Part 5/17:
Once found, other nodes validate quickly.
If a miner attempts to cheat (double-spending, invalid transactions), their work is invalid, and they lose their upfront investment or electricity costs.
The longest chain (most accumulated work) becomes the accepted history.
This makes attacking the system prohibitively expensive, especially in Bitcoin, which has hundreds of thousands of miners with specialized hardware (ASICs), making 51% attacks costly and risky.
In contrast, Ethereum's PoW used Ethash, designed to be memory-hard so that ordinary GPUs could mine, democratizing participation initially. But over time, this facilitated the rise of GPU farms, raising centralization concerns and hardware scarcity.