Bitcoin maintains a high level of immutability on two levels. Firstly, Bitcoin’s consensus protocol is resistant to change, and the core aspects of the protocol, most importantly Bitcoin’s strict monetary policy, can never be changed. This immutability is enforced by the tens of thousands of Bitcoin nodes who independently run the same code and agree to the same ruleset. No single party, neither miners, nor governments, is able to change Bitcoin’s consensus rules without convincing these tens of thousands of nodes to agree to their proposed changes. This immutability gives institutional and retail investors faith in Bitcoin’s scarcity and longevity.
Secondly, Bitcoin’s blockchain is immutable in the sense that its history cannot be easily rewritten. This allows merchants to trust Bitcoin payments more easily than fiat payments. Bitcoin’s blockchain is append-only, meaning that once a block is embedded in the chain, it is practically infeasible to remove or alter it. This makes Bitcoin’s history immutable. This property is enforced by the SHA-256 hash function. When a miner hashes a block hoping to find a valid hash, the hash of the previous block is included in that block. Thanks to the properties of a hash function, if the hash of the previous block changes, this will change the current block’s hash, invalidating the Proof-of-Work and thus the entire block.
For example, if the blockchain has 500 blocks, Block #400’s hash will include Block #399’s hash. If a single piece of Block #399 is altered, Block #399’s hash will change, causing Block #400’s hash to change and so on, all the way until Block #500. Every block after #399 will be invalidated. This trait prevents anyone from altering a block once it is part of the blockchain without completely rebuilding the blockchain.
Bitcoin’s immutability is neither absolute nor unassailable. If an attacker were to control a majority of all computing power on the bitcoin network, they could alter past blocks in what is called a 51% attack. It is imperative that Bitcoin maintain a significant and decentralized hash rate in order to keep the cost of such an attack beyond the means of any entity.