Somewhere there exists a completely homogeneous speaking culture. A giant spaceship comes about and teleports 1,000,000 people at random onto the craft. This ship is a complete isolated ecosystem capable of supporting these people.
How much time would have to elapse before the plant bound people’s language became impossible to understand?
Is there any way to estimate?
Depends on what you mean by “impossible to understand,” but a ballpark range would be 500-1000 years. We can generally understand dialects separated by 500 years or less (although some may have developed strong differences within only a few hundred years); we are generally unable to understand languages/dialects separated from English by 1000 years or more (those separated by 700-800 years are pretty unintelligible). The large size of the population involved, and its lack of outside influences, could slow divergence down somewhat.
Of course, details may vary based on a variety of factors. This is just a very rough estimate.
I think the general rule of thumb is about 1,000 years, but that can vary widely, depending on the circumstances. Icelandic hasn’t changed much at all in the last 1,000 years, whereas English from that time is almost unrecognizable to modern speakers. If the spaceship had living conditions significantly differenet than those of earth, then it might happen faster.
Bear in mind that the situation described in the OP isn’t quite the same as asking how long it takes before, say, English speakers become incapable of understanding Old English. Modern English is constantly changing; Old English is static, since no-one speaks it any more. In other words, in this scenario only *one *of the two languages is changing.
Here we have two populations (the people on the spaceship and the people left behind) whose languages will *both *change over time but in different and unrelated ways. So they should become mutually unintelligible that much quicker; presumably twice as quick as the case of one language changing over time. So 500 years rather than 1000.
That’s seems like an unwarranted presumption to me. The speed obviously varies substantially depending on several variables. Iceland, with a small, fairly isolated (for most of the time period) population has a language that has changed little. England, not particularly small, and definitelly not isolated, has a language that’s changed quite a bit.
Now this abducted group are much more numerous than the Icelanders, but both they and the people not abducted… who we don’t know the number of, for some reason have developed a homogenous speaking culture. If the reason for their linguistic history so far doesn’t drastically change it could be more than a thousand years, rather than fewer.
It depends partly on what the abductees take with them. Do they have a library, including literary works going back to Chaucer and Shakespeare? Do they have a collection of DVDs, going back to the start of the talkies around 1930? If they do, and they read and watch those things, it will be a conservative influence on their language. (They will also be a conservative influience on present-day English: people will probably still be watching 20th-century movies 1000 years from now, from endless delivery on whatever system has replaced cable TV and the Internet).