some days ago the following scenario came to my mind. Let's imagine, bitcoin is run by two miners A and B. At some point, they disagree with each other and build two forks that grow independently of each other but both forks strictly stick to the rules. Therefore, both forks are valid bitcoin block chains. Now, let's say one year later, A and B decide to work together again. They would like to merge their two forks.
You cannot merge any forks. Once the forks has blocks that has different rules or blocks that are different, the entire blockchain after that point in time will be different completely.
A has the longer fork. In my understanding, the rules now state that the fork of A dominates that one of B and becomes the only valid chain. Is that correct?
Both are valid. If both chains do not contain blocks or transactions that violates the protocol rules, they won't be invalid. The nodes would see two different chains with differing proof of work. The network will see the different chain and only accept the one with the longest proof of work, discarding the others.
In diffrerent words: What happens, if at some point a node of the mining system presents to the world a completly valid block chain that is longer than the one the world is working on? Does theoretically the world accept that new chain or not?
Yes. If its longer difficulty-wise, nodes will take that chain and build on it.
