Decoherence relationship to the measurement problem

[bhobba]

I suppose that that is actually what I'm asking about. What defines the "environment" that imparts the environmental control.

That is an issue called the factoring problem.

The math of decoherence uses a decomposition of a system into measurement apparatus, system being measured, and environment.

But some have raised the issue of if decoherence is simply a result of that decomposition - they suggest if you decompose it another way you may not get decoherence. In physics one usually assumes that it doesn't matter how you decompose a problem you get the same answer. Strictly speaking however theorems are required to show regardless of decomposition you get the same answer - its part of the ongoing research.

Thanks
Bill

 

[bhobba]

I suppose a better question would be... Why does what you refer to as the environment/system/apparatus in the double slit experiment allow the maintained superposition of the fundamental particles as they pass through the slit, but not a partially dead cat in Schroedinger's box because of a maintained superposition of alpha decay.

Its to do with interaction with the environment.

Photons interact weakly, cats VERY VERY strongly. If you interact strongly you are entangled and hence are decohered to effectively have classical properties.

Thanks
Bill

 

[Maui]

You need to study the math in standard trexts like Schlosshauer's
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

For example it turns out radial type interactions, of which most are, leads to position being singled out - see page 83 of the above text. Its got to do with the interaction commuting with position.

Thanks
Bill

The irresolvable problem with decoherence and all other interpretations, in plain English, seems to be the transition of Psi to classical probabilities and i think you know this quite well. This is where magic, religion and philosophy is added to science so that some structure can carry all that baggage.

The seemingly easier way out of this to assume that Psi has objective existence and it somehow splits, measures, does whatever it does so that a world appears out of the haze. But the Psi can't exist in space time due to multiparticle states - these have too many dimensions to fit 3+1 spacetime.
So the essence of the transition is unknown and unknowable.

The harder way is to assume that Psi is about the knowledge of the observer - here one has to stick closer to the shut up and calculate camp(ala Bohr) and not engage with the MP at all. There is no way to define how measurements select outcomes and this is why Bohr gave up his quest to understand Nature and said physics is about what we can say about Nature(and it's certainly not all that much but his measurements certainly get things done in the 'real' world).


People easily become disillusioned after all the talk of resolution to the MP only to find that the structure is always supported on philosophical beliefs that have no scientific value and little logical support.

Reality is emergent and a shadow of a different reality and this is not surprizing at this age - the recent discovery of the Amplituhedron comes to mind. If one accepts quantum theory as the fundamental theory of the world, the conclusion is pretty much inescapable and this is a blow to the whole classical paradigm. Decoherence, whatever it is, cannot be a classical phenomenon, as there is no such thing as classicality in quantum theory. It's all make belief. And even if it worked, it wouldn't save the classical world with its causal relationships so what benefit could it offer?

 

[craigi]

I have heard the argument that "decoherence" in quantum states causes the quantum collapse to occur, and that this solves the "measurement problem". But I'm still left with a nagging question... Does decoherence only produce quantum collapse when there is an observation made of the quantum system with sufficient specificity to identify the decoherence. If so, how can that possibly be considered a solution to the "measurement problem"?


Sent from my iPhone using Physics Forums

I've seen this quoted in recent literature too, but as far as I'm concerned dechorence does not solve the measurement problem and I believe that this is the scientific consensus.

Although decoherence has been an active area of research since the 1980's the concept isn't particularly new. It explains why superpositions aren't more commonly observed in the classical realm.

After a system undergoes dechorence, superpositions remain, though many states, very quickly attain vanishingly small probabilities.

This looks like a useful paper, if you want more detail:
http://arxiv.org/pdf/quant-ph/0312059v4.pdf

 

[subquantumboy]

I have heard the argument that "decoherence" in quantum states causes the quantum collapse to occur, and that this solves the "measurement problem". But I'm still left with a nagging question... Does decoherence only produce quantum collapse when there is an observation made of the quantum system with sufficient specificity to identify the decoherence. If so, how can that possibly be considered a solution to the "measurement problem"?Sent from my iPhone using Physics Forums

Everyone knows observing collapses the wave function. It means observers can affect the quantum system. The measurement has nothing to do with the quantum world.

 

[bhobba]

Everyone knows observing collapses the wave function. It means observers can affect the quantum system. The measurement has nothing to do with the quantum world.

Everyone does not know that because its simply not true.

Collapse, if it occurs at all, is very interpretation dependent.

Only very backwater interpretations, held by a small minority these days, have anything to do with observers.

The most common interpretations, Copenhagen and Ensemble, have no role for observers - only for observations. Confusing the two is a common semantic fallacy made by beginning students or those influenced by populist accounts.

Thanks
Bill

 

[craigi]

Everyone knows observing collapses the wave function. It means observers can affect the quantum system. The measurement has nothing to do with the quantum world.

I think you're misunderstanding a couple of things. Try this for a start:
http://en.wikipedia.org/wiki/Quantum_measurement#Measurement_from_a_practical_point_of_view

 

[Feeble Wonk]

Everyone does not know that because its simply not true.

Collapse, if it occurs at all, is very interpretation dependent.

Only very backwater interpretations, held by a small minority these days, have anything to do with observers.

The most common interpretations, Copenhagen and Ensemble, have no role for observers - only for observations.

Thanks
Bill

Just for the sake of discussion... To the extent that the observer makes an observation, could he/she not be considered the apparatus in the environment/system/apparatus entanglement? Would he/she not play a role in collapse?

 

[Jilang]

Just for the sake of discussion... To the extent that the observer makes an observation, could he/she not be considered the apparatus in the environment/system/apparatus entanglement? Would he/she not play a role in collapse?

I'd say not. If it's amplified by the measurement process enough for us to see it, it is already collapsed.

 

[Maui]

Just for the sake of discussion... To the extent that the observer makes an observation, could he/she not be considered the apparatus in the environment/system/apparatus entanglement? Would he/she not play a role in collapse?

There is no such thing as classical environment.

 

[Feeble Wonk]

There is no such thing as classical environment.

Ugh... >_< My ignorance exposed yet again. Could you please help me understand the definition of "environment" in this (quantum) context.

 

[atyy]

There is certainly a classical world in some interpretations. And in some interpretations the classical observer causes the collapse. Some people don't like that, so they say the classical measuring apparatus causes the collapse. I don't think it's a big deal whether one says the classical observer or the classical measuring apparatus causes the collapse, because all these variants impose a cut between the classical and quantum realms, with measurement results on the classical side. Once you impose the cut, you have something not quite satisfactory.

To find an interpretation of quantum mechanics without this cut, one has to try de Broglie-Bohm theory, or perhaps the many-worlds approach.

Decoherence, however, always refers to the quantum side of the cut. It refers to a quantum system interacting with a quantum measuring apparatus immersed in a quantum environment. Decoherence is fully quantum, and is common to all interpretations of quantum mechanics. It also does not solve the measurement problem.

 

[craigi]

Just for the sake of discussion... To the extent that the observer makes an observation, could he/she not be considered the apparatus in the environment/system/apparatus entanglement? Would he/she not play a role in collapse?

Yes they could be considered apparatus.

Yes they would play a role in collapse, under interpretations where collapse takes place, but wave function collapse doesn't take place in all interpretations. In those that it does, this role varies.

In some interpretations, a consciousness actually causes the collapse, in others where collapse does take place, the conscious mind plays no role more significant than any other form of detector.

This is an area that people tend to feel passionately about.

Physicists are much more comfortable in the belief that the mind is simply a manifestation from elementary particles obeying the laws of physics excatly like all others. It follows the tradition by which almost all of us learn physics in and it allows us to separate ourselves from the problem that we're considering, at least conceptually. In reality an experimenter can never be truly isolated from the system under observation. It is possible to maintain sufficient isolation for a period of time, so as not to destroy a system under observation, but inevitably to become aware of a result requires ending such isolation.

To give the mind a significant role, is favoured by some non-physicists, with an interest in the subject. Typically a physicist will see these as hijacking the subject to publicise their own delusionary thinking and sell books of no academic merit. That is not to say that everyone who has an interest in such interpretations is not a physicist and is delusional.

Personally, I think that many physicists, too readily neglect the role of anthropic selection bias, which I believe is relevant no matter which interpretation that you favour. I'm unsure as to whether such anthropic considerations should cause us to tend towards any particular interpretation, but I would say that it is unachievable to completely remove yourself from the problem.

 

[Maui]

Ugh... >_< My ignorance exposed yet again. Could you please help me understand the definition of "environment" in this (quantum) context.

This would be a question for nothing less than a Nobel Prize.
My own research leads me to believe that there is only one consistent 'interpretation' - 'Shut up and calculate'. If you are willing to inject philosophy and personal theories, you can arrive at a multitude of interpretations - mainstream or otherwise.

 

[Feeble Wonk]

Thanks atyy. I haven't been able to find Wallace's book yet that you recommended. Still working on it. My understanding is the de Broglie-Bohm pilot wave concept is not well accepted by consensus within the physicist community. What is the primary objection?

 

[atyy]

Thanks atyy. I haven't been able to find Wallace's book yet that you recommended. Still working on it. My understanding is the de Broglie-Bohm pilot wave concept is not well accepted by consensus within the physicist community. What is the primary objection?

The de Broglie - Bohm theory (and its variants) are well accepted as solutions to the measurement for non-relativistic quantum mechanics.

However, no one has constructed a version of de Broglie - Bohm theory that respects the symmetry of special relativity. Because there is no known violation of the symmetry of special relativity, de Broglie - Bohm theory is not generally accepted as a solution to the measurement problem for relativistic quantum mechanics.

Here is a good overview of various interpretations of quantum mechanics.

http://arxiv.org/abs/quant-ph/0209123
Do we really understand quantum mechanics?
Franck Laloe

 

[craigi]

Ugh... >_< My ignorance exposed yet again. Could you please help me understand the definition of "environment" in this (quantum) context.

When we talk of the "environment" in quantum physics, we're really talking of a complex system where thermodynamically irreversible processes take place. It's basically everything else apart from the isolated quantum system.

 

[Feeble Wonk]

Physicists are much more comfortable in the belief that the mind is simply a manifestation from elementary particles obeying the laws of physics excatly like all others.

It's a bit of a departure from the primary thread subject, but I've got to ask... I understand the desire to frame the concept of mind "mechanisticly"... a manifestation of fundamental particle mechanics in the neuronal tissue of the brain, as you said. But, I'm curious how you (or the typical physicist for that matter) envision the general nature of a fundamental "particle" at this relatively advanced stage of theoretical physics. Do you professionals, or advanced academics, still view the elementary particle as a "solid" piece of something... anything? Or is it really more about the information?

 

[craigi]

It's a bit of a departure from the primary thread subject, but I've got to ask... I understand the desire to frame the concept of mind "mechanisticly"... a manifestation of fundamental particle mechanics in the neuronal tissue of the brain, as you said. But, I'm curious how you (or the typical physicist for that matter) envision the general nature of a fundamental "particle" at this relatively advanced stage of theoretical physics. Do you professionals, or advanced academics, still view the elementary particle as a "solid" piece of something... anything? Or is it really more about the information?

I'm not a professional quantum physicist nor would I describe myself as particulary advanced academically in the field, but I'd say that the contemporary view of fundamental particles is as stable fluctuations in a field.

I'm not sure that there's a good way to visualise them. You could try searching for videos for string theory, but any depiction will miss key properties. I think, in the end you just stop trying to visualise them.

I'd avoid thinking of them as 'solid' since they frequently pass through each other without interacting. 'Information' seems to imply something abstract and devoid of a physical form.

 

[bhobba]

Just for the sake of discussion... To the extent that the observer makes an observation, could he/she not be considered the apparatus in the environment/system/apparatus entanglement? Would he/she not play a role in collapse?

As I have explained collapse is entirely interpretation dependent and may or may not exist.

Yes of course a human being can be the point where the observation occurs - but in most instances that is not the place it happens.

Aside from that these days the whole issue is avoided by placing the observation directly after decoherence avoiding such things entirely.

Thanks
Bill

 

[subquantumboy]

Everyone does not know that because its simply not true.

Collapse, if it occurs at all, is very interpretation dependent.

Only very backwater interpretations, held by a small minority these days, have anything to do with observers.

The most common interpretations, Copenhagen and Ensemble, have no role for observers - only for observations. Confusing the two is a common semantic fallacy made by beginning students or those influenced by populist accounts.

Thanks
Bill

No. It's true. That's self-evident. Observing depends on observers. So observers are the key. Copenhagen or Ensemble doesn't make sense. I disagree.

 

[bhobba]

Do you professionals, or advanced academics, still view the elementary particle as a "solid" piece of something... anything? Or is it really more about the information?

Its viewed in whatever way that's consistent with experimental evidence and is convenient at the time. In the photoelectric effect its convenient to view a photon as a particle is the commonsense way. In the double slit experiment its viewed as a wave. In QED it's viewed as the excitations of a field. Theory shows, in each of those circumstances it is valid, which is all that really matters.

Most physicists do not really give a hoot about such questions - that's really a philosophers game.

Its perfectly understandable that people just starting out in coming to grips with QM ask questions like that, but after you have been exposed to it for a while you understand its a dead end.

Its similar to applying probability in everyday life. Most people don't give much thought to exactly what underlying process is responsible for a coin landing heads or tales, or the weather that will occur tomorrow etc etc. They simply use it. The twist QM adds, is in those cases if you investigate it carefully enough you can understand the underlying process, but in QM that has turned out to be not possible (at least no one has been able to figure out how to do it) - so you just accept it.

Thanks
Bill.

 

[bhobba]

No. It's true. That's self-evident. Observing depends on observers. So observers are the key. Copenhagen or Ensemble doesn't make sense. I disagree.

Mate - before you can argue QM you must understand QM.

Everything I said is from standard textbooks such as Ballentine - Quantum Mechanics - A Modern Development:
https://www.amazon.com/dp/9810241054/?tag=pfamazon01-20

See Chapter 9 - Measurements And Interpretations Of States.

You may not agree with Copenhagen or Ensemble but saying it doesn't make sense is simply exposing your ignorance. The reason is both are simply the formalism with a slightly different take on the role of the state - Copenhagen is more like the Bayesian view of probability, Ensemble is more like the frequentest view. So if they do not make sense QM doesn't really make sense.

But just in case you have spotted some Earth shattering issue the innumerable people that have been exposed to it haven't, mind explaining exactly in what way they do not make sense?

My suspicion is you have not been exposed to the professional literature like the textbook above where the meaning of observation in the sense its used in QM is explained carefully, even some beginning textbooks don't do that, and certainly populist books do not. Its technical meaning is not the usual everyday meaning, which is often a cause of confusion.

Thanks
Bill

 

[WannabeNewton]

Do you professionals, or advanced academics, still view the elementary particle as a "solid" piece of something... anything?

It depends on the context. Fundamentally no they don't because particles are just quanta of fields i.e. photons-EM field and electrons-Dirac field etc. but for various problems yes without a doubt they do. Simple examples wherein the semi-classical picture is useful are relativistic conservation of 4-momentum problems deriving from scatterings and decays.

 

[Feeble Wonk]

Its viewed in whatever way that's consistent with experimental evidence and is convenient at the time. In the photoelectric effect its convenient to view a photon as a particle is the commonsense way. In the double slit experiment its viewed as a wave. In QED it viewed as the excitations of a field. Theory shows, in each of those circumstances it is valid, which is all that really matters.
Most physicists do not really give a hoot about such questions - that's really a philosophers game.
Thanks

Bill.

Of course this is how you would address the nature of the fundamental particle functionally, as a working physicist... because that allows you to make the calculations, and that perspective has resulted in incredible advancements in scientific understanding. But in your more meditative moments, do you really not ponder about this kind of question?

 

[bhobba]

do you really not ponder about this kind of question?

Sure - that's why you come to realize its a dead end.

I think you can see that is philosophy which is an off topic here, but I ask you to consider exactly what philosophers agree on, to me they just seem to go around in circles with endless word games. All physicists however agree on the formalism of QM.

And when you understand it you realize that at its root it is something conceptually quite simple:
http://www.scottaaronson.com/democritus/lec9.html

But each person must come to grips with that one themselves.

Thanks
Bill

 

[Feeble Wonk]

Sure - that's why you come to realize its a dead end.

Fair enough. According to Feynman, "That way madness lies". I was just curious.

 

[bhobba]

Fair enough. According to Feynman, "That way madness lies". I was just curious.

Madness doesn't really lie that way.

It's simply until you can figure out an answer, and it is experimentally verifiable, and such experiments have been done, its a useless endeavor.

Occasionally, some genius like Bell, figures out exactly how to do that, and a bit more light is shined on the issue.

But the fact it only happens occasionally attests to how hard it really is.

Thanks
Bill

 

[Feeble Wonk]

I'd avoid thinking of them as 'solid' since they frequently pass through each other without interacting. 'Information' seems to imply something abstract and devoid of a physical form.

I understand your hesitation with the "information" description. But in the absence of "solidity" even at the fundamental particle level, what image would you consider less abstract, or possessing a more "real" physical form. Non-solid analogies of liquid, gas or plasma don't work, because they are obviously just different phases of the "solid" particles. What physical form does a field fluctuation have? The field is a mathematical construct. So is the quantum wave form. Yes, the "particle" sometimes behaves in a seemingly solid manner, but that's just a description of it's behavior... not it's substance. Why shouldn't we think of the information content of the "particle", field or wave as being the essence of physical form that is "real". I know that this isn't an overly constructive line of discussion Bill, but I can't help myself.

 

[bhobba]

Why shouldn't we think of the information content of the "particle", field or wave as being the essence of physical form that is "real". I know that this isn't an overly constructive line of discussion Bill, but I can't help myself.

Well it depends on what is thought as real.

The particle state, mathematically, is like probabilities, and most would say probabilities are not 'real'.

Classical fields however are a different matter. You have probably seen definitions of fields like an electric field is defined by the force experienced by a charge. In that sense you may think it too is simply something that's introduced for convenience.

Actually it isn't. Classical fields are necessary so that momentum and energy are conserved - if they were not real in some sense then such conservation laws would not be possible - and there are very deep reasons associated with symmetry (look into Noethers Theorem) why it is believed such are true.

Physicists are not driven to these ideas without reason.

Thanks
Bill

 

[bhobba]

But in the absence of "solidity" even at the fundamental particle level, what image would you consider less abstract,

Oh - another point.

The everyday solidity of objects is actually a quantum effect from Pauli's exclusion principle:
http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-hard/

But exactly how it does that is not particularly simple. Freeman Dyson was the first to figure it out in a famous paper in 1965.

Solidity at the atomic level is much much more nebulous.

Thanks
Bill

 

[craigi]

I understand your hesitation with the "information" description. But in the absence of "solidity" even at the fundamental particle level, what image would you consider less abstract, or possessing a more "real" physical form. Non-solid analogies of liquid, gas or plasma don't work, because they are obviously just different phases of the "solid" particles. What physical form does a field fluctuation have? The field is a mathematical construct. So is the quantum wave form. Yes, the "particle" sometimes behaves in a seemingly solid manner, but that's just a description of it's behavior... not it's substance. Why shouldn't we think of the information content of the "particle", field or wave as being the essence of physical form that is "real". I know that this isn't an overly constructive line of discussion Bill, but I can't help myself.

The problem is that you're asking for a depiction of something that has very peculiar properties in time and space. If you follow through an introductory QM course this will become apparent to you.

To give you an analogy: if I give you a pen and paper, could you draw the complete spectrum of human emotion? How would you go about it? Maybe you'd draw a heart or an angry face. It might be an abstract represention of an aspect of what you're trying to draw, but it's just not possible to depict it in its entirety. You don't have the necessary medium. That's the same problem with depicting quantum mechanical objects. You'd be using the classical world to try to represent something that is non-classical.

 

[atyy]

Thanks atyy. I haven't been able to find Wallace's book yet that you recommended.

Wallace's home page http://users.ox.ac.uk/~mert0130/index.shtml links to a short essay which summarizes his recent views on many-worlds: http://philsci-archive.pitt.edu/8888/1/Wallace_chapter_in_Oxford_Handbook.pdf.

 

[Feeble Wonk]

"My suspicion is you have not been exposed to the professional literature like the textbook above where the meaning of observation in the sense its used in QM is explained carefully, even some beginning textbooks don't do that, and certainly populist books do not. Its technical meaning is not the usual everyday meaning, which is often a cause of confusion."

It's been a long time Bill, but I'm still rolling this thread around in my head trying to come to grips with it. And I think your comment that I pasted above might be at the crux of my confusion.
Would you please try to explain to me what the "formal" definition of observation/measurement is with regard to QM, and how that differs from the customary usage. If I fully understand that, maybe the rest will come into focus better.

 

[Feeble Wonk]

My suspicion is you have not been exposed to the professional literature like the textbook above where the meaning of observation in the sense its used in QM is explained carefully, even some beginning textbooks don't do that, and certainly populist books do not. Its technical meaning is not the usual everyday meaning, which is often a cause of confusion.

Thanks
Bill

 

[bhobba]

"Would you please try to explain to me what the "formal" definition of observation/measurement is with regard to QM, and how that differs from the customary usage. If I fully understand that, maybe the rest will come into focus better.

The formal definition unfortunately requires exposure to technical aspects of QM. Its when decoherence has reduced interference terms way below detectability and you have an improper mixed state.

You need to build up to it.

I suggest starting with Susskind:
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20

The customary one is some variant of everyday observation where somebody observes something. In QM it requires no conscious observer. Its assumed to exist in a classical common-sense world.

The problem is that you're asking for a depiction of something that has very peculiar properties in time and space. If you follow through an introductory QM course this will become apparent to you.

Now a question for you - what actual professional literature have you consulted to try and sort this out?

Thanks
Bill

 

[Feeble Wonk]

Thanks Bill... I'll check out your Susskind reference. Was the last question for me or Craigi?

 

[bhobba]

Thanks Bill... I'll check out your Susskind reference. Was the last question for me or Craigi?

It was for you.

But its not an issue now because you will be reading Susskind.

The reason I asked it is people here go to a lot of trouble answering questions and recommending things to read. I just like to know those on the other side make use of it.

Thanks
Bill

 

[Feeble Wonk]

I've got Susskind's book ordered. Looking forward to it. I'm almost afraid to ask, but how mathematically challenging is it. My skills in that arena are worse than pitiful.

 

[Feeble Wonk]

...and for whatever it's worth, I recognize and very much appreciate the time and effort you and the other physics forum contributors provide. I think I've told you in the past that I'm not a physicist. Biology is my field. But the concepts I'm struggling with at present require guidance and insight by real physicists. I'm trying very hard not to be dependent on material in various pseudoscience types of publications. So, thank you.

 

[bhobba]

I've got Susskind's book ordered. Looking forward to it. I'm almost afraid to ask, but how mathematically challenging is it. My skills in that arena are worse than pitiful.

It requires a smattering of calculus.

Thanks
Bill