What Is State Root?

On Ethereum, each account's balance, nonce value, contract code and storage data are kept in a tree structure called a Merkle Patricia Trie. The nodes of the tree are hashed from the bottom up and a single root value forms at the top. After the transactions in a block are executed, the root of the resulting new state is written to the block header; even the smallest change in the state changes this root completely.

Thanks to the state root, nodes can check that they have reached the same state after executing the same transactions by comparing a single value. Light clients can also verify the balance of a particular account against the state root in the block header with a Merkle proof, without downloading the entire state. Rollups also send their own state roots to the main chain so that withdrawal requests can be verified; in optimistic rollups, challenge processes run over these roots.

As the state tree grows, the size of proofs also grows, and this is an obstacle for light and stateless clients. Verkle trees on Ethereum's roadmap are a proposal that aims to perform the same function with much smaller proofs. Bitcoin, on the other hand, uses the UTXO model instead of an account model, so there is no general state root in its block header.