⛓️ The Blockchain
What is the blockchain?
The blockchain is a public ledger that records every bitcoin transaction ever made. Anyone can read it, no one can secretly rewrite it, and thousands of computers around the world hold identical copies. It's the answer to the question that stumped digital-money designers for decades: how do you stop someone spending the same digital coin twice, without appointing a bank to keep the books?
How it works
- Transactions are grouped into blocks. Roughly every ten minutes, pending transactions are bundled together by miners.
- Each block is cryptographically linked to the previous one. Every block contains a hash — a digital fingerprint — of the block before it, forming a chain back to the very first block from January 2009.
- Every node validates and stores the chain. Thousands of independent computers on the peer-to-peer network check every rule on every transaction and reject anything invalid.
Why it can't be quietly changed
Because each block's fingerprint depends on the block before it, altering an old transaction would change that block's hash, which would break every block after it — a change instantly obvious to every node on the network. To rewrite history you'd have to redo all the mining work since that point, faster than the entire rest of the network combined. That's what people mean when they call the blockchain immutable: not that change is forbidden, but that it's computationally absurd.
Common misconception
"Blockchain" isn't a magic ingredient — it's a deliberately expensive way to keep a database honest without an owner. It makes sense when no single party can be trusted with the ledger (money being the prime example), and it's usually overkill when one can.
Related: Peer-to-Peer Networks · Cryptography · Mining