Proof-of-Work

Proof-of-work is Bitcoin's mechanism for deciding which version of history is real. A block is only valid if the SHA-256 hash of its header, read as a 256-bit number, is below a target value the network sets and adjusts. Miners search for a nonce that produces such a hash — there's no shortcut, only guessing.

The asymmetry is the point: finding a valid hash takes the global network roughly ten minutes of work, but anyone can verify it in microseconds. Energy goes in; certainty comes out. To rewrite history, an attacker must redo all that work *faster* than the rest of the network is doing new work — a race they lose unless they control more than half the hashrate.

The phrase "proof of work" predates Bitcoin (Hashcash, 1997, by Adam Back). Satoshi's contribution was wiring it into a chain of blocks with a difficulty adjustment, so the cost stays calibrated as hardware improves. The system has no central scheduler. It coordinates through cost.