I Some (unrelated) questions about the measurement problem

[Derek P]

Where does the idea come from that the MWI doesn't refer to physical objects like particles but only to the state vector of the universe? Max Tegmark's Mathematical Universe Hypothesis states this but this is not the MWI.

Well the WF's evolution is all that appears in the theory. You can sprinkle particles onto the wavefunction if it makes you feel good, but they can't take part in any interactions and they can't affect the WF. They must be puppets of the WF. You then have Bohmian Mechanics, which, for this very reason, has been described as "Many Worlds in denial".

 

[Derek P]

But if all that exists is the wave function, then there is no sense in which the second interpretation is implied by the physics of the wave function.

Sure there is. Decoherence imposes a preferred basis. Stationary state superpositions, intriguing though they may be, are not phenomenal. The Rorchasch cards have plain backs.

 

[stevendaryl]

Sure there is. Decoherence imposes a preferred basis.

A basis preferred by US, since it is more like the classical physics we're used to. Why should it be preferred by the physics?

 

[stevendaryl]

A basis preferred by US, since it is more like the classical physics we're used to. Why should it be preferred by the physics?

I understand that the basis that gets selected by decoherence has certain desirable properties, such as stability. But why does it make that basis more real than other bases?

 

[akvadrako]

I understand that the basis that gets selected by decoherence has certain desirable properties, such as stability. But why does it make that basis more real than other bases?

It doesn't have to be more real from an objective viewpoint; it just happens to be our perspective. The other perspectives might be real too. If they are unstable, they won't contain configurations like us and won't be observed. The state can even be static as long as our perspective keeps evolving.

 

[Derek P]

I understand that the basis that gets selected by decoherence has certain desirable properties, such as stability. But why does it make that basis more real than other bases?

I don't think it does. In fact I don't think physics talks about degrees of reality - Bell realism and Einstein realism are misnomers. Being real just means that something exists without qualification. MWI asserts the onticity of the wavefunction - although, of course, it may be derivative on path integrals etc. I don't think it is the job of physics to declare that such-and-such a subset of possible worlds is more real than others. More likely, that's all.

 

[Derek P]

But as I said, the wave function by itself is compatible with a universe in which nothing at all happens.

At best, such a universal wave function is a rorschach that you can see birds and trees in, but it's unclear that they're really there.

Having thought about it, I don't think I agree. Yes you can decompose the state into stationary states but they don't exist in isolation. They are in superposition, which means that the universe as a whole is not static. Specifically, if the superposition of stationary states is projected on a "world" base, we see interference between the "stationary" components, which, of course, have huge phase frequencies. This "interference pattern" is identical to the phenomenal world. Which means that the dynamic phenomena of our familiar world emerge through interference between the static states.

 

[Derek P]

They assume that, in the setup they consider, the interference can be seen at a single detector. But it cannot. It is only seen in coincidences (correlations) between two detectors. When this is taken into account, their argument does not longer work.

I don't think it's only their setup. Intuitively: if the correlator has a non-zero output corresponding to a single-slit pattern then the real pattern at the detector cannot have any dark regions.
I'm thinking that to see an interference pattern, the state must be of the form (|L>+|R>)*|xyz>, which is not an entanglement. And whatever you do with xyz you can't tag an |L> (or an |R>) state. Which is very nice because I'd been wondering for a while now why the Kim et al DCQE uses a state where no interference pattern is seen. Looks like the requirement to be able to tag signal photons as belonging to different patterns actually rules any visible-interference setup out. Am I correct?

 

[Derek P]

I don't think it does. In fact I don't think physics talks about degrees of reality.

Well perhaps Heisenberg did but I can't help feeling it shouldn't

 

[stevendaryl]

Having thought about it, I don't think I agree. Yes you can decompose the state into stationary states but they don't exist in isolation. They are in superposition, which means that the universe as a whole is not static. Specifically, if the superposition of stationary states is projected on a "world" base, we see interference between the "stationary" components, which, of course, have huge phase frequencies. This "interference pattern" is identical to the phenomenal world. Which means that the dynamic phenomena of our familiar world emerge through interference between the static states.

It seems to me that you can't really talk about interference in QM without specifying initial and final states. If you prepare a system as a superposition of two states, |\psi\rangle = \alpha |A\rangle + \beta |B\rangle, then the probability of finding it in state C will involve interference:

P = |\alpha \langle C|A\rangle + \beta \langle C|B\rangle|^2 = |\alpha|^2 |\langle C|A\rangle|^2 + |\beta|^2 |\langle C|B\rangle|^2 + 2 Re(\alpha^* \beta \langle A|C\rangle \langle C|B\rangle)

The last term is the interference term.

So yes, if you choose to project the state of the universe onto a basis other than the energy eigenstates, then you will find interference terms. But doing that projection is not forced on you by the universal wave function.

To me, it seems that you need something external to, or in addition to, the universal wave function in order to meaningfully say that the universal wavefunction should be interpreted as alternative possible worlds.

 

[Derek P]

To me, it seems that you need something external to, or in addition to, the universal wave function in order to meaningfully say that the universal wavefunction should be interpreted as alternative possible worlds.

I don't see the need to justify interpreting it one way or the other! The fact that it can be decomposed into phenomenal worlds is sufficient to explain... phenomena. What more could you want?

 

[stevendaryl]

I don't see the need to justify interpreting it one way or the other! The fact that it can be decomposed into phenomenal worlds is sufficient to explain... phenomena. What more could you want?

I think that's back to the Rorschach images. If it can be interpreted as a picture of a butterfly, it is a picture of a butterfly.

 

[Demystifier]

I don't think it's only their setup. Intuitively: if the correlator has a non-zero output corresponding to a single-slit pattern then the real pattern at the detector cannot have any dark regions.
I'm thinking that to see an interference pattern, the state must be of the form (|L>+|R>)*|xyz>, which is not an entanglement. And whatever you do with xyz you can't tag an |L> (or an |R>) state. Which is very nice because I'd been wondering for a while now why the Kim et al DCQE uses a state where no interference pattern is seen. Looks like the requirement to be able to tag signal photons as belonging to different patterns actually rules any visible-interference setup out. Am I correct?

No. Kim et al consider a setup in which interference is only "seen" in the correlations.

 

[Derek P]

I think that's back to the Rorschach images. If it can be interpreted as a picture of a butterfly, it is a picture of a butterfly.

With the additional factor that the only entity that can see the picture of a butterfly is the butterfly in the picture.

 

[Derek P]

No. Kim et al consider a setup in which interference is only "seen" in the correlations.

I thought I was being perfectly clear - I was talking about visible interference at the signal detector. There isn't any.

 

[haushofer]

I got the book by Travis Norsen and started reading, so hopefully it will address some of my questions. I would like to return to my "consciousness questions". To me, ontologically, the idea that concsiousness collapses the wavefunction is ontologically as reasonable as the idea that Chuck Norris collapses the wavefunction by his telepathic abilities, but it is a persistent idea by many. So that's why I'd like to hear more comments on my questions 2 and 3, if any. Are there any clear arguments directly against the (Von Neumann-) Wigner interpretation involving consciousness from e.g. certain experimental results?

 

[kith]

why not simply use the simple double slit experiment, put a detector near one of the slits, and only look at the screen? I assume we'll still see the interference pattern (right?), so doesn't this conclusively exclude the possibility that "a conscious observation near one of the slits is needed to make the interference pattern disappear"?

I don't think that proponents of the von Neumann-Wigner interpretation would say that a conscious observation is needed near the slit. They would would say that collapse happens neither at the detector near the slit, nor at the screen, but only as soon as the conscious observer looks at the screen.

About the Von Neumann-Wigner interpretation: is there any way to "break out of the Von-Neumann chain of regression" without hidden variables or imposing extra dynamics on top of the Schrodinger equation?

The Many Worlds interpretation goes one step further by saying that even consiousness as the last link in the von Neumann chain doesn't collapse the state vector but is itself in superposition. Copenhagen at least removes the problem of the interation between consiousness and the state vector as physical entities by stating that the state vector is an epistemic entity and not a physical one. Time-symmetric interpretations may also get around it although one can consider them hidden variable theories where the hidden variables are in the future, so they wouldn't meet your criterion.

I see that people often use this Von-Neumann chain to motivate the Von Neumann-Wigner interpretation. And how do adherents of this interpretation explain cosmic events like e.g. the CMB we're receiving from events billions of years ago?

Do you really see this often? I have never met someone who actually follows the von Neumann-Wigner interpretation. My impression is that it is mostly a historical interpretation and that people who were in it for the realism part now prefer some sort of Many Worlds, and people who were in it for the consiousness part now prefer Copenhagen, for the reason that it doesn't give good answers to questions like yours. See e.g. Penrose's quote in the Wikipedia article.

 

[haushofer]

I don't think that proponents of the von Neumann-Wigner interpretation would say that a conscious observation is needed near the slit. They would would say that collapse happens neither at the detector near the slit, nor at the screen, but only as soon as the conscious observer looks at the screen.

Ok, fair enough. What I meant was: put a detector at one of the slits and don't look at its outcome. Then the interference pattern disappears without being conscious about the precise outcome the slit detector.

Do you really see this often? I have never met someone who actually follows the von Neumann-Wigner interpretation. My impression is that it is mostly a historical interpretation and that people who were in it for the realism part now prefer some sort of Many Worlds, and people who were in it for the consiousness part now prefer Copenhagen, for the reason that it doesn't give good answers to questions like yours. See e.g. Penrose's quote in the Wikipedia article.

Actually, "often" is not the right word. I see it with people who don't know their quantum mechanics and mix Von Neumann-Wigner with esoteric or religieus beliefs. That Penrose quote is interesting, thanks!

 

[haushofer]

What I still don't get is the following: it is sometimes claimed, see e.g. here by Bhobba (maybe he's reading along),

https://www.physicsforums.com/threa...e-observer-the-double-slit-experiment.765350/

that the cut in the Von-Neumann chain/regression is most naturally put at the moment decoherence kicks in. I don't get that. I understand the Von Neumann chain as the claim that everything between process and measurement is described by the Schrodinger equation and hence you need something "nonmaterial" to make the wavefunction collapse (i.e. to give non-unitary evolution). But decoherence doesn't make the wavefunction collapse, it gives "merely" a classical probability distribution. So why is it claimed that decoherence is the natural place to put the cut? How can it be if decoherence doesn't solve the measurement problem? Or is this again a "FAPP"-thing?

 

[Lord Jestocost]

So why is it claimed that decoherence is the natural place to put the cut?

Who claims this? Decoherence cannot explain the process of factualization of potentiality; that's outside quantum theory and has to be put in "by hand".

 

[kith]

Ok, fair enough. What I meant was: put a detector at one of the slits and don't look at its outcome. Then the interference pattern disappears without being conscious about the precise outcome the slit detector.

I'm not sure what you are getting at. The disappearance of interference isn't equivalent to collapse. Unitary evolution of the combined system of the particle and the detector may lead to a macroscopic superposition like |\text{particle went through the left slit} \rangle \otimes |\text{detector didn't click} \rangle + |\text{particle went through the right slit} \rangle \otimes |\text{detector clicked} \rangle which corresponds to a fully decohered state of the subsystem of the particle.

So there's no problem with saying that the collapse to one term of the superposition doesn't happen until the consious observer performs a measurement on the particle.

 

[Lord Jestocost]

2) About the Von Neumann-Wigner interpretation: I am puzzled about this interpretation...

3) About the Von Neumann-Wigner interpretation: is there any way to "break out of the Von-Neumann chain of regression" without hidden variables or imposing extra dynamics on top of the Schrodinger equation?

Why is the von Neumann-Wigner interpretation puzzling? Casually speaking, the von Neumann-Wigner interpretation is one of the two principal options we have when considering the “measurement problem”. Either you discard Descartes’ idea that nature is intrinsically divided into two parts, viz. mind and matter, or you hang on to this idea following the approach of classical physics.

Following M. Esfeld:

“To sum up, Wigner’s earlier papers on the measurement problem and his later change of mind reflect the two principal options which we have: (a) we can regard quantum mechanics including the superposition principle and the Schrödinger dynamics as universally applicable in the physical realm. In this case we face the problem how to square the ensuing view of physical reality with our experience. (b) We can maintain that state reductions occur in nature. In this case we face the challenge to develop a dynamics that accounts for state reductions. Because there is as yet no overall convincing physical solution to this latter problem, it is still an issue of philosophical argument which one of these two principal options one should adopt.”

M. Esfeld “Essay Review Wigner’s View of Physical Reality”, Studies in History and Philosophy of Modern Physics, 30B (1999), pp. 145–154

 

[bhobba]

that the cut in the Von-Neumann chain/regression is most naturally put at the moment decoherence kicks in.

In his seminal book this consciousness stuff originated from (Mathematical Foundations of QM) Von-Neumann also showed the quantum classical cut can be put anywhere. His argument for placing it at consciousnesses was no place is really any different except there - so that's where he placed it. We now know a place that is different - just after decoherence so is the natural place to put it. You can place it anywhere you like - or not place it anywhere at all - it makes no difference - Lord Jestocost would likely emphasize do not read more into it than the math says and not place it anywhere. That's a very Dirac like view - nothing but the math ma'am, nothing but the math; although, while he is associsated by some with Copenhagen, his view was in fact rather subtle and more in line with Einstein:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.485.9188&rep=rep1&type=pdf

Lord Jestocot is correct - there is nothing wrong with what Von-Neumann did - it just leads to a rather strange view - but strangeness is not what science is about - take your pick of the many interpretations that exist - its entirely up to you.

Thanks
Bill

 

[bhobba]

Who claims this? Decoherence cannot explain the process of factualization of potentiality; that's outside quantum theory and has to be put in "by hand".

That's a nice name for it. The technical detail is how does an improper mixed state become a proper one. But your name is as good as any.

Thanks
Bill

 

[haushofer]

Why is the von Neumann-Wigner interpretation puzzling? Casually speaking, the von Neumann-Wigner interpretation is one of the two principal options we have when considering the “measurement problem”. Either you discard Descartes’ idea that nature is intrinsically divided into two parts, viz. mind and matter, or you hang on to this idea following the approach of classical physics.

Following M. Esfeld:

“To sum up, Wigner’s earlier papers on the measurement problem and his later change of mind reflect the two principal options which we have: (a) we can regard quantum mechanics including the superposition principle and the Schrödinger dynamics as universally applicable in the physical realm. In this case we face the problem how to square the ensuing view of physical reality with our experience. (b) We can maintain that state reductions occur in nature. In this case we face the challenge to develop a dynamics that accounts for state reductions. Because there is as yet no overall convincing physical solution to this latter problem, it is still an issue of philosophical argument which one of these two principal options one should adopt.”

M. Esfeld “Essay Review Wigner’s View of Physical Reality”, Studies in History and Philosophy of Modern Physics, 30B (1999), pp. 145–154

Well, it's puzzling because of e.g. Wigner's Friend or the question which amount of consciousness is needed to collapse the wavefunction. To speak with Bell: do we need a PhD to collapse it? Or can a bacteria do the job?

 

[haushofer]

I'm not sure what you are getting at. The disappearance of interference isn't equivalent to collapse. Unitary evolution of the combined system of the particle and the detector may lead to a macroscopic superposition like |\text{particle went through the left slit} \rangle \otimes |\text{detector didn't click} \rangle + |\text{particle went through the right slit} \rangle \otimes |\text{detector clicked} \rangle which corresponds to a fully decohered state of the subsystem of the particle.

So there's no problem with saying that the collapse to one term of the superposition doesn't happen until the consious observer performs a measurement on the particle.

Yes, you're right, thanks.

 

[haushofer]

In his seminal book this consciousness stuff originated from (Mathematical Foundations of QM) Von-Neumann also showed the quantum classical cut can be put anywhere. His argument for placing it at consciousnesses was no place is really any different except there - so that's where he placed it. We now know a place that is different - just after decoherence so is the natural place to put it.

I don't get this. I read the "cut" as the point where unitary evolution stops and collapse kicks in. Decoherence doesn't collapse anything, it merely makes the probability distribution classical.

I have the feeling that you regard the "cut" as the border between classical physics (no interference, classical prob.distributions) and quantum physics. That's fine, but Von Neumann's chain is still problematic. You still need some sort of collapse or new principles to explain definit outcomes.

 

[haushofer]

Who claims this? Decoherence cannot explain the process of factualization of potentiality; that's outside quantum theory and has to be put in "by hand".

Well, Bhobba for instance :P I thought it was claimed also in papers, but to be honest I can't find them right now, so maybe I'm wrong.

 

[AlexCaledin]

which amount of consciousness is needed to collapse the wavefunction... do we need a PhD to collapse it? Or can a bacteria do the job?

As Feynman said, it's always the whole Nature that is doing the job of incrementing her knowledge; so PhD, bacteria, cat etc. are mere various ways to participate in the game of Nature, like very different chess pieces (appearing as players).

 

[bhobba]

Well, it's puzzling because of e.g. Wigner's Friend or the question which amount of consciousness is needed to collapse the wavefunction. To speak with Bell: do we need a PhD to collapse it? Or can a bacteria do the job?

It has a number of issues. There is an anesthesiologist that has some theory about it:
https://en.wikipedia.org/wiki/Stuart_Hameroff

For me it has far too many issues - but each to their own - it can't be disproved.

Thanks
Bill

 

[bhobba]

I don't get this. I read the "cut" as the point where unitary evolution stops and collapse kicks in

Cut is where you can consider things classically from then on. If you put it after decoherence you are saying one of the possible outcomes is now objectively real ie it actually is in that state - but we don't know what state - such are by definition proper states. An improper state gives exactly the same probabilities of outcomes as proper ones - but is it in that state prior to observation? BM and MW would say yes - but others say no or who cares. There is no way to tell. But putting the cut right after decoherence is a simple way for things to be more understandable - for me and others anyway - of course each to their own who may think it total rubbish. For example the high priest of the Ensemble Interpretation Ballentine thinks its rubbish:
https://marcofrasca.wordpress.com/2009/03/10/ballentine-and-the-decoherence-program/

This is science - if it isn't what you like then you can view it anyway you want as long as its consistent with the formalism that everyone agrees on.

Thanks
Bill

 

[bhobba]

Well, Bhobba for instance :P I thought it was claimed also in papers, but to be honest I can't find them right now, so maybe I'm wrong.

I never claimed that. I claimed one can, if they wish, that an improper mixed state can be considered a proper one. This is the modern clear and unambiguous view of collapse - others for me don't make much sense - but each to their own. Its also a choice of where to place the Von-Neumann cut.

Thanks
Bill

 

[Lord Jestocost]

Well, Bhobba for instance :P I thought it was claimed also in papers, but to be honest I can't find them right now, so maybe I'm wrong.

It was "claimed" by some, but Stephen L. Adler, for example, cleared up the story:

“Why decoherence has not solved the measurement problem: a response to P.W. Anderson” by Stephen L. Adler (Studies in History and Philosophy of Modern Physics 34 (2003) 135–142) https://arxiv.org/abs/quant-ph/0112095

 

[StevieTNZ]

Isn't there, in accordance with the fundamental Schrodinger equation, that the system photon+detector+environment has an observable which could be used to find out if it is in a proper or improper mixture?

 

[bhobba]

Isn't there, in accordance with the fundamental Schrodinger equation, that the system photon+detector+environment has an observable which could be used to find out if it is in a proper or improper mixture?

There is no way to tell the difference because they are exactly the same state.

Thanks
Bill

 

[Zafa Pi]

Here's the way it works in practice:

• After decoherence, there are no more interference effects.
• Without interference effects, quantum probabilities work just like classical probabilities.
• So you can give the post-decoherence probabilities the same interpretation that you do classical probabilities---that the probabilities reflect ignorance of the true state of the system.

In other words, after decoherence, you might as well assume that a "collapse" has happened. The system is either in state \psi_1 or in state \psi_2, you just don't know which.

This is intellectually incoherent, in my opinion, but it works fine as a rule of thumb.

All of this is evolution via the postulate of the continuous unitary process (Schrodinger), plus a tad of chaos. But there is also the postulate that measurements are random variables and the probabilities are not in principle derivable.
How am I to reconcile the two postulates? I've received many conflicting answers and remain conflicted. Is this "the measurement problem"?
You're my last hope, otherwise it's quantum suicide.

 

[Derek P]

#

All of this is evolution via the postulate of the continuous unitary process (Schrodinger), plus a tad of chaos. But there is also the postulate that measurements are random variables and the probabilities are not in principle derivable.
How am I to reconcile the two postulates? I've received many conflicting answers and remain conflicted. Is this "the measurement problem"?
You're my last hope, otherwise it's quantum suicide.

Under unitary evolution there is no selection of an eigenstate and no randomness.
Histories are thus superposed.
Probabilities emerge as frequencies in a history.
The emergence of a preferred basis seems to be explained okay.
All that's left of the measurement problem is dealing with the idea that there are actual probabilities as well. Seems simplest to say there aren't any but I'm told that that is philosophy and not part of science. :)

 

[StevieTNZ]

Here is some information from a chapter written in the book referenced below, by Professor Jeffrey Barrett:

Corradini, A., & Meixner, U. (Eds.). (2014). Quantum Physics Meets the Philosophy of Mind: New Essays on the Mind-Body Relation in Quantum-Theoretical Perspective. Berlin/Boston: Walter de Gruyter GmbH.

 

[haushofer]

I never claimed that. I claimed one can, if they wish, that an improper mixed state can be considered a proper one. This is the modern clear and unambiguous view of collapse - others for me don't make much sense - but each to their own. Its also a choice of where to place the Von-Neumann cut.

Thanks
Bill

Ok, I was quoting from e.g. this topic, https://www.physicsforums.com/threads/measurement-and-basics-of-qm.855073/page-3, which says

...the most natural place to put the Von Neumann cut is just after decoherence which in the Schroedinger Cat experiment is at the particle detector.

 

[haushofer]

Cut is where you can consider things classically from then on. If you put it after decoherence you are saying one of the possible outcomes is now objectively real ie it actually is in that state - but we don't know what state - such are by definition proper states. An improper state gives exactly the same probabilities of outcomes as proper ones - but is it in that state prior to observation? BM and MW would say yes - but others say no or who cares. There is no way to tell. But putting the cut right after decoherence is a simple way for things to be more understandable - for me and others anyway - of course each to their own who may think it total rubbish. For example the high priest of the Ensemble Interpretation Ballentine thinks its rubbish:
https://marcofrasca.wordpress.com/2009/03/10/ballentine-and-the-decoherence-program/

This is science - if it isn't what you like then you can view it anyway you want as long as its consistent with the formalism that everyone agrees on.

Thanks
Bill

Ok, then I get your point. Thanks!