EPR & Many-Worlds: Understanding the Basics

[mysearch]

I am in the process of trying to understand, in basic terms, how the many-worlds interpretation (MWI) might explain the EPR experiment. In this context it is hoped that the scope for what appears to be almost unlimited speculation might be reduced. It seems that the original EPR experiment of 1935 was initially trying to disprove Heisenberg’s uncertainty principle. Later this description was modified, by Bohm (?), into a pion system disintegrating into an electron and positron, which then puts the issue of locality into better perspective. Based on limited reading, see links below, it seems that the MWI (1957+) assumes that the wave function does not collapse and does not depend on an observer. As such:

Does the MWI argue that the wave function has to have some form of physical existence?
Presumably, no physical verification of this wave has ever been detected?

While the following example is more than a little contrived, it is only seeking to clarify a limited number of basic issues within the context defined above, if at all possible. The example is orientated around two observers on two space-stations, e.g. A & B, separated by 10 light-years. Therefore, any ‘normal’ signalling would take a minimum of 10 years, if restricted to light speed. At a location close to A, a quantum system is evolving in time, according to Schrodinger’s wave equation (?), such that a composite pion particle with zero charge and spin disintegrates into an electron and a positron, where each must ultimately have opposite spin. While presumably still in a quantum entangled state, the electron and positron are described as ‘propagating’ in opposite directions, through the vacuum of space, towards A & B respectively.

Does it make sense to describe the pion particle disintegrating into an electron and a positron, while in a quantum superposition wave state?

Can quantum waves propagate energy and momentum, if only a mathematical construct?

On the assumption that the original pion was close to A, the electron arrives at A in a matter of days, while it would take the positron 10+? years to reach B. At A, an observer measures the spin of the electron, but within the context of the MWI, it is assumed that this observer now has to split into two realities, i.e. one who measures up-spin, and one who measures down-spin, as the MWI appears to suggest that the wave function doesn’t collapse, therefore both spin probabilities have to exist, albeit in different realities.

Is the description above basically valid within the many-worlds interpretation?

In the first split reality of A, it would seem that there is an electron with down-spin, such that there must be a positron with up-spin heading towards B; while in the other split reality, there is an electron with up-spin with a positron with down-spin heading towards B. As such, there appears to be the suggestion that two split observers must eventually exist at B, i.e. in different but associated realities, destined to measure a positron with the corresponding spin, but this interpretation then appears to raise a number of questions:

As an entangled system, do both spin permutations instantly exist when the observer at A makes the measurement on the electron?
When do the two split observers at B come into existence?
Does the inference of the MWI ultimately propagate outwards to become many-universes and, if so, is the propagation assumed to be instantaneous?
Does the MWI account for the apparent duplication of energy in each reality?
Does it make sense to assume that a quantum superposition exists in normal spacetime?

Sorry, this is possibly too many questions for one post, but I wanted to just table the issues I am trying to resolve within this thread as a whole. Thanks

Initial cross references:

Wikipedia:
http://en.wikipedia.org/wiki/Many-worlds_interpretation
http://en.wikipedia.org/wiki/Quantum_decoherence

The Everett FAQ:
One of the reasons in raising this thread was to help in the process of understanding all the arguments put forward in this link:
http://www.hedweb.com/manworld.htm

PF Threads:
Two recent PF discussions involving many exchanges.
https://www.physicsforums.com/showthread.php?t=522754&highlight=EPR+many-worlds
The first post of this thread is worth a read. I haven’t really reviewed all 655 entries.May be somebody can point to any that are particularly relevant.

https://www.physicsforums.com/showthread.php?t=497228&highlight=EPR+many-worlds
Thread discussed some of the perceived problems. Again, I have not yet reviewed all 210 entries, but didn't come across any that seem to specifically addressed my more basic issues.

 

[juanrga]

I am in the process of trying to understand, in basic terms, how the many-worlds interpretation (MWI) might explain the EPR experiment. In this context it is hoped that the scope for what appears to be almost unlimited speculation might be reduced. It seems that the original EPR experiment of 1935 was initially trying to disprove Heisenberg’s uncertainty principle. Later this description was modified, by Bohm (?), into a pion system disintegrating into an electron and positron, which then puts the issue of locality into better perspective. Based on limited reading, see links below, it seems that the MWI (1957+) assumes that the wave function does not collapse and does not depend on an observer. As such:

Does the MWI argue that the wave function has to have some form of physical existence?
Presumably, no physical verification of this wave has ever been detected?

While the following example is more than a little contrived, it is only seeking to clarify a limited number of basic issues within the context defined above, if at all possible. The example is orientated around two observers on two space-stations, e.g. A & B, separated by 10 light-years. Therefore, any ‘normal’ signalling would take a minimum of 10 years, if restricted to light speed. At a location close to A, a quantum system is evolving in time, according to Schrodinger’s wave equation (?), such that a composite pion particle with zero charge and spin disintegrates into an electron and a positron, where each must ultimately have opposite spin. While presumably still in a quantum entangled state, the electron and positron are described as ‘propagating’ in opposite directions, through the vacuum of space, towards A & B respectively.

Does it make sense to describe the pion particle disintegrating into an electron and a positron, while in a quantum superposition wave state?

Can quantum waves propagate energy and momentum, if only a mathematical construct?

On the assumption that the original pion was close to A, the electron arrives at A in a matter of days, while it would take the positron 10+? years to reach B. At A, an observer measures the spin of the electron, but within the context of the MWI, it is assumed that this observer now has to split into two realities, i.e. one who measures up-spin, and one who measures down-spin, as the MWI appears to suggest that the wave function doesn’t collapse, therefore both spin probabilities have to exist, albeit in different realities.

Is the description above basically valid within the many-worlds interpretation?

In the first split reality of A, it would seem that there is an electron with down-spin, such that there must be a positron with up-spin heading towards B; while in the other split reality, there is an electron with up-spin with a positron with down-spin heading towards B. As such, there appears to be the suggestion that two split observers must eventually exist at B, i.e. in different but associated realities, destined to measure a positron with the corresponding spin, but this interpretation then appears to raise a number of questions:

As an entangled system, do both spin permutations instantly exist when the observer at A makes the measurement on the electron?
When do the two split observers at B come into existence?
Does the inference of the MWI ultimately propagate outwards to become many-universes and, if so, is the propagation assumed to be instantaneous?
Does the MWI account for the apparent duplication of energy in each reality?
Does it make sense to assume that a quantum superposition exists in normal spacetime?

Sorry, this is possibly too many questions for one post, but I wanted to just table the issues I am trying to resolve within this thread as a whole. Thanks

Initial cross references:

Wikipedia:
http://en.wikipedia.org/wiki/Many-worlds_interpretation
http://en.wikipedia.org/wiki/Quantum_decoherence

The Everett FAQ:
One of the reasons in raising this thread was to help in the process of understanding all the arguments put forward in this link:
http://www.hedweb.com/manworld.htm

PF Threads:
Two recent PF discussions involving many exchanges.
https://www.physicsforums.com/showthread.php?t=522754&highlight=EPR+many-worlds
The first post of this thread is worth a read. I haven’t really reviewed all 655 entries.May be somebody can point to any that are particularly relevant.

https://www.physicsforums.com/showthread.php?t=497228&highlight=EPR+many-worlds
Thread discussed some of the perceived problems. Again, I have not yet reviewed all 210 entries, but didn't come across any that seem to specifically addressed my more basic issues.

There is not a «many-worlds interpretation (MWI)». MWI is an inconsistent theory that is not equivalent to QM.

This has been discussed in PF and references were given as well.

 

[mysearch]

There is not a «many-worlds interpretation (MWI)». MWI is an inconsistent theory that is not equivalent to QM. This has been discussed in PF and references were given as well.

Succinct, despite the duplication of the entire opening post, but ultimately not that helpful. Is it PF policy that nobody is allowed to raise this topic from now on or just your opinion?

http://www.hedweb.com/manworld.htm#believes
https://www.physicsforums.com/poll.php?do=showresults&pollid=1995
http://arnold-neumaier.at/physfaq/topics/manyworlds

However, I am also confused as to whether your statement about MWI, based on the polls above, is your personal opinion or now reflected in accepted opinion?

I believe my first post, as you point out, acknowledged that the MWI has been previously discussed, many times, but doing a search on ‘EPR many worlds’ produces no threads that identify both topics in the title. Doing the same search on posts identifies 468 threads, which would take a very long time to read through. While the PF can be an excellent source of helpful information, it is not always the easiest to used as a cross-reference. Likewise, while the web offers much source material, the diversity of opinion, especially in the field of QM, can be problematic when trying to ascertain what is accepted opinion. For example, it is not easy to reconcile your opinion with the Wikipedia reference given in post #1.

However, taking your advice, I have attempted to review the two PF threads identified in post #1 for references to MWI and EPR:

https://www.physicsforums.com/showpost.php?p=3304755&postcount=64
https://www.physicsforums.com/showpost.php?p=3321416&postcount=136

I am not sure that these brief comments are enough to help me resolve the questions raised in this thread. I also searched for your contributions to the MWI discussion and was initially confused by the fact you appear to have changed your ID from ‘kith’. Apologises if this is not so. Under the ‘kith’ ID I can find a number of equally succinct posts in the ‘Problems with Many Worlds Interpretation’ thread. The following example seems representative of your assertion in this thread:

https://www.physicsforums.com/showpost.php?p=3572121&postcount=609

However, I see no specific discussion of EPR and MWI, which was the root of my questions in this thread. Please understand, I am not arguing for MWI, simply trying to learn about it from anybody willing to lend a helping hand. Of course, if nobody wants to discuss this topic that is fair enough and the thread will simply die a ‘natural death’.

 

[juanrga]

There is not a «many-worlds interpretation (MWI)». MWI is an inconsistent theory that is not equivalent to QM. This has been discussed in PF and references were given as well.

Succinct, despite the duplication of the entire opening post, but ultimately not that helpful. Is it PF policy that nobody is allowed to raise this topic from now on or just your opinion?

Where in my post (which you quote) you read what you are asking?

http://www.hedweb.com/manworld.htm#believes
https://www.physicsforums.com/poll.php?do=showresults&pollid=1995
http://arnold-neumaier.at/physfaq/topics/manyworlds

However, I am also confused as to whether your statement about MWI, based on the polls above, is your personal opinion or now reflected in accepted opinion?

Where I said that my opinion about MWI is based in polls?

I believe my first post, as you point out, acknowledged that the MWI has been previously discussed, many times, but doing a search on ‘EPR many worlds’ produces no threads that identify both topics in the title. Doing the same search on posts identifies 468 threads, which would take a very long time to read through. While the PF can be an excellent source of helpful information, it is not always the easiest to used as a cross-reference. Likewise, while the web offers much source material, the diversity of opinion, especially in the field of QM, can be problematic when trying to ascertain what is accepted opinion. For example, it is not easy to reconcile your opinion with the Wikipedia reference given in post #1.

I would not trust wikipedia at this point, several alternatives to it are based in its general low quality as academic reference.

However, taking your advice, I have attempted to review the two PF threads identified in post #1 for references to MWI and EPR:

https://www.physicsforums.com/showpost.php?p=3304755&postcount=64
https://www.physicsforums.com/showpost.php?p=3321416&postcount=136

I am not sure that these brief comments are enough to help me resolve the questions raised in this thread. I also searched for your contributions to the MWI discussion and was initially confused by the fact you appear to have changed your ID from ‘kith’. Apologises if this is not so. Under the ‘kith’ ID I can find a number of equally succinct posts in the ‘Problems with Many Worlds Interpretation’ thread.

Really? Also when kith discussed by me :rofl:

https://www.physicsforums.com/showpost.php?p=3574558&postcount=618

Sorry but (s)he is not me. Best ask her/him about what said/think.

The following example seems representative of your assertion in this thread:

https://www.physicsforums.com/showpost.php?p=3572121&postcount=609

Yes, I think that is very representative of the status of MWI as a misinterpretation of QM.

However, I see no specific discussion of EPR and MWI, which was the root of my questions in this thread. Please understand, I am not arguing for MWI, simply trying to learn about it from anybody willing to lend a helping hand. Of course, if nobody wants to discuss this topic that is fair enough and the thread will simply die a ‘natural death’.

Basically you want to discuss (your own words) «how the many-worlds interpretation (MWI) might explain the EPR experiment». I.e., how an internally inconsistent 'theory' that does not agree with experience (see references cited) would explain an experimental result

Moreover, I will add now that many of your red-color-questions are explained in ordinary textbooks on QM.

 

[mysearch]

Juanrga,
I fear that responding to your last post on a point by point basis will do neither of us any good and not benefit the goals of this forum. However, I will happily apologise for the confusion of mistakenly linking you with ‘kith’.

I guess many people come to the forum with questions, which those who have already acquired knowledge of the subject might reasonably expect them to find in textbooks, assuming that they have access to the right ones. However, the issues surrounding the various interpretations of quantum mechanics seems to be very broad and to some extent subjective, i.e. depending on whether it is sourced by a physicist, mathematician or philosopher. As such, my hope was to get the perspective of some of the members of this forum, who clearly already understand the subject better than I.

As I stated in my last post, I did attempt to review previous threads and other sources, but felt that they did not directly address the questions raised in post #1. So rather than engage in a pointless exchange of issues, which seem somewhat tangential to my original purpose of this thread, may I simply ask you which interpretation you favour or alternatively for you point to any references that might help answer my specific questions. Of course, I will understand if you wish to refrain from further comment.

 

[kith]

Really? Also when kith discussed by me :rofl:
https://www.physicsforums.com/showpost.php?p=3574558&postcount=618
Sorry but (s)he is not me.

I can confirm that.

As far as the MWI is concerned, I do think that it is a valid interpretation and I think that this is the mainstream scientific view. At least there is no consensus that the MWI has been "disproven". There are of course arguments against certain aspects of certain versions of the MWI. However, I am in no way an expert on the MWI.

I don't have much time to contribute to your actual questions in this thread, so I'll just give you a brief general comment. You seem to divide waves in two categories: physical ones like electrodynamical waves and "mathematical" ones like "quantum waves". I don't think such a separation is justified. Both kinds produce the same observable phenomena (interference on the one hand and absorption as a whole on the other hand). In the old days, this was called the wave-particle-duality, while a modern description would probably involve quantum field theory (unfortunately, it is not so straightforward to treat single photons with non-relativistic QM).

 

[mysearch]

I can confirm that.

Kith, again, my apologises for the confusion. When I was searching the posts by ‘juanrga’ it seemed to be pointing me to posts by you.

As far as the MWI is concerned, I do think that it is a valid interpretation and I think that this is the mainstream scientific view. At least there is no consensus that the MWI has been "disproven". There are of course arguments against certain aspects of certain versions of the MWI. However, I am in no way an expert on the MWI.

I also got the impression that quite a few people supported the MWI. To honest, while my opinion carries no weight, I find it difficult to accept this interpretation and initially favoured something like the many-minds interpretation – see issue below for one possible reason. However, it is unclear to me how any of the interpretations might be subject to what might be called empirical verification.

You seem to divide waves in two categories: physical ones like electrodynamical waves and "mathematical" ones like "quantum waves". I don't think such a separation is justified. Both kinds produce the same observable phenomena (interference on the one hand and absorption as a whole on the other hand). In the old days, this was called the wave-particle-duality, while a modern description would probably involve quantum field theory (unfortunately, it is not so straightforward to treat single photons with non-relativistic QM).

You could well be right, but I have previously tried to get clarification of the following comment:

According to the standard formalism of QM, there's no more distinction between particles and waves, as these 2 concepts, as I said before, actually pertain to classical physics, namely the mechanics of point particles and waves (including electromagnetism). So <quantum waves> as a concept does not exist. The fundamental concepts of QM are: (quantum) system, states and observables of a system and virtual statistical ensembles. The rest is essentially mathematics.

As I understood this comment, it would appear to suggest that classical concepts of particles and waves, including EM waves, do not align to the QM description at the quantum level of existence, i.e. they cease to be meaningful? However, if I take the inference that quantum waves are essentially a mathematical construct, I am left wondering as to what, if anything, is left on which any form of physical quantum reality could be built, i.e. what transports energy and momentum from A to B? In part, it was this type of issue that led me to start reviewing the scope of interpretations to see whether any of them help explain this situation.

 

[kith]

People like Schrödinger thought that elementary physical objects could be explained by a dual viewpoint: during propagation through space, they behave as waves, while in measurements, they behave as particles. For simple single-particle systems, such a viewpoint is possible. In general, it is not.

Consider a system of two particles. Classically, it is always possible to assign a state (x,p) to each of the individual systems. Quantum mechanically, it is not! There are entangled states psi(x1,x2), which cannot be expressed by individual particle states psi(x1) and psi(x2). This means, you can't view psi(x1,x2) as a wave which propagates in the 3D physical space, but in an abstract 6D space, which is called configuration space. This has led to the more general Hilbert-space-formalism (which is what dextercioby briefly summarized in his comment).

So the standard interpretation (Copenhagen) states that quantum mechanics simply does not tell you what's going on in physical space between measurements. Schematically, you start your experiment with a physical preparation procedure, which puts your system in an initial (quantum) state. This state lives in an abstract space. Then, it propagates in the abstract space until the moment you perform another measurement. The quantum mechanical formalism now assigns probabilities for every possible outcome to your final state. And one of this possibilities is realized.

The question what exactly constitutes a measurement, is the core of the so-called measurement problem. Certainly, every "classical" measurement apparatus like a phosphor screen does.

 

[mysearch]

Really appreciate you taking the time to provide a very useful summary, which I will attempted to read up in more detail. As such, my comments are mainly as notes to myself to follow-up:

People like Schrödinger thought that elementary physical objects could be explained by a dual viewpoint: during propagation through space, they behave as waves, while in measurements, they behave as particles. For simple single-particle systems, such a viewpoint is possible. In general, it is not.

Of course, even as a single particle system, one might question what ‘substance’ the particle is made of. In part, my focus on the wave description simply reflects the only mechanism I know that appears ‘physically’ capable of transporting energy and momentum at the ‘lowest’ level of reality. Of course, this doesn’t mean that it is, only that it is the mechanism I know of.

Consider a system of two particles. Classically, it is always possible to assign a state (x,p) to each of the individual systems. Quantum mechanically, it is not! There are entangled states psi(x1,x2), which cannot be expressed by individual particle states psi(x1) and psi(x2). This means, you can't view psi(x1,x2) as a wave which propagates in the 3D physical space, but in an abstract 6D space, which is called configuration space. This has led to the more general Hilbert-space-formalism (which is what dextercioby briefly summarized in his comment).

I do not have the mathematical background to comment too much at the moment. However, based on limited reading, I thought Hamilton mechanics helped defined a coordinate system based on 3 degrees of momentum [p] and 3 degrees of position [q], i.e. 6 numbers in total. Is this what you call 6D space? In contrast, I thought configuration space requires 3 numbers to define the position of each particle + time. Therefore, configuration space is defined by 3N+1, where N is the number of particles in the system. This comment is not an assertion, simply an issue I need to later clarify.

So the standard interpretation (Copenhagen) states that quantum mechanics simply does not tell you what's going on in physical space between measurements. Schematically, you start your experiment with a physical preparation procedure, which puts your system in an initial (quantum) state. This state lives in an abstract space. Then, it propagates in the abstract space until the moment you perform another measurement. The quantum mechanical formalism now assigns probabilities for every possible outcome to your final state. And one of this possibilities is realized.

This is the bit I am finding the most difficult to grasp in QM, i.e. what is applied physics that is verifiable by experiment and what is theoretical physics based on mathematical or even philosophical assumption/premise. In this context, the number of interpretations, which are said not to contradict QM, only seems to make the factual basis even more difficult to pin down, but I like your summary. Thanks

The question what exactly constitutes a measurement, is the core of the so-called measurement problem. Certainly, every "classical" measurement apparatus like a phosphor screen does.

Cross reference for later.
http://en.wikipedia.org/wiki/Measurement_problem

 

[juanrga]

Juanrga,
I fear that responding to your last post on a point by point basis will do neither of us any good and not benefit the goals of this forum. However, I will happily apologise for the confusion of mistakenly linking you with ‘kith’.

I guess many people come to the forum with questions, which those who have already acquired knowledge of the subject might reasonably expect them to find in textbooks, assuming that they have access to the right ones. However, the issues surrounding the various interpretations of quantum mechanics seems to be very broad and to some extent subjective, i.e. depending on whether it is sourced by a physicist, mathematician or philosopher. As such, my hope was to get the perspective of some of the members of this forum, who clearly already understand the subject better than I.

As I stated in my last post, I did attempt to review previous threads and other sources, but felt that they did not directly address the questions raised in post #1. So rather than engage in a pointless exchange of issues, which seem somewhat tangential to my original purpose of this thread, may I simply ask you which interpretation you favour or alternatively for you point to any references that might help answer my specific questions. Of course, I will understand if you wish to refrain from further comment.

Of course, you are welcome to ask anything and you are, of course, welcome to have your own opinion about any issue. Another thing is that I cannot say my opinion as well.

About MWI I have already stated my opinion in a precise and concise way. I also cited (in the threads that you know) a FAQ, a website, and a journal article explaining why MWI is not an interpretation of QM.

QM is a well-defined theory, well-covered in many excellent textbooks (e.g., that by Cohen et al.). Besides that one finds misconceptions of QM under the names of MWI, ensemble, and others...

EPR was a failed attempt (from people who never understood QM) to discredit what they considered 'spooky action at a distance'. Essentially you are asking my about the relation of nonsense to nonsense

 

[mysearch]

Of course, you are welcome to ask anything and you are, of course, welcome to have your own opinion about any issue. Another thing is that I cannot say my opinion as well…
EPR was a failed attempt (from people who never understood QM) to discredit what they considered 'spooky action at a distance'. Essentially you are asking me about the relation of nonsense to nonsense

Juanrga, I appreciate your balanced comments. I suspect you may well be in a better position than me to have an informed opinion. I accept that I am still at the stage where I have lots of questions, possibly quite annoyingly so, therefore a helping hand is much appreciated! However, I thought Feynman said that nobody understood QM?

 

[kith]

Of course, even as a single particle system, one might question what ‘substance’ the particle is made of.

Well, elementary particles are the substance. They are the true atoms in the sense that they can't be divided any further. If you ask for a meaning beyond this, I think your question becomes philosophical.

I do not have the mathematical background to comment too much at the moment. However, based on limited reading, I thought Hamilton mechanics helped defined a coordinate system based on 3 degrees of momentum [p] and 3 degrees of position [q], i.e. 6 numbers in total. Is this what you call 6D space? In contrast, I thought configuration space requires 3 numbers to define the position of each particle + time. Therefore, configuration space is defined by 3N+1, where N is the number of particles in the system.

No, I was referring to the configuration space not phase space. Since position and momentum cannot be known simultaneously in QM, phase space is "too large". You get the basic concept of these spaces, but the mathematical details are not that important. The bottom line is: in classical mechanics, you can always reduce motion in abstract spaces to motion in physical 3D space, in QM you can't.

This is the bit I am finding the most difficult to grasp in QM, i.e. what is applied physics that is verifiable by experiment and what is theoretical physics based on mathematical or even philosophical assumption/premise.

As far as simple QM is concerned, every theoretical result is backed by experimental evidence. The mathematical formalism of QM (in the form of axioms) is definately the heart of QM. But I agree that it sometimes hard to distinguish if a statement is about the formalism or about the (more philosophical) interpretation of the formalism.

 

[kith]

QM is a well-defined theory, well-covered in many excellent textbooks (e.g., that by Cohen et al.). Besides that one finds misconceptions of QM under the names of MWI, ensemble, and others...

This is your personal opinion and in no way the scientific mainstream view. Please state this explicitly when you make such claims in beginner's threads.

Also, it's an opinion I still don't understand. Do you think that every interpretation other than the Copenhagen interpretation is a misconception?

 

[mysearch]

Again, my comments are primarily to myself to follow up on issues raised, but would welcome any clarifications on offer.

Well, elementary particles are the substance. They are the true atoms in the sense that they can't be divided any further. If you ask for a meaning beyond this, I think your question becomes philosophical.

I understand why you might say this, but the Greeks defined the atoms as an indivisible elementary particle, more as a philosophical statement, because their science didn’t allow them to investigate further – see comment about Copenhagen Interpretation at the end of the post. Therefore, I would argue that my question about the ‘substance’ of a particle is scientific and that many of the answers floating around in QM are more philosophical, at least, in terms of many of the interpretations being suggested and the apparent mathematical nature of the wave function.

No, I was referring to the configuration space not phase space. Since position and momentum cannot be known simultaneously in QM, phase space is "too large". You get the basic concept of these spaces, but the mathematical details are not that important. The bottom line is: in classical mechanics, you can always reduce motion in abstract spaces to motion in physical 3D space, in QM you can't.

Thanks for the clarification. Some references to check out:
http://en.wikipedia.org/wiki/Phase_space
http://en.wikipedia.org/wiki/Configuration_space
As a slightly tangential issue, I was wondering if anybody had any opinions on the following article, i.e. does QM take place in 3N+1 configuration space or just 3D space. I am not sure whether we are talking physically or mathematically at some points in this discussion.
Quantum Mechanics and 3N-Dimensional Space

As far as simple QM is concerned, every theoretical result is backed by experimental evidence. The mathematical formalism of QM (in the form of axioms) is definately the heart of QM. But I agree that it sometimes hard to distinguish if a statement is about the formalism or about the (more philosophical) interpretation of the formalism.

From my initial reading into QM, the single biggest issue that I am trying to get into perspective concerns a ‘science’ that seems able to define a start state |A> and then predict end state |B> with great accuracy, but not really able to quantify what happens in between. I am sure those who have a better understand might jump on this statement, but all I am referring to is my ‘perception’ that the time evolution of the Schrodinger equation, i.e. simple free particle case, seems to be predicated entirely on a mathematical premise with only the end state conclusion empirically verified. Again, if this is a wrong perception, I would welcome any corrections. Thanks.

Finally, on the issue of QM interpretations, a PF poll in 2011, still seems to favour the Copenhagen Interpretation. Is this because it appears to side-step many of the fundamental issues or do people really think certain aspect of quantum reality can never be answered?

 

[juanrga]

Juanrga, I appreciate your balanced comments. I suspect you may well be in a better position than me to have an informed opinion. I accept that I am still at the stage where I have lots of questions, possibly quite annoyingly so, therefore a helping hand is much appreciated! However, I thought Feynman said that nobody understood QM?

I know that Feynman quote. Also Gell-Mann has said that QM is not weird or strange but just QM. I am not saying that QM was easy to learn, but I think that there exists a lot of misconceptions and myths behind QM. And I suspect this thought was behind Gell-Mann quote.

 

[juanrga]

This is your personal opinion and in no way the scientific mainstream view. Please state this explicitly when you make such claims in beginner's threads.

That QM is a well-defined theory, well-covered in many excellent textbooks (e.g., that by Cohen-Tannoudji et al.) is a fact.

That one finds misconceptions of QM under the names of MWI, ensemble, and others... is also a fact. The literature is there and I have cited often.

Also, it's an opinion I still don't understand. Do you think that every interpretation other than the Copenhagen interpretation is a misconception?

I have not said that. The Copenhagen interpretation is the standard developed by the fathers of QM and it is what one finds in textbooks, including the well-known two volume set by Cohen-Tannoudji et al. (https://www.amazon.com/dp/0471569526/?tag=pfamazon01-20) {*}

Besides that one finds misconceptions as those cited, but I have not studied any published or imagined interpretation and cannot say.

{*} In another thread I saw perplexed as people who was discussing about the foundations of QM recognized that they did not even read this basic book, which must explains why they say so many nonsense about QM.

 

[kith]

From my initial reading into QM, the single biggest issue that I am trying to get into perspective concerns a ‘science’ that seems able to define a start state |A> and then predict end state |B> with great accuracy, but not really able to quantify what happens in between.

If by quantify you mean quantify in 3D physical space, that's correct.

I am sure those who have a better understand might jump on this statement, but all I am referring to is my ‘perception’ that the time evolution of the Schrodinger equation, i.e. simple free particle case, seems to be predicated entirely on a mathematical premise with only the end state conclusion empirically verified.

That's correct. But this is also the case in classical mechanics. Scientific knowledge is obtained by measurements. What happens between measurements can be extrapolated mathematically, but not verified. I think it really boils down to the already mentioned motion in different spaces. The reason why we intuitively prefer the 3D physical space to the abstract Hilbert space is, that it corresponds better to our daily experience. And this experience could very well just be an emergent property of fundamentally different physical laws for elementary particles.

I think this is the main point, why many people favor agnostic interpretations like Copenhagen. Other important reasons are, that it was advocated by the founding fathers and has become the "standard language" in textbooks, and that most physicists are more interested in doing science than in philosophical rambling. ;)

 

[kith]

That QM is a well-defined theory, well-covered in many excellent textbooks (e.g., that by Cohen-Tannoudji et al.) is a fact.

Yes.

That one finds misconceptions of QM under the names of MWI, ensemble, and others... is also a fact.

No. Two website links and one journal article are not sufficient to make a claim a fact. There are numerous peer-reviewed articles about the interpretations you reject.

{*} In another thread I saw perplexed as people who was discussing about the foundations of QM recognized that they did not even read this basic book, which must explains why they say so many nonsense about QM.

Just because someone hasn't read Cohen-Tannoudji doesn't mean he hasn't read any standard text about QM. Or do you think CT is "more" standard than all others? If yes, why?

 

[mysearch]

I would not trust wikipedia at this point, several alternatives to it are based in its general low quality as academic reference……Moreover, I will add now that many of your red-color-questions are explained in ordinary textbooks on QM.

The Copenhagen interpretation is the standard developed by the fathers of QM and it is what one finds in textbooks, including the well-known two volume set by Cohen-Tannoudji et al. (https://www.amazon.com/dp/0471569526/?tag=pfamazon01-20)

Just because someone hasn't read Cohen-Tannoudji doesn't mean he hasn't read any standard text about QM. Or do you think CT is "more" standard than all others? If yes, why?

Is it possibly to focus the comments above into a general issue? Clearly many members who come to this forum operate in an academic environment, either as students or lecturers. As such, access to textbooks and other sources of information may be relatively easy and cheap. On the other hand, there is probably another group of members who don’t and who have to rely heavily on free sources like Wikipedia and the Cornell Library to get what appears to be latest ideas on a certain topic. In this respect, the PF can be a very important source of ‘arbitration’ for the enormous scope of information and opinion in the field of QM.

In the comments above, Juanrga not unreasonably raises a note of caution about Wikipedia and suggests referencing a QM textbook, possibly the Cohen-Tannoudji book. However, this book cost $185 on Amazon and so might not be an option for many when just starting to read into QM as the following review comment implies:

“I just finished a graduate level quantum mechanics course where Cohen-Tannoudji's "Quantum Mechanics" was the primary text….While CT may be the most comprehensive text I have seen, I would not recommend it being the only text used for a class.”​

Therefore, I was wondering if some of the scientific advisors who help out in this forum might organise a ‘sticky’ post alongside the forum rules with some incremental recommendations for a range of references, i.e. textbooks, Wikipedia and Cornell articles. For example, a post by DrChinese appears to provide some very useful references, although I haven’t had a chance to read any in detail. However, finding such references can be a bit of a pot-luck affair when they are buried in one of thousands of threads and posts. Of course, I realize the scope of QM might cover many sub-fields, but the issue of interpretation and mathematical scope would seem a sensible, and smaller, starting point

Again, by way of example, the link to the Cornell article ‘Quantum mechanics: Myths and Facts’ seems, at face value, to provide a very useful insight to some of the key arguments surrounding the interpretation debate, but would appreciate any cautionary comments. However, the opening statement in the introduction appears to nicely capture the divide of opinion that sparks so much discussion in this forum.

“On the technical level, quantum mechanics (QM) is a set of mathematically formulated prescriptions that serve for calculations of probabilities of different measurement outcomes. The calculated probabilities agree with experiments. This is the fact! From a pragmatic point of view, this is also enough. Pragmatic physicists are interested only in these pragmatic aspects of QM, which is fine. Nevertheless, many physicists are not only interested in the pragmatic aspects, but also want to understand nature on a deeper conceptual level. Besides, a deeper understanding of nature on the conceptual level may also induce a new development of pragmatic aspects. Thus, the conceptual understanding of physical phenomena is also an important aspect of physics. Unfortunately, the conceptual issues turn out to be particularly difficult in the most fundamental physical theory currently known – quantum theory.”​

 

[juanrga]

No. Two website links and one journal article are not sufficient to make a claim a fact. There are numerous peer-reviewed articles about the interpretations you reject.

I have given three accessible references that people can consult {*}, including a rather recent and complete review analyzing those articles that you allude to. Of course, there are many more articles showing why MWI is incorrect, but people can search by themselves.

Just because someone hasn't read Cohen-Tannoudji doesn't mean he hasn't read any standard text about QM. Or do you think CT is "more" standard than all others? If yes, why?

It is considered kind of a bible on the topic. I do not know if people has read another textbooks, but many seem did not.

{*} Others, promoting MWI, have given none reference, in their discussions with me.

 

[juanrga]

Is it possibly to focus the comments above into a general issue? Clearly many members who come to this forum operate in an academic environment, either as students or lecturers. As such, access to textbooks and other sources of information may be relatively easy and cheap. On the other hand, there is probably another group of members who don’t and who have to rely heavily on free sources like Wikipedia and the Cornell Library to get what appears to be latest ideas on a certain topic. In this respect, the PF can be a very important source of ‘arbitration’ for the enormous scope of information and opinion in the field of QM.

In the comments above, Juanrga not unreasonably raises a note of caution about Wikipedia and suggests referencing a QM textbook, possibly the Cohen-Tannoudji book. However, this book cost $185 on Amazon and so might not be an option for many when just starting to read into QM as the following review comment implies:

“I just finished a graduate level quantum mechanics course where Cohen-Tannoudji's "Quantum Mechanics" was the primary text….While CT may be the most comprehensive text I have seen, I would not recommend it being the only text used for a class.”​

Therefore, I was wondering if some of the scientific advisors who help out in this forum might organise a ‘sticky’ post alongside the forum rules with some incremental recommendations for a range of references, i.e. textbooks, Wikipedia and Cornell articles. For example, a post by DrChinese appears to provide some very useful references, although I haven’t had a chance to read any in detail. However, finding such references can be a bit of a pot-luck affair when they are buried in one of thousands of threads and posts. Of course, I realize the scope of QM might cover many sub-fields, but the issue of interpretation and mathematical scope would seem a sensible, and smaller, starting point

Again, by way of example, the link to the Cornell article ‘Quantum mechanics: Myths and Facts’ seems, at face value, to provide a very useful insight to some of the key arguments surrounding the interpretation debate, but would appreciate any cautionary comments. However, the opening statement in the introduction appears to nicely capture the divide of opinion that sparks so much discussion in this forum.

“On the technical level, quantum mechanics (QM) is a set of mathematically formulated prescriptions that serve for calculations of probabilities of different measurement outcomes. The calculated probabilities agree with experiments. This is the fact! From a pragmatic point of view, this is also enough. Pragmatic physicists are interested only in these pragmatic aspects of QM, which is fine. Nevertheless, many physicists are not only interested in the pragmatic aspects, but also want to understand nature on a deeper conceptual level. Besides, a deeper understanding of nature on the conceptual level may also induce a new development of pragmatic aspects. Thus, the conceptual understanding of physical phenomena is also an important aspect of physics. Unfortunately, the conceptual issues turn out to be particularly difficult in the most fundamental physical theory currently known – quantum theory.”​

Well, I have given three online references accessible for free, including a physics FAQ.

Regarding the preprint by Nikolic, some parts are right and other wrong (chapter 6 is particularly misleading). My responses are (see his Contents)

2 myth
3 fact
4 undecided
5 fact
6 QM is both local and nonlocal
7,8,9,10 myths

 

[kith]

Well, I fear this discussion will remain fruitless. I'm no expert in MWI and have no time for a technical debate. So maybe just two simple questions.

What's wrong with the most simple MWI, where we keep all axioms of the CI and only add/modify that whenever collapse occurs, the world splits into non-interacting branches and the probability to land in one of them is given by the Born rule? I don't see how someone could disprove such an interpretation, because
(a) it is also possible for classical probabilities
(b) it is experimentally indistinguishable from the CI, because the branches don't interact (well, it's an interpretation)

And similarily, a simple ensemble interpretation can be constructed. Since the CI is inherently probalistic, you always need an ensemble to check it's predictions. This ensemble can consist of many equally prepared systems or of one system which is prepared over and over again. How can you distinguish CI from an interpretation which assigns states only to these ensembles instead of single systems, when you always need such an ensemble to verify CI statements?

Please don't just link to technical papers, but tell me where my reasoning goes wrong.