New experimental proof of wave-function collapse?

[bhobba]

bhobba's Ensemble - I would like to have bhobba comment. I think it is a problem, because Ensemble is essentially Copenhagen, and the cut should be shiftable and subjective. If the cut if shiftable, won't any cut that is placed by decoherence be too objective?

Its a problem for my ignorance ensemble, indeed most interpretations (BM is the only one I can think of that it isn't - but there may be others) that uses decoherence. Its not a problem for Ballentine's ensemble because an observation simply selects an outcome from the conceptual ensemble of outcomes with that state and observable. Remember Ballentine doesn't believe decoherence has anything to say about interpretation issues - its a very real phenomena of course - and he thinks its VERY VERY important to the practical realisation of quantum computers - but of no interpretational relevance.

Thanks
Bill

 

[Edward Wij]

Hmmmm. I think you are misunderstanding things in those texts because QFT is not an effective field theory - effective field theory's are examples of QFT's. We have zero evidence that the much vaunted theory of everything that lifts the veil beyond about the plank scale may not be a QFT - string theory for example is a QFT - but more general; than the usual QFT in 3+1 dimensions. Although I have read string theory may be a bit different in that ordinary QM may be sufficient for its description - but I am not expert enough to say and some say QFT and string theory are the same thing.

I think it might be wise for you to more carefully study those texts, and the one by Griffiths, before I discuss it with you again.

Thanks
Bill

The following entry in wiki is wrong then (you'd better correct it)?

http://en.wikipedia.org/wiki/Quantum_field_theory

"Quantum field theory of the fundamental forces itself has been postulated to be the low-energy effective field theory limit of a more fundamental theory such as superstring theory."

 

[bhobba]

"Quantum field theory of the fundamental forces itself has been postulated to be the low-energy effective field theory limit of a more fundamental theory such as superstring theory."

English was never my best subject - I in fact failed it at High School.

But can you please read this stuff with your thinking cap on and cognate on 'Quantum field theory of the fundamental forces'

Thanks
Bill

 

[Demystifier]

Its a problem for my ignorance ensemble, indeed most interpretations (BM is the only one I can think of that it isn't - but there may be others) that uses decoherence.

BM uses decoherence:
http://en.wikipedia.org/wiki/Quantum_decoherence#In_interpretations_of_quantum_mechanics

 

[Demystifier]

Its saying nothing can happen in MW. Yet we have things like vacuum fluctuations causing inherent randomness. Since MW is cooked up to be indistinguishable from standard QM it should include that. So I don't necessarily accept that papers analysis as correct. In saying that, I am appealing to Quantum Field Theory which I am not as familiar with as I would like. I would like someone with more knowledge of QFT to comment of the exact cause of random vacuum fluctuations.

I think you misunderstood the concept of a vacuum fluctuation. Here the word "fluctuation" does not refer to a time-dependent process. It is merely a statistical fluctuation, meaning only that some probability distribution is not a delta-function, i.e. that the probability distribution assigns a finite probability to a value different from the average value. The vacuum fluctuation is very similar to the fact that quantum harmonic oscillator in the ground state has a finite probability to be at a position x not equal to 0.

 

[bhobba]

BM uses decoherence:
http://en.wikipedia.org/wiki/Quantum_decoherence#In_interpretations_of_quantum_mechanics

Yes I know.

I just don't think the factorisation problem is an issue for it.

Thanks
Bill

 

[bhobba]

The vacuum fluctuation is very similar to the fact that quantum harmonic oscillator in the ground state has a finite probability to be at a position x not equal to 0.

Yes.

My point is it is generally assumed, for example, that spontaneous emission is a random process explained by vacuum fluctuations. This could explain the very intuitive fact the environment is correctly modeled as having random phase.

Thanks
Bill

 

[Demystifier]

My point is it is generally assumed, for example, that spontaneous emission is a random process explained by vacuum fluctuations.

Fundamentally, spontaneous emission happens because the initial state is not an eigenstate of the full Hamiltonian (including the interaction term). Perturbativelly, the effect can be calculated in terms of loop diagrams which can be interpreted as "vacuum fluctuations", but I don't think there is anything fundamental about such a picture.

 

[kith]

My point is it is generally assumed, for example, that spontaneous emission is a random process explained by vacuum fluctuations. This could explain the very intuitive fact the environment is correctly modeled as having random phase.

I don't think this works the way you intend because your intuition here seems to be rooted in semi-classical thinking where the atom is treated quantum-mechanically but the field is not.

Using this approximation to describe the interaction between a two-level system and a field mode, spontaneous emission looks indeed like a random interruption of a coherent time evolution (Rabi oscillation). But if you use a full quantum description like the Jaynes-Cummings model, the randomness in the time evolution goes away.

 

[vanhees71]

Indeed, the spontaneous emission is the effect of the quantization of the em. field. In the semi-classical picture there's no spontaneous emission, and also the excited states of, e.g., the electron in the hydrogen atom are stable.

 

[bhobba]

As other have said spontaneous emission is not explained by normal QM - if its in an eigenstate of energy it should remain so. That's my whole point - one has to go to QED to explain it. My suspicion is that is the rock bottom reason for the randomness we see in things like the phase of photons in decoherence.

Thanks
Bill

 

[bhobba]

spontaneous emission looks indeed like a random interruption of a coherent time evolution (Rabi oscillation). But if you use a full quantum description like the Jaynes-Cummings model, the randomness in the time evolution goes away.

But is there any way to predict when it will spontaneously emit? If that's not the case then we know why photons have random phase.

Thanks
Bill

 

[kith]

But is there any way to predict when it will spontaneously emit? If that's not the case then we know why photons have random phase.

Maybe I didn't understand completely what the problem is, you think the vacuum fluctuations may solve.

Let me set the stage: when two separate quantum systems interact, we get entanglement and therefore decreased coherence if we look at one system only. Usually, the unitary time evolution of the combined system may lead to increasing entanglement / decreasing coherence in the subsystems, as well as decreasing entanglement / increasing coherence in the subsystems.

In measurements, we don't observe coherences between the possible final states of the system at all, which implies that the interaction with the measurement apparatus is such that the coherence is suppressed (a) strongly and (b) in a long-lasting manner. This can be referred to as approximate decoherence.

Is the problem now how to derive this approximate decoherence or do you want to show that the decoherence is more permanent and that there's no recoherence? Or is it something else?

 

[bhobba]

Is the problem now how to derive this approximate decoherence or do you want to show that the decoherence is more permanent and that there's no recoherence? Or is it something else?

Its to do with some decoherence models requiring a random environment eg:
http://quantum.phys.cmu.edu/CQT/chaps/cqt26.pdf

Now to me its a very obvious reasonable assumption that the phase of the photons (for example as in the above) is random and doesn't require any explanation simply due to the fact the number of disorderly phases is much much greater than orderly ones. I personally wouldn't even count it as a formal assumption - but that's just me - it is an assumption. Now Ruth who has been referred to in this thread thinks it needs explaining - in fact she believes this assumption really assumes what you are trying to show so its circular. I don't believe that - but that's her argument.

My view is there seems to be a natural randomness in photons from QFT due to spontaneous emission - any photon we observe likely has been randomly emitted by spontaneous emission eg:
http://www.famaf.unc.edu.ar/~vmarconi/moderna1/emision_estimulada_AJP.pdf

As the above points out the modern explanation is vacuum fluctuations of the quantum EM field that permeates all space. My understanding of QFT is not as good as I would like it but I do know something of it. The explanation of vacuum fluctuations I have seen is the Heisenberg uncertainty principle - you can't say it has a definite value for the same reason. My suspicion is this is the cause of the randomness. Its inherent and removes any possibility of circularity.

Thanks
Bill

 

[vanhees71]

Where is there a problem? Everyday matter provides such randomness. Just the cosmic microwave background radiation, which is the most accurate realization of black-body radiation ever achieved (literally in the universe ;-)), is sufficient to make objects like the moon behave classically FAPP.

For "quantum research/applications" the opposite is a problem, namely how to avoid decoherence over a sufficiently long time and keep quantum coherence stable long enough!

 

[bhobba]

Where is there a problem?

Your preaching to the converted. There is no problem. But Ruth will not be dissuaded. I am simply trying to come up with an argument with no holes that can be exploited. She believes even statistical mechanics has this circularity. First I have heard of it - the only issue I have read about is actually proving the ergodic hypothesis.

Thanks
Bill

 

[kith]

Its to do with some decoherence models requiring a random environment eg:
http://quantum.phys.cmu.edu/CQT/chaps/cqt26.pdf

Now to me its a very obvious reasonable assumption that the phase of the photons (for example as in the above) is random and doesn't require any explanation simply due to the fact the number of disorderly phases is much much greater than orderly ones. I personally wouldn't even count it as a formal assumption - but that's just me - it is an assumption. Now Ruth who has been referred to in this thread thinks it needs explaining - in fact she believes this assumption really assumes what you are trying to show so its circular. I don't believe that - but that's her argument.

I think it depends on what you want to show. Approximate decoherence can be explained by arguments along your lines (namely statistical reasoning which is very similar to Boltzmann's molecular chaos argument). But because of things like Poincarés recurrence theorem, many arguments in the foundations of QM get weakened if decoherence is only approximate. For example, the intuitive picture of splitting worlds whenever decoherence occurs makes less sense if you keep in mind that after a certain time span -which can be really big- recoherence and therefore the merging of worlds occurs.

If Ruth is talking about this, she is correct. If you ask yourself, "how can decoherence in a system be permanent?" and get the answer "because of the random phases of photons which interact with the system" the immediate follow-up question is "how can the phases of photons be permanently random?". After all, the randomness of phases is essentially equivalent to decoherence in the field. So the question about permanent decoherence has been shifted from the system to the field but not answered.

It is really a pattern in these discussions that people are talking past each others because of this.

 

[bhobba]

It is really a pattern in these discussions that people are talking past each others because of this.

Yes.

I have always said regarding this stuff more research is required.

Thanks
Bill

 

[kith]

I have always said regarding this stuff more research is required.

I don't think I agree with you here. To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

 

[atyy]

I don't think I agree with you here. To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

However, that is only true for classical statistical mechanics. If one uses quantum mechanics as the basis for statistical mechanics, it is less clear (unless one is not using Copenhagen, but maybe some version of BM).

 

[kith]

However, that is only true for classical statistical mechanics. If one uses quantum mechanics as the basis for statistical mechanics, it is less clear (unless one is not using Copenhagen, but maybe some version of BM).

I think Copenhagen fits in because it is about people doing science.

 

[atyy]

I think Copenhagen fits in because it is about people doing science.

What I mean is that in classical statistical mechanics, irreversibility is not fundamental, because we take Newton's laws as fundamental and statistical mechanics and thermodynamics as emergent. However, in quantum mechanics, in Copenhagen, we need an observer to decide when an irreversible macroscopic mark has occurred. Since the observer is fundamental, irreversibility is fundamental.

 

[kith]

What I mean is that in classical statistical mechanics, irreversibility is not fundamental, because we take Newton's laws as fundamental and statistical mechanics and thermodynamics as emergent. However, in quantum mechanics, in Copenhagen, we need an observer to decide when an irreversible macroscopic mark has occurred. Since the observer is fundamental, irreversibility is fundamental.

The "irreversible macroscopic mark" is left in a classical system, so I don't think it is fundamentally irreversible. Sure, Copenhagen includes measurements as key elements but whether a measurement has taken place is a matter of practice and not of principle. As you say, it is a decision which the scientist makes. I think it is misleading to call this "fundamental irreversibility".

 

[TrickyDicky]

To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

Isn't this arguing against your #77? Since when is irreversibility not fundamental? Last I checked the second law was still alive and well, both in cosmology/GR and QFT. This has never been settled by statistical mechanics to my knowledge. A special initial state IS a way to define irreversibility as fundamental.

 

[atyy]

The "irreversible macroscopic mark" is left in a classical system, so I don't think it is fundamentally irreversible. Sure, Copenhagen includes measurements as key elements but whether a measurement has taken place is a matter of practice and not of principle. As you say, it is a decision which the scientist makes. I think it is misleading to call this "fundamental irreversibility".

But the classical world in Copenhagen is not fully lawed - in particular, it is not fully lawed by Newton's laws or classical relativity, which are falsified by quantum mechanics. So in Copenhagen the decision a scientist makes is fundamental. For the observer to be not fundamental, one needs an interpretation in which the observer is not fundamental, ie. BM or MWI.

 

[Derek Potter]

Yes, that's among the papers I know about. I have tried to read almost all your papers with great interest! I guess I'm not enough of an expert to evaluate its correctness by myself, and I don't know if there is consensus about whether it really works, at least not the way Bohmian Mechanics for non-relativistic quantum mechanics has been examined for all sorts of tricky situations, and really does seem to work. Would it be fair to say that this is still pretty much at the frontier of research, rather than textbook knowledge? I have the same reservations about MWI - is it really an alternative interpretation to Copenhagen - or is it still an approach that it is unclear whether all the problems have really been worked out?

So would it be fair to say that at the consensus level - eg., what one can teach to undergraduates - Copenhagen is still the only interpretation of quantum mechanics?

(Consistent histories, maybe - but it essentially has collapse and all the same problems as Copenhagen, just declared not to be problems)

Well I understand "elegance" and I understand "mathematical consistency" but this is the first time I've come across "being able to teach it to undergraduates" as a criterion for accepting an interpretation :)

Consistent histories can, I think, be formulated without collapse. it then becomes a many histories theory, which only needs a small dash on ontology to turn it into a Tegmarkian world.

 

[atyy]

Well I understand "elegance" and I understand "mathematical consistency" but this is the first time I've come across "being able to teach it to undergraduates" as a criterion for accepting an interpretation :)

Consistent histories can, I think, be formulated without collapse. it then becomes a many histories theory, which only needs a small dash on ontology to turn it into a Tegmarkian world.

No, what I said was that being unquestionably right was a criterion for teaching it to undergraduates.

 

[kith]

But the classical world in Copenhagen is not fully lawed - in particular, it is not fully lawed by Newton's laws or classical relativity, which are falsified by quantum mechanics. So in Copenhagen the decision a scientist makes is fundamental. For the observer to be not fundamental, one needs an interpretation in which the observer is not fundamental, ie. BM or MWI.

The decision a certain observer makes is not fundamental because another observer can make a different decision. If different observers disagree whether a measurement has been performed, they also disagree about whether a process is irreversible. So what Copenhagen needs is the observer and his subjective notion of irreversibility, which is the second kind in my post #79.

 

[atyy]

The decision a certain observer makes is not fundamental because another observer can make a different decision. If different observers disagree whether a measurement has been performed, they also disagree about whether a process is irreversible. So what Copenhagen needs is the observer and his subjective notion of irreversibility, which is the second kind in my post #79.

Yes, I agree. But there is no fundamental reversibility either.

 

[kith]

Yes, I agree. But there is no fundamental reversibility either.

Yes, in the sense that Copenhagen doesn't try to remove the observer and his subjective notions.