On the relation to decoherence, maybe "completely unrelated" is too strong but they say their model is "distinct from" and "not in the class of" environment-induced decoherence models. I think it's debatable if it is an "improvement" or not, but I don't think they get any closer to "solving" the measurement problem than Zurek does.
I find interesting that London and Bauer proposed a similar model already in 1939 (ref. [12] in the "Quantum mechanics of measurement" pre-print):
[9. Statistics of a System Composed of Two Subsystems]
"The state of a closed system, perhaps the entire universe, is completely determined for all time if it is known at a given instant. According to Schroedinger's equation, a pure case represented by a psi function remains always a pure case. One does not immediately see any occasion for the introduction of probabilities, and our statistics definitions might appear in the theory as a foreign structure
"We will see that that is not the case. It is true that the state of a closed system, once given pure, always remains pure. But let us study what happens when one puts into contact two systems, both originally in pure states, and afterwards separates them. [...]
"While the combined system I + II, which we suppose isolated from the rest of the world, is and remains in a pure state, we see that during the interaction systems I and II individually transform themselves from pure cases into mixtures.
"This is a rather strange result. In classical mechanics we are not astonished by the fact that a maximal knowledge of a composite system implies a maximal knowledge of all its parts. We see that this equivalence, which might have been considered trivial, does not take place in quantum mechanics. [...]
"The fact that the description we obtain for each of the two individual systems does not have the caracter of a pure case warns us that we are renouncing part of the knowledge contained in Psi(x,y) when we calculate probabilities for each of the two individuals separately. [...] This loss of knowledge expresses itself by the appearance of probabilities, now understood in the ordinary sense of the word, as expression of the fact that our knowledge about the combined system is not maximal."
[10. Reversible and Irreversible Evolution]
"I. Reversible or "causal" transformations. These take place when the system is isolated. [...]
"II. Irreversible transformations, which one might also call "acausal." These take place only when the system in question (I) makes physical contact with another system (II). The total system, comprising the two systems (I + II), again in this case undergoes a reversible transformation so long as the combined system I + II is isolated. But if we fix our attention on system I, this system will undergo an irreversible transformation. If it was in a pure state before the contact, it will ordinarily be transformed into a mixture. [...]
"We shall see specifically that measurement processes bring about an irreversible transformation of the state of the measured object [...] The transition from P to P' clearly cannot be represented by a unitary transformation. It is associated with an increase of the entropy from 0 to -k Sum_n |psi_n|^2 ln |psi_n|^2, which cannot come about by a unitary transformation."
[11. Measurement and Observation. The Act of Objectification]
"According to the preceding section, [the wave function after the measurement] represents a state of the combined system that has for each separate system, object and apparatus, the character of a mixture. [...] But of course quantum mechanics does not allow us to predict which value will actually be found in the measurement. The interaction with the apparatus does not put the object into a new pure state. Alone, it does not confer on the object a new wave function. [...]
"So far we have only coupled one apparatus with one object. But a coupling, even with a measuring device, is not yet a measurement. A measurement is achieved only when the position of the pointer has been observed. It is precisely this increase of knowledge, aquired by observation, that gives the observer the right to choose among the different components of the mixture predicted by the theory, to reject those which are not observed, and to attribute henceforth to the object a new wave function, that of the pure case which he has found."
They go on to discuss how "the observer establishes his own framework of objectivity and acquires a new piece of information about the object in question" while there is no change for us if we look at the "object + apparatus + observer" system from outside. The combined system will be in a pure state and we will have correlated mixtures for each subsystem.
London and Bauer say that the role played by consciousness of the observer is essential for the transition from the mixture to the pure case. Cerf and Adami claim to do without the intervention of consciousness but it's not clear how do they explain the transtion from the mixture to something else. They say things like the following, which doesn't look different from London and Bauer to me:
"The observer notices that the cat is either dead or alive, and thus the observer’s own state becomes classically correlated with that of the cat, although, in reality, the entire system (including the atom, the γ, the cat, and the observer) is in a pure entangled state."
From a very superficial look at the blog posts you linked to, I think I agree with some things (v.g. that most of this ideas have been around for quite some time), I disagree with others (v.g. that there is no measurement problem) but I guess most of the discussion is meta-physical so it cannot be "wrong".
I think dodging the measurement problem is not solving it. The core of the problem is here:
"But, while QM predicts that you will be classically correlated, it does NOT (and cannot) predict what the outcome of your measurements will actually be."
QM does not predict that the measurement will have an outcome. The transition from that probability distribution to one definite result is the measurement problem.
I am not an expert and I am not sure to what extent a solution to the measurement problem is possible, or even needed, but I think it would mean that the current QM theory is incorrect or at least incomplete. In the first case (v.g. the "objective collapse" theories) the Schroedinger function may be very good approximation and it may be practically impossible to get a experimental/observational verification. In the second case (v.g. the "non-local hidden variables" theories) the predictions of the Schroedinger function may be exact so a verification could be impossible even in principle.
He gives more details in section 3.7 of his "Lectures on Quantum Mechanics" (Interpretations of Quantum Mechanics) which ends as follows:
"My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared. Indeed, many physicists are satisfied with their own interpretation of quantum mechanics. But different physicists are satisfied with different interpretations. In my view, we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is only a good approximation."
The last chapter in Commins' "Quantum Mechanics: An Experimentalist’s Approach" (The Quantum Measurement Problem) gives a nice description of the problem and the proposed solutions which fall into three categories:
1.- there is no problem. [Decoherence doesn't completely avoid the problem; it makes the off-diagonal elements of the density matrix zero but the issue of going from a mixture to a specific outcome remains.]
2.- the interpretation of the rules must be changed but this can be done in ways that are empirically indistinguishable from the standard theory. [De Broglie-Bohm pilot-wave theory postulates "real" particles compatible with QM predictions, but currently only works well in the nonrelativistic setting.]
3.- deterministic unitary evolution is only an approximation. [Adding non-linear and stochastic terms produce "spontaneus localization", tuning the parameters we can explain both the microscopic "unitary" and the macroscopic "non-unitary" behaviours.]
"In conclusion, although many interesting suggestions have been made for overcoming the quantum measurement problem, it remains unsolved. We can only hope that some future experimental observation may guide us toward a solution."
[By the way, I sent you an email but I don't know if you got it.]
Weinberg either does not understand the multiverse theory or he intentionally misrepresents it. It is not the case that the universe splits when a measurement is made. That description begs the question because it doesn't specify what counts as a measurement. It's putting lipstick on the Copenhagen pig. A much better (though still not very good) description of the multiverse can be found in David Deutsch's book "The beginning of infinity" chapter 11. Universes do not "come into being" when measurements are made. The entire multiverse always exists. This is the pithy summary:
"Universes, histories, particles and their instances are not referred to by quantum theory at all – any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the multiverse."
Indeed, time itself is an emergent phenomenon:
"[T]ime is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks – or of any objects usable as clocks) into the same history."
Here is Weinberg's fundamental problem:
"[T]he vista of all these parallel histories is deeply unsettling, and like many other physicists I would prefer a single history."
Well, I'm sorry Steven, but you can't have it. That's just not how the world is, and wishing it were so sounds as naive and petulant as an undergrad wishing Galilean relativity were true, or Einstein wishing that particles really do have definite positions and velocities at all times. Yes, it would be nice if all these things were true. But they aren't.
> Weinberg either does not understand the multiverse theory or he intentionally misrepresents it. It is not the case that the universe splits when a measurement is made.
I don’t know if your point is that the universe splits more often than that or never. Note that Weinberg is referring to the usual MWI formulations, I don’t know what is the precise definition of the “multiverse theory” you mention. And actually he says that “the fission of history would not only occur when someone measures a spin. In the realist approach the history of the world is endlessly splitting; it does so every time a macroscopic body becomes tied in with a choice of quantum states.”
“when an experiment is observed to have a particular result, all the other possible results also occur and are observed simultaneously by other instances of the same observer who exist in physical reality – whose multiplicity the various Everettian theories refer to by terms such as ‘multiverse', ‘many universes', ‘many histories' or even ‘many minds'. “
The “plain English” description in the book you cite lacks any rigour and can be confusing if you know a bit of QM because it postulates that the universe splits already (but in what basis?) in the cases where single-universe QM works fine (the wave function can describe a pure quantum state which is a superposition). These “soft splits” can be undone and allow for interference (corresponding to the unitary evolution of the Schroedinger equation).
But, he says that “interference can happen only in objects that are unentangled with the rest of the world” and “once the object is entangled with the rest of the world [...] the histories are merely split further”. These “hard splits” are the branches in the MWI. And correspond perfectly to Weinberg’s description: “[the world splits] every time a macroscopic body becomes tied in with a choice of quantum states.”
In Deutsch’s multiverse the number of universes doesn’t grow because there is always an uncountable infinite number of them, but I’m not sure this makes the theory much better.
Unfortunately I don’t have time to continue this interesting discussion in the near future. At least you have to concede that in an infinite number of alternative universes you found my arguments convincing. That’s good enough for me.
> it does so every time a macroscopic body becomes tied in with a choice of quantum states
But again this just begs the question. How big does a body have to be before it counts as "macroscopic"?
> The “plain English” description in the book you cite lacks any rigour and can be confusing
Yes, that's why I said that it wasn't a very good description (despite being better than Weinberg's).
> These “hard splits” ... correspond perfectly to Weinberg’s description
No, they don't. "A macroscopic body" and "the rest of the world" are not synonyms.
> I’m not sure this makes the theory much better.
I agree with you. That's why I prefer the "zero-universe" approach, and consider our universe and the wave function to be in different ontological categories.
> Unfortunately I don’t have time to continue this interesting discussion in the near future.
:-(
> At least you have to concede that in an infinite number of alternative universes you found my arguments convincing.
I find interesting that London and Bauer proposed a similar model already in 1939 (ref. [12] in the "Quantum mechanics of measurement" pre-print):
[9. Statistics of a System Composed of Two Subsystems]
"The state of a closed system, perhaps the entire universe, is completely determined for all time if it is known at a given instant. According to Schroedinger's equation, a pure case represented by a psi function remains always a pure case. One does not immediately see any occasion for the introduction of probabilities, and our statistics definitions might appear in the theory as a foreign structure
"We will see that that is not the case. It is true that the state of a closed system, once given pure, always remains pure. But let us study what happens when one puts into contact two systems, both originally in pure states, and afterwards separates them. [...]
"While the combined system I + II, which we suppose isolated from the rest of the world, is and remains in a pure state, we see that during the interaction systems I and II individually transform themselves from pure cases into mixtures.
"This is a rather strange result. In classical mechanics we are not astonished by the fact that a maximal knowledge of a composite system implies a maximal knowledge of all its parts. We see that this equivalence, which might have been considered trivial, does not take place in quantum mechanics. [...]
"The fact that the description we obtain for each of the two individual systems does not have the caracter of a pure case warns us that we are renouncing part of the knowledge contained in Psi(x,y) when we calculate probabilities for each of the two individuals separately. [...] This loss of knowledge expresses itself by the appearance of probabilities, now understood in the ordinary sense of the word, as expression of the fact that our knowledge about the combined system is not maximal."
[10. Reversible and Irreversible Evolution]
"I. Reversible or "causal" transformations. These take place when the system is isolated. [...]
"II. Irreversible transformations, which one might also call "acausal." These take place only when the system in question (I) makes physical contact with another system (II). The total system, comprising the two systems (I + II), again in this case undergoes a reversible transformation so long as the combined system I + II is isolated. But if we fix our attention on system I, this system will undergo an irreversible transformation. If it was in a pure state before the contact, it will ordinarily be transformed into a mixture. [...]
"We shall see specifically that measurement processes bring about an irreversible transformation of the state of the measured object [...] The transition from P to P' clearly cannot be represented by a unitary transformation. It is associated with an increase of the entropy from 0 to -k Sum_n |psi_n|^2 ln |psi_n|^2, which cannot come about by a unitary transformation."
[11. Measurement and Observation. The Act of Objectification]
"According to the preceding section, [the wave function after the measurement] represents a state of the combined system that has for each separate system, object and apparatus, the character of a mixture. [...] But of course quantum mechanics does not allow us to predict which value will actually be found in the measurement. The interaction with the apparatus does not put the object into a new pure state. Alone, it does not confer on the object a new wave function. [...]
"So far we have only coupled one apparatus with one object. But a coupling, even with a measuring device, is not yet a measurement. A measurement is achieved only when the position of the pointer has been observed. It is precisely this increase of knowledge, aquired by observation, that gives the observer the right to choose among the different components of the mixture predicted by the theory, to reject those which are not observed, and to attribute henceforth to the object a new wave function, that of the pure case which he has found."
They go on to discuss how "the observer establishes his own framework of objectivity and acquires a new piece of information about the object in question" while there is no change for us if we look at the "object + apparatus + observer" system from outside. The combined system will be in a pure state and we will have correlated mixtures for each subsystem.
London and Bauer say that the role played by consciousness of the observer is essential for the transition from the mixture to the pure case. Cerf and Adami claim to do without the intervention of consciousness but it's not clear how do they explain the transtion from the mixture to something else. They say things like the following, which doesn't look different from London and Bauer to me:
"The observer notices that the cat is either dead or alive, and thus the observer’s own state becomes classically correlated with that of the cat, although, in reality, the entire system (including the atom, the γ, the cat, and the observer) is in a pure entangled state."
From a very superficial look at the blog posts you linked to, I think I agree with some things (v.g. that most of this ideas have been around for quite some time), I disagree with others (v.g. that there is no measurement problem) but I guess most of the discussion is meta-physical so it cannot be "wrong".
I think dodging the measurement problem is not solving it. The core of the problem is here:
"But, while QM predicts that you will be classically correlated, it does NOT (and cannot) predict what the outcome of your measurements will actually be."
QM does not predict that the measurement will have an outcome. The transition from that probability distribution to one definite result is the measurement problem.