. Hence the question of why the dog did not bark. i.e., why hasn’t the experiment been carried out and why is no reference made to it.

 

[Haelfix]

Yea I agree with Vanesch on this in general. You can take the phenomonological viewpoint (shut up and calculate) but even there there are mismatches in various regimes.

Anyway, I think there is still an open question on foundational interpretation issues in Quantum mechanics. Not so much in the wave/particle duality, but more so in the nonlinear nature of the collapse coupled with the quantum to classical limit (somewhat helped by decoherence). Every possible method tends to have various logical pitfalls at some point, and when you poll most theorists they are all over the place on exactly which formalism they prefer. I still consider it an unsolved and dare I say it, unsatisfactory *question* in physics and I also don't believe any of the current proposals is ultimately the *right* answer.

As far as QFT, the mathematical axiomatization thereof is very unsatisfying, tons of open issues and a very hard and unrewarding subject in general. Throw in curved backgrounds, and all hell breaks loose. This is not just an academic problem, there are plenty of experiments you could think off that could give quite different results depending on what you choose. Some very fundamental gap in our knowledge is clearly lurking in the midst in all of that.

Keep in mind, we are still very far from *perfect* theories, even in the standard model on flat space at a physicists lvl of rigor. For instance QED is plagued by the Landau pole, and more or less strongly suggests that it only effective and there is something more fundamental lurking in the UV.
For QCD we still have inabilities to show a correct mass gap.

 

[McQueen]

The tranparency of many dielectric is dictated by the phonon structure. If that phonon mode is available, then chances are, the material will absorb a photon with that particular energy. The lattice vibration that can either convert this into heat, or cause another transition. If the vibrational mode is not available, the ions will simply get displaced and retransmit the same energy since it cannot sustain that mode!

I agree with what you have stated . But this again leaves the question of how electrical energy is actually conveyed through a metal. QM regards electrons as the charge carriers , that is to say it is electrons that actually convey electrical energy ; loosely bound valence and free electrons are drawn to the positively charged ions. Unfortunately , this theory does not actually work. Take the circumstance where electrical energy flows over an open circuit without any electrons being observed. One could say , oh well ! that’s a simple one to deal with , Maxwell’s equations say it all. But the point is that Maxwell’s equations do not say it all , if they did we would once again be left with something like the Ultra violet catastrophe. If this question is raised , the answer that is usually given is that yes electrical energy is conveyed through electrons but that the energy passes from electron to electron via virtual photons . This is very confusing , is it admissible ? I was under the impression that virtual particles had to conform to Heisenberg’s Uncertainty principle , which in turn has to conform to the Conservation Laws , which means in effect that the interactions of virtual particles can not be real , except possibly in the sense of altering the direction of a real particle , they ( virtual particles ) cannot under any circumstances impart any extra momentum or energy to the real particle they are interacting with. At least this is my understanding. Thus although the concept of virtual particles might be acceptable from the QFT point of view viz-a-viz low frequency EM radiation , it cannot be applied to the conveying of electrical energy. My question is this : every interaction between electrons and other particles is mediated by real photons. This has been made especially clear in recent years , with the creation and observation , of low energy photons ( 1.4eV , 800 Nm. ) . Photons of this energy and slightly lower can only be absorbed by loosely bound valence electrons , which would make them the ideal candidate for the conveying of electrical energy. Note , that this would also escalate the wave OR particle question of light to a whole new level. It just doesn’t seem right that only in the phenomenon of electricity , exclusively , do we see electrons directly delivering energy. Isn’t it time that this was changed.

 

[ZapperZ]

I agree with what you have stated . But this again leaves the question of how electrical energy is actually conveyed through a metal. QM regards electrons as the charge carriers , that is to say it is electrons that actually convey electrical energy ; loosely bound valence and free electrons are drawn to the positively charged ions. Unfortunately , this theory does not actually work. Take the circumstance where electrical energy flows over an open circuit without any electrons being observed. One could say , oh well ! that’s a simple one to deal with , Maxwell’s equations say it all. But the point is that Maxwell’s equations do not say it all , if they did we would once again be left with something like the Ultra violet catastrophe. If this question is raised , the answer that is usually given is that yes electrical energy is conveyed through electrons but that the energy passes from electron to electron via virtual photons . This is very confusing , is it admissible ? I was under the impression that virtual particles had to conform to Heisenberg’s Uncertainty principle , which in turn has to conform to the Conservation Laws , which means in effect that the interactions of virtual particles can not be real , except possibly in the sense of altering the direction of a real particle , they ( virtual particles ) cannot under any circumstances impart any extra momentum or energy to the real particle they are interacting with. At least this is my understanding. Thus although the concept of virtual particles might be acceptable from the QFT point of view viz-a-viz low frequency EM radiation , it cannot be applied to the conveying of electrical energy. My question is this : every interaction between electrons and other particles is mediated by real photons. This has been made especially clear in recent years , with the creation and observation , of low energy photons ( 1.4eV , 800 Nm. ) . Photons of this energy and slightly lower can only be absorbed by loosely bound valence electrons , which would make them the ideal candidate for the conveying of electrical energy. Note , that this would also escalate the wave OR particle question of light to a whole new level. It just doesn’t seem right that only in the phenomenon of electricity , exclusively , do we see electrons directly delivering energy. Isn’t it time that this was changed.

How do you make the LEAP from "optical conductivity" which was what I was trying to describe, straight into "electrical conductivity of open circuit"? Doesn't an abrupt change of topic like that causes nosebleeds?

Charge transport in metals, semiconductors, etc. is a WELL-STUDIED area of solid state physics. Open any solid state physics text if you do not believe me. One can take in everything from the semi-classical Drude model, all the way to the Landau's Fermi Liquid theory, and going into the more exotic Luttinger Liquid theory. Can you tell me where exactly in these descriptions that you see the need for a change?

Zz.

 

[Sherlock]

Sherlock while I appreciate that you have taken a lot of trouble to answer my questions and doubts and have furthermore done a good job of it , I also get the feeling that many of the pertinent points I had raised have been glossed over in your replies or simply ignored. I hope you don’t mind my making this criticism and will try to justify my statements during the course of this reply.

Criticism offered in the spirit of learning is a good thing. If I've glossed over or missed an important point, then I'm glad to have it brought to my attention.

Take first your statement that FTL is at the most a marginal issue viz-a-viz quantum mechanics.:
In actual fact FTL turns out to be very much a central issue as far as QM is concerned. Take for example the phenomenon of Quantum Tunneling which is inevitably raised whenever QM is discussed. FTL is implicit in the QM explanation of quantum tunneling. I clearly remember a reference to a group of scientists who had claimed to have transmitted a Mozart composition using FTL at 4.7 c ! If I remember rightly the subject was discussed in Physics Forums and eventually these claims were proved wrong. (I am unable to find the exact post) . Nevertheless , the fact remains that FTL is central to Quantum tunneling.

How is it central? Nothing that I've read about quantum tunneling says anything about FTL.

Again , take your insistence that EPR has nothing whatsoever to do with QM .
Yet many people feel that the EPR is a decisive issue as to whether Quantum Mechanics , at least as regards the superposition of states , is viable or not. Which is precisely the point I had repeatedly made in this thread. Also your claim that the Quantum Encryption system used in the transfer of money , posted at New Scientist and to which I had given a link in one of my earlier replies , would result in an anomalous result , seems to me quite incredible.

I don't remember saying that EPR has nothing whatsoever to do with QM. Anyway, the *viability* of qm is determined by how closely it approximates experimental results. Quantum theory is, essentially, a wave theory, so the principle of linear superposition is a necessary part of it.
I also don't remember saying that the quantum encryption system would result in an anomalous result. What do you mean by "anomalous result"?

Consider the fact that this is an encryption system , where even the slightest mistake made could result in catastrophic misunderstandings. For instance take a three letter word such as cat if any of the three letters are changed the whole meaning would be drastically changed. As for instance , hat , mat , fat , cot ,etc., etc., . Yet this is a system which is now being manufactured and sold commercially , you or I could just go to a shop and buy one.

Well, maybe you could.

Given that this is so it should be possible to conduct the experiment by changing the polarization of one of the spatially separated photons and determining if the other photon also undergoes the relevant change.

You're going to have to have to describe exactly what you mean here. I tried the link you provided (to virginia.edu) and it didn't work. To avoid any misunderstanding on my part, just describe the setup you're talking about, and what you expect to happen, because maybe I'm not getting what you mean by "changing the polarization of one of the spatially separated photons and determining if the other photon also undergoes the relevant change."

Lastly the quote from “Silver Blaze” referred to just this obtuseness which is demonstrated by (a) either not conducting the experiment or (b) obfuscating the results if the experiment has been conducted. Hence the question of why the dog did not bark. i.e., why hasn’t the experiment been carried out.

As I mentioned, according to my understanding of what you mean wrt "the experiment", it's been conducted many times and no FTL effects have ever been observed. But I await your clarification.

 

[McQueen]

How do you make the LEAP from "optical conductivity" which was what I was trying to describe, straight into "electrical conductivity of open circuit"? Doesn't an abrupt change of topic like that causes nosebleeds?

I am sorry for the sudden shift from optical conductivity to electrical conductivity. It was an attempt to try to return to the original topic in the thread.

Charge transport in metals, semiconductors, etc. is a WELL-STUDIED area of solid state physics. Open any solid state physics text if you do not believe me. One can take in everything from the semi-classical Drude model, all the way to the Landau's Fermi Liquid theory, and going into the more exotic Luttinger Liquid theory. Can you tell me where exactly in these descriptions that you see the need for a change?

Here is a link to the Wikepedia Encyclopaedia :
http://en.wikipedia.org/wiki/Electrical_conduction [/URL] on the subject of “electrical conduction.” You can imagine my amazement when I saw nothing at all about
the Landau Fermi Liquid and the even more esoteric Luttinger Liquid Theory or the Weidman- Franz Law in the article , although the enyclopaedia does list separate articles on these subjects. The point is , if the theories you had quoted namely the “Lanadau Liquid Theory “ and the “Luttinger Liquid Theory” were well accepted , well articulated theories on the subject of “electrical conduction” and remember that the Landau Fermi Liquid Theory was formulated almost fifty years ago , there should at least be some reference to the subject in accepted sources . The Hyperphysics web site
[PLAIN] http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html [/URL]( which is an accepted authority ) states that “ ……. it is electrons which are the mobile charge carriers which are responsible for electric current in conductors. “ The Landau Fermi Liquid Theory seems to relate more to the behaviour of electrons in metals , under certain conditions , resulting in such interactions as Umklapp scattering than in any explanation as to how electrical energy is actually conducted in a metal. To simplify my question , are electrons , in this single instance , the actual agents of electrical energy , or are they not ?

 

[ZapperZ]

I am sorry for the sudden shift from optical conductivity to electrical conductivity. It was an attempt to try to return to the original topic in the thread.
Here is a link to the Wikepedia Encyclopaedia :
http://en.wikipedia.org/wiki/Electrical_conduction [/URL] on the subject of “electrical conduction.” You can imagine my amazement when I saw nothing at all about
the Landau Fermi Liquid and the even more esoteric Luttinger Liquid Theory or the Weidman- Franz Law in the article , although the enyclopaedia does list separate articles on these subjects. The point is , if the theories you had quoted namely the “Lanadau Liquid Theory “ and the “Luttinger Liquid Theory” were well accepted , well articulated theories on the subject of “electrical conduction” and remember that the Landau Fermi Liquid Theory was formulated almost fifty years ago , there should at least be some reference to the subject in accepted sources . The Hyperphysics web site
[PLAIN] http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html [/URL]( which is an accepted authority ) states that “ ……. it is electrons which are the mobile charge carriers which are responsible for electric current in conductors. “ The Landau Fermi Liquid Theory seems to relate more to the behaviour of electrons in metals , under certain conditions , resulting in such interactions as Umklapp scattering than in any explanation as to how electrical energy is actually conducted in a metal. To simplify my question , are electrons , in this single instance , the actual agents of electrical energy , or are they not ?[/QUOTE]

I'm sorry, but do you not seem something GLARINGLY wrong here? You are judging the STATE of KNOWLEDGE of physics based on what you learn from Wikipedia?! You do not see the absurdity in this? If it doesn't exist on Wikipedia, it then doesn't exist? Is this where you get all your primary source of info, and this is how you are trying to contradict what I am telling you??!

Then there is no point in my pointing out Ashcroft and Mermin's Solid State Physics Text, or even Mahan's Many-Particle Physics text, etc...etc. Because they are not Wikipedia, they are useless. Why we make graduate students go through these books when there's Wikipedia available for free, AND with simpler, naive version of physics, I have no idea.

Zz.

 

[McQueen]

How is it central? Nothing that I've read about quantum tunneling says anything about FTL.

If you look at the reference :
http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html [/URL] you can see what I meant by the statement that FTL is central to QM. [I] “So, has special relativity been disproved,[b] now that FTL speeds have been measured?[/b] (N.B. Bold letters are mine , for emphasis) The first problem with this naive conclusion is that, while in special relativity neither information nor energy are allowed to be transmitted faster than light, but that certain velocities in connection with the phenomena of wave transmission may well exceed light speed. For instance, the phase velocity of a wave or the group velocity of a wave packet are not in principle restricted below light speed. The speed connected with wave phenomena that, according to special relativity, must never exceed light speed, is the front velocity of the wave or wave packet. “[/I] Since QM is to a large extent committed to Schrodinger’s wave function , it follows that ,logically at least , QM is also committed to FTL .
[QUOTE=Sherlock] As I mentioned, according to my understanding of what you mean wrt "the experiment", it's been conducted many times and no FTL effects have ever been observed. But I await your clarification.[/QUOTE] I agree that over the years several experiments have been conducted on the lines of the EPR , none of which conclusively proved anything one way or another. This was mainly due to lack of the right equipment. Which is why the reference I had given to a commercially viable quantum encryption system is so vitally important. It means that suppose you produce two quantum entangled photons using PDC and send them to two spatially separated locations , the result is consistent enough to use commercially. i.e ., if the two entangled photons have the same polarization at point of origin , then when the polarization of one of the photons is found the polarization of the other is the same to a consistent degree. Do you agree with this so far ? My point is that since the process is consistent , it should be possible to [I] change [/I] the polarization of one of the spatially separated entangled photons and to determine if the other spatially separated photon also undergoes a change in polarization. This would prove conclusively whether FTL does or does not exist . If in fact FTL is found not to exist it would doubts about the whole wave function and QM in general. More important what does this say about the work of Deutsch , Aspect and others ?
[QUOTE=ZapperZ] I'm sorry, but do you not seem something GLARINGLY wrong here? You are judging the STATE of KNOWLEDGE of physics based on what you learn from Wikipedia?! You do not see the absurdity in this? If it doesn't exist on Wikipedia, it then doesn't exist? Is this where you get all your primary source of info, and this is how you are trying to contradict what I am telling you??![/QUOTE]
Right , anticipating such a reaction , if you recall my final question was a fairly simple one . “…….,are electrons , in this single instance , the actual agents of electrical energy , or are they not ?
Also I had visited your web-site and have noted a posting where you yourself had given a link to
[url]http://groups.yahoo.com/group/undernetphysics/message/7[/url]
the same hyperphysics web-site that I had mentioned in my previous post. What am I to make of this?

 

[ZapperZ]

If you look at the reference :
http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html [/URL] you can see what I meant by the statement that FTL is central to QM. [I] “So, has special relativity been disproved,[b] now that FTL speeds have been measured?[/b] (N.B. Bold letters are mine , for emphasis) The first problem with this naive conclusion is that, while in special relativity neither information nor energy are allowed to be transmitted faster than light, but that certain velocities in connection with the phenomena of wave transmission may well exceed light speed. For instance, the phase velocity of a wave or the group velocity of a wave packet are not in principle restricted below light speed. The speed connected with wave phenomena that, according to special relativity, must never exceed light speed, is the front velocity of the wave or wave packet. “[/I] Since QM is to a large extent committed to Schrodinger’s wave function , it follows that ,logically at least , QM is also committed to FTL . [/quote]

This is wrong. You are confusing "non-localitiy" with "FTL". No one who does the EPR type experiments ever claim "FTL". Read any of the papers on this if you don't believe me. What is being claimed to be violated is the EPR notion of "local realism".

And if you do a search on here, I've written at least in a couple of threads why FTL claim in tunneling is also dubious.

[quote]Right , anticipating such a reaction , if you recall my final question was a fairly simple one . “…….,are electrons , in this single instance , the actual agents of electrical energy , or are they not ?
Would you be kind enough to quote from the numerous text-books you had mentioned.[/QUOTE]

Charge carriers in ordinary metals and semiconductors are carried by "quasiparticles" as defined within the Landau Fermi Liquid theory. These can be quasielectrons or quasiholes. These are called quasiparticles because the behavior (such as their effective mass) have been renormalized from the bare particles ("bare" holes are not well-defined, actually). The behavior of these quasiparticles are described by the single-particle spectral function, which in the propagator/field theoretic picture, is defined by the imaginary part of the single-particle Green's function.

Zz.

 

[McQueen]

Zapperz , I sincerely appreciate the thought you have put into your reply and the time you have taken to answer my questions . However bear in mind that what I have to say next might be irritating or even aggravating , depending on your state of mind. I hope you will have the patience to hear me out.

Charge carriers in ordinary metals and semiconductors are carried by "quasiparticles" as defined within the Landau Fermi Liquid theory. These can be quasielectrons or quasiholes. These are called quasiparticles because the behavior (such as their effective mass) have been renormalized from the bare particles ("bare" holes are not well-defined, actually). The behavior of these quasiparticles are described by the single-particle spectral function, which in the propagator/field theoretic picture, is defined by the imaginary part of the single-particle Green's function.
Zz.

What are quasiparticles , when the term was introduced fifty years ago , it referred to ‘virtual particles’ the two terms are often used interchangeably in Russian text-books on QM and I must presume it still has the same connotation. Take the quasiparticle term phonon , it is the QM term for the quantization of vibrations of the lattice and is derived from the Greek word for sound , and in fact is a resultant of the classical physics equivalent of sound vibrations . But just consider for a moment how absurd this sounds ?
Phonons travel at the speed of sound , they are massless particles , so this doesn’t make sense , theoretically they should travel at the speed of light. In the classical usage it makes perfect sense because sound is a wave traveling through a medium . But presumably phonons still exist in space , where there is no medium for them to travel through. How then is this particular phonon speed possible in space ? How the, if the argument is extended , can phonons exist at all in space? OK let’s put another meaning to the word quasiparticle and consider it to be something the electron resembles when it interacts with vibrations within the crystal lattice of the conductor , (Note: these vibrations still have to travel at the speed of sound when in space without the help of a medium and this is something that has to be explained.) But suppose it is possible and that the electron in this state is able to project something very similar to itself , each of which carries the same spin, charge and momentum as the original particle. So now the electron is capable of making ‘virtual’ clones of itself. Not only that but these ‘virtual’ clones are able to influence matter just as if they were the real thing , this is truly extraordinary ?? Consider for just a moment , you had accused me in your second last post of wanting simplistic physics. Is this theory a viable alternative. Alright , consider next the drift velocity of electrons in a conductor , the mean free path etc ., A man walking at 4 Km/hr is traveling at about a billion times the speed of an electron in a conductor. Wouldn’t it be simpler to theorize that electrical energy is carried by photons and to determine how free electrons and valence electrons in a conductor are able to absorb and emit low energy photons in order to do this ?

 

[ZapperZ]

What are quasiparticles , when the term was introduced fifty years ago , it referred to ‘virtual particles’ the two terms are often used interchangeably in Russian text-books on QM and I must presume it still has the same connotation.

No, this is wrong. Quasiparticles as defined in the Fermi Liquid theory are NOT virtual particles. In ordinary metals, if you solve the propagator, the quasiparticles have INFINITE lifetime, and thus is a delta function in the energy spectrum. This is not the behavior of a virtual particles. Besides, you can MEASURE direction the behavior of quasiparticles. When you measure the dispersion in a material, you are measuring the dispersion of the quasiparticles. When was the last time you could do the same with virtual particles. So your whole description of quasiparticles being identical to virtual particles is wrong.

Take the quasiparticle term phonon ,

Wrong. A phonon is not a quasiparticle. A phonon is a collective excitation, similar to a photon being an excitation out of a vacuum state.

Zz.

 

[vanesch]

Phonons travel at the speed of sound , they are massless particles , so this doesn’t make sense , theoretically they should travel at the speed of light. In the classical usage it makes perfect sense because sound is a wave traveling through a medium . But presumably phonons still exist in space , where there is no medium for them to travel through.

That's not correct. "Phonons" are eigenstates of the energy/momentum 4-vector of the vibrational degrees of freedom of a lattice, and because of the formal ressemblance to free field theories in fundamental QFT, you can call them "particles" of some kind, because the mathematics is the same. It is a useful mental picture, that's all. In reality you're working with eigenstates of the hamiltonian of your lattice. But because the mathematics is the same as the QFT of free particles "in space", of course a lot of properties of these solutions look exactly like the solutions you have for free particles in space, so it is a very very useful mental picture. You will for example also have the equivalent of energy and momentum conservation in perturbative interactions and all other kinds of things.

But, of course, quasi-particles cannot leave the crystal :-)

Mind you, when I was in high school, my idea was to have a positron source by accelerating holes in a semiconductor to get them out of the xtal... but I've grown up in the mean time

 

[Sherlock]

If you look at the reference :
http://www.aei-potsdam.mpg.de/~mpoes...nelingftl.html you can see what I meant by the statement that FTL is central to QM. “So, has special relativity been disproved, now that FTL speeds have been measured? (N.B. Bold letters are mine , for emphasis) The first problem with this naive conclusion is that, while in special relativity neither information nor energy are allowed to be transmitted faster than light, but that certain velocities in connection with the phenomena of wave transmission may well exceed light speed. For instance, the phase velocity of a wave or the group velocity of a wave packet are not in principle restricted below light speed. The speed connected with wave phenomena that, according to special relativity, must never exceed light speed, is the front velocity of the wave or wave packet. “ Since QM is to a large extent committed to Schrodinger’s wave function , it follows that ,logically at least , QM is also committed to FTL .

Nice link, lots of good references, thanks.

As the author of the review articles says, the consensus is that no FTL signal has been measured.

I've read one of Chiao and Kwiat's papers (involving quantum non-local correlations) in which they explicitly state that no energy has been transferred FTL.

My textbook (Bohm, 1950) states in a few places that quantum non-local correlations do not imply FTL transmissions.

So, you'll have to lay out your reasons for thinking that QM is committed to FTL

I agree that over the years several experiments have been conducted on the lines of the EPR , none of which conclusively proved anything one way or another. This was mainly due to lack of the right equipment. Which is why the reference I had given to a commercially viable quantum encryption system is so vitally important. It means that suppose you produce two quantum entangled photons using PDC and send them to two spatially separated locations , the result is consistent enough to use commercially. i.e ., if the two entangled photons have the same polarization at point of origin , then when the polarization of one of the photons is found the polarization of the other is the same to a consistent degree. Do you agree with this so far ? My point is that since the process is consistent , it should be possible to change the polarization of one of the spatially separated entangled photons and to determine if the other spatially separated photon also undergoes a change in polarization. This would prove conclusively whether FTL does or does not exist .

I know little about PDC. Are you saying that the two photons (which, as I understand it, are harmonics of the pump photon) transmitted by the nonlinear crystal are always polarized the same way --- or is it that their polarizations are always related?

The encryption systems involve parallel settings of the polarizers, don't they? I don't know for sure.

Anyway, say you've detected photon 1 wrt a certain setting of your polarizer. Now, you can change the setting of the polarizer that photon 2 is incident on while photon 2 is in flight. The probability of coincidental detection, in the ideal, is given as cos^2(theta) by qm, where theta is the angular difference between the polarizers. In effect, as you change the setting of the polarizer that photon 2 is incident on, you change the probability of detecting photon 2. How does this tell you whether FTL does or does not exist?

If in fact FTL is found not to exist it would doubts about the whole wave function and QM in general. More important what does this say about the work of Deutsch , Aspect and others ?

I don't know about Deutsch's work, but I've read a few of Aspect et al.'s papers and from what I understand they don't say anything about FTL, but only about whether local hidden variable (lhv, or local realist) formulations for their experimental setups are consistent with the results (and also whether the qm formulation is consistent with the results).

The consensus is that the lhv formulation isn't empirically viable. But, it's the hidden variable or realist part of the lhv formulations that is at odds with the results, not locality --- since the locality condition isn't really a locality condition but rather just an independence condition. A and B aren't independent of each other -- that is, the results at A and B are related to each other. But this has to do with the experimental setup, and not with them being causally related to each other during a given coincidence interval. As qm has it anyway, even without the Bell tests, the polarization of photon 1 is not defined prior to detection, and the polarization of photon 2 is not defined prior to detection (only the relationship between the polarizations of photon 1 and photon 2 is defined prior to detection). So, given the current understanding of polarization, the photons incident on the polarizers during a given coincidence interval can't be assigned specific polarization prior to detection, and hence a local realist (ie., a 'classical') description (at least wrt the current state of the art of polarization) of the incident photons isn't viable for all joint polarizer settings.

But again, at least afaik, quantum theory is not committed to FTL. (Of course, as far as anybody knows, the correlations might be due to FTL transmissions, but such an explanation isn't necessitated, so the assumption of locality is retained.) So far, from what you've written and referred to, such an FTL committment doesn't seem to follow. So, it might be good if you spell out your logical chain of reasoning that leads you to the conclusion that you're advocating.

 

[McQueen]

Anyway, say you've detected photon 1 wrt a certain setting of your polarizer. Now, you can change the setting of the polarizer that photon 2 is incident on while photon 2 is in flight. The probability of coincidental detection, in the ideal, is given as cos^2(theta) by qm, where theta is the angular difference between the polarizers. In effect, as you change the setting of the polarizer that photon 2 is incident on, you change the probability of detecting photon 2. How does this tell you whether FTL does or does not exist?

You don’t have to detect the photon before changing the polarization, because that has already been established to a consistent degree. And if your argument is to hold water , shouldn’t the probability of a photon being detected at A also change with the detection of the photon at B , something which apparently does not seem to happen ?

But again, at least afaik, quantum theory is not committed to FTL. (Of course, as far as anybody knows, the correlations might be due to FTL transmissions, but such an explanation isn't necessitated, so the assumption of locality is retained.) So far, from what you've written and referred to, such an FTL committment doesn't seem to follow. So, it might be good if you spell out your logical chain of reasoning that leads you to the conclusion that you're advocating.

If we look at the background of QM , it becomes clear that FTL is in fact central to many of the basic precepts of QM. I think that we can all agree that the matter waves postulated by Louis de Broglie , which became central to wave-particle duality and Schrodinger’s wave function are one of the key tents of QM. Yet according to de Broglies theory matter waves ( waves of probability according to Schrodinger ) which travel with an electron , move faster than the speed of light . And “……..the slower the electron the faster the velocity of its associated wave. (N.B ., Quote from Sir George Thomson , winner of the Nobel Prize in Physics. From his book “The Atom”) Thus it is the matter wave which guides the electron as to where to go. This being so , how is it possible to claim that FTL is not central to QM. Even the most basic foundations of QM have this concept of FTL inbuilt into them. If we take another approach , we get the same result. What is the size of an electron ? From the fact that an electron can give up all its energy on impact , it must be reasoned that its dimensions are fairly compact and localized , on the other hand since an electron can be influenced by another charge placed at a distance in space , the influence of the electron can be taken as being almost infinite. It is therefore difficult to come to a conclusion as to the exact size of an electron. This interaction or influence that the electron experiences from other electrons is what led to Schrodingers blurred or smeared (electron cloud ) description of the electron within the atom , which later was proved to be wrong. One way to avoid all this confusion is by the particle view of light . Thus if by the wave view of light we have : particle –> electromagnetic field –> particle . Then on the particle view we would have : particle -> photon -> particle . i.e., a charged particle generates a photon which is absorbed by another particle. This is the mechanism which is responsible for the force acting on these particles.

 

[McQueen]

Wrong. A phonon is not a quasiparticle. A phonon is a collective excitation, similar to a photon being an excitation out of a vacuum state.

Shouldn't the correct term be a "Virtual photon".

But, of course, quasi-particles cannot leave the crystal :-)

Is that why phonons travel at the speed of sound in space where there is no medium for which for them to travel through.

 

[ZapperZ]

Shouldn't the correct term be a "Virtual photon".

Why are you so hung-up on the NAME but not the physics involved? I could call it "cow photon", and it won't change a thing about the physics.

I still think you are "learning" these things via bits and pieces that you get off the web. I would be very suspicious of my knowledge if I were you.

Zz.

 

[McQueen]

Why are you so hung-up on the NAME but not the physics involved? I could call it "cow photon", and it won't change a thing about the physics.

I still think you are "learning" these things via bits and pieces that you get off the web. I would be very suspicious of my knowledge if I were you.

OK why can phonons travel at the speed of sound in space. Or are they restricted to within the crystal and if so why ?

 

[vanesch]

OK why can phonons travel at the speed of sound in space. Or are they restricted to within the crystal and if so why ?

They are of course restricted to the crystal. In fact, it does not even make sense to say so, because they are really a numbering scheme of the stationary states of the lattice vibrations ;

You could just as well ask if reciprocal vectors are restricted to the crystal or not.

The reason why phonons are a bit particle-like is because the mathematics looks much the same as the mathematics of the stationary states of the free Dirac field ; these stationary states are called electrons.

 

[ZapperZ]

OK why can phonons travel at the speed of sound in space. Or are they restricted to within the crystal and if so why ?

Considering that a phonon is DEFINED as the quanta of LATTICE VIBRATIONS, how do you propose to have this thing travel in space where there are no LATTICE VIBRATIONS? Phonons are not even well defined in liquid (even though there are loose usage of that term within that scenario), and certainly not defined in a gas. So think of how absurd it is to to talk about it in vacuum. You are taking the concept and then twisting it into ways it wasn't meant to be used.

Zz.

 

[McQueen]

Considering that a phonon is DEFINED as the quanta of LATTICE VIBRATIONS, how do you propose to have this thing travel in space where there are no LATTICE VIBRATIONS? Phonons are not even well defined in liquid (even though there are loose usage of that term within that scenario), and certainly not defined in a gas. So think of how absurd it is to to talk about it in vacuum. You are taking the concept and then twisting it into ways it wasn't meant to be used.

Lattice vibrations , yes , but how do they travel?Are they specially empowered , so that they can control their speed ? I know phonons have , or were , for a long time a well accepted part of physics. But still do they have some kind of special dispensation , that allows them to travel at specific speeds namely that of sound. The whole of Coulomb's theory can be derived from the fact that the photons has no mass. OK so the phonon is not actually a partcile , but surely it still is a vibration?

 

[Sherlock]

You don’t have to detect the photon before changing the polarization, because that has already been established to a consistent degree.

If the photons are polarization-entangled (which is what we want isn't it?), then their polarization isn't established prior to detection. Their polarization is random. Only the entanglement relationship is established -- and, depending on the photon pair source, then with polarizers aligned, A and B will always register the same results, or A and B will always register opposite results.
So let's say we're dealing with polarization-entangled photons that, with polarizers aligned, always register the same result at A and B for a given pair (either both detect, or both don't detect, with polarizers aligned). This is the entanglement relationship between paired photons that are produced by atomic calcium cascades, which were used by Aspect et al. in their 1982 experiment involving time-varying analyzers. Pairing the photons involves associating them with intervals controlled by coincidence circuitry (in the case of the Aspect experiment they wanted to pair photons emitted by the same atom). Since the emission time is a random variable, a coincidence interval is initiated by a detection at either A or B.

And if your argument is to hold water , shouldn’t the probability of a photon being detected at A also change with the detection of the photon at B , something which apparently does not seem to happen ?

The average photon count at A (or B) for, say, a 5 minute run, with polarizer in place is 1/2 what it is without the polarizer. The photon flux at A doesn't depend in any way on what you might do to the B side, and vice versa.
Individual detection is random, uncontrollable, unpredictable. What is controllable is the rate of coincidental detection, which is a function of the angular difference between the polarizers. This suggests that properly paired photons refer to disturbances that are related prior to detection, presumably because, in the process we're considering, they were emitted by the same atom. And, that's how qm treats the situation. No FTL implied.
As I mentioned before, there's no experiment (yet), afaik, that can definitively ascertain that FTL transmissions aren't happening between A and B during a given coincidence interval. But, there's no particular reason to suppose that the correlations must be due to anything FTL. It seems likely that the entanglement is produced via the emission process. The fact that there isn't any geometrical or mechanical visualization accompanying the qm account doesn't reinforce the notion that FTL transmissions must be involved.

If we look at the background of QM , it becomes clear that FTL is in fact central to many of the basic precepts of QM. I think that we can all agree that the matter waves postulated by Louis de Broglie , which became central to wave-particle duality and Schrodinger’s wave function are one of the key tents of QM. Yet according to de Broglies theory matter waves ( waves of probability according to Schrodinger ) which travel with an electron , move faster than the speed of light . And “……..the slower the electron the faster the velocity of its associated wave. (N.B ., Quote from Sir George Thomson , winner of the Nobel Prize in Physics. From his book “The Atom”) Thus it is the matter wave which guides the electron as to where to go. This being so , how is it possible to claim that FTL is not central to QM. Even the most basic foundations of QM have this concept of FTL inbuilt into them.

How is it possible that David Bohm wrote a 646 page textbook on quantum theory without mentioning any of these FTL considerations, except to say that, wrt quantum correlations such as we're considering here, events at A and B are not affecting each other. (Maybe I've missed something.)

Bohm and deBroglie were around at the same time weren't they? If what you say is true about matter waves moving FTL, then how did Bohm get by without mentioning it in a qm textbook?

I'm aware that Bohm published a non-local hidden variable formulation of qm after he wrote the textbook. But, afaik, that formulation was not meant to be a serious contender, but was presented to show that such an alternative, hidden variable formulation (albeit an explicitly non-local one) was possible. Maybe he did it to show that hidden variable formulations are required to be explicitly non-local. I don't know. But, it's clear in his quantum theory textbook that he's proceeding under the assumption that nature obeys the principle of locality. There are several places where he emphasizes this, and also several places where he emphasizes that hidden variable theories are not possible in a local universe wherein the experimental determination of natural processes is limited by a fundamental quantum of action and the uncertainty relations.

 

[DrChinese]

I know little about PDC. Are you saying that the two photons (which, as I understand it, are harmonics of the pump photon) transmitted by the nonlinear crystal are always polarized the same way --- or is it that their polarizations are always related?

A quick note about PDC, because it can be a little confusing. There are 2 types of PDC:

Type I PDC:
The output photons of a tuned non-linear crystal have parallel and known spin, and are NOT in superposition of spin states initially. Their spin is orthogonal (perpendicular) to that of the pump photon. So they are entangled, but not as to spin - which sounds wrong. But this is easily fixed so that we have a usable Bell state for an experiment.

This is accomplished by using 2 crystals instead of one. The crystals are placed in series but 90 degrees apart. The pump stream is placed at a 45 degree angle relative to the crystals. There is a mixing of 2 output beams - i.e. the photon pair comes out of one crystal or the other, but you don't know which. The result is a good superposition of H>H> and V>V> states that can then be tested per usual, with identical polarization.

Type II PDC:
The output photons of a tuned non-linear crystal have perdendicular (orthogonal) and UNknown spin, and ARE in superposition of spin states initially. The superposition is H>V> and V>H>. This only needs a single crystal.

It's a little more complicated than my explanation implies, but this al least disctinguishes the 2 types. I am working on a new page for my site that will explain this pictorially. I will post this when it is available.

 

[Sherlock]

A quick note about PDC, because it can be a little confusing. There are 2 types of PDC:
Type I PDC:
The output photons of a tuned non-linear crystal have parallel and known spin, and are NOT in superposition of spin states initially. Their spin is orthogonal (perpendicular) to that of the pump photon. So they are entangled, but not as to spin - which sounds wrong. But this is easily fixed so that we have a usable Bell state for an experiment.
This is accomplished by using 2 crystals instead of one. The crystals are placed in series but 90 degrees apart. The pump stream is placed at a 45 degree angle relative to the crystals. There is a mixing of 2 output beams - i.e. the photon pair comes out of one crystal or the other, but you don't know which. The result is a good superposition of H>H> and V>V> states that can then be tested per usual, with identical polarization.
Type II PDC:
The output photons of a tuned non-linear crystal have perdendicular (orthogonal) and UNknown spin, and ARE in superposition of spin states initially. The superposition is H>V> and V>H>. This only needs a single crystal.
It's a little more complicated than my explanation implies, but this al least disctinguishes the 2 types. I am working on a new page for my site that will explain this pictorially. I will post this when it is available.

Ok, thanks DrC. I've been meaning to read up on this stuff. :-)

As I understand McQueen's proposed experiment, it would have to involve polarization-entangled photons wrt which the polarization has "already been established to a consistent degree", as he puts it, prior to detection. So, his experiment is impossible to implement. Or is it?

Do you understand what he's talking about?