Polarization of a photon how do we know?

[vanhees71]

Hmm, since I think I am one of the few people on this forum to claim to believe in Copenhagen, let me say Motl is from a different denomination of the church than me :P

So let me link two Copenhagenist views that I think are more doctrinally sound:
http://arxiv.org/abs/1308.5290v2 (ok, maybe this one's a little extreme)
http://mattleifer.info/wordpress/wp-content/uploads/2008/11/commandments.pdf (this one's interesting, since he also advocates hidden variables in http://arxiv.org/abs/1107.5849, footnote 9)

Of course, I don't know whether other Copenhagenists would recognize me as a true believer either!

Hmm, the 1st paper (arXiv:1308.5290) (Sect. 3.1) seems not Copenhagenian to me but rather like the minimal statistical interpretation. It's just to take Born's rule as the prediction of probabilities and nothing else. I don't see that in this description any collapse argument is made, and in Sect. 6 he explicitly denies it in the sense of the minimal interpretation: It's just an update of the probability description after making a (filter) measurement, leading to a new preparation of the system. If you strip Copenhagen from the (imho unnecessary) collapse hypothesis in this way, it's the minimal interpretation, and no trouble occurs. That there are no "spooky actions at a distance" is made also clear in the sense of the minimal interpretation (Sect. 7). That's I also have stated it some time ago in this forum. There's nothing instantaneously changing for Bob's photon when Alice is measuring the polarization of hers. The interactions of Alice's photon with her measurement apparatus is local and doesn't act on Bob's photon. Since there is no collapse in the sense of a dynamical physical process assumed, no problems with Einstein causality occur with this typical "EPR setup". So this is in fact not the Copenhagen interpretation with a collapse but the minimal statistical interpretation. If you want to call it a flavor of Copenhagen, it's fine with me. It's close to Bohr's point of view, although he often referred to an artificial "cut" between quantum behavior of micro systems and macroscopical behavior of measurement apparati. This contradicts, however, QT, because there is no such cut inherent in QT. There are, however, convincing treatments of many-body systems clearly showing, how classical behavior occurs as a coarse-grained description of macro systems, made up by very many microscopic constituents (e.g., the vapor molecules in a cloud chamber or the silicon pixels in a CCD chip detecting particles/photons). This is very clearly described in Sect. 9. I think this is one of the clearest papers describing the needlessness of nonsensical debates on "interpretation", I've ever read! Thanks for pointing it out.

I'd only be a bit more careful concerning the "completeness of quantum theory". As any other theory, also QT is subject to experimental tests. So far it has been confirmed by all these tests, among them some of the highest precision ever achieved in the art of measurements. So I'd say, that so far, there is no evidence for the necessity to revise QT as a physical theory. I'd never say, however, it is impossible to observe a new phenomenon one day that contradicts QT. Then we have to think about a new theory which is more comprehensive than QT as we know it today. That has been the case with all "thought-to-be complete theories" in the history of science. Newtonian mechanics had to be revised by special-relativistic mechanics and classical field theory, which in turn had to be refined to general relativity to also incorporate gravity within a consistent framework. So far that's the closure of classical physics. Maybe one day even at this level the theory has to be adopted to new observations. So far there's nothing in sight I know of. This classical picture had then be revised by quantum theory. So far quantum theory marks also the end of a development of microscopic physics at a certain level. One should, however, admit that still the last stone is missing, namely a consistent description of a quantum (field?) theoretical description of gravity. So far, we have no real theory about it. There are some speculations around, and one might hope to find a quantum description incorporating all phenomena, including gravity.

 

[Johan0001]

hmm, so many interpretations , don't know where to start.

 

[atyy]

Hmm, the 1st paper (arXiv:1308.5290) (Sect. 3.1) seems not Copenhagenian to me but rather like the minimal statistical interpretation. It's just to take Born's rule as the prediction of probabilities and nothing else. I don't see that in this description any collapse argument is made, and in Sect. 6 he explicitly denies it in the sense of the minimal interpretation: It's just an update of the probability description after making a (filter) measurement, leading to a new preparation of the system. If you strip Copenhagen from the (imho unnecessary) collapse hypothesis in this way, it's the minimal interpretation, and no trouble occurs. That there are no "spooky actions at a distance" is made also clear in the sense of the minimal interpretation (Sect. 7). That's I also have stated it some time ago in this forum. There's nothing instantaneously changing for Bob's photon when Alice is measuring the polarization of hers. The interactions of Alice's photon with her measurement apparatus is local and doesn't act on Bob's photon. Since there is no collapse in the sense of a dynamical physical process assumed, no problems with Einstein causality occur with this typical "EPR setup". So this is in fact not the Copenhagen interpretation with a collapse but the minimal statistical interpretation. If you want to call it a flavor of Copenhagen, it's fine with me. It's close to Bohr's point of view, although he often referred to an artificial "cut" between quantum behavior of micro systems and macroscopical behavior of measurement apparati.

It's fine if we call this "Ensemble" or "Minimal Statistical". I don't object to those - for example, I think bhobba's Ensemble Interpretation is correct. What I do object to is Ballentine's, which is wrong. Anyway, in Copenhagen or Ensemble or Minimal Statistical, collapse or state reduction is not real, because the wave function is not real. It is only a tool to calculate the probabilities of events, which are real. It's in the spirit of the famous quote attributed to Bohr, even if modern Copenhagen viewpoints are subtly different: "There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature".

However, the cut is also necessary, as I will explain below. The reason I don't like Ballentine is he fails to mention the cut, and it seems that he is lacking collapse or an equivalent postulate that is necessary to describe filtering experiments.

This contradicts, however, QT, because there is no such cut inherent in QT. There are, however, convincing treatments of many-body systems clearly showing, how classical behavior occurs as a coarse-grained description of macro systems, made up by very many microscopic constituents (e.g., the vapor molecules in a cloud chamber or the silicon pixels in a CCD chip detecting particles/photons). This is very clearly described in Sect. 9. I think this is one of the clearest papers describing the needlessness of nonsensical debates on "interpretation", I've ever read! Thanks for pointing it out.

In both Copenhagen and Ensemble the cut is necessary. Although this means that not eveything is quantum, it doesn't mean that not everything can be part of something quantum. So atoms, solids with ~10²³ molecules, and even a huge subsystem like the early universe in Mukhanov and Chibisov's calculation can be treated by quantum mechanics. But in this interpretation, it simply doesn't make sense to assign a "wave function of the universe". So there always is a cut, analogous to the cut in statistical mechanics. The cut in statistical mechanics is due to the fact that equilibrium is subjective. The expanding universe clearly means nothing is in true equilibrium, yet we can treat small slices of time in the expanding early universe as "in equilibrium".

I'd only be a bit more careful concerning the "completeness of quantum theory". As any other theory, also QT is subject to experimental tests. So far it has been confirmed by all these tests, among them some of the highest precision ever achieved in the art of measurements. So I'd say, that so far, there is no evidence for the necessity to revise QT as a physical theory. I'd never say, however, it is impossible to observe a new phenomenon one day that contradicts QT. Then we have to think about a new theory which is more comprehensive than QT as we know it today. That has been the case with all "thought-to-be complete theories" in the history of science. Newtonian mechanics had to be revised by special-relativistic mechanics and classical field theory, which in turn had to be refined to general relativity to also incorporate gravity within a consistent framework. So far that's the closure of classical physics. Maybe one day even at this level the theory has to be adopted to new observations. So far there's nothing in sight I know of. This classical picture had then be revised by quantum theory. So far quantum theory marks also the end of a development of microscopic physics at a certain level. One should, however, admit that still the last stone is missing, namely a consistent description of a quantum (field?) theoretical description of gravity. So far, we have no real theory about it. There are some speculations around, and one might hope to find a quantum description incorporating all phenomena, including gravity.

Yes, I agree, but perhaps for a different reason. Once there is a cut, provided one doesn't rule out a more complete theory without a cut, then quantum mechanics is incomplete. So in a sense, the two great conceptual advances in physics in the second half of the twentieth century both had to do with showing how it is reasonable to think of our best present theories as being necessarily incomplete, even in though they have not been falsified by experiments. In Bohm's case, he showed that the Copenhagen interpretation is consistent with quantum mechanics as an effective theory. Similarly, Wilson showed that QED is best understood as an effective field theory.

Gravity is interesting, I think there are very strong indications that at least in AdS spaces there seems to be a UV complete quantum theory of Einstein gravity, but the matter content and cosmology are not realistic.

 

[Johan0001]

Everything should be made as simple as possible, but not simpler

 

[bhobba]

hmm, so many interpretations , don't know where to start.

Start with Ballentine - QM - A Modern Development:
https://www.amazon.com/dp/9810241054/?tag=pfamazon01-20

There the Ensemble, also sometimes called the Minimal Statistical Interpretation, is carefully explained.

Once you understand that you can branch out.

Griffith's book on Consistent Histories is also very good, and he has been kind enough to post it up:
http://quantum.phys.cmu.edu/CQT/index.html

But I personally think that interpretation, while nice, is a bit more complex than necessary. In that interpretation observations don't exist - QM is a stochastic theory of histories - a history is a sequence of projection operators. Already you can see its a bit deep - some call it many worlds without the many worlds.

I hold to a slightly modified version of the ensemble to get around the observation occurring in a classical world thing.

I define an observation as when decoherence has occurred which is a purely quantum process. I then say the improper mixture is a proper one and without any further ado no issues - other than the assumption you make of course - which, from many discussions here I can assure you is not accepted by many as really resolving anything - but that is a whole new thread . Its sometimes called the ignorance ensemble interpretation. It's really the same as the usual Ensemble interpretation - but only applied to improper mixtures.

Thanks
Bill

 

[bhobba]

Everything should be made as simple as possible, but not simpler

Yea - trouble is what is simpler is not universally agreed.

If you want to delve into interpretations best, like I said, to start with Ballentine - then branch out.

He not only carefully explains the Ensemble interpretation, but you will learn the correct axiomatic treatment and understand where things like Schroedinger's equation etc really comes from (symmetry) and the QM is contained in just two axioms. They are the axioms that need interpreting which greatly simplifies the issue.

Thanks
Bill

 

[vanhees71]

However, the cut is also necessary, as I will explain below. The reason I don't like Ballentine is he fails to mention the cut, and it seems that he is lacking collapse or an equivalent postulate that is necessary to describe filtering experiments.

But there is no such cut in quantum mechanics. The macroscopic behavior of macroscopic systems is an emergent phenomenon which is, to a certain extent, understandable from quantum many-body theory. Of course, Bohr is right in saying that we can learn about quantum systems only by making measurements with macroscopic measurement devices which allow to fix (practically) irreversibly the outcome of this measurement. Nevertheless, the measurement apparatus or any other macroscopic system that behaves according to classical physics, itself is not something contradicting quantum theory, but the relevant macroscopic observables are coarse grained (averaged) quantities over very many microscopic degrees of freedom.

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

 

[bhobba]

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

These days now we understand decoherence a lot better there is no need for a cut.

We simply say an effective observation has occurred after decoherence.

Thanks
Bill

 

[atyy]

But there is no such cut in quantum mechanics. The macroscopic behavior of macroscopic systems is an emergent phenomenon which is, to a certain extent, understandable from quantum many-body theory. Of course, Bohr is right in saying that we can learn about quantum systems only by making measurements with macroscopic measurement devices which allow to fix (practically) irreversibly the outcome of this measurement. Nevertheless, the measurement apparatus or any other macroscopic system that behaves according to classical physics, itself is not something contradicting quantum theory, but the relevant macroscopic observables are coarse grained (averaged) quantities over very many microscopic degrees of freedom.

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

The cut doesn't mean that things on the classical side cannot also be included in a quantum description. What the cut means is that although we can shift it and place it in many different places, we cannot get it of it completely, because within Copenhagen and Ensemble interpretations, we don't know what physical meaning the "wave function of the universe" can have. The coarse graining doesn't solve the problem, because the underlying fine grained theory must also make sense, but it doesn't appear that an underlying fine grained quantum theory with only unitary evolution of the wave function can be extended to the whole universe, unless one takes something like a Many-Worlds approach.