Proof of work vs proof of stake

Why does a blockchain need a consensus mechanism?

A blockchain is a ledger copied across thousands of computers that do not know or trust each other. Anyone can broadcast a transaction, and anyone could try to broadcast a conflicting one — for example, spending the same coin twice. The network needs a rule for deciding which block of transactions gets added next, and for making that decision expensive to fake.

That rule is the consensus mechanism. It answers three questions: who is allowed to propose the next block, how everyone else checks it, and what an attacker would have to give up to rewrite history. Proof of work (PoW) and proof of stake (PoS) answer the third question in opposite ways. One makes cheating cost energy that has already been burned; the other makes cheating cost collateral that can be taken away.

How does proof of work choose the next block?

The Bitcoin whitepaper describes the process in six steps. New transactions are broadcast; each node gathers them into a candidate block; nodes race to find a valid proof of work; the winner broadcasts its block; others accept it only if every transaction is valid and unspent; and they show acceptance by building the next block on top of it.

The "work" is a guessing game. A miner repeatedly changes a small number in the block header, called a nonce, and runs the header through the SHA-256 hash function. A block counts only if the resulting hash is below a target set by the network. There is no shortcut: finding one means trying enormous numbers of guesses. Bitcoin Core sets the target spacing at 10 minutes and resets difficulty every 2,016 blocks so the average stays near that pace. We walk through the full process in how Bitcoin mining works.

Nodes follow the chain with the most accumulated work. To rewrite past blocks, an attacker would have to redo their work and then outpace everyone else, which the developer documentation says requires a majority of the network's hashing power.

How does proof of stake choose the next block?

Proof of stake replaces the guessing race with a lottery among people who have posted collateral. ethereum.org describes it as a way to prove that validators have put something of value into the network that can be destroyed if they act dishonestly.

On Ethereum, a validator deposits 32 ETH into a deposit contract and runs the required software. Time is cut into slots of 12 seconds and epochs of 32 slots. In each slot one validator is chosen at random to propose a block, and a randomly selected committee of other validators votes on, or "attests" to, whether it is valid. When validators holding at least two-thirds of all staked ETH vote for the same checkpoint pair, the earlier checkpoint becomes finalized — reversing it would require destroying a large amount of stake.

Validators who go offline lose small amounts; validators who sign contradictory messages can be slashed, meaning part of their deposit is destroyed and they are removed. ethereum.org notes slashing ranges from under 0.1% of a validator's balance in isolated cases to 100% when many validators misbehave together. For how ordinary holders take part, see what is staking.

What are the key differences between proof of work and proof of stake?

The table below compares the two designs as they run on Bitcoin and Ethereum today. Figures come from the Bitcoin Core source code, the Bitcoin developer guide and ethereum.org.

ethereum.org also lists PoW's strengths: anyone can start mining without owning the coin first, and the model has a long security record. Its weaknesses are heavy energy use, costly specialized equipment and the risk that mining pools concentrate power. Proof of stake has its own weak spot: stake can pile up with a few large providers, a risk ethereum.org itself flags on its staking pools page, warning that it creates conditions for censorship and single points of failure.

Why did Ethereum switch from proof of work to proof of stake?

Ethereum launched with proof of work. In December 2020 it started a separate proof-of-stake chain, the Beacon Chain, and ran the two side by side. On September 15, 2022, in an upgrade called The Merge, the original chain's transactions moved onto the proof-of-stake system and mining on Ethereum ended. ethereum.org estimates the network's energy consumption dropped by about 99.95%.

Bitcoin still uses proof of work, and the choice remains a live debate in crypto. Which trade-off is "better" depends on what you value — open entry and physical cost, or efficiency and explicit finality — and the blockchain trilemma explains why no design maximizes everything at once.

What mistakes do beginners make about proof of work and proof of stake?

• Thinking miners or validators can create any transaction they like. They choose the order and contents of a block, but every node checks the rules. A block with an invalid transaction is simply rejected.
• Assuming "51% attack" means the same thing in both systems. In PoW it is about hash power; in PoS the thresholds are about stake, and different shares enable different attacks (33%, 51%, 66%).
• Believing proof of stake means anyone holding coins earns rewards automatically. Rewards go to validators doing the work, and they carry penalties and slashing risk.
• Treating a single confirmation as final on a PoW chain. Settlement strengthens with each block added on top; services usually wait for several.
• Expecting a consensus change to make fees cheaper. The Merge shows it does not by itself.