The Next Generation Event Horizon Telescope Collaboration: History, Philosophy, and Culture

Round 1

Reviewer 1 Report

This is a very interesting paper, and I would recommend to any member of a large collaboration to read this. I have learned a lot from this paper. My thanks to the authors and I look forward to reading a number of their planned studies in the future.

The document has more than one author, and a reader clearly notices different kind of language and style used in the various sections (though the language use itself of every section is fine and one can read the paper well). Though I am not a native English speaker, Section 3 stands out the most in language style. I would recommend to make the language style more uniform across all sections. I'd also recommend to look through all the sections, harmonize the use of "" (single ' versus double ") and remove those where the word can be used with commonly accepted meaning (bottom-up can be used regularly for example, no need for ""). I highlighted where I thought those removals of "" were called for.

In Section 4, which discusses the governance structure, various collaboration structures are investigated and a draft structure for the ngEHT is presented. In particular in 4.2 and 4.4 I wondered how the ngEHT governance relates to the EHT governance as this was strangely enough not mentioned. I do understand (and appreciate) that the authors wish to have a much broader comparison of collaborations than only the EHT one, but not to mention it at all is a bit strange. I think a comment on how the ngEHT structure relates to the EHT governance structure would be important in this Section (specifically in 4.2 and 4.4).

Other comments by line number:

Section 1

line 67: The EHT had 8 observatories on 6 sites in 2017, but in 2018 the array was already expanded to 11 observatories on 9 sites. I suggest to rewrite the sentence to "The EHT observatories, eight on six sites in 2017 and expanded to eleven observatories since," or something similar to indicate that the EHT is a growing array.

line 70: Data is plural to my understanding, so "These aligned data".

line 72: Cite EHTC et al. 2019a for the first image of M87*

line 74: Cite EHTC et al. 2022a for the first image of Sgr A*

line 78: Remove "first ever". while the EHT has not yet produced any movies this is not yet out of the scope of the EHTC's goals. No doubt that the ngEHT movies will be of higher quality, but "first ever" is not a correct adjective at this moment.

line 121: I suggest to use ngEHT rather than "next generation EHT" because the acronym has already been introduced and it causes less confusion with EHT.


Section 2

line 206: Instead of a footnote, the authors should cite the full list of the EHTC papers on M87* and SgrA*.

line 258: Give a citation (or citations) for the synthetic image library. I would suggest EHTC et al. 2019e, 2021b and 2022e.

line 272: A citation to the M87* Imaging paper by the EHTC 2019d would be appropriate here.

line 275: Perhaps it is interesting to add (for readers outside of physics) that blue has shorter wavelengths and therefore actually "hotter" than orange?

line 281: Add * for M87 (M87*)

line 301: Maybe consider to  add "19th century" between "sublime" and "views" because I was first confused whether American West was the name of a Nebula (like the North American Nebula) rather than a geographical location at a certain time in history as idealised by artists.


Section 3

line 347: It would be useful to add a reference to this sentence.

line 366: It would be useful to add a reference that no-hair theorems apply to stationary black holes only.

line 369: Replace "various mathematical assumptions that are high unrealistic" with "analyticity which is not realistic" which is more clear in my opinion. In "various mathematical assumptions" where one has no feeling for the size of the problematics. Also, add a reference for this claim.

line 377: replace "many" by "some", reading this sentence as it is makes the reader wonder why one would ever use the Kerr metric at all, which I think is not what is intended here.

line 380: I would invite the authors to add a few lines on how the EHT measurements of Sgr A* (EHTC et al. 2022f) tested the Kerr metric also to search for possible deviations. These were not found, but the important point is that the data were used to test these metrics (and also alternative metrics).

line 385: What is ADM?

line 392: Lambda > 0 universe? case? Please add the word meant.

line 392: "such quantities"; what quantities are meant here? The mass, charge and angular momentum?

line 393: I get confused what quantities are meant by the "above" ones and by the "new" ones. Could the sentence be rewritten so that the meaning is clearer?

line 394: what is meant with "the different quantities"?

line 394: What is meant by "former"?

line 401/409: This part is written in a rather complicated fashion and I have difficulty of following the argumentation. Maybe you can describe the problematics more generically, that the BH models as used in astromony to interpret data are inconsistent with a Lambda > 0 universe as a large scale structure and how this (non-zero) Lambda term affects the BH parameters? Or perhaps you can give more details/explanation?

line 406: I would remove the "" of far away and too far away, and instead define these more clearly.

line 411: I would change Problems to Situations and remove the "", because one would have to investigate how problematic these really are before naming them as such.

line 413: Why is strictly speaking (third time that this appears in Section 3) in italics?

line 425: Replace problems by issues (problems that are found to problematic sounds a bit weird)

line 432: Replace problems by issues and remove ""

line 433: Remove ""?

line 434: Why are these words in italics?

line 440: Remove ""?

line 442: helps to explain or helps explaining

line 445/447: This sentence is a bit vague to me, processes to build what? Models to do what? Physical processes? Physical models or the work approach?

line 448: The word "we" is probably a typo. Could beginning of the sentence be rewritten to make the meaning more clear?

line 45a1: Remove ""?


Section 4

line 490: Remove ""?

line 500/501/508/523/525/529/532/536: Remove "", all these words can be used without the need for "" (or I'm missing the additional value of the "")

line 572: replace "its" by the collaboration's

line 580: remove  ""?

line 614: remove ""?

line 737/754: About the Ethics Committee, I wondered if the Ombudspanel would be part of the Ethics committee or if the Ombudspanel would be a separate entity (and thus with its own box in Figure 1). If I understand the text well, the Ethics committee would form (and maintain/adapt) the procedures and other committees would carry these out (as in Publication committee). As such the Ombudspanel would be an separate entity just as the Publication committee?

line 757/758: I don't completely understand this phrase, the ngEHT would need to have additional sites with reference to what? The EHT? And when is thereafter? Why not directly 10? Perhaps some references can be added here?


Section 5

line 793: I think that "first ever" term is not correct here, at the moment the movie making with EHT is not yet excluded.

Author Response

Thank you for your comments. We attach a document detailing your advised revisions and our responses.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is a white paper rather than a classical research paper.  Therefore I recommend to change the paper type to 'Perspective', see https://www.mdpi.com/about/article_types.

The paper is well written, pleasant to read, and has an interesting reference list.  I wish the authors good luck with the ngEHT endeavour.

Author Response

Thank you very much for your comments!

We are open to changing it to a "Perspective" type but cannot see how to make the change ourselves in the online submission system. We have added a note to the editor to this effect.

Round 2

Reviewer 1 Report

Dear Authors,

Thank you for considering my comments with care and for your detailed replies. I accept the revised version of the paper.

I found only one minor comment: in footnote 4 (line 79) you reference EHTC+ 2022 paper IV, but presumably you wished to refer to EHTC+ 2022 paper III about the imaging.

I look forward to your future publications on these interesting matters.

Author Response

Thank you for catching this. The manuscript has now been corrected to cite paper III rather than paper IV.