Optimistic rollups are additionally referred to as fraud proof rollups as a result of they use fraud proofs to make sure safety. Mainly, when the rollup posts its knowledge to Ethereum, there’s a 7-day window the place any rollup validator can submit a dispute with a fraud proof with a purpose to appropriate one thing improper concerning the submitted knowledge.
If the fraud proof is created correctly, the rollup will appropriate the errors. Which means that you solely want a single trustworthy validator on the rollup to make sure safety, as a result of that one validator can watch the chain for any fraud. You do not want tons of or 1000’s of validators to vouch for the information as a result of it is posted to Ethereum, the place 1000’s of nodes agree on its state.
With an alt-L1, it does not essentially put up any knowledge to Ethereum. It’s ruled by its personal consensus guidelines on what’s a legitimate block, and more often than not if 51%+ of the validators are trustworthy there is not any concern. If 51%+ are dishonest, they will roll again blocks or in some circumstances validate invalid blocks.