The Hidden Clash: Spacetime Outlook and Quantum-State Reductions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have enjoyed reading the manuscript entitled
"The Hidden Clash: Space-Time Outlook and Quantum-State
Reductions".
It is a brief but fresh accounts of the difficulties
in reconciling quantum mechanics with special relativity.
I fully agree that such difficulties are found in the
different ways in which simultaneity is treated by
the two theories: there is no absolute simultaneity
in special relativity, while quantum  mechanics
requires the notions of absolute simultaneity.

This problem is presented, although briefly, from
various points of view, except one that I feel is
particularly interesting and not very well-known.
I am referring to superfluid vacuum theory.
Superfluid vacuum theory was born in 1951 when
Dirac published two papers
[P. A. M. Dirac, Is there an Æther?, Letters to Nature.
Nature 168, 906–907 (1951).  doi:10.1038/168906a0]
and [ P. A. M. Dirac, Is there an Æther?, Nature 169,
702 (1952). doi:10.1038/169702b0]
where he suggested that, because of the uncertainty
principle the state of motion of the ether can only
be specified by a wave function.
Following Dirac's idea, in 1975 Sinha, Sivaram,
and Sudarshan published three papers
[K. P. Sinha, C. Sivaram, and E. C. G. Sudarshan,
Aether as a superfluid state of particle-antiparticle pairs
Foundations of Physics 6, 65–70 (1976). doi:10.1007/bf00708664],
K. P. Sinha, C. Sivaram, and E. C. G. Sudarshan, The superfluid
vacuum state, time-varying cosmological constant, and nonsingular
cosmological models, Foundations of Physics 6, 717–726 (1976)
doi:10.1007/bf00708950], and [K. P. Sinha, C. Sivaram,
and E. C. G. Sudarshan, The superfluid as a source of all interactions,s
Foundations of Physics 8, 823–831 (1978) doi:10.1007/bf00715056]
in which they considered the vacuum as a non-relativistic superfluid,
whom relativistic excitations give rise to relativistic space-time.
Notable books that discuss such a theory are [G. E. Volovik, The Universe
in a helium droplet (Oxford University Press, Oxford, 2003)] and
[K. Huang, A Superfluid Universe (World Scientific, Singapore, 2017].
Superfluid vacuum theory appears to solve the problem discussed in the
present manuscript. For this reason, such an approach deserves to be discussed in a paragraph or two.

I recommend the publication of a revised manuscript, containing the discussion
of superfluid vacuum theory for establishing a clear connection between quantum theory and special relativity.



that suggested a new model for the aether according to which it is a superfluid state of fermion and anti-fermion pairs, describable by a macroscopic wave function.[3][4][5] They noted that particle-like small fluctuations of superfluid background obey the Lorentz symmetry, even if the superfluid itself is non-relativistic. Nevertheless, they decided to treat the superfluid as the relativistic matter – by putting it into the stress–energy tensor of the Einstein field equations. This did not allow them to describe the relativistic gravity as a small fluctuation of the superfluid vacuum, as subsequent authors have noted[citation needed].

Author Response

Thanks for your comments. In the new version I included two paragraphs about the contributions of the superfluid vacuum theory to the problem my paper addresses.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Editor

The article "The hidden clash: Space-time outlook and quantum state reductions" contains important conceptual errors which invalidate the whole analysis. Sentences such as "The essentially non-relativistic character of elementary quantum field theory, which distinguished time t from the three space coordinates (x,y,z), entailed in a certain way implicitly admitting a concept of absolute simultaneity" are technically wrong. Also, talking about a "tension [that] persists between the conception of the universe that special relativity defends, as a geometric theory of space-time, and the assumption about physical reality that support quantum theory in any of its versions" makes no sense from the perspective of our current understanding of theoretical physics. As I see it, the work is based on a misconception of QFT and is aimed at solving a problem that does not exist as such.

 

Author Response

I think that the paper does not contain substantial flaws or conceptual errors and I do believe that the foundational problem this paper addresses really exist, it is extremely relevant, and no current undertanding of theoretical physics has been ablle to solve it up to this moment.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper purports to highlight a deep conceptual incompatibility between relativistic and quantum conceptions of spacetime that goes beyond the mere technical difficulties in formulating a Lorentz-invariant version of quantum mechanics or a theory of quantum gravity.

The paper displays a good understanding of the issues discussed, and there are interesting points being made, but I am a bit less sure about its novelty.

Section 2 could benefit from being clearer on the distinction between ontology and nomology, and the fact that some structures of the theory can be considered one or the other depending on the interpretation (see for example "Primitive Ontology and Quantum State in the GRW Matter Density Theory" from Egg and Esfeld). In a flash ontology, it's not clear that there are superposed states per se: these could be viewed as nomological entities, not actual "states", prefiguring or constraining the distribution flashes, thus avoiding the ontological duality suggested in the paper.

The presentation of section 3 is a bit messy in my view, mixing aspects having to do with the quantum measurement problem and with entanglement. The compatibility with relativity is barely addressed despite the title of the section: the measurement problem is a problem in itself, irrespective of the theory of relativity. Furthermore, all these issues have been presented extensively elsewhere (in Maudlin's "three measurement problems" for instance), so maybe the paper could restrict itself to what is strictly necessary for specific topic of the paper.

Section 4 mentions that Hilbert space is not amenable to classical space, but this is only one formulation of quantum mechanics. Configuration space is easier to relate to classical space. The section also implicitly focuses on collapse theories, which, as aknowledged in the paper, are not the only way of interpreting quantum theory. This is not a problem, but I think it should be more explicit, from the abstract, that this is the focus of the paper. Or maybe the problem I had reading the paper is that it is not clear that the specific problem that is being addressed can be addressed without assuming one interpretation of quantum or another, but the paper is not very upfront on this aspect. Finally, the non-locality of collapses (or of quantum mechanics in general, as per the violation of Bell's inequality), and the tension with relativity it creates, are well known, so it's not clear what novelty the first part of section 4 is supposed to bring to the debate.

The really interesting part, according to me, concerns the tension between objective probabilities and relativity as analysed starting from line 324 and developed in section 5. This specific aspect is indeed somehow neglected in the literature as far as I know (but I might be wrong!) and it seems to run deep into the fundamental assumptions of relativity and quantum mechanics respectively (putting aside Everettian and Bohmian interpretations at least). My feeling, overall, is that the paper could gain in focus and clarity by focusing more on this specific issue from the start, and only laying out the framework in which the discussion takes place before (Are we assuming a collapse-interpretation of quantum mechanics? What constraints on probability ascription from various reference frames are we assuming? etc.), instead of dwelling on other related, but already well known issues.

typo line 222: yo?

Author Response

I appreciate very much your acurate comments on my paper and in the new version I tried to include them as much as possible. The discussion about objective possibility is extended and part of the exposition about quantum collapse was suppresed. I did not reduce more the previous text because another reviewer agreed on the original content of my paper and encouraged me to add some paragraphs about the superfluid vacuum theory. 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

I would like to thank the author for the corrections. My feeling is that some of my comments could have been taken a bit more seriously (often an addition merely acknowledges them, but this doesn't affect the rest of the paper). I'm thinking in particular about the claim that a dual ontology is needed in collapse theories. Reading the passage again, (1) it's not clear why this should be the case, even without the nomological interpretation, since collapsed and non collapsed vectors have the same mathematical form (a vector being a superposition or not depends on which basis is used to describe it, but the choice of a basis is naturally considered "in the head" of the modeller rather than in reality), (2) it's not clear why it is important for the paper, since all the rest of the argumentation could be carried out in the same way if there were no ontological duality in collapse theories.

My suggestion, which I think is the wisest solution, is to remove the passage entirely since it seems inessential: this would improve the focus and impact of the paper, which already addresses many different aspects. But perhaps the author prefers to add some clarifications instead in response, which is acceptable to me as well. This would mean: explain why the dual ontology would come out naturally in collapse interpretation (how are non collapse states any different) and optionally how this relates to the main issue addressed in this paper.

Author Response

Dear reviewer,

Could you indicate exactly the section that should be removed?