Why the Many-Worlds Interpretation?
Round 1
Reviewer 1 Report
In the article "Why the Many-Worlds Interpretation?", the author stated the clear intent to argue "that the MWI is the only interpretation which removes action at a distance and randomness from quantum theory". It "is a preface to the Special Issue of Quantum Reports devoted to the results of (a) workshop". As a reader, who knows hardly anything about the workshop, and a non-believer of the MWI, I have to say I am very disappointed with the article. As a reviewer, however, given the nature of the article, I do not exactly think I should recommend against its publishing. I do not think one should be demanding on such a preface, asking it to do a job beyond what has been achieved in the workshop itself. So, while I see the article as completely failing to attain the claimed intention, I will leave it to the editors, or the workshop participants, to decide if the article serves its purpose as that preface. I sketch below my personal dissatisfaction with the author's case for the MWI as presented in the article.
I am definitely in support of the author's clear stand for determinism in a physical theory. Simply put, a (successful) deterministic theory has stronger predictive power than a non-deterministic one. Yet, the author's claim about determinism in the MWI versus the non-deterministic projection postulate of the "collapse interpretation" only replaces the probabilistic "collapse" by the probably even more mysterious "self-location probability". At least until one has a deterministic theory on the latter, the claim seems to be an empty one.
More importantly, with the development of the decoherence theory as a physical description of the "collapse" within the framework of the quantum theory itself, matching the MWI versus the original naive notion of a non-deterministic "collapse" is of little interest. In the picture of the decoherence theory, the microstate of the whole system (object measured, apparatus, and environment) maintains a deterministic unitary evolution as predicted by the quantum theory, resulting in a macrostate described by the reduced density matrix for the part of the apparatus as a pointer state for the answer of the measurement and the object measured. The so-called "collapse" never truly happens, the unitarily evolving quantum state is always there. Only our practical ignorance about the initial condition of the full microstate keeps us from retrieving the deterministically predicted macrostate (and microstate at any instant) from the quantum theory. There is no randomness. To me. it is this understanding of the physical picture from the decoherence theory that whoever believes in the MWI should match the latter against. And I do not see any argument on the matter in this article. It is a practical experimental fact that decoherence is hard to avoid, even if no projective measurement has been performed. It is also noteworthy that recent experiments have essentially been able to monitor and manipulate the "collapse" as a physical process (Minev et.al. Nature 570, 200), which I see to be in line with the picture from the decoherence theory.
I have not touched on the question of locality or action-at-a-distance for a reason. I appreciate a different picture of quantum reality (Stud.Hist.Philo. Sci. 92, 186) that goes along with a different picture or model of the physical space. The Newtonian space being a good model of the physical space is really an integral part of the theory of Newtonian dynamics. There is no reason at all to assume the model still works well with the quantum theory, and it quite obviously does not so long as any realistic picture of that space or position in that space is concerned. The locality question hence should not be addressed as it has been based on the Newtonian model of the physical space. Of course, there would not be any Born probability interpretation either.
Author Response
I made a significant revision making multiple style and English corrections and adding a few sentences following the comments of the referee. My replies to the referee are below between ====== and -----.
Report of the referee I
In the article "Why the Many-Worlds Interpretation?", the author stated the clear intent to argue "that the MWI is the only interpretation which removes action at a distance and randomness from quantum theory". It "is a preface to the Special Issue of Quantum Reports devoted to the results of (a) workshop". As a reader, who knows hardly anything about the workshop, and a non-believer of the MWI, I have to say I am very disappointed with the article. As a reviewer, however, given the nature of the article, I do not exactly think I should recommend against its publishing. I do not think one should be demanding on such a preface, asking it to do a job beyond what has been achieved in the workshop itself. So, while I see the article as completely failing to attain the claimed intention, I will leave it to the editors, or the workshop participants, to decide if the article serves its purpose as that preface. I sketch below my personal dissatisfaction with the author's case for the MWI as presented in the article.
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I am grateful to the referee for his attempt to understand my paper and very sorry that he was not convinced. Maybe my case is weak and the workshop will show this. I mentioned in my preface that this result also can be considered as a success of the workshop. However, the referee report was not enough to persuade me that my case is weak. I briefly explain why, by adding my replies to the comments of the referee.
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I am definitely in support of the author's clear stand for determinism in a physical theory. Simply put, a (successful) deterministic theory has stronger predictive power than a non-deterministic one. Yet, the author's claim about determinism in the MWI versus the non-deterministic projection postulate of the "collapse interpretation" only replaces the probabilistic "collapse" by the probably even more mysterious "self-location probability". At least until one has a deterministic theory on the latter, the claim seems to be an empty one.
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I added a few sentences in section 3 and 8 to stress that the MWI is deterministic as a physical theory which describes all worlds together. Within a world there is nothing which will tell us what will be the result of a quantum measurement. The way to reconcile this with determinism is to realise that this is an illegitimate question because all results will take place. The interpretational part which connects deterministic evolution of the universal wave function with our experiences in different worlds has the self-location Born-type rule postulate which explains our illusion of probability. The statement that the physical part of the theory has no random processes is not an empty claim.
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More importantly, with the development of the decoherence theory as a physical description of the "collapse" within the framework of the quantum theory itself, matching the MWI versus the original naive notion of a non-deterministic "collapse" is of little interest. In the picture of the decoherence theory, the microstate of the whole system (object measured, apparatus, and environment) maintains a deterministic unitary evolution as predicted by the quantum theory, resulting in a macrostate described by the reduced density matrix for the part of the apparatus as a pointer state for the answer of the measurement and the object measured. The so-called "collapse" never truly happens, the unitarily evolving quantum state is always there. Only our practical ignorance about the initial condition of the full microstate keeps us from retrieving the deterministically predicted macrostate (and microstate at any instant) from the quantum theory. There is no randomness. To me. it is this understanding of the physical picture from the decoherence theory that whoever believes in the MWI should match the latter against. And I do not see any argument on the matter in this article. It is a practical experimental fact that decoherence is hard to avoid, even if no projective measurement has been performed. It is also noteworthy that recent experiments have essentially been able to monitor and manipulate the "collapse" as a physical process (Minev et.al. Nature 570, 200), which I see to be in line with the picture from the decoherence theory.
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It seems that the referee believes that the decoherence theory solves the measurement problem. He writes: "our practical ignorance about the initial condition of the full microstate keeps us from retrieving the deterministically predicted macrostate (and microstate at any instant) from the quantum theory." I strongly believe that there is no one macrostate at the end of the evolution described on my Fig. 2b. The literature mentioned by the referee is vague about exact meaning of the results. Quantum trajectories can be considered just as an effective description without ontological meaning.
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I have not touched on the question of locality or action-at-a-distance for a reason. I appreciate a different picture of quantum reality (Stud.Hist.Philo. Sci. 92, 186) that goes along with a different picture or model of the physical space. The Newtonian space being a good model of the physical space is really an integral part of the theory of Newtonian dynamics. There is no reason at all to assume the model still works well with the quantum theory, and it quite obviously does not so long as any realistic picture of that space or position in that space is concerned. The locality question hence should not be addressed as it has been based on the Newtonian model of the physical space. Of course, there would not be any Born probability interpretation either.
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Yes, our starting points are very different. I do believe we can understand reality of our quantum world in Newtonian space. The referee writes that quantum theory quite obviously cannot do so. It would be very helpful if the referee would explain it to the workshop participants by attending it, or by submitting a paper to the proceedings.
(I do agree with the referee that the MWI does not have a strong case without assuming Newtonian three-dimensional space.)
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Reviewer 2 Report
Dear Editor,
Please find below my report:
The manuscript by Lev Vaidman is an interesting preface for a workshop devoted to the MWI (many worlds interpretation) . The author reviews his own interpretation of the MWI and explains clearly why he thinks that this approach is the best for interpreting quantum mechanics (this includes the absence of explicit nonlocality that would lead to faster than light communication). The MWI is controversial because of the debate concerning probability and Vaidman explains why his own interpretation solves the issue. This review constitutes a good introduction to the work shop and I believe it will induces reactions from the other participants. I strongly encourage the publication of this text.
Author Response
I thank the referee for encouragement.
I made another proofread of the paper to improve syle and Englsih grammar.
