The ideal state cannot be modified, and the blockchain can only be added, not modified or deleted. Although Ethereum can modify and delete account data, it is only at the level of state machine, and the state is transferred through transactions or events, and the previous state will not be modified and deleted.

If it is a power algorithm: for example, bitcoin. Users often keep pace with the best new chain stores. And each transaction will be delayed by 6 blocks. So whenever we make a fake square, we usually need to make all the POWs of the first six squares at the same time, and the cumulative POWs of these six squares need to be higher than the real main chain (the main chain is maintained by many honest users) to ensure success every time, so we need to master the computing power of 5 1.