Immutability

Immutability refers to Bitcoin’s fundamental resistance to change, both in its rules and its blockchain history.

On one level, the protocol rules—especially Bitcoin’s monetary policy—are designed to be extremely difficult to alter. These rules are enforced not by one authority, but by thousands of independent Bitcoin nodes distributed worldwide. Each node runs the same software and enforces the same rules. No miner, government, or any single group can change Bitcoin’s core rules unless they gain agreement from the vast majority of node operators. This decentralized consensus underpins trust in Bitcoin’s predictable supply and continued operation.

On another level, immutability applies to Bitcoin’s blockchain. The blockchain serves as a permanent, append-only record: once a block is added, it’s nearly impossible to modify or remove it. This reliability comes from cryptographic hashing—specifically the SHA-256 hash function. Every block contains the hash of the previous block. If anyone tries to change an earlier block, its hash changes, breaking the chain and invalidating every block that comes after. This ensures that transaction history remains tamper-resistant, giving merchants and users confidence that confirmed payments can’t be undone.

However, Bitcoin’s immutability isn’t absolute. If an attacker were to gain control of most of the network’s computing power (a scenario known as a 51% attack), it would become possible—though extremely costly and difficult—to rewrite parts of the blockchain. Maintaining a decentralized, robust hash rate is critical to keeping such attacks impractical and preserving Bitcoin’s integrity.