Immutability Mechanics: The Domino Effect

The use of the Previous Block Hash creates a "Domino Effect" that provides Bitcoin with its Immutability. Immutability doesn't mean the data cannot be changed; it means that changing it is so computationally expensive that it is practically impossible.

1. The Sensitivity of SHA-256

SHA-256 is highly sensitive to even the smallest change. This is known as the Avalanche Effect. * Original Header: Block Hash A * Header with one bit changed: Block Hash B (completely different)

2. Breaking the Chain

If an attacker wants to change a transaction in Block 700,000: 1. They modify the transaction data. 2. This changes the Merkle Root in the Block 700,000 header. 3. Changing the Merkle Root changes the Hash of Block 700,000. 4. Since Block 700,001 contains the prev_hash of 700,000, Block 700,001 now has an invalid link. 5. To make 700,001 valid again, the attacker must re-mine it (find a new nonce). 6. This change cascades through every subsequent block (700,002, 700,003, etc.).

3. The Cost of Revision

To successfully change the past, an attacker must re-mine the modified block and every block that follows it faster than the rest of the network can mine new blocks. * If the attacker has 10% of the hashrate, they will never catch up. * The deeper a block is in the chain (more "confirmations"), the more immutable it becomes.

4. Finality Depth

This is why exchanges wait for 3 to 6 confirmations. By the time a block is buried 6 layers deep, the amount of energy required to "undo" it is equivalent to billions of dollars in electricity and hardware.

In the next section, we will look at the only block that has no parent: the Genesis Block.