I would like to make this a semi-challenge. As I would like to investigate how the different alternate timelines shaped up at the point we visited them and what regular and Earth-shattering events happened in them. I would like to focus on TNG Yesterday's Enterprise, TNG All Good Things..., DS9 The Visitor, VOY Non Sequitur, ENT Shockwave, ENT Twilight. There is no need to address TAS Yesteryear, after all it already got the story treatment...
ENT Shockwave Lets start chronologically with ENT Shockwave. It is a strange timeline. As it contains several premises: 1. no advanced computer technology -> Humans disappear in the 22nd/23rd century 2. knowledge of the Romulan Star Empire 3. buildings are mostly the same -> millennium old buildings? ----------- There is the problem with the Xindi. By this point there are already set on sending a probe. It could be explained that the Sphere Builders persuade the Xindi to use the bioweapon. It would take perhaps several decades and by that time Earth has the opportunity to learn of the Romulans. As it is we have a similar development of the Vulacan Protectorate seen in Forgotten History. V'Ger shows up does not find anything on Earth, so it continues its search for the creator... Delphic Expanse covers the entire galaxy. In thousands of years the shockwave created by the Body Electric destroys all life. As to what happens to the Borg, Caeliar, Q,... your guess is as good as mine.
Re: ENT Shockwave Likely not millennium old. There was a very interesting show (history or discovery channel) called Life After People, once we stop maintaining them, the majority our buildings would disappear suprisingly quickly. A couple of centuries.
Re: ENT Shockwave That was an awesome show! I caught it on History channel, but they probably have shown it on discovery too. That said, we could argue that the buildings of the 22nd Century last longer than modern ones.
Here is one for you. Did the mirror universe also get reset? Is it the nearest universe where it also saw Narada go back into time? Is it a universe somehow corrupted by a mad Q where it isn't just another timeline--but a true alternate universe modified by an evil entiry to where everyone seems to be in a bad mood--and not just because the way history turned out with conquests. We had a history like that but people are generally nice. Now I'm starting to sound like Rom, in wondering what exactly is alternate.
^Yup. Have seen it too. But we have to work wit what we are given. Daniels says that the difference was only the missing Federation memorial and that the library was not digitalized. The place he was working at was still there. The early 24th century would be the last point when digitalization were to take place. Although yes, this particular library could have been done much much later. But one should expect that future buildings ought to last longer, perhaps even repair old buildings to last longer.
Well first from our limited mortal view we can say that all parallel universes are alternate too. And the one or two mirror universes we know of are among them. I would say that the mirror universe is only one of the neighboring universes. I don't know where the inversion of the good people are bad and bad people are good came from. From on screen evidence it only shows that the humans are a bit more evil, and once they are broken for some decades they develop goodness. As for the source of the mirror universe, no idea, my take is that the genetic defect of the Augments as observed by Arik Soong actually naturally developed in mirror humans in "prahistory". As to the nuMirror universe, then, yes, sooner or later it will change. Now whether that will be when nuDefiant disappears or in the 22nd century, we shall never now, for I don't believe it will be ever addressed.
Re: ENT Shockwave The actual structures might decay rather quickly, but the traces would remain on the landscape for millennia. You only have to look out at the countryside to see the traces of Roman roads, mediaeval land use, motte and bailey castles, and other earthworks many centuries old. If we ceased to exist overnight, our cities and roads would scar the surface of the earth for thousands of years.
Seconded. Christopher quite literally wrote the book on Star Trek time travel, and it's packed with hypothesis on how timelines interact and influence each other.