Of the Germanic consonant shift

[KathTheDragon]

I'm quite partial to *d as implosives, following the Cao Bang theory.

I'm honestly sceptical about voicing [tʰ] leading to "[dʰ]", given that the latter are not phonetically aspirated, they're breathy-voiced. At best, I'd expect straight voicing to lead to [d͡tʰ] (initially voiced, devoicing partway through closure, then a delayed onset of voicing during release), but more likely just [d].

[cedh]

*rīks < PCelt *rīgs is admittedly a challenge to fit into this picture...

It has also been suggested that Gmc. *rīkja in its territorial sense might instead be formed from a native root *rik-, *raik- 'stretch, expand' (cf. English reach) < PIE *rēǵ-, with the political sense a secondary development. If this is true, there may have been some cross-contamination with Celtic *rīgs, the latter influencing the semantics of the former, and the former influencing the phonetics of the loanword *rīks (which may thus have been borrowed at a later stage). Doesn't sound all too implausible to me...

[KathTheDragon]

That's a very interesting suggestion.

[hwhatting]

[url=https://www.dwds.de/wb/etymwb/Reich] a native root *rik-, *raik- 'stretch, expand' (cf. English reach) < PIE *rēǵ-

You're misquoting somewhat - the root Polenz assumes for Germanic is different root from PIE *reg-, namely *rēiĝ-. Otherwise you wouldn't get the i-diphthong in WGmc. *raikjan or the long /i:/ in ri:k-. *e: > i: is only a Celtic, not a Germanic development. LIV has that root as *reiĝ- (p. 503).

[Otto Kretschmer]

This past week I've read in two different books that the changes known as Grimm's Law were the most wide-ranging ever known to occur in any human language. To what extent is this known to be true?

What exactly is wide ranging about it/ O.o

It's literally just three consonants turing into other consonants.

Just the list of phonological changes from Old to Middle English is several times more extensive than this.

[cedh]

[url=https://www.dwds.de/wb/etymwb/Reich] a native root *rik-, *raik- 'stretch, expand' (cf. English reach) < PIE *rēǵ-

You're misquoting somewhat - the root Polenz assumes for Germanic is different root from PIE *reg-, namely *rēiĝ-. Otherwise you wouldn't get the i-diphthong in WGmc. *raikjan or the long /i:/ in ri:k-. *e: > i: is only a Celtic, not a Germanic development. LIV has that root as *reiĝ- (p. 503).

OK, thank you for this correction. IIUC, the meaning 'stretch, expand' seems to be associated with two very similar roots, PIE *rēĝ- and *rēiĝ-, both of which are mentioned in that DWDS article, and both of which (together?) form the basis for the alternative explanation why Gmc. *rīkja might not be a straightforward Celtic borrowing, which I wanted to mention. And it seems I overlooked the *i in the second root...

[hwhatting]

IIUC, the meaning 'stretch, expand' seems to be associated with two very similar roots, PIE *rēĝ- and *rēiĝ-, both of which are mentioned in that DWDS article, and both of which (together?) form the basis for the alternative explanation why Gmc. *rīkja might not be a straightforward Celtic borrowing,

Still not quite right - *reĝ- is the root behind the Celtic word (the long /e/ in Celtic rīks is due to lengthened grade in the Nom.Sg.), while *reiĝ- is the basis for the alternative explanation.