Proof of Work (PoW)
Proof of work (PoW) is a consensus mechanism in which miners compete, using computing power, to find a valid block. The winner adds the next block to the chain, and the cost of that work is what makes rewriting history expensive.
How proof of work works
Miners bundle pending transactions into a candidate block and then hash its header, running it through a function that outputs a seemingly random fingerprint. The block is only valid if that hash is below a target number set by the network. The only way to find one is trial and error: change a small value called the nonce, hash again, and repeat, often trillions of times per second across the network.
Finding a valid hash is hard, but checking one is instant. Any node can verify the work in a fraction of a second, so miners cannot fake it. The network adjusts the difficulty so that, in Bitcoin's case, a block is found about every ten minutes on average regardless of how much computing power joins.
Why it provides security
Nodes follow the valid chain with the most accumulated work. To reverse a confirmed transaction, an attacker would have to redo the work of that block and every block after it, and outpace the rest of the network while doing so. On a large network like Bitcoin, that requires a vast amount of specialized hardware and electricity, which makes such an attack extremely costly.
Energy use and criticism
The security comes directly from spending real resources, so PoW consumes a lot of electricity, comparable to that of some countries according to widely cited estimates. Supporters argue that this cost is the point and that miners increasingly use cheap or stranded energy; critics point to the environmental footprint. Ethereum switched from proof of work to proof of stake in September 2022, in an upgrade known as the Merge, largely to cut energy use.
Limits to keep in mind
PoW is only as strong as the computing power behind a chain. Smaller proof-of-work networks, which share mining hardware with larger ones, have suffered 51 percent attacks in which attackers rented enough power to rewrite recent blocks and double spend. Mining has also become concentrated in large pools, which is a centralization concern even if individual miners can switch pools.
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Frequently asked questions
Which cryptocurrencies use proof of work?
Bitcoin is the best-known example. Litecoin, Dogecoin and Monero also use variants of proof of work.
What is the difference between proof of work and proof of stake?
In proof of work, miners spend computing power to propose blocks. In proof of stake, validators lock up coins as collateral and can lose part of them for misbehaving.
Is proof of work wasteful?
It depends on what you value. The energy buys security without trusted parties, but it has a real environmental cost that critics consider too high.
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