Proof of Work (PoW)

Proof of Work requires miners to repeatedly hash candidate block headers until one produces a result below the network target. Finding such a hash is hard; verifying it is instant. This asymmetry lets anyone confirm that real energy was spent to produce each block, anchoring the blockchain's security in physics rather than trust.

Because rewriting history would require redoing all the accumulated work faster than the honest network produces new blocks, PoW chains like Bitcoin become exponentially harder to attack as hashrate grows. The energy cost criticised by outsiders is precisely the security budget that makes the ledger immutable.