What is proof of work in Bitcoin?
Correct answer
Answer A: Proof of computational work done to find a block.
Quick explanation
Proof of work requires miners to use electrical energy for a huge number of computational attempts and compete to find the next valid block.
Detailed explanation
In proof of work, miners repeatedly calculate a block header's hash. Because hash values cannot usefully be predicted, they must try many variations. A block satisfies the proof only when its header hash is below the permitted target. The lower the target, the more attempts are needed on average.
Generating this proof is costly, while checking it is very fast: a full node calculates the hash and checks the target and block rules. Since each block refers to its predecessor's hash, every block adds work to a valid chain. Nodes compare competing valid histories by greatest cumulative work, not simply block count or the number of miners.
Proof of work is not a vote on arbitrary rules and does not prove that all block contents are valid. Independent node verification therefore remains necessary. The mechanism orders valid block proposals and makes later changes increasingly costly.
Example or everyday application
An attacker changes an older transaction. This changes the block hash and all subsequent references. They would have to redo the work from that block onward and catch up with the public chain.
Common misconception
Proof of work is often understood as a miners' vote or complete validity check. It is the work proof used for chain selection, not a full check of every consensus rule.
Sources used
Bitcoin: A Peer-to-Peer Electronic Cash System
Check original sourceBitcoin Developer Guide - Block Chain
This source is mainly intended for developers. The relevant information may therefore be harder to find.
Check original source