What qualifies as an observer in quantum mechanics?

[bhobba]

Surely Wigner was aware decoherence doesn't solve the measurement problem, so abandoning his, what I call, excellent idea, wasn't required.

Well maybe like many he thought it did FAPP. I am not going to get into discussions about it again - they go nowhere and can get quite heated. Here is a fair minded article about it I have posted before - anyone can form their own view - we know yours:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Mine is the ignorance or ensemble interpretation:
'Ignorance interpretation: The mixed states we find by taking the partial trace over the environment can be interpreted as a proper mixture. Note that this is essentially a collapse postulate'

But there are a plethora to choose from - and consciousness causes collapse is just one of them.

Thanks
Bill

 

[stevendaryl]

Mine is the ignorance or ensemble interpretation:
'Ignorance interpretation: The mixed states we find by taking the partial trace over the environment can be interpreted as a proper mixture. Note that this is essentially a collapse postulate'

Does "can be interpreted as a proper mixture" means "you can pretend that it is a proper mixture, and not get in trouble"?

 

[ktx49]

It depends on what you mean by "the measurement problem". I think there are three different "measurement problems":

1. The second effect of measurement is the disappearance of all alternatives (among values for observables) except one. The difficulty here is that it's not clear whether this is a problem or not, because there is no way to observe the multiple alternatives. It's sort of like a coin: both heads and tails are present, but it's not possible to see both at the same time.

lets take the coin analogy a step further...

it is possible to "measure" the coin(along a certain "unstable" axis) in a manner so that both outcomes are possible yet neither outcome is actually decided...so with this measurement both alternatives are actually present, however we can only speak of meaningful outcomes in terms of the probabilities of future events. imagine the coin in the world of some 2D flatlanders...they would never be able to see both outcomes at once unless they were able to move into a higher dimensions yet they would clearly be able to determine the 50/50 probability of said coin...yet whenever they actually observe and measure the coin they would only be able to see head or tails.

we could also perform this same sort analogy with a normal dice, or an 8 sided dice, or a 30 sided dice...and again we would only be able to talk about the probabilities of the outcomes...yet for the flatlanders they would only get meaningful observations about ONE SIDE at a time.

lets say anytime they want to measure these dices they have to interact with the object in some manner and this physical interaction causes the unstable coin/dice to "fall" into a definitive state in their dimensions which is then observable. until the coin/dice has been disturbed, its only meaningful to talk about the probabilities.

just realized how this would completely squash many-worlds interpretationss :)

 

[bhobba]

Does "can be interpreted as a proper mixture" means "you can pretend that it is a proper mixture, and not get in trouble"?

That's part of it - but its slightly more subtle than that. Without going through exactly what the go is and generate these long posts that go nowhere (largely because it boils down to semantics about appear, FAPP etc etc), and I have detailed in plenty of other threads, I will simply give the Wikepedia article on it that explains it pretty well:
http://en.wikipedia.org/wiki/Quantum_mind%E2%80%93body_problem
'Decoherence does not generate literal wave function collapse. Rather, it only provides an explanation for the appearance of wavefunction collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the universal wavefunction still exists (and remains coherent at the global level), but its fundamentality remains an interpretational issue. "Post-Everett" decoherence also answers the measurement problem, holding that literal wavefunction collapse simply doesn't exist. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".'

Basically my view is wavefunction collapse doesn't actually exist. Its replaced by decoherence that only gives the apperance of collapse. But since its observationally exactly the same as if it collapsed no wonder it looks like its there. Instead of collapse the measurement postulate, in my interpretation, is replaced by a simple ensemble postulate. The measurement postulate is still there - but its form is now benign.

Also my view is only one of a number of interpretations that use dehoherence in its foundations.

Thanks
Bill

 

[patfada]

But if our detector does not leak information into the environment, then it will of its own accord be unable to introduce decoherence.

.

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

 

[bhobba]

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

Of course it doesn't.

There is a confusion here about leaving a mark in the classical world.

Think of a particle detector that say clicks when a particle is detected. Its not the click that does it - its the appearance here in the classical world. If you look at how a particle detector works and trace back to exactly when it makes its first appearance - that's when an observation is made. In the pencil thing its the movement of the mechanical arm - and if you go back further the currents in the wire controlling it and even further back the electric fields controlling those currents.

This also lays the issue of Copenhagen bare - deciding on when that happens is not spelled out in the interpretation. For that you need a fully quantum theory of measurement. The modern view is it occurs once decoherence has happened, which generally happens very very quickly and before it actually leaves some kind of mark. For example in the particle detector decoherence localizes the particle even before it is detected. That's when the interference pattern disappears.

Thanks
Bill

 

[Fiziqs]

There is a confusion here about leaving a mark in the classical world.

Think of a particle detector that say clicks when a particle is detected. Its not the click that does it - its the appearance here in the classical world. If you look at how a particle detector works and trace back to exactly when it makes its first appearance - that's when an observation is made...

For example in the particle detector decoherence localizes the particle even before it is detected. That's when the interference pattern disappears

Bill, is there any experimental evidence to support this interpretation? Or is it simply theoretical?
When it comes to decoherence any additional evidence is always welcome.

Thanks

 

[bhobba]

Bill, is there any experimental evidence to support this interpretation? Or is it simply theoretical? When it comes to decoherence any additional evidence is always welcome

That's why its called an interpretation and not a theory. Its experimentally equivalent to the mathematical formalism - which is basically an interpretation with minimal interpretative aspects - there are a number like that eg Copenhagen and the Ensemble interpretation - the difference being their interpretation of the state which is slightly different in each - but no need to go into it here - the Wikipedia article explains it fairly well:
http://en.wikipedia.org/wiki/Copenhagen_interpretation

Since decoherence follows from the principles of QM every interpretation has it - they just differ in the importance they ascribe to it. Decoherence normally occurs so quickly its hard to detect, but some experiments have been performed under special conditions to slow it down and it has been observed. But that proves nothing since its in all interpretations of QM.

Thanks
Bill

 

[stevendaryl]

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

I think that the appearance of mixed states from pure states doesn't require decoherence to understand. Suppose you have a system in state $|\psi\rangle = \alpha |\psi_1\rangle + \beta |\psi_2\rangle$ Now, you let it interact with another system described by state $|\varphi\rangle$. Then the composite state $|\Psi\rangle = |\psi\rangle \otimes |\varphi\rangle$ will make a transition:

$(|\psi\rangle \otimes |\varphi \rangle) \rightarrow \alpha (|\psi_1'\rangle \otimes |\varphi_1\rangle) + \beta (|\psi_2'\rangle \otimes |\varphi_2\rangle)$

To see interference effects, you have to disentangle the two systems. When the two systems are small (a few particles), this is possible, but when they are large, it's not.

To me, the only thing that decoherence adds to the story is that there is always two systems interacting, since every particle interacts with the environment (the electromagnetic field).

 

[Prathyush]

Measurement is not different from an entanglement of the recording device to the state of the Particle. There are limitations on how the interaction Hamiltonian can be written, which restricts the possibilities of measurement. How the record is later seen is hardly relevant to the question any more. All future records made, will be entangled with the past records. it is always possible to reverse quantum measurement in principle, Provided all the copies of the record are collected and appropriately evolved using a suitable Hamiltonian. In this process all copies of the record are destroyed. Thus there is no records of the state left in the universe, only the state itself. Only in this condition will measurement be reversible.

 

[StevieTNZ]

I think that the appearance of mixed states from pure states doesn't require decoherence to understand. Suppose you have a system in state $|\psi\rangle = \alpha |\psi_1\rangle + \beta |\psi_2\rangle$ Now, you let it interact with another system described by state $|\varphi\rangle$. Then the composite state $|\Psi\rangle = |\psi\rangle \otimes |\varphi\rangle$ will make a transition:

$(|\psi\rangle \otimes |\varphi \rangle) \rightarrow \alpha (|\psi_1'\rangle \otimes |\varphi_1\rangle) + \beta (|\psi_2'\rangle \otimes |\varphi_2\rangle)$

To see interference effects, you have to disentangle the two systems. When the two systems are small (a few particles), this is possible, but when they are large, it's not.

Much like the quantum eraser experiment.

 

[ktx49]

so it would not be adequate to describe "measurement" as any exchange of information between the environment and a quantum system that lowers the entropy of said system?

 

[bhobba]

so it would not be adequate to describe "measurement" as any exchange of information between the environment and a quantum system that lowers the entropy of said system?

Its simply when a quantum system leaves a mark here in the macro world. Nothing deep about it other than philosophical waffle such as if a tree fell in the forest and no one heard it did it make a sound. Just apply a bit of common sense eg in a particle detector it's when a flash or a click happens. If that aspect has been disabled something must have triggered it - then that is when it makes its appearance unless it's like say the mark on a photographic plate when a photon hits - that's the first place its makes its appearance.

Thanks
Bill

 

[ktx49]

so its not useful to think about measurement in terms of information theory?

(PS, don't be offended by my ignorance here, I'm learning so much about QMs just by asking you guys these sort of noob questions...thanks!)

 

[bhobba]

so its not useful to think about measurement in terms of information theory?

(PS, don't be offended by my ignorance here, I'm learning so much about QMs just by asking you guys these sort of noob questions...thanks!)

Of course it is. But information in what? Think about it.

Thanks
Bill

 

[Badvok]

Its simply when a quantum system leaves a mark here in the macro world.

Where is the border between the quantum system and the macro world?

 

[bhobba]

Where is the border between the quantum system and the macro world?

Ahhh. Now we are getting to a REAL issue with QM. Although a lot of progress has been made that issue still has not been resolved entirely satisfactorily.

The real answer is there is no border - everything is quantum. But how does a classical world that conforms to everyday intuition emerge. That's the 64 million dollar question.

The latest thinking is it emerges when objects are in constant entanglement with its environment and constantly decohered. Although it's difficult to do, when objects have been isolated from the rest of the world, some very very weird effects occur eg:
http://www.newscientist.com/article/dn18669-first-quantum-effects-seen-in-visible-object.html

Thanks
Bill

 

[stevendaryl]

Where is the border between the quantum system and the macro world?

The way that I interpret "leave a mark in the macroscopic world" is "cause an irreversible change". For example, if electromagnetic radiation causes a dot on a photographic plate to darken, that's irreversible. I don't know whether "irreversible" is better-defined than "macroscopic", but I think it's a more accurate description.

 

[meBigGuy]

I'm going to try some words. When a measurement device and a particle become entangled the degrees of freedom for the state of the particle become reduced to the point that it appears classical. That to me is more accurate than "left its mark" and "irreversible change". The borderline between quantum and classical is one of probabilities.

For example, if the spin indicator correctly says up, the particle can no longer be down. They are entangled.

 

[stevendaryl]

I'm going to try some words. When a measurement device and a particle become entangled the degrees of freedom for the state of the particle become reduced to the point that it appears classical. That to me is more accurate than "left its mark" and "irreversible change". The borderline between quantum and classical is one of probabilities.

For example, if the spin indicator correctly says up, the particle can no longer be down. They are entangled.

I'm not sure what you mean by "the degrees of freedom for the state of the particle become reduced". Do you mean going from a superposition of spin-up and spin-down to definitely spin-up or definitely spin-down? If so, that's what people mean when they say that the wave function has "collapsed". The question is: what caused such a collapse?

 

[meBigGuy]

I'm not sure what you mean by "the degrees of freedom for the state of the particle become reduced". Do you mean going from a superposition of spin-up and spin-down to definitely spin-up or definitely spin-down? If so, that's what people mean when they say that the wave function has "collapsed". The question is: what caused such a collapse?

I am quibbling about the black and white line that represents collapse. When a particle becomes entangled with a measurement system its possible states obviously become limited. People call that collapse as if it is something different than simply the results of further entanglement. As if you crossed some collapse line that no one can accurately define. I'm saying there is no line as such. Entangled systems have to correlate. We live in a quantum reality, not a classical reality that somehow "collapsed" from quantum elements. It's just that the complexity and definiteness of measurement devices and macro interactions limits the possibilities so we can approximate it as classical.

Be gentle.

 

[DennisN]

Ahhh. Now we are getting to a REAL issue with QM. Although a lot of progress has been made that issue still has not been resolved entirely satisfactorily.

The real answer is there is no border - everything is quantum. But how does a classical world that conforms to everyday intuition emerge. That's the 64 million dollar question.

The latest thinking is it emerges when objects are in constant entanglement with its environment and constantly decohered. Although it's difficult to do, when objects have been isolated from the rest of the world, some very very weird effects occur eg:
http://www.newscientist.com/article/dn18669-first-quantum-effects-seen-in-visible-object.html

Thanks
Bill

I just noticed a relatively new experiment which I think should be quite interesting to check out for those who are reading this thread.

Local emergence of thermal correlations in an isolated quantum many-body system
Tim Langen, Remi Geiger, Maximilian Kuhnert, Bernhard Rauer, Joerg Schmiedmayer
(Submitted on 16 May 2013, Published 8 September 2013)
http://arxiv.org/abs/1305.3708
[/URL]http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2739.html

We experimentally demonstrate how thermal properties in an non-equilibrium quantum many-body system emerge locally, spread in space and time, and finally lead to the globally relaxed state. In our experiment, we quench a one-dimensional (1D) Bose gas by coherently splitting it into two parts. By monitoring the phase coherence between the two parts we observe that the thermal correlations of a prethermalized state emerge locally in their final form and propagate through the system in a light-cone-like evolution. Our results underline the close link between the propagation of correlations and relaxation processes in quantum many-body systems.
(4 pages)

Article 1: Scientists manage to study the physics that connect the classical the quantum world
(Note: a little sloppy language in the article, perhaps, but I link to it anyway)

Article 2: Quantum Temperature: Scientists Study the Physics That Connects the Classical to the Quantum World (ScienceDaily)

EDIT:
I thought I'd might quote the final section of the paper as a teaser:

"In our experiment thermal correlations emerge locally.

A local observer would see thermal relaxed correlation function appear immediately after the splitting and spread through the system in a light-cone horizon-like fashion, while long-range phase coherence remains outside. This leads us to conjecture a general pathway to relaxation and the emergence of classical properties in isolated quantum many-body systems: the decay of quantum coherence starts locally and then spreads through the system to establish a globally relaxed (dephased) state. In systems where interactions manifest themselves in excitations with a linear dispersion relation the decay of quantum coherence takes the form of an effective lightcone."

No conscious observers seem to have been harmed in this experiment.

 

[StevieTNZ]

As Brian Cox and Jeff Forshaw say in their book "The Quantum Universe", QM applies to both the micro and macro world. This is also echoed in 'Quantum Enigma'. There is no divide, according to QM formalism.

Decoherence rests on the fact that Quantum Mechanics applies also on the macro scale.

As stated in Roland Omnes book 'Philosophy of QM', a rock can appear at one place (position 1) at t=2. At t=3, it could have suddenly jumped to position 3. Decoherence doesn't stop this chance from happening.

 

[DennisN]

As stated in Roland Omnes book 'Philosophy of QM', a rock can appear at one place (position 1) at t=2. At t=3, it could have suddenly jumped to position 3. Decoherence doesn't stop this chance from happening.

That must be a philosophically oriented book, and not a physics book, am I correct? Because that quote goes straight against special relativity.

 

[StevieTNZ]

That must be a philosophically oriented book, and not a physics book, am I correct? Because that quote goes straight against special relativity.

It is a philosophy orientated book, written by a physicist.
https://www.amazon.com/dp/0691095515/?tag=pfamazon01-20

How does the example go against special relativity?
Micro systems act in a similar manner, and thus also(?) violate special relativity (SR). But as far as I'm aware, QM and SR are combined, so I don't know how it goes against SR.

 

[DennisN]

It is a philosophy orientated book, written by a physicist.
https://www.amazon.com/dp/0691095515/?tag=pfamazon01-20

Well, I was confused by the quote since it mentions a "rock"; a rock can not pop in and out of existence like that. That would obviously mean faster-than-light travel, and this is not possible according to special relativity, which you might be aware of. The author may have tried to use a figure of speech, I don't know. But anyway, I do not know if you were quoting or simply recalling those words? Were these the exact words in the book? (By the way, there are no experimentally confirmed violations of special relativity).

 

[meBigGuy]

That's like saying brownian motion can evacuate all the air molecules in front of your face and you suffocate.

Jumping from 1 to 2 can't be instantaneous by relativity.
I have a problem with it though.
Think about it. The rock has to be in a superposition of 1 and 2 while completely entangled with its environment (that is, the environment has to "agree" it could be 1 or 2) and then something happens that says it has to be in 2. But that means it was never really in position 1, but a superposition.

I don't think it can just be in one and suddenly the universe thinks it should have been in 2. The air molecules, photons of reflected light, pressure on the ground, all of that has to be in superposition and then it was never really in 1.

That is, the environment, the universe, has to be entangled with the rock in a way that such a thing would happen.

Word soup --- sorry.

 

[DennisN]

Oops, StevieTNZ, you have to excuse me that I missed that you actually linked to an article in post #30 which was about the experiment I posted about above in post #57. I don't know why I missed that post of yours, it was an excellent link! EDIT: I've added your link to my post.

 

[Prathyush]

I'm not sure what you mean by "the degrees of freedom for the state of the particle become reduced". Do you mean going from a superposition of spin-up and spin-down to definitely spin-up or definitely spin-down? If so, that's what people mean when they say that the wave function has "collapsed". The question is: what caused such a collapse?

What is the physical mechanism by which knowledge is obtained?
The Wavefunction of an election describes the past History a particle. When A Measurement Apparatus Interacts with a particle, It forms an entanglement, which serves as the record. Now what does it mean the particle Has been Recorded onto a Device. It means all future records made will be entangled with the past record.

The State of the record serves to define the State of the particle, for any further experimentation on the particle. Note that any further observation of the record,involves formation of more records by any physical mechanism chosen. These records will also be entangled with the state of the particle.

Is record Permanent? How is it consistent this unitary evolution? Can a record be destroyed and the particle state be restored?
When a record is made, Multiple copies of it are made, each time you observe it and so on. For instance shine light on the record, then the photons are entangled with the state of the Record. It is clear that one can only restore the state, if all the copies of the record are collected and then evolved back using the correct unitary evolution, Usually in practice its not possible but still it has been observed in experiments like the Quantum Eraser experiment.

 

[stevendaryl]

What is the physical mechanism by which knowledge is obtained?

Wave function collapse is not exactly the same as acquiring knowledge. That's the lesson of decoherence. If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears. In effect, the wave function has collapsed. But nobody necessarily acquired any knowledge about the particle.

The Wavefunction of an election describes the past History a particle. When A Measurement Apparatus Interacts with a particle, It forms an entanglement, which serves as the record. Now what does it mean the particle Has been Recorded onto a Device. It means all future records made will be entangled with the past record.

The State of the record serves to define the State of the particle, for any further experimentation on the particle. Note that any further observation of the record,involves formation of more records by any physical mechanism chosen. These records will also be entangled with the state of the particle.

Is record Permanent? How is it consistent this unitary evolution? Can a record be destroyed and the particle state be restored?
When a record is made, Multiple copies of it are made, each time you observe it and so on. For instance shine light on the record, then the photons are entangled with the state of the Record. It is clear that one can only restore the state, if all the copies of the record are collected and then evolved back using the correct unitary evolution, Usually in practice its not possible but still it has been observed in experiments like the Quantum Eraser experiment.

I think that's right. Measurement or memory involves something like entanglement between the past and the present state. As I said in a previous post, I think that this requires irreversibility.

 

[Fiziqs]

Jumping from 1 to 2 can't be instantaneous by relativity.
I have a problem with it though.
Think about it. The rock has to be in a superposition of 1 and 2 while completely entangled with its environment (that is, the environment has to "agree" it could be 1 or 2) and then something happens that says it has to be in 2. But that means it was never really in position 1, but a superposition.

I don't think it can just be in one and suddenly the universe thinks it should have been in 2. The air molecules, photons of reflected light, pressure on the ground, all of that has to be in superposition and then it was never really in 1.

But doesn't HUP say that you can never know both the position and momentum of a particle at the same time? So on a very micro scale isn't the rock always in a superposition of these two states? Does the wave function ever really collapse, or does its uncertainty just get transferred from one property to another? So while on a macro scale it may look like the position and momentum of the rock are pretty darn certain, on a micro scale they're not.

For that matter isn't every particle in the universe in a constant state of superposition? Solid and substantial in appearance, but at the same time ethereal in nature? Is the world really as tangible as it appears?

I realize that this sounds extremely metaphysical, but isn't that an unavoidable consequence of HUP.

Either HUP is wrong, or reality isn't as solid as it appears.

If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears.

Do they?

 

[stevendaryl]

(In response to my statement: "If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears.")

Do they?

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state. But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed. (The effects spread out at the speed of light and are soon way beyond the boundaries of your experimental setup.) So in these circumstances, there usually cannot be a single final state that is consistent with both alternative intermediate states.

 

[Fiziqs]

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state. But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed. (The effects spread out at the speed of light and are soon way beyond the boundaries of your experimental setup.) So in these circumstances, there usually cannot be a single final state that is consistent with both alternative intermediate states.

Thank you for the clarification. Nice explanation, with which I wholeheartedly agree. As you point out "you only see interference between two intermediate states if they both lead to the SAME final state." Thus most particles never exhibit quantum effects even if they are in a quantum state.

Again, thanks for the clarification.

 

[Fiziqs]

But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed.

I know that the idea of irreversibility is a popular one, and I do for the most part agree with it, but I'm not sure that irreversibility is the actual determining factor.

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state.

I kind of think that this is the relevant factor. If the final state is such that it precludes either intermediate state, then only the intermediate state which leads to the final state will be observed. The final state may indeed be reversible, and as such altering the final state may alter the observed state, but I don't think that this is the determining factor. To me the deciding factor is whether the final state precludes one or the other of the intermediate states. Regardless of whether the final state is reversible or not.

 

[StevieTNZ]

Well, I was confused by the quote since it mentions a "rock"; a rock can not pop in and out of existence like that. That would obviously mean faster-than-light travel, and this is not possible according to special relativity, which you might be aware of. The author may have tried to use a figure of speech, I don't know. But anyway, I do not know if you were quoting or simply recalling those words? Were these the exact words in the book? (By the way, there are no experimentally confirmed violations of special relativity).

It is information that cannot travel faster than light, as Brian Cox points out in his book.

According to the Schrodinger equation, at t=0 a micro (and in principle macro) system is at position 1. Then at t=1, it is potentially everywhere else in the universe (until observation occurs). That's why we can observe the rock to be at position 1 at t=1, and then at position 2 at t=2, because at t=2 it is a potentiality to be there.