I have long been skeptical, from a strictly conceptual standpoint, of the notion of relative simultaneity. I am reading a recent (2021) paper which, using the sagnac effect as a basis, claims that relative simultaneity is inconsistent with experiment. Here is the link to that paper: https://www.tandfonline.com/doi/full/10.1080/09500340.2021.1887384
This paper contains a number of references and has been cited twice. In some respects it is quite technical, being chock-full of mathematical formulae and calculation. I will now post a few excerpt from the paper which I found interesting. These excerpts pertain primarily to the methodology and the conclusions of the paper, not the specific basis for the conclusions. First, from the abstract:
We thoroughly examine the role of absolute and relative simultaneity in the interpretation of the Sagnac effect using an approach that allows for determining the local speed along the light path...a rigorous and coherent interpretation of the Sagnac effect favours absolute over relative simultaneity...The implications for the Lorentz transformations and synchronization by means of the Global Positioning System are considered.
From the main body of the paper:
Authors adhering to the conventionalist thesis [2,4,10,12–15] assume that the LT and LTA are physically equivalent. However, the equivalence of Einstein and absolute synchronization implies the equivalence of relative and absolute simultaneity, while the two are conceptually incompatible. ...
[We are] using an approach that assumes the non-equivalence of absolute and relative simultaneity and takes into account the decisive role of clock synchronization.
Moreover, a recent work has shown that relative and absolute simultaneity (and the LT and LTA) can be discriminated experimentally with the implication that the one-way speed of light is measurable in principle and, therefore, the conventionality of the speed of light no longer holds.
For determining the value of the local light speed, in this paper we use a novel approach that emphasizes the essential roles of absolute and relative simultaneity and the related clock synchronization procedures in the interpretation of the Sagnac effect.
Then, taking rigorously into account clock synchronization, our approach reveals the inconsistencies that emerge by requiring the local light speed to be c in both the linear and circular Sagnac effect, here considered in the context of Relativity Theory.
We highlight that the inconsistencies we found with relative simultaneity can be related with the ‘undesirable consequences’ pointed out recently by Lee  for light propagation in (closed) cylindrical spacetime. As already shown by Selleri , by us , and now by Lee, these inconsistencies, or undesirable consequences, disappear when absolute synchronization is adopted.
The many attempts aimed to rebut Sagnac's rational claim are quite understandable from some perspectives, although it is unreasonable to try to justify and preserve the paradigm of the constancy of c at any cost, for example, by claiming that synchronization is conventional. Considering that Sagnac's effect can be coherently interpreted with absolute simultaneity, then it is difficult to support the conventional arguments that Einstein synchronization is also confirmed [12–15,27] when relative simultaneity is met with the discontinuity of the time gap.
...Einstein synchronization fails when applied to a closed contour [6,8,9,16–18,20] and this unphysical prediction of a discontinuity in time on a rotating disc arising from Einstein synchronization has been appropriately described by Klauber  as ‘bizarre’....Einstein synchronization, or more precisely the equivalent assumption of the invariance of c, leads to a result in disagreement with observation.
...since the Sagnac effect can be described coherently by means of absolute simultaneity, but not with relative simultaneity, absolute and Einstein synchronization cannot be physically equivalent, as has been proved independently in the work of ...
...for several decades there has been a recognition, visible in recent literature [6,8,9,12,16–18,23] that conservation of simultaneity and the related transformations offer a simpler and physically meaningful way to interpret optical experiments.
The conclusion is that a coherent interpretation of the Sagnac effect favours absolute over Einstein synchronization, indicating that transformations based on absolute simultaneity (such as the LTA) are likely candidates for describing the whole body of natural phenomena."
As I said, I have mainly just quoted conclusory statements. I have little doubt that advocates of relative simultaneity would argue with these conclusions until the cows come home.
That said, does anyone here have comments on the merits of this paper? Are there mathematical mistakes, interpretative errors, insufficient "evidence," illogic, for other flaws in this paper which would negate its conclusions? I have not read all the papers referred to in this paper, which would presumably allow for a more complete understanding, but maybe one of you has (or will).