Unit 3 · Level 2 · Layer 2s
Optimistic vs ZK
How does Ethereum know a rollup's batch is honest? Two schools. Optimistic rollups (Arbitrum, Optimism) assume batches are valid, but leave a challenge window (about seven days) during which anyone can submit a fraud proof showing a batch was wrong. ZK rollups skip the waiting: every batch ships with a validity proof, a piece of cryptographic math that demonstrates correctness up front. Innocent-until-challenged versus proven-innocent-on-arrival.
Free to play. No ads, no token, no account needed to start.
What you get asked
Match each term to its meaning
Two different ways to answer one question: why should the L1 believe what the L2 tells it?
Why do optimistic rollups keep a roughly 7-day challenge window?
'Optimistic' means unverified by default; the window is the safety net. One honest watcher with a fraud proof is enough to stop a bad batch.
What's the headline advantage of ZK rollups?
The math arrives with the batch, so the L1 can accept it immediately, with no seven-day suspense. ('ZK' is short for zero-knowledge, the proof family used.)
Optimistic rollups assume batches are valid unless someone submits a ___ proof.
Fraud proofs punish lies after the fact; validity proofs prevent them up front. Same goal, opposite timing.
Honest question: if ZK skips the wait, why does anyone run optimistic rollups?
Generating validity proofs takes serious computation and engineering; optimistic designs shipped earlier and stay simpler. It's tradeoffs all the way down, as usual. 🐜
The rest of this unit
Rollups: do the work upstairs, settle the bill on Ethereum.