- #36
tom.stoer
Science Advisor
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give me a hint; where's a contradiction?nanosiborg said:Thanks tom.stoer. I think I should reread your and others' comments and think about this some more.
give me a hint; where's a contradiction?nanosiborg said:Thanks tom.stoer. I think I should reread your and others' comments and think about this some more.
Not necessarily any contradictions but, for example, I said that ... quantum superposition is, in a most important sense, an expression of our ignorance of deep reality. To which you replied: Not necessarily; it could be an ontological feature ...tom.stoer said:give me a hint; where's a contradiction?
bhobba said:As Schlosshauer says it transforms a superposition into an 'improper' mixed state. Here improper means it mathematically looks exactly the same as a mixed state and no experiment can tell it from one but in reality it isn't. But it is this 'mimicking' of a mixed state that allows it to be interpreted as one, and as an interpretational thing solve the measurement problem. It doesn't by itself solve the measurement problem but by allowing the improper mixed states to be interpreted as proper ones does for all practical purposes. The wavefunction collapse issue is still there but swept under the rug so to speak.
Sorry I should have posed the question less tounge-in-cheek. (And I agree that decoherence really doesn't address the ultimate measurement problem.) However, doesn't decoherence explain why you don't notice macroscopic superposition of the observer, i.e. because the wave-function evolves into multiple non-interacting components? (Not to mention the requisite (unresolved) issues of 'recovering the born rule', preferred basis, etc. if you stop there though.)tom.stoer said:Accortding to the QM formalism they are; according to my perception they aren't. That's the core of the problem. QM doesn't tell us what we will observe, it only tell's us something about the probabilities of observations. If there is a 50% probability for "dead" I will never observe these superpositions or mixed states. I will always either observe "dead" or "alive". But there is nothing in the QM formalism which tells us how the 50% in the density matrix become the 100% in my perception.
I could not agree more with the 'feel' of this, btw!..tom.stoer said:The problem is deeper. If you insint on some ontological status of QM you immediately run into these problems. But if you give up an ontological interpretation and introduce "our ignorance of reality" then logically it follows that either QM is not complete in the description of nature or our understanding of QM is not complete. So the problem is not only a philosophical one but a physical one as well. We are feeling uncomfortable with the situation that there "is" or "seems to be" more than we can calculate. We can then never be sure where the problem resides and whether there may be a physical but yet unkown solution. I think your interpretation regarding "our ignorance of reality" is something we don't like b/c it may be an interpretation only.
The case of decoherence tells us that (partially !) we can solve the measurement problem. And there's some hope - so we don't stop.
I don't think that makes sense. Researchers study, manipulate and make use of quantum superpositions all the time (quantum computation, etc.)nanosiborg said:To which I would reply that I think the mathematics of quantum superposition, necessarily, does not correspond to any ontological feature of fundamental reality.
eloheim said:However, doesn't decoherence explain why you don't notice macroscopic superposition of the observer, i.e. because the wave-function evolves into multiple non-interacting components?
I don't think that makes sense. Researchers study, manipulate and make use of quantum superpositions all the time (quantum computation, etc.)
Jazzdude said:If you say that a proper mixed state and an improper mixed state cannot be practically distinguished by experiment really means that they cannot be distinguished by quantum measurement. So you still need the quantum measurement postulate to argue like that, and that leaves you where you started. So this doesn't solve anything.
Maui said:Yes, decoherence requires that there be interactionally-real components of the wavefunction which is in sync with your next statement:
Yes, mathematically. It's part of a calculus that assigns values to possible measurement results. It's reasonable to infer that there are wavefunction components that correspond in some way to reality. It's not reasonable to infer that expressions of mutually exclusive measurement results refer to real ontological states.eloheim said:I don't think that makes sense. Researchers study, manipulate and make use of quantum superpositions all the time (quantum computation, etc.)
nanosiborg said:Yes, mathematically. It's part of a calculus that assigns values to possible measurement results. It's reasonable to infer that there are wavefunction components that correspond in some way to reality. It's not reasonable to infer that expressions of mutually exclusive measurement results refer to real ontological states.
nanosiborg said:It's reasonable to infer that there are wavefunction components that correspond in some way to reality.
nanosiborg said:I like your take on this. And some others. My two cents is:
Quantum superposition is a mathematical representation, based on classical wave mechanics, of the extent of our knowledge of possible instrumental behaviors. Quantum superposition has the nonclassical character it does precisely because of our ignorance of the reality underlying instrumental behavior. That is, quantum superposition is, in a most important sense, an expression of our ignorance of deep reality.
Quantumental said:This used to be how I viewed the situation, but then what about PBR theorem?
tom.stoer said:PBR theorem? this is new to me; seems that reading LQG and LHC Higgs papers is the wrong scope
bhobba said:No, decoherence is interpretation independent. For example decoherent histories doesn't require that and yet it is central to it.
bhobba said:There are also ways of evading it such as if you believe QM is incomplete then small blemishes like that don't really matter - that would be Einsteins view.
Maui said:Yes, I've read claims that decoherence doesn't have to involve any real-world interaction(and nothing physical is decohering) but it seems like fitting the facts to the theory instead of changing the theory. Decoherence rates gave been measured and they vary depending on the setup so the states act in ways that do not imply they represent knowledge of the system. Does it make sense to say that by changing the temperature at which an atom is stored, you can decouple the atom from the environment and turn it into information about the system?
Quantumental said:But doesn't this theorem put restraints on whatever fundamental theory that lies beneath QM anyways?
Quantumental said:I am in the "QM can't be 100% correct" group because none of the interpretations to date are satisfactory to me. Collapse is just philosophically bad, indeterminism is not acceptable as a scientific explanation anymore than magic or God is. Plus the entire "when" does collapse occur is a problem. de-Broglie Bohm is the best way to visualize QM and it sort of makes sense, but at the end of the day I don't buy it, it's just too ad hoc for me.
Everett is invalidated by the Born Rule and in addition you have the preferred basis issues.
So yeah, QM *HAS* to be wrong, but I would think that PBR's results will still have a impact on narrowing down the field of possible more fundamental theories? Just like Bells theorem restrict it.
tom.stoer said:So why the hell should (1) be wrong and in which sense??
But can't (1) itself also be seen just as one more interpretation of QM (e.g. an instrumental approach).tom.stoer said:We should make a clear distinction between
1) QM as a theory of nature = a formalism to predict experimental results
2) our ideas about or philosophy of reality
3) an interpretation of QM and its relation to 2)
4) the language we are using to talk about 1-4)
5) ...
Doing that I come to the conclusion that something in this web of relationships (1-4) evades our naive model of nature we have before starting to think about QM. But I would not dare to deduce that QM in the sense of (1) has to be wrong. QM has always proven to be "correct" in the sense of (1). The problems appear at the level of (2-4).
So why the hell should (1) be wrong and in which sense??
bhobba said:I would be careful about letting your prejudices lead you to believe anything 'HAS' to be right or wrong. Choose an interpretation based on what makes most sense to you, or even reject them all, but don't think it must be like that - nature has a way of confounding that sort of view.
tom.stoer said:1) QM as a theory of nature = a formalism to predict experimental results
...
So why the hell should (1) be wrong and in which sense??
Quantumental said:For anyone believing in indeterminism, I wonder how you can justify the Born Rule.
No, not really.bohm2 said:But can't (1) itself also be seen just as one more interpretation of QM (e.g. an instrumental approach).
I understand what you are asking for. And I agree with you in some sense, and I am therefore interested in the same kind of questions.Quantumental said:If you followed up with "but why would God kill cats according to what we percieve as the Born Rule?" ... It's incoherent, stupid and not science. Science seeks explanations, ...
No. No matter how the measurement problem is formulated it isn't solved by the "decoherence Program", as you put it.Prathyush said:As the Title describes, Is the measuremet problem completely solved by the decoherence Program?
Yes. The physical referents of the term "recorded information" are amenable to our sensory apprehension.Prathyush said:In specific I would like the following question addressed.
Is there is clear explanation as to what it means to Record Infromation?
No.Prathyush said:Can it explain the behaviour of a photographic plate?
Open question.Prathyush said:What happens to the appratus after measurement?
Does this do anything for you? (And being in line with the thread title.) A small part of this (just to set up the issue):Quantumental said:Everett is invalidated by the Born Rule and in addition you have the preferred basis issues.
An argument against this resolution is that the limit only holds in the case of an infinite number of measurements (which seems unphysical). However, as Aguirre and Tegmark point out here, in a spatially infinite universe there actually will be an infinite number of such measurements being made. This is suggests to them the measurement problem can be resolved by appealing to a duality, of sorts, between the many worlds of quantum mechanics and cosmology.Jan-Markus Schwindt said:How can the EI [Everett Interpretation] explain the observed probabilities in quantum measurements? I.e.,
why is the squared norm |ca|2 of a branch equivalent to the probability an observer
encounters for measuring the value a? If an observer performs the “same” (equivalent)
measurement many times, the state vector branches each time, and in the
end there will be a branch for each combined result of the measurements. Each
branch also contains one version of the observer. Each observer will conclude the
probability for each value a from the statistics of the individual results he got. One
can show that the norm of the part of the state vector corresponding to branches
where observers don’t get the right probabilities converges to zero when the number
of measurements is increased [2]. The remaining question is whether or not this
argument solves the problem (I think it does). In this paper, I will not deal with
the probability problem, so I won’t discuss this issue any further.
nanosiborg said:Some of it is beyond my current ability to fully understand (or maybe I'm just trying to read too fast). So, expect some questions ... and I hope they don't sound too naive.
eloheim said:An argument against this resolution is that the limit only holds in the case of an infinite number of measurements (which seems unphysical). However, as Aguirre and Tegmark point out here, in a spatially infinite universe there actually will be an infinite number of such measurements being made. This is suggests to them the measurement problem can be resolved by appealing to a duality, of sorts, between the many worlds of quantum mechanics and cosmology.
Jacques Mallah said:The frequency operator is the operator associated with the observable that is the number of cases in a series of experiments that a particular result occurs, divided by the total number of experiments. If is assumed that just the frequency itself is measured, and if the limit of the number of experiments is taken to infinity, the eigenvalue of this frequency operator is unique and equal to the Born Rule probability. The quantum system is then left in the eigenstate with that frequency; all other terms have zero amplitude, as shown by Finkelstein (1963) and Hartle (1968).
This scheme is irrelevant for two reasons. First, an infinite number of experiments can never be performed. As a result, terms of all possible frequencies remain in the superposition. Unless the Born Rule is assumed, there is no reason to discard branches of small amplitude. Assuming that they just disappear is equivalent to assuming collapse of the wavefunction.
Second, in real experiments, individual outcomes are recorded as well as the overall frequency. As a result, there are many branches with the same frequency and the amplitude of anyone branch tends towards zero as the number of experiments is increased. If one discards branches that approach zero amplitude in the limit of infinite experiments, then all branches should be discarded. Furthermore, prior to taking the infinite limit, the very largest individual branch is the one where the highest amplitude outcome of each individual experiment occurred, if there is one.
A more detailed critique of the frequency operator approach is given here(http://arxiv.org/abs/quant-ph/0409144).
The same basic approach of using infinite ensembles of measurements has been taken recently by certain Japanese physicists, Tanaka (who seems unaware of Hartle's work) and (seperately) Wada. Their work contains no significant improvements on the old, failed approach.
tom.stoer said:Btw.: we never ask the question "why" things work as they do in classical mechanics. Why do we not ask this question? Why do we ask this question in QM? Is it really true that we have a full understanding of these ideas in classical mechanics? I am sure the answer is "no".
Quantumental said:If you ask why and their reply is simply: indeterminism!,
tom.stoer said:now there are two options
1) change nature
2) change our ideas about nature
for me 2) is acceptable, but that's a matter of taste ;-)