Decoherence relationship to the measurement problem

[Feeble Wonk]

I shouldn't ask this, but I can't help myself... stevendaryl raises a very pertinent question. What determines the limit of "perceived" quantum superposition? I can't be in Georgia and New York at the same time, because it wouldn't be a logically coherent quantum state. But, I can perceive the interference bars in the double slit experiment. They form a macroscopically observable structure that represents a confluence of superpositional outcomes. What does this say about the state of the entire quantum system that includes the brain of the observer?

 

[atyy]

I shouldn't ask this, but I can't help myself... stevendaryl raises a very pertinent question. What determines the limit of "perceived" quantum superposition? I can't be in Georgia and New York at the same time, because it wouldn't be a logically coherent quantum state. But, I can perceive the interference bars in the double slit experiment. They form a macroscopically observable structure that represents a confluence of superpositional outcomes. What does this say about the state of the entire quantum system that includes the brain of the observer?

In a double slit experiment you still perceive definite outcomes. The interference bars are made of macroscopic spots.

 

[atyy]

Well, yes, that's how I think it's usually presented. But I do not see how the bolded part is warranted so the whole idea is mostly philosophy. You either accept it or don't.

If there is only one decomposition into environment/system/apparatus in which decoherence is perfect, the problem is solved. Of course, decoherence is not perfect, so the problem is open. But in a sense, solving the imperfection of decoherence could solve the environment/system/apparatus decomposition problem. I don't know if it really works, but I'd say try David Wallace's approach for a serious attempt at addressing these issues.

 

[Feeble Wonk]

In a double slit experiment you still perceive definite outcomes. The interference bars are made of macroscopic spots.

Well, yes. That's my point. They ARE macroscopic spots. Spots that are what they are, and are WHERE they are because of the unresolved quantum state of the fundamental particles' passage through the slits. They are a macroscopic representation of a quantum state superposition.

 

[atyy]

Well, yes. That's my point. They ARE macroscopic spots. Spots that are what they are, and are WHERE they are because of the unresolved quantum state of the fundamental particles' passage through the slits. They are a macroscopic representation of a quantum state superposition.

Yes, but even a dead cat is a macroscopic representation of a quantum superposition - it is a superposition of "dead-plus-alive" and "dead-minus-alive". I know I just seem to have contradicted myself here, but here unlike above I'm assuming there's no environment that has decohered the cat, because in the double slit experiment there is enough experimental control so that the environment has not decohered the quantum system.

 

[Feeble Wonk]

Yes, but even a dead cat is a macroscopic representation of a quantum superposition - it is a superposition of "dead-plus-alive" and "dead-minus-alive". I know I just seem to have contradicted myself here, but here unlike above I'm assuming there's no environment that has decohered the cat, because in the double slit experiment there is enough experimental control so that the environment has not decohered the quantum system.

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

 

[Feeble Wonk]

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.

 

[atyy]

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

Difficult question. For the issue in many-worlds, I'd take a look at David Wallace's https://www.amazon.com/dp/0199546967/?tag=pfamazon01-20 . I don't know if it works, but I think Wallace does try to address all the difficulties in a very detailed and serious way.

As that is really where decoherence is useful, all I can say is that in the standard shut-up-and-calculate Copenhagenish interpretation, this is subjective and requires common sense. If an experimentalist can control the environment, then she can reverse the decoherence of the cat. So it basically depends on the skill of the experimentalist as to what constitutes an "uncontrolled environment". But of course this is the measurement problem again. I would basically say if you want to have the measurement problem solved, ignore decoherence - look at de Broglie-Bohm or many-worlds. Then within those interpretive structures, decoherence is a useful tool. But taking decoherence alone without specifying an additional interpretive structure will just go round in circles, because as far as we know, decoherence does not solve the measurement problem.

 

[Jilang]

The environment is whatever has enough degrees of freedom to make the probability of a system's evolution and interaction with it being reversible extremely unlikely. It's all in the statistics, but that's just my opinion.

 

[Feeble Wonk]

The environment is whatever has enough degrees of freedom to make the probability of a system's evolution and interaction with it being reversible extremely unlikely. It's all in the statistics, but that's just my opinion.

But in the double slit experiment, once the interaction occurs at the photographic film that demonstrates the interference pattern, the quantum system is irreversible... yet it still represents an event in superposition. I'm sure I could think of a way to magnify the outcome over a greater amount of material and arbitrary amount of mass. The differentiation in environmental control regarding the double slit experiment versus Schroedinger's cat seems like it is not a question of magnitude of material involved, but logical implication of the outcome.

 

[Feeble Wonk]

I've tried to find Wallace's book. Not at my library unfortunately. Maybe I'll just have to buy it. I'll look at Broglie-Bohm and see if it clears things up for me.

 

[Jilang]

Why do you think it is still in superposition when it's measured? It's in superposition when it's passing through the slits.

 

[Feeble Wonk]

Why do you think it is still in superposition when it's measured? It's in superposition when it's passing through the slits.

Yes... But the "measured" outcome is a result of the superposition during the he particle's passage through the slit. The superposition is maintained, resulting in the event that is measured, and is no longer reversible.

 

[Maui]

But in the double slit experiment, once the interaction occurs at the photographic film that demonstrates the interference pattern, the quantum system is irreversible... yet it still represents an event in superposition.

This only happens with small size systems - photons, electrons, etc. It's all due to the HUP and shows why it's only observable at that scale of things.

This should answer your previous question:

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.

 

[Feeble Wonk]

This only happens with small size systems - photons, electrons, etc. It's all due to the HUP and shows why it's only observable at that scale of things.

I guess that's what I'm trying to come to terms with. I'm trying to picture a multiverse application to the double slit experiment. It seems like there would be a set of universes where the particle goes through one slit and another set for the other slit. But because of the uncertainty at the quantum level, allowing the particle to remain in superposition throughout its passage, we experience (observe) a macroscopic universe altogether different than either would be. And the outcome is not reversible. The entire universe evolves from this point forward BECAUSE the particle remained in superposition.

 

[Jilang]

I think the worlds are supposed to split on measurement (wavefunction collapse).

 

[Feeble Wonk]

I think the worlds are supposed to split on measurement (wavefunction collapse).

But the worlds don't split into any of those in which the solitary fundamental particle goes through one slit or the other. The observed macroscopic world is from an altogether different set in which the quantum state of the particle remains in superposition. In fact, the observed macroscopic world is BECAUSE it was in superposition... because of the uncertainty.

And I retract my previous question...

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.

It's a bad comparison. The Geiger counter simply acts in the same way as the particle detector in the double slit experiment, removing the uncertainty and collapsing the wave function (quantum superposition) of the system.

 

[bhobba]

How is the pointer basis selected?

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

 

[bhobba]

This seems as relevant a place as any to ask a question I've had. What would it mean to "observe a quantum world?" I mean we see non-classicality in experiments all the time. I assume when people say the above they mean macroscopic non-classicality? Or how would the world appear exactly, so that we knew it was the case?

In Copenhagen one of the issues is defining precisely what an observation is. In practice its invariably utterly trivial such as in Schrodinger's Cat it happens at the particle detector, but when dealing with matters of principle greater precision is necessary.

In modern times its generally considered to have occurred once decoherence has happened. Indeed if you put collapse right there many issues (but far from all) are easily resolved - eg this silly consciousness created of Von Neumann and Wigner world that some still harp on about (despite the fact Wigner abandoned it when he learned about decoherence) goes out the door.

Thanks
Bill

 

[bhobba]

So, if I understand you properly, the concept of decoherence is really the logical/mathematical process that addresses the entire quantum system ("environment") and rules out the event outcomes that cannot logically exist AT ALL, but still leaves all potential quantum states that CAN logically exist. It then requires something like Everett's "multiverse" theory to whittle down the remaining states to one that is actually manifest. Right?

The rock bottom essence of decoherence as it relates to the measurement problem is that, via the process of tracing over the environment a pure state is converted to a mixture. A mixture can be interpreted as apparent collapse occurring.

Its very hard to go into the details without the math. But if you know that, or are willing to learn it, the following gives the detail, as well as the issues:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Its basically a cut down version of Schlosshauer's text as it pertains to the measurement problem.

Regarding Many Worlds it is far from the only interpretation that uses decoherence - as is explained in the above link.

I personally hold to the ignorance ensemble interpretation which is almost trivially simple. Since there is no way to tell the difference between an improper mixture and a proper one I simply assume its a proper one. But the above link goes into the details of that as well as its pros and cons.

Thanks
Bill

 

[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.