With the right technology, could the entire blockchain be rewritten? Imagine that someone had access to an alien super computer such that they could mine any block in nanoseconds. At that point, could that person create a chain with the same BTC genesis block, generate any/all the blocks and transactions they want, to the point where this new chain had a greater cumulative difficulty than the current chain? And if that person did this, would the network accept this chain, thus erasing the past 10+ years of BTC?
I realize the computational power to do this is theoretical, like this question. However, I still wonder what, if any, safe guards are in place to prevent something like this?
In theory yes but the alien super computer would have to have hash power at an incredibly large multiple of the hash power pointed towards the Bitcoin blockchain today. According to blockchain.com today's network hash rate is around 130 million terahashes per second. If the alien super computer had more than this it could likely produce the next block with greater than 50 percent chance. However, to generate a new chain from genesis (over 10 years ago) it would need to create 10 years of blocks rather than just one. There are checkpoints in Bitcoin Core at historical intervals for when you are doing an initial block download but my understanding of these is that they don't overrule a new chain with a heavier accumulated proof of work even if it goes all the way back to genesis.