Unit 1 · Level 1 · What is a blockchain
Consensus: how strangers agree
Thousands of computers around the world each hold a copy of the ledger. So who decides which new block is the real one? There's no CEO, no headquarters, no referee to ask. The rules that let total strangers converge on one shared history are called consensus. The two famous flavours: Proof of Work and Proof of Stake.
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What you get asked
What core problem does consensus solve for a blockchain?
If two versions of history existed, you could double-spend across them. Consensus is how strangers with no boss all land on the same single story.
In Proof of Work, why does burning real electricity make Bitcoin secure?
Miners race to solve a costly puzzle for the right to add a block. To fake history you'd have to redo all that work faster than the whole honest world. The energy spent becomes a wall.
In Proof of Stake, validators lock up coins that get ___ if they cheat.
It's literally called slashing: cheat and the network destroys part of your locked stake. Capital at risk replaces energy burned. The idea is the same: cheating must hurt.
Match each term to its role in consensus
Both systems make one thing true: attacking the network costs far more than playing fair. Only the currency of the cost differs: electricity or locked coins.
What happened in Ethereum's famous 2022 'Merge'?
In September 2022 Ethereum swapped miners for validators mid-flight, cutting its energy use by roughly 99.9% without stopping the network. Like changing a plane's engine while flying it. 🐜
The rest of this unit
Blocks, consensus, and why any of this works without a boss.