51% Attack

A 51% attack is an attempt to control a majority of a proof-of-work network's mining power. An attacker with more than half the hashrate can reorganize recent blocks, reverse their own transactions to double spend, and censor payments, but cannot steal other users' coins, forge signatures, or change bitcoin's 21 million supply cap.

Why it matters

The threat defines the economic security model of every proof-of-work chain. Bitcoin's defense is scale: its network hashrate is measured in hundreds of exahashes per second, produced by millions of specialized ASIC machines consuming gigawatts of power. Acquiring and running a majority of that capacity would cost billions of dollars, and the attack would likely destroy the value of the very coins and hardware the attacker holds.

Smaller networks enjoy no such protection. Ethereum Classic suffered multiple 51% attacks in 2019 and 2020, with attackers renting hashpower to reorganize thousands of blocks. Security, in other words, is proportional to the honest work behind a chain, not to the elegance of its code.

In the gold vs bitcoin debate

Gold advocates point to the 51% attack as a risk category gold simply does not have: no one can reorganize a vault. Bitcoin advocates respond that gold's equivalent attack surface is custodial and political, from confiscation orders to rehypothecated paper claims, while bitcoin's worst-case attacker still cannot counterfeit supply. Each asset concentrates its risk in a different place, physics and custody for gold, economics and energy for bitcoin.