Chapter 2 · Mechanics · lesson 5
Consensus, and what finality means
10 min
Builds on Mine a block by hand
Chapter 2 · Mechanics · lesson 5
10 min
Builds on Mine a block by hand
Thousands of nodes, no referee, and messages that arrive in different orders. Somehow they all have to agree on one history.
That agreement is consensus, and it is the last of the two problems from track 0.
You mined a block by hand in track 0, so you already know the proof-of-work answer: make producing a block cost real electricity, and rewriting history means out-spending everyone.
Ethereum replaced that in 2022. Under proof of stake, validators lock up 32 ETH as collateral. Propose two conflicting blocks or attest to something false and a chunk of that deposit is destroyed — slashed.
Both mechanisms do the same job through different currencies. Proof of work prices attacks in electricity you must keep buying. Proof of stake prices them in capital you have already handed over and can lose.
Proof of stake has one genuine advantage beyond energy: with work, an attacker who fails keeps their hardware. With stake, an attacker who fails loses the stake. Failure itself becomes expensive.
Predict
Ethereum groups blocks into epochs of 32 slots, about 6.4 minutes. Validators vote, an epoch becomes justified, and once two consecutive epochs are justified the earlier one is finalised.
Reversing a finalised block requires at least a third of all staked ETH to be slashed — billions of dollars, deliberately destroyed. That is what "final" means here: not impossible, but priced beyond reason.
Check
Both mechanisms fail at the same threshold, for the same reason.
An attacker with a majority of the hash rate — or of the stake — can consistently outpace everyone else and decide which history wins. Below that they are just an expensive participant.
What a majority attacker cannot do is forge signatures. They cannot spend your ETH or invent transactions you did not sign. Their power is over ordering and censorship: reversing their own recent payments, or refusing to include yours. Serious, but narrower than the phrase "51% attack" suggests.