After i read this:
Honest generators only build onto a block (by referencing it in blocks they create) if it is the latest block in the longest valid chain. "Length" is calculated as total combined difficulty of that chain, not number of blocks, though this distinction is only important in the context of a few potential attacks. A chain is valid if all of the blocks and transactions within it are valid, and only if it starts with the genesis block. https://en.bitcoin.it/wiki/Block_chain
Would it be possible to mine a block after the genisis block, which has a higher difficulty then all blocks in the mainchain together and override the blockchain with my own one? --- just hypothetical, i know that such a difficulty is ismost impossible to reach... but with some luck?