I Is the collapse indispensable?

[naima]

You can discuss at length about the collapse of one particle. It will have no physical content if you neglect the no cloning theorem.

 

[zonde]

We do not usually consider the beam splitter to be conscious, so it is not necessarily a measurement. If a measurement is made, but the outcome is discarded or not retained by the conscious observer, then there is no need for collapse.

To support this, the beamsplitter can be modeled using unitary evolution. http://arxiv.org/abs/quant-ph/0305007 (section 4.1)

If one doesn't like the term "conscious", one can replace it with the term "classical".

If using BPS does not count as measurement then what does? Please give real example.

 

[zonde]

MW does not require an observer.

I don't see how this is possible. Predictions are made for observer. And correspondence principle requires observer.

 

[bhobba]

I don't see how this is possible. Predictions are made for observer. And correspondence principle requires observer.

Its based of the concept of history the same as decoherent histories:
https://www.math.rutgers.edu/~oldstein/papers/qts/node2.html

Thanks
Bill

 

[vanhees71]

We do not usually consider the beam splitter to be conscious, so it is not necessarily a measurement. If a measurement is made, but the outcome is discarded or not retained by the conscious observer, then there is no need for collapse.

To support this, the beamsplitter can be modeled using unitary evolution. http://arxiv.org/abs/quant-ph/0305007 (section 4.1)

If one doesn't like the term "conscious", one can replace it with the term "classical".

Well, if something is unobservable for physics it is totally unimportant which other properties (being realistic or not is one such property) it may have. It's simply not part of physics, because physics is about objectively observable facts about Nature.

Conceptually the objectivity of a collapse is, however, highly problematic in the context of the relativistic space-time structure and causality. This doesn't matter much either, because the collapse is not observable and thus one doesn't need to introduce it. In this sense collapse is a short-cut description of what we mean when we say we prepare a system in a certain (pure or mixed) state. It's of course much more natural to describe the quantum-theoretical state simply by the description of a preparation procedure. Then you can make a model in terms of quantum theory for this state, i.e., you assume a statistical operator and then make measurements on an ensemble of such prepared systems to check whether the probabilistic predictions of quantum theory with the postulated description in terms of the statistical operator are correct or not. That's all, what's behind "collapse". One should not speak about it as if it were a real process in the sense of an instantaneous change of the state due to the interaction of the system with the measurement apparatus. According to the best working quantum theory, i.e., local relativistic quantum field theory there is no such instantaneous interaction and no violation of the relativistic causality structure!

 

[ddd123]

In this sense collapse is a short-cut description of what we mean when we say we prepare a system in a certain (pure or mixed) state. It's of course much more natural to describe the quantum-theoretical state simply by the description of a preparation procedure. Then you can make a model in terms of quantum theory for this state, i.e., you assume a statistical operator and then make measurements on an ensemble of such prepared systems to check whether the probabilistic predictions of quantum theory with the postulated description in terms of the statistical operator are correct or not. That's all, what's behind "collapse".

Could you point me to an article or a book which explains how this and collapse are equivalent? Thanks.

 

[vanhees71]

No, that's just the conclusion I came to when thinking about the meaning of collapse.

 

[zonde]

Well, if something is unobservable for physics it is totally unimportant which other properties (being realistic or not is one such property) it may have. It's simply not part of physics, because physics is about objectively observable facts about Nature.

If something is part of the model that makes testable predictions, then it matters and it is part of the physics.

 

[rkastner]

I studied lots of points of view, and lots of how physicists actually use quantum mechanics in the applications. I came to the conclusion that there is an objective and a subjective side to quantum mechanics.

The collapse belongs to the subjective side, since it is associated with ''knowledge'' of which nature is ignorant.

This is a common assumption, but it is not necessarily true. In fact I have argued that this assumption is part of the longstanding problem in interpreting QM. In a direct-action theory of quantum fields, you DO get collapse as an objective, physical process.
This is what the transactional interpretation (TI) is based on. I argue in my published research that this solves the measurement problem by providing a physical account of 'measurement' that is not observer-dependent. Also, if there are non-unitary collapses in nature, this would also explain where the 2nd Law of thermodynamics comes from. Non-unitary collapse is an irreversible process and would constitute the 'seed' of irreversibility that is ubiquitous in micro-processes. For example, under TI, thermal interactions are non-unitary collapses in which energy is exchanged between gas molecules.
In my view the big mistake in QM interpretation has been assuming that all QM dynamics must be unitary. See my (peer-reviewed) papers and books for presentation of the TI alternative. Yes, the direct-action theory has been ignored and marginalized for quite some time, but there is nothing wrong with it. In fact John Wheeler was enthusiastically endorsing it in 2003, see e.g.: http://www.ijqf.org/archives/2004

 

[vanhees71]

If something is part of the model that makes testable predictions, then it matters and it is part of the physics.

How is the "collapse of the state" observable? I'm not aware of any example.

 

[A. Neumaier]

See my (peer-reviewed) papers

Just point to one, if possible in an arXiv version. (You can place it there if it isn't already there.)

 

[eltodesukane]

Whether the wavefunction collapses into an (unpredictable) specific state in of the Copenhagen interpretation, or whether the wavefunction branches into an (unpredictable) specific world in the many-worlds interpretation (MWI), it is really the same thing, the same problem. Why the collapse into some state? Why the branch into some world?

 

[MarkPercival]

>I studied lots of points of view, and lots of how physicists actually use quantum mechanics in the applications. I came to the conclusion
>that there is an objective and a subjective side to quantum mechanics.

I don't understand why QM needs a subjective side.

"Objectivity" usually is taken to mean something like "Any observer stationed *here* will observe *this* under *these* conditions". That is to say, there's something going on that will look the same to anyone who happens to be there looking at it., and it will do so in the same way even if there's nobody there looking at it which must be the case for all possible observers to see the same thing. To put an ever finer point on it, reality is real in and of itself- it doesn't require an audience to be real.

This strongly implies that non-conscious physical entities (particles, fields, macroscopic objects) are valid observers of each other and don't need us to validate them.

>The collapse belongs to the subjective side, since it is associated with ''knowledge'' of which nature is ignorant.

A philosophical assumption. How can Nature be ignorant? Every subatomic particle in existence, even the virtual ones, "knows" its own state(s) and the state(s) of its environmental variables (field vectors and strengths) to which it couples. If this were not the case physics couldn't happen. I'm not arguing "hidden variables" mind you- I'm simply restating objectivity. All of the allegedly infinite possible outcomes of every quantum interaction that has ever happened and that are happening right now unfailingly unitarily add up to what we observe. Each particle collapses the eigenstates of its environmental variables constantly.

What we see when we look at them depends solely on how we choose to look at them. Isn't that what quantum eraser experiments are about? How are they different from imposing constraints all of the possible paths from here to there and then being surprised when the outcome changes?

>''shut up and calculate'' belongs to the objective side. it couldn't work if the collapse were indispensable. Properly distinguishing between
>an objective and a subjective side clears up a lot of the confusion prevailing in the foundations of QM.

I think the whole concept of subjectivity needs to be put on trial to justify its existence.

 

[A. Neumaier]

I don't understand why QM needs a subjective side.

It may not need one, but given the history of the subject, it obviously has one, as can be seen empirically from the discussions.

 

[atyy]

"Objectivity" usually is taken to mean something like "Any observer stationed *here* will observe *this* under *these* conditions". That is to say, there's something going on that will look the same to anyone who happens to be there looking at it., and it will do so in the same way even if there's nobody there looking at it which must be the case for all possible observers to see the same thing. To put an ever finer point on it, reality is real in and of itself- it doesn't require an audience to be real.

That is the point. We don't know whether the moon exists if nobody looks at it.

In QM it is difficult to assert "Nature doesn't care what we like", since there is no model of "Nature" that exists apart from "us".

 

[Jilang]

MarkP, QM only has a subjective side. It is totally silent on the objective side.

 

[rkastner]

MarkP, QM only has a subjective side. It is totally silent on the objective side.

That's a matter of interpretation. The appeal to subjectivity only arose because no one could provide a physical account of collapse.
You get the latter in TIQM. See eg http://transactionalinterpretation....tivistic-and-non-relativistic-quantum-theory/

 

[rkastner]

Just point to one, if possible in an arXiv version. (You can place it there if it isn't already there.)

An introduction to the basic principles of TI and to my extension of it is here:

http://transactionalinterpretation....tivistic-and-non-relativistic-quantum-theory/

 

[rkastner]

Whether the wavefunction collapses into an (unpredictable) specific state in of the Copenhagen interpretation, or whether the wavefunction branches into an (unpredictable) specific world in the many-worlds interpretation (MWI), it is really the same thing, the same problem. Why the collapse into some state? Why the branch into some world?

The Born Rule gives the probability that one outcome occurs. So clearly, either there really has to be 'collapse' to that outcome, or we have a many worlds situation (which doesn't work, as I've noted here: http://arxiv.org/abs/1406.4126 )
The problem has been accounting for collapse in physical terms. In a direct-action theory, this can be done (through the transactional picture). I discuss other benefits of the direct-action theory here: http://www.ijqf.org/archives/2004

 

[strangerep]

MarkP, QM only has a subjective side. It is totally silent on the objective side.

Not true.

Proof: The half-integral spectrum of quantum angular momentum is independent of the observer, hence objective. But that spectrum arises from representing rotational symmetry on a Hilbert space. Hence the Hilbert space is not subjective. $\Box$

 

[atyy]

Not true.

Proof: The half-integral spectrum of quantum angular momentum is independent of the observer, hence objective. But that spectrum arises from representing rotational symmetry on a Hilbert space. Hence the Hilbert space is not subjective. $\Box$

The Hilbert space is subjective because the Hilbert space depends on the division of the universe into the unreal quantum part (described by a vector in Hilbert space) and the real classical part (not described by a vector in Hilbert space).

 

[strangerep]

The Hilbert space is subjective because the Hilbert space depends on the division of the universe into the unreal quantum part (described by a vector in Hilbert space) and the real classical part (not described by a vector in Hilbert space).

That would mean half-integral quantum angular momenta are "unreal". To disprove this, one must derive the half-integral spectrum using purely classical means. (Good luck.)

 

[atyy]

That would mean half-integral quantum angular momenta are "unreal". To disprove this, one must derive the half-integral spectrum using purely classical means. (Good luck.)

Is there anything wrong with half-integral quantum angular momenta being "unreal"?

 

[strangerep]

Is there anything wrong with half-integral quantum angular momenta being "unreal"?

That sounds like a very fine drop of port you're enjoying right now.

[ @Greg Bernhardt : we need another icon in your enhanced list: something portraying a party girl staggering around enjoying herself... ]

[Edit: Let us terminate this subdiscussion, lest we offend the OP and moderators.]

 

[atyy]

That sounds like a very fine drop of port you're enjoying right now.

[ @Greg Bernhardt : we need another icon in your enhanced list: something portraying a party girl staggering around enjoying herself... ]

Not any little girl. Has to be this one.

 

[bhobba]

In my view the big mistake in QM interpretation has been assuming that all QM dynamics must be unitary.

I don't know if its a big mistake, but I do believe that it must be unitary is open to question. Curios though about the status Wigners theorem if it isn't.

See my (peer-reviewed) papers and books for presentation of the TI alternative. Yes, the direct-action theory has been ignored and marginalized for quite some time, but there is nothing wrong with it. In fact John Wheeler was enthusiastically endorsing it in 2003, see e.g.: http://www.ijqf.org/archives/2004

Of course its a valid interpretation. But these things go thorough fads etc for no apparent reason. My favourite interpretation, ignorance ensemble, virtually no one knows about. And there are many others like that eg primary state diffusion. I don't think its anything to get worried about.

Thanks
Bill

 

[zonde]

How is the "collapse of the state" observable? I'm not aware of any example.

You are inventing your own rules.
Here are the rules: if a model makes consistent predictions and predictions are tested and verified in experiment it's valid for now.
So the only question is whether collapse is indispensable part of valid model (QM).

 

[vanhees71]

If this is so, then it should be easy for you to provide an example for a real experiment, where you need the collapse hypothesis to describe its result within quantum theory. I don't know of any. So far, the most simple description is in terms of the minimal interpretation. You just take the Born rule as one more independent assumption, i.e., the quantum mechanical state describes probabilities for the outcome of measurements and nothing more.

 

[atyy]

If this is so, then it should be easy for you to provide an example for a real experiment, where you need the collapse hypothesis to describe its result within quantum theory. I don't know of any. So far, the most simple description is in terms of the minimal interpretation. You just take the Born rule as one more independent assumption, i.e., the quantum mechanical state describes probabilities for the outcome of measurements and nothing more.

Challenge: Derive the generalized Born rule from the Born rule, but without using collapse!

 

[vanhees71]

What's the "generalized Born rule". For me the Born rule is a postulate saying that for any state, represented by a statistical operator $\hat{R}$ the outcome of the measurement of an observable $A$ to be the value $a$, represented by a self-adjoint operator $\hat{A}$ defining a (generalized) orthonormalized eigenvector basis $|a,\beta \rangle$ is given by
$$P_A(a|\hat{R})=\sum_{\beta} \langle a,\beta|\hat{R}|a,\beta \rangle,$$
where the sum can also be an integral or both a sum and an integral, depending on the specific spectral properties of $\hat{A}$.

For me that's a postulate and nothing that can be derived. Weinberg has given a thorough analysis of whether the Born rule is derivable from the other postulates (all well hidden above ;-)) coming to the conclusion that it can't be derived. I don't need an assumption about what happens to the state of the system due to the interaction between the measured object and the measure device, and I can't give a general one, because of course it depends on the details of this device. For sure I don't need a collapse for formulate the Born rule. It simply tells me that I have to do the measurement on a large ensemble of equally stochastically independent prepared systems to check whether the prediction of the Born rule concerning the probabilities is correct or not (within a given significance according to standard statistical rules).

 

[atyy]

What's the "generalized Born rule". For me the Born rule is a postulate saying that for any state, represented by a statistical operator $\hat{R}$ the outcome of the measurement of an observable $A$ to be the value $a$, represented by a self-adjoint operator $\hat{A}$ defining a (generalized) orthonormalized eigenvector basis $|a,\beta \rangle$ is given by
$$P_A(a|\hat{R})=\sum_{\beta} \langle a,\beta|\hat{R}|a,\beta \rangle,$$
where the sum can also be an integral or both a sum and an integral, depending on the specific spectral properties of $\hat{A}$.

For me that's a postulate and nothing that can be derived. Weinberg has given a thorough analysis of whether the Born rule is derivable from the other postulates (all well hidden above ;-)) coming to the conclusion that it can't be derived. I don't need an assumption about what happens to the state of the system due to the interaction between the measured object and the measure device, and I can't give a general one, because of course it depends on the details of this device. For sure I don't need a collapse for formulate the Born rule. It simply tells me that I have to do the measurement on a large ensemble of equally stochastically independent prepared systems to check whether the prediction of the Born rule concerning the probabilities is correct or not (within a given significance according to standard statistical rules).

The generalized Born rule is Eq 37 on p67 of http://arxiv.org/abs/quant-ph/0209123 .

 

[vanhees71]

No having read the complete paper, I think that's just the description of a measurement at time $t_2$ after performing an ideal von Neumann filter measurement at $t_1<t_2$. Where do you need a collapse here? It's just filtering out subensembles. I just need to block beams, i.e., local interaction of the partial beams with some "beam dumps", not an instantaneous collapse of whatever. One must not loose the foundation of physics in real-world setups of experiments to the abstract formalism! Then all esoterics concerning "interpretation" is usually absent from our description of this real-world experiments.

 

[atyy]

No having read the complete paper, I think that's just the description of a measurement at time $t_2$ after performing an ideal von Neumann filter measurement at $t_1<t_2$. Where do you need a collapse here? It's just filtering out subensembles. I just need to block beams, i.e., local interaction of the partial beams with some "beam dumps", not an instantaneous collapse of whatever. One must not loose the foundation of physics in real-world setups of experiments to the abstract formalism! Then all esoterics concerning "interpretation" is usually absent from our description of this real-world experiments.

Well, that's the Peres and Ballentine claim. Is it correct that with only unitary evolution you can derive collapse? Till this day you have never exhibited a derivation, neither have Peres nor Ballentine. It's a pity that quantum mechanics is still not understood even by experts.

 

[vanhees71]

Well, I'm pretty sure I cannot described a "beam dump" in all microscopic detail, but that's not necessary to know that fact that it filters out unwanted beams! Why should I derive something unobservable and unneeded like the collapse from QT?

 

[atyy]

Well, I'm pretty sure I cannot described a "beam dump" in all microscopic detail, but that's not necessary to know that fact that it filters out unwanted beams! Why should I derive something unobservable and unneeded like the collapse from QT?

The collapse gives the correct prediction of your uncalculatable filtering. Which should I take - collapse which makes the prediction, or filtering which you cannot calculate?

 

[vanhees71]

The collapse gives the correct prediction of your uncalculatable filtering. Which should I take - collapse which makes the prediction, or filtering which you cannot calculate?

I prefer just to use the projection operators as given in the text without assuming an instantaneous collapse, which violates fundamental principles of physics like causality.

 

[atyy]

I prefer just to use the projection operators as given in the text without assuming an instantaneous collapse, which violates fundamental principles of physics like causality.

So you do accept the collapse as necessary, just not its physicality!

In the standard interpretation, collapse is not necessarily physical.

However, the physicality of collapse cannot be rejected on the basis of relativistic causality.

 

[vanhees71]

There is no collapse in this very expression! It's just filtering out unwanted states, which is precisely described by the projection operators (for an idealized filtering). The filtering itself is not due to instantaneous action at a distance but due to local interactions (at least as long as you consider standard relativistic QFT as a correct (effective) description of nature). I'm so strictly against the collapse assumption, because it denies the fundamental property of the locality of interactions in standard relativistic QFT and it assumes dynamics outside of quantum theory.

 

[atyy]

There is no collapse in this very expression! It's just filtering out unwanted states, which is precisely described by the projection operators (for an idealized filtering). The filtering itself is not due to instantaneous action at a distance but due to local interactions (at least as long as you consider standard relativistic QFT as a correct (effective) description of nature). I'm so strictly against the collapse assumption, because it denies the fundamental property of the locality of interactions in standard relativistic QFT and it assumes dynamics outside of quantum theory.

Sorry, this is just wrong.

 

[ddd123]

Sorry, this is just wrong.

Maybe he means something like this? http://arxiv.org/abs/hep-th/0205105

 

[atyy]

vanhees71 is wrong for the following reasons.

1. Unitary evolution and the "filtering" that he imagines will allow the projection to be derived cannot do it, because the unitary evolution and partial trace caused by the "filtering" only produce an improper mixture. To get the definite outcome, one must further assume that the improper mixture is converted to a proper mixture, which is the same as assuming collapse. Ballentine and Peres are probably missing this assumption in their erroneous books.

2. The "locality" of QFT that is enforced by the "local" interactions has the meaning of "no superluminal transmission of classical information" (and a little more). It does not mean local interactions and local causality. vanhees71 consistently confuses multiple meanings of "local".

3. Collapse is consistent with the "locality" of quantum field theory. It is not consistent with relativistic causality, but neither is quantum field theory.

 

[A. Neumaier]

Ballentine and Peres are probably missing this assumption in their erroneous books.

I think you should moderate your language.

The books by Ballentine and Peres are highly respectable books that provide all the information one ever needs to understand the basics of quantum mechanics in theory and practice. Calling them erroneous based on your own subjective view of the interpretation issues is inappropriate.

Your arguments are not that impeccable that you would be justified to call their treatment erroneous. (Write your own book and you'll see that it will most likely contain even more glaring problems.)

 

[atyy]

I think you should moderate your language.

The books by Ballentine and Peres are highly respectable books that provide all the information one ever needs to understand the basics of quantum mechanics in theory and practice. Calling them erroneous based on your own subjective view of the interpretation issues is inappropriate.

Your arguments are not that impeccable that you would be justified to call their treatment erroneous. (Write your own book and you'll see that it will most likely contain even more glaring problems.)

I'm quite sure I am right, and the standard interpretation is done that way for good reasons. Ballentine and Peres are wrong.

 

[A. Neumaier]

I'm quite sure I am right, and the standard interpretation is done that way for good reasons. Ballentine and Peres are wrong.

Ballentine and Peres were also sure of what they wrote (this is visible from how they defend it elsewhere in their publications). Moreover, the way they wrote it was done for good reasons.

So it is view against view. In such a case the credentials count, and you as an outsider should be temperate about your bold assertions.

By the way, there is no standard interpretation. You probably mean your favorite interpretation, or your favorite version of the Copenhagen interpretation.

 

[ddd123]

Unitary evolution and the "filtering" that he imagines will allow the projection to be derived cannot do it, because the unitary evolution and partial trace caused by the "filtering" only produce an improper mixture. To get the definite outcome, one must further assume that the improper mixture is converted to a proper mixture, which is the same as assuming collapse. Ballentine and Peres are probably missing this assumption in their erroneous books.

Wouldn't it be strange if they missed it? A book cannot contain every retort to every attack, maybe they've elaborated further on this, elsewhere.

 

[atyy]

Ballentine and Peres were also sure of what they wrote (this is visible from how they defend it elsewhere in their publications). Moreover, the way they wrote it was done for good reasons.

So it is view against view. In such a case the credentials count, and you as an outsider should be temperate about your bold assertions.

By the way, there is no standard interpretation. You probably mean your favorite interpretation, or your favorite version of the Copenhagen interpretation.

I am not an outsider. I am stating that the standard texts are right.

Ballentine and Peres are the outsiders.

 

[atyy]

Wouldn't it be strange if they missed it? A book cannot contain every retort to every attack, maybe they've elaborated further on this, elsewhere.

Wouldn't it be strange if standard quantum mechanics were wrong? Wouldn't it be strange if even Nielsen and Chuang were wrong?

 

[ddd123]

Wouldn't it be strange if standard quantum mechanics were wrong? Wouldn't it be strange if even Nielsen and Chuang were wrong?

I have no idea :D
I guess we should just stick to the pure arguments (or rather, you, I'm too low level).

 

[A. Neumaier]

I am not an outsider. I am stating that the standard texts are right.

Ballentine and Peres are the outsiders.

Which standard texts are you referring to? Who but you decided that they are the standard?

These two books, together with the commented reprints in Wheeler and Zurek, are the modern standard! (There is also decoherence theory, which is newer than these; but this is silent on collapse.) They devote considerable space to the foundations, whereas typical textbooks on quantum mechanics only have short sections where they parrot what they glean from elsewhere, often from the long past.

 

[ddd123]

I'm afraid this thread is at risk of getting closed. Would be a pity if nobody actually answered or provided a source with an answer to atyy's point I quoted in post #104.