My favourite paper

It is a privilege to contribute to the inaugural issue of the Phi Delta Alpha newsletter. Let me use this opportunity to write about my favourite physics paper, “Firewalls in the CMB Revisited” by R. P. Weinkind, Phys. Rev. D 88, 271828 (2013). As most PDA readers are not specialists on the subject, I will assess chiefly the presentational aspects.

Let us begin with the title. It is crisp, yet informative. Plus intriguing: how could firewalls, an idea linked to black holes, appear in the cosmic microwave background? A good problem has been picked, one that is likely to impact the progress of the field – the paper was not written for the sake of writing one. The “revisited” assures us that the author has something genuinely interesting to add. And that their input to the theme isn't being spun as a whole new paradigm, sadly an all-too-often occurrence in today’s competitive times.

The section headings are descriptive enough to already give a feel for the narrative. It primes the reader to dive in. When we do, a five-star introduction meets us. It doubles as a concise, clear review for an early-stage graduate student, and packs countless references. Notice that all the relevant papers by T. Menderkar have been cited. Weinkind and Menderkar have long been feuding on adjacent topics, at times openly at conferences. But you couldn’t guess this from their generosity of credit where it is due. 

The plots are superlative. Enough of a tale is told through them that an expert can skip the rest of the paper. Of help here are the thorough captions detailing the variables plotted and the physics point illustrated. At least we are grateful for not being hit with “Plot of ξ vs k for various values of η. See text for more details.” The dynamical ranges of the axes are so well-chosen as to focus solely on interesting regions. Where the range spans orders of magnitude, the plot is on a log scale, so relevant features are unobscured. The axis and plot labels are the size of the main text’s font. The colours are few, with differences in curves conveyed more by their dash style: solid, dashed, dotted, etc. All of which is to say, the author is clearly aware of chart junk. 

The paper has many other things going for it. For one, it has multiple entry points. It is aware that not all eyeballs start at the introduction – some do at the results section, others at the conclusions, and yet others at the plots. Each can catch the narrative thread right away thanks to enough repetition and hyperlinking. As for run-on sentences, there are none – zero. On average there appear to be 25 words per sentence, set down in an English clear, simple, and correct. The appendix is two-thirds of the paper. That is because the author did not shove the hard-mastered technical details into the main body, which they had reserved for the central message. Both footnotes are mildly amusing. Weinkind appears to be aware of the irony of footnotes: designed to be an aside, yet luring attention via their unusual placement. The acknowledgements are fairly detailed – it’s always nice to see someone thanked for reading a draft; the published version hat-tips the anonymous referee. And there is a tongue-in-cheek reference to Schwarzschild’s original paper when the metric is specified.

My second favourite thing about the paper is that it has exactly one Big Thought. Let me explain. To begin with, this is not a null-idea affair, or in David Mermin’s phrasing, a “superluminal paper” that conveys no information. But again, see the second paragraph in the Discussion section? All those “We leave these to future work” inspirations? The author surely had them near the end of the project as their expertise grew, but did not cram them into the premise of the present paper. They have adhered to Howard Georgi's doctrine of “A paper must convey at most one idea. One is OK.” 

What then is my favourite thing about "Firewalls in the CMB revisited"? It is that it doesn't exist. Much like Prof. Weinkind, its fictional author, and his rival Prof. Menderkar. But it combines, in one way or another, qualities of the most pleasurable papers I have been amongst.

December 2024
Bengaluru