Examining the Myth of Decoherence & the Measurement Problem

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In summary, decoherence is often touted as the solution to the "measurement problem" in quantum mechanics, but upon closer inspection, it appears to be nothing more than a practical tool for working with the theory. The majority of physicists do not believe that decoherence actually resolves the "measurement problem" and view it as a glorified FAPP (for all practical purposes) construct. Even among the minority who claim it does resolve foundational issues, there are alternative approaches and evidence that this is not a definitive solution. There is also a concern about the politics behind different interpretations of quantum mechanics and how they may influence the understanding of reality and existence."
  • #1
Coldcall
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"decoherence" - the myth

It's interesting when one hears supposedly fact-based scientists claim that decoherence has resolved the "measurement problem".

Having been brought up on a staple of QM texts I was flabbergasted when i heard these kind of statements claiming decoherence was the answer to foundational problems of qm. So i decided to investigate these claims and allow myself to be convinced. Having now come to my conclusion that decoherence is nothing other than bunk; its made me wonder about the ethics of qm interpretational research.

I'm not criticising decoherence as a practical framework for working with qm but i am severely critical of the extra weight which has been given to it as a solution to the "measurement problem". For anyone who wants a fair appraisal or what decoherence actually achieves and what it does not i believe the best paper available at the moment is:
2005 updated version:

http://arxiv.org/abs/quant-ph/0312059v4

To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox. So what we basically have here is not the truth but a glorified FAPP type construct.

But let's forget the majority for a moment. What about this minority who claim that decoherence resolves foundational issues such as the "measurement problem". If i as a layman can work out they are talking rubbish (re the real problem) then what does it say about their motives or their intellect that they feel they can sweep this incredible paradox under the rug with some made-up false construct?

Frankly i am gobsmacked.
 
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  • #2


Coldcall said:
So what we basically have here is not the truth but a glorified FAPP type construct.
A criticism rejected as unjustified by the very author you cite...
 
  • #3


Hurkyl said:
A criticism rejected as unjustified by the very author you cite...

The author of that paper says the same thing except he is far more diplomatic and "round the houses" in his manner of objecting to the false claims about "decoherence" solving the measurement problem. He is a phycists after all and i don't expect him to come out with guns blazing.

He praises decoherence as a practical tool, as a way of seeing the wave collapse in a different manner. That would be like praising algebra as a useful tool but it tells us nothing about objctive reality.

Its all about "appearances". Seriously if you want to fall into the FAPPTRAP be my guest but do not confuse a practical framework for a foundational truth. And its the foundational truth which is what we are after in concerns to the "measurement problem".

Decoherence fails in providing any more knowledge about the meaning of qm for our reality.
 
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  • #4


Coldcall said:
It's interesting when one hears supposedly fact-based scientists claim that decoherence has resolved the "measurement problem".

Having been brought up on a staple of QM texts I was flabbergasted when i heard these kind of statements claiming decoherence was the answer to foundational problems of qm. So i decided to investigate these claims and allow myself to be convinced. Having now come to my conclusion that decoherence is nothing other than bunk; its made me wonder about the ethics of qm interpretational research.

I'm not criticising decoherence as a practical framework for working with qm but i am severely critical of the extra weight which has been given to it as a solution to the "measurement problem". For anyone who wants a fair appraisal or what decoherence actually achieves and what it does not i believe the best paper available at the moment is:
2005 updated version:

http://arxiv.org/abs/quant-ph/0312059v4

To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox. So what we basically have here is not the truth but a glorified FAPP type construct.

But let's forget the majority for a moment. What about this minority who claim that decoherence resolves foundational issues such as the "measurement problem". If i as a layman can work out they are talking rubbish (re the real problem) then what does it say about their motives or their intellect that they feel they can sweep this incredible paradox under the rug with some made-up false construct?

Frankly i am gobsmacked.

Can you point to me physics papers that actually claim that decoherence has resolved the measurement problem? It makes no sense, even among those who have studied decoherence, because we continue to study these things even today.

There are certainly many papers that try to link decoherence as the "culprit", but I have never seen one that claim to have the definitive "smoking gun". In fact, there are other alternative approaches to decoherence published in major journals, such as this:

https://www.physicsforums.com/showpost.php?p=1520644&postcount=59

So this is ample evidence that no one is thinking that this is a done deal.

Zz.
 
  • #5


The point i am really trying to get at is the "politics" behind qm interpretations. I'm not knocking Zurek's work. It's a very clever way to explain the wave collapse mechanism simply as a leakage of a quantum state/superposition into the wider (collapsed) environment. And equally there is nothing wrong with the interpretation as a FAPP placeholder. You want students to shut up and calculate; give them the "decoherence" version of events and unless they had previously read various qm books they would be forever ignorant of the deeper implications of what it is telling us about the nature of reality.

Problem is physics has finally encountered nature at a level which has significant implications about being, existence, and other ontological subjects. So far, it appears we have some of the physics community in a sort of self delusional state where they are so desperate to disprove the observers role in wave function collapse that they will believe almost anything else as long as it appears to resolve the "measurement problem".

And its not just phycists - who at least have an excuse that they need to see the world in objective terms to do good science. Many modern philosophers are woefully ignorant even to the most basic tenets of qm and hence they develop ideas which appear grounded in classical physics! That is almost funny if not so sad.
 
  • #6


ZapperZ,

"Can you point to me physics papers that actually claim that decoherence has resolved the measurement problem? It makes no sense, even among those who have studied decoherence, because we continue to study these things even today."

The paper i linked has various quotes from phycists implying if not explicitly stating that "decoherence" had solved the "measurment problem". Its in the first section where he gives the "for and against" quotes from various phycists.

Yes i agree it is nonsense but it is often used as a disingenuous way to reject the observer's role.
 
  • #7


ZapperZ,

There are certainly many papers that try to link decoherence as the "culprit", but I have never seen one that claim to have the definitive "smoking gun". In fact, there are other alternative approaches to decoherence published in major journals, such as this:

https://www.physicsforums.com/showpos...4&postcount=59 [Broken]

So this is ample evidence that no one is thinking that this is a done deal.


Yes the above theory kind of makes my original point. Why is everyone so desperate to eject the observer's role? Its like a religious movement to prove that the world is flat after all! Get used to it folks, QM tells us something peculiar about the nature of our reality and we have a distinct role in defining the world around us.

No extra interpretational work is necessary until we understand what constitutes an observer. When we understand that we may be closer to understanding what is the actual causal factor in wave collapse.
 
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  • #8


Coldcall said:
ZapperZ,

There are certainly many papers that try to link decoherence as the "culprit", but I have never seen one that claim to have the definitive "smoking gun". In fact, there are other alternative approaches to decoherence published in major journals, such as this:

https://www.physicsforums.com/showpos...4&postcount=59 [Broken]

So this is ample evidence that no one is thinking that this is a done deal.


Yes the above theory kind of makes my original point. Why is everyone so desperate to eject the observer's role? Its like a religious movement to prove that the world is flat after all! Get used to it folks, QM tells us something peculiar about the nature of our reality and we have a distinct role in defining the world around us.

No extra interpretational work is necessary until we understand what constitutes an observer. When we understand that we may be closer to understanding what is the actual causal factor in wave collapse.

You have not answered my question. Please point to me evidence that the physics community has accepted decoherence as the definitive reason for the measurement problem. Point to me reputable, peer-reviewed papers that make such claims.

If not, your starting premise of your complaint is faulty. You want us to explain to you a non-existent scenario, which would be a waste of time for everyone involved. So it is you who need to examine how you were able to draw up such a conclusion in the first place.

Zz.
 
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  • #9


Coldcall said:
ZapperZ,

The paper i linked has various quotes from phycists implying if not explicitly stating that "decoherence" had solved the "measurment problem". Its in the first section where he gives the "for and against" quotes from various phycists.

Where does it say that? Read the final paragraph:

It is fair to say that the decoherence programsheds new light on many foundational aspects of quantum mechanics. It paves a physics-based path towards motivating solutions to the measurement problem; it imposes constraints on the strands of interpretations that seek such a solution and thus makes them also more and more similar to each other. Decoherence remains an ongoing field of intense research, in both the theoretical and experimental domain, and we can expect further implications for the foundations of quantum mechanics from such studies in the near future.

Where does it say that this is settled?

Zz.
 
  • #10


ZapperZ,

What's funny is you are now behaving defensively like i once did when i was told that decoherence removes the observer's role. :approve:

"Please point to me evidence that the physics community has accepted decoherence as the definitive reason for the measurement problem. Point to me reputable, peer-reviewed papers that make such claims."

Excuse me; I've always said the "majority" agree it doesn't resolve the "measurement problem". You are putting words in my mouth. My previous post was clear on this point:

"To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox."

Where does it say that this is settled?

Never said it was. I'm arguing the other way - if you had not yet noticed :smile:

By the way the quote i was suggesting to you from that paper is:

"In his monumental book on the foundations of quantum
mechanics (QM), Auletta (2000, p. 791) concludes that
the Measurement theory could be part of the interpretation
of QM only to the extent that it
would still be an open problem, and we think
that this is largely no longer the case.
This is mainly so because, according to Auletta (2000,
p. 289),
decoherence is able to solve practically all the
problems of Measurement which have been discussed
in the previous chapters
"

my emphasis.

By the way, there are lots of these tricky statements which suggest decoherence means we no longer need to worry about the "measurement problem". They are out there..look them up.

However i agree that most mainstream phycists now accept that decoherence has not resolved the "measurment problem". ie. observer controversy - what it should really be called.
 
  • #11


Coldcall said:
ZapperZ,

What's funny is you are now behaving defensively like i once did when i was told that decoherence removes the observer's role. :approve:

Er... defensively? I have no idea what this is about, actually because I haven't made any decision on whether decoherence actually plays any significant role in the measurement problem.

"Please point to me evidence that the physics community has accepted decoherence as the definitive reason for the measurement problem. Point to me reputable, peer-reviewed papers that make such claims."

Excuse me; I've always said the "majority" agree it doesn't resolve the "measurement problem". You are putting words in my mouth. My previous post was clear on this point:

"To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox."

Then I may have interpreted your intention incorrectly.

However, I still do not see who you are criticizing here. Are you up-in-arms against the MWI adopters, who, btw, have bigger fish to fry with their model than the "measurement problem"? They have their hands full already addressing the sharp criticism form Leggett that MWI is nothing more than "window dressing". What brought about this thread in attacking decoherence as a "myth"?

Zz.
 
  • #12
I have seen an exercise essay written by an undergraduate student, which claimed that the Schrödinger's cat paradox is basically solved with the decoherence explanation. Okey, this is not a published paper, like what ZapperZ was asking for, but notice: It was not an undergraduate's own personal project. It was a supervised exercise, and the student got credit for it when approaching his graduation.

Perhaps there're researchers out there, who are more interested in teaching their beliefs to the students, than publishing them in peer-reviewed journals? :biggrin: :rolleyes:

There should be surveys carried out, to figure out what kind of beliefs really are dominant. I understood from the discussion in this thread, that the majority of physicists are not believing that decoherence is here to solve the measurement problem, but I would still be curious to know some percentage numbers. The evolution of beliefs should be observed in this business.
 
  • #13


ZapperZ,

"However, I still do not see who you are criticizing here. Are you up-in-arms against the MWI adopters, who, btw, have bigger fish to fry with their model than the "measurement problem"? They have their hands full already addressing the sharp criticism form Leggett that MWI is nothing more than "window dressing". What brought about this thread in attacking decoherence as a "myth"?"

Yes i constantly attack MWI on various fronts as well - my critcism of interpretations is not restricted to decoherence :smile:

I'm for Copenhagen and even a literal Copenhagen taking the observer into expilicit account as the causation for wave function collapse. Experiments have repeatedly demonstrated that a human observer has a special, and as far as we know *unique*, relationship with matter/energy at its most reductive scale. How and why we don't know but that is the raw fact of the matter.

Each of these other interpretations ignore the obvious and demonstrated causal agent in a wave function collapse. What they all appear to be attemtpting to do is falsify the essentiality of an observer. We don't need a different framework FAPP because qm is still the same beast mathematically. So what's the point of these alternatives?
 
  • #14


jostpuur said:
I have seen an exercise essay written by an undergraduate student, which claimed that the Schrödinger's cat paradox is basically solved with the decoherence explanation. Okey, this is not a published paper, like what ZapperZ was asking for, but notice: It was not an undergraduate's own personal project. It was a supervised exercise, and the student got credit for it when approaching his graduation.

Perhaps there're researchers out there, who are more interested in teaching their beliefs to the students, than publishing them in peer-reviewed journals? :biggrin: :rolleyes:

There should be surveys carried out, to figure out what kind of beliefs really are dominant. I understood from the discussion in this thread, that the majority of physicists are not believing that decoherence is here to solve the measurement problem, but I would still be curious to know some percentage numbers. The evolution of beliefs should be observed in this business.

Yes i agree. Its a bit like the argument about Darwinism vs Creationism in teaching. Of course its in no way as serious as that issue but I feel that neglecting to inform physics students of the real enigma, or observer paradox in qm, is a "bit" like refusing to teach Darwinism. Interpretations such as decoherence and even MWI allow a different interpretation to be taught which if taken on face value, would be mis-leading to the students about the deeper meaning of qm.
 
  • #15


Coldcall said:
ZapperZ,

Experiments have repeatedly demonstrated that a human observer has a special, and as far as we know *unique*, relationship with matter/energy at its most reductive scale. How and why we don't know but that is the raw fact of the matter.

Each of these other interpretations ignore the obvious and demonstrated causal agent in a wave function collapse. What they all appear to be attemtpting to do is falsify the essentiality of an observer. We don't need a different framework FAPP because qm is still the same beast mathematically. So what's the point of these alternatives?



"It would seem that the theory [quantum mechanics] is exclusively concerned about "results of measurement", and has nothing to say about anything else. What exactly qualifies some physical systems to play the role of "measurer"? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system ... with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less "measurement-like" processes are going on more or less all the time, more or less everywhere. Do we not have jumping then all the time?"

- J.S. BELL
Against "Measurement"
 
  • #16


Maaneli said:
"It would seem that the theory [quantum mechanics] is exclusively concerned about "results of measurement", and has nothing to say about anything else. What exactly qualifies some physical systems to play the role of "measurer"? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system ... with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less "measurement-like" processes are going on more or less all the time, more or less everywhere. Do we not have jumping then all the time?"

- J.S. BELL
Against "Measurement"

Maaneli,

I love the way Bell wrote - a real poet type phycist! He would have made learning physics a pleasure with the way he attacked the BIG issues.

I was kind of thinking about this yesterday about my cat. For instance, its odd that cats cannot see themselves in a mirror. They clearly see something but their perception cannot stretch to the concept that they may be looking at themselves.

What if each level of organism can only observe certain physical realities? Hence from a qm perspective they can only collapse certain wave functions which are accessible to them. If this is the case then one can imagine that before humans appeared the world may have looked very different. One could use that same scaling to go all the way back to the first primitive organism.
 
  • #17


Actually the idea about scaling the level of biological/observer awareness into a correlation with physical reality is less crazy than it sounds.

For instance, this grapgh represents (roughly) the growing awareness/consciousness of biology as it has evolved on earth:

humans..................
monkeys............
rats...........
cats........
reptiles......
vegetation...
first microbes.

So this represents a sort of horizon of reality. Each new evolutionary jump adds a little bit more reality to the universe based on what that organism can observe. For instance each biological predecessor kind of prepares the universe for the next evolutionary jump in perception. Of course, as we know with Wheeler's delayed choice experiment, our choices, and i would assume the same for simple organisms though their choices are far more limited, have a retro-causal effect on history. So saying that the universe existed as it does today even when there were only microbes existing is a paradox in itself because all the extra definiton our minds demand from the universe was not existent then.

Is it not plausible that each new biological evolution which makes our minds more complex adds increasingly more detail to our universe?

Again i refer to animals that cannot see themselves in a mirror. Its got nothing to do with their eyes, but all to do with their ability to perceive and self awareness. So the extent of an organism's consciousness/awareness could be what controls "scalable" wave function collapse.

Further more i would argue that based on the above theory; Schrodinger's cat is a valid observer and would collapse the wave function before anyone looked to see if it was alive or dead. The wave function would collapse for the simple reason we have set up the experiment in the first place. The cat has the ability to perceive a gunshot, or anything else which is harmful to it, so there is no paradox.

Am i missing something?
 
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  • #18


Coldcall said:
Yes i constantly attack MWI on various fronts as well - my critcism of interpretations is not restricted to decoherence :smile:

Ok, try me :cool:

MWI is my preferred *interpretation* of quantum mechanics. But you will be disappointed: I'm not claiming that nature IS that way. I'm claiming that MWI is a good way to *picture* the way the quantum formalism is working. I keep open the possibility that against all odds, quantum mechanics might be a "true" description of nature, but I don't really think that we can say so until we have a better understanding about the interplay of gravity and quantum mechanics.

I'm for Copenhagen and even a literal Copenhagen taking the observer into expilicit account as the causation for wave function collapse. Experiments have repeatedly demonstrated that a human observer has a special, and as far as we know *unique*, relationship with matter/energy at its most reductive scale. How and why we don't know but that is the raw fact of the matter.

Let me go in your direction. I think that solipsism is then even better. I think that there is actually only ONE important observer in nature, and that's me. I think that before I was born, the classical world didn't exist, and I think that after I'm dead, it won't exist anymore. All the other human beings, animals and rocks are not true conscious beings, some only act behaviorally that way. I'm the universal wavefunction collapser. Not you. Just me. Now you again.
 
  • #19


Vanesch,

"I'm claiming that MWI is a good way to *picture* the way the quantum formalism is working. I keep open the possibility that against all odds, quantum mechanics might be a "true" description of nature, but I don't really think that we can say so until we have a better understanding about the interplay of gravity and quantum mechanics"

Yes i would not argue with any of that - sorry to dissapoint :smile:

You are distinguishing a way to "picture" qm and you have your favoured way to see things. I also understand why MWI appears a good interpretation because it allows all probabilities to actually exist instead of them all dissapearing into thin air once a measurement or decoherence occurs. I also think MWI seems more reasonable from the Schrodinger's cat paradox, if its a genuine paradox. However as I've said before i think Schrodinger's cat is not the paradox it appears to be and the cat will collapse the wave function before any observer looks in the box.

By the way has a (non-lethal) Schrodinger's cat experiment ever actually been done? Surely its pretty simple to test using a water gun or something that won't kill the cat?

"Let me go in your direction. I think that solipsism is then even better. I think that there is actually only ONE important observer in nature, and that's me. I think that before I was born, the classical world didn't exist, and I think that after I'm dead, it won't exist anymore. All the other human beings, animals and rocks are not true conscious beings, some only act behaviorally that way. I'm the universal wavefunction collapser. Not you. Just me. Now you again."

I think that's going too far. I do actually believe that we are all capable of collapsing wave functions, but on a scalable system which means that animals can only collapse a wave function of which they can percieve because of their limited sensory devices. So humans can work with atomic matter because we can set up the double slit. A cat could not because they don't have the consciousness or self-awareness, nor the sensory devices to force a photon to behave with particle/wave duality.
 
  • #20


Coldcall said:
However as I've said before i think Schrodinger's cat is not the paradox it appears to be and the cat will collapse the wave function before any observer looks in the box.

The Schroedinger cat paradox is a paradox for the quantum formalism - but surely not "for real". The Schroedinger cat paradox illustrates that IF we accept a unitary time evolution all the time, and IF we accept that everything is described by quantum mechanics, then it follows that the "cat state" must have a live and a dead component. It is a dramatic way to illustrate what it really means to have unitary time evolution and the superposition principle: it means you cannot get rid of the extra terms the way you do using a projection.

In other words, the Schroedinger cat paradox pushes your nose on a FORMAL problem: it is impossible for a complicated physical process, described nevertheless by unitary dynamics, to bring about genuine state projection.

No problem, you say. Make time evolution slightly non-unitary (non-linear). There are people who do this. However, we now run into a second difficulty. If we modify the time evolution in such a way as to make projection a physical process, then it becomes impossible to make that Lorentz-invariant. In other words, a physical process that implements a true projection will require an absolute ether frame. This is nothing else but Bell's theorem (with a few additional assumptions, such as no "gods book" correlations, and no superdeterminism).

And here we have a REAL paradox, at least in the idealized gedanken experiment of EPR.

If we can build a real EPR experiment, we will have data that will - independently of any quantum theory - give us a genuine paradoxial situation, unless we give up on relativity. If we insist on keeping the spirit of relativity (we live on a 4-dim spacetime manifold, with no preferred slicing) then true EPR data (as predicted by quantum theory) present a real headache. Funnily, unitary quantum theory can describe it perfectly, and respect relativity.

So, if we keep relativity, and if we keep strict unitarity, then we have the formal problem of the Schroedinger's cat.
If we give up on unitarity and introduce genuine collapse, then, through EPR, we have a problem with relativity. Even without quantum mechanics, the pure data of the EPR paradox would collide with relativity, were it not that unitary quantum theory can explain it and is compatible with relativity.

In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid.

By the way has a (non-lethal) Schrodinger's cat experiment ever actually been done? Surely its pretty simple to test using a water gun or something that won't kill the cat?

What would be the observable effect ? You would open the box, and observe in 50% of the cases, a wet, angry cat, and in 50% of the cases a purring, dry cat.
 
  • #21


Vanesch,

Very interesting post thank you.

"In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid."

Exactly. I don't accept the cat is alive and dead at the same time. In fact, i don't accept the experimental set up would constitute a quantum state in superposition until a human observer opens the box.

The wave function collapse would occur somehwere in the chain between the atom and the cat.
 
  • #22


Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?
 
  • #23


peter0302 said:
Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?

Yes it nicely reduces the phenomemon into an objective process. I think "ensemble/statistical interpretation" fits into that category as well except it kind of side steps any philosophical assumptions whatsoever.

We have interpretations to suit all needs..step right up folks :biggrin:
 
  • #24


Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc. We've got an objective explanation for why classical objects don't display interference, Schrodinger's cat, etc, that requires no new assumptions (like pilot waves or extra worlds), though is compatible with either one or many world scenarios. That makes it a very powerful concept.

Does it solve the measurement problem? Define "solve." If solve means moot otherwise valid objections to QM, then it does that. If solve means "prove this is the way it is", well no theory can do that.
 
  • #25
peter0302 said:
Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?

Removal of the interferences is quite far from the explanation of the collapse.

peter0302 said:
Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc.

What does CI stand for? Collapse Interpretation?
 
  • #26


jostpuur said:
Removal of the interferences is quite far from the explanation of the collapse.
I disagree. Decoherence shows that the branches of the wave function have less and less, and eventually no chance of future interaction as the quantum system becomes more entangled with the environment. While what happens to these other branches cannot be known (i.e. MWI or single-world), "wave function collapse," i.e., our obtaining a single definite value for an observeable, is fully explained. We just don't know why we get the one we get.

What does CI stand for? Collapse Interpretation?
Well, I meant Copenhagen Interpretation, but Collapse Interpretation isn't a bad label for it.
 
  • #27
peter0302 said:
Decoherence shows that the branches of the wave function have less and less, and eventually no chance of future interaction as the quantum system becomes more entangled with the environment.

I agree so far.

While what happens to these other branches cannot be known

What "other" branches? It sounds like you are implicitly assuming, that one branch becomes more special than others, under decoherence. Otherwise it wouldn't make sense to talk about something happening to the other branches.

If some particle is in superposition of several possible eigenstates of some observable quantity, the particle's entanglement with the macroscopic environment is not going to make one eigenstate more special than others. The particle maintains non-zero amplitudes for being on all initial eigenstates. Only the interferences vanish, due to the entanglement with macroscopic environment.
 
  • #28


jostpuur said:
I agree so far.



What "other" branches? It sounds like you are implicitly assuming, that one branch becomes more special than others, under decoherence. Otherwise it wouldn't make sense to talk about something happening to the other branches.

If some particle is in superposition of several possible eigenstates of some observable quantity, the particle's entanglement with the macroscopic environment is not going to make one eigenstate more special than others. The particle maintains non-zero amplitudes for being on all initial eigenstates. Only the interferences vanish, due to the entanglement with macroscopic environment.

I guess peter calls "other branches" those that do not correspond to the observed outcome (by you?).

What decoherence helps to explain is that we don't see livecat+deadcat interference, or some spooky cat state which is in between. Whatever branches will appear, will be "consistent dead cat" and "consistent live cat" branches which, each by themselves, look entirely classical.

What decoherence doesn't explain, is why we are only aware of one of these branches, and, indeed, what happens to those we aren't aware of.
Do they "disappear in a puff of logic" (that's Copenhagen's stance) at a certain point (which is called the Heisenberg cut, and which must be part of whatever is called a measurement) ?
Do they continue their existence, but "we" aren't part of it (but "they" are) ? That's MWI's stance.
Is QM simply a statistical description of some underlying process - in other words, is the quantum wave function simply some description of our knowledge about a system, and not a physical description of the system itself ? That would then explain the "collapse" as just an "update of our information" ? But it begs then the question of what IS a physical description.
Is the wavefunction just part of the physical state, and is an extra physical quantity present which indicates which branch of the wavefunction is the "right" one ? That's Bohm, or any other hidden-variable theory.

That's still the remaining interpretational difficulty. Decoherence didn't solve that. Decoherence removed unnecessary objections to the continued existence of the different branches, namely the "obvious objection" of seeing a cat in a superposed state of dead and live.
In other words, decoherence indicated us that we probably won't be able to find out experimentally whether the branches "really continue to exist" or whether "they collapse", as experimentally, they will give identical, classical results.
 
  • #29


peter0302 said:
Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc. We've got an objective explanation for why classical objects don't display interference,

I think peter really captured the curx of the arugument here, in capturing the real value of decoherence. Though certainly decoherence likely does not solve the measurement problem by itself (though there a fair number who believe it may) it removes one of the most disconcerting aspects of QM, ultimate subjectivity. Decoherence seems to be highly effective at restoring an objective world without the need of an observer, and articulating the mechanism which helps bridge the ambiguous quantum world, with the familiar classical one. It needs further refinement yet, but many in the physics community believe that it will be an important part of any eventual solution to the measurement problem. I personally find it unsettling the some tend to try to minimalize its importance in closing the chasm between two different realms of nature.
 
  • #30


Coldcall said:
Maaneli,

I love the way Bell wrote - a real poet type phycist! He would have made learning physics a pleasure with the way he attacked the BIG issues.

I was kind of thinking about this yesterday about my cat. For instance, its odd that cats cannot see themselves in a mirror. They clearly see something but their perception cannot stretch to the concept that they may be looking at themselves.

What if each level of organism can only observe certain physical realities? Hence from a qm perspective they can only collapse certain wave functions which are accessible to them. If this is the case then one can imagine that before humans appeared the world may have looked very different. One could use that same scaling to go all the way back to the first primitive organism.





Hey Coldcall,

I appreciate and respect your comments on this post about the overselling of decoherence as a myth to the measurement problem and the way you defended yourself against ZapperZ.

But, I'm not sure if you know that the above quote by Bell is actually intended to ridicule the CI projection postulates. Bell himself was a stauch advocate of hidden variable (or more precisely "beable") formulations of QT like the de Broglie-Bohm pilot wave theory and GRW spontaneous collapse - neither of which require the existence of human observers to invoke projection postulates.

By the way, I believe your idea of using the consciousness of animals to cause collapse might have also been along the same lines as the ideas of Eugene Wigner on the measurement problem. I think he also believed that consciousness causes collapse. However, I would like to point out some obvious questions and difficulties with the view you propose:

When there were no living organisms around on Earth (~4.5 billion years ago) there would be nothing to collapse the wavefunctions of the universe; do you think that means that the universe was in a coherent superposition state? Also if living organisms did cause collapse, what would be the range of their ability to collapse the wavefunctions of nonliving matter? Would organisms on Earth be able to collapse the wavefunction of matter 100 light years away or only here on earth? How would you quantify such an ability?

Even if you said there may be other life on other planets, even they did not exist a certain time back such as 11 billion years ago. So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort.

Anyway, just some things to consider.
 
  • #31


Coldcall said:
"In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid."

Exactly. I don't accept the cat is alive and dead at the same time.

But could you accept two worlds, one in which there is a live cat, and one in which there is a dead cat, you living in one of them ?
 
  • #32


Maaneli said:
So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort.

Anyway, just some things to consider.

First of all, I would say that the application of quantum theory to cosmology is certainly as of now ill-defined, as we don't know how to deal with gravity in quantum theory. But apart from that, what do you think would be the observable consequences of coherent unitary state vector evolution during the early universe ? In what way would that imply anything different than the classical evolution one would have otherwise ?
 
  • #33


Maaneli,

"But, I'm not sure if you know that the above quote by Bell is actually intended to ridicule the CI projection postulates. Bell himself was a stauch advocate of hidden variable (or more precisely "beable") formulations of QT like the de Broglie-Bohm pilot wave theory and GRW spontaneous collapse - neither of which require the existence of human observers to invoke projection postulates."

Yes I'm aware of his views on Bohm and i don't agree with him there, and I think the be-ables is again just another "treatment" of the problem. However he himself admitted the demise of locality, and most phycists would say hidden variables are not viable. Ironic because he falsified his own argument for hidden variables.

But all this does not stop me respecting him and his deep views on qm :)

"By the way, I believe your idea of using the consciousness of animals to cause collapse might have also been along the same lines as the ideas of Eugene Wigner on the measurement problem. I think he also believed that consciousness causes collapse."

Yes though I don't know know whether "observership" consitutes "consciousness" or vice versa. I do think its related to biology in some causal fashion. And considering we now know that quantum effects occur in biology the argument that it does not occur in our brains is unsupportable. Max Tegmark argued this in 2000 and new evidence fasilfies that argument against quantum "consciousness". That does not prove its happening but its taken away another reason for it not to be happening.

"When there were no living organisms around on Earth (~4.5 billion years ago) there would be nothing to collapse the wavefunctions of the universe; do you think that means that the universe was in a coherent superposition state? Also if living organisms did cause collapse, what would be the range of their ability to collapse the wavefunctions of nonliving matter? Would organisms on Earth be able to collapse the wavefunction of matter 100 light years away or only here on earth? How would you quantify such an ability?"

Good questions :) But this is not my theory; it was first proposed by Wheeler in PAP. First of all, if Quantum mechanics is really fundamental to the universe then a universal wave function scenario would be quite natural. In fact Hawking & Harte have used the same idea but their conclusions don't involve the biological scope achieved by Wheeler.

As i understand the theory; In a wave of universe probabilities the one which evolves subjectively to produce the first self-evolved biological system collapses in a retro-causal manner. The microbe would be able to sense something, so the universe at that moment would be as defined or developed as was necessary for that microbe to have evolved in the probablity wave. Consistency is maintained. So in essence that first little microbe or nanobe or whatever it was held open the reality wave - our universe. I don't think the microbe has to have the "collapsing range" in the way you are looking at it. If QM, through entanglement or whatever function insists on consistency then the microbe could have very far reaching retro-causal effects.

"Even if you said there may be other life on other planets, even they did not exist a certain time back such as 11 billion years ago. So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort."

I'm not quite sure what you mean in the last part but I think the state of the universe was more or less fixed by the evolution of the first microbe, again because of consistent histories. However I'm not saying later observations did not cause further retro-causal history to occur. In fact we know the universe has changed in certain profound ways during its development, dark energy, acceleration etc..

If you can point out some sort of cosmological evidence that rules out a retro-causal universe wave form i'd be interested in reading it. Wheeler's Delayed choice showed retro-causality is real. I believe the quantum eraser is kind of similar.

Just to add: Why i like this theory so much is because it

1) Accepts the observer as causal agent as every qm experiment has demonstrated.
2) Solves the biocentric tuning problem - anthropic coincidences
3) Solves the reason why it appears to be a fluke anything self-organised at all, because in a wave of universal probability, the HUP kicks in and the practically impossible becomes possible.
4) Gives a part for consciousness/awareness as opposed to zombie like machine biology.

Atleast Quantum physics actually answers how the universe could have got started and produced biology against all the odds.
 
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  • #34


Coldcall said:
I'm not quite sure what you mean in the last part but I think the state of the universe was more or less fixed by the evolution of the first microbe, again because of consistent histories.

You now have a problem here: that cosmic first microbe (as a material structure) only appeared in certain terms in the wavefunction, at a certain time. In fact, there are terms of the wavefunction which had other microbe-states way way way before microbes appeared in "our" universe. They were just in branches with smaller complex coefficients. So how come that *they* didn't collapse the wavefunction, but our "latecomer" microbes did ?

See, we have:

|wavefunction_at_time_0> = |quarks>|photons>...

evolution of the wavefuntion, still no microbes:

|wavefunction_at_time_t1> = |dusty universe> + |universe with stars1> + |universe with stars2> + ... + |universe with stars3234> + ...|universe with one big black hole>

evolution of wavefunction:

|wavefunction_at_time_t2> = |dusty universe> + |universe with stars 1 and a unicorn> + |universe with stars 2 and no microbes> + ... |universe with stars3234> + ...|universe with one big black hole>

evolution even further:

|wavefunction_at_time_t3> = |dusty universe> + |universe with stars 1 and a unicorn and some flying bananas> + |universe with stars 2 and our microbes> + ... |universe with stars3234 and very intelligent giant ants> + ...|universe with one big black hole>

evolution still further:

|wavefunction_at_time_t3> = |dusty universe> + |universe with stars 1 and a unicorn and some flying bananas> + |universe with stars 2 and humans and all that> + |universe with stars 2 which looks like ours, but in which the dinosaurs weren't destroyed> + ... |universe with stars3234 and very intelligent giant ants who killed themselves> + ...|universe with one big black hole>

So why did the unicorn at time t2 not collapse the wavefunction, but why did our microbes do so at t3 ?

If you reply by saying that our microbes were the first, then that's definitely not true. There WAS an immensely small amplitude to have a universe with a unicorn at almost any time in the sense that the amplitude for the particles to be "by concidence" in the right configuration to make a unicorn pop out of dust at any time (even right now) wasn't strictly 0. These amplitudes are so small that they represent almost zero odds. But they are not 0.
 
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  • #35


Vanesch,

"You now have a problem here: that cosmic first microbe (as a material structure) only appeared in certain terms in the wavefunction, at a certain time. In fact, there are terms of the wavefunction which had other microbe-states way way way before microbes appeared in "our" universe. They were just in branches with smaller complex coefficients. So how come that *they* didn't collapse the wavefunction, but our "latecomer" microbes did ?"

i think i understand what you are asking. Not sure i can answer 100% but I'll give it a try :smile:

The way I see Wheeler's PAP is that because its based on an observer-centric version of qm only once a biology has occurred which is capable of meeting the requirments needed for observer status does the universal wave function collapse. So the first universe to create a microbe capable of being an observer is the one that collapses, so there cannot have been any previous microbe-states otherwise they would have come into existence instead. However that doesn't rule out that sometimes, or maybe more often than not, the microbe evolves in the superposition, the wave function collapses into a real universe and then the microbe can't reproduce and dies. hence with no observer the universe disappears down the memory hole. Like a recycling bin.

"If you reply by saying that our microbes were the first, then that's definitely not true. There WAS an immensely small amplitude to have a universe with at almost any time in the sense that the amplitude for the particles to be "by concidence" in the right configuration to make a unicorn pop out of dust at any time (even right now) wasn't strictly 0. These amplitudes are so small that they represent almost zero odds. But they are not 0."

But the odds that a universal wave function evolves a viable Unicorn-inhabited universe before the one with microbes makes it almost impossible. Let's face it, the unicorn-centric fine tuning which would need to go into the wave function for such an outcome is highly unlikely. Our microbe universe, even though it is wildly coincidental as it is, is a darn sight more probable than a universe which spontaneously evolves unicorns as its first valid observer.
 
<h2>1. What is the measurement problem in quantum mechanics?</h2><p>The measurement problem in quantum mechanics refers to the issue of how a quantum system, which exists in multiple states simultaneously, can collapse into a single state when it is observed or measured. This is in contrast to classical physics, where measurements do not affect the state of the system being measured.</p><h2>2. What is decoherence and how does it relate to the measurement problem?</h2><p>Decoherence is the process by which a quantum system interacts with its environment, causing its superposition of states to collapse into a single state. This explains why macroscopic objects appear to follow classical physics, as their interactions with the environment quickly lead to decoherence. Decoherence does not fully solve the measurement problem, but it offers a possible explanation for why we only observe one state of a quantum system.</p><h2>3. Is decoherence the only explanation for the measurement problem?</h2><p>No, there are other proposed solutions to the measurement problem, such as the Copenhagen interpretation, which states that the act of measurement itself causes the collapse of the wave function. There is also the many-worlds interpretation, which suggests that the universe splits into multiple parallel universes when a measurement is made, with each possible outcome occurring in a different universe.</p><h2>4. How do scientists study and test the concept of decoherence?</h2><p>Scientists study and test decoherence through experiments that measure the state of a quantum system and its interactions with its environment. They also use mathematical models and simulations to understand how decoherence may occur in different systems. Additionally, advancements in technology have allowed for more precise measurements of quantum systems, providing further evidence for the role of decoherence in the measurement problem.</p><h2>5. Can decoherence be applied to other fields of science?</h2><p>Yes, the concept of decoherence has been applied to other fields such as biology, chemistry, and cosmology. In biology, it has been proposed as an explanation for how living organisms maintain their quantum coherence despite interacting with their environment. In chemistry, it has been used to explain why certain reactions occur more frequently than others. In cosmology, it has been suggested as a possible explanation for the origins of the universe and the formation of large-scale structures like galaxies and clusters of galaxies.</p>

1. What is the measurement problem in quantum mechanics?

The measurement problem in quantum mechanics refers to the issue of how a quantum system, which exists in multiple states simultaneously, can collapse into a single state when it is observed or measured. This is in contrast to classical physics, where measurements do not affect the state of the system being measured.

2. What is decoherence and how does it relate to the measurement problem?

Decoherence is the process by which a quantum system interacts with its environment, causing its superposition of states to collapse into a single state. This explains why macroscopic objects appear to follow classical physics, as their interactions with the environment quickly lead to decoherence. Decoherence does not fully solve the measurement problem, but it offers a possible explanation for why we only observe one state of a quantum system.

3. Is decoherence the only explanation for the measurement problem?

No, there are other proposed solutions to the measurement problem, such as the Copenhagen interpretation, which states that the act of measurement itself causes the collapse of the wave function. There is also the many-worlds interpretation, which suggests that the universe splits into multiple parallel universes when a measurement is made, with each possible outcome occurring in a different universe.

4. How do scientists study and test the concept of decoherence?

Scientists study and test decoherence through experiments that measure the state of a quantum system and its interactions with its environment. They also use mathematical models and simulations to understand how decoherence may occur in different systems. Additionally, advancements in technology have allowed for more precise measurements of quantum systems, providing further evidence for the role of decoherence in the measurement problem.

5. Can decoherence be applied to other fields of science?

Yes, the concept of decoherence has been applied to other fields such as biology, chemistry, and cosmology. In biology, it has been proposed as an explanation for how living organisms maintain their quantum coherence despite interacting with their environment. In chemistry, it has been used to explain why certain reactions occur more frequently than others. In cosmology, it has been suggested as a possible explanation for the origins of the universe and the formation of large-scale structures like galaxies and clusters of galaxies.

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