Consensus
Consensus in Bitcoin is the mechanism by which all network participants agree on the current state of the blockchain without a central authority. It is achieved through a combination of proof-of-work mining and independent rule enforcement by full nodes.
How It Works
Bitcoin consensus operates on two levels. First, every full node independently validates all transactions and blocks against a shared set of rules. These rules cover everything from signature verification to supply limits to block size constraints. If a block violates any rule, it is rejected — no exceptions, no appeals, regardless of how much hash power is behind it.
Second, proof-of-work determines which valid chain of blocks is the "correct" one when multiple valid chains exist. Nodes follow the chain with the most cumulative proof-of-work (commonly called the "longest chain," though "most work" is more accurate). This elegantly solves the problem of ordering transactions in a distributed system without a central timestamp server.
Changing consensus rules is intentionally difficult. It requires overwhelming agreement among node operators, miners, developers, and users. This difficulty is a feature — it protects minority participants from having rules imposed on them. Proposed changes go through extensive review via the BIP process, and implementations are deployed as soft forks when possible to maintain backward compatibility.
Key Points
- Every full node independently enforces identical consensus rules
- Proof-of-work determines which valid chain has the most cumulative work
- Invalid blocks are rejected regardless of the hash power behind them
- Changing consensus rules requires near-universal agreement across the ecosystem
- The difficulty of changing rules is a feature that protects all participants