The Copenhagen interpretation was first posed by physicist Niels Bohr in 1920. It says that a quantum particle doesn't exist in one state or another, but in all of its possible states at once.
https://science.howstuffworks.com/innovation/science-questions/quantum-suicide4.htm
Okay Mister.
I'm ready for your - longest yet - post on the temp:
Particles <=> History
Feel free to use spoilers wherever neccessary...
As it was getting into serious asides I just wanted to give whatever people that cared a choice term to search for if they wanted. Many worlds vs copenhagen being the usual entry point to the debate if you want to do parallel universes (which would arguably encompass time travel owing to the whole changing history thing, neither preclude it either).
You said in as many words that whatever happens then everything remains the same. That is another view of things, rather out of fashion owing to said quantum mechanics (if you are sticking with that then it would be Newtonian physics -- that being with enough computing power and knowledge of the initial state you could predict the whole course of events of every particle created/interacting for the whole of time. Quantum stuff being random does appear rather to step all over that though, even more so as we appear to be able to make decisions based upon that* and butterfly effects, even more so over longer time scales).
It also ends up in a debate about free will as well.
However that gets all a bit navel gazing for the tastes of most, and even if it was the case that we are all automatons we appear to be able to hypothesise about free will in some form of illusion anyway. To that end we can handwave it all away and get onto the business of causation.
Economics, biological drives, geography, psychology (in as much as it is distinct from biology), philosophy, said quantum physics (we only have what was around when some earlier stars went supernova, and did not get blown off by solar wind)... all good stuff to contemplate in the study of history beyond "what, when and where something happened" that most leave school thinking it is (and frankly I would have been happy to even have that -- my main history teacher was atrocious, and even if it did not kill my desire to learn history, I did have some good teachers and sources otherwise, it put it into a coma for about a decade).
Similarly there is the phrase "those who do not know history are doomed to repeat it", which I am not entirely on board with but there is some truth to it, one that even sits nicely in your more physics centric approach of this last post (same start conditions the experiment should repeat). To that end "history rhymes" is perhaps a more agreeable one for me. Much like music you can also scale up and down quite happily to look at local policy and how it might do stuff for you, or (inter)national and how you hedge your bets to enjoy an easier life (or go the other way), this goes even if you figure you are a leaf in a stream if we are going to start mixing up history related metaphors.
While you may figure you are but a leaf in the stream, one of many before and after that will be letters in an old book before the century is out, there is the idea that not everybody is. Some things in history appear to be discovered time and time again; after wheat a like is made someone will leave it around for a day or so and thus you get bread, leave that a bit more and you have booze (there is even some argument, with not inconsiderable archaeological evidence, that booze came before bread).
At the same time human genetics is incredibly variable with random aberrations popping up all the time. It might well be that the greatest mathematician (pre cybernetics and genetic enhancement anyway) was born centuries ago and just pondered things while sowing fields for some leader, or indeed the greatest general for ancient times was born thousands of years late. Sometimes though coincidence happens, brilliance happens to be in the right place (or wrong place, at least for your value system, to take it back to the thread's main premise). Inherent with the question of the thread is also the idea that you might be able to drop a modern encyclopaedia through the time portal (suitable translated of course), maybe just a note, and change things, or you might be able to go back with a small strike team and fire bullets various ways to change things (or maybe less violently if it was a random rock that led to the discovery of something then move that).
*if doing videos