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In Bitcoin's protocol (and similar PoW systems), at one point chain might temporarily split in two and two different chains will be maintained up until one outpaces the other and proves itself as the correct one.

I thought this phenomena is not possible in Cardano since for every slot a delegator is already selected beforehand so we theoretically know from which validator/delegator to expect new block i.e. we cannot have two validators creating the same block for the slot. This is what I thought up until watching a video by Cardano's chief scientist who made be doubt my premise (https://youtu.be/nB6eDbnkAk8?t=1563).

I would love to know if I am getting something wrong? From the video it seems he is explaining the scenario that I thought cannot even happen in Cardano's protocol.

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That's incorrect. The ouroboros praos protocol is based on random distribution, and there's still a low probability of multiple pools being assigned the same slot. With as many slots as there are, even with a low probability this still happens fairly regularly. Furthermore you can not have a collision on the exact slot and not have consensus with the next pool and have a fork because a pool after yours made a block without yours in the chain. The security window is 3k/f which is the largest a chain can ever rollback which is 36 hours on Cardano mainnet.

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Cardano is a permissionless public blockchain with a fully dynamic validator set, which means that the validators/miners can join/leave the network at any time. Much like Bitcoin and Ethereum, this means that anyone can join and contribute to block production. However, this also means that it is possible for honest parties to propose multiple valid chain extensions, thus creating a fork. The network must then employ some kind of chain-selection rule to eventually converge on a canonical chain.

Cardano can have two validators creating the same block for the same slot, even though the leadership scheduled is (privately) known. Such temporary (a.k.a. "shallow") forking is healthy and natural behavior for permissionless systems with probabilistic finality, like Bitcoin, Ethereum, or Cardano. In such systems, security guarantees are probabilistic in nature - the deeper a given block is from the current local height, the higher the probability of that block being on the canonical chain.

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