I don't want to be the translator from @vanhees71 to English, but I think that non-local randomness is something he would agree is part of QFT.gentzen said:
Yes, according to some interpretations/theories. Example is Bohmian mechanics out of quantum equilibrium.martinbn said:
I don't think that he would agree that randomness is non-local.martinbn said:
The correlations between far-distant parts of a quantum system (e.g., an entangled two-photon state with the photons measured at far-distant places) described by entangled states of course a consistent with relativistic microcausal QFT, and for me this implies that these long-ranged correlations are described by a local theory (QFT) and thus that what's violated in Bell's local realistic HV theories is "realism" (i.e., the assumption that all observables take determined values).gentzen said:
I don't know, what "non-local randomness" means though.martinbn said:
But according to QM (and current experiments) it is not possible. Of course you can find theories that have all kinds of interactions. Newtonian gravity has action at a distance.Demystifier said:
It just means randomness.vanhees71 said:
Non-local here is superfluous.Demystifier said:
But they aren't in the standard SG experiment, which is what we're discussing. Nothing is done to the wave function between exiting the SG magnet and hitting the detector.Demystifier said:
If we want to get really, strictly technical, yes, the position of the flash is a measurement of the position of the particle, which gets entangled with its momentum (which output beam it is in) by the detector, and the momentum gets entangled with the spin by the SG magnet. So strictly speaking there are two stages of deduction required to get from the observed position of the flash on the detector to the "measured" value of spin.Demystifier said:
"Not predetermined" random outcomes at spacelike separated (spacetime) points/events which are not independent.vanhees71 said:
I see this claim more often on PF, but why exactly? To me, the duality states that quantum objects show both particle and wave behaviour, which is captured in a new ontological category for the quantum object we call "quantum particle". This is how I've understood this concept for a long time. What's outdated about that? I'd say the concept has evolved, not that it's outdated.vanhees71 said:
Of course. Let me remind you that we discuss examples that illustrate the difference between superluminal signalling and superluminal action. Standard QM is not a good example, which is why we discuss other examples.martinbn said:
Nothing is outdated about notion of quantum particle. What is outdated is that sometimes it behaves like a classical wave and sometimes like a classical particle.haushofer said:
But what's wrong with that? In certain cases the wavefunction is sharply peaked, which means that the quantum particle exhibits "classical behaviour". But that doesn't make it a "classic particle".Demystifier said:
RQ: Measurements result in definite, singular values, don't they?vanhees71 said:
No, we are discussing whether a statement was conraversial or not. If it is a consequence of QM it shouldn't be.Demystifier said:
I don't think you will ever be able to eliminate such deductions entirely, as we will always need to construct a logical implication between the macroscopic measurement outcome read by the experimenter, and the measured variable of the quantum system. I.e. There will always be an intermediate dynamical model of both the measured system and relevant degrees of freedom of the measurement apparatus that justifies any inferences we make from an experiment.PeterDonis said:
Yes, this is obvious since by hypothesis the measuring device and the measured system are different degrees of freedom. But one can still look at how many stages of deduction are required to get from the measuring device to the observable of the measured system that we are interested in.Morbert said:
Yes, and I'd say this nomenclature is somewhat outdated.haushofer said:
martinbn said:
Demystifier said:
Do I miss something? Did you perhaps got a satisfying answer to your question above and then switched to another topic? Discussions with you would be much easier if you could occasionally say something like: "OK, now I understand this, thank you for explaining it to me, now I have another question.". Or if that would be too much, a simple "like" would be enough to signal that I answered your question at least partially satisfying. Or if a "like" would be too much as well, you could still indicate somehow that you are no longer interested in the initial question. Otherwise, I have an impression that all your later questions are motivated by the initial one.martinbn said:
Because the classical scale of everyday objects is just too imposing.haushofer said:
In other words that's what I call "far-distant correlations".gentzen said:
That's true for "von Neumann filter measurements". Usually in experiments with photons the photons get absorbed by the detector. Then there are no photons with any properties prepared. In such a case we simply get a measurement result with probabilities according to the quantum state (prediction), which we can test with these experiments on an ensemble of equally prepared systems.gentzen said:
That's my point! It's "unproblematic", but particularly this is what's denied by all those who think that there's a "measurement problem", because it's very hard to accept that completely undetermined properties can be strongly correlated, even if the measured properties are measured at parts of the system at very far distant places. This inseparability feature of entangled state was what Einstein really bothered (according to his Dialectica article of 1948 which is much clearer than the (in)famous EPR article).gentzen said:
I would like to stress that all POVM measurements, not only projective measurements, are described by a collapse postulate in standard QM. The QM postulates are summarized nicely in the following excerpt from the book "Quantum Computations and Quantum Information" by Nielsen and Chuang. Eq. (2.160) is the collapse postulate for general (not necessarily projective) measurements.vanhees71 said:
To my mind, Einstein’s illusion was that the experiential reality could be represented by a physical theory that could be based on the assumption that the entities and their characters the theory is about might exist objectively in an ontological sense and thus literally experiencer-independently (“scientific realism”).vanhees71 said:
[...]martinbn said:
Wow, martinbn was indeed right!vanhees71 said:
I had a conversation with Ruth E. Kastner (a long time ago) on Mateus Araújo's blog, where she agreed thatvanhees71 said:
Except for using ‘shut up and calculate’ as a bad name for instrumentalism (I implicitly proposed 'let me calculate and explain' instead in that exchange), I had no problems (and still don't have) with that reaction. At about the same time also Steven Weinberg's attack on quantum interpretations and specifically instrumentalism appeared, which I found totally unacceptable back then. At some later point I read something which temporarily made me understand his attack (it had something to do with being able to explain such stuff in a textbook in a satisfying way), but I forgot it again in the meantime.
vanhees71 said:
Eq. (2.160) (which first appeared as Eq. 2.92) does not describe a POVM measurement. Section 8.2.4 explains something about the interpretational background of that formulation, but (from my POV) this doesn't change that vanhees71 is "not wrong" here.Demystifier said:
N&C never mention Kraus operators, but refer to [Kra83] (K. Kraus. States, Effects, and Operations: Fundamental Notions of Quantum Theory. Lecture Notes in Physics, Vol. 190. Springer-Verlag, Berlin, 1983) via "The theory of generalized measurements which we have employed was developed between the 1940s and 1970s. Much of the history can be distilled from the book of Kraus [Kra83]." and "We mention just a few key references, primarily the book by Kraus [Kra83] , which contains references to much earlier work on the subject."
The point is, of course, that you are free to "want more" from science than it "promises" to do, i.e., to deliver a way to find "laws of nature" that describe, how nature behaves (or more precisely how we observe it to behave). That's why there are so many "interpretations" of the QT formalism. In principle it indeed describes what we observe (except that there's this fundamental problem of "quantum gravitation", i.e., the incompatibility between GR and QT) when just accepting the probabilistic interpretation of the "quantum state", using Born's rule, as well as the quantum-mechanical time evolution. That's in a way a "shut up and calculate" paradigm, because we restrict ourselves of the purely scientific meaning of the theory and don't expect to find some "deeper truth" in it. What I never understood is the fact that such a demand nobody ever had in view of classical physics ;-)).gentzen said:
I don't think that the measurement problem, whatever you think it might be, is solved by generalizing the very idealized idea of von Neumann filter measurements (POV) with POVMs, which is simply taking into account the possibility for "incomplete measurements", which is almost always what we are able to do in the lab. The probabilistic nature of the quantum state persists also in this more general formulation. I don't see that POVMs is something that goes much beyond standard QT, although of course you can take POVMs as the measurement postulate as in the book quoted above (this seems to be a very concise formulation of the postulates, which is much more careful than the standard formulations in the introductory textbooks).gentzen said:
Sounds interesting. My source for this direction of more recent research is A. Peres, Quantum Theory, Concepts and Methods.gentzen said:
Then what does Eq. (2.160) describe? Perhaps it doesn't describe the POVM measurement itself, but it certainly describes the update of information after the POVM measurement.gentzen said:
With that, I absolutely agree.
Indeed, today POVM is considered to be a part of standard QT. In fact, POVM measurement of a measured system can be described as a projective measurement of a larger system, which includes not only the measured system but also a part of its "ancilla" environment. From this bigger point of view, all measurement are projective.vanhees71 said:
With that I agree. But my point was to deny a frequent misconception that "a collapse is something that is associated only with projective measurements, not with POVM measurements".gentzen said:
Interesting. What exactly is the misconception that you have in mind? You agree that a collapse postulate (a change of the quantum state) is not included in a POVM measurement. I guess you also agree that a typical description of a projective measurement includes a collapse postulate.Demystifier said:
Sure, that's why I don't think that it changes any of the "fundamental quibbles" some people have, and I'd also take only the part giving the probabilities for the "POVM measurement" result but not the "collapse postulate" since as in the case of the more restricted POV measurements it depends on the specific apparatus used to realize the POVM measurement. So whether or not the measured system is described after the POVM measurement by the assumed "collapsed state", cannot be generally stated either but maybe for some specific POVM measurement it's possible to realize such a specific "state preparation by measurement".Demystifier said:
It's a misconception to say that one of those includes a collapse postulate and the other doesn't. In a consistent joint treatment of projective and POVM measurements, either both include a collapse postulate or neither does. Unfortunately, some "typical" presentations of projective and POVM measurements look as you described above, which is a misconception.gentzen said:
Yes, exactly!gentzen said:
Exactly!vanhees71 said:
The only problem is that the definition of POVM explicitly doesn't include a collapse postulate. The general case presented by N&C seems to be called "quantum measurement" by them. Wikipedia claims that also the name quantum instrument would be in use, but the reference section of that article does not convince me. More convincing is Wikipedia's claim that the mathematical formalism is actually called quantum operation, and used to describe the effects of measurement and transient interactions with an environment. The reference section seems to support that claim, and it is also plausible: Why on Earth would anybody want to call every manipulation or preparation of a quantum system a measurement?Demystifier said: