Gerry here: I'm not sure who is writing below, but I am familiar with
glottochronology that I believe John Croft presented on the list a week
or so ago. Supposedly it measures the "divergence" between languages
but John then changed the word to "convergence". It's very
controversial and IMO, I'm not sure that it stands as a verifiable means
by which to assess the age of a language. Actually someone on the list
did comment that Chaucer's grandchild 20 generations removed would NOT
be able to speak to great granddad even though both both were born in
the same location.

Gerry

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Our discussion so far, however, has been nothing but an exchange
of opinions. It would be interesting to have some more precise and
verifiable data based on maths/stats. Do you, Piotr, or does anyone
of phoNet-icians know about such studies of the phonetic distance
between languages, or between various stages of one particular
language? Comparative studies could be also of interest. What would
you say for example about the phonetic distance between Modern Polish
and, say, 15th cent. Polish vs. the same (or perhaps another period)
for English? Or Byzantine and Modern Greek? How could it be measured?
And what about measuring the distance between spelling and
pronunciation?