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The FröhlichMorchioStrocchi mechanism and quantum gravity
by Axel Maas
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Submission summary
Authors (as registered SciPost users):  Axel Maas 
Submission information  

Preprint Link:  https://arxiv.org/abs/1908.02140v2 (pdf) 
Date submitted:  20200115 01:00 
Submitted by:  Maas, Axel 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
Taking manifest invariance under both gauge symmetry and diffeomorphisms as a guiding principle physical objects are constructed for YangMillsHiggs theory coupled to quantum gravity. These objects are entirely classified by quantum numbers defined in the tangent space. Applying the Fr\"ohlichMorchioStrocchi mechanism to these objects reveals that they coincide with ordinary correlation functions in quantumfield theory, if quantum fluctuations of gravity and curvature become small. Taking these descriptions literally exhibits how quantum gravity fields need to dress quantum fields to create physical objects, i.e. giving a graviton component to ordinary observed particles. The same mechanism provides access to the physical spectrum of pure gravitational degrees of freedom.
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Reports on this Submission
Anonymous Report 1 on 2020118 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1908.02140v2, delivered 20200118, doi: 10.21468/SciPost.Report.1458
Strengths
1 The work applies ideas originating from particle physics in the context of gravity. This results in a highly innovative (and partially speculative) new perspective which will certainly trigger interesting discussions in the field.
2 The construction of observables in the context of quantum gravity is a highly nontrivial problem, see e.g. arXiv:1507.07921. The work makes interesting progress in this direction.
3 The article outlines a potential connection between fundamental gravity research and a phenomenologically interesting dark matter candidate, the “geon”.
Weaknesses
1 In some places, the work is short on details. In particular the discussion about the status of a vanishing vacuum expectation value for the spacetime metric could have taken more room.
Report
I find the work interesting and rich of original ideas. It certainly deserves publication in SciPost Physics.
Requested changes
I leave it up to the author whether he still would like to comment on and suitably implement the following suggestions:
1 The vanishing of the vacuum expectation value for the spacetime metric is an interesting statement. The arguments leading to this conclusion are clear. There are two potentially farreaching consequences where it would be interesting to learn the author’s opinion.
Firstly, does the author suggest that one should focus on directly computing expectation values of the physical length $<ds^2> = < g_{\mu\nu} dx^\mu dx^\nu >$ without referring to the concept of a vacuum expectation value for $g_{\mu\nu}$?
Secondly, it is commonly accepted that one could compute a nonvanishing expectation value $<g_{\mu\nu}>$ by solving the equations of motion derived from the effective action of gravity. At this stage, it is not clear to me how to reconcile these two pictures, so it may be worthwhile to clarify this connection.
2 The role of eq. (14) is not clear. In principle $J_\mu^u$ is independent of the spinconnection. The equivalence principle would nevertheless suggest that the conservation law should be constructed from a covariant derivative including the LeviCivita connection. Since in general $\partial_\mu$ will not commute with the metric $\partial_\mu J^\mu \not = \partial^\mu J_\mu$ which seems odd.
3 There are a few stylistic remarks which would improve the readability of the work.
E.g., the definition of the operators $O_1$ and $O_2$ could be put into an equation and then referred back to when their twopointfunctions are studied.
The paragraph containing eq. (23) provided a stumbling block. My first reaction was that this cannot be true in general (the remaining paragraph explains this in detail). So perhaps one could describe the generic situation first and then use eq. (23) as a particular example.
Author: Axel Maas on 20200204 [id 725]
(in reply to Report 1 on 20200118)Dear referee,
thank you for the kind report, and leaving me the option, whether I want to answer. However, the editor has decided that I have to address the issues, so my own decision is not necessary.
In response to your questions, I have the following answers and made the following changes:
Yes and no. If a gauge and coordinate system is fixed, one can talk also about the metric itself. However, just as with other quantum fields, I expect still that any expectation value <g(x)> can still either only vanish if no event is special (as long as the gauge condition treats all event equally), or as an alternative would be a metric, which is essentially xindependent, i.e. an isotropic and homogeneous one. Though to express it also a coordinate system needs to be fixed. Thus, rather correlation functions <g(x)g(y)> would make more sense.
Without fixing the gauge a similar statement would apply to <ds^2>  as long as it is evaluated at fixed events, and with an indefinite metric, I expect this to average to zero. Rather, histograms over all events would be interesting, giving average distributions of such quantities. Alternatively, integrated curvature would probably be an alternative to characterize the average geometry of spacetime.
I have added these statements in section 3 when discussing expectation values.
If the quantum effective action is gaugefixed and a coordinate system is chosen, the same should apply as before, and one can get a nonvanishing <g_mn>. If it is not gaugefixed, I would expect that only zero would be such a solution. But in the latter case, a diffeomorphisminvariant formulation of the quantum effective action would likely be qualitatively similar to having YangMills theory formulated in Wilson loops, and then as little can be said about the metric as can be said about the gauge fields in such a formulation of YangMills theory. However, I am not sure how to formulate such a quantum effective action of either YangMills theory or gravity at this stage, and can thus not provide any substantial support for my expectation.
At any rate, as before, if the quantum effective action treats all events equal, so should any such expectation value of the metric. Hence, I expect for such solutions the same as said above.
Of course, if for some reason events become special, anything can happen.
The argument would be that J_mu is actually an expression, which involves only the scalar fields and the covariant derivative Delta_mu. On each of these D_mu should act only like partial_mu, when decomposing it into its elements. This would be like the electromagnetic current (see [18]), and comes about because the charge is actually carried by scalars, which do not involve a nontrivial coupling to the metric. I have added a corresponding statement. This could also be seen by explicitly expanding all terms, and executing the covariant derivatives explicitly. In this case, and only this case, because pd_mu J^mu=0 and pd^mu J_mu=0, there is no contradiction.
I have followed the recommendations. I have also added a citation to the mentioned article, which I found very useful.