Bitcoin Proof of Work demonstrates that a block header’s double SHA-256 hash meets a target condition. Difficulty describes expected attempts, not exact electricity use or transaction validity.
Proof of Work is neither an electricity receipt nor proof that a block is valid. A node checks that the numerical header hash does not exceed the target encoded in nBits. Verification is cheap, while finding a suitable hash is random and a lower target requires more attempts on average. [Bitcoin whitepaper] [Bitcoin Core v29.0: Proof of Work]
An ASIC may exhaust the four-byte header nonce without success. Mining software can then change the extra nonce in the coinbase transaction, changing Merkle Root and therefore the header searched next. Mining is not simply increasing one number indefinitely. [Bitcoin Developer Guide: Mining]
nBits compactly encodes the target. A smaller target admits fewer hashes and implies greater difficulty; it is not a share of computing power, a count of zeroes, or a quantity of joules. Independent attempts mean even a high hashrate cannot guarantee who finds the next block. [Bitcoin Core v29.0: Proof of Work] [Bitcoin whitepaper]
Besides work, a node checks block structure, signatures, unspent inputs and coinbase rules. Implementation ordering does not imply all these checks precede hashing. A hash below target cannot repair an invalid signature or double spend; a valid candidate without sufficient work also cannot join the active chain. [Bitcoin whitepaper] [Bitcoin Core v29.0: block validation]
When valid branches compete, nodes compare accumulated work derived from block targets, not just length. Rewriting confirmed history requires alternative valid blocks and catching up with and surpassing the honest branch’s continuing work. This is an economic and probabilistic barrier, not immediate irreversibility. [Bitcoin whitepaper] [Bitcoin Core v29.0: chainwork]
On mainnet, Bitcoin Core changes the required target at difficulty interval boundaries. It compares observed and intended time spans, clamps the observed span to one quarter through four times the intended span, and caps the new target at the mainnet limit. BTC price, node count and miners voting for new rules do not determine this calculation. [Bitcoin Core v29.0: Proof of Work]
A pool may set an easier internal share target to estimate contributions. Most shares do not meet the network target and never enter the blockchain. A network-target hit is different: the pool broadcasts the full candidate block, whose acceptance independent nodes decide. [Bitcoin Developer Guide: Mining]
Your node or public block data lets you compare the header hash, bits, target, height and chainwork. A winning hash alone cannot reveal exact energy use, the physical miner’s identity or location. Security claims should specify payment value, confirmation count and the assumption of predominant honest work. [Bitcoin Core v29.0: Proof of Work] [Bitcoin whitepaper]
For the clearest picture, read this entry together with Mining, Difficulty adjustment, Timechain, Full Node, Block Subsidy. The reverse links also lead from Bitcoin, Satoshi Nakamoto, Bitcoin Whitepaper, Timechain.
01Why does Bitcoin need proof of work?+
Proof of work gives independent nodes an objective way to compare competing valid histories. The chain with the most accumulated work is expensive to replace and needs no central timekeeper or voting registry.
02Can miners change Bitcoin’s supply with enough hashpower?+
No. Hashpower can choose among blocks that satisfy consensus rules, but full nodes reject a block that creates too much bitcoin or spends coins without valid authorization.