Quantum Mechanics without Measurement

[craigi]

I agree that MWI and dBB have no measurement problem. The ensemble interpretation is just CI in density matrix language, and has a measurement problem because it has a classical/quantum cut.

Does CH really not have a measurement problem? If CH requires a single framework to be chosen, then who chooses the framework? Or must the observer lie outside, and choose the framework? Or do all frameworks occur?

I might be wrong here, but as I understand it, in the EI, since the wave function describes an ensemble rather than an individual system, there is no collapse, so there is no measurement problem.

Again, I could be wrong here, but in the CH, I think this single framework is the entire classical world containing all obeservers. I don't have a reference for this, so I could be way off the mark.

 

[atyy]

I might be wrong here, but as I understand it, in the EI, since the wave function describes an ensemble rather than an individual system, there is no collapse, so there is no measurement problem.

Again, I could be wrong here, but in the CH, I think this single framework is the entire classical world containing all obeservers. I don't have a reference for this, so I could be way off the mark.

EI has collapse. This is in the assumption that an improper mixture can be treated as a proper mixture.

My question in CH is there is more than one possible framework, so if all observers are in all frameworks, then who chooses which framework is "reality"? In other words, the single framework rule requires a choice, whose choice?

 

[forcefield]

Let P(i, j) be the probability that Alice measures spin-up along axis i and Bob measures spin-up along axis j.

Why do you need Alice and Bob ? Why don't you just describe it in terms of measurement settings and measurement results ?

 

[craigi]

EI has collapse. This is in the assumption that an improper mixture can be treated as a proper mixture.

My question in CH is there is more than one possible framework, so if all observers are in all frameworks, then who chooses which framework is "reality"? In other words, the single framework rule requires a choice, whose choice?

I'm far from an expert on this. I haven't read Griffiths' book and I'm really just extrapolating from what I know here, so please someone tell me if I'm wrong, but I'm going to try to explain this, as I understand it.

Decoherence causes the entire classical world, that is you and everyone on the planet to be connected in a non-quantum manner, though constant exchange of particles and virtual particles in thermodynamically irreversible processes.

When I say planet, I'm being conservative here, because I don't understand to what extent dechorence is taking place on a cosmological level. Certainly our solar system is going to be undergoing decoherence, due to the stream of particles from the sun. Perhaps the lesser intensity of particle exchange going on between stars is a candidate for a less significant form of decoherence and solar systems can be sufficiently isolated to be considered separate quantum systems. Outer space is very cold, but it's not absolute zero. I just don't know if it's cold enough to prevent decoherence on a cosmological scale, but let's stick to planet, where we can be confident that all histories on the macroscopic scale are consistent.

So you asked about observers. In CH, no observer has a privilged role. Humans, animals, plants, computers, rocks and so on, are all just entities in the classical world, all kept consistent with each other. So there is no special role for an "observer", and no "choice" to be made. If there is a privilged role for anything, it's simply the classical world, which is so active that all quantum behaviour dissipates almost instantanously.

Within that, we have systems that become isolated from the classical world, or environment, on a quantum mechanical level. This is where other "histories" are. Typically, these systems are small or weakly interacting, such as the famous QM experiments. Some of which require careful isolation from their environment, often thermal isolation. If this isolation is broken, we lose the quantum behaviour of the system as it decoheres with the classical world and its histories become consistent with it.

This is largely just a description of decoherence, but I think it's necessary in order to explain where there separate sets of histories lie and to answer your question about the role of observers.

Arguably, an aspect of the measurement problem is still relevant, in that there could be a yet unknown mechanism selecting a pure state from probabilities of a mixed state, that would seem more plausible than the process being intrinsically stochastic in nature.

 

[stevendaryl]

Why do you need Alice and Bob ? Why don't you just describe it in terms of measurement settings and measurement results ?

That's what it is: P(i,j) is the probability that one particle will have spin-up along axis i and the other particle will have spin-up along axis j. "Alice" and "Bob" are just traditional.

 

[atyy]

I'm far from an expert on this. I haven't read Griffiths' book and I'm really just extrapolating from what I know here, so please someone tell me if I'm wrong, but I'm going to try to explain this, as I understand it.

Decoherence causes the entire classical world, that is you and everyone on the planet to be connected in a non-quantum manner, though constant exchange of particles and virtual particles in thermodynamically irreversible processes.

When I say planet, I'm being conservative here, because I don't understand to what extent dechorence is taking place on a cosmological level. Certainly our solar system is going to be undergoing decoherence, due to the stream of particles from the sun. Perhaps the lesser intensity of particle exchange going on between stars is a candidate for a less significant form of decoherence and solar systems can be sufficiently isolated to be considered separate quantum systems. Outer space is very cold, but it's not absolute zero. I just don't know if it's cold enough to prevent decoherence on a cosmological scale, but let's stick to planet, where we can be confident that all histories on the macroscopic scale are consistent.

So you asked about observers. In CH, no observer has a privilged role. Humans, animals, plants, computers, rocks and so on, are all just entities in the classical world, all kept consistent with each other. So there is no special role for an "observer", and no "choice" to be made. If there is a privilged role for anything, it's simply the classical world, which is so active that all quantum behaviour dissipates almost instantanously.

Within that, we have systems that become isolated from the classical world, or environment, on a quantum mechanical level. This is where other "histories" are. Typically, these systems are small or weakly interacting, such as the famous QM experiments. Some of which require careful isolation from their environment, often thermal isolation. If this isolation is broken, we lose the quantum behaviour of the system as it decoheres with the classical world and its histories become consistent with it.

This is largely just a description of decoherence, but I think it's necessary in order to explain where there separate sets of histories lie and to answer your question about the role of observers.

Arguably, an aspect of the measurement problem is still relevant, in that there could still be a yet unknown mechanism selecting a pure state from probabilities of a mixed state, rather than being intrinsically stochastic in nature.

But if the sun is needed then isn't there still a part of the universe that isn't quantum?

If the whole universe is quantum, then given that there are multiple frameworks (a framework is a family of consistent histories), and that a single framework must be chosen, who chooses that framework?

 

[Autochthon]

But if the sun is needed then isn't there still a part of the universe that isn't quantum?

If the whole universe is quantum, then given that there are multiple frameworks (a framework is a family of consistent histories), and that a single framework must be chosen, who chooses that framework?

This may be way off base but, Wouldn't the framework be defined by overlapping wavefunctions. ie. I make a measurement, the wavefunction of that measurement acts as a "filter" on family of histories?

 

[stevendaryl]

But if the sun is needed then isn't there still a part of the universe that isn't quantum?

If the whole universe is quantum, then given that there are multiple frameworks (a framework is a family of consistent histories), and that a single framework must be chosen, who chooses that framework?

To me, it seems like choosing a framework is purely pragmatic. You want to know the answers to specific questions, then you choose a framework for which those questions have answers.

 

[atyy]

This may be way off base but, Wouldn't the framework be defined by overlapping wavefunctions. ie. I make a measurement, the wavefunction of that measurement acts as a "filter" on family of histories?

As I understand it, there is no you sitting outside the system making a measurement. If there is then measurement is reintroduced as a fundamental concept, and the measurement problem is not solved.

So given multiple frameworks, and the need to choose one framework, who chooses the framework? Does the single framework rule reintroduce the measurement problem?

 

[craigi]

But if the sun is needed then isn't there still a part of the universe that isn't quantum?

If the whole universe is quantum, then given that there are multiple frameworks (a framework is a family of consistent histories), and that a single framework must be chosen, who chooses that framework?

For the first part, I just don't know if we can consider the solar systems of the universe as a collection of isolated macroscopic quantum systems, each with their own classicality. Perhaps gravity plays the dominant role in decoherence at this scale. I'm not sure if anyone understands this, yet. However, the universe is filled with neutrinos and photons, which have coherent quantum properties and their own histories, independent of our classical world.

For the second part, I think what you're talking about is how CH phrases the reduction from mixed to pure state, that I was referring to.

So given multiple frameworks, and the need to choose one framework, who chooses the framework? Does the single framework rule reintroduce the measurement problem?

I need to read up on this.

 

[stevendaryl]

As I understand it, there is no you sitting outside the system making a measurement. If there is then measurement is reintroduced as a fundamental concept, and the measurement problem is not solved.

So given multiple frameworks, and the need to choose one framework, who chooses the framework? Does the single framework rule reintroduce the measurement problem?

I thought the idea was that all frameworks are equally valid, but are not equally useful. A framework in which cats are in a superposition of dead and alive is perfectly valid, although it would be useless.

 

[atyy]

I thought the idea was that all frameworks are equally valid, but are not equally useful. A framework in which cats are in a superposition of dead and alive is perfectly valid, although it would be useless.

All frameworks are equally valid, but as I understand it there are multiple frameworks and one must choose one, because different valid framework as are not compatible. If one must choose one framework from many, who chooses it?

We can't talk about usefulness for the measurement problem, since there are no observers, for whom the choice is useful.

 

[stevendaryl]

All frameworks are equally valid, but as I understand it there are multiple frameworks and one must choose one, because different valid framework as are not compatible. If one must choose one framework from many, who chooses it?

We can't talk about usefulness for the measurement problem, since there are no observers, for whom the choice is useful.

I don't quite understand the issue. You can use any framework you like. But the questions you can ask depend on the framework. So choose a framework that is appropriate for the questions you want answers to.

You can choose a different framework than I choose, and presumably the formalism works for both of us.

 

[atyy]

I don't quite understand the issue. You can use any framework you like. But the questions you can ask depend on the framework. So choose a framework that is appropriate for the questions you want answers to.

You can choose a different framework than I choose, and presumably the formalism works for both of us.

But choosing is subjective. Hasn't the subjective nature of the classical/quantum cut been reintroduced?

If there are no observers, who chooses? Can the multiple frameworks coexist without an observer to make a choice? It's not obvious to me they can, since they are incompatible.

Edit: According to Gell-Mann and Hartle's http://arxiv.org/abs/1106.0767, in the no observers case, one can consider one history from each framework or realm to be "real", but because the frameworks are incompatible, each "real" history has nothing to do with any other "real" history. They consider this a challenge to the notion of reality in CH, so they introduce negative probabilities, which according to them makes sense in an extended Bayesian framework (in the sense of de Finetti). After they do this, they say that there is a single "fine-grained history" which is real. Their comments seem to support that the measurement problem has not been solved in CH, because in the absence of an observer, there is no one to choose a single framework from the multiple valid, but incompatible frameworks.

 

[Jilang]

Atyy, I am stuck already with this paper which looks very interesting, QM as a classical stochastic theory with negative probabilities. What is a fine grained history in simple words?

 

[craigi]

But choosing is subjective. Hasn't the subjective nature of the classical/quantum cut been reintroduced?

If there are no observers, who chooses? Can the multiple frameworks coexist without an observer to make a choice? It's not obvious to me they can, since they are incompatible.

Edit: According to Gell-Mann and Hartle's http://arxiv.org/abs/1106.0767, in the no observers case, one can consider one history from each framework or realm to be "real", but because the frameworks are incompatible, each "real" history has nothing to do with any other "real" history. They consider this a challenge to the notion of reality in CH, so they introduce negative probabilities, which according to them makes sense in an extended Bayesian framework (in the sense of de Finetti). After they do this, they say that there is a single "fine-grained history" which is real. Their comments seem to support that the measurement problem has not been solved in CH, because in the absence of an observer, there is no one to choose a single framework from the multiple valid, but incompatible frameworks.

As I understand it, you can choose whichever framework suits your purpose, on a theoretical level. The Single Framework Rule, ensures that inferences can only be generated from compatible properties. For example, spin x isn't int the same framework as spin z on an electron.

When it comes to making measurements, we must use the framework of the classical world. What we can do is manipulate the classical world, for instance, by changing the orientation of our detector, to modify which other frameworks are compatible with it. Just before measurement the frameworks combine through the process of decoherence and the quantum system goes from a superposed state to a mixed state. A pure state is then selected stochastically.

I don't consider that the classical world has a special role here. It's just a large framework where the superposition has dissipated and has a strong propensity to merge with other compatible frameworks. Any other framework could have equally played that role. Observers just happen to live in the this large framework. I don't see how they have a more significant role than that, under this interpretation.

 

[stevendaryl]

But choosing is subjective. Hasn't the subjective nature of the classical/quantum cut been reintroduced? If there are no observers, who chooses?

Who says there are no observers? You're an observer, I'm an observer. The goal of the CH is that there is no special physics associated with an observer.

Can the multiple frameworks coexist without an observer to make a choice? It's not obvious to me they can, since they are incompatible.

I think that all possible frameworks exist simultaneously, in the MWI type way.

 

[atyy]

As I understand it, you can choose whichever framework suits your purpose, on a theoretical level. The Single Framework Rule, ensures that inferences can only be generated from compatible properties. For example, spin x isn't int the same framework as spin z on an electron.

Yes, but this assumes an observer exists to choose a framework.To solve the measurement problem, observers cannot be fundamental, so let's say there are no observers. Do all frameworks coexist then? How can they if they are incompatible?

 

[atyy]

Who says there are no observers? You're an observer, I'm an observer. The goal of the CH is that there is no special physics associated with an observer.

To solve the measurement problem, observers cannot be fundamental.

I think that all possible frameworks exist simultaneously, in the MWI type way.

I can accept this as a solution to ehat happens at the fundamental level at which no observers exist. But now since different frameworks don't interact, can't we just throw all but one away?

 

[stevendaryl]

I can accept this as a solution to ehat happens at the fundamental level at which no observers exist. But now since different frameworks don't interact, can't we just throw all but one away?

We can ignore all but one. But the point is that there is nothing about the physics that would select one.

 

[craigi]

Yes, but this assumes an observer exists to choose a framework.To solve the measurement problem, observers cannot be fundamental, so let's say there are no observers.

Ah.. I think I understand what's going on here now. You're saying that an observer is required to contemplate a property of a particle and in doing so, is selecting a framework, right? This selection is happening on a purely conceptual level, in the mind.

In the CI, an observer is required to physically interact with matter. I don't think we should consider these the same thing at all.

Under CH, the universe would function just the same without us (ignoring any anthropic selection effects). Under the CI that question is left open.

Do all frameworks coexist then? How can they if they are incompatible?

I think we can say that frameworks coexist, though I'd be cautious about the word 'exist'.

When we say that they're incompatible, that means that they cannont decohere and we cannont make inferences by combining properties from each.

I can accept this as a solution to ehat happens at the fundamental level at which no observers exist. But now since different frameworks don't interact, can't we just throw all but one away?

Observers aren't required for frameworks to interact. They interact through decoherence, which is independent of any observer. All you need is a mass containing some hadrons at a little bit of temperature, for example. Nothing that we could call an observer.

 

[atyy]

I think we can say that frameworks coexist, though I'd be cautious about the word 'exist'.

When we say that they're incompatible, that means that they cannont decohere and we cannont make inferences by combining properties from each.

At a fundamental level, there is no decoherence and the separate frameworks never interact. So would you agree with stevendaryl that these are separate universes, and that since they never interact we can just arbitrarily pick anyone framework to discuss as reality?

 

[craigi]

At a fundamental level, there is no decoherence and the separate frameworks never interact. So would you agree with stevendaryl that these are separate universes, and that since they never interact we can just arbitrarily pick anyone framework to discuss as reality?

I would say that separate universes are part of the MWI. In CH, the histories are considered part of the same universe.

I don't yet understand why you want to pick a framework as a reality and exclude all others, but I'm working on it. I'm confident that this isn't the purpose of the Single Framework Rule. I'm pretty sure that you can't just pick anyone framework for your reality, because you're likely to find that you're not in it.

I recall Griffiths' writing that CH had been misinterpreted, along similar lines but I can't find where this was at the moment. I think the term "history", has lead to some confusion.

 

[atyy]

I would say that separate universes are part of the MWI. In CH, the histories are considered part of the same universe.

I don't yet understand why you want to pick a framework as a reality and exclude all others, but I'm working on it. I'm confident that this isn't the purpose of the Single Framework Rule.

I recall Griffiths' writing that CH had been misinterpreted, along similar lines but I can't find where this was at the moment. I think the term "history", has lead to some confusion.

Well, it seems to me that the histories in one framework are part of the same universe (or "realm" in Gell-Mann and Hartle's terminology). Different frameworks are not compatible and in the absence of decoherence, they never interact. So although there are many frameworks, since the frameworks don't interact, and since one framework is complete and consistent in itself, we can just focus on one.

Gell-Mann and Hartle say something about non-interacting frameworks or realms in the introduction of http://arxiv.org/abs/1106.0767.

 

[craigi]

Different frameworks are not compatible and in the absence of decoherence, they never interact.

This sentence doesn't match my understanding at all.

Some frameworks are compatible with each other, in fact the need to be to undergo decoherence.

You do realize that decoherence isn't a rare phenomenon, don't you? The entire classical world is incoherent as a result of it.

 

[atyy]

You do realize that decoherence isn't a rare phenomenon, don't you? The entire classical world is incoherent as a result of it.

Decoherence is rare for the universe, because there is nothing to decohere it.

 

[craigi]

Decoherence is rare for the universe, because there is nothing to decohere it.

What makes you think that? Do you have a reference for it?

If this were true the entire universe would be in a superposition of states and would contain no classicality, right? Are you taking this from another interpretation? I don't think it's part of CH.

 

[atyy]

What makes you think that? Do you have a reference for it?

No reference. My understanding is decoherence needs an environment. But by definition there is no environment for the universe.

 

[craigi]

No reference. My understanding is decoherence needs an environment. But by definition there is no environment for the universe.

The environment is within the universe already. It's just the classical world.

Quantum systems within the universe lose coherence though interaction with the classical world.

 

[DevilsAvocado]

But by definition there is no environment for the universe.

Have you been reading Lee Smolin's latest book!?

(I like it very much)