For a sentence such as "physics’ greatest...", would you use physics’s or physics’ ?
Microsoft Word highlights physics's as incorrect; however, I have seen it be used.
My understanding is that both may be used, but that in the case of physics, the addition of 's would render the word ungainly in pronunciation, and so physics' might be preferred.
f pronunciation would be awkward with the added -‘s, some writers use only the apostrophe. Either use is acceptable.
Diana Hacker, A Writer’s Reference
With some singular nouns that end in -s, pronouncing the possessive ending as a separate syllable can sound awkward; in such cases, it is acceptable to use just an apostrophe.
Kirszner & Mandell, The Brief Holt Handbook
Since writers vary in the use of the apostrophe, it is not possible to make a hard and fast rule about the apostrophe in singular words ending in s.… Punctuate according to pronunciation.
John E. Warriner, English Grammar and Composition