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PeterDonis
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Ah, I see.Demystifier said:It provides that the Bohmian particle cannot enter more than one branch of the wave function
Ah, I see.Demystifier said:It provides that the Bohmian particle cannot enter more than one branch of the wave function
Can you name one interpretation that does not solve the Hyperion problem? I think they all do, whatever Hossenfelder says.PeterDonis said:In the Bohmian interpretation, that's true, as I've already said. And since that is your preferred interpretation, I can see why it seems this way to you. But there are other interpretations.
As I understood it, this is one the reason that pure "interpretations" alone are getting us nowhere. I agree on that.PeterDonis said:I would not state it this way, because if we believe that QM can't explain the observed motion of Hyperion, and by extension of any system that shows classical dynamics beyond the Ehrenfest time, then why would we describe what we are looking for to take its place as "solving the measurement problem", since the measurement problem itself is a problem of QM, and if we accept the first part of her argument, we are discarding QM?
I don't understand her problem with the textbook response. Given some initial state that includes both Hyperion and its environment, the linear evolution of this state in Hilbert state will predict a chaotic evolution of Hyperion in phase space that lasts longer than 20 years. She likens this to a model of a die that gives the right average but the wrong relative frequencies (e.g. predicting the die will return 106 approximately half the time). What is the Hyperion analogue to the 106 case?A. Neumaier said:See also one of Sabine Hossenfelder's blog entries:
https://backreaction.blogspot.com/2022/05/chaos-real-problem-with-quantum.html
Berthold-Georg Englert in "On Quantum Theory" (The European Physical Journal D, 2013 - Springer):Madeleine Birchfield said:Summary: preprint by Jonte Hance and Sabine Hossenfelder on the measurement problem and quantum foundations
Yesterday Jonte Hance and Sabine Hossenfelder published this preprint on the arXiv: https://arxiv.org/abs/2206.10445
What does it take to solve the measurement problem?
The perspective in that paper IMO misses a whole important facet - which at least for me, and from the interacting agent perspective - is the most important one: Unification of interactions and measurements, for the purpose of understanding unification of forces and the nature of causality.Lord Jestocost said:Berthold-Georg Englert in "On Quantum Theory" (The European Physical Journal D, 2013 - Springer):
"One preexisting concept of quantum theory is the event, such as the emission of a photon by an atom, the radioactive decay of a nucleus, or the ionization of a molecule in a bubble chamber. The formalism of quantum theory has the power to predict the probabilities that the events occur, whereby Born’s rule [4] is the link between formalism and phenomenon. But an answer to the question Why are there events? cannot be given by quantum theory.
. . . . . .
Fifth, since neither decoherence nor any other mechanism select one particular outcome (see Sec. 8), the whole “measurement problem” reduces to the question Why is there one specific outcome? which is asking Why are there randomly realized events? in the particular context considered. This harkens back to Sec. 1, where we noted that quantum theory cannot give an answer.
In summary, then, the alleged “measurement problem” does not exist as a problem of quantum theory. Those who want to pursue the question Why are there events? must seek the answer elsewhere." [bold by LJ]
I just saw Hossnefelder's video, and that was my first reaction - quantum mechanics should reproduce the classical motion over long-time scales. I didn't realise it hadn't been proven yet. Interesting.A. Neumaier said:That's not her argument. Her argument is:
The long term motion of Hyperion is in agreement with classical chaotic mechanics. Thus to explain it by quantum mechanics we need to show that quantum mechanics reproduces the classical motion for these long time scales, which has not been done so far. This, in turn, is related to the measurement problem.
Lord Jestocost said:In summary, then, the alleged “measurement problem” does not exist as a problem of quantum theory. Those who want to pursue the question Why are there events? must seek the answer elsewhere." [bold by LJ]
The correct statement is that it hasn't been proven yet in an interpretation independent way. Different interpretations reproduce classical motion in different ways, but without an interpretation one cannot get it.bhobba said:I just saw Hossnefelder's video, and that was my first reaction - quantum mechanics should reproduce the classical motion over long-time scales. I didn't realise it hadn't been proven yet.
It (Namely that ''quantum mechanics should reproduce the classical motion over long-time scales'') also hasn't been proven yet in an interpretation dependent way, thus the additional phrase only obscures matters. If you object, please point to a proof.Demystifier said:The correct statement is that it hasn't been proven yet in an interpretation independent way.
Yes, and the eikonal approximation is valid only at short times. Nobody so far proved something interesting for very long times, at the constant physical value of ##\hbar##.vanhees71 said:For the most simple case of a single particle in non-relativistic QM, you get the classical limit by the eikonal approximation of the (time-dependent) Schrödinger equation. In the path-integral formulation that's equivalent to the saddle-point approximation and is valid when the typical values of the action around the classical trajectory under investigation are very large compared to ##\hbar##. I think there's nothing interpretation-dependent in this argument.
By "proven" I didn't mean in the strict mathematical sense. Different interpretations explain (rather than prove in the strict mathematical sense) it, but it cannot be explained in interpretation independent way.A. Neumaier said:It (Namely that ''quantum mechanics should reproduce the classical motion over long-time scales'') also hasn't been proven yet in an interpretation dependent way, thusd the additional phrase only obscures matters. If you object, please point to a proof.
The classical limit is physical, but we don't fully understand how it arises. That's one of the reasons why we have different interpretations, to explain the classical limit.vanhees71 said:Then you say that the classical limit is unphysical? Any physically observable phenomenon must be, of course, independent of the interpretation of QT used.
This is only part of the story. Decoherence also plays an important role in the classical limit, while the argument above does not involve decoherence.vanhees71 said:For the most simple case of a single particle in non-relativistic QM, you get the classical limit by the eikonal approximation of the (time-dependent) Schrödinger equation. In the path-integral formulation that's equivalent to the saddle-point approximation and is valid when the typical values of the action around the classical trajectory under investigation are very large compared to ##\hbar##. I think there's nothing interpretation-dependent in this argument.
I currently do understand how preparation and "interaction" of sufficiently small systems (certainly not the whole universe) can be modeled within Bohmian mechanics. The way how to model measurement is not fully clear to me. (What is unclear to me is that the individual trajectory is not (supposed to be) observable, but maybe "many particle statistics" of trajectories are "supposed to be" observable.)A. Neumaier said:It (Namely that ''quantum mechanics should reproduce the classical motion over long-time scales'') also hasn't been proven yet in an interpretation dependent way, thusd the additional phrase only obscures matters. If you object, please point to a proof.
''proofs'' in the sense of a formal limit ##\hbar\to 0## don't mean much for long times; the time limit for which these limits are valid are of the order of a power of ##\hbar## (times context-dependent constants that producing the right units), and are microscopically small for the physical value of ##\hbar##.Demystifier said:By "proven" I didn't mean in the strict mathematical sense. Different interpretations explain (rather than prove in the strict mathematical sense) it, but it cannot be explained in interpretation independent way.
zekise said:There is no such thing as the measurement problem.
The experts in the field don't agree with you. Have you read Schlosshauer's book on decoherence?zekise said:There is no such thing as the measurement problem. This is the creation of some very dated and wrong ideas
A. Neumaier said:The experts in the field don't agree with you. Have you read Schlosshauer's book on decoherence?
One of Rovelli's sound points is that there are no absolute observations, he even goes on to acqknowledge that there are no absolute relations between observations, it should take a third observer to assess it. But onfortunately at some point he claims that communication between observers follows the rules of QM. So he ends up explaining nothing IMO. I think the reasons is that while he at the same time wants to make a nice interpretation, he does not wish to CHANGE the theory, so his solution is conservative. And I think that is a mistake.zekise said:There is no such thing as the measurement problem. This is the creation of some very dated and wrong ideas, picked up by science writers to create some drama in their profession.
In fact observer A can take a measurement of a particle and the particle wave function would not "collapse"!
Imagine we have two observers A and B. A is coherent w.r.t. the environment E and the observer B, and observer B is embedded in E (entangled with E).
A particle arrives in a superposition of states for both A and B. A measures the particle and it "collapses" (an unfortunate word) for A. But the particle has NOT collapsed for B.
The state of the particle depends on who is asking or measuring. There is no such thing as an absolute wave function. A wave function is always between two (or more) parties. Just like velocity. You can't have V(a) the absolute velocity of object a. You can only have V(a, b).
This is the Relational Interpretation of Carlo Rovelli (1994).
I'm not sure this is a sound point. Taken as it is stated, it denies the possibility of irreversible observer-independent experimental results, and without those, we have nothing on which to build a theory.Fra said:One of Rovelli's sound points is that there are no absolute observations
bhobba said:It fits in nicely with the QFT view of things IMHO, as it should.
PeterDonis said:I'm not sure this is a sound point. Taken as it is stated, it denies the possibility of irreversible observer-independent experimental results, and without those, we have nothing on which to build a theory.
Another way of putting this point is that, when we start considering scenarios in which observers are supposed to be modeled using QM, we have a serious problem: any such QM model will have to include the possibility of reversing any observation (because time evolution in QM is unitary and any unitary operation can be reversed). But that amounts to allowing the possibility of reversing decoherence, and again, once you allow that, you have no irreversible observer-independent experimental results, and thus nothing on which to build a theory.
I get why it can seem this way. And it certainly complicates things. But the same can be said qbout science. How can we do science if we are never sure about anything? Here the answer is corroboration.PeterDonis said:In short, it seems to me that this kind of "relational" viewpoint undermines the possibility of doing physics at all.
We don't have to be "sure" to make predictions and decide whether or not to act on them. You can do that based on probabilities--how likely is it that this number we just calculated from our scientific theory is correct? And it doesn't have to be exactly correct; it just has to be close enough for whatever purpose we are using it for.Fra said:How can we do science if we are never sure about anything?
Of course this helps in collecting data on how accurate a scientific theory's predictions are; it's a lot easier to get a large amount of data to work with if you have multiple people doing it.Fra said:Here the answer is corroboration.
Fair enough. That choice is word isnt meant to play down science and suggets it's just politics. I meant it a also deeper way where reality is emerget.PeterDonis said:However, I'm not sure I would describe this as "negotiation".
https://iopscience.iop.org/article/10.1088/2399-6528/ac96cf#jpcoac96cfs5-4malawi_glenn said:isn't Hossenfelder on the verge of crackpot soon?
The main sources which are to be discussed in this thread is an arxiv manuscript and a blogpost. I thought only peer-review published work were allowed on this forum? I am just trying to understand
It is sometimes questioned whether the Collapse Postulate is actually necessary (e.g. in [6]). Without it, quantum mechanics would still correctly predict average values for large numbers of repetitions of the same experiment. This is the statistical interpretation suggested by Ballentine [7].
However, we do not merely observe averages of many experiments: we also observe the outcomes of individual experiments. And we know from observations that the outcome of an experiment is never a superposition of detector eigenstates, nor is it ever a mixed state (whatever that would look like)—a detector either detects a particle or it doesn't, but not both. As Maudlin put it [2], 'it is a plain physical fact that some individual cats are alive and some dead' (emphasis original). Without the Collapse Postulate, the mathematical machinery of quantum mechanics just does not describe this aspect of physical reality correctly.
There cannot be a generally valid collapse postulate, because it depends on your experimental setup, what happens to the measured object. E.g., if you detect a photon with a photo detector you use the photoelectric effect, i.e., the photon is absorbed by the detector and for sure not in an eigenstate of the measured quantity (e.g., the polarization in a given direction). Of course there are (approximations of) ideal von Neumann "filter measurements", e.g., using a polarization filter, which lets through only photons with linear polarization in a given direction. Then the projection postulate holds, and you have a kind of "collapse of the state".Morbert said:Just to re-emphasise an issue I have. From the paper:The bit in bold could be interpreted two ways:
i) Without the Collapse Postulate, the mathematical machinery of quantum mechanics does not describe this aspect of physical reality.
I don't know, what you mean here. There is no problem with filter measurements nor with other kinds of experiments. The "proof" is simple: QT works with great success whereever it is applied!Morbert said:ii) Without the Collapse Postulate, the mathematical machinery of quantum mechanics incorrectly describes this aspect of physical reality.
The latter would mean the measurement problem really is a problem, in the sense that it is a point of incorrectness.
The former is a better reading of interpretations like those presented by Ballentine: QM is correct everywhere in its domain, and it is not a problem that QM returns probabilities for possibilities rather than a definite actuality.
There can be, and in fact there is:vanhees71 said:There cannot be a generally valid collapse postulate, because it depends on your experimental setup, what happens to the measured object.
See also section 8.2.4 (because it "explains something about the interpretational background" of that concept) ...A. Neumaier said:(in Section 2.2.3 on quantum measurement, without introducing a name for the concept)
Adding to the point made by A. Neumaier above, we can also recover a collapse postulate if we generalise our description of the experiment to possible histories ##\{C_\alpha\}## of the joint system (measured system + instrument). Collapse is the change ##\rho \rightarrow \frac{C_\alpha \rho C^\dagger_\alpha}{\mathrm{tr}C_\alpha \rho C^\dagger_\alpha}##vanhees71 said:There cannot be a generally valid collapse postulate, because it depends on your experimental setup, what happens to the measured object. E.g., if you detect a photon with a photo detector you use the photoelectric effect, i.e., the photon is absorbed by the detector and for sure not in an eigenstate of the measured quantity (e.g., the polarization in a given direction). Of course there are (approximations of) ideal von Neumann "filter measurements", e.g., using a polarization filter, which lets through only photons with linear polarization in a given direction. Then the projection postulate holds, and you have a kind of "collapse of the state".
What Hossenfelder is arguing (unsuccessfully imo) is that, without a collapse postulate, QM's inability to account for the realisation of one possibility over others is a point of incorrectness of the theory.I don't know, what you mean here. There is no problem with filter measurements nor with other kinds of experiments. The "proof" is simple: QT works with great success whereever it is applied!