Bitcoin Universe · Model 03

The chain of blocks

Why the past is so hard to change: every block is sealed by its own proof of work and chained to the one before. Open blocks, change a payment, try to repair the damage, and see which branch nodes follow.

Simulated blockchain · real hashes
Topic 1 of 6

Linked blocks

Competing branches

After the forkBlocksValidWorkNodes

Work is the expected number of attempts: the front row's blocks each needed about 4,096, the back row's about 1,024.

Block 3

Transactions

    Header

    Merkle tree

    Valid seal, intact link, changed data and the followed branch Broken seal: the contents no longer match the header Simulated blockchain

    Every hash, Merkle root and proof-of-work check on this page is computed for real (SHA-256d). The transactions are short demonstration texts rather than serialized transactions, and the targets are educational, so a block needs thousands of attempts rather than about 10²³. The competing branch is invented to show that chain length and chainwork are different things; its easier target is set by hand. In real Bitcoin the target is fixed by consensus for every height, so branches within the same 2016-block period share it.