dextercioby said:From my perspective, this thread is done may posts ago.
atyy said:But MWI can be falsified. For example, if collapse is found to be real :) Maybe something like in http://arxiv.org/abs/1410.0270.
stevendaryl said:I think that's backwards. MWI doesn't "invoke" an assumption of infinitely many parallel universes. Rather, the assumption that there is only one value for macroscopic observables is an ADDITIONAL assumption, not strictly speaking required.
I understand that it is convenient to make the assumption, and I'm fine with just saying: Well, let's assume something along those lines so that we can along with doing science. But to me, it's ad hoc, and it's only a rule of thumb, and not to be taken seriously. "The mind collapses the wave function" or "Measurement collapses the wave function" or "Decoherence collapses the wave function", or (Bohm-DeBroglie), "the only thing that is real is position, so it's already collapsed at all times" are all essentially equivalent. They are just ad hoc rules to get past interpretational difficulties in QM. I think that's fine, but I don't think people should pretend that there is anything other than an ad hoc choice being made.
Even if it is true that the preferred basis can be derived from the working of memory, the point is that somehow the existence of a preferred basis can be derived. The paper we discuss in this thread can also be viewed as a derivation of its existence.stevendaryl said:Each copy has a complete memory of definite observations. I guess that could be considered a "preferred basis" assumption, in some sense, but to really investigate the extent to which this needs to be an assumption, I think you would have to look at how "memories" work. It's possible that the mechanism of memory only works in one basis. The basis in which the states consists of superpositions of "seeing heads" and "seeing tails" has nothing like a memory to record such things.
I strongly hope they will not do it!stevendaryl said:They should just go ahead and put it on the list of forbidden topics.
This sentence contains a semantic trap in the word "either" that is related to the thread's paper. Because in practice for the theory to have any applicability and empirical and predictive validity only the case with determined values(so in practice always mixed states) is actuallly considered. That is ultimately why the standard quantum theory state of the first postulate isn't physical.vanhees71 said:According to standard quantum theory the state of a quantum system can be such that a given observable either has a determined value or it has not a determined value.
What is science all about is determined by scientists. Unfortunately, there is no complete agreement among scientists on that. In particular, many scientists would not agree that the above is all what science is about.vanhees71 said:because that's all what science is about: To find an as comprehensive quantitative description of that part of the world that is objective.
I think that's a great idea.vanhees71 said:What I'd recommend is to make an extra subforum, labeled "Interpretational issues of Quantum Theory" or something like that. Then people, who don't like to discuss those issues can stay out of these threads more easily, and the Quantum Mechanics forum can concentrate on the "hard science and mathematical" part of quantum theory.
Aside from interpretations, the "hard", FAPP quantum theory applied by experimentalists with so much success includes a form of collapse (that it is of course epistemic, nothing physically collapses instantly, no ftl implications whatsoever) whether one wants to acknowledge it explicitly or not, most people applying QM don't think about it or even care about it. They just set up experiments, measure and calculate. They don't need to be explicitly aware of the quantum/classical interface, they are actually implementing it by doing measurements, and any scientific theory in physics(an emprical discipline) must include measurements in their formulation, otherwise they are pure math and have no connection with nature. I think the first three sections of the Ronde and Maasri paper make this clear with different words.vanhees71 said:The minimal interpretation without collapse is a fully satisfactory one from a scientific point of view. Everything beyond this is philosophy
Well, many interesting things in theoretical physics, cannot be addressed by analytical means (yet?). An example is lattice QCD, which is the only way to study full QCD without making use of perturbation theory, and there are a lot of fundamental things, one cannot study perturbatively (confinement, the hadron spectrum, the QCD phase diagram of strongly interacting matter) but with lattice. Are you saying that's not a satisfactory method to test QCD against observations and doesn't this lead to further progress in our understanding of hadrons and the strong interaction? I'd disagree with this entirely!Demystifier said:What is science all about is determined by scientists. Unfortunately, there is no complete agreement among scientists on that. In particular, many scientists would not agree that the above is all what science is about.
For example, even theoretical physicists who are not interested in philosophical issues are not satisfied by having only quantitative description without a conceptual understanding. In particular, many theoretical physicists feel that a numerical simulation on a computer without a simple (even if approximate) analytical solution does not provide a completely satisfying description. To quote the Nobel laureate Eugene Wigner "It is nice to know that the computer understands the problem, but I would like to understand it too".
I've never heard or read a convincing argument for the necessity of a collpase. The naive collapse is unphysical, and it doesn't help to call it epistemic. I have minimally interpreted quantum theory without a collapse, and that's all what's needed by experimentalists and theorists alike to describe quantitative observations with a high accuracy. So again: What do I need a collapse for?TrickyDicky said:Aside from interpretations, the "hard", FAPP quantum theory applied by experimentalists with so much success includes a form of collapse (that it is of course epistemic, nothing physically collapses instantly, no ftl implications whatsoever) whether one wants to acknowledge it explicitly or not, most people applying QM don't think about it or even care about it. They just set up experiments, measure and calculate. They don't need to be explicitly aware of the quantum/classical interface, they are actually implementing it by doing measurements, and any scientific theory in physics(an emprical discipline) must include measurements in their formulation, otherwise they are pure math and have no connection with nature. I think the first three sections of the Ronde and Maasri paper make this clear with different words.
No, I'm saying that many physicists (not necessarily including myself) think that QCD would be even better understood if could solve it analytically, without computer simulations.vanhees71 said:Are you saying that's not a satisfactory method to test QCD against observations and doesn't this lead to further progress in our understanding of hadrons and the strong interaction?
To account for the information which is available to you after the measurement, i.e. to make measurable predictions for a measurement performed after a measurement.vanhees71 said:What do I need a collapse for?
Of course. But if both approaches eventually give the same numbers, then what is the advantage of the analytical approach? Many think that one advantage is that the analytical approach gives a conceptual understanding that the purely numerical approach lacks. My point is, the mere fact that some physicists think so is a proof that physics is not only about numbers.vanhees71 said:No question about that. If one could get an analytic solution of QCD that would for sure be the best...
You cannot do further measurements with the photon, but you can do further measurements with the atom which absorbed the photon. The claim that your new state is an atom in an excited state can also be expressed in terms of a collapse.vanhees71 said:If I destroy, e.g., my photon in absorbing it in detecting it, I can't do further measurements on it.
It is needed. You can prepare the state to be ##\sigma_z=+1/2##, or you can prepare the state to be ##\sigma_z=-1/2##. In the first case you can say that the state collapsed to ##|+1/2>##, while in the second case you can say that the state collapsed to ##|-1/2>##. If you do not use the word "collapse", then what would you say, how did the state come to one of those two states? Certainly not by unitary evolution governed by the Schrodinger equation!vanhees71 said:... which then is prepared in a state with a determined ##\sigma_z##. ... Nowhere is a collapse needed to describe state preparation!
Demystifier said:Of course. But if both approaches eventually give the same numbers, then what is the advantage of the analytical approach? Many think that one advantage is that the analytical approach gives a conceptual understanding which the purely numerical approach lacks. My point is, the mere fact that some physicists think so is a proof that physics is not only about numbers.
And if can agree that theoretical physics is not only about getting the correct numbers but also about understanding the origin of those numbers, then we should not be very far from agreeing that physics may also be about philosophy of physics.
Nothing has collapsed in my SG example. There was just unitary time evolution of one particle beam into two spatially separated particle beams with (FAPP) determined spin-##z## components (one has ##\sigma_z=1/2##, the other ##\sigma_z=-1/2##). Nowhere did I envoke non-quantum dynamics, let alone an instantaneous action at a distance, to explain that I have well-separated beams with determined spin-##z## components, and that's very important, because a collapse would be an unphysical process envoked as an "explanation" for a physical process, and that's not physics but esoterics!Demystifier said:You cannot do further measurements with the photon, but you can do further measurements with the atom which absorbed the photon. The claim that your new state is an atom in an excited state can also be expressed in terms of a collapse.Of course there is. You can prepare the state to be ##\sigma_z=+1/2##, or you can prepare the state to be ##\sigma_z=-1/2##. In the first case you can say that the state collapsed to ##|+1/2>##, while in the second case you can say that the state collapsed to ##|-1/2>##. If you do not use the word "collapse", then what would you say, how did the state come to one of these two states? Certainly not by unitary evolution governed by the Schrodinger equation!
How do you distinguish the "real" open problems from the "cold" ones? Most lists of the most important open problems in physics, such as this onevanhees71 said:This example also shows that from a scientific point of view it's more fruitful to try to attack the real open problems of contemporary physics than to ponder "cold cases" like the "collapse" or the "measurement problem".
If your initial beam contains only one particle, the experiments show that there are no two separated beams, but only one! How can you explain that, by unitary means without a collapse?vanhees71 said:There was just unitary time evolution of one particle beam into two spatially separated particle beams with (FAPP) determined spin-zz components (one has σz=1/2\sigma_z=1/2, the other σz=−1/2\sigma_z=-1/2).
Wait, but what about (irreversible) amplification of signal? Single excited atom won't work as detector.vanhees71 said:The same with the photon part. Here you have a single atom as "detector" and a photon to begin with. Then you look at the case where a photon gets absorbed by the atom which is excited after that. Great, but no collapse either. The absorption of the photon can be well described with quantum dynamics (at least perturbatively).
vanhees71 said:The reason is simply that on the one hand from a scientific point of view, there's no problem with quantum theory
Demystifier said:Even if it is true that the preferred basis can be derived from the working of memory, the point is that somehow the existence of a preferred basis can be derived. The paper we discuss in this thread can also be viewed as a derivation of its existence.
Yes, that's why many physicists try to solve it without making new experiments or even without making new measurable predictions.stevendaryl said:It seems to me that if there is a solution to the interpretation problem for QM, we already have enough information to solve it--we're just not smart enough, maybe.
Yes, some variants on MWI already look into that direction.stevendaryl said:Yes, I think that a preferred basis might very well be derivable. My suggestion, which I don't know whether anyone has looked into it, is that the preferred basis isn't inherent in the physics of particles and fields, but instead comes from the human need to be able to remember past observations. The physics doesn't care about one basis versus another, but WE do.
The "beam" can consist of single particles. By assumption the location of the particle at the 2nd Stern-Gerlach apparatus is enough to be sure that there enters a particle with definite ##\sigma_z##.Demystifier said:If your initial beam contains only one particle, the experiments show that there are no two separated beams, but only one! How can you explain that, by unitary means without a collapse?
Ah, now I think I finally see what interpretation of QM do you have in mind. In the ensemble interpretation, the wave function does not represent individual measured objects, but only statistical properties of the ensemble. With that view, you don't need collapse.vanhees71 said:The "beam" can consist of single particles. By assumption the location of the particle at the 2nd Stern-Gerlach apparatus is enough to be sure that there enters a particle with definite ##\sigma_z##.
Of course, to test the predictions of QT you always need "beams" in the sense that you have to perform the experiment with single particles often enough to collect "enought statistics" to have a large enough confidence level to confirm disconfirm the probabilistic prediction. That's why this point of view is usually called "(Minimal) Statistical Interpretation" or "Ensemble Interpretation".
Strange. I have never heard about a minimal version without collapse.vanhees71 said:I've never heard or read a convincing argument for the necessity of a collpase. The naive collapse is unphysical, and it doesn't help to call it epistemic. I have minimally interpreted quantum theory without a collapse, and that's all what's needed by experimentalists and theorists alike to describe quantitative observations with a high accuracy. So again: What do I need a collapse for?
But you do admit that probably there is something which is not the wave function (otherwise you would need either the collapse or many-worlds), even if you don't care what exactly that something might be. Am I right?vanhees71 said:I'm agnostic about this question. As long as there's no clear evidence against standard QT, there seems not to be a chance to find a more comprehensive theory.
I think "smart" is not the problem. Most are not ready to accept the hypothesis that relativistic symmetry is not fundamental, not even as a working hypothesis, which one accepts for the sake of the argument, to have a look at how it fails, no, it is rejected out of hand, without any discussion. Like here, where even to discuss the Lorentz ether interpretation vs. Minkowski interpretation is forbidden (BTW with a completely wrong argument that there is no difference in the predictions, in a situation where one can derive Bell's inequalities only in the Minkowski interpretation, but not in the Lorentz interpretation, which allows hidden causal influences, SCNR).stevendaryl said:It seems to me that if there is a solution to the interpretation problem for QM, we already have enough information to solve it--we're just not smart enough, maybe.
Where do you see the preferred basis problem in the de Broglie-Bohm interpretation?Demystifier said:With such a more common terminology, their theorems say that the physical content of QM is not basis independent. But this claim is not new at all. This is nothing but a restatement of the preferred basis problem appearing in one way or another in all interpretations of QM.