If writing in a Descriptive Book simply links to an age, then how does editing that book change the linked age?

Question

In both the books and games of the Myst series, it is established that Descriptive Books do not create worlds, but simply link to already created worlds.

If this is true, then wouldn't editing a Descriptive Book change the link to a different world, not alter the preexisting age? If it is indeed linking to a different age, then how can that be reconciled with a Descriptive Book being altered while main characters are in the age and meeting up with them later with them having visually watched the changes happen? If one changes a descriptive book and then goes into the age, their linking book is still in the last place they hid it as well.

Example: In Myst: The Book of Atrus...

Gehn edited a descriptive book in an effort to show his power. He made the oceans warm in an attempt to dispel the thick fog and eddies that surrounded the island. He succeeded, but at the cost of severe instability. The oceans and lakes eventually drained, and terrible weather descended on the island. When Gehn and Atrus returned to the island they were approached by villagers that fully remembered who they were and pleaded for their help. The villagers were fully aware that the mist had once been there, had vanished, and then the oceans had drained. This all happened in real time alongside Gehn's edits.

Gehn believes that they create worlds, yet Atrus (who we are to believe is smarter and more logical) thinks the worlds already exist and that they are simply linking to them. This fact is repeated throughout the games and books without Atrus ever being convinced that Gehn may have been right. However, the only way I can seem to make the editing of a descriptive book work is to follow Gehn's theory.

Using Atrus' theory: If writing in a Descriptive Book simply links to an age, then how does editing that book change the linked age?

Answer 1

Using the example in either the book of Atrus (or perhaps Ti'ana, I can't remember at the moment), the Descriptive book makes the link to an Age in the Great Tree of Possibility (or was it time?).

Once that link is made, you are on one branch of the Great tree, and subsequent alterations take you to branches on that branch. Because you're on the same branch of the Great Tree, you're still fundamentally dealing with the scenario that was created, which is why you can watch the changes happen. In the book of Atrus, note what happens when Ghen undoes the changes:

the entire culture that had formed because of his (Ghen's) arrival is reverted to what existed prior to Ghen's arrival. While it might be a bit of a plot hole as to why the reversion went this far, it was Atrus' own conclusion that the link went to a different branch of the same Age, just one that had never had contact with Ghen and Atrus.

Edit -- here's a link to a Journal from RealMYST, which describes Atrus' experiments with the crystal viewer to see Riven's state. It has some discussion on the topic at hand of what happens when you make changes to an Age, though it might muddy the waters rather than clear anything up.

Answer 2

Very short version: Because quantum.

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Richard Watson, out-of-universe one of the Cyan developers and in-universe a member of the D'ni Restoration Council, has written about this. Note that the letter is written from an in-universe perspective:

Many of the interpretations of quantum theory say that until a state of matter is observed, it exists in many states simultaneously - it creates a bizarre "probability wave" that contains all of the possible states of that matter. Therefore, as was proposed in Schrodinger's famous cat analogy, bizarre things happen on the quantum level that allow things like Schrodinger's cat to be both alive and dead at the same time, until one ov the states of observed, locking it in a single state, and collapsing the "probability wave."

What the D'ni seem to have concluded (proved?), is that those waves don't actually cease to exist altogether, instead each possibility continues to exist in an alternate quantum reality (read "parallel universe"), until a state is observed in that quantum reality, and the possibilities not observed in that quantum reality continue to exist in still another, and so on ad infinitum. This makes the universe infinitely complex, with every possible quantum combination since the creation of the universe existing in a quantum reality somehere (even the "unstable Ages"). The Books somehow allow observation of (thus the locking of) and travel to those quantum realities.

So, you can make "unobserved" changes (probabilities that haven't been locked down by description in the Book, or by physical observation in the Age itself) without forcing the Book to link to a new quantum reality.

This is why being careful of contradictions is so important. The problem with contradictions is that the Book attemps to link to a quantum reality that matches a contradictory description, and the closest thing it can find is usually fairly unstable.

[...]

"What about the changes [Catherine made to] to Riven? You still haven't answered that."

The changes made to Riven near the end of the Book of Atrus (pg 268 in the hardcover edition), were a collaboration between Anna and Catherine. Anna's main contribution was probably keeping the Book free of contradictions. Catherine's intuitive (but D'ni rule-breaking) style was so bizarre that earlier Atrus had claimed that her Books wouldn't even work - yet they did.

The daggers which mysteriously appeared around the island, and the lava filled fissures were made possible by her odd style - which I cannot explain. And although Catherine and Anna intended for the lava filled fissures as part of their plan to rescue Atrus while still leaving Gehn trapped in his Fifth Age, the Star Filled Fissure was not intentional or anticipated.

To me, it remains the most mysterious object in all the D'ni histories.

TL;DR: You can edit things which have not been observed and which were not previously mentioned in the Descriptive book, without breaking the link. Also, Catherine and later Yeesha do weird things with their writing and break the rules.

In response to a fan query, Watson had this to say:

I was reading BoD the other night and something finally made sense to me.

When you link, you can link to any place at ANY TIME. In BoA, Atrus linked back to a time before they were ever there.

Sorry to add to the confusion, but linking to an earlier time doesn't explain the situation in the Book of Atrus. Atrus actually linked to a completely separate, albeit remarkably similar, Age.

Gehn's removeal of the phrases in the Age 37 Descriptive Book forced the Book to choose another Age which still fit the description. The Ages were very similar, but distinct.

[italics in original, bold added]

Based on these writings, it appears to me that Gehn's initial change to Age 37 was realized as a large-scale shift in the local climate. Because nobody had the technology or data to accurately predict said climate, this was an unobserved change and therefore did not break the link. But simply negating the change was not an unobserved modification; Gehn was asking for a world in which the climate shift had never occurred. To do this without breaking the link, Gehn would have had to carefully write in a second climate shift to cancel out the first.